Stumped with backfire from carburator

[naukkis79]

Chokeless carbs will backfire at engine startup and when given more gas at lower than normal engine temps. Absolutely normal, as engine gets plenty of lean mixture it might burn so slowly that most of burn happens at TDC of exhaust stroke which will cough back to carb.

That video backfiring is almost non-existent compared to some big cammed independent runner setup startup backfiring. And little backfiring is usually totally harmless, you need long lasting fire to melt boosters - or really crap carbs if they will broke from little cough.

[Firedome8]

Does it have a choke? Why are plugs fouled?

[John Haskell]

Several times in the up-right coil old days, the two leads -/+ at the coil will pass the spark from terminal to terminal. Try it at nite & it will show as a blue spark jumping out of the coil hat or circling the terminals.

[lg1969]

Does it have a choke? Why are plugs fouled?

cold plugs will do that.

[sc2dave]

If it was due to cold plugs, it would do it all the time, not once in a while. Plug heat range doesn't matter when you're just cranking the engine to start it. How could a plug ignite fuel based solely on heat range at cranking?????

[Firedome8]

If it was due to cold plugs, it would do it all the time, not once in a while. Plug heat range doesn't matter when you're just cranking the engine to start it. How could a plug ignite fuel based solely on heat range at cranking?????

HEI is sensitive to fouling . what does it do with fresh plugs?

[Geoff2]

HEI is not sensitive to fouling if it has the full 12v.

[MadBill]

That's an important consideration!

A few years back a guy had me go through the Quadrajet on his early seventies Buick due to driveability issues and cutting out under load, that had previously stumped a couple of shops. I found nothing of consequence in the carb, but looking around afterwards I realized that it was sporting an HEI distributor, which first came into service years after the car was built. A little detective work revealed that it was wired through the original factory non-HEI resistor lead, cutting several volts from the power supply.

Amazing how much better an HEI system works with ~ 13.5 v instead of ~9...

[1989TransAm]

"Amazing how much better an HEI system works with ~ 13.5 v instead of ~9... "

I can imagine.

[Firedome8]

Autolite 25 should be ok, but a black plug can arc from the porcelain to shell and can initiate a very long ignition delay time resulting in a very retarded burn. In my experience a gm hei will misfire under load if the plugs look black regardless of why .
Just thinking.

[sc2dave]

so did you ever get it fixed?

[MadBill]

Always feel bummed-out when we track someone's tough problem, throw all our best stuff at it and then it just....stops. Like watching the Daytona 500 and having the feed go down on lap 198.

[midnightbluS10]

For anyone wondering what the yb post said:

So I finally got a chance to get over there with enough time and light to have a good look. We switched the distributor polarity that seemed to make it better but it still had the popping up through the intake when the rpms come up. Vacuum was about 14-15 and steady. I pulled passenger side valve cover and checked all the rockers and did a leak down they were OK. So here is where I’m an idiot, I pulled the drivers side and find the number 1 exhaust rocker isn’t moving nearly as much as it should. Not sure how I missed that first go around, the battery was down so I probably thought it just kicked back but what got me though was the pre load was still correct. So long story short I talk to another buddy and I find out that he has already been over there a few weeks ago and re adjusted the rockers and the oil has been changed ETC... so motor is coming out. But I also found that the poly locks only have about 3 1/2 turns of thread engagement. The set screws are all way down into the nuts. The rockers have larger trunion so I’m assuming the rocker studs aren’t long enough. The valves springs don’t fit the retainers. I bet there will be other stuff we find too. I’m trying to talk him into a Hyd. Roller instead of the flat tappet this time. Thanks again for all the replies.

From http://www.yellowbullet.com/forum/showt ... ?t=2440298

[huero]

Hello gentlemen,
I apologize for the delay but, i had but the car on the back burner for a while while life took over. Anyhow, That was not me on yellow bullet.
I still have the backfire issue.
Here are what I have done since.
Readjusted the valves with with half turn past 0 lash (wanted to make sure valve not hanging open from too tight of an adjustment)
Did leak down test. Results were all done at 90 psi.
cyl 1 = 2% leak
cyl 2 = 12% leak
cyl 3 = 12% leak
cyl 4 = 8% leak
cyl 5 = 9% leak
cyl 6 = 10% leak
cyl 7 = 2% leak
cyl 8 = 10% leak

Again these results were done @ 90 psi as well as on a cold engine. All cylinders leaked past the rings as the only air I heard was out the valve cover.
Installed a new set of autolite 25 sparked plugs all gapped at .040.
Took car for drive and it runs good, but when left sitting Backfire on startup is still there causing hard starts. Seems like this is when the situation is worst. Drive car to shows and park it about 30 mins later a white fog of mist coughs out of carb and out the air cleaner.
Also, set a fuel pressure regulator at 4 psi to make sure it wasn't a fuel pressure problem.
As well, the fueled plugs I uploaded earlier in the thread were from the car idling. As soon as i took the car for a drive, the plugs looked in really good condition.



Yes, these are the same plugs. Once I took the car for a spin, this is what they looked like.
Regardless, New plugs were put in the car and still it coughs on start up.
Thank you all for your help. I hope I can get direction as to where to turn next.

Could the carb be bad ?

[huero]

Runs and drives well. Coughs hot or cold.
Tried timing at 8-18 dtdc and verified timing mark
pumped gas pedal before start still no help