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MG MGB Technical - Missing Under Load

I have a '74 1/2 B with dualHIF-4's., no polution equipment,no vacuum to distributor. Has run perfectly for the first year of ownership. Now I get a sputter/miss? in 3rd & 4th gear under almost any load. In first or second I can go to redline with no trouble. I have set the timing at every setting possible, and the carbs tuned to every combination possible. Where do I look next? Thank You Tom
Tom Johnston

Tom

I had this exact problem with a Grand-am. Turned out to be the plug wires but could also have been the cap, rotor, or the most common I am told, the plugs themselves.

Mike
Mike Phillips

Tom. There are many possiblities. Spark plugs are first. Get a set of NGK BP6ES or BPR6ES and gap them to .035". If this does not correct the problem, use an ohm meter to measure the resistance of your spark plug wires. They should all be close to the same resistance. If not, replace. Distributor caps and rotors are cheap. Replace them and eliminate that variable. Why "no vacuum to distributor"? Have you replaced it with an aftermarket dizzy or is the "can" bad and it has been disconnected. Do you still run points? When were the points and condensor last replaced?

Set the jets in the carbs about .060" below the "bridge" and then trouble shoot the ignition system. (Peter Burgess recommends .060" as an initial setting. I find, on my HS-4s, that about .055" is correct for my engine.) When the ignition system is proved good, then, is the time to fool with the carbs. I have some tech articles which may be of use to you. My website is http://www.custompistols.com . Click on the MG pages, then articles and see what may be of use to you. Post what you find, after the troubleshooting, and we should be able to help diagnose the problem. Les
Les Bengtson

Les & Mike, Thanks for a quick response. I put new Platinum plugs in, new distributor cap. solid copper wires...........I don't see a connection point on the carb for a vacuum to the distributor. The manuals show one but It's not there. When I bought this car one year ago the timing was near 35 degrees BTDC and it was running perfect. I reset it to 11 BTDC. Low end was much better but this problem appeared over a two week peroid. I see other responses here that say remove the hose and set timing at 35BTDC!?
I have tried the carbs set with the screw all the way in and then tried it at one revolution increments with minimum change to the sputter (and why can I hit red-line in firtst and second gear? I'm confused..Tom
Tom Johnston

If it was running so well at 35 why change it? At 35 with no vacuum you are likely to get pinking under load. At 11 you are likely to get poor cruising economy and mid-throttle reponse. It is the job of the vacuum capsule deliver the best combination according to conditions - albeit crudely and approximately.

The US went to manifold vacuum in 71 so you wouldn't find a carb port much after that. Have you checked the dwell? Have you rigged up a timing light so you can see what the flashes do when the missfire starts? Accellerating in higher gears usually results in higher cylinder pressures for longer than in lower gears, which makes it harder for the spark to jump the plug gap, so the HT voltage rises higher, so any marginal HT components can break down. I diagnosed a bad cap and/or rotor under similar conditions.

Does pulling out the choke make any difference? Screwing the mixture screw in on HIFs richens the mixture. Don't adjust the carbs until you are sure the ignition has no faults, and by now you will have to do a full airflow and mixture balancing setup.
Paul Hunt

In my experience, missing under load is nearly always HT leads, either breaking down internally, or shorting to earth.
Being 'new' doesn't mean you can assume they're OK.
Dave
Dave Wellings

ThankYou!ThankYou!ThankYou all. Now I have something to aim at. Every manual I've found is so vague about what model has what setups. i.e "Hook up vacuum hose if you have one" That doesn't tell me if I'm suppose to have one!
I changed the timing from 35 to11 BTDC because the books said that was where it should be. I didn't want to risk cooking an engine.
I can't spot the misfire with a timing light since it goes to redline without a problem in the driveway.
Applying the choke makes no difference to the "miss". I have no problems adjusting the carbs but like most of you have implied, it sounds like the first step is electrical in the HT area......Thanks Again..Now I need a break in the weather....TomJ
Tom Johnston

Tom,

you might also check the Platinum plugs you are using. I and others have not had the greatest luck with platinum plugs, for some reason only an engineer might understand. All I know is that for me, the NGKs Les recommends, tho copper cored, really seem to work the best in these engines, while all the various platinum plugs I have experimented with seem to eventually foul with soot. Doesn't seem to matter what the gap, the timing, the energy of the coil (a megablast hot rod ignition like an MSD might make a difference) or the type of plug wires used ~ they just seem to foul, even with no change in timing from what was good for NGKs. As they say, you luck may vary - depending on which tuning god smiles down on you while you are work. Good luck.
Bob Muenchausen

Les,
You suggested NGK plugs with a setting at .035 for Tom's 74 MGB with the missing problem in 3rd & 4th gear. Does that same advice apply to the 66 18GB engine? The book specifies Champion N-9Y set at ,025 My plugs also show a cool burn ( sooty) even after a long hot run of 100 miles plus. In other words the plugs don't look good or those suggesting a complete hot combustion. My timing is set at 10 degrees BTDC and tied to that 25D Lucas distributor we talked about a day or so ago. It seems to have zip when it runs, but always looking for good advice. If you have only driven one 'B". then you don't have a good basis to qualify "Zip" By the way, my vacuum advance takes off from the left carb, through a tube to a capsule and then to the distributor. I assume the capsule is a filter or water collector or some such device. Any info appreciated.

Jim
jim Hanks

Jim. I am running NGK BP6ES plugs gapped at .035" in my 68 and have for years. I am not having any problems doing so, but I replace the rotor and distributor cap every years at emissions time and the plug wires every two years or when they check bad. Arizona summers are a bear on rubber and rubberized components. I used to use Champion plugs many years ago, but they never seemed to last long. What their quality is today, I have no idea.

What do you mean by the "left carb"? Left and right refer to sitting the the driver's seat. Hence, the carbs are on the left side of the engine and the dizzy is on the right. Carbs are "front and rear". The rear one is the one that is, funny enough, on your left when you are working on the dizzy. The rear carb is the one that I have always seen have the nipple or connection for the vacuum advance line. The bulb in the line is a "flame trap" or vapor trap depending on whom you read and what era the book was written in. Many years ago, it was stated that the purpose was to trap any gas vapors that got into the tube and prevent them from damaging the rubber diaphram in the vacuum advance can. Later versions described the purpose as being to prevent damage to the vacuum advance can if there were a back fire through the carb. Later MGBs did not seem to have this device and I do not remember seeing it on any of my US cars back in the 60s. So, I do not really know exactly what it is or what it does, but do not worry about it as long as the vacuum advance works properly.

As to "zip", etc. most of us are not all that well qualified. I currently have three cars running between my daughter and myself. Steve S. has several between his and his sons. During most of my life, I have only had access to one example of any model at one time. I have, over the last few years, found that my 68GT has an 18V timing chain cover and crankshaft pulley. Recently, I found I have a different thermostat housing (or water elbow) than was standard on most people's 18GF engine. Hence, most of us are only marginally qualified and are narrowly focused. If you want to know how much Zip your car has, recruit an assistant (preferably a young female--less weight added to the car) and do some stop watch testing. See how long it takes to get up to 45, 55, 65, etc., then compare the results you obtain with some of the old, published results. You can also post what you find and see if anyone else is willing to do some testing. Gives you an idea and lets the rest of us know what is realistic if enough people will join in the testing. Les
Les Bengtson

Les,

Thanks for your quick response and as always, good advice! You are correct, no left and right on the carbs, but more properly front and rear. My mentality on the left and right scenario is from the reference of leaning over the left fender ( or wing if you prefer) and constantly trying to balance up the carbs, or set the choke properly. So the vacuum take off as you say , is from the rear carb. I am not as familiar with the "B" as I am with my 65 Sprite and it's 1098 cc engine, which had "Bugeye" carbs on it when aquired and I have since applied the stock items as well as learning the hard way on engine, clutch, etc over three engine overhauls plus a ton of bodywork. In the course of this activity ( over 20 yrs) I learned some do's and don'ts. The car was a daily driver for my daughter in High school and was quite dependable. once you accepted the no rear crank seal and oil leaking. I have subsequently corrected that with an aftermarket seal. The point is that I tend to know this little green (BRG) machine better than the '"B" which I bought about 8 years ago. It seems the "B" is not as forgiving. Everything has to be just so or it won't start. Many of the problems are those which you talk about on your website and have been corrected. I suppose I am just doing the same things to the "B", I did to the Sprite, but have forgotten the time it takes and being older, don't have as much patience. Actually the "B" was in much better condition initially, running, where as the Sprite was not. Expectations are likely a factor here. Thanks!!
Jim Hanks

Jim. My first LBC was a 61 Sprite Mark II. It took a lot of sorting, more than I was truly competent to do when I was 18, in college and working part time. Over the years I have always found that there is a "sorting out" process involved after the purchase of an old car. Once the car is properly sorted, most of the major problems disappear. But, there will always be minor problems with an old car due to age of components and quality of replacement parts available. If you can sort a Sprite well enough for your daughter to use it as a daily driver, you can sort the B well enough to have good, reliable transportation. All it takes is time. Les
Les Bengtson

The comments on this thread are very helpful. I'm having a similar problem with my 69 BGT in that it misfires under acceleration in any gear. It had been running well for several months, but then went in the tank. Cap, rotor, points, cond, all new within 100 miles. The resistance on the wires all measure closely so I figure they're not the problem. When I took off the points they seemed to be a bit cooked. Timing is set to about 10 deg and the dwell is between 55 and 60. Any thoughts on this would really be appreciated. I miss not being able to drive my B.
John Larabee

Les,
When you are retired you are supposed to have tons of time, but it seems you can;t keep your mouth shut and you end up being the volunteer for various projects. Besides, you get lazy---no pressure, no work. I'll get after it and "sort it all out", but I might have to hit your web site or the BBS tech area, cause the mind gets a tad slow also.
Thanks for the advice and comments

Jim
Jim Hanks

Jim. I have been "retired" for years and find that I work harder than when I was "working" Was Travis McGee the fictional detective who took his "retirement" in batches when he could afford it? I have been doing the same. Have just spent four hours trying to adjust the timing on my daughter's car for emissions testing. Base strobe timing is 10 deg BTDC at 1,500 rpm. Opus system (45DE4) with a Luminition retrofit. It will go to 15 deg BTDC before hitting the oil pressure take off line. Moving the dizzy drive one cog to the right (advances timing) results in the "box" covering the dip stick. Have finally got it back to original setting, had it set at 10 deg BTDC and found I could not attach the vacuum advance line. Now, it is back to 15 deg BTDC, the vacuum advance line is hooked up and I will drop a line to Bob Ford at Brit-Tek about a Euro Spec, points type, 45D4. Synonyms for "Enthusiast"--Idiot, Severely Frustrated, Stupid, Old and Cranky. Les
Les Bengtson

Jim
One thing you might want to check, if you haven't already, is whether the points plate is advancing properly under vacuum or not. If memory serves, the dizzy is combination centrifugal and vacuum. Can anyone confirm, I don't have my books handy.

Mike
Mike Phillips

John

Almost sounds like fuel problem. Especially if the ignition system has been gone through thoroughly. If its not bogging like its getting too much fuel (stuck choke, make sure they're seating back to the carb) and is a misfire, that's a lean condition. Can you make it go away by applying a little choke? Great advantage to having a manual choke BTW I've got a auto.


Mike
Mike Phillips

John. At what rpm was the advance set and did you disconnect and plug the vacuum advance when setting the timing? Did you clean the mating surfaces of the points before installing them? Dwell should be about 60 degrees. Are you getting good spark from all the wires? The spark from the coil lead should be about the same as the spark from each plug lead. Are you getting a good spark from the coil lead? A bad coil can cause this problem. If the ignition system is good, you have to look to the fuel system and carbs as a possible problem. Les
Les Bengtson

Mike, the position of the choke doesn't seem to have any impact on the problem, and it does appear to be seating properly. Car doesn't bog, but the warmer it gets the more severe the cutting out gets. Just like someone is playing with a switch to the ignition.

Les, The advance was set at about 800 RPM with the vacuum disconnected and pluged. I will reset the dwell, then check the spark on the leads and let you know.

Thanks for the help. It's great to have the support of this BBS for MG newbies like myself. It really helps to keep the frustration levels in check.
John Larabee

Tom,

Try changing the condenser. These can cause a miss under load when they begin to break down and this is often heat related.

George
George Bowick

If the (electronic) tach is steady when the missfire occurs you can pretty-well ignore the ignition LT. If you can reproduce it when stationary clip a timing light to the coil lead and each plug lead and see if you can spot any changes in the flashing when the missfire occurs. If not could still be plugs which you don't mention. You mention cooked points - if you have been running with a 6v coil on a 12v system that burns the points. A 6v coil primary measures about 1.5 ohms, the 12v for a 69 should measure about 3 ohms, or possibly 2.4 if a 12v sport. Otherwise fuel, check delivery by removing the pipe from one carb (watch out for a spurt if the ignition has been on recently), should pump about 1 pint in 30 secs.
Paul Hunt

I haven't been able to reproduce the problem when stationary. I did forget to mention the plugs, which were new at the same time as the other parts (Champion N9Ys set at .025) The car is new to me, so I don't know if it has the correct coil or not. I'll measure and report back my findings.

Thanks again!
John Larabee

John

I just read back through your original post.
Has the car ever run right?

Mike Phillips

Mike, I've had the car now for around seven months. And while it has run much better than it is currently, it has never ran the way I believe it should. The car is cosmetically great. However, it was in dire need of mechanical attention and tuning, which I've been attempting to complete. I've replaced the ignition components listed above, adjusted the valves, flushed and cleaned cooling system, and synced the carbs, and oil and filter. Standard tuning stuff. The car was running pretty solid before the problem started.

Paul, I measured the resistance on the coil last night and it reads 2.6 Ohms between the terminals. From the articles I read on Les' web site it should be around 3.1 or so for a 12V coil. Didn't have time for anything else last night.
John Larabee

2.6 sounds like a Sport coil, will put a bit of extra load on the points but shouldn't cause a problem. What does the tach do when the miss/splutter occurs? I.e. does it stay more or less where it is or drop noticeably?
Paul Hunt

I don't recall. I'll attempt to get the car out this weekend and see what happens.

Thanks for the help!
John Larabee

George ir correct
Its the condenser!
Spent a frustrating weekend once at Cadwell Park with exactly the same problem.
Bob

John. I will check the workshop manual and measure the resistance of my Lucas Sports Coils and post that. Factory spec for the 12V coil is something over 3 ohms, but will post the factory data for the OEM coil and what I find my measuring the Sports Coil today. Les
Les Bengtson

Measured the resistance of a Sports Coil, cold, and it was 2.8 ohms and the car passed emissions testing yesterday, so that is probably an accurate reading. The factory "General Data" lists an HA-12 coil as being standard. Cold, it should read between 3.1 and 3.5 ohms resistance. Do not have any true feeling for what will happen if the coil is warm from being run. I checked the Sports Coil on my 68GT (12V coil, just as on the car just through emissions) and it was reading 1.7 ohms, but it had just been run and the engine and coil were still warm. Perhaps Paul Hunt can provide some information on whether heat will affect the readings. If not, Paul, does it sound like my Sports Coil is shorting out the primary windings? The car runs very well, passed emissions about two months ago and the Sports Coil is four years old. Should I consider replacing it?

On the subject of condensors. Motorhead has a "bad parts" page that someone mentioned some years back. The Lucas condensor has been listed there for years, with the comment, to the effect, "Some fail from new and others in a short time causing the car to run poorly". Thus, replacing the condensor is always a good idea. But, remember the old service manual advise for replacing condensors and coils, "Replace with a known good unit". Hard to do unless you can pull one out of a car that is working well already. The Motor Head web page for bad parts is www.motorheadltd.com/bad_parts.htm and may be of use. Les
Les Bengtson

Typically resistance increases with heat. I measured my Lucas Sport coil a while ago, conditions unknown, simply as a comparison with the 12v and 6v and I had recorded it at 2.4. Today, cool, it measured 2.8 and the 6v measured 1.7. After a few minutes as Gas Mark 12 it was warm to the touch (but nowhere near as hot as when I inadvertanly left the roadster (12v coil) ignition on for about 20 mins) and neasured 3.3 ohms.

1.7 ohms is getting very close to a 6v coil - 1.43 to 1.58 ohms according to Haynes, 1.7 on my V8 - and people have reported rapid burning of the points when running a 6v coil at 12v. My Sport coil measured 8.3k at the secondary, what's yours, Les? (and don't say "a pint of Speckled Hen"), and if the 1.7 was a hot reading what is the cold?

The only coil I have ever had to replace was on the roadster a few years ago when it started briefly cutting out, a known failure mode of coils with rivetted spades as opposed to studs and nuts.
Paul Hunt

I recently worked through a similar problem with a friend of mine on his 1972 MGB, also with HIF4 carbs. We went through the whole electrical search with no success. I would suggest that you check these two things to see if they might be affecting your car's performance.

1. Does the heat shield between the carbs and the exhaust manifold have any insulating material on the back of it? On that vintage car, it may have been asbestos. Be careful. We installed pieces of a heat shield fabric from an Oatey Flame Protector (fabric sold at home supply plumbing departments to protect the surrounding walls when sweating copper pipe joints). We installed this fabric with pop rivets. One sheet provided enough material to cover all of the sensitive areas of the heat shield. With no heat shielding material, you could be experiencing vapor lock in the carbs when the engine heats up.

2. My friends ultimate problem was the fuel pickup in the fuel tank. His was rusted and had holes in it. The car would run great at idle, or in the garage adjusting the carbs and reving the engine. On the road after a few miles, the car exerienced fuel starvation and would sputter and lose power. The fuel pump was experiencing the same sensation that you would get while trying to drink through a straw with holes punched in it.

You can examine the fuel pickup line by draining the fuel tank and removing the fuel sending unit/feed line connection on the side of the tank, and looking in with a flashlight. My friend burned up a new fuel pump before he figured this out.

I hope that some of this information proves useful to you.

Good luck!

Greg Qiog;eu

Just finished working on the GT for the night with no apparent progress. I replaced the points and condenser again for good measure ( I had the parts on hand and this time they are not Lucas, but Beck I believe) and believe that I have come across another problem. When installing the new points I noticed that the plate in the distributor that they mount to seems to be loose and the arm of the points that rides on the distributor cam is sitting at an angle with only the botom portion of the arm touching. The mechanism does rotate and spring back like it should but it doesn't seem right that it should sit like that. The dwell won't budge off of about 40 now and the car is idling so poorly I don't want to take it out of the garage. I guess I'm going to have to take the distributor out inspect it, but the problem is that I've never worked on a good one to have someting to compare to. I do have the manual though. Any helpful hints would be greatly appreciated.

BTW.. the factory heat shield with the backing is still in place on the car.

Thanks for the help.
John Larabee

Had Same problem in my 1980 MGB IT WAS THE CONDENSER
Blake J.

The points plate should rotate against spring pressure from the vacuum capsule and spring back. It should not have any slop except for possibly a very small amount where the capsule bar engages with a pin on the plate. If the points are sitting square on the plate but not against the cam then the plate must be misplaced somehow, but are you sure that the pips and holes on the points and the plate are properly engaged?

The dwell is a measure of how long the points are closed, so if altering the points gap has no effect on dwell then either your measuring instrument is faulty or you are not changing the gap when you think you are. Are you sure you have the right points correctly installed?
Paul Hunt

I wanted to post a follow up to give the results to date and thank everyone for their help. It appears that I had bad fuel along with a bad condenser and more. I had just filled the tank right before I started having the problems and hadn't put two and two together. I was able to start the car again after completely draining the tank, and pumping out the lines with the fuel pump then refilling with fresh fuel. I also drained the float bowls and refilled them with fresh fuel. The bowl of the rear carb was very nasty.
Once it was at least running again, I went back to the distributor. I removed it and upon a detailed inspection realized that the hole in the points mounting plate where the distributor cam passes through was elongated significantly. Its actually a two piece mechanism with the lower plate fitting up into the center of the upper plate around the distributor cam opening. This portion of the lower plate is worn through almost completely on th side opposite where the points mount. This is allowing the plate to flop. Also, the springs on the advance weights appear to have been modified earlier in the car's life, and they don't appear to be working properly. No wonder the poor MG won't run right.
I can see from the Moss web site that these plates are not offered for sale so I guess that I'll have the buy a new distributor. Has anyone done the 45 in place of the 25D? I can't see buying another used 25 and havng similar problems.
Thanks for the help.
John Larabee

A 45 is a straight swap for a 25 and easier to install/adjust/remove, but you need to be aware of how its curve compares with the original. Too far off and you could get bad performance and economy or pinking.
Paul Hunt

John,

Why not take the opportunity to swop the whole thing out for one of the electronic ignition systems available from Moss.
From the outside they look just like the original.

Best regards,

Angus.
angus

Paul, thanks for the tip. I wouldn't have thought about that.

Angus, If I'm thinking of the same one you are, (the pertronix units) I didn't consider that because the plate where it would mount is where I'm having the problem with the current distributor.

Thanks again to eveyone for all the helpful hints and suggestions. Now I'm off to buy a distributor....
John Larabee

John,

What I meant was... why not replace the entire distributor and convert it to the 'electronic' right away. You would then have perfect timing, a better spark, probably better fuel consumption and no adjustment for life.

Just a thought... good luck anyway...

Angus
Angus

This thread was discussed between 18/11/2002 and 03/12/2002

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