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Runs great for 30 minutes then not so well

Van Bagley

Donation Time
Now that I am able to actually drive my car I still have a few issues.

First start up with full choke the engine will fire and run as it should. Idle around 900 rpm when warm and choke in. Driving for the first 2o miles or so everything is OK with the exception of a few random coughs. The longer the drive the lousier the engine runs. Cool air,(outside air around 60F) not over heating, oil pressure good, premium fuel (Chevron). New fuel filter, the fuel pump filter is clean. I have a single Weber with headers. The dizzy is a new rebuilt unit , points and the timing are dead on. The balancer is new. Compression is 180/181/179/170/ Plugs are clean, wires are recent.
Why does it fall off after 20 miles or so? I will cough and sputter, pop back thru the carb, back fire.If I let it sit over night it will run good then repeat itself. Could it be a vacuum problem? This is not a random event.
 

husky drvr

Platinum Level Sponsor
Van,

From your description, the first thing I would check is the ignition condenser for a failure when it heats up.

Either try a new condenser or maybe carry a spare and change it when the engine starts acting up. If the engine runs better, then you've most likely found your issue. Yes, even new condensers can be bad.

HTH,
 

dansun22

Silver Level Sponsor
good suggestion HTH. I was thinking old fuel lines deteriorating but the filters would avoid that contamination...
 

RootesRooter

Donation Time
You mentioned a lot of new parts. Has the car run OK at any time since the most recent new part(s) was added?

My own experience with a bad condenser is that the ignition would intermittently cut out altogether for minutes at a time. No grey area of just running badly.
 

Bill Blue

Platinum Level Sponsor
Very good description of a bad condenser. Could possibly be the coil, but they usually act up under load. Back in the Good Old Days, new condensers were so variable in quality that if the points were not pitted, the old condenser was good and not removed.

Bill
 

nsbluenose

Silver Level Sponsor
There are lots of bad rotors to be had, but I find rotors work or they don't. Kinda like a light bulb, either they light or they are burnt out. Just my opinion.
 

jumpinjan

Bronze Level Sponsor
...will cough and sputter, pop back thru the carb, back fire.
Is it backfiring and missing at the same time? Keep a timing light with you and when it does that, pull over and check timing.
If the engine wants to bog when you open the throttle and/or backfire, pull the choke out a little bit and see it that helps smooth it out?
Is the coil 12vdc power running through the ballast resistor? And is the generator operating okay?
Jan
 

Bill Blue

Platinum Level Sponsor
There are lots of bad rotors to be had, but I find rotors work or they don't. Kinda like a light bulb, either they light or they are burnt out. Just my opinion.
I had one that developed a hairline crack, allowing spark to ground to the dizzy shaft. Started as an intermittent miss under load, within 50 miles it became a problem to keep the car running.
Bill
 

jumpinjan

Bronze Level Sponsor
developed a hairline crack, allowing spark to ground to the dizzy shaftl
That got me thinking.....I can high potential (hipot) test dielectric parts up to 10kv here in the shop and measure current leakage down to 1 uA.
If I have some time, I'll test a few rotors both used & new
Jan
 

Van Bagley

Donation Time
Great information, thank you all.
I will replace the condenser and the coil straight away. The newest part is the distributor which is a rebuilt 25D from Jeff Schlemmer in MN
There is back pressure when opening the fuel cap, I will check the venting as well.
Also it will diesel when shutting it off. This is new as well. The timing is set to Jeff's specs
 

Thor 1211

Silver Level Sponsor
Great information, thank you all.
I will replace the condenser and the coil straight away. The newest part is the distributor which is a rebuilt 25D from Jeff Schlemmer in MN There is back pressure when opening the fuel cap, I will check the venting as well.
Also it will diesel when shutting it off. This is new as well. The timing is set to Jeff's specs
by back pressure do you mean vaccuum.? If when you open the gas cap there is any noise like air going in or out that is a problem. +1 on making sure the gas cap is properly vented. Don't ask how I know except to say that all Pep Boys replacement gas caps aren't vented.
 

nsbluenose

Silver Level Sponsor
I had one that developed a hairline crack, allowing spark to ground to the dizzy shaft. Started as an intermittent miss under load, within 50 miles it became a problem to keep the car running.
Bill
Bill, Similar thing happened to me in the Pocono Mountains with the Singer. Except that when mine shorted to ground it quit immediately. Had less than 3000 miles. Brand new before our trip from Nova Scotia to Pittsburg, so didn’t have a spare. Lesson learned. Now have spare; rotor (2), cap (2)wires, 4 plugs, and distributor.
 

Bill Blue

Platinum Level Sponsor
by back pressure do you mean vaccuum.? If when you open the gas cap there is any noise like air going in or out that is a problem. +1 on making sure the gas cap is properly vented. Don't ask how I know except to say that all Pep Boys replacement gas caps aren't vented.
Stock Series 5 caps (don't know about earlier ones) are "smart" in that they will close to carry pressure and open to alleviate a vacuum. The closed position is spring loaded to prevent dangerous pressure build up.

Bill
 

v13311

Silver Level Sponsor
They are "not so smart" if the spring mechanism doesn't work for any reason. Mainly corrosion is the culprit I always spray a little WD 40 into the tiny hole on the spring mechanism, and then push in and out many times to lubricate the inside of the cap. Had many overflows of gas on a hot day because of a faulty spring mechanism. Lesson learned.
 

Van Bagley

Donation Time
The gas cap could be the problem. After sitting for 24 hours, there is zero pressure when opening the cap. When running just for a few miles there is noticeable pressure when opening
RootesRooter; yes the dizzy came with a new cap
 
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