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MG Midget and Sprite Technical - No fuel in no 4 on rebuilt 1098 engine.

Recently got my rebuilt 1098 engine running, however it is very lumpy. All plugs have spark and fuel is getting to tboth float bowls. However the plug in No. 4 cylinder is intermittently dry after running. Should I suspect sticking/leaking valve or could it be carburettor issues. The carbs are not completely balanced as it will not run consistently without choke to set them up.
A Potter

It ought to be possible to set the carbs on a basic setting that would be able to run the car until it can be fine tuned on the carbs.

However responding to you valv sticking question?

I would be very very surprised if your valve was sticking on low revs? Generally a valve will stick when the engine is being used to extrmes.

Also if it stuck open it would sound terrible as it would if it stuck closed, either would alert you well before loss in performance caused concern.
Robert (Bob) Midget Turbo

Bob, I did set the carbs up with the basic settings, 2 turns on the jet adjusting nuts etc. but in order to get the engine to run I had to richen the mixture on both carbs by another full turn, not sure if this is normal or not? It is difficult to start but once it does it will run a t quite high revs 1500rpm with choke but the engine hunts as if missing. I checked the plugs and number 4 was still shiny as I had tried a new set, with no sign of any action. I then tried removing the plug cap on no. 4 and there was no change in the hunting pattern. Each plug has a strong spark when held against the block. It seems as if the fuel doesn't get sucked into no.4 somehow but gets into the other three cylinders. Any ideas? Thanks.
A Potter

Hi mate

Of course the ultimate test is always a compression test but short of a tester you can remove all the plugs and turn the engine over on the starter and by placing your thumb over each plug hole judge it the comp is the same on each?

The problem does sound like a problem perhaps with the carb for 3/4 are you sure it is balanced with the front carb?
Robert (Bob) Midget Turbo

If you had to richen the mixture that much to get it to start, and it runs at a very fast tickover it sounds as if you have an induction air leak. I don't know if this is linked to the poor firing on #4

It seems to me odd that #4 should be weak when #3 is Ok when the inlet manifold is siamesed.
Guy

Guy
funny you should say that, last night it wouldn't start at all, when I removed the plugs 1 and 2 had been firing and 3 and 4 were as clean new. Maybe Bobs comment about the carbs being balanced is pertinent. I think I'll return both to the basic settings tomorrow and check the manifold nuts. Thanks for the pointers.
A Potter

Think Guy is giving some good pointers but I always have the throttles disconnected on both carbs and can then use each idle screw to get both carbs singing. :)
Robert (Bob) Midget Turbo

Thanks Bob I'll bear that in mind when I carry on the fight tomorrow.
A Potter

I may be going off at a tangent here, but what condition are the butterfly spindles in, - particularly on the back carb? Slack on the spindle can draw in a considerable amount of air, weakening the mixture to nos 3 and 4. Would also make it very difficult to balance them!
Guy

Guy, I'll check that too. I did remove the flaps recently when I refurbed the carbs. Must admit I re-used the old screws maybe that could cause an issue. Cheers.
A Potter

The danger of reusing the screws is that you don't want one to work loose and end up being sucked into the engine! I have re-used them, but use locktight and also make sure that the split ends are well spread to lock them in.
Guy

Well made some progress of sorts today before I ran out of time.
Firstly I returned the carbs to the basic reccommended settings, disconnecting linkages and aliging choke cams. I then tried starting and had no joy, all plugs remained dry. Then I checked fuel flow, this was just a dribble but was consistent, I decided to remove the inline fuel filter I fitted and improved flow considerably. I then managed to start with lots of choke and eventually managed to keep it at a fast tick over still with the basic carb settings which must be better all round. But, you knew there must be one, the tickover races of its own accord, a blip of the throttle makes it settle down again for a few seconds then it races again. Also I still have the hunting but does seem slightly better and the plugs are all a similar colour now. The butterfly flaps both look ok visually. So to sum it up re setting carbs has helped, and also the improvrd fuel flow, maybe this is compensating for induction leaks as suggested by Guy. I am going to get a compression tester in the week for peace of mind and remove the carbs and re check manifold nuts next weekend. any other pointers gratefully received. Andy.
A Potter

Did you look into the throat of the carbs while running?
Does the piston lift on both carbs? Did you accidental swapped the pistons or covers perhaps? are the jets centered or are they of the biased type? What needles did you fit, the old ones or new? What type?

There are many possibilitys that could be off, and of which the symptoms can sometimes be corrected by tweaking something else.

Sounds like a weak mixture, but what causes it is the question.
Alex G Matla

Alex, both pistons seem to drop freely with soft clunk when not damped so I think the needles are centred ok. the rate is pretty much the same on both. I did re-use the old needles though, they are just the standard ones. I was careful to keep the pistons etc with the correct carb, but it is a fair point. It seems like the fuel builds up and the engine revs quickly and then either the flow slows down or sucks a lot of air from somewhere and the revs drop. I'll keep looking......... Cheers.
A Potter

Tends to work the other way - revs rise with a weak mixture.

Check for grit or dirt trapped inside the small flexibile tube feeding the rear carbs jet.
I am still puzzled about how the fuel mixture can be different between nos.3 and 4 !?
Guy

Guy, thanks for pointing that out about mixture. I replaced the jet assemblies complete with new ones but I guess thecould have got contaminated. Regarding the difference between 3 and 4, the plug in 4 hasn't got much of a sooty build up where as the other three are black with a tinge of white, it was my guess that the mixture must be slightly different but I agree this does seem illogical after your comment about the manifold. Cheers.
A Potter

Ok, here is an out of the box long shot - what is the possibility that the cam lobe on valve 7 is badly worn so that the inlet valve isn't opening sufficiently?

You could remove the rocker cover and measure the valve lift as you turn it over by hand. Never heard of this on an A series although have had it on a tvr V 8 camshaft where one lobe had worn completely round !
Guy

Guy, the camshaft is a reground item, assumption is it is ok? Could a badly adjusted rocker give the same issue? Probably worth checking though. Thanks.
A Potter

Only if adjustment very loose, or if #7 pushrod had jumped off the rocker. That would stop the valve opening and give you no fuel ! But l think it would make an odd noise when running. Easy to check though.
Guy

Finally got engine running smoothly. I slackened all manifold nuts and carb mounting nuts and retightened. The only other thing I spotted on the carbs when comparing to an old pair I had in another garage was that the spring on the damper piston stop pin was inside the counterbore in the carb body not underneath, so I swapped this around too. I think I copied the way the previous owner had assembled this. The engine then fired instantly without choke. The tickover seems a little fast(not sure what speed as the tacho has stopped working for no apparent reason). Could this mean I need to change the type of needle (currently got standard ones)as I can't slow it down anymore on the throttle screws. When I lift the damper piston the engine speed doesn't fluctuate so seems to indicate correct mixture? Thanks everyone for their previous advice.
A Potter

The only way engine speed remains high can only be due to air entering the engine. This is generally because either the idle screws are not clear or more likely that the throttle disks are not closing correctly in the carb throats. It definately is not the needles.
Robert (Bob) Midget Turbo

Bob, think I'll have to check the butterly and spindles, as you previously mentioned, more closely. There were no obvious gaps I could see this morning. Are there any other areas air could be getting sucked in, apart from the manifold, seems the induction leak suggested by Guy was my problem all along.
A Potter

Maybe.
But it is still very puzzling why the mixture in cylinders 3 and 4 should appear to be so different, when they are supplied by the same carb! Did you check the valves were lifting correctly on both cylinders? It could be a bent pushrod or worn cam lobe. Unless the observation was just misleading, and they looked more different than they actually were?
Guy

Generally if air is entering through a leak in the manifold area the engine will tend to run rough as the mixture will be weak. If air is entering via the carbs then it is fueled at the same time and so the engine runs well.

Normally when carbs have been apart most times the problems are with poor orientation of the throttle disks. It is almost impossible to see a gap large enough to speed up the idle. In normal operation throttle disks are very tightly closed
Robert (Bob) Midget Turbo

Just to add to Bob's advice there - the throttle discs have a chamfered edge, top and bottom, such that they close neatly against the choke. If not spotted, they can be put in back to front in which case they don't close properly. There is also a little side to side adjustment available in order to centre them on the shaft.
Guy

Guy
the mixture seems consistent across all cylinders now so must have been linked to manifold leaks or as you say me not being as observant as I should. So just the throttle discs to check now. Cheers.
A Potter

This thread was discussed between 21/04/2012 and 09/05/2012

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