Technical Revisiting exhaust backfire on decel issue

Spoke to a mechanic I know this morning and showed him the video of my vac gauge. This guy has built more hot rods and engines than anyone I know. He thinks given the cam specs that vacuum doesn't really point to a bad valve since my compression and leakdown are good. He said if anything I may have a valve that's not rotating and with such low mileage on the engine that would likely correct itself with break in. I may pull the valve cover and try to mark the valves so I can check them after running it again.

He gave me a few other suggestions to try from least to most invasive as well. Colder plugs on that side to see if the current ones are glowing too hot and causing the issue, temporarily messing with the timing advance by putting a t fitting into the vacuum line and using pin drills to introduce a small vac leak I to that fitting just to see if it helps, facing my exhaust turnout down to see if it sucks less fresh air into the exhaust, and then possibly drilling a small hole into the throttle plates. If it was his car he said he'd do that right off the bat but I'd rather do that as a last resort.....aside from eventually building a full exhaust still.

 

Lakes headers on my sbc, with internal baffles, never backfire on deceleration or when snapping the throttles closed. No full exhaust system, and no backfires. I believe your problem is related to too lean air/fuel ratio, and/or leaks in the header gaskets. I've had motorcycles that consistently backfire on heavy deceleration, and it is always one of those 2 issues causing the backfires.

 

I'd say with good certainty I don't have an exhaust leak. The header was both tested with a shop vac blowing into it while I sprayed Simple Green around the gaskets and welds as well as smoke tested at my buddy's shop with their smoke machine.

As for lean idle...it sure doesn't smell lean. And wouldn't my vacuum continue to rise as I adjusted richer until the screws were all the way out if it was liking the richer mixture?

 

I'm starting to think about just putting a piece of flexible dryer hose over the turnout with some foil in between so it doesn't damage my ceramic coating and see if that longer length makes the problem go away

 

take the baffles out and drive it a little. If I drive my roadster with lake headers (small Chevy with dual quads) on it and let off the throttle quick at speed it wisps small flames out of the headers (only seen at night run open with no baffles), a little unspent fuel in the header will make a nice bang when you let off the throttle. Id bet you take the baffles out the back fire stops. Which would mean its a little too rich at part throttle cruise most likely.

 

I spoke with GM Performance Tech just now as well as Pace Performance and another large distributor of GM crate engines who do dyno breakins. They all confirmed that with the cam in that engine they wouldn't expect to see a steady vacuum at idle. GM stated idle vac should be between 14 and 21hg which also seemed a bit of a company man answer with such a large swing lol.

 

More thoughts...a broken valve spring may cause the valve to be sucked off the seat under high vacuum. Like a poppet valve.

 

I didn't disassemble the heads but a visual inspection of the springs with the valve cover off didn't show any issues. Doesn't mean there isn't but it was nothing apparent.

After speaking with GM it appears as if I have plugs in there that are 1 heat range hotter than suggested. I've seen 2 different sets of instructions for this same engine....1 lists R44LTS and the other R43LTS. I have the 44s in there but I suppose I'll try 43s.....not expecting it to fix my issue however

 

pops and bangs sounds fun to me... until its as bad as my '57s 292 is where it does it at idle glass packs, rumbling, and popping and banging sounds like hot rodding to me.

 

Are the flanges for the street hook up open?

 

No they have blockoff plates and gaskets

 

If I remember correctly in your previous thread,,,,you had times when certain cylinders exhaust would be wildly different temperatures ?
Then they would correct at different rpm ranges,,,,,if I remember right ?
That sounds like an intake flow ,,,or carb issue.
Which would equate out to a very close example of this gauge reading .
Those intakes,,,,look very cool,,,,,but usually run less than desired .

Good luck.

Tommy

 

Tommy you're correct at idle the 4 outer most cylinders measure lower in temp with an IR gun than the inners....I'm assuming definitely less fuel reaching those cylinders. Aside from going to a 4bbl setup not sure how I'd remedy that. I'm kicking around the idea of richening up the mixture screw substantially more on the driver side to see if there's a change or even using 2 different size jets left and right to see if it helps.
Spoke with another local hotrod shop today who also said it could be low speed fuel distribution and he suggested seeing if I could get the end carbs open just a hair at idle to bring fuel in at the ends....I need to look at my linkages tomorrow but not sure I have that option

 

So I'm bouncing back and forth between this engine today and painting a pedal car body. After letting it warm up good I went back after the header primaries with the IR gun and I think number 1 cylinder is my issue. At idle it was at least 50* cooler than all the other ones. I can hear the slight misfire on that side too. I'm going to swap the plug out yet again on that side but I'm pretty certain that's not the culprit. Next I'll pull the valve cover and relash both valves on that side AGAIN. Maybe when the lifters pump up its enough to hang a valve open? After that I'll pull the header to inspect the exhaust valve. If that's no help I guess I'll try to pull the front carb and get a bore scope down to the intake valve? Just not sure why compression and leakdown are good??

 

Compression and Leak Down are good because they are not the problem.

 

Wouldn't that mean no valve issue then though?

 

That's exactly what it means. If the number's are high and relatively even, the engine is sealing.

 

I've seen bent pushrods and broken valve springs give good Comp. and LD readings. It's when the engine is run the issues show themselves.

 

I will have to pull the valve cover tomorrow or Friday and yank those 2 pushrods although I think they're going to be ok.
Interestingly enough I started treating it like a fuel issue just now and backed the idle screws out even more. My max vacuum didn't go up however the swings in my vacuum reading calmed down to only about 1/2" hg and the reading on that #1 primary with the IR gun came up closer to the rest. Took the truck for a ride and still getting the backfire....maybe not as often though. Seems like I may be getting more afterfire warble from both sides now though. Still get the big backfire during the 1-2 shift and sometimes if I let it snap shut at cruise speed. I'm wondering if that #1 cylinder still needs more fuel? The idle screws are reaching the end of their adjustment so I can't play much more. Main jet sizing will not effect the idle fuel on the 2GC right?

I did put a fresh plug in that #1 cylinder and while it was out I was able to see both valves opening and closing in the chamber so if it's a bent pushrod it's not completely toast. After maybe 5 or 6 miles of spirited driving and a bunch of idle time that new #1 plug looks a bit lean....bright white still and possibly some flecks on the porcelain.

 

Ok I went back at it this morning for a bit. I pulled the driver side valve cover and redid the lash. All valves ended up basically in the same spot give or take 1/4 turn. A visual inspection of all the springs showed nothing atleast completely broken and when turning over by hand all valves appear to be opening the same amount. Given all that plus the good leakdown and compression numbers I'm moving away from valves for now. I suppose if 1 is sticky it could still be the cause but without removing the head not sure how to check this. Again it's a brand new GM crate engine.

So back to ignition or fuel I guess. My initial timing is all the way up around 15* currently and I'm all in by about 2600 or so....have to check the MSD book for actual number again. Vac advance currently hooked up but I get the afterfire either whether it is or not.

Maybe I am in fact too rich. I know the end cylinders get less fuel obviously. My center plugs however are definitely a bit sooty and the porcelain is black and sooty on a few. I need to replace a few carb gaskets next week that are seeping fuel from my pressure being too high. While I'm in the center carb I'll experiment again with going down a jet size or two. Last year it didn't seem to help but worth trying again with my fuel pressure back down to 3.5 vs 6.5.

Any other thoughts? Oh and yes I know.....full exhaust. That's still coming sometime in the near future. Just not sure why most people can run lake headers and not have the massive afterfire. The popping I get out of my passenger side is completely acceptable. I also still need to try pulling the baffle one day next week when it's not raining just to see.

 

Have you tried fuel pressure lower than 3.5?
What brand/type regulator is it?
Are your air filters paper elements or the oiled cotton type?

 

I have not turned it down below 3.5 yet. I'll drop it to 3 when I put the new gaskets in the carbs next week. Regulator is from Spectre...it's the same as the Holley and another one....same China plant dufferent name stamped on it. I've confirmed the setting with a gauge though. Filters are paper element and brand new as I just changed to a different style air cleaner.

 

Once you clean up your carbs, I might try running without the filters and play with smaller jets.
I may be off base here, but I have heard that the 4" paper elements are very restrictive. There is a dyno test with a tripower Yblock that shows this...
Reduced air will of course make it run rich, but also may cause havoc with how the fuel is pulled out of the carb. Unstable fuel atomazation= more waste in your pipes to burn.
if they are a problem, that may be why previous jet changes seemed to make little difference before.

 

My new air cleaners require the filter as they make up the center of the whole assembly. I suppose I could throw my old chrome ones back on for a test though as I can leave the filters out of those. I'll try anything at this point.

I did have it idling with the center air cleaner off today and when I really revved it good on just the center carb I could get the afterfire and see flames out of both sides of the exhaust.

 

Just take the whole assembly off for test and tune.
I would pair it with the rejet and reset of the idle screws.

 

I agree ^^^^^^^ ,,,,, I would test and tune without any air cleaners right now .
They might be contributing to the problem,,,,it’s at least worth a try .

Tommy

 

I'll definitely give it a try .

The more I'm reading about "reading" spark plugs the more I'm thinking I'm too rich. Most of the plugs have a few hundred miles on them and the porcelain is fairly dark/black in the center and some even up towards the electrode. I'm gathering black closer to the electrode could be from extended idling but down more towards the base and center would be my normal driving possibly. If I'm that rich could definitely be unburnt fuel forced into the exhaust and lighting off.

Like I said other than the loud bang out of the driver side most driveability is good.

 

If the vacuum advance wasn't advancing at the time of throttle let-off/very high vacuum, say, due to a collapsing vacuum hose, timing would be retarded for engine speed until vacuum dropped enough for the hose to open, quickly advancing timing. The retarding may be loading fuel into the header, the quick advance may be igniting it.

 

Interesting theory....I could put a t-fitting in line right before the distributor and hook the vac gauge up to it and see.
I got a bit lost in the theory the other day but a mechanic I know who's considered the engine whisperer at work and has tons of vintage rods told me to put a vac coupler into the line going to the distributor and play around with some pin drill holes in it and see if they changed my backfire. I don't fully understand the theory behind that though.
Also not that it really matters but GM actually calls for no vac advance on this engine. I've run it both with and without and have the same backfire.

 

If the problem only seems to be effecting one side, than the source of the problem is not something that would effect both sides. It has to be located on the driver's side only.