backfire through carbs

bostonmeche · 02-27-2004, 12:05 PM

Hi all,
I got a bit of a problem. Cylinder #3 tends to back fire through the carb every now and then when it is not idling. Idling it will go all day long no problems.

I took the filters off and put a hand over each velocity stack. Only cylinder #3's stack doesn't cause the engine to quit. It bogs it down a little but it keeps on running just the same.

A car guy at work here says its probably a blown exhaust seat and that a quick and dirty fix is to replace the motor oil with transmission oil, run it for a little while, and then change the oil with motor oil a couple of times. This should remove the carbon and swell up the seats.

First, what are there other things to check first? Second, is this transmission oil thing a real possible solution?

Some background, I only need the engine to last another 5000-8000 miles. I plan on doing a 3.2 swap by then.

Thanks for any help!

Zeke · 02-27-2004, 12:23 PM

People have used ATF to clean the inside of their motors for decades. ATF is high detergent. Whether I would do this or not is questionable. Well, in fact, I myself wouldn't do it.

But that is not the real issue. I don't see how the oil in the sump is gonna help out with your valve seats. You could put some fuel injector cleaner in the gas and try to loosen carbon on the seat. Other tricks include spraying Berrymans carb cleaner in the stack while the car is running at mid to high rpm's. Same with water.

bostonmeche · 02-27-2004, 12:33 PM

Well at least he wasn't talking out of his ass...

Are there other things to check first though? Or is the concensus that it is an exhaust seat issue.

Zeke · 02-27-2004, 01:19 PM

There is no concenus here. I'm the only one who posted

! How about a compression check? If your mechanic is a serious guy, he will do this before pouring ATF in you motor. BTW, you only need one qt. if you do this. You will unleash all kinds of dirt and debris in your engine. Do you really want to do that, or leave sleeping dogs lie? Find out what's wrong before trying to fix it, eh?

bostonmeche · 02-27-2004, 02:59 PM

I'm one of those fools who is his own mechanic. Perhaps mechanic is somewhat misleading, I'm my own disassembler and "loser of parts". I haven't really trouble shooted and fixed anything yet.

My compression is crappy.
Cyl #1 100
Cyl #2 85
Cyl #3 100
Cyl #4 75

I thought it was interesting that the cylinders that are opposite each other have silmilar compression results. BTW, I screwed up before. The problem cylinder is #4 NOT #3. Interesting how it is the lowest number.

914Ghost · 02-27-2004, 03:23 PM

Didja do the compression check while engine is warm?
Do it warm. Are the valves adjusted? Low compression CAN be a tight valve..and would save you a lot of trouble. You dont really have to adjust them, just make sure none are tight and you can eliminate that.
Vacuum leaks..? Check for those.
Unscrew the jets you can get to next and make sure they're clean.
Then try adjusting carbs...when you think its adjusted right take it out for a drive and REV THE HELL OUT OF IT.
And what kinda carbs are they?
Webers Dellortos Edelbrock?
Maybe something's stuck in there..but I dunno about the engine-enima on an old engine...you can let loose some bad demons.
Bob O

Zeke · 02-27-2004, 04:39 PM

If you have a bad intake on no. 4, that fits the backfire problem. Check the adjustment and then pour the Berryman's in. If that doesn't do it, let it go until you have to rebuild.

david c. · 02-27-2004, 04:45 PM

Are you sure the ignition is set up properly? How have you made certain the timing is correct? Any chance you have miswired the ignition or have a bad plug, plug wire, dizzy cap?

I once had a similar situation boil down to someone (who was that?) cross a couple of plug wires.

David

bostonmeche · 02-27-2004, 08:47 PM

Thanks for all the advice! I did the compression test cold so I will try it again warm. I am running dual Dellorto 40s. I don't know about the engine timing. I will have to get a timing light an check it out. With a little luck it will be a nice New England day tomorrow and I will be able to implement some of the suggestions.

Jake Raby · 02-27-2004, 09:17 PM

sounds like a cam going flat (they share lobes)

914Ghost · 03-01-2004, 08:33 AM

Jake-
You ever have nightmares about a 4 cam engine with the cam(s) going flat!?!?
Bob O

Toast · 03-01-2004, 10:02 AM

Backfire thru carbs can be timing, valve problems (burnt, maladjusted), cam problems (flat lobes), preignition, too short of a manifold, etc -

Sorry for the long list. Did you find the problem yet? If not you may have more checking to do.
:P

bostonmeche · 03-01-2004, 06:40 PM

I didn't get a chance to check yet. I ended up doing some work on the house instead. Thanks for the advice though!

914Ghost · 03-01-2004, 09:43 PM

Is the house backfiring too?
Mine does that... .probably just some dirt in there.
Bob O

bostonmeche · 03-02-2004, 07:48 AM

You know, I'm a sucker for a basket case. If I'm not battling termites, I'm battling rust worms. With a little luck, the backfiring won't spread to the house. Though with my luck, I should probably start putting sand bags in front of my bathroom doors.