Backfire

[Rifter_Chevy]

Hello all, so today on my way to work the truck started backfiring. Not real bad but definatly noticable. It only does it at above 20-30% throttle, if i keep my foot off it it runs smooth as butter. Doesnt do it at idle at all, does do it in neutral if i rev it up.

Background on truck, never backfired before ever. Owned for 9 months. Changed carb 2 months ago has been running fine since. Wires/plugs/HEI distrubutor all brand new 9 months ago when purchased. Checked/adjusted timing 2 months ago when i installed new carb.

Its a small block 350 all stock except edelbrock 650 AVS off road carb.


What do you think i should look at first?

[sumran]

I would verify timing and be sure you are getting proper advance. Carefully check distributor cap and internals.

[old Rusty C10]

definately sounds like something in the vacuum advance , but like he said check out the cap for a hairline crack or deposit

[njlimbaugh]

Look for a vacuum leak.

[Rifter_Chevy]

found the issue, was a loose/barely connected spark plug wire.

The Previous owner decided it would be a good idea to use 5, yes 5, different spark plug wire brands/types so all the boots are a differnt design/thickness/color so when i visually look at them they all looked connected but when i took the cap off one fell off so it obviously wasnt connected properly. All is well now.

I hate people who shouldnt be working on cars(like the guy i bought this from)

[sumran]

Glad you fixed it. If the P.O. hadn't frustrated himself, he might not have sold it to you.

[Rifter_Chevy]

Ok new issue!

So since i fixed this last week its been fine, put over a tank of gas through it and no issues. Then comes the drive to work today.

So now its i think a different issue as it does it at different times.

Now it backfires ONLY at low throttle, if i floor it it will backfire like CRAZY for 1-2 seconds(like 20-40 pops in that 1-2 seconds, sounds like a popcorn machine is under my feet) then it kicks in and runs fine and pulls hard right till 5000+ rpms. If i apply like 40-60% pedal sometimes it backfires a little sometimes it does not.

Does not do it cold at all only after warmup. Unlike last week this time it DOES NOT backfire at all in neutral, can rev it up all i want with all kinds of pedal pressures in neutral it will not backfire.

So before i drive it off a cliff what should i look into this time.

EDIT to add, what makes me think its unrelated is last week was very small pops through the tailpipe, very small backfires. This week its much louder and much closer together with the poping sound.

[Your41Plague12]

Sounds like the difference in vacuum created when putting the transmission in gear is causing some problems. My guess is the vacuum advance isn't working. Check to see if your advance weights are rusted together. They should be free to spin inside the distributor. If you have a spare distributor with a working vacuum advance (or if a buddy has one), install it and see if that fixes it. I'm guessing it has something to do with your distributor, whether the advance weights or a bad vacuum advance itself, providing all your vacuum lines are connected and not dryrotted.

[Rifter_Chevy]

Originally Posted by Your41Plague12

Sounds like the difference in vacuum created when putting the transmission in gear is causing some problems. My guess is the vacuum advance isn't working. Check to see if your advance weights are rusted together. They should be free to spin inside the distributor. If you have a spare distributor with a working vacuum advance (or if a buddy has one), install it and see if that fixes it. I'm guessing it has something to do with your distributor, whether the advance weights or a bad vacuum advance itself, providing all your vacuum lines are connected and not dryrotted.

vac advance line is brand new as of 2 months ago when i did the carb swap, the old line was dryrotted really bad.

Might just end up dropping a new dizzy in it i know a guy with a new GM dizzy with less than 5000 miles on it i could get cheap.

[Your41Plague12]

I'd try that if you can get it cheap. Never hurts!

[irishman999]

So if its backfiring out of the exhaust its valve train problems, if it backfires out of the carb its timing. That is supposed to be the rule.

[donut]

How many miles?
Verify all of your vacuum lines and connections, (and look for any obvious vacuum leaks) I'm assuming tune is good with your prior problems. If everything still checks out, pull a valve cover and all the plugs. Rotate engine by hand both ways, see if valves move equally. ie no "lag" forward to backward.

Had an old Furd years ago, could make it backfire on demand, just hit the gas hard. (sounded like a machine gun). Digging into it, I was able to pull my timing chain off without pulling the gears. It would run fine once the slack in the chain was taken up.

Not saying this is your problem, but unless the chain and gears have been changed in the past several years... IIRC some of these also had nylon timing gears.

[Rifter_Chevy]

Originally Posted by donut

How many miles?
Verify all of your vacuum lines and connections, (and look for any obvious vacuum leaks) I'm assuming tune is good with your prior problems. If everything still checks out, pull a valve cover and all the plugs. Rotate engine by hand both ways, see if valves move equally. ie no "lag" forward to backward.

Had an old Furd years ago, could make it backfire on demand, just hit the gas hard. (sounded like a machine gun). Digging into it, I was able to pull my timing chain off without pulling the gears. It would run fine once the slack in the chain was taken up.

Not saying this is your problem, but unless the chain and gears have been changed in the past several years... IIRC some of these also had nylon timing gears.

Thanks for the reply.

ODO reads less than 6,000 miles, was at less than 2000 miles when i bought it 9 months ago so obviosuly it has been wrapped a few times i dont know how many. PO said engine had been rebuilt when he bought it 6 years ago and he only used it in the summer to haul his camper and put less than 50,000 miles on it while he owned it but who knows if thats true.

Vac lines are all good, replaced most when i replaced the carb a while back.

I know the sound of a loose chev timing chain, when they start to stretch you can hear it when you crank as it takes up the slack on the chain as the starter starts to engage, ive herd it a few times before but my truck is not making this noise when cranking. I mean im looking for a excuse to gear drive it so it wouldnt be the end of the world i just dont think this is my issue right now but who knows.

Will advise when/if i get it home in a few hours after i look at the dizzy closer.

[CoggedBelt75]

I chased this problem myself for a month or so. A while back I started experiencing this problem also. At first there were just minor backfires thru the exhaust. Then it wouldn't do this again for maybe a week or so. Replaced plugs and seemed fine. As time progressed, the backfiring would increase on a more consistant basis along with more intensity. Then one day it became real obvious that one , maybe two cylinders were not firing. Pulled the wires off and checked each one for resistance with a volt meter. #5 and #7 wires showed zero resistance and were totally dead. Replaced them and the problem is gone. Might not be your problem but hope it's something as simple and over looked.

[Rifter_Chevy]

Originally Posted by CoggedBelt75

I chased this problem myself for a month or so. A while back I started experiencing this problem also. At first there were just minor backfires thru the exhaust. Then it wouldn't do this again for maybe a week or so. Replaced plugs and seemed fine. As time progressed, the backfiring would increase on a more consistant basis along with more intensity. Then one day it became real obvious that one , maybe two cylinders were not firing. Pulled the wires off and checked each one for resistance with a volt meter. #5 and #7 wires showed zero resistance and were totally dead. Replaced them and the problem is gone. Might not be your problem but hope it's something as simple and over looked.

This was pretty much the issue.

Didnt sleep at all today and am back at work, yay me 36 hours no sleep with 24 hours of that being at work and the other 12 driving or mechanical time! but at least the truck is fixed.

So i got home and decided screw it i got the cash im going to replace EVERYTHING ingnition related.

Issues discovered while replacing everything:

1. Cracked spark plug, litterally in half with the electrode completly burned out of the middle of it.

2. 3 Spark plug wires with broken/corroded or otherwise not functioning terminals, was driving a 5 banger 350 lol.

3. PO wired in a kill switch to the ignition coil power WITH 22 GAUGE wire.

4. Burnt contacts on bottom of cap on 3 diff terminals

5. Very weak timing advance in dizzy, loose and worn out and needed replacing.

6. Ignition module was literally burned, as in was colored black all over.

7. firing order was not correct, like by alot, like not even 2 in the order correct.


Honestly i have no idea how the hell it ran as good as it did with that disaster of a ignition system. i honestly was stunned. It must be a miracle this thing lasted 9 months of every day driving in this condidtion.


Now its got brand new plugs, wires, whole complete brand new dizzy(MSD streetfire HEI), new coil power feed wire.

It runs great and sounds alot better to.

Thanks for helping me through this disaster guys.

[sumran]

That is quite a list. one or two of those items would be a problem. To have all of them on a running truck is amazing. It must feel like a different truck.

[406 Q-ship]

Put a vacuum gauge on it at idle, if the needle jumps up and down.........flat cam or burnt valve.

[RastaRobb]

Sounds like you have a truck that wants to run. Now that you are giving it a fair chance, never let it go. How much money did it take to change all that out? What's next?

[Rifter_Chevy]

Originally Posted by RastaRobb

Sounds like you have a truck that wants to run. Now that you are giving it a fair chance, never let it go. How much money did it take to change all that out? What's next?

cost me $120 for the MSD streetfire HEI dizzy(included all parts, cap, rotor, coil, control module, vac advance, gasket etc)

And another $75 for the plugs and wires.

The wire to replace the ignition power wire i had on hand already, call it maybe $10 in shop supplies for the wire/connectors/heatshrink and a can of brake clean i used to clean it all up.



Next will probably be a rad support as mine is bent/cracked and welded back together as has been in a collision i guess. Which will also give me a good oppertunity to replace the timing belt with a gear drive with the rad out of the way. Will also do waterpump at that time as well as maybe a better fan.

[sumran]

Originally Posted by Rifter_Chevy

Which will also give me a good oppertunity to replace the timing belt with a gear drive ...

??? You lost me at timing belt. What setup do you have and what are you planning to install?

[Rifter_Chevy]

Originally Posted by sumran

??? You lost me at timing belt. What setup do you have and what are you planning to install?

should have said CHAIN was into the imports for to long.

[sumran]

That explains that. Are you going to build this to a level that needs a gear drive? I thought those were only needed if you were going high RPM, full-race.

[gchemist]

Your motor, unless it's been rebuilt, had a timing chain gear with nylon on it. When the nylon starts to fall off, timing is affected. I had mine fail one morning.
Here are the pictures:

What type of carb are you using? Is the advance set to ported or full vacuum? Connect at $15 vacuum gauge to the carb. How does the needle move? Compare the results to this site. http://www.centuryperformance.com/tu...e-spg-148.html


Is your truck stock? Do you have any pictures of it? Is it a nice survivor?