Carbs popping at idle - can it be the timing?

ber911 · 04-08-2005, 02:00 PM

We have just rebuilt a 2.5 motor with E cams and 40 Webers and cannot get it to idle without popping badly. The carbs were bought second hand and stripped and cleaned out twice and have 55 idles, f3 emulsions, 180 air correctors and 140 mains. Adjusting the air correctors has very little effect on the idling.
Do I definitely have a problem with the idle circuit on the carbs - I am about to strip them again to double check the secondary venturi orientation - or is it possible that there is a problem with the timing that is causing this? One can see a flame down the venturi when the carb pops indicating the intake port is open when the plug is firing.

Robert

jluetjen · jluetjen's Garage

It most likely doesn't make a difference (except to give a context to your other jetting information), but what sort of venturi are you running?

Are you sure that the cams are timed correctly and that the valve clearance is set right?

ber911 · 04-08-2005, 02:23 PM

34 venturis and tall secondary's. The motor was rebuilt by an experienced engine rebuilder so I am pretty sure that the cam timing and valve clearances are correct. Was hoping that something like the dizzy being one tooth out or 180 deg out could cause these symptoms.

Robert

FrayAdjacent911 · 04-08-2005, 03:02 PM

The timing seems like it would be the culprit to me. Firing the spark while the intake valve is still open? Boom up the carburetor. Maybe the dizzy IS a tooth out. I'd check the timing.

patkeefe · 04-08-2005, 03:24 PM

Timing definite maybe.
Valve lash OK?
Intakes seated real well?
140's seem a tad big...how is it with the throttle kicked in?

Pat

Fishcop · Fishcop's Garage

Rob(?), make sure you don't have any air leaks in either the intake manifolds or in the exhaust. I know this goes without saying - but I initially had some popping with my carbs after a 2.7 build, I traced it to an air leak in one bank of carbs. Another thing you should double check is that your plug leads are correct... I had 3 & 5 swapped - the car ran and seem okay until reved, then I'd get severe back firing.

Cheers

Superman · 04-08-2005, 05:03 PM

Quote:

Originally posted by Fishcop
Rob(?), make sure you don't have any air leaks in either the intake manifolds or in the exhaust. I know this goes without saying - but I initially had some popping with my carbs after a 2.7 build, I traced it to an air leak in one bank of carbs. Another thing you should double check is that your plug leads are correct... I had 3 & 5 swapped - the car ran and seem okay until reved, then I'd get severe back firing.

Cheers

The traditional problem to assume when carbs are popping is a too-lean mixture. Here, Fishcop mentions vacuum leaks, which cause the mixture to be too lean. It's common to have plenty of 'sorting out' issues between the time carbs are bolted on, and the time when they're running smoothly. Jetting, etc. That's my guess. Mixture is too lean at idle. And maybe elsewhere. This is where you need to be John Walker.

Here's another traditional tip: If you're curious as to what's going on in the carburetor throat, use a mirror to look. The penalty for not using a mirror is loss of eyebrows. Don't ask me how I know this.