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Old 04-06-2005, 10:42 PM
mahoney mahoney is offline
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SUCCESS-Backfire through carburetor

I have a rochester quadrajet on a 1973 GM 307 engine(marine). I got everything up and running and it idles great up to 3000 rpm even while running under no load on the garden hose, however under load(when secondary chokes open) it will backfire through the carb. I have checked timing, fuel pump, all lines and it all seems to lead back to the carb and it backfiring through the throttlebdy as soon as the secondaries are opened. I blew out all passages with compressed air and they are clean. Any suggestions?

Last edited by mahoney; 04-09-2005 at 03:52 PM. Reason: Solution!
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Old 04-06-2005, 10:47 PM
dougbfresh dougbfresh is offline
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Secondary metering rods too big(rich)?? Did the carb come on this engine>
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Old 04-06-2005, 10:54 PM
mahoney mahoney is offline
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Carb came on this engine when it was given to me and ran fine prior to storage. It had set for 5 years unused. Consensus on other forums is that it is due to a lean mixture when it backfires out the carb.
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Old 04-06-2005, 10:59 PM
dougbfresh dougbfresh is offline
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Well, if this carb came on this setup and noboby has "dinked" with the jetting the plugs should help give it away. Pull a couple out and check their color. I seem to recall that some marine application do not use vacuum advance, is this true with yours? The other likely possibility is that the mechanical advance in the distributor is all crusty and not working. Have you looked in the distributor lately??
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Old 04-06-2005, 11:36 PM
danny danny is offline
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not saying that the carb is good or bad but all too often people go after the carb when something like this happens. the problem is in the electrical part of the engine. i would start by checking all the electrical components like doug said. start with the plugs and distributor. check the cap and rotor for cracks and deterioration. rarely does the carb create a backfire, you have to remember that a carb supplies fuel and air not spark or timing.
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Old 04-06-2005, 11:56 PM
Racerock Racerock is offline
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Quote:
Originally Posted by mahoney
Carb came on this engine when it was given to me and ran fine prior to storage. It had set for 5 years unused. Consensus on other forums is that it is due to a lean mixture when it backfires out the carb.
Now this I totally agree with. It sounds like the classic lean backfire, or too much advance. Most likely lean backfire. I have personally been through this once before on an old boat, but different engine/carb.

What it boiled down to was that the accelerator pump was working, but not well enough to keep up with the demand. You can try this to verify, sort of. Try running the RPM's up slowly beyond the 3,000 mark, say up to 4,000. Did it backfire? Probably not, as the accelerator pump did not need to kick in.

Now, have it running at say 1,500 RPM, and snap the throttle (Outdrive in Neutral, or not. Does not matter, just make sure the kids are away from the prop. Backfire? I'd bet the accelerator pump is giving out, given that it did not happen before.

You can check the accelerator for general performance by turning off the motor, and look down the primaries for really strong squirts the entire stroke from idle to full throttle basically. If not a nice strong stream, time for a rebuild.

My carb had an accelerator pump that was in a bore in the carb and had seals. It would appear to squirt well, but in reality, it was not. Put a new one in, and whamo, good to go. It was on a 165 Hp Fresh Water Cooled Merc Built Aluminum 4 cyl that was basically a 460 Ford bore/stroke but 4 cyls. I think it had a GM Carb on it...

Is it a marine carb? The other option on the secondary's it to change the secondary metering rods to richen them. Had to do that on a car once after changing the cam, etc. Similar symptoms, but for sure the accel pump was working well.
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Old 04-06-2005, 11:58 PM
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I had a Chebby van once that did this. When the secondaries tipped in she'd backfire back through the carb. Ran great otherwise. I suspected a burnt valve. A buddy of mine suggested swapping out the carb and it ran great again.
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Old 04-06-2005, 11:59 PM
Racerock Racerock is offline
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Quote:
Originally Posted by danny
not saying that the carb is good or bad but all too often people go after the carb when something like this happens. the problem is in the electrical part of the engine. i would start by checking all the electrical components like doug said. start with the plugs and distributor. check the cap and rotor for cracks and deterioration. rarely does the carb create a backfire, you have to remember that a carb supplies fuel and air not spark or timing.
Hate to disagree with you, but a lean backfire is a classic, and can be a really misunderstood symptom. Have had it a few times.

One time, the backfire on acceleration was a combination of too much vacuum advance, and lean secondary metering rods. Had Joe Mondello Jr.(Sr. & Jr. - Olds Gooroo's)confirm my thoughts, and he was happy/surprised to see I was on the right track. He recommended the new sized metering rods, and he was right on.
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Old 04-07-2005, 01:10 AM
danny danny is offline
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it's all good, that's why i put in the disclaimer of not ruling out the condition of the carb. i am in agreeance with checking all the electrical components first though before going through/to the carb. i feel that this is the proper way to go and is easier for most to check.
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Old 04-07-2005, 01:44 AM
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You guys are killin' me :-) Mahoney, please post what you find out. Its not one of my better areas, but I have had a number of quadrbawgs on cars so I'm curious.
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Old 04-07-2005, 11:56 AM
Ben There Ben There is offline
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You got me thinking about those oldies if that's an orginal 73 Quad thet were a lean drop carb, opening of the secondary flies [the gap] was somewhat important you had to use a drill bit size? and adjust the linkage not running One other thing was timming it had to be done at 1100 rpm with vac/add plugged set at 20 degree's as it is an OLds block After the carb secondary linkage was set then you ajusted the mixture and after done you gave it a lean drop
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Old 04-07-2005, 12:03 PM
mahoney mahoney is offline
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Still working it

I agree with all the statements here, but am really confused. The symptom seems to indicate a lean backfire, however the plugs are always black, not wet just black, when I remove them. As stated earlier in the above post, running slowly up to 3000 rpm does not backfire, it is only when snapping the throttle that it does this.

I repleaced the accelerator pump and rebuilt the carb. I am a novice at this, and am going to pick up a book today for more info of what might have went wrong with that task(I am mechanically inclined though).

On another note this distributor is electronic ignition(converted) and is mechanical advance(which I checked and it is working as it should). I also checked advance while timing and it does advance as rpm's increase.

Here is one of my conserns. When I initially went to time the engine, I got the engine running at roughly 700 rpm, and it was running somewhat roughly(surging a bit like it could quit anytime) I set timing where it should be. What was odd is that if I turned the distributor from that intitial, what I thought of as good timing, I could make more rpm just by turning the cap and the engine ran smoother. I had the idea of doing this by ear and tach untill I milked the most rpm, smoothest engine running, then started backing off the throttle to see if it would settle down to idle rpm then shoot timing again?

Wow, this one has eveyone stumped! I would love to get this nipped, and am open to any/all suggestions!

Thanks Guys

Last edited by mahoney; 04-07-2005 at 12:04 PM. Reason: spelling
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Old 04-07-2005, 12:07 PM
dougbfresh dougbfresh is offline
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Do you have an adjustable timing light? You need to check to make sure the advance is working like you think. The black plugs make me thing it is running rich. You could get some leaner and richer secondary metering rods and experiment a little. What does a vacuum guage show out of curiosity?
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Old 04-07-2005, 12:14 PM
Ben There Ben There is offline
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What degree are you set at
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Old 04-07-2005, 12:38 PM
mahoney mahoney is offline
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Vacuum gauge showed a pretty steady 15 lbs, give or take a 1 lb wavering(which decoded mostly to mixture problems) I knew I was having those. That tells me it is most likely not a sticking valve as I would see a 4lb +- wavering at 1500RPM.

Timing light is not adjustable, and I thought I had the setting at 8 Degrees BTD, which is correct in the manual, but I am suspicious as I can rotate the cap from this timing and get better engine performance.
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