Backfiring when clutching
 

Hi all - I know that the subject of backfiring has come up many times but not exactly as my car is experiencing it. Mine backfires when I clutch, especially when shifting from 1st to 2nd. It happens more when the GAV is set lean, so I set it at 1 full turn out and it mostly goes away. The problem, of course, is that I get about 5 miles per gallon and the plugs are fouled quite a bit. So, if I flood the carb (basically) I get little to no backfiring, but when I put the GAV at a quarter to half turn like you're supposed to, it backfires each time I push the clutch in. The carb has been rebuilt (and the backfiring happened both before and after the rebuild) and the points have been cleaned. As always, any ideas would be greatly appreciated. Thanks in advance!

Re: Backfiring when clutching
 

Try Marvel Mystery Oil in your gas. Helps with the valves sticking.

How is the car running once in third gear? Does it back fire when you take your foot off the gas or going down a hill?

Re: Backfiring when clutching
 

The car runs fine in all gears, and doesn't backfire in third or when running. It's just when I clutch. No, it doesn't backfire when I take my foot off the gas or when going downhill. Thanks!

Re: Backfiring when clutching
 

Too lean. You most likely have a intake leak.

Re: Backfiring when clutching
 

You might check your battery cable that runs from the battery to the starter. If it's routed incorrectly and/or incorrect length, it might be contacting the pedal arm and almost worn through the insulation - causing it to ground out resulting in weak spark to the distributor.

Re: Backfiring when clutching
 

I bet you clutch is shorting out the ignition circuit. If it is a very loud bang, I bet that’s it. Same as turning the key off and on.

Re: Backfiring when clutching
 

Is the "back firing" in the exhaust (after fire) or carb.

If in the exhaust I think you may have an exhaust leak and are drawing air into the exhaust system, allowing the mixture (unburned gas and leaked in air) to "explode" in the muffler. By opening up the GAV you are dumping in so much gas that the mixture is too rich to explode, thus less backfire. And the plugs are sooting up.

Re: Backfiring when clutching
 

I suspect this is the problem as it occurs often. In bead blasting and painting countless numbers of clutch pedals I have found the majority to have copper transfer where they at one time shorted the battery cable.


Tom Endy

Re: Backfiring when clutching
 

I set my idle just a hair higher, 7mph in low instead of 5 and that solved the problem.

Re: Backfiring when clutching
 

Wow - thanks all! I'll look into all of the ideas. Thanks again for all you do!

Re: Backfiring when clutching
 

The backfire is definitely coming from the muffler/exhaust pipe, not the carb.

Re: Backfiring when clutching
 

I am thinking you have a slow closing or sticking valve problem.

At higher RPM’s things are working out. Push in the clutch, foot off the gas, engine down to idle for a couple of seconds. Exhaust valve or valves sticking for a moment.

Model A’s are simple cars. Try simple fixes first. I would again say that you should try Marvel Mystery Oil in your gas. It can’t hurt.

Are you using detergent oil?

The good thing is everything seems fine at speed. Enjoy.

Re: Backfiring when clutching
 

Thanks WHN - I'll try the gas additive. No, I'm not using detergent oil that I'm aware of. But you're right - easy things first!! Thanks again.

Re: Backfiring when clutching
 

I don’t want to get started on another “what oil to use” conversation.

However, you should understand what type of oil your using.

Most oil sold today is detergent oil.

If the car is new to you, I would try to find out what type of oil the previous owner used. There are old school owners out there that still use non detergent oil.

If you are using non detergent oil. Take a moment to read up on the pluses and minuses of both types of oil.

Detergent oil should help with a sticking valve problem along with keeping your engine cleaner. Also helping with sludge buildup in engine and oil pan.

We all have our formula. Enjoy.

Re: Backfiring when clutching
 

Hi all - update: I tried the MMO and still backfiring. I tried finding an exhaust leak, but with the fan going, it's virtually impossible. I tried looking at the pedal/battery cable and no evidence of them even touching, let alone rubbing to a point of fraying. However, I did notice that the backfiring did not occur when the engine was still cold - only when it warmed up. So, I appeal to the group again - could the fact that the backfiring only happens when it's warm make a difference? Thanks!

Re: Backfiring when clutching
 

You have to give the MMO a chance to work if your problem is a sticking valve.

Engine heats up, metal expands, dirty valve guides bind.

You might want to also check your rotor and the contacts on the distributor body. Clean them with a light nail file. Check your points while you have things open. Look at plugs also.

Because it runs fine at speed. I don’t think it’s a manifold problem.

Could be something else. But whatever it is, I think will be simple.

Re: Backfiring when clutching
 

Hi again. I subsequently noticed that the backfiring was occuring more when the engine was warm, so I looked that up and most everyone agreed that that situations occurs when the coil is on the outs. So, I replaced the coil and that sure seems to have cleared a lot of the backfiring up! Now I'm playing with the GAV to see where I should put that - too lean and it backfires, so I'm leaving it at about a half turn (perhaps a little more) and that seems to be working so far. So, MMO in the tank, new coil, properly (I think) adjusted GAV, and I should be in business again. Thanks again!!

Re: Backfiring when clutching
 

Riv, take off your fan belt and start it up to check for leaks. That way you don't have the fan air blowing around.

Re: Backfiring when clutching
 

With the GAV setting at half a turn you are leaning the mixture from the typical 1/4 turnout, for low to medium speeds Perhaps it is running rich? Pull a plug and look at the plug condition, if the center insulator is black motor is running rich.


Do you ever see any black smoke out the exhaust? if yes running rich.


Have you recently adjusted the idle and idle mixture screw, perhaps needs adjustment. Is set with engine warm and GAV set to 1/4 turn.


Pull the distributor up, the block where the distributor sits on the block should be bare metal, free from paint and corrosion. This surface is the major mechanical ground point for the distributor points and the condenser. From experience this was painted by a motor rebuilder on my motor and caused me a lot of grief including some occasional backfires when it would miss and the exhaust built up with gas/air mixture. It is mre prevalent with a misadjusted idle mixture screw, when tuning off the motor.


Motor cold, with the Fuel shut off on and sitting not running for 15 minutes do you have leakage from the Carb? Go for a drive and with motor up to temp, turn off motor, wait 15 minutes check for carb leak,. Let sit for a couple of hours and check again. If leaking after the 15 minute checks - could be carb gas bowl is overfilling due to a leaky float valve and causing the jets to run rich. The two hour test should not be an issue as long as you are in the habit of turning off the shut off, a perfect carb should never leak.


Get a Float gauge, if do not have one purchase from Renners which has the larger diameter sight tube, check if your carb float is set to 5/8 ths. inch. If not the float valve requires shimming.