MTFCA Forum

Yesterday I drove my 1927 coupe 7 miles for the first time in six months. Question 1: At times, the engine backfired and coupe lurched badly in high gear. What are the main causes of backfiring?

Question 2: I checked the temperature of the upper part of the radiator. It was 214F. The temp of the head was 214F. The temperature of the exhaust manifold was 400F. The air temp was 74F. I assume these temps are normal, correct?





































a

Temps are acceptable...lurching & backfiring = lean mixture

Do you have a heat pipe?
Hi humidity can freeze the intake on a non vaporizer motor.

Bruce,
A bad ignition switch or a loose connection will do that. If the ignition is momentarily cut off under load and back on again you will have a momentary loss of power followed by a gunshot and then normal performance until it cuts back out again. Good way to blow a muffler apart. That happen to me, had the switch restored and problem solved.

I think we determined the cause of the backfiring and lurching. After we got back to the garage, my grandson noticed that the back clamp that holds the cover to the coil box was loose. The front clamp was tight.


Today, we drove the coupe with cover off and the engine ran fine. We then put the cover back on with the back clamp loose and experienced misfiring, but no backfiring. I would appreciate thoughts on what is going on.

It was 214F. The temp of the head was 214F.

Boiling point at sea level is 212ºF. Somebody who knows better can correct me, but I think a cooling system that's working properly should keep you not only below the boiling point, but under 200º.

Today, we drove the coupe with cover off and the engine ran fine. We then put the cover back on with the back clamp loose and experienced misfiring, but no backfiring. I would appreciate thoughts on what is going on.

My guess would be loose coils not keeping proper contact at all points. The cover isn't just to keep dust out. It's also supposed to keep the coils where they have good, steady contact. Or maybe the loose cover is touching the business end of the coils.

In addition to the above, running the engine with the spark set too late can cause it to run hot and have reduced power. It can also cause backfire through the carburetor. Be sure your spark lever linkage is adjusted correctly and that you are advancing the spark enough when up to speed on the open road. Above 20 to 25 MPH, the spark lever should be at or near full advance, especially if running on battery.

Look for any evidence of arcing between a coil and the box lid, particularly the prongs that hold the coils down tight. I've seen this on my '27 and on other's coils. Spark erosion is pretty easy to see and would cause at least a miss in the engine. May or May Not be consistent.

Good Luck

Are you running gas with ethynol in it? Or are you running "clean" gas? If the issue comes and goes, probably not fuel related, but my first thought after reading not driven in 6 months was the possibility of fuel gumming up the jets in the carb slightly and creating some issues. If you are running fuel with ethynol in it, might not hurt to just spray some carb cleaner through the carb while running a high idle. JMO

I don't know how the gas in your state is, but here in California, if you leave unburnt gas in the carburetor or fuel line for more than a few months it will jell up and plug things up. It could be clogged carburetor passages or a clogged fuel line. Too lean a mixture would cause it to run hot.
Norm

Isn't it also true that a too-rich mixture can send unburned fuel out the exhaust that can ignite and backfire?

IMHO 214 is way too hot.

I agree... the car should run around 180 to 190 F under most conditions. Late timing and other things mentioned can promote over heating.

Should I measure the radiator temp while the engine is running?

Antifreeze will increase the boiling temperature, but I agree, the thermometer should be checked.

Art Mirtes