VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Points gap on 1926

I have been trying to get my hands around a back firing issue after and engine rebuild, to no end. However, I did set the points to the book (0.030 in gap) only to have a friend (un-named, but an expert that has been in the antique chevy field for 60 years), tell me to set the pints to at least 0.040, to prevent back firing. finally, it should be noted that the points, prior to rebuild, were set to 0.080, and it never back fired.

I am about the check the coil strength, and though I would consult the board on the subject. Any opinions?

Dave

The Chevrolet Repair handbook for the series AA-AB shows the information for the points gap on the REMY 374-A distributor as being 0.025".It's in a table with all the electrical data for the starters etc; on page 227.The point gap is also mentioned on page 185 as well,along wih how to adjust it.

The correct distributor point gap should be .020" with the cam follower of the points resting on one of the four distributor cam lobs. Unsure where anyone came up with .030, .040, or .080" for a point gap, spark plug gap at .030 maybe. The spark plug gap spec. for the 26 is .025". Fix these items first and move on if the problem still present.

The firing order is 1-2-4-3 for your vehicle. Do you have that correct?

It would be handy to know more about the backfiring. When is it happening? On the road, going down a hill, idling? I set the points at .020" on my K and it runs nicely. The figures you quote seem terribly excessive to me.

Al

Mate,the information is printed in the repair manual in black & white for the REMY 374-A distributor,have a look at in the book.The plug gap on Chev 4's should be set at 0.030",not 0.025".

Hi,
Have a good look for wear on the distributer shaft and for endfloat of the shaft.Also check the point gap when you operate the advance retard lever as this may also show up wear on the shaft.
For such a large point gap I think it is needed to compensate for wear.

With the REMY 374-A distributor you need to check how much play there is between the locating spigot for the breaker points plate(this is cast integral with the distributor housing)and the bore of the breaker plate,as too much clearance between the two will affect not only the point gap,but the amount of advance/retard you can get.

Correct, points gap is all about dwell. Everyone here has plenty of advice but we still have no idea when this backfiring is occurring and so, our advice may not relate at all.

Al

Good point.We need to know if the engine is backfiring out of the exhaust system,or the intake system.If it backfires from the exhaust system,it's a pretty good indicator that the ignition timing/valve timing is out,and if it backfires from the intake/carb side,it's a good indicator of a problem in the fuel delivery side.

thanks all, I appreciate the input. This is related to an earlier post titled "engine sputtering at idle". I checked the coil, strong as can be. As stated, inset the points to spec, and it putters "small back firing through exhaust", but upon increasing the gap and resetting the timing, it runs much smoother. Perhaps the cam-crank timing gears are worn. But as for dwell, how does one set the dwell when the only adjustment is timing cam and points gap?

One additional note that is sure bring comment, at the time of manufacturing, 1926, the flame front and associated combustibility of gas was different than now. We can credit Kettering of GM for developing stable burning gas while he was at DELCO (which he started). Because of the new octain valves and the combustion profiles of modern gas, some ignition parameters may have changed. However, back on planet earth, I thing I have bad exhaust valves.

Dave

There is much more research into flame front propagation today then ever before. That is due not only to the changes in molecular makeup of gasoline but also the fuel economy and power required of modern engines. Therefore modern fuels are much more consistent refinery to refinery, brand to brand, load to load.

OK, to check the valves without removing the head, you need to do a cylinder leakage test. If you are mechanically handy and have a welder (to make a test fitting)and air compressor, you'll be able to do it yourself. If not, get some help. If you want to have a go, let us know and we'll help you through it.

Regards
Al

I do have a modified spark plug with a hose barb to pressurize the cylinder, and I have a pressure decay tester. I will couple the two and check the bleed down rate. I do not know what is standard.

All you'll really need to do is bring it up to TDC, one cylinder at a time. Put it in top gear and set the park brake. Connect the compressed air and then have a good listen at the carb and then at the tail pipe. Repeat on each cylinder. If the valves are good, you shouldn't hear any air escaping. Have a listen to the crankcase breather too for worn rings, although they shouldn't cause the back firing.

Regards
Al

AlBrass,

Will do. I have done this before, used surgical gloves over carb and tail pipe to see flow. Thanks for advice, and for reminding me to put in high gear, nothing like a car rolling around the garage!

Dave