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MG Midget and Sprite Technical - Firing puzzle

Sometimes I think I must be imagining things but the following puzzle concerns me. I found that if I disconnect the plug leads on #1 or #4 cylinder there is a noticable drop in revs and increased poor running in each case. If I disconnect #2 or #3 I get little noticable effect in either case. How could that be?

I thought of plug failure or poor compression but I did a compression test and all 4 cylinders are exactly the same (210 psi). I also changed all four plugs for new ones but the effect remains the same - #2 and #3 definitely do not seem to produce the same effect on running. Thinking about it now I guess that leaves failing plug leads but they are newish. Could it be something to do with the firing order or what?

Puzzled Chris
Chris H (1970 Midget 1275)

Yes Chris, but why would you? A question I ask my clients at least twice a week.
Dave Squire (1500)

Chris,

I have seen this sort of behaviour before and it is due to the charge robbing resulting from the siamesed ports and firing order IIRC. I'm not sure how much the carb arrangement effects it but it was quite obvious on my engines fitted with 40 Dellortos side draughts. I don't think I ever tried it with the SUs fitted so can't comment on those. Other than it being a bit odd it didn't seem to effect the engine running otherwise.
David Billington

Hmmm...

Removing 50% of the engine capacity had an effect on engine performace....wow I cant imagine Thats even possiable....hahaha

Id imagine the engine running on 2 cly.at a time would be effected the simease head...as mentioned above

I wouldnt run it on 2 cly long or damage will happen

Prop
Prop and the Blackhole Midget

Thanks David for the sensible comment! I have a single HIF44 rather than twin SUs. Not sure if that would be comparable to the Dellortos. As you say, not noticed anything untoward on the road.

Prop - I was only disconnecting one cylinder at a time, not two!

Chris
Chris H (1970 Midget 1275)

Prop,

This is now normal on some of the latest Audi engines - when on light load they cut fuel to two cylinders.

Fuel economy thing.

R.
richard boobier

This thread was discussed between 22/05/2013 and 23/05/2013

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