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Help. Truck issue....

badmix

badmix

1/2 ton status
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Nov 7, 2004
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Location
West Virginia
1984 K5 305CID, Holley 600 Street Avenger, Mallory Dizzy. Rest,of motor stock 180k .

Truck starts fine, idles fine, when give past 1/4 throttle , get loud (pop) bang up thru carb. Replace cap and rotor , timing at 4* vacuum advanced discord and 8* hooked up. Sprayed around carb, no leaks vacuum gauge reads 17-18hb at idle.

Help !!
 
Back firing through the carb usually means it is lean or an intake valve is not closing.

Is it constant, or just occasionally?
 
Have you double checked the plug wire routing and that the rotor is at #1 when the timing tab is? Also, did you do any other work before the issue started? If it just happens on throttle opening, the accelerator pump might be failing.

It seems weird to only get 4* of advance from the vacuum can. SBC can typically take a lot of advance at idle. But it might not be related to this problem since the vacuum is probably small while you're accelerating.
 
It's a repeatable issue if I go past 1/4 throttle, even jus sitting parked.

No work was done to truck prior to this. Did a tune up last August 2022.

Could bad gas or water in gas cause this?

The pop is loud and sounds metallic thru carb

Plugs, wires, cap and rotor all new. When I put new cap and rotor today, made sure fire order correct, it starts right up and idles great
 
Vacuum gauge reading will help, diagnosis.
Could be the timing chain is stretched or maybe even slipped a tooth.
Bent or brunt valve, at 180k miles wouldn't be surprised.
 
Are you blipping the throttle or is it a gradual increase?

I'm with @Blue85 and would suspect accelerator circuit issue. Especially if you're worried about bad gas I'm assuming because it has sat for a while.
 
Forgot to add the low vacuum advance seems odd like @Blue85 also mentioned. I wonder if the mechanical advance could also be messed up.
 
Just did new plugs and wires on my K20 and had basically the exact same issue, start and idle fine, even rev ok half the time, but the other half of the time it would backfire through the carb bad, and even worse with any kind of load.



Had 11* initial advance, with 19 inches of vacuum, but still backfired. Checked my firing order no less than a dozen times, switched the cap for another, and nothing changed, richened it up, backed the timing off, no luck.

Then basically because phuck it, I switched #5 and #7, and all was better. Despite me literally checking my firing order 12 times, I didn't catch 5 and 7 being flipped.

20230916_141926.jpg
 
It is normal for the advance mechanisms to get gummed up. Have you verified it all moves freely? You're saying it's dependent on throttle position alone, or could it be partly RPM?
 
When I pulled cap and rotor, I checked the weights, they moved freely a d snapped closed.

I can run RPM up to any RPMs slowly with no banging, if I stab or go 1/2 throttle it'll bang (pop)
 
I think it's either in dizzy or carb. It's a pretty loud bang tnru carb, not a normal "pop"
 
Ok, I ordered new dizzy and holley carb "renew" kit. Gonna replace vacuum lines as well.

On accelerator pump, what to look at?
 
if it's a holley it will be leaking fuel diaphragm is bad. Other than that adjustments, plugged passages.
 
Was it during acceleration? Under load? Wouldn't cam issues be present in RPM range no matter how slow its rev'd
 
Its been a while. I think it was just barely noticeable at idle and got way worse when driving. Loud popping right up out of the carb. I had a noisy lifter after an engine swap. It was used parts and very little money piecing an engine together. Went back into the engine and tightened things up and went too far on 2 rockers. Ran good for a week or two and started having trouble. Wiped a cam lobe off smooth and had a bunch of wear on another. Learning engines the hard way back in the day. New cam, oil change and it ran great again. Lesson learned.
 
Ok, It doesn't seem like cam or valve train to me, i could be wrong but would think running RPMs up slowly, valve train issues would present itself.

It's the quick moving of the throttle that'll cause the banging thru carb.

Gonna put new dizzy in, found out my Mallory is a discontinued item.

Holley is back ordered on carb parts. Anything I can check in the mean time ?
 
Still brain storming til,parts get here. What would failing dizzy module act like? If I had issues with mechanical advance what would be symptoms? The weights move freely and springs retract easily
 
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