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Discussion Starter #1
Some of you may remember I thought my engine had a rod knock awhile ago. It was making a really ugly knocking noise everywhere I went. It turns out this noise is actually backfiring through the carburetor, I witnessed it today while I was fooling around with it (Naturally while I was standing over it:rolleyes:). The problem is very consistent, it backfires everytime I move off idle and continues to backfire at any speed on throttle. It was so regular I thought something must be knocking, but NOPE. What could cause this? My timing is at 12 btdc and I don't see any broken valve springs. I haven't touched the valves or anything lately, and they aren't making any discernible noise. Fuel pressure is good, vacuum is 18. I used to hear the same noise when I would really punch it, never figured out what it was, but it mostly disappeared when I increased my fuel pressure a little bit. Is anything in an eddy 1405 carb known for causing this?
 

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couple of things can contribute to this condition.. lean, cross firing, burnt valves(lean) point dwell, weak coil.... for starters....
 

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Could be an exhaust valve not functioning properly(because of cam/lifter/push rod/rocker arm). Try disconnecting a spark plug wire one at a time and see if you can get the poping to stop, that may help you isolate it if is that.
 

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Discussion Starter #6
Wouldn't a cam lobe make noise? I have a fairly new hei distributor, so can't be dwell. I just got shocked pretty bad by the coil, so i bet its good. Shouldn't be lean, unless there is carb trouble. I ran out of gas a day or so before this started happening, and had a stuck needle and seat afterward. That's why I'm leaning toward something in the carb is screwed up. I'll check thoroughly for crossfiring. I can't understand why it idles perfectly, and then anything off idle starts backfiring. I also made sure my centrifugal advance is working.
 

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I have a brand new 500+ h.p. 383 that will be ready to install during the week--the shop is in Pomona too! But I don't allow Edelbrock carbs on any of my engines.
 

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Discussion Starter #8
Hehe I know you hate edelbrock carbs from your posts before, but until summer I have to figure out some way to make this work. Funny I never had any trouble before, maybe edelbrocks don't last. Although, given the condition of my engine, it could be nearly anything thats wrong right now. I've seen engines with bad cams before, and they act really weird even at idle. My engine idles fine, but backfires under throttle. Somehow I'm betting carburetor.
 

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My engine idles fine, but backfires under throttle. Somehow I'm betting carburetor.
Oh, I mis-read your post. I thought it was back firing on throttle lift.. Tip in throttle backfire makes me think ignition, not fuel. Timing. JR
 

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Discussion Starter #10
It backfires more readily and harder if I blip the throttle, If I ease on the throttle it gets to ~2500rpm without backfiring. I made sure to disconnect the vacuum advance, so its not that. I'll try a compression check today, and probly take the distributor apart to make sure nothing weird is going on.
 

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Discussion Starter #11
OK, I did a compression check, and compression was between 145 and 160 on all cylinders. Not great, but its the same as it was when I checked it several months ago, before I had problems. All of the plugs were fairly sooty this time, so I doubt it's running lean.
 

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Backfiring through the carb while accelerating is usually timing-related, although it can be other things too. Late ignition timing (not enough advance) is the usual culprit.

I know you mentioned that your initial timing is set at 12 degrees BTDC but what is your total mechanical advance?

Most unaltered distributors produce around 20 degrees advance (give or take). This means that you may only be seeing 32 degrees total advance. It may even be less than that. Try advancing your timing to 15 degrees initial and see what happens. It should get better if the problem is retarded timing (which almost always causes a backfire). If it gets better at 15 degrees initial but still isn't perfect, advance it to 17 and try it again. If it pings, back-off a tad.

Remember to disconnect the vacuum advance line from the distributor and plug the hose going to the carb when setting your initial advance.
 

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Discussion Starter #13
I have run the car (with this distributor) with 12* initial advance for a long time now, there is no reason I should need to run more. I don't have a way to measure total timing, but assuming my fairly new summit distributor is operating properly to full advance, I shouldn't need to change my timing. I've already played with the timing anyway, to see if it affected the backfiring, and it didn't. I did notice today that the backfire only seems to come out of the driver side of the carburetor. I have a dual plane intake, so it looks like perhaps something is amiss on that side of the engine. Maybe I do have some kind of cam problem.
 

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If you had a cam problem, you probably would get some valve noise. Have you checked valve adjustment? What cam do you have? Can you put a dial indicator on the rockers to check the lift? It will tell you if you have a bad lobe.
I know you did a compression check, but have you tried a cylinder leak test? Maybe you have a bad valve.
 

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OK, I did a compression check, and compression was between 145 and 160 on all cylinders. Not great, but its the same as it was when I checked it several months ago, before I had problems. All of the plugs were fairly sooty this time, so I doubt it's running lean.
Oh, sooty plugs, she is rich. Ok, I had the same issue sorta with my engine. Sounds like alotta work but pull all the plugs and clean them. See if she does it (backfiring), I think it wont.

So then you need to see why she is rich. I wonder if a hotter plug would help. Try the cleaning of the plugs, not for a fix but just to see if that is gonna help for a second till they get sooty again. If after you clean them and you dont have the problem initially then you know its a rich condition. The soot will make a rich engine "appear" richer due to the resistance (reduced spark) in the ignition cycle. Then you know its a rich condition and can work from there.

With faults you need to fix the obvious issues (dirty plugs) to narrow down the real cause of the trouble. Hard to troubleshoot a fault while there are faulty components in place. JR

Oh yeah, one last thing. You have an over all issue. Not just one cylinder, so I rule out the valve train. They dont go out as a set for the most part. Look at whats common to all the cylinders, ignition and carb. Sure, cam but the lobes dont go away all at the same equally. And a good plug read will see if the cylinders are consistent with each other. If they are then its a common element, ignition, carb.
 

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I did notice today that the backfire only seems to come out of the driver side of the carburetor. I have a dual plane intake, so it looks like perhaps something is amiss on that side of the engine. Maybe I do have some kind of cam problem.
I hope you realize that with most dual plane intakes, the driver side of the carb supplies fuel to the passenger side of the engine.
 

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Discussion Starter #17
Oops I definitely overlooked that...:eek: I did find a problem that was about to occur on the driver side though; I went to readjust the valves and a stud started unscrewing...didn't seem to have unscrewed any before that point though. It got dark but I'll have to check the other side, maybe it has a similar problem.
 

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Discussion Starter #18
Well obviously I don't work too fast but I think I've pretty much ruled out everything but fuel system trouble. I've done a compression test, readjusted the valves (not really for any reason though), opened the timing cover to look at the timing chain, played around with the timing, played around with the fuel pressure, put injector cleaner in the gas, checked vacuum, checked and re gapped every plug, wire, and connector, replaced a slightly loose screw-in rocker stud, and nothing made any difference to the backfiring:mad: Must be the carburetor... backfires through the carb like a metronome after about 2000 rpm. :confused:
 

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You mentioned in an older post that you got shocked by the coil....how did that happen with an H.E.I ?
is it a Chevrolet style (big cap) distributor?
and just for clarity tell me what your firing order is..
 
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