fast backfire popping

H

Hillbilly38

Member
1978 Scout traveler
345motor
727 tranny
new Holley 7448 350cfm carb
new plugs autolite 303 gap 035
new wires cap rotor
prestolite dist pn 484778091

ok the problem I am having is at high rpm's or heavy load like pulloing a hill and having to gas it I get a fast small popping out of my carb. Not big backfires. No flames ect. But its noticable and seems to hinder the performance in the upper rpm's. I can drive it down the road at 65mph with no issues until I have to pull a big hill or try to pass someone then it starts. Any ideas would be very appreciated. My only theory thus far is a intake valve issue
 
Let's define "backfire" and "popping".

Backfire occurs when it comes out the tailpipe and is normally caused by an over-Rich fuel mixture that was not completely burned in the combustion chamber and the exhaust mix is lit off again by heat from the exhaust system. That is not the only cause of the phenomena...but the most common.

If the "popping" or "spit back" is evident as being exhibited coming back through the carburetor venturis, that indicates a problem with the fuel mixture in the cylinder(s) being lit off (burned) too early or too late (when an intake or exhaust valve is still "open"). That indeed could have a root cause in a leaking intake valve or any thing that creates a out-of-spec valve timing occurance (flat cam lobe, bent push rod, stuck/siezed valve, etc., etc.

If a cylinder compression issue is suspected, then a cold and then hot compression test must be performed and the data recorded. That should be followed by a cylinder leakdown test, and manifold vacuum test...it takes all those diagnostic tests to come to any meaningful conclusion.

And those tests will show up a flat cam lobe, etc. If one knows how to interpret the data.

Much of time regarding this kind of rejuvenation of old, unknown junkiron, what we'll find is a combination of many issues creating the drivabilty complaint.

If you want the "quick and easy" test if you are experiencing spitback or "pop" through the carb venturis that is caused by an ignition event that is not optimum, then power time the bitch like we've discussed over and over. That will rule in or rule out basic ignition timing as an issue.

If the carb itself is jetted too lean so there is a big "gap" in liquid fuel delivery that has to be "made up" when the power valve actuates, that could also cause what you May be experiencing. And this becomes more of an issue every day as more folks are being forced to run e10 fuel. All Holley carbs I'm servicing now automatically receive a two step increase in main jet size as part of the reconditioning process.
 
Ok let me add some info to see if that helps change any of the answers. This is a brand new Holley 7448 this has had the same issue with the original carb. I have power timed this whore about 50 times with no positive results. I have pulled and inspected each pushrod and lifter and see no signs of cam wear bent rods broken springs. I have not pulled the heads off nor put a compression check on the cylinders it does this hot or cold only at high rpms ive tried cleaning the lifters the old way by squirting tranny fluid into the carb while keeping it running. If I drive it without heavy gas pedal pushing it does fine but as soon as go to pass or climb a steep hill with heavy throttle it happens. Thanks.
 
Well I never checked, they were what was in it when I bought it so I assumed they were stock and replaced them. Ill call the auto parts store and see what they recommend.
 
Autolite 303 plugs are the exact same spark plug heat range/spec as an autolite 85. The difference between the two plugs is the tip design. Either plug callout is suitable/correct for your engine. Don't ever trust some partscounter monkey to make a spark plug recommendation!

An autolite 85 is an "extended" tip design based on plug technology of the late 60's/early 70's. That is a design used for engines that have sufficient plug tip-to-piston/valve clearance to prevent any collision damage. In some cases regarding engines that consume oil due to poor ring and valve stem seal, they tend to "self-clean" a bit better than a conventional tip plug. Also, they worked best in "emissions" engines running very lean mixtures across the board and along with the crappy base ignition timing "spec" that emissions setups demanded. Nowadays we have to throw in this shit e10 fuel which does not want to light off under compression. Talk to your congressman about that clusterfuck.

Comparable champion, ngk, bosch, denso, etc. Plugs are available also in both designs with interchangeable heat range crossover.

Squirting atf ar any other kinda snake oil through the carburetor cannot do anything regarding "cleaning/unsticking" a hydraulic lifter! Think about it...how can any solvent get into the lifter gallery if not added to the crankcase???? Another bullshit/urban myth that was propagated first in the 1920's regarding "sticking valves".

To service a "stuck" lifter, ya remove, disassemble, clean, and reassemble. But 80% of the time that is wasted effort since the lifter base and cam lobe is most likely toast. 20% of the time it might be doable.

The distributor must be verified as being in first class condition...does the mechanical advance function per spec? Does the vacuum advance function per spec? Is the coil breaking down under load after a heat soak (only a coil tester will prove that up)?

What is the jetting on the carb...same as what a new/out-of-the-box Holley 7448 is set up with? What is the fuel pressure/volume at idle??

We have to look at this as a whole...with many multiple parts to the puzzle. I'm firm in my belief that you will end up with at least three identifiable areas to correct once the diagnostics are done. And a compression test is imperative in order to rule in...or rule out any mechanical issues.

A flat or "weak" cam lobe will only be found through either actual measurement with a micrometer/caliper, or a dial indicator to measure lift if installed in an engine. It will also show as an anomaly in a vacuum test but not which cylinder and which lobe.
 

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Putting the solvent through the carb is not to get a response out of the lifter. It was meant to clean carbon off of valves. The jets I changed to #56 as per your carb tech.
 
hillbilly38 said:
putting the solvent through the carb is not to get a response out of the lifter. It was meant to clean carbon off of valves. The jets I changed to #56 as per your carb tech.
Click to expand...

The #56 main jets are a "starting" point for dialing in that carb...each engine is gonna respond differently to that number based upon it's overall mechanical condition and whether you are being forced by congress to burn e10 in your local area.

When I jet these carbs, I "bracket" the lean and Rich ends so I can find what doesn't work. You May have found an overly lean end already with the #56. So to make an extreme difference to go the other way, I'd use #64 and you will most likely find the plugs pretty sooty after a run from being over-Rich on the mains. So then split the difference and see what happens.

But none of that is gonna work well unless all other gremlins are purged!
 
Back in my Chevy days I was always rounding a cam. If you lose an exhaust lobe you will get a very fast popopopopop when working the engine. It pops out the carb because the burned fuel cant get out the exhaust valve. It can only escape out the intake valve during valve overlap.

The original poster said that he checked for cupped lifters so we can rule out a flat cam.

I bet that it's a weak or broken valve spring on an intake valve. At higher rpm's the valve cant close fast enough and the exhaust goes out the intake valve.

Ya a vacuum reading would be nice.
 
I did on Monday run a compression check and all cylinders were around 120 give or take a couple did not run a vacuum though ill have to get a vac gauge set. Im now wondering if its maybe a worn dist shaft. I put bigger jets in the carb and no change there. So I kinda ruled the lean carb theory out. Guess all I can do it just one thing at a time and eventually ill find it lol. Thanks for all the insight so far guys.
 
hillbilly38 said:
did not run a vacuum though ill have to get a vac gauge set. Im now wondering if its maybe a worn dist shaft. I put bigger jets in the carb and no change there. So I kinda ruled the lean carb theory out. Guess all I can do it just one thing at a time
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Definatly run a vac test. Bad thing is, if it runs smooth when you are doing the vac test it May show nothing.


The best way is one test/thing at a time. You will find it for sure. If you get frustrated just step away from it for a day. We will still be here.:p
 
hillbilly38 said:
I did on Monday run a compression check and all cylinders were around 120 give or take a couple did not run a vacuum though ill have to get a vac gauge set. Im now wondering if its maybe a worn dist shaft. I put bigger jets in the carb and no change there. So I kinda ruled the lean carb theory out. Guess all I can do it just one thing at a time and eventually ill find it lol. Thanks for all the insight so far guys.
Click to expand...
A lean carb wont cause 1 cylinder to constantly pop up through the carb. It May cause a stumble, hesatation or lean misfire under light load.

A worn distributer shaft can make the timing move a little but not enough to make it fire when the intake valve is open. It will have very little if any effect on timing with an electronic ignition.

The popping effects the power because it is reversing the intake air flow in the intake manifold and goofing up the intake charge. Only 3 things can cause this. A flat cam, weak valve spring or broken spring.


I'm sticking with the weak or broken valvespring since the cam is not flat.

So quit chasing your tail with jetting changes and guessing.

You need to pull the valve covers and look at every spring very closely. Probly need to pull the rockers off too. Grab the springs and try to wiggle and push them around. See if one feels different.
 
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Well I will not rule out a weak spring b ut can rule out a broken spring for sure. Im going tomorrow to buy a valve spring tension check tool.
 
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