Inliners International Forums General Discussion Engines Cause of Exhaust backfires?

Stock '67 230. Still has orginal everything (monojet/stock intake and exhaust heated) I have hesitation at cruising speed and some backfires when I get off the gas. I have Lots of power when accelerating and idles great.

I do have 2 cyl's that have 30 psi lower compression.... but I dont think that is the issue because after driving it the plugs all look great... no fouling at all! The lower compression I have isolotated to rings and hope they will free up with use since the car sat for 10 yrs before I got it. I have checked the timing too and the advance seems to be working. I have rebuilt the carb a wile back.

I was thinking for a backfire to happen unburned fuel and spark would be needed.

Any ideas?
Thanks as always, Clyde

Is this a points ignition? If so, check your dwell. It sounds like an ignition problem.

------------------
1950 Chevy pickup with '62 261, 4 speed.

I dont own a dwell meter... but the I have them gapped at .019 per the book. Guess I will buy a dwell meter.

Clyde

I did adjust the points to .016... no change. So when should the vacuum advance peak out? I seems to only move about 1/2 way through the popssible full travel?

Clyde

Check the side play of the dist. shaft and also for carbon tracks inside the dist. cap. Sam

Don't see much on the cap contacts. All the ignition parts are new...other than the coil. Blew out my muffler today with a backfire!... LOL

Clyde

Clyde: It could possibly be a vacuum leak or retarded timing. Your distributor may not be advancing correctly due to a worn breaker plate or shaft, bad diaphram or vacuum lines. Don

------------------
Stuck in the 50's
1951 Ply Concord truck
1954 Nash Rambler 2 dr
http://www.picturetrail.com/elplymino

The dist. shaft is tight. I have replaced all the vaccum lines. I checked the diaphram. I advanced and retarded the timing today and the engine runs worse when I do that. Hummmm. maybe I will look at the advance mechanics tomorrow. What is someone torqed down a rocker to far? Have not had the valve cover off yet.

Thanks, Clyde

Clyde, I recently had a back-fire problem and it turned out to be an exhaust leak. The flange gasket between the manifold and exhaust pipe was split. Headers are notorious for leaking, too.

The exhaust gas that exits from the cylinders is hot enough to burn, but with no oxygen present, it will not. However, if any outside air can get into the pipe, especially on deceleration and a rich mixture then present, the exhaust will ignite......hince your backfire. Check it out! Dennis

More suggestions Clyde. Recheck vacuum line/ spark plug wire routing. Check the accelerator pump for flow/wear or clogging. Spray some carb cleaner around the base of the intake manifold while the engine is running to see if the idle picks up, indicating a vacumm leak. Do same with the distributor diaphram.

If the car backfires when you rev it up, pull the ignition wires off the 2 low cylinders (one at a time) to see if the backfiring stops. If it does, the cylinder(s) are your problem. Rings/valve spring may be broken.

If the car sat for 10 years all your gaskets are shot. Rebuilding the carb would help. Likewise, any gas that sat in the tank would have grown hair and the tank probably needs cleaning. Replace fuel filter. Hope one of these does the trick because I am out of cures! Don

I think my hesitation and backfires are related. when it is hesitationing at cruising speed... if you punch it... it goes! back fire happens when you get back off of it.
Yea.. the fuel system was trash when I bought it! I replaced with new parts from the gas cap up to the carb. Lots of good ideas, Thanks! I will look into the today and let you know.

Clyde

If this motor sat up for 10 years and has some low comp. cylinders I would suspect a crusty exhaust valve or two. If a valve or two were open during the sit they would surely crust up. Might not show up under acceleration as the other cylinders would take up the slack but when you back off the open valve would let the passing exhaust light up in the pipe.This may cure itself with some more miles, might not.

OK scraching my head here! Rechecking the vacumm lines and pluging and unplugin and testing. I found that if I disconnect the vacuum advance line that goes to the dist. from the carb all my issues are gone? I checked the timing and I still have advance? So is there mechanical advance in the dist too? I guess so! Then what is the vacumm advance for? Quicker response? I know I had a inline chevy 4 cyl in my boat years ago which only had 1/2 moon weights with springs (no vacumm).

Clyde

Clyde: reading here: http://www.stoveboltengineco.com/howto/vac.htm seems to indicate you have distributor problems. Don

------------------
Stuck in the 50's
1951 Ply Concord truck
1954 Nash Rambler 2 dr
http://www.picturetrail.com/elplymino

I agree. At least I know where to look. Will take apart the dist. and clean things up. Also... I don't know if the advance is connected to the right port on the carb. But I have tried them all with out improvement.

Thanks, Clyde

Hello, noticed that there was quite a bit of talk about the points, but know one seemed to mention replacement of the Condenser which when faulty can cause backfire and shorten the point life drasticly..... Just a thought--Paul

Thanks Paul. The condenser is new also. When I bought the car it did not even have a condenser... LOL.

OK.. I looked into the dist. and found the bushings that the weights pivot on were toast. I replaced the bushings and springs and cleaned it all up. Got a new vacuum advance too. Issue is still there.

So which port on the carb should the vacuum advance plug in to? Should it be the port below the throttle plate (manifold presure)?
Thanks, Clyde

Clyde,

Silly question. Have you checked/set the valve clearances? Also, since the motor sat for a while, any chance the any valves are getting stuck in the guides?

Bruce

I keep looking at the advance because it runs fair when the vacuum adv is disconnected. I would think that would say they are OK?

Clyde

Clyde: It appears you have replaced all ignition parts. I think I would take a vacuum gauge and go thru all the tests for mechanical problems in the motor.
The car starts, runs but not well, so maybe the two low cylinders or their valves are the problem?
The vacuum gauge can tell many defects in the motor. Good luck. Don

------------------
Stuck in the 50's
1951 Ply Concord truck
1954 Nash Rambler 2 dr
http://www.picturetrail.com/elplymino

Clyde, try hooking up the vacuum advance to ported vacuum. What's your timing set at? I would also try to find out why those 2 cylinders were low. The other guys have some good advice. Just a thought. John

OK guys... I just got in and read your post... but here is what I was just looked at in the garage.

I also have a 62 Impala with a 235 which runs great ... so I compared the timing advance looking at it with the timing light... just watching the mark move with the RPM.
The 235 looks about the same as my nova (230) but when the nova vacuum advance is disconnected (only has centrifical advance). I would say 8 deg advance with 1500 RPM or so. When the nova (230) has the vacuum advance connected, it advances alot more... maybe 15 or 20 degrees and runs bad! I have tried every port on the carb. I even tried it strait to manifold with it would barely idle with that setup.

So I think some how?? The vacuum advance is advancing to much. Anyone have a feel for how much advance is normal? I know I may still have issues with the 2 low cyl's... but it runs too well without vacuum advance and not ready to tear into it yet.

Thanks, Clyde

Well, pull out the distributor and take it to a shop that has a distributor testor. They will have the specs and can adjust the mechanical and vacuum advance, check for lobe wear, reset the cam angle, and check spring tension.
While its out check the wire leads as they may be broken off inside the wire insulation. Your block "may" also have a distributor bushing to support the lower end of the distributor. It may be worn.

------------------
Stuck in the 50's
1951 Ply Concord truck
1954 Nash Rambler 2 dr
http://www.picturetrail.com/elplymino

Ahh....check you distributor number to see that it is the correct one for you car...

Hello, Reading from a "motors" manual:
Centrifugal advance
"0"degrees @ 450rpm-"15"degrees@1600rpm
Vacuum advance
6in of vacuum required to start plunger
max. advance--10 degrees @ 14 in of vacuum
I think i have a "GM" manual and I will check the specs. in it for you.
I am begining to think you have a leaking intake manifold gasket. Check the alingment between the exhaust and intake manifolds to the head with you gasket combination. I am not sure if I typed that understandable. I can scan in a diagram and e-mail it to you, if you want. I have seen some intakes leaking the will not show up at idle(Spraying with solvent or water test).Also you may trying using a small spray bottle of water to spray the spark plug wires at the dist. and at the head. Do both these test at idle and with the motor reved to 2000 rpm. May this will help---Paul

I forgot to say if you see any "ARCing" when the Spark Plug wires and connections at the head,coil,or dist.are spayed " lightly" with water they are faulty.
Paul

Another thought--Clyde does the motor have a smog pump?(Calif. built car?) If it does the anit-backfire valve on the smog pump may be defective. Noticed you live hear in Texas so, you can junk-it and all the brackets and still get a Sticker,hehehehehe

Thanks Paul,
The car is not Cal. so no pump. Looks like I have more test to do.. I have done some spraying around the intake but not found any leaks. It's not just backfires... it runs rough with the vacuum advance above idle. Like it is loading up (engine shakes).
I just don't see that much advance on my 235 (Impala).I will check the 235 again tonight. The dist on it rotates with the vacuum advance.... so you can see it and verify it is functioning. On the 230 (Nova) the diaphram is connected to the point plate.
So lets say at 2000 RPM you could have 15 deg of Centrifugal plus 10 deg of vacuum advance... so 25 degrees advance total?

Thanks, Clyde

Clyde, 25 degrees would be correct but remember that your timing light is "late" or it's telling you what has already happened. high quaility timing lights are accurate to about 1 1/2 degrees and that is per 1000rpm, so at 2000, the light is late 3degrees. cheaper ones are 3-4 degrees per 1000rpm. I thinking I would want to see at least 30 degrees "total" with the timing light for your perticular motor.
Off the subject but are you going to the car show in Houston? ---Paul

Hey Paul,
What kind of show and when in houston? Not sure the car is road worthy enough yet.
I have found the problem..The wire lug that connects to the points and goes to the condenser was digging into the cap and not allowing the vacumm adv. to move properly. I have had that cap off many times and even replaced the advance bushings ... but some how managed to return it to the right position to bind up everytime.

Thanks, Clyde

Thats cool that you got it fixed ---it's usually something small that causes a big headache..hehehe Sorry I did not answer you your e-mail--- for some reason my e-mail is not working right but I will try again--- Houston Car show/Swap meet (normally scheduled on ThanksGiving Day) was moved to this past weekend ---I attended with Fellow Inlinner John Matejka from Houston. It was Big and great(very similar to a West Coast happening) and I picked up a 2-1 set-up for
my '50 Chevy( 235). Paul

Thanks for the info... I could not have gone this weekend with or without the car anyway. Just replaced the exhaust that I blew the hole in on a backfire yee ha.. Running better now and sound better too!

Clyde