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dellorto 40 miss-fire under acceleration

Hi, Yet again i'm in a tad of trouble ..... i've developed a bad miss-fire under acceleration (i.e as soon as the engine is under load ). It also seams only to be out of the one carb details engine 1776cc

009 dissy (point set to 16thou ) engle 110 cam dellorto 40's (55 idle , 142 main , 9164.1 emulsion tubes , 180 air - i think ) I know the actual configuration of the engine is ok because it was fine for a week after a rolling road tune up and check (80 bhp at wheels, 2.25% CO @ idle ). Before the rolling road i replaced the plug leads so i know there ok , it's got new plugs (that made the idle better) Just incase i've replaced the breaker points, and condenser .... that was no good. The jets are clear in the carb, the cylinders respond to the mixture screws. Do i have an actual blockage in the carb casting or am i missing something here ?

The engine idles fine but as soon as i apply the gas pedal to pull away , loads of poping back through carbs and miss-firing

Confused

John

snipped-for-privacy@blueyonder.co.uk

Reply to
John Skeldon
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The engine idles fine but as soon as i apply the gas pedal to pull away , loads of poping back through carbs and miss-firing

Confused

John

snipped-for-privacy@blueyonder.co.uk

Loss of fuel pressure? Blocked supply to pump? Blocked filter? Tank vent blocked? Floats "hanging"?

J.

Reply to
P.J.Berg

Read this article by John Connerlly at aircooled.net it was a big help to me.

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Joe Cali -
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Reply to
Joe Cali Next Generation-usa

John, I had a similar problem with those carbs, but at higher rpm. It turned out to be too small needle valves. I would suspect fuel starvation in your case too. Popping is very often caused by a lean mixture, so again it points to fuel starvation.

Check your pump, pipes, tank breather and then the needle valves in the top of the float chamber. A quick test might be to remove the fuel filler cap and take it for a blast. If that cures it then you have an air blockage at that end.

Gaz

The engine idles fine but as soon as i apply the gas pedal to pull away , loads of poping back through carbs and miss-firing

Confused

John

snipped-for-privacy@blueyonder.co.uk

Reply to
Gaz Pike
009 dissy (point set to 16thou ) engle 110 cam dellorto 40's (55 idle , 142 main , 9164.1 emulsion tubes , 180 air - i think ) I know the actual configuration of the engine is ok because it was fine for a week after a rolling road tune up and check (80 bhp at wheels, 2.25% CO @ idle ). Before the rolling road i replaced the plug leads so i know there ok , it's got new plugs (that made the idle better) Just incase i've replaced the breaker points, and condenser .... that was no good. The jets are clear in the carb, the cylinders respond to the mixture screws. Do i have an actual blockage in the carb casting or am i missing something here ?

The engine idles fine but as soon as i apply the gas pedal to pull away , loads of poping back through carbs and miss-firing

Confused

John

snipped-for-privacy@blueyonder.co.uk

Also check for air leaks around manifold/carb flanges bellows etc. Make doubly sure that the ignition system is in tip top condition and correctly adjusted before blaming the fuel system. 90% of misfires and poor running are caused by ignition faults. You say you have changed plug leads, condenser and points etc. you haven't said anything about adjusting the timing and this could be different from stock if you have changed the cam. Also ensure that the advance system is working correctly. Andy

Reply to
Andrew Wheeldon

Of course i've dicked farted and messed about and now i'm in aright mess. I'm pissed off , high on fumes, and have a right head ache from the exhaust aswell Things i've tried. Ignition timing .... 10degrees advance at idle , max advance when revved =

30degrees Ripped apart the carb , checked for blockages , all jets ok , re-set the float (as in dellorto book)

is it possible for the needle valve to get partly blocked ? it works (i.e shuts off fuel when float rises ) but i haven't taken it out and checked it.

Thanks all for the advise

John

Reply to
John Skeldon

I forgot to add that it only misses pop and bangs through ONE bank of carbs so maybe it is the needle valve. ???????

John

Reply to
John Skeldon

I think they have a filter in each carb where the fuel goes in. It's been a long time since I worked on Dells. If there is muck in one filter then that carb will have trouble getting the fuel as the revs rise.

Gaz

Reply to
Gaz Pike

Gaz,

I dont think there is a filter BUT i will change the needle valve because its the only thing i haven't checked that is common to both cylinders that could cause this problem

Thanks

John

Reply to
John

I don't know about dellortos but on Webbers the needle valve is a calibrated item. It could be that the ones you have are too small and could be causing fuel starvation. Andy

Reply to
Andrew Wheeldon

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