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45mm Dellortos Misfire Nightmare...HELP!
lordbenny - 9/2/14 at 04:44 PM

Had an ongoing problem with my #1 cylinder Dellorto in that it misfires and backfires when revved. If I put my hand over the trumpet remove it and then rev the car it runs sweetly for 5 seconds then goes back to running lumpy again. I've pulled out all the jets inspected and blown them through, looked into the carb itself and not seen any grime. The only other thing is that when I turn the idle mixture screw all the way it tightly it doesn't make a lot of difference. I dont think its a balancing problem and I don't think its a blocked jet. What could it be?

[Edited on 9/2/14 by lordbenny]

sjmatthews - 9/2/14 at 07:15 PM

Possibly an air leak. Check the inlet manifold gasket area and carb mounting/spacer by spraying wd40 in these areas. An increase in revs will indicate a leak. I expect it's the latter.

ross05 - 9/2/14 at 07:28 PM

What engine are the carbs on ? Reason for asking had a similar problem with a 2ltr blacktop zetec, turned out to be the plug lead not seated correctly on the spark plug.Pull plug lead out and push back onto plug..

just a silly fault but easy fix when spotted.

lordbenny - 9/2/14 at 08:23 PM

quote:
Originally posted by ross05
What engine are the carbs on ? Reason for asking had a similar problem with a 2ltr blacktop zetec, turned out to be the plug lead not seated correctly on the spark plug.Pull plug lead out and push back onto plug..

just a silly fault but easy fix when spotted.


Zetec Silvertop....both those replies seem pretty fathomable, will investigate further!

lordbenny - 9/2/14 at 09:05 PM

Just checked with my 'mechanic' who was with me today...he says that a loose or dodgy spark plug or air leak wouldn't change things if you put your hand over the trumpet.

rusty nuts - 9/2/14 at 09:14 PM

Sounds like the cylinder that is misfiring isn't getting enough fuel, blocking off the inlet to that cylinder is momentarily enriching the mixture Blocked idle jet/circuit , leaking diaphragm,manifold.duff plug/lead or possibly a leaking valve on that cylinder

sjmatthews - 9/2/14 at 09:21 PM

quote:
Originally posted by lordbenny
Just checked with my 'mechanic' who was with me today...he says that a loose or dodgy spark plug or air leak wouldn't change things if you put your hand over the trumpet.


Then why didn't he fix it?!

At idle, carbs rely on the vacuum created by the almost closed butterfly to draw fuel through the idle circuit. If you have a leak, even a small one, the vacuum is lost and the idle circuit doesn't work. I have seen this time and time again with weber dcoes. Often caused by a damaged o ring or damaged faces, or on dcoes the lead plugs can sink and cause a leak on the o ring face.

Placing your hand over the trumpet will increase the vacuum and draw through fuel, improving running temporarily.

Let us know how you get on

davidimurray - 9/2/14 at 09:28 PM

You mentioned you have blown the jets through. I had a set of bike carbs that I had a nightmare to get running. Blew the jet through numerous times. Eventually I soaked them in carb cleaner for 24 hours and the scum that came out of them was unbelievable! Totally transformed the carbs and ran beautifully after that.

lordbenny - 12/2/14 at 08:07 PM

So, took the carb off and cleaned everything out....quite a lot of gunk! Also, the screw that holds the auxiliary venturi in place on all 4 carbs needed screwing up tight. the aux venturis were all loose! just need to get them balanced now! Thanks for your help.