Stumbling and backfiring carb question (1 Viewer)

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Couldn't agree more! The idle is ok at 7 BTDC but it stumbles and backfires as soon as you take off. Fuel pump looks relatively new, I am going to replace the fuel filter next. I'll check the advance and see if the timing changes at higher RPMs.
This is important information that you did not disclose previously.. This isn’t stumbling under load, it is stumbling and backfiring off idle. This is usually a problem of lack of idle fuel or a intake manifold vacuum leak. It is hard to distinguish these possibilities, but try to rule out the manifold leak and then test whether disconnecting the idle fuel cut off makes any difference. Turning up the idle speed screw to make it run off the main nozzle often helps it run better, but this is bandaid.
 
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I have pretty solid vacuum numbers so I don’t think there is a intake leak. Would the sight glass have fuel if there was a fuel starvation problem? I can manually rev it and watch the glass, it will backfire but the glass still shows fuel. The idle speed screw is currently not doing anything when turned.
 
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I also tried planing the intake/manifold with good results. You can also pull one plug wire at time if there is is no change in idle that could also be an issue. May have several problems. Run some sea foam in gas tanks. Good. Does it back fire while at idle and Rev up the engine and than you get backfire ?
 
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I have just completed a carb rebuild and found that same weight and ball were missing. It ran good for the previous owner, so I put it back together and runs better now. I just need to adjust the carb to slow the idle. I do not have any backfiring issue at idle or underload without the weight and ball. I did pick up a spare carb left over from another build and plan on using it for those parts.
 
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I also tried planing the intake/manifold with good results. You can also pull one plug wire at time if there is is no change in idle that could also be an issue. May have several problems. Run some sea foam in gas tanks. Good. Does it back fire while at idle and Rev up the engine and than you get backfire ?
At 7 BTDC it will idle ok, but backfire/sputter as soon as it is revved. Pretty much not driveable. At 20 BTDC it seems to run fine with no backfire or sputter. The only issue is the idle is high and it may not be good to run long term with that much advance.
 
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14 inches of water is not good vacuum. Did you check for leaks, especiallu underneath the manifold where it bolts to the head? What happens when you disconnect the idle fuel cut off solenoid wire?
 

thebigredrocker

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I see manifold vacuum of 13-14 in the pic you posted. That's low at idle. Sign of non-functioning Idle circuit is—>At idle you shine a light down the primary and can see fuel spray from the main nozzle. While your shining your light in the primary, pump the throttle arm a watch for a fuel squirt. No discharge chamber squirt will contribute to the stumble.
 
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Not saying this is your problem, but the 2F in my '79 FJ40 started showing the same symptoms you are describing after it had sat for about a month while I did a complete valve job to upgrade to Chevy exhaust valves. After getting it back together it stumbled badly and had a tough time gaining speed. Sputtering and backfiring was embarrassing. I tracked it down to the distributor. The insides showed rust. Took the dizzy out of the block and went through it thoroughly. Cleaned all the rust out, checked for cracks in the cap and rotor (they were ok) and found that the mechanical advance was bound up. The hose that gets a vacuum from the side of the air cleaner was blocked, so the dizzy wasn't getting air drawn through it. Got the guts of the dizzy cleaned and lubed and the truck fired right up and runs like a top again. Just thought I should throw this out there in case someone else has a similar problem...... it's not always a fuel issue.
Doug
 
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@jjrowe
E2EEBF34-7482-4B9C-B006-E0CC7FF78B65.jpeg
 
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i was thinking along the same line as Doug above. What distributor do you have and where is it connected to carb? I recently picked up a 69 that was running poorly, stumbling backfiring stalling, and it turned out the PO had the vacuum advance distributor connected to manifold vacuum instead of ported vacuum...
 
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Did some more testing.
1. Disconnecting fuel solenoid wire causes the engine to die immediately.
2. Vacuum I measured before was teed into the manifold port that went to the VSV. If I measure directly from manifold I get a good 21 in. So I’m losing vacuum inside the VSV. Not sure if that is a problem or not. Seems to run a little better when VSV is removed from system.

F49E61CE-F1AC-4012-9C68-9A3FF0307D4C.jpeg
 
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i was thinking along the same line as Doug above. What distributor do you have and where is it connected to carb? I recently picked up a 69 that was running poorly, stumbling backfiring stalling, and it turned out the PO had the vacuum advance distributor connected to manifold vacuum instead of ported vacuum...
I believe I have a retarding distributor and its vacuum is hooked to the VSV. I’m not measuring any vacuum at that port.
Edit: I connected the distributor directly to the port at the bottom of the carb and it started to run rough. So vacuum does seem to retarding the timing.
 
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Fuel should not be dribbling out of the primary nozzle at idle. Can you adjust the idle speed down to 625 RPM? The fact that the engine stalls when you disconnect the solenoid indicates that the idle circuit is providing fuel.

Does the engine stumble when you rev it in neutral and parked?
 
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Fuel should not be dribbling out of the primary nozzle at idle. Can you adjust the idle speed down to 625 RPM? The fact that the engine stalls when you disconnect the solenoid indicates that the idle circuit is providing fuel.

Does the engine stumble when you rev it in neutral and parked?
In the video I am parked in neutral and there is some hesitation. I’ll try it again and see if I can get the idle down. I brought the timing back down, and with the VSV disconnected, I am not seeing the carb backfire like I was. That vacuum loss through the VSV appears to be at least part of my problem.
 
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The point is to find and eliminate any intake manifold vacuum leaks and see if the problem goes away. Also pop the distributor cap and twist the rotor back and forth. It should move freely against spring pressure about 20 degees. A little spray lube wouldn’t hurt.
 
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The main issue seems to be my VSV isn't working properly. To bypass that for now, I connected my distributor vacuum directly to the base of the carburetor (manifold vacuum) and reset the timing. A retarding distributor cannot be timed to the bulb without manifold vacuum or it will be too retarded when the rpms increase. The mechanical advance is not enough alone. I was compensating for that by running 15-20 BTDC before. The sputtering and backfiring is gone now. Next step is to diagnose the VSV.
 

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