302 backfiring

[eflow]

Hey ya'll, long time no post...

I've got a problem of backfiring thru the carb. I've run thru the check list of things that could be wrong and none of them seem to be the problem. here goes-

New plugs, wires, coil, dist and cap
No smog equip
no vacume leaks
timing is on
done compression test - all seem good
firing order checked
getting good spark

I'm down to a couple of options and I'm looking to see if I could have missed something. First, the firing order is right out of the book, but it could be the HO firing order is needed, I don't think I have an HO motor though. Second, we get the majority of backfiring at idle, so it could be the idle components in the carb are damaged or bad. Maybe a bad jet?

SOS, please send help! Thanks guys, any info will really help since I'll be back in the shop again tomorrow working on this issue.

-E

[ca]

what was the last thing changed before it started backfiring? double check TDC. more likely its electric n not carb

[eflow]

(darn nice avatar...)

Umm....yeah, I'm TDC on #1. We already checked that (heck, a few times to make sure!)

As for electric, I'll double check spark on all plugs. I've got a new coil and ignition box, wires are new, the distrib is rebuilt and the cap and rotor are fresh.

What about vacume? Maybe I've missed something?

[arie]

Make sure that the # 7 and 8 plug wire are not running next to each from cap to plug, they have been known to cross fire, double check cap to check if there is a arc or carbon track between any of the points.

[ca]

shouldnt have yur vacuum hooked up when timing anyway ;D shouldnt be guessin bout the HO firin order
its sumpthin you missed ;D wrong order or 180 off. i'ma way too medicated to even try to make sence

[Meteorgirl]

Ok, so a few of us are sitting around the club garage and I check the site. Here's a few more opinions.
Like Ca says, you could be 180 out and the plugs are firing as the intake opens not closes.
You MIGHT need the HO order, try it, it won't break anything just to try.
You say the dist is rebuilt. If it's still points did you check the point gap? Also, if it's not your old core is the shaft straight? We had a beefed small block that we pulled the intake off twice and had to pull the heads off once because 2 valves got smoked. Checked both Holleys, tried different carbs, new wires, cap...everything. Finally found it. The rebuilt dist had a bent shaft and the points were going stupid. Acted just like a super leanout.
You also mention new coil and box. If its like an MSD type, make sure the box AND the car/motor have good grounds.
If you've converted from points to electronic, did you get the right coil? Point distributors use a coil with a high resistance like 3 or more ohms. Electronic distributors like 1 ohm or less. Some new electronic dist will smoke and have no warranty if used with a point style coil.
Check the rotor and make sure the blade is firmly attached to the flat tower spring.
If the dist is the same as you used to have, put the old cap, rotor and wires on and see if the problem goes away. If it does, you know where the prob is. If it doesn't, you know where the prob isn't.

[eflow]

I guess we might be 180 out, I don't think so, but we'll double check that.

The distrib is converted and rebuilt and has been matched to the right coil and box. All of that was done by the same guy who works with MSD. I'll double check the grounds

I'll try the HO order.

Ca, I was thinking about a vacume leak from say the carb or manifold not from the advance.

Thanks, y'all.

[w2zero]

Is the backfire on one cylinder, on all? Only at idle? All throttle positions? Maybe one tooth off on stabbing the distributor?

[arie]

Here is another try e flow. with a tach hooked up run the engine at 900rpm and then pull one plug wire off at a time and see how many revs you drop at each cyl. this may point to which cyl is causing the problem. a sticky valve will cause a back fire.

[ca]

ok ehole, how did it come out

[eflow]

Hey thanks fpr all the help everyone! Turns out it was just the firing order. The manual I was using showed only the HO firing order and didn't list the non HO order except down at the bottom of the page in very small print. DOH! And in the trouble shooting section it didn't mention there were two different orders. Another big problem was I hadn't realized until last friday that there were two different motors (HO and non HO) so I was assuming the order was okay since I didn't know to look for another order. Oh well, now I know.

The 302 is firing right up, sounds great coming right out of the headers! Woohoo! It will be road worthy this weekend.

Thanks again for all the help and knowledege.

[ca]

are you at the house or the shop? - in case i feel like takin a drive this weekend

[eflow]

The Merc is now down at the Corona airport. I have a hanger with my buddy Brent. WAY more room to work in.

I'll have the phone on so come by on Saturday if you can. Just give me a buzz.