Visit The Dirt Forum for More Information

Author Topic:   backfire/miss
dode
Dirt Freak

Total posts: 417
posted September 10, 2002 09:31 AM UIN: 13209501
Ok, here is the deal, this has come and gone all year. Some weeks it doesn't happen at all, sometimes it happens a lot. Last weekend the motor was "coughing" a lot. I could hear it from where I was standing off the track, so it wasn't hard to hear. I can't tell if it is a backfire or a miss or what. I know it pops pretty good, and the more it happens, the hotter the engine gets. Last week we got really hot, and when we pulled off the track, both manifolds were GLOWING red, and stayed that way for quite some time as we were cooling the motor down. I just put new plugs and wires on last week, so I don't think that is the problem. I guess that brings it down to the distributor or carb. Both are stock, with an HEI and a 4 barrel quadrajet. Can anyone give me any ideas of what to look for? I can't get it to replicate this other than running hard out on the track, so it makes it hard to diagnose. We are running a stock fuel pump (new at mid year) and turn about 5200 rpm max. We only have one week left, so we don't have a lot of time left to play with it, but would like to finish the season on a good note.

John

outlawstock17
Dirt Forum Champ
Total posts: 620
posted September 10, 2002 10:47 AM
if the exhaust manifolds were glowing red, it sounds like the timing is retarded.

find ACTUAL top dead center by removing the #1 plug and using a TDC stop in the spark plug hole. you have to slowly back off the stop until the engine rotates or if you have a degree wheel, you can rotate the engine backwards back to the stop and split the difference. this is the only way that i know of to find true TDC with the heads on. once you've found TDC, mark your balancer and set your timing.

you might be surprised when you locate TDC and then look at where the marks are on your balancer.

dode
Dirt Freak

Total posts: 417
posted September 11, 2002 08:31 AM UIN: 13209501
Yes, I have done that, and in fact the timing mark was only off about a degree. I am not going to fret over that much. I am going to try and advance the timing some this week. Also, I pulled out the secondary metering rods and hanger from the carb, and I am going to replace them as they are a quite a bit on the lean side of the scale, according to my book. I have a few different options that I would like to try. Can anyone give me any recommendations as to what to use? I am not a big carburator guy, so any pointers would be helpful. Thanks.

John

KPLugnut
Dirt Freak

Total posts: 408
posted September 11, 2002 10:00 AM
John,
Lots of things can cause symptoms like that in the quadrajets. a partially stuck power piston, restrictions or dirt in the primary air bleeds, secondary air bleed tubes, or even a clogged secondary well tube.
Just changing the secondary metering rods isn't going to solve it, I'd say.
I know you only have one week left, but you may want to have a carb builder go thru that carb for you after the season.

Also, verify your fuel pressure isn't too high or too low. Around 6lbs at idle is enough.

Check to make sure your distributor is set up right, too, and that it is advancing properly and at the right rpm. And I'm not sure you said so, but don't run the vacuum advance.

Hope you find it.
KPLugnut

dode
Dirt Freak

Total posts: 417
posted September 11, 2002 10:08 AM UIN: 13209501
Yes, I have checked over the distributor pretty well. It seems to be advancing properly. 15 degrees base up to 35 at about 3300 rpm. Oh, and of course no vacuum advance.

I dunno...I just happened to think about the secondary metering rods when I was thinking of stuff I could look at, and according to my Rochester book, they are like the 5th from the "leanest" that I could have. Surely this can't be good in this application correct? I have another set, labeled DA I think, which are much smaller and seem to have a longer tip, which it seems would be better for out application. At this point I figure what can we lose. I do plan on getting a good carb next year with our new car, but they may change over to holley's, so I am not doing anything until I find out what's going on there. I have very little carb knowledge, but I am trying to learn...slowly but surely. Anything else that I could look at without tearing down the whole carb?

John

dode
Dirt Freak

Total posts: 417
posted September 11, 2002 10:10 AM UIN: 13209501
Oh, and our fuel pressure is right about 5.5 lbs.

John

avila
Dirt Full Roller

Total posts: 55
posted September 11, 2002 10:53 AM
try this. get a new set of plugs, gap them at 22 thousands. if the problem goes
away, the problem lies within the ignition system. otherwise, it might be carb related or maybe you need a valve adjustment.

dode
Dirt Freak

Total posts: 417
posted September 11, 2002 12:52 PM UIN: 13209501
wow...that's a small plug gap. I might try that though. By the way, I have set the valves at a quarter turn on hydraulic lift. That's what jammin recommended to me. I hate to change a bunch of things at once, but should I loosen this up at all? The cam we are running is a Comp Cams 270H. It is a street cam, but it seems to do pretty well for us when the car is running ok. We have lead a couple of races this year with it. Thanks for all your help.

John

smallrock98
Dirt Roller

Total posts: 19
posted September 17, 2002 09:40 PM
Sounds like moisture in the distributor cap to me.

Geffy95x
Dirt Maniac

Total posts: 122
posted September 21, 2002 09:50 AM
Did you check for broken springs ?

BK19
Dirt Freak

Total posts: 285
posted September 21, 2002 01:01 PM UIN: 40696038
do a compression check in your cylinders sounds like a broken piston ring or a bent valve

Back to the Archives