VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 1927 chev runing rough

Hello,
I have a 1927 chevy that starts to miss,backfire through the exhaust and inlet after running for 5 minutes at medium speed.It starts up and idles fine and runs at medium speed for a short period then the problems start.I have swapped the sparkplugs,coil,condensor and carburettor.Plugs and points set at correct gap.There is plenty of fuel left in the carburettor so I dont think it is a fuel issue.When it cools all is well untill it warms up again.
I have noticed that the air preheat tube is very warm to touch.
Car was a sweet runner up till now.
Any suggestions please.
Regards
Rusted

Just a couple of other things.
Tappits are all adjusted to specs and other carb was rebuilt(carb late 27).
Have recently rebuilt distributor and installed to specs.(early 1927)
Running a 28 chev head,a 28 inlet manifold and a rxo carb.

Rusted

Rusted
here ya go,
1) fuel , well if its backfire from carb you could be lean, backfire from exhaust you are running rich, usually its one or the other so I would start with ignition.
2) I would carry hot wire in car under seat ( 5 ft of 14 gauge with alligator clips) that I would use to jump from battery to coil when problem arrived, jump past all connections between battery and small connection on switch side of coil , if condition persisted I would assume all ok on that side of coil. I would then focus on distributor side of coil the primary (point) side. With engine off after problem arrives take a volt meter and check volts at small coil connection distributor side. should get 6 volts or so, then with distributor cap off rotate engine via fan blade until points are open, check voltage at coil distributor side again should be zero, if you read 6 volts you have a short in distributor to fix. if you read zero I would assume primary side wire good.
3) check spark at coil wire (top of distributor to top of coil) see if it is strong and in cycle.

got my own problems with ignition coil that quits when it gets hot on my 1927 chevy capital aa. I also have to run with some choke when hot to find that sweet spot. I think driving around in 90 degree weather also strains situation with coils. this could be your problem.

if your timing is off your engine will run hot and your coil may take a dump on you. do you have distributor timing set with the number thing you view flywheel mark thru the bell housing hole ?

probably if it runs good then when hot you have problem I gotta think its the coil overheating

Ken 45145

Hi,
Thanks for your reply.
After 2 wasted days,I think I have found the problem,after swapping coils,condensors,plugs and a carb.
As I said before I have just rebuilt the distributor trying to eliminate a slight miss at 40 mph.Early 1927 manual advance/retard.
After a close examination it appears that the post that holds the adjustable point contact is a bit wider than the others.As a result when the plate is rotated(advance or retard )the post is very close to the lobe on top of the distributer shaft that the points breaker arm operates on.This is causing the spark to jump across to the lobe and not through the points in certain positions.The reason for backfiring through both the inlet and exhaust.When I rebuilt the distributor I picked the best parts from 3 units not thinking there would be any difference in manufacture.
Once i remove some material from the post all should be fine.
(well hopefully)
Cheers
Rusted

Rusted
If you're running a 28 head, you should have a late 27 RAJXO carby or a 28 RAKXO as the 28 head has bigger valves than the early 27 and usuing a RXO carby will cause it to starve for fuel when using, especially under load
Chris