Engine backfire on acceleration

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scooter@bothbarrels

Well-known member
Joined
Sep 7, 2008
Messages
105
Have tried to adjust the timing and carb to get it out, no luck, doesn't seem to change it. Just rebuilt the heads, valves sound good after adjustment..

thing is it did this before hand and I think then I replaced the coil, before the head gasket blew...is it possible it's a electrial issue??

77- 454
 
Yeah - what RB said - sounds like you have a dist. thats's off a tooth or so or it could be a cam lobe problem or a lifter problem or a cracked valve seat or a burnt valve or ? A lot of things could cause this - but it sounds as if a cylinder is firing when a intake valve is slightly open
 
pop goes the chebby

is it a one time pop, as in a lean backfire. or a steady pop pop pop like a flat cam lobe or valve train issue when accellerating?.

in theory a backfire is in general from inproper timing(a spark during the intake valve cycle) or firing order off check 5 and 7 these 2 get crossed often and chevys will run ok untill accellerating then pop pop pop.

restricted acellerator passageways in a carb or inproper adjustment on the accelerator pump it self will cause a lean condition and a pop when accelerating.... dp1
 
rebuilt heads, new carb...

did not pull cam, but what I could see in the valley APPEARED OK.. ah man, I hope that's not it, but just my luck

I am thinking one tooth as well, tried to time with the light, it idles smooth, but the balancer mark is off of the timing plate, close but off, and I run out of room on the dist. before I can get it to move to the tab.. I read somewhere one tooth equals about 17 deg of timing??

What is bugging me is that the pop existed before the rebuild, and I thought it was because of a bad valve, so cam damage is highly possible..
 
Quick & easy

The quick & easy way to tell is... take the #1 spark plug out... stick a screwdriver down the hole... turn the motor until it pushes the screwdriver up so u know it's at the top... the timing mark should be on TDC & the rotor should be on #1 on the dizzy.

BoB
 
If you think its the cam pull the valvecovers pull the coil wire so it wont fire and have someone turn the engine over while you observe the rocker arms to see if they are all moving about the same amount. This should give you some idea on the cam.
 
check plug wires running next to other plug wires. If it did it before check that because the firing order sometimes will jump wires if firing or is in align with wires. Do not know what motor but had an issue with racers back in the day
 
How are you trying to set the timing on this motor? Does it have a vac advance on it? You could possibly set the timing correct even if the dist is one tooth off.

If you have a HEI a bad pick up coil will cause this also, if you have a vac advance hooked up. Heres how. The pickup coil rotates inside the dist. The vac advance turns it. The wire that goes from the pickup coil gets broken, but makes connection. Untill the vac advance pulls the coil around and pulls the wire apart. Then it is like you turned the key off, the engine losses vac and it connects again. Before the engine can die it runs again and it back fires like turning the key off and on.
 
I set the timing buy getting the #1 piston tdc, then setting the rotor to point to # 1. At this seting it runs ok but a little ruff, backfiring at high speed on acceleration, not just one, several, altough I can get it to improve, it stills pops some. Not enough adjustment in the dist to smooth the engine out, so next I moved it one tooth.

Now when the dist is moved enough to smooth it out, it is hard to start like it's too advanced, backfiring seems to be getting better.. I moved it clockwise when I moved it one tooth, maybe I should have moved it the other way.

Just put a new Accel HEI coil on it to see if that was the problem, no change..

I am trying to rule out the heads, because they are fresh. I looked at the cam lobes when I tore it down, the could be worn, but each had a lobe! Also have a Holley 650 on it//

This sh%* is frustrating, I don't usually have such problems. going to readjust the timing and carb and see if that helps, if not I am going to move the dist the other way..
 
I mean that by the time it smoothed out, it was hitting the the carb. I checked the vac advance by putting a pump on it and creating vacum, this caused it to move..

Used the light, but getting it on the mark, it won't idle right..
 
Run the car with partial choke locked on. This will richen the mixture at all speeds. If it clears up the miss, you are running too lean. Step up the jets 2 sizes, probably in the secondary. I hope you don't have a big vacuum leak causing it to run lean. You may want to put some vice grips on the pcv hose.
You can throw enough spark and close the plug gap enough to hide some lean conditions. Work on one thing at a time, do not change more than one thing before making a pass.

good luck,
railroad

If a hammer won't fix it, it's an electrical problem.
 
I mean that by the time it smoothed out, it was hitting the the carb. I checked the vac advance by putting a pump on it and creating vacum, this caused it to move..

Used the light, but getting it on the mark, it won't idle right..


how much vacuum is needed to pull the advance? it shouldn't need much. I had that issue on a 390 ford and it was taking 13 # of pressure to move the advance. it should be 2# or less. I would go and check the stupid stuff again. plug wires for cross wire and proper seat, TDC, Dizzy postion, etc then go from there.

it is possible that the cam is off a tooth?

are you running a newer dizzy with an ecm? that could be bad.

just somethings to consider.

Good luck
 
I mean that by the time it smoothed out, it was hitting the the carb. I checked the vac advance by putting a pump on it and creating vacum, this caused it to move..

Used the light, but getting it on the mark, it won't idle right..

You are setting the timing with the vac advance disconnected and vacuum port plugged right? If not, you made the same mistake I did, and your symptom sounds similar. It will result in timing not advanced enough.

Disconnect vac advance, try setting the timing at proper idle speed again. Then reconnect vac advance, see how you did....
 
i had a problem like that, drove me nuts, i had bought a cam too big for the motor to handle, ran good on idle, but as soon as i gaver it started back fireing, was getting way too much gas for the almost stock engine to handle, i had to down size on the lobes of the next cam i bought...
 

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