61 Roadster

I’m experiencing occasional backfire from the drivers’ side; typically about 3-4 seconds after heavy acceleration if you let it drop to idle speed. Slowly lowering the RPMs will often avoid the backfire. Otherwise, the motor runs great, idles smoothly and has reasonable power.

The motor is a hydraulic lifter 1961 230 HP 283 with a 62 AFB intake and 64 AFB carburetor. The distributor has an old Pertronix ignition. Again, the motor has been running smoothly and reliably for several years with this set up using 93 octane pump gas.

Since the backfire began about two months ago, I’ve replaced plugs, wires, cap and rotor. Checked the plugs again and they looked quite clean as did the previous set.

Had a friend with a timing light help me; we tried to set timing to the recommended 4 degrees, but found it ran very rough and started poorly until we returned it to about 8 degrees. It continues to backfire after heavy acceleration.

After someone’s suggestion, I added twice the recommended dosage of Fuel Injector/Carburetor cleaner (two bottles!) to a tank. That stopped the backfiring for a few days.

Now, even after fresh fill ups and two bottles of cleaner, the backfire will more than likely occur, though the severity is affected by the ratio of cleaner to fuel mixture in the tank at the time.

Again, I stress that no changes have been made to the motor before the backfiring began two months ago.

Do I learn to live on carb cleaner additives from now on? Any thoughts are welcome.

GTOguy

Is it backfiring through the carburetor or exhaust? Can not offer any advice without this information.

SDVette

I would pull the 4 plugs on that side and have a look.

EDIT: Sorry, you already said they look OK.

Frankie the Fink

Since the OP said "backfire from the driver's side" its unlikely its a carb backfire...so my money says its raw gas lighting off in the exhaust after rapid deceleration, meaning the throttle plates are closed and vacuum is spiking (I fought this problem on my '67 Chevelle)....first thing to check is the timing, the next is that the exhaust manifolds are snugged down good and tight, and, if gaskets are used that they are in good shape. Make sure the exhaust manifold "doughnuts" are good and that that joint is nice and tight too.

Anything loose around the cylinder will suck air under these conditions, lean out the mixture and cause these symptoms.

BTW - that lil ole 230hp mill should run like a bandit on 87 octane all day long -- no need for 93 juice...

tuxnharley

But, since he said it's on the driver's side I'm guessing that implies exhaust. If that's the case I'd look at the carb, maybe the power valve hanging up after the described hard acceleration and dumping too much fuel and the burning somewhere in the exhaust, maybe the manifold.

Carb cleaner/additive might be temporarily easing the problem by partially loosening whatever gunk is in there.

Admittedly this is just a SWAG, but if it were me I'd look at pulling the carb and cleaning/rebuilding - especially since the additive temporarily helps the problem. I think that may be the big clue here.

bluzrocker

Have had this issue....was leaking air some where in the piping...sucking air in on decelerate .....had pin holes in pipe..might check both exhaust pipes and at connections...and at manifold

did you check timing with vacuum plugged?

hope2

using all that fuel additive could dilute oil.

tuxnharley

I think that's unlikely since he said he's using two bottles to a full tank of gas, which is what - 16.74 say 17 gallons in a '61? If the raw gas was getting past the rings anyway that's a whole different problem, but apparently not what the OP is talking about here.....

sub006

Who sells 93 octane gas in Mass?

-Another Californian, where 91 is the max...

tuxnharley

yeah, and that with 10% ethanol to boot...........

MikeM

This is where I would look first.

DUB

I know I am adding 'something' even though '61 Roadster' has not replied to any of the questions asked that need answers.

BUT...if timing is a possible culprit.... All I can say it that I rarely trust the notch in the harmonic balancer due to it can move over time. Use a vacuum gauge and when you get it right...then put your timing light on it and see what it tells you. I DO KNOW this does not make sense due to no engine work was done and the backfiring just started...

DUB

61 Roadster

Lot's of great responses here and plenty for me to check tonight; namely, snugging down all the exhaust bolts, etc.

I'll try to get together with my friends' timing light this weekend. We did plug the vacuum line during the timing.

I believe 93 octane is commonly available here but with 10% ethanol

For what it's worth, the motor runs great other than the described conditions.

The spark plugs look clean just like the previous set.

I'll try again tomorrow; after that it looks like carburetor work; not my forte!

Thanks again to everyone!

GTOguy

Doubt it's the carb, fairly certain it's exhaust leak(s). Clean plugs and good drivability indicate that the carb is not the issue. The sudden introduction of fresh oxygen via an exhaust leak will re-ignite the exhausted mixture and cause a backfire in the exhaust.

tuxnharley

Hmmm - so why does the carb cleaner additive temporarily resolve the problem then?

MikeM

GTOguy

The carb cleaner additive probably reduces the volatility of the gasoline, so it's not as easily re-ignited in the exhaust system. That would be my guess. YMMV.....

ILBMF

If you are losing ignition spark all together or a single cylinder intermittently it can cause a backfire in the exhaust

Twan Sloot

ILBMF maybe on to something, because carberateur mixture settings won't change from one day to another without touching it. But distributor points tend to wear, maybe one cilinder doesn't ignite properly. But on the other hand that doesn't explain why it would only happen when throttle is released. And the engine is operating normal apart from the backfire

Twan Sloot

Maybe putting in some octane booster will make it better. At least the fuel would ignite at a higher temprature resulting in less back fire. Here in europe I always run my classics on 98 or 102