I crawled under my car last night to see why I might have backfire. I figured maybe some piece of exhaust hardware had loosened itself up. Nope, everything looks good.

I'm thinking the issue is carbs, since that's the last thing I played with. What should I check next?

TIA,
Rusty

where on the rpm range does it happen? cruise, what, 70 or 90? snapping back through the carbs (i refer to it as sneezing) at 3000 rpm. i know you have a six. but it's one more barrel per bank, i'm starting to figure out my idf's.

kevin

Morning, Kevin,

When crusing... but most often when the RPMs are falling, or I let up on the gas.

Backfire happens ore often when the engine has warmed up a bit.

-Rusty

good morning, i mean where does it happen @ cruise? exactly what rpm. at warm idle do the carbs flow the same? just saying "sounds lean" is pretty vague. when stuff happens, when it happens is key. what are you using? a unisyn or the ball in the tube. jp has the tricks for your carbs.

kevin

i also regret not putting ina o2 bung before i had my exhaust coated. what form of exhaust are you running?

kevin

Carbs are out of sync.....

here is where the speculation starts. at 3000 to 3100 rpm my car would sneeze on the left side. the transition point. pulled idle jets, blewout circuit, gone. then i threw away the idle screw on the pass side carb and used the drop rod to adjust flow. one idle screw does the work. got to have a good linkage tho.

kevin

Hi Lawrence,

While this might not be of any help, when i'm WOT, and back off, i get a pretty big backfire. enough to have reports of flames coming out of the tailpipes at the last DE. But this was at the track.

So what is it that you are hearing? Pops in the intake or backfires in the exhaust while cruising?

When you say that you are getting this while crusing, any idea of the rpm rang. Check it under 3000 lets say 2000-2800, then at 3000-3500, then at 3500-4000.

take a look at http://www.inglese.com/tech.htm. this will give you an idea of the magic that is happening in the carbs regarding idle jets, main jets, emulsion tube, air correctors and so.

Are the floats set and double checked, is each cyl in a bank is in sync drawing the same air, aret both banks in sync also (at idle and a 3500rpm), ignition timing is set, fuel pressure is in the 3-4psi rang.

Id go warm up the car to 180 and and reset the idle jets. turn the idle jet in till that cylinder drops (rpm goes down and you can hear that you lost that cylinder) then bump it back up till it catches back. I do this slowly in 1/8 increments. I give it a 1/2 turn out after that cylinder came back and i move on to the other.

HTH

Marc-André

just to rule some other stuff out. Check for leaks at the heads/intake manifolds and intake manifolds/carbs with propane.

Marc-André

The carb floats are roughed in, but at least they don't overflow and cause hydrolock anymore. I don't have the float level tool - someday.

Sounds like full tune-up time. I really need to get that tach fixed so I can check timing again.

A valve adjustment is on my to-do list... I'll play with it this weekend and next week. Thanks for all the replies.

Rusty

P.S. I'm pretty sure there are no leaks (exhaust/intake manifolds).

Well, if you don't have that handy float lever tool, you could build one. We did this when checking what was going on with the carbs on a friends 914-6 conversion.

After all, it's a liquide level.

Basically we took one of the drain plugs out, ran to the hardware store, to find a bolt that had the same thread and some clear tubing that could fit tight over the bolt head. We drilled out the center of the bolt, put the clear tubing tight over the head and put the bolt into the carb. we made a little fixture to hold the clear tubing pointing up next to the carb with some metal wire.

We put a line on the tubing to align it to the carb flang and 2 other line where the one on the float tools are located (got the specs somewhere).

It worked pretty good. The fun part of it was that you only need to bend back the tube to drain the chamber in a cup instead of spilling gas all over while unscrewing the float lever tool.

Marc-André

i agree with mikey.

sync the carb linkage at cruise ( 3k ) and see what you have.

I'd also check the mechanical/vac advance of the dizy. Could be sticky.

I've had similar problems with mine with poor quality gas.

I try to run a 50/50 mix of race gas and unleaded premium (NO ETHONOL)

I haven't had a lot of time to play with it... but I agree with you.

Ethanol is evil.