Engine stumbles on acceleration only after starting up again after about 15-30 minutes

[vineyardpinz]

Hey there I’m new to the forum and hope someone can help me out I’m working on a 73 pinz 712 for my work so far I’ve siphoned out about 18 gal of stale gas and replaced the fuel filter fuel pump and spark plugs it runs a lot better but I’m still getting a weird stumbling after leaving it off for about 15-30 it’s used for tours and this condition happens regularly but what’s odd is the first start up and drive doesn’t have this issue I’ve attempted to attach a video of what’s happening thanks in advance https://youtube.com/shorts/la2ydcvj5os? ... bQgkYIJogy

[rmel]

Hard one to diagnose.

But... You say first start up it runs fine, does it continue to run well for an extended
period of time without stumbling?

After you stop and then let it rest for 15 to 30 minuets THEN it runs like crap, if you
run it for some time after it ran poorly does it then eventually run well again?

BTW where is your location.

[vineyardpinz]

If I just drive it around without letting it sit it will run fine no hesitation it’s only after the hot start that it gives me issues and it can get better just driving it but not always and I’m in California with a low elevation

[rmel]

What you are describing fits a heat soak condition into the intake manifolds.

These are under the tin, inside the cooling plenum. They connect directly to
the intake ports of the heads and can get hot when the engine is turned off
with no airflow for cooling. If they get too hot atomized fuel in the intake
manifolds gets pre-heated to the point were you have a VERY lean condition
and ignition could be poor.

There's a couple of possibilities that could cause this. Improper timing, poor
airflow through the plenum or uninsulted intake manifolds.

The last issue was a know problem that was improved on late 73 builds by adding
insulation to both intake manifolds. Easy to check to see if they are insulted by
remove the front tin under the forward carburetor and inspect the manifolds to
If they are not insulted, it would be good to do so. SAV sells a kit.

Now this all said, your not running at altitude or super hot desert? So it's a bit
of a surprise your seeing this. Good to check the insulation and make sure your
timing is advanced enough.

[vineyardpinz]

Thank you for the advice I’ll check it out tomorrow when I go back in and hopefully I’ll have some good news thank you again I really appreciate it

[vineyardpinz]

What you are describing fits a heat soak condition into the intake manifolds.

These are under the tin, inside the cooling plenum. They connect directly to
the intake ports of the heads and can get hot when the engine is turned off
with no airflow for cooling. If they get too hot atomized fuel in the intake
manifolds gets pre-heated to the point were you have a VERY lean condition
and ignition could be poor.

There's a couple of possibilities that could cause this. Improper timing, poor
airflow through the plenum or uninsulted intake manifolds.

The last issue was a know problem that was improved on late 73 builds by adding
insulation to both intake manifolds. Easy to check to see if they are insulted by
remove the front tin under the forward carburetor and inspect the manifolds to
If they are not insulted, it would be good to do so. SAV sells a kit.

Now this all said, your not running at altitude or super hot desert? So it's a bit
of a surprise your seeing this. Good to check the insulation and make sure your
timing is advanced enough.

Hey there you were totally right I checked the intake runners and there was no insulation I just ordered the parts hopefully that fixes the issue!

[rmel]

OK, well that should help. But it's still odd that this 712 is that hot
given the conditions are not that severe.

Don't forget to check timing. There's a recent thread on this forum
for how to do so. Rev her up and make sure your getting progressively
more advance with RPM. If you run with insufficient advance exhaust
could get very hot -- thus the heads. Running lean can result in a hot
condition, check your carb Jets.

OH, BTW you will have to pull the rear carb to get the read tin off.
The insulation has adhesive on the carb side, should stick OK. I've
done two installs with this material and used Stainless Steel Tie wraps
to keep them snug.

Good luck.