I have twin 440s in my boat with AFBs. I pulled one of the carbs because of a fuel leak. Turns out, floats were set too high. Guy who fixed it, solved the problem, but also moved my mixture screws from where I had them. Now, when I cold start, I am getting backfiring until she catches and then runs.

I thought I followed directions on setting the mixure screws by listening for RPM changes, but apparently I'm way off base. I will ask the guy again, but I would welcome second opinions as I watch Sunday football.

Timing was on prior to pulling carb.

How do I solve the backfiring? It did not do this prior.

Thanks

 

A backfire thru the carb usually means its running too lean. Do you use a choke on those carbs? I start with 1 1/2 turns open, then when at operating temp, adjust for best idle. But adjusted this way, it will always be lean when cold, but fine when it warms up. So thats why you need a choke on the carb.

 

backfiring through the carbs or the exhaust ?

 

The higher float levels it had before were maybe helping an other wise lean condition. I dont run a choke on my Holley 780, and the 451 was very flat when cold. And still bogged a little when warmed up. This pointed to a too lean condition. So I increased jet sizes 4 steps larger than stock. Boy did that ever wake that motor up! Went from 72 primary to 76. And 76 secondary to 80, on this 3310. Now just a slight bog when cold, and not any hesitation after warming up. And it lays rubber for about 20 feet when it hits 2nd gear. Never did that before! Good luck.

 

Do these carbs run automatic chokes? Could be as simple as the choke set too lean, or defective, sticking, etc, or even if manual, could be that the cable is adjusted incorrectly, no closing.

Is there any possibility that you NOW have a vacuum leak? (Carb base gasket?)

Just how did you adjust idle?

Engine MUST be up to operating temp. "Smog" cars run on the "slightly lean side" but in this case I would use a tach or vacuum gauge, back the idle screw out (rich, ccw) until you see a peak if too lean, or if already too rich, no real change, or a downwarn change.

Then adjust the screw back in (cw, lean) until you get a "peak" in vacuum or tach.

Go back and forth a couple times on both screws, and you may have to re-adjust idle speed if it goes up some.

For vacuum leaks, try a small piece of fuel hose held to your ear and "sniff" around the carb base. A spray bottle of water will also affect idle speed when sprayed in the vicinity of a leak.

Some people use propane, etc for this, but not necessary, and obviously in a boat............................

 

I will back the screws out first and then give it a shot. No on auto choke-manual set and intake heats up the winding to release choke. I don't even think I use the choke as I adjust rpm at my throttle. I sure hope I don't have a vacuum leak. I thought I did a pretty good job of cleaning base and alternating my torque sequence.

440roadrunner, I did just as you describe on how to adjust. I am a work in progress as are my wrenching skills.

Psycho, it's backfiring through the carb on cold start up. On warm start, no prob.

Thanks for the input

 

if it backfires through the Carb when cold it is too lean so the choke is not closed enough or you do not have enough accelerator pump shot , if the car doesn't stumble when warm the accel pump is ok

 

I'm not clear on what you meant on the choke

I believe what you meant is that you do have an auto choke. ALL of them are "manual set" IE floor the throttle to set the auto choke

A manual choke has a push/ pull cable, controlled from the dash

Don't you have the black round plastic choke assembly? If so, you can adjust them for rich/ lean. You loosen up the clamp screws and rotate the black housing. This may be all that's the matter for cold starting.

Try to compare relative spring tension between the two choke butterfies, IE with both engines cold, floor the throttle once to set both chokes, bring both back to idle or starting position, then gently flick each choke and compare how far they are closed and spring resistance.

"Pull off" is also important. If you have the black pastic style choke, there's a vacuum piston in there that must be properly operating. It's job is to pull the choke just slightly open after starting. It may not be working properly, or sticking.

You can easily check for vacuum leaks with a scrap of fuel hose held to your ear, and probe "down around" as well as a spray bottle of water. "Mist" around suspect areas and listen for a change in the engine sound

 

Its a boat, with twin 440's if i read it right....

 

I figured it out....Since I do have twin 440s, I was able to compare both carbs to see what I was missing or not attached. Sure enough, the choke spring/winding that is tucked in the intake was not attached properly to the shaft connected to the choke plate. The choke wasn't working. Attached and working great.

An overlook on my part but I happy it was an easy fix.

Thanks for the help!

 

Very good!!!!