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MG Midget and Sprite Technical - Hesitation at low revs when accelerating

I've just got back from Gaydon and had some engine problems during both legs of the trip (about 150 miles each way). Seems to be when the engine bay gets hot. When pulling away at junctions etc the engine coughs and splutters until about 3000 rpm. I thought it was electrical originally (found only one clip fastened on the distributor cap) but same problems today. I removed the heater ducting to get more cold air into the engine bay and it seemed to reduce the effect. I also noticed that the idle revs were up to 1300 rpm from 1000rpm. Carburetor float bowl not overly hot (HIF44 and large heat shield). Could this be caused by a manifold gasket leak? It runs fine until I hit some traffic.

Four cars went from Norwich and apart from my problems we had two dirty fuse problems and one broken rear spring. Oh and of course numerous leaks in the monsoon weather coming home today.
Steve Church

Sounds to me like the mixture maybe a bit rich, although the 1300 tickover speed needs to be sorted. That is either the throttle plate not closing correctly (poor alignment or tight cable) or as you say an airleak which could be why you have got the mixture set too rich. How did you tune the mixture?
Bob (robert) Midget Turbo

Steve, nice to meet you yesterday, i'd say a 1300 rmp tickover sounds like a manifold leak. maybe strip it all off again and build up carefully with new gaskets.

good luck.

Nick

I had it adjusted to the point where the engine ran smoothly. It ended up about 2 1/2 turns of the screw. I haven't checked the plugs today but when I got to Gaydon they where not sooted up but the insulators where a dark grey. The car goes ok in normal country driving, just behaves poorly in traffic. Wouldn't I notice a rich mixture all the time?
Steve Church

Nick,
Yes, nice to put a face to the names. Really good day. But really bad weather coming home today. By far the worst conditions I've driven the Sprite in.
I'll maybe try that tomorrow if I get time after changing the anti roll bar drop links on my VW tomorrow.
Steve Church

No a rich mixture can be undetectable when the engine is at normal road speeds or is cold. It generally only manifests itself when you stand at idle for a short time, then the engine slowly clogs up and begins to run rough and eventually dies. This can also be caused by the float bowl jet passing and allowing excess mixture into the float bowl which would cause the same effect.

It would be best to sort out the 1300 idle speed first before trying other actions.
Bob (robert) Midget Turbo

You can search for manifold leak using eg WD-40; the engine will speed up if it sucks in WD...

A
Anthony

Silly question maybe, but you do have adequate oil in the carb damper, right?

Perhaps irrelevant, but you never know.

-:G:-
Gryf Ketcherside

Yep, oil in dashpot.
I've since stripped the manifolds down and replaced gaskets. Used sealant on all carb gaskets but not on the main exhaust/inlet gasket. Cleaned out the dashpot/piston/damper assembly and refitted. Hopefully will have eliminated any possible air leaks. Filled dashpot with 20/50 engine oil.

Steve
Steve Church

Well I've had a chance to do a longish trip (30 miles). I called in to get some petrol having driven 15 miles at 60 mph-ish and the car still played up when I pulled out of the petrol station. When I got to my destination I stopped for my entry ticket (went to Snetterton for thew historic car racing), the car stalled and I had a hard job starting it again. Had to pull the choke to get it to start up. I pulled the heater vent tube to get some cool air into the engine bay on the way home and things were better.

When I got back I noticed that if I blipped the throttle the car would run roughly and not pick up. Sounded like it was firing on 3. I tried different oil in the dashpot thinking that the mixture was weak on acceleration, no difference. I tried a richer needle (from a BDL top a BDK), no difference. I'm running out of ideas. I have set the jet to the point where the engine runs smoothly so am happy with that. Other info. Engine is 1330 with 10:1 MG Metro head HIF44 with Titan and K&N. Morspeed phase 2 cam (similar to SW5). LCB with standard twin box exhaust. Standard distributor with Lumenition Optronic timed to 14 degrees. All this has been added after I changed the cam over the winter. I am considering changing back to the standard ignition setup in that this I knew worked before. I have changed the distributor cap and leads.

This problem seems to start when the engine bay gets hot. Water temperature is normal. It's getting so that I don't want to take the car out any more.

Steve
Steve Church

Do you still have the original cooling fan or an electric one? Do you have a good heat shield? The symptoms point towards a heat problem maybe affecting the float in the carbs?

Carl
C Bintcliffe

I have a full width 3mm aluminium heat shield which shileds the carb from all branches of the manifold. When I have checked the float bowl temperature when this problems occurs it is only warm. I have been running with an electric fan for a couple of years and have removed the mechanical fan.
Steve Church

I've got a HIF above a big-bore Maniflow, and with a full width stainless heatshield I don't get any evaporation problems. Do you have the water connection on your titan connected? If so, then your set-up is so similar to mine that I don't believe you have fuel heat problems. In fact your engine package sounds like its well balanced to me, and should be capable of delivering.

Some suggestions:

1) Do you have the original twins lying around? Can you bang them back on and see if the problem goes away? - this will verify that your ignition system is doing what it should be and that the problem is in the HIF set-up

2) Whats the history of the carb? has it been rebuilt? do you know that the float valve is working properly?


It would be nice to get to the bottom of this (I sure you agree Steve!) as I know how nicely these carbs can run.

Nick

Sounds like the fuel is boiling due to excess engine bay heat. Check fuel line routing, keep it away from all heat sources & insulate it where you can. Also look into a small heat exchanger like a power steering cooler or small oil cooler. Mount it out front behind the grill, it will make an enormous difference in performance & help avoid the percolation of fuel.
Mike G

This was part of my solution to the under bonnet heat problem.(see photo)Judging by the amount of heat that comes out of it at idle it must be helping.

Carl


C Bintcliffe

Hey carl,

I was thinking of you the other day, do you think you might be making up a set of planes on how to make those....or making several sets...Id still like to buy one or the other....I have tried to make those a couple differant times but it keeps turning out looking like abstract art...tho there getting better...but along way off from having something nice like yours ...

prop
Prop

Prop I may be able to do something soon, I haven't forgotten.

Carl
C Bintcliffe

I Vizardised the carb and rebuilt with a rebuild kit. New float valve. I do not have the water connection connected as I was under the impression that the cooler the air into the inlet the more fuel load it could carry. I am not convince about fuel vapourisation. All fuel lines are not in the line of fire of any heat source and the float chamber is not hot.

Next step is to run the engine in the garage with the bonnet closed til the fault occurs. It's always nice to get a fault to be repeatable under controlled conditions. Failing that then heating up various parts under the bonnet (hair dryer/heat gun) to get the problem to appear.

I'm sure I'll find it in the end. Just a bit discouraged at the moment.

Steve

Steve Church

Steve,
I've had this experience on both my 1500s, sitting in traffic and on pulling away the car suffers from fuel starvation.
Both cars are fitted with electric fans.
I am convinced that the (my) problem was that heat built up in the engine compartment to such a degree that it caused fuel evaporation.
I have since re-fitted the belt driven fan, and that has improved the problem out of all recognition, probably due to the fact that cool air is being drawn into the engine compartment even when the car is stationary. The electric fan very rarely switches on now. (belt & braces)
Give it a try with the mechanical fan re-fitted, it worked for me.

Dave
Penwithian

WHOOOOaaa there trigger,

Back up the cart there neally, See If we got us a clue...

Are you running a cold air tube TO the carb. (straight at the carb)...and you Vizardised the carb...to which of the 3 levels he descibes in his book?....if you did level 3...then yeah, there the problem, that level is to be used primarly of full race cars...not street applications.

If you got a cold air tube going right into a Vizardised carb., then thats going to be a problem as well...becuase the V.carb is already flowing a minuimim of 30% better with a stage 2 V.Carb add to the fact that this is a HiFF...who knows what your flowing efficancy is....BUT adding a Cold air tube from the perivious heater vent directly into the carb...and if your using the dry hose tube that spirals, creating a focased vortex of sorts.... well, what you got you is basically a poormans supercharger...

In a nut shell, your doing to air what a 50psi fuel pump pressure does to a SU Hs2....

agian this is just my theory, So it may have no realaventcy at all.

Prop
Prop

Maybe I missed the thread but why don't you just take the car to a rolling road. They'll find the problem and set up the engine at the same time. The last one I used found me 18 bhp within 20 minutes!
f pollock

Those were my findings exactly Dave. Replacing the fan stopped my problems and ever since I have had none of these problems. My engine produced 98 bhp at the wheels on Peter Burgess,s rolling road and I have never had any overheating problems with the standard fan. I think the Dellortos solved the fuel vaporisation problems for good!


C Bintcliffe

A long shot, but worth checking because it's so simple to check:
Though none of our midgets suffered from this, I had an identical problem with my MGB. It took a lot of finding - checked all the variuos fuel-side aspects including those suggested above and still to no avail.

It got progressively worse, feeling more and more like a major air leak with the power not cutting in till around 4000 RPM, after which it was all fine and went like a rocket. The car was also hard to start when hot, which suggested a heat soak problem with the carb (Weber). Tried several things, but still nothing.

One day in frustration I threw the timing light on when cranking, and found nothing ... as in no flash. Five minutes later I'd traced it to the coil, replaced it and the car went back to normal. What a relief!

I've never had those symptoms from a coil before, but there you go - never too old to learn new tricks.
Paul Walbran

An Add-on thought...

If the intake manifold is longer and still to wide in the throat.... Then add the fact of extra air your flowing thur the Vizardised carbs and the cold air tube into the carbs... thus atomising the fuel mixture even more, then most likely with a really hot engine the fuel mixture is cooking off (vaperising) before it gets to the head... You may need more fuel, shorter intake manifold, narrower manifold throat...or cooler suroundings.


prop
Prop

Oh good suggestion paul....I like it...got so hot that it out heated the coil....knowing how hot my car gets ...I can see it.

prop
Prop

I have an incling its ignition related too.

Whats known good on that score?

Theres quite a bit to check:
Coil inc LT connections
Electronic ignition (replace points and test maybe)
Rotor Arm
Leads (should have a resistance of about 10kOhm per meter length)
Plugs (although you say they're fine)


Failing this I'd too be tempeted to take it to a rolling road, or even a mobile tuner (if you can find one that understands old cars). I used one a few years ago when I just couldn't get my car running properly. Within 25 seconds he'd identified a manifold air leak, then he set everything up in ten mins and off I went. INSTANT power and better fuel economy.

Nick

Thanks for all your comments guys. Putting the mechanical fan back on should be easy enough if I can find the original (longer) bolts. Can someone remind me if I need to remove the radiator to do this? I have a club run on Wednesday so should be a good test. I have changed the coil to a standard one from the Lucas Sports coil that was on. Not recommended for use with an Optronic system I gather. Similar symptoms with either. Also had different dissy cap and leads with similar results. I have not changed the rotor arm which I should have done given the terminal failure of one just after I got the car. Will try that as well. My engineering background says to only change one thing at a time, then test each change. However the driver in me just wants to get the problem sorted. Guess I should stick to slow and careful.

Another observation. When I had the air filter off I noticed that the piston could move with no resistance for the first 2 to 3 mm of travel when warm. In looking at the damper I notice that the damper piston (little 'bullet' at the end) is free to move by this amount and forms a sort of one way valve which means the oil has less resistance when then carb piston is going down. Thus the carb piston immediately rises by 2mm when I blip the throttle. Can someone advise if theirs does something similar. Should there be this 2 to 3mm of 'free play' when the carb piston is bottomed (idle). Wouldn't this cause an immediate weakening of the mixture. Just a thought. Will try to follow this through in more detail tonight.

Thanks again for your support.
Steve Church

I constantly hear about the idea of only changing one item at a time! Fair enough if you really want to know the root cause. But for problems like this where the real goal is getting it running nicely I always feel that for the price of the bits, just change everything back to known good and then add bits back on. Thats why I reckon, take your carb off, take you optronic off. Stick a standard dizzy in and standard carbs back on a give it the well tried and tested standard settings. Given, this may not provide the optimum for your cam but it lets you know where you are.
Nick

I have done that a time or 2 nick....I think we all have.

prop
Prop

I may be with you on this one Nick. I have my daily driver, my wife's and the Sprite to get through MOTs within the next 6 weeks so spending inordinate amounts of time fault finding may not be prudent. Change as much as I can while I'm under the bonnet I think.

Steve
Steve Church

Steve with the carb pistons I have found one or 2 anomolies with them. If you remove the air filters and watch the pistons rise and fall with the throttle you can check that they do it in unison. If one of them moves up and down more or quicker than the other one then you have a problem. Some times this can be simply due to the oil in the dashpot, othertimes it can be mechanical friction. Another issue could be that the filter is fitted incorrectly blocking the small air hole between carb and filter.
Bob (robert) Midget Turbo

Steve,
Further to the issue of re-fitting the belt driven fan, had it not sorted the problem I had intended to replace the K&N filters with the original fitment air filter, which on the later 1500s came with a pair of hoses which drew cool (and Pressurised?) air from the front of the car, rather than the K&Ns which draw their air from a hot environment. They must have introduced these air hoses on the later cars for a good reason.

Dave
Penwithian

Steve,

Did you find the problem in the end? I fiddled with my SU's last night and managed to get them from quite rich and a bit off balance to massive hesitation as the pistons try to lift under rev/acceleration to a final state of won't start at all, (this last may be flooding). All I altered was the mixture and the idle screws!

Su's 1 - 0 Sean

sean griffiths

I changed my Lumenition Optronic system to the standard points and I had no problem on the club run last week. Not had much of a chance to test around town yet.
Steve Church

I distrust Luminition. Last summer, a friend reported his TR-6 engine stopped suddenly in traffic. I towed him home (using the Focus) and then set about finding the problem.

He had no spark. Take the dissy cap off and turn over the engine, and lo! sparks at the coil. Put the dissy cap on again, and it won't start - no sparks again.

I was beginning to tug at the last few strands of hair above my forehead.

It seemed the light in the Luminition unit under the dissy cap had failed. With the dissy cap off, there was enough sunlight for the system to operate... sort of.

So I put an old set of points in and it's all fine.

Luminition has too many elements that can fail...


A
anthony

Well I'm just about to install a Megajolt system (will keep the standard points system but disconnected). More electronics. Just have to see how reliable it is. Assuming I get it going in the first place of course.

Steve
Steve Church

Steve
When it's done, pls post a thread to report on your megajolt experiences.
I'll be doing one when I source a trigger wheel setup.
Mike
Mike Allen

This thread was discussed between 25/05/2008 and 10/06/2008

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