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MG MGB Technical - Sputtering, popping, waking the neighbors - MGB

Sorry for the double post - first in MGB General then realized it should be here in Technical...

My MGB started acting up a few days ago, especially when cold - the worst is when first starting up in the morning.

It sputters, pops, backfires and generally acts horrible.

This morning was the absolute worst of all, so much that I attempted to turn around in the middle of the street to take it back home.
It died when I was broadside in the road, so I turned the key and it started and VROOM! It ran smoothly. I trundled off to work and it ran perfectly strong!

What would cause this kind of on again, off again horrible running?

I have pulled & cleaned plugs, I have checked the cap & rotor. My new distributor seems to be working fine, the one I got from Advanced Distributors.
I have rebalanced the carbs.

When it's not sputtering & popping it is running perfectly smooth.

Could a block in the fuel line cause this?

I had motor oil in the dash pots and I replaced this with 3 in 1 oil yesterday which didn't make any difference.

A couple weeks ago the hose connecting to the oil pressure sender blew out & oil got all over the place on that side. I think I cleaned it all up, but could there be oil in something causing this?

I'm at Wits End here.
Rick Bastedo

Rick...give us more details...year of car, carb set up..since you mention block in the fuel line...is that a possibility. do you have rust in your tank, etc..do you have a transparent fuel filter...do you have smog equipment? does it ever do the same thing when it is warm?
jjralston

Check your spark...if its good...Look at the plugs in the midst of one of these events...Then I'd say carbs...maybe float valve issue in one...or mixture...how is the vacuum capsule?...
Pete

1970 MGB, dual SU carbs, clear glass inline filter (looks clean).

It has been running good all the time I've owned it (about 15 months). Great since I got the distributor from Advanced (Jeff Schlemmer).

Seems to be "ok" when warm. The weirdest thing is that starting it again after it died this morning it ran great. Before dying it was popping & sputtering so bad that I was just going to try to roll it down the hill back home. Immediately after restart it was perfect.

Rick Bastedo

Make sure your jets are not sticking in the choke position. I've been having that problem on my 65 that is in need of new or rebuilt carbs.

I thought it was a sticking needle valve but when poking around during a poor running after a start I found the jet was stuck down. A push on it and it snapped into position.

Until I really repair things I open the hood after starting and snap the jet back up. A real pain.
Robert McCoy

Rick - been having the same, and again, it was since the cold weather arrived. Again, mine (73BGT, HS4 carbs) would be pretty awful then once warmed up, no worries, but a little hesitant, and just not as responsive as it was a week ago. I know my carbs were good as I have just reconned them and all is well. My vac capsule is good. My spark is strong, plugs are good, leads are good. Havent checked timing with a strobe yet, but as of this morning I know (well, I think) thats the problem. I advanced the timing by a small amount (barely preceptible turn of the distributer, no more than a couple of degrees advance). reason I did this was last night i suddenly relaised that the onset of cold weather could be coincidental, or just exacerbating something already there. I noted that the usual slight pinking/pinging I had when on a hill in top gear at a low speed was no longer with me. So, figured I am as well to advance it a wee bit this morning to see what happens - and it seems to have improved - not perfect, so need to do a proper job at the weekend, but seems I might be on the right lines. However, I now ask what the hell has retarded my timing in this way? I never touched the timing your honour, and the only thing I have done is rebush the rear suspension, which is, well, unlikely!
Mick
Mick

Rick,

Check for a vacuum leak around the intake manifold, hoses, and carbs. Check tightness of carb to manifold nuts. Spray WD-40 around all intake connections to see if you get a change in operation.

Regards,

Larry C. '74 B/GT & '69 midget

Larry C '69 Midget

Thanks guys, I did spend a lot of time on the carbs a couple days ago thinking that was the cause. They are working very smoothly, but I will check again for leaks.

I will also remove the fuel filter and see if it's got some foreign material in there.

Should the top of the coil be at all loose? Mine moved a slight bit when I was reconnecting wires there while I was looking at the spark side). I re-did the connections, still this didn't show me a "smoking gun".
Rick Bastedo

Mick, Rick,


definitively not an issue with the rear spring bushing.. To setup more advance only allow bypassing the initial problem. Dampness might be the solution, have a look inside the dizzy cover and check its condition at first. When restarting the engine does it keep on running fine or not, i mean the necessary time to fill empty the float bowl? If so, may be a fuel pump supply. Oil may have polluted your dizzy too, there was a flush at oil sender blow out time.
Regards.
RG
Renou

BTW when it's running right it idles at about 1000 rpm and runs up to 6000 rpm in 4th gear. Yeah that's kind of a bit over the recommended speed for our freeway driving, but I had to test. :)
Rick Bastedo

"Oil may have polluted your dizzy too, there was a flush at oil sender blow out time"

Should I open it up & spray it with something? Carb cleaner?

It is a pretty fresh Distributor but it may well have oil inside due to the quarts that leaked out just above it when that hose blew.
Rick Bastedo

I sprayed the inside of the cap and the distributor with carb cleaner & let it sit to dry out for about 90 minutes.

Then I drove home. The car started and ran great for about 10 miles freeway speed.

I got to the top of the hill and it started again.
I pulled off the hiway and it seemed to clear up so I pulled back on the road and drove home.

Either there's something bad in the gas or ???

I have a Facet fuel pump here I'm going to put that inline and see what happens.
Rick Bastedo

I just ran it about 10 miles round trip after putting a Facet fuel pump inline. It ran ok, no popping or missing.

Will this harm my SU pump?
Rick Bastedo

Rick,

Are you familiar with David DuBois' web site? He has some instructions for installing the back up pump in tandem with the SU. He also rebuilds the SU pumps, and he is in Washington.

Charley
C R Huff

Rick, No, it won't harm the S.U. pump as the fuel will go past the inlet and outlet valves. RAY
RAY

Rick,

Intriging issue, do tou mean that since the fuel pump has been swapped, the problem no longer occurs?
Or may be a reluctant car to motorway, it sounds this problem happens each time in same conditions (lol) ?
Rgds
RG
Renou

The additional fuel pump did not fix the problem.
This morning was not as bad as yesterday, however it was still bad.

It always happens first thing in the morning.
The conditions are:

5:00AM it is dark and about 35F.
Engine starts, but runs rough with or without choke on.
Pull onto street and it starts missing, if I give it more gas then it will pop and sputter.
I let it idle a minute, then I can go a bit farther.
Getting up to speed, about 30 mph, if it starts to pop & miss and I give it more pedal it will backfire.
Pull off the road, turn engine off, wait a minute, start engine and pull away.
Rinse & Repeat.

Eventually it has straightened out and ran good enough to drive it to work. This is at Interstate highway speeds, then I get off the highway and drive about a half mile on surface streets about 25 mph to the office.

This morning it missed a few times on the freeway, but nothing bad.

Maybe there is a stuck valve in a float chamber, I will take them apart tonight.

I removed the inline fuel filter and put the facet pump in its place. The filter had no restrictions to flow.
Rick Bastedo

It sounds like condensation somewhere in the HT system. It's the right time of year for it and the problem always goes away if you let it sit for a while. Try warming up the HT leads and dizzy cap in the airing cupboard for a while, then when you refit them spray with ignition sealant to keep the damp out.
Miles Banister

At lunchtime I pulled the tops off the float bowls.
Looks perfect in there, valves are operating correctly.

One of the backfires blew a 2 inch hole out of the bottom of my muffler. The car is a bit too loud now.
The muffler was rusty, so the metal was weak.

The cap & wires are relatively new, all bought this year.
I'll keep looking and checking all the suggestions as long as you all want to keep giving them or until this is fixed.
Rick Bastedo

The ongoing saga continues...

It was horrible driving home today, I'd rather drive the Midget with one broken motor mount than try driving this thing again.

Yes, I am about to admit defeat.

At least until I can get some parts together and replace a bunch of stuff.

I'll start with the coil, then the condenser, then the plugs, etc...
Rick Bastedo

Hi Rick
I would imagine a bad condenser could cause this
or a bad coil. I know it's geting pretty chilly up your way this time of year why don't you try a bottle of good isopropyl gas line antifreeze? if you don't keep the tank full you could be getting excessive amounts of condensation in there. May be iceing up first thing in the AM then you may be getting moisture from the tank after it's warm.
Mike
'70 BGT in NY
MK Mike

Hi Rick,

A quick check for the coil. Is it really hot when the issue occurs ? Not a definitive idea but a way to go with. Capacitor may become complex to diagnose. Yet more infos, the new ones are really prone to strange failures too. I don't believe that much to carbs icing: it should take a bit time to get at it and is easily checkable.
Did you recently check out your automatic advance devices at dizzy bottom, they regularly should be oiled and cleaned. The return springs are in good order too?
Good luck and please thanks for the feed backs.
rgds.
RG
Renou

Rick, is this one of those rotor problems where some recent rotors were made with too high a carbon content? The result is shorting through the rotor spring to the distributor shaft.
Another possibility is a broken wire inside the distributor, the small one with the loose cloth cover that provides a ground for the plates.
Barc Cunningham

Yesterday afternoon it was worse than ever, it was warm & sunny outside and about 60F.
The car was well warmed up and ran horrible.

The Distributor is one I got from Jeff Schlemmer of Advanced Distributors this year. It's a great unit and hasn't given me any trouble. The cap & rotor and all the innards came from Jeff. It was totally set up & ready to install when I received it and it ran great.

The Distributor had several quarts of 20W-50 sprayed on it when the oil pressure sensor hose blew out.

That seems to be about the time this problem started too, but it seems to be getting worse.

Could that hot oil have cooked my cap or rotor?

I also have a tachometer that drops out, just goes to zero then bounces back up and works right randomly.
Since it works and doesn't work while the car is running right I haven't mentioned it before. Now I'm reaching for anything.

I plan to get a new condenser, cap, rotor, plugs & coil and replace them all. Maybe I'll get lucky.

At least I know it wasn't my fuel pump.
Rick Bastedo

" also have a tachometer that drops out, just goes to zero then bounces back up and works right randomly.
Since it works and doesn't work while the car is running right I haven't mentioned it before..."

A random electrical issue? Any link with the oil spray? As you mentionned it at this thread start, there seems to be a narrow connection between your actual situation and the under bonnet oil spray.
Good luck.
Renou

I had the EXACT same problem, turned out to be a dodgy ignition switch. Loads of suggestions, no solutions. Not condenser, as electronic ignition. Clearly not fuel, as it wouldn't backfire if there was no fuel! Obviously the bad connection inside the switch was wasting some volts (sorry, mechanical engineer...) - the ignition is fed from the switch - and that was causing a problem only when engine was cold (presumably needs a stronger spark then). Try putting a 12V bodge wire direct from the battery to the coil and see if it still happens.

Neil
Neil

Neil,


Amazingly, your suggestion confirms the random character of this kind of problem.
RG
Renou

Wow Neil, that would definitely do it.
Just hadn't thought along those lines. I will do as you say on the weekend.
Rick Bastedo

Rick, have you tried giving your points/contact breaker a clean (presume you have points?) I had similar experience, and a quick scrub with some rough paper cured things.

Good luck.

E S BOWDEN

I did spray them but assuming that wasn't enough I will also hit them with some fine sandpaper.
Rick Bastedo

If it is anything to do with the ignition switch, points or anything else (bar the condenser) in the ignition LT circuit including the points or ground wires inside the distributor you will see the tach flicking about all over the place. Problems with the condenser are the only thing in the LT circuit that doesn't show up on the tach, to detect those you need to measure the spark strength, a timing light may reveal it, other than that just swap the condenser. However if the tach is flicking about when the car is running normally, this can be misleading. 73 and earlier cars can have problems in the tach circuits which don't affect running (unless the sense wire is shorting out), with 72 and earlier if the tach trigger circuit is faulty i.e. going open-circuit or briefly short-circuit it will affect running. If the tach was flicking about *before* this running problem started, you should have fixed the tach, then it would have been a better diagnostic!

An ignition (LT or HT) problem will also cause backfiring in the exhaust - fuel gathering during the non-firing cycles and igniting with a bang when you get the next firing cycle. This can indeed cause exhaust damage. You won't get this with a fuel supply problem.
Paul Hunt

I take the point about the tacho, and I'n no sparky as I said, but I'm assuming that the tacho would work as long as there is a spark. But if the spark is very weak, then the engine will not run very well and there may be incomplete combustion (hence the backfiring). I cannot remember what my tacho was doing, but I do recall that the engine ran fine if not loaded to much - it it would not actually stall if I dipped the clutch and would keep pulling weakly if I kept the revs up. Then it would be fine after a while ie when warm. That to me would indicate weak spark rather than intermittent no spark.

Neil
Neil

You can have tach without a spark, also a spark without tach, it all depends what is wrong. In the later case it is unlikely to affect the running of the engine ... unless you have more than one problem which is something that must always be borne in mind.
Paul Hunt

Thanks guys,

I'm kind of in a quandary about just where to start.
Yes Paul, it would have been better if I had fixed that tacho before this started.

Now I just feel like I should start bypassing things until it runs right (if ever).
Rick Bastedo

"I also have a tachometer that drops out, just goes to zero then bounces back up and works right randomly.
Since it works and doesn't work while the car is running right I haven't mentioned it before."...

Check the large brown wire connection at the starter
solenoid (underneath the car). On my '71 B/GT it's
a push-on tab connector.

A blown oil transmitter could've dripped oil all over it
and engine vibration may have loosened the connector.

Maybe it's worth a look.
Daniel Wong

Thanks Daniel,

I will start there as it really is a very reasonable possibility!!
Rick Bastedo

Rick, my 79 had a similar problem that I just fixed yesterday.

I had backfiring through the carburetor Weber DGV

Starts right up, tach working fine, Lumitronic Ignition. New plugs

Lived with the backfiring on Wednesday

Thurday, start up and missing - run on 3 cylinders.

#1 plug wet, not firing, all othr plugs are fine. tach is working fine - no bouncing. oil is clean, coolant is full.

Open up distributor, check inside cap for arcing or carbon tracking. Clean off "fuzzy" corrosion on the contact ends and clean the rotor.

97% of problem is solved.

I will buy a new distributor cap and rotor as I still get a slight "stuttering" under load.

Thinking it through, the engine was rebuilt about 10k ago and the distributor, cap and rotor replaced.

Cheers

Gary :>{D
79mgb

gnhansen

Well I put in new plugs and cleaned up that big brown wire connection at the starter today.

No joy.

Still runs very rough, sputters & backfires.

I know there's a whole list of possible things to switch out for new in the process of elimination, but I'd rather not spend that much money!

I can get a new cap & rotor, that's the next thing I would normally do anyway.
Rick Bastedo

Rick,

Fisrt test to carry on, disconnect tacho as mentionned by Paul Hunt: an erratic short ( heat? ) inside it might turn into random ignition system losses. Further checks: control connectivity onto electrical wiring.
Regards.
RG
Renou

I had a VERY similar problem. turned out the be a dodgy connection on the little pig-tail wire in the dissy.

Easy to try. Connect a wire from the dissy base plate (use an existing nut/bolt/screw fitting) direct to any convenient earth (chassis/engine). If the problem goes you know the cause. My wire looked fine, checked out fine with a meter and a test lamp, but was making and intermittant and resistive connection. Idled OK, popped and banged under load and would not pull at all. I thought it was condenser but actuall the wire. Now has a permanent fix - new wire and backup wire.

Gary
GN

We are supposed to get some clearing skies later today so I'll see about getting out there to look at it.
Thanks!
Rick Bastedo

I have just spent some "quality" time with the B.

Before I started it would start, run roughly, spit & cough & backfire.

After a bit of work it wouldn't start at all.

After a bit more work I got it back to how it was when I started. frown

Here is what I know:

The points were set a bit tight, I adjusted them to .014
When it "idles" it backfires and spits a bit of fuel spray out the carbs.
When I can get it to run up at 4000 - 4500 rpm it runs a little rough but not as bad as at 'idle'.

When I turn the key on and manually open the points I can't see a spark.
If I open up the points by turning the engine a bit then cross between both sides of the points with a screwdriver I do get a spark.

Meter from plate to ground = 0 ohms

Meter from points to coil = 7 ohms (what should it read?) That's to the wire that connects to the side of the distributor via the spade lug.

Meter from one side of points to other when closed = 0 ohms
Rick Bastedo

I put in a different coil - same (if not worse).
I could almost get it to run.

I put the old coil back in and it ran but very badly.

I thought I had a winner when I started the swap as the lead to the positive side slipped out of its connector.
I put on a new connector but that clearly wasn't the problem.

I suppose I will order a new cap, rotor & condenser just for grins.
Rick Bastedo

Rick,

I know this post has been going for a while, so forgive me if I am repeating previous advice.

I think your last post was getting on to the mark. I think you need to do all the stuff you normally do in a tune up.

That is, replace the points, condenser, rotor, cap, plugs, and plug wires. Then go back to the factory recommended motor oil in the carb dampers, and make sure your carb pistons move freely and drop at about the same rate. While you are at it, pull the dash pots and see if both jets are descended about the same distance in the jet holder, and check to see that the "choke" is not hanging up.

Check to see if your vacuum advance plumbing is correct, check to see if your vacuum advance pod is working and that it does indeed move the point plate in the distributor. Check to see that the mechanical advance is free and that it springs back to the retarded position. Then set the timing. Clean the connections that feed the low tension side of the ignition Then adjust the valves and see that all valves move freely.

Check your PCV system to see that it is not at times dumping too much air into the intake.

When you get that done, you should be back to a baseline. From there, if you still have the problem, you can look for more obscure problems knowing that the basics are in order. Maybe run a compression test several times and see if you can get the same results each time?

If you still have the problem, I would try a hot wire straight from the battery to the coil, but give consideration to your car's need for a resistor.

Charley
C R Huff

Rick. Clean up all the terminals at the fuse box and replace all the terminal ends on all the wires. Also pull all the fuses and clean the terminals inside the fuse box. That cleared up all my electrical issues. I too had a backfire issue this past summer. Check to see if the coil wire is fully inseted into the coil. If you have an old condensor, from your last tune up,
swap it with your newly installed one. Just because the condensor is new does not mean it is a good unit. Then try it out. You have nothing to lose and you will spend about $5 on new terminal ends - cannot hurt.

cheers

Gary :>{D
79 MGB
gnhansen

No point cleaning up the fusebox - for an ignition problem anyway - as the ignition feed on a 72 and earlier goes nowhere near it. Even on 73 and later whilst the white from the ignition to the coil does feed other things on the fusebox, it doesn't go through any of its connections.

Is the tach still flicking around? A 1970 should have an RVI (on the faceplate) tach and on these the ignition feed goes through the pickup in the tach and in your case probably via spade or bullet connections.

Bypass the tach by jumpering from the white at the fusebox to the coil +ve, making sure there is a good clean connection at each end of course.

If you still get the misfiring then it isn't the tach, unless perhaps the white is intermittently shorting to ground. To eliminate this jumper from the brown on the fusebox to the +ve on the coil, removing the white from the +ve, and turning off the ignition. Factory wiring on a chrome bumper car has a 12v coil and no harness ballast so this won't cauyse any problems. This will stop the fuel pump but the engine should continue to run for a few minutes, if the misfiring carries on you know it is nothing to do with the feed to the coil +ve being intermittent open-circuit *or* short-circuit.

I note it happens with two coils which tends to eliminate them, but both could have a similar problem, if you can try both on other cars or borrow a third that would eliminate the oil still further.

It is also important to check you have the correct coil and the only way this can be done is by measuring the primary resistance, which should be between 2.5 ohms and 3 ohms for a chrome bumper car. Ignore any labelling on the coils which can be confusing if not downright wrong. If you measure 1.2 ohms to 1.5 ohms it is a 6v coil which needs a similar ballast resistance between the coil +ve and ignition supply. This is contained inside the harness of a rubber bumper car, it should not be necessary in a chrome bumper car which should have a 12v coil as I say.

If you have a 25D4 distributor (knurled adjuster on the end of the vacuum capsule) the coil wire goes to a spade on the body of the distributor. There are two braided very flexible wires inside the distributor - one from this spade to the points and the other a ground wire from the points plate to the distributor body. These wires are continually flexed back and fore under changing vacuum advance, and eventually fray and the conductors break, giving intermittent connections. The first thing to do is disconnect the distributor vacuum pipe at the carb and plug the carb port. If the misfiring stops now then it quite probably is one or both of these wires faulty. If it still carries on it could still be these wires, with the vacuum advance temporarily disconnected it should be possible to solder additional wires to efefctively bypass the existing wires. If *that* stops the problem then bypassing just one of them should show whether one or both is faulty. In understand the points wire is available, but the points plate wire is spot-welded to the plate and on the face of it not replaceable. However Moss UK sells a suitable ground wire which is really intended for the 45D distributor, but may also fit the 25D between the condenser screw and the screw on the body.

If you have a 45D then much the same conditions apply, except that the points wire goes through a hole in the body to a flying spade connector and is plastic insulated, although more flexible than standard wire. This wire comes with a replacement condenser and the Moss ground wire should be a direct replacement.
Paul Hunt

Paul, when you say: "jumpering from the white at the fusebox to the coil +ve" do you mean I should remove the wire from the + side of the coil and jumper from white at fusebox to that terminal or simply attach a jumper with the other wire in situ?

Thanks again for all this help, I'll get out there again today and see what I can find.

This is a 25D distributor, it was received from Jeff Schlemmer of Advanced Distributors earlier this year.
As soon as I installed this unit the performance of my car went from granny to hotrod - it really was a night & day experience where before it would hardly go past 4000 rpm it went right up to 6K after the switch from the old tired unit to the one from Jeff. It felt like it would just keep on going but I rarely took it up past 5000 rpm just to be "safe".

It sounds like I need to do some process of elimination with the tach. Today I will receive new cap, rotor and condenser so I will install those and when that doesn't fix anything I will go ahead and bypass the tach also.
Next I'll replace some wiring in the distributor, however it is all new stuff and Jeff's work has earned him a great reputation.

I guess if that doesn't help it will be time to go back to the drawing board.

They have a new coil at the local auto parts place so I could give that a try.
Rick Bastedo

Yes, the coil I swapped in was a good working unit from a running car that was also running great when I put it back in the original car.
So I doubt I have a coil problem.

I read again about jumpering, I understand what to do.
I think I'll go straight to the brown wire and if I still have problems... At least I will know it's not caused by the coil or the ignition switch.
Rick Bastedo

"At least I will know it's not caused by the coil or the ignition switch"

I should have said: "At least I will know it's not caused by the tach or the ignition switch"
Rick Bastedo

It Lives!!!

New cap = no difference
New rotor = no difference
New condenser = no difference
Jumpering everything out = no difference

After all that I was beginning to see the end of the rope so I metered things that really "looked" like they were right to see if the meter agreed.

With the points closed I was getting odd readings, like they really were not closed.

I opened and closed them several times and got differing readings.

Next I pulled the points out and hit the contacts with a file, the meter then showed closed when they touches and open when not.

After reinstalling the points and makins sure everything else was right I tried it and - IT RAN!!!

I guess sometimes it's just that one "obvious" thing that puts you over a barrel.

Thanks again for all the help here.
Rick Bastedo

Excellent! You are right. It is the overly simple things.

cheers

Gary
gnhansen

Hi Rick,

Little cause but huge effects as it's case too often!
Thanks for the feed back for this hard to diagnose issue.
Regards.
Renou

This thread was discussed between 28/10/2008 and 12/11/2008

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