Falling on face/missing/backfiring (through intake)?

[Ride The Snake!]

I believe I must have a carb issue on my '70, but I'm wondering what you all think. I've just been sorta teaching myself how to work on this thing for years.

Anything above light demand on the throttle causes the motor to miss or sputter, or more violently backfire--out the intake? Sudden, moderate or heavier acceleration is most likely to induce the backfire, but even if I very gently rev the motor--cruising or stationary--it starts to stumble and miss past a certain point (no tach but I would estimate in the low to mid 2,000s). Progressively, slowly increasing rpm just causes the problem to become more pronounced and often induce a backfire.

Basic-ass stock 350 setup except for a slightly better cam grind:
-Roch. 2G (replaced with a remanufactured unit maybe 7 years ago, after I had attempted rebuilding the old one twice and found no improvement in whatever issue I was having at that time). New carb has maybe a few thousand miles on it.
-GM HEI (I rebuilt it a few years ago)
-stock GM heads from '74
-the Crane cam is nothing inappropriate to the application, I would have to look up the specs but it's a bit of a lower-end 1500-2200 cruising rpm type for trucks
-4 spd MT

-I did the timing kit with a dual roller at the time of the camshaft.
-Last week replaced fuel pump, filter, and sending unit.
-A few years ago (and few miles since) I replaced every part of the HEI including the pickup coil. Plugs gapped, wires look good.
-Timing set to 12 BTDC, no digital timing light or timing tape but the mechanical advance seemed to come in appropriately or within ballpark.
-Idles fine and at low rpm seems to cruise okay.

It must be running lean somehow, no?

If I've left out any other pertinent info just ask. I appreciate any input.

[fdbdw]

I would double check everything in the ignition system first, backfire out the top is usually fuel that is not being ignited at the right time . Bring your number one cylinder TDC, make sure your number one terminal is in line with the rotor then work out from there. I’ve had my wires in the wrong order before even, after double checking.

[kidatheart]

Possibly flat lobe on the cam?

[geezer#99]

Easy test.
Warm the motor up completely. Loosen distributor enough to move by hand. When you rev it up to induce the symptoms just increase the advance slowly and see what it does. If it runs better it’s ignition timing related.
Could be an intermittent short in the distributor, stuck weights, broken/missing springs, cracked cap, bad wires etc.
Other things like spun balancer or dreaded flat cam could be the culprit.

[landarts]

There are a few things that can produce a backfire through the intake. I recently had this issue and after trying serval things it came down to bad gas that had some water in it. An old school mechanic suggested that I siphon out some gas from the tank and put into a glass jar. You could see where the water separated from the gas. I had to drain tank, flush fuel line and rebuild the brand new carb that only had about 100 miles on it.

Another way you could do a test is to disconnect the fuel line going to the carb and run a rubber fuel line from a 1 gallon gas can to the fuel pump and let it run for a bit until it is just getting the new gas and see if that changes anything.

[mikeshome]

Just wondering if problem just appeared or was anything done that it started doing this.Has truck been parked for winter etc?

[jbarron]

I would check the accelerator pump in the carb. If it does not give a good shot of fuel when you hit the throttle it will lean out and backfire.

[Ride The Snake!]

Boy I sure hope it isn't flattening the new cam. That's what happened to the old one.

Old cam had flattened at #6 intake valve. I never knew why; my guess was that the lifter collapsed from oil starvation, or the fact that I never used oil with ZDDP (though neither has my dad in his '71 Suburban and he's never flattened a cam). Weak theories maybe, but I didn't think it was due to a condition that would be bound to repeat.

When that cam had flattened, I could easily hear that I had a dead cylinder even at idle.
Vacuum tested it, checked compression at the time.
The lifter valley had a fair amount of crud in it but nothing spectacular. Cleaned it out as best I could and used compressed air in the smaller channels iirc.
Another oddity, the oil filter had somehow collapsed on itself from negative pressure.

Now, I know what you're all thinking--that it was time for a complete tear-down. Well, I didn't really have the funds, or the skill, or patience, and try finding a place to hot-tank your block in the Bay Area of CA, haha.

I found no metal in the oil pan, nor in the lifter valley. I cut open the collapsed filter and found the lobe's metal fragments in there, so my hope was that they collected there and I wasn't going to spin a rod or cam bearing.

New cam, balancer, timing kit (the old cam sprocket was like, NYLON plastic or some such, with significant wear), new plugs, etc. Valve lash set to 0 and then the preload as defined by the Haynes manual and the instructions that came with the cam and lifter kit. Everything seemed good.

Cam break-in seemed normal. Changed out the break-in oil and added Joe Gibbs 10w-30.

The problem I have now didn't take long to creep up, after very sporadic driving over the last ~8 mo or so since that work was finished.

I'll recheck my plug wire order yet again just to be sure, play with the timing a bit. If it ran well on initial cam break-in on what was several year-old fuel at the time, I kind of doubt water-contamination is my issue now. But maybe it is.

The dizz didn't appear to have any signs of arcing, the mechanical advance springs/weights are opening appropriately. I should note I disconnected the vac advance and plugged it, because i think the diaphragm was blown out (only thing I didn't replace in it). But that wouldn't have any effect on timing during acceleration/load.

I'll report back on any findings.

[Sheepdip]

Originally Posted by jbarron

I would check the accelerator pump in the carb. If it does not give a good shot of fuel when you hit the throttle it will lean out and backfire.

^^^^^^^^^^^I second looking down the carb with the engine off and opening the throttle to see how big a squirt you get, I'll bet not much. Very common for a intake backfire.

[Ride The Snake!]

Squirts from accelerator pump look plenty good.

I did a vacuum test:

~17 in.-Hg at a slightly low idle (no tach to measure accurately)
~19 at a slightly higher idle when fully warmed up (no fast idle cam linkage set up, choke is disabled and fully open at all times).

At idle I get a little bit of a bounce that's <1 notch/in.-Hg

Aggressive throttle snap drops vac pretty much all the way to zero if the snap is WOT.

Under gradual acceleration the needle moves up to ~22-23", becomes almost completely steady.

Closing the throttle from cruising RPM causes the needle to bounce up to 25-26" before returning to normal.

As I said before, initial timing is ~12btdc, and I don't know the precise total timing but it seems to advance appropriately when revved up, seeing the mark on the balancer climb to roughly 12 o'clock if I'm facing the engine from the front. That all being said, I'm ordering a piece of timing tape.


What am I missing here, fellas? Could it be anything besides maladjusted ignition timing? If it is timing, given the symptoms described, am I likely too advanced or retarded?

[68 P.O.S.]

Timing is always job #1

[bigmac73]

Timing set to 12 BTDC, no digital timing light or timing tape



To me this is the first issue, how do you know it is at 12? Go buy a light and make sure your timing tab is correct or add a tape, once you find Top Dead Center,

[Ride The Snake!]

Originally Posted by bigmac73

Timing set to 12 BTDC, no digital timing light or timing tape



To me this is the first issue, how do you know it is at 12? Go buy a light and make sure your timing tab is correct or add a tape, once you find Top Dead Center,

I have a basic timing light. I know it's at 12 because there is a timing tab (albeit for a slightly larger diameter harmonic balancer). The assumption I'm making is that it's actually correctly set so that the 0 mark truly corresponds to TDC. If it's inaccurate, it's not by much, but I guess it wouldn't hurt to get a piston stop and double check that it is exact, add timing tape and go from there.

Even if I'm a couple degrees off, would that really be causing the horrible running condition I'm seeing?

[68 P.O.S.]

Originally Posted by Ride The Snake!

I have a basic timing light. I know it's at 12 because there is a timing tab (albeit for a slightly larger diameter harmonic balancer). The assumption I'm making is that it's actually correctly set so that the 0 mark truly corresponds to TDC. If it's inaccurate, it's not by much, but I guess it wouldn't hurt to get a piston stop and double check that it is exact, add timing tape and go from there.

Even if I'm a couple degrees off, would that really be causing the horrible running condition I'm seeing?

Since you have the wrong timing tab for your balancer size, you don't actually know what your timing is. Get the correct timing tab for your balancer, get a correctly sized timing tape, find TDC with a piston stop to verify timing marks, start her up and set your total timing for 36 degrees btdc (or whatever you're comfortable with).

Yes, it is possible for the timing to cause your issues. It all depends on how far off your timing is.

[Ride The Snake!]

Originally Posted by 68 P.O.S.

Since you have the wrong timing tab for your balancer size, you don't actually know what your timing is. Get the correct timing tab for your balancer, get a correctly sized timing tape, find TDC with a piston stop to verify timing marks, start her up and set your total timing for 36 degrees btdc (or whatever you're comfortable with).

I agree that it would help, but I remember measuring and calculating the likely balancer size this tab was meant for, and it's not a massive difference, i think I estimated it was degree'd for a 7.5" balancer. But given that the ideal degree of advance is going to vary slightly even among similar mild/stock configurations, I think having absolute accuracy is probably not THAT important, since in this situation one is making adjustments based on subjective perception of how the motor sounds and "feels" to drive. Having some precision in your adjustments is more important, I think.

Even the wrong timing tab should still be accurate at 0 and wouldn't be that off at 12 btdc. A 6.25" diameter balancer would be 1 degree every ~1.38 mm. On an 8" balancer, it would be 1 degree every 1.77mm.

So at 12 degrees the discrepancy on the tab would be a distance of ~4.7mm.

12 degrees might really be somewhere like 15. Okay that's significant, but can be accounted for.


All that said, I do appreciate the value there would be in confirming that the tab is positioned correctly such that the 0 mark actually corresponds to precise TDC. Otherwise, any other number is meaningless. I'll pick up a piston stop and do that.

[BigBird05]

I don't think anybody has asked, but how old are your spark plugs and wires?

[phlegm]

I just went through all this, my starvation issue was pump/line/filter/tank related. The truck had sat with bad gas. Once that was cleared I had a rich issue. That I cleared by puling the carb and doing a quick clean out of it, since it had just been rebuilt last year turns out there was rusty residue from my tank issues in the bowl at what I call the pin valve and the bowl was loading up. Got that straight and leaned out the mixture with the adjustment screws....

Once all that was done it was still backfiring ,that was all about timing. I am running HEI, stock 350/stock cam/ quadrajet at 14 BTDC using a light. Worth noting 10 - 12 BTDC took the backfire out but i had reduced power compared to how my truck ran after a large backfire before (I know, still doesnt make sense to me) 14 BTDC gave me the go I needed in third while still solving for the backfire.

[Bobbysgmc]

I had a carb run rich and heat from header would ignite fuel and cause it to back fire, also a bad spark plug will do the same thing.Check simple stuff first.

[wesmigletz]

Double check your number 5 and 7 wires to make sure they arent switched

[Ride The Snake!]

Just a quick update:

I definitely think my main issue is timing. I've re-tuned the idle circuit. I had the mixture set too rich and the only way it idled decently before was with the idle speed screw turned in too far, opening the throttle plates. I figure I was actually idling on the cruise circuit before. Oopsie poopsies.

I haven't found anything stating clearly whether it should be able to idle with the plates fully seated. In my case, it won't, but now the throttle is far more closed, the idle is smoother, and the idle vacuum is now at a solid steady 20 in-Hg and without that 1 in-Hg wander I had before at 18-19.

I also backed off the static timing about 3-4 degrees, but until my timing tape and piston stop arrive, I can't accurately measure total advance and verify that my timing tab is in fact accurately set to TDC at the zero mark.

I also suspect my vac advance diaphragm is blown, and after reading some very insightful posts last night, I'm convinced of the value of vac advance, and renounce my false belief that vac advance is unnecessary. My only question for this is, should I replace the same diaphragm and use ported vacuum, or buy one spec'd for use with manifold vac? I've read pretty convincing arguments for manifold vac advance being superior so long as you dont have it paired with excessive/aggressive mechanical advance. Manifold vac will give more advance to the lean idle mixture, resulting in cooler idle temps, better throttle response, and more efficient combustion. Ported vacuum advance was introduced to early smoggers to retard idle timing, so cars sitting in traffic would "light the fire late" and burn cleaner, hotter exhaust at the cost of higher idle temps and a crappier timing curve that needed to be offset by more mechanical advance to regain the engine's pep at higher RPM.

Took it for brief test drive after fully warmed up, and it felt more responsive, but still misses at WOT a bit and it backfired once out the carb, but it was a quieter pop than what I was getting previously. So, that's some progress.

Will update when I discover more. I welcome any thoughts y'all have.

[HO455]

Originally Posted by Ride The Snake!

Just a quick update:

I definitely think my main issue is timing. I've re-tuned the idle circuit. I had the mixture set too rich and the only way it idled decently before was with the idle speed screw turned in too far, opening the throttle plates. I figure I was actually idling on the cruise circuit before. Oopsie poopsies.

I haven't found anything stating clearly whether it should be able to idle with the plates fully seated. In my case, it won't, but now the throttle is far more closed, the idle is smoother, and the idle vacuum is now at a solid steady 20 in-Hg and without that 1 in-Hg wander I had before at 18-19.

I also backed off the static timing about 3-4 degrees, but until my timing tape and piston stop arrive, I can't accurately measure total advance and verify that my timing tab is in fact accurately set to TDC at the zero mark.

I also suspect my vac advance diaphragm is blown, and after reading some very insightful posts last night, I'm convinced of the value of vac advance, and renounce my false belief that vac advance is unnecessary. My only question for this is, should I replace the same diaphragm and use ported vacuum, or buy one spec'd for use with manifold vac? I've read pretty convincing arguments for manifold vac advance being superior so long as you dont have it paired with excessive/aggressive mechanical advance. Manifold vac will give more advance to the lean idle mixture, resulting in cooler idle temps, better throttle response, and more efficient combustion. Ported vacuum advance was introduced to early smoggers to retard idle timing, so cars sitting in traffic would "light the fire late" and burn cleaner, hotter exhaust at the cost of higher idle temps and a crappier timing curve that needed to be offset by more mechanical advance to regain the engine's pep at higher RPM.

Took it for brief test drive after fully warmed up, and it felt more responsive, but still misses at WOT a bit and it backfired once out the carb, but it was a quieter pop than what I was getting previously. So, that's some progress.

Will update when I discover more. I welcome any thoughts y'all have.

Some comments.
It is very rare you will get a vehicle to idle with the primary plates completely shut.

Why wait for a timing tape? You understand the math just scribe the balancer yourself and save the cash.

If your vacuum advance is "blown" then it is a vacuum leak and it could be the cause of some of your issues.

Read Tim_MC's attachment lots of good information on selecting a advance canister.

And once again Ported vacuum advance DID NOT come about because of emissions. My 1951 GMC came from the factory with ported vacuum advance. As a pre PCV valve engine they didn't even know the word emissions existed. Heck that engine was designed during WWII. There are reasons to run both and one needs to understand when to use which one.

Have you inspected the mechanical advance mechanism? Do the weights move freely?

Have you verified the voltage to the HEI? Under load?
HEI's are not as bulletproof as folks think especially with aftermarket components. You could be suffering from a bad module or coil in the distributor.

[tim_mc]

I've read the same convincing arguments for manifold over ported sources for vacuum advance and the use of ported source for emissions reasons. I tried to keep my vac advance on manifold, but had to move it to ported in my specific case - at least for now. I've been sorting out what seemed like a lean condition when accelerating from cruise, which would cause a flat spot stumble and often a small backfire. Taking off from a dead stop was usually OK, or just not noticeable, but it didn't seem to have the power I was expecting. My setup is an Edelbrock 1905 650cfm carb on a GM 350 crate engine, a mild Comp Cams roller cam, Brodix IK180 heads, EPS intake and full-length headers, about 9:1 compression, 93 octane fuel. I was having lean surges on the highway and suspected too much total timing, so I changed out the HEI canister (20 degrees at 7.5"Hg) to a Standard Motor Products VC204 can with 14 total degrees advance, 10-12"Hg max advance, starts at 7-9"Hg (should be the same as Napa Echlin VC1838/AR12). Both the AC Delco HEI and Edelbrock carb instructions state to use a ported source for vacuum advance.

Yesterday I disconnected my vacuum advance, blocked the port, and added a few more degrees of static timing at the distributor (I have 12 degrees BTDC intial at the crank, new AC Delco HEI with 22 degrees mechanical). Took it for a drive and WOW, it finally runs awesome! I then hooked up the vac advance to the ported source with no issues (14 degrees max). I believe my issue is that when I would give it throttle, the timing added from the vacuum advance was removed when the manifold vacuum dropped, and that my current vacuum canister doesn't work well with a manifold source. Moving it from manifold to ported eliminated the quick drop in timing and kept the timing curve advancing as it should be.

I would get it running smoothly on intial+mechanical advance first, and try setting the vacuum advance up on a manifold source given the advantages. If you run into issues as I did, you can always get the canister specific to your application and put it back to stock (ported) as a Plan B. I've attached a list of different GM points and HEI vacuum canisters and their specs by Lars Grimsrud, which I found interesting.

[tim_mc]

That's right...I stand corrected. I also read that ported vacuum advance configurations existed well before the emissions era. I guess it doesn't matter if vacuum advance is manifold or ported so long as the timing curve for is appropriate for the specific application. When using ported vacuum advance, more static timing can be added for the best idle and throttle response instead of relying on the vacuum advance to pull it up. One argument for using the manifold source is that it might add a few more degrees of timing at idle (static+vacuum), otherwise ported and manifold sources should provide the same vacuum signal under light acceleration and while cruising. Just make sure that the vacuum advance canister doesn't add too much and exceeds 50-52 degrees BTDC or so when cruising. I think that was one of my issues before changing out the canister.

I also agree with using AC Delco components in an HEI. An aftermarket module left me on the side of the road years ago.

[68 P.O.S.]

Here's a good article written by a former GM engineer who helped develop the systems that explains it very well.

http://www.camaros.org/pdf/timing101.pdf

[Ride The Snake!]

Thank you guys very much. Food for thought. And thank you for that vacuum advance spec sheet. I think I stumbled across that same .pdf a couple nights ago and have it somewhere in one of my browser tabs.

I had previously been running the truck with the ported vac port plugged on the carb, but I reconnected it to the canister before my test drive yesterday after playing with the idle mixture and timing. I will plug it again for now and see how it goes.

True enough about the timing tape but for 5 bucks I figured why not. I was contemplating measuring and marking the balancer at 36 degrees for now to check my total timing.

Yes, my mechanical advance weights are moving freely. I opened the dizz and checked em, and during operation I can see the mark on the balancer advance when revving the motor. I do not know the specs of my mech advance kit, but iirc I once looked at them and found numbers stamped into the weights. Is there a spec sheet somewhere for these weights? I guess there's no way to know exactly which springs I have to know the shape of the curve without measuring its behavior somehow on my motor.

I might have measured my balancer incorrectly, but using a tailor's measuring tape to get the circumference and calculating for diameter, my balancer came out to 6.17" diameter; that can't be right. Can anyone confirm if these GM balancers are 6.25" or are some of them 6.375"? I would hope I wouldn't be so inaccurate as to be off by a whole 3/10" (in the case of a 6.375" diameter balancer), which corresponds to a circumference discrepancy of ~.64" between that and which I measured.