Runs with choke only ... mostly

[GregH]

Ok, it seems to be my turn for this kind of problem, and I hope it is something simple that I just dont know enough about.

The engine this spring has been toyed with a little in terms of idle mixture and power tuning at idle.

Started the engine today (starts as expected with choke and throttle ) and after a min closed the choke and let it warm up around 1200-1500 rpm. After around 10 min it started losing power and then died.

Restarted with choke on and it would take enough throttle for it to run around 2500 rpm, eventually climbing to 3000 if I did nothing. With the choke on, if I reduce throttle it dies.

About 10% of the time, I can get the choke off and the engine running around 2000 rpm for a min or two, then it will lose some power, come back, but eventually dies within that time frame.

The carb choke opens and closes fully - VISUALLY checked

The idle mix is at 1.25 turns

Fuel pressure is constant at about 2.5 psi

During the brief times it runs, the vacuum is at 15

I did not have a fuel safe container to check fuel from carb, etc. With me.

Time to remove carb and clean?

Could it be water causing this? Time to hook up another fuel source and see if any different ?

I'VE BEEN trying to search all the tidbits of info on here but would appreciate any input.

[lat 64]

First guess from me; carb float.*

*NOT a qualified opinion

[roadnsky]

Originally Posted by GregH

The engine this spring has been toyed with a little in terms of idle mixture and power tuning at idle.

Can you give us more detail on what you did?

Have you cleaned or rebuilt the carb in the recent past?

[JOHN COOKSON]

Could be a weak spark plug. Pull the spark plug wires one at a time while the engine is in the "low power" mode and note any changed in performance.

Could be a blocked tank vent. Would explain the 10 minute normal run time. To diagnose loosen the fill cap.

More as I think on this a bit more.

TRUE GRIT

[GregH]

Originally Posted by JOHN COOKSON

Could be a weak spark plug. Pull the spark plug wires one at a time while the engine is in the "low power" mode and note any changed in performance.

Could be a blocked tank vent. Would explain the 10 minute normal run time. To diagnose loosen the fill cap.

More as I think on this a bit more.

TRUE GRIT

I did try the open deck fill and no difference. The fuel pressure gauge always around 2-3 psi

[ndutton]

The symptoms point to an inadequate fuel/air ratio so what are the possibilities? Partially clogged main jet, intake vacuum leak, fuel contamination, fuel level low in the bowl. Do you have a PCV system? If so, check it for leaks and proper valve operation.

However, there was one other thing you said:

The engine this spring has been toyed with a little in terms of . . . . . power tuning at idle.

If by tuning you mean adjusting the timing, there is a directive from Universal about power timing at idle but the preferred method by far is to power time at greater RPM with the advance fully engaged. My earlier power timing efforts were at idle but checking it carefully a few years ago with a timing light showed it was grossly advanced. Getting it right made a huge difference in performance.

[GregH]

Originally Posted by ndutton

The symptoms point to an inadequate fuel/air ratio so what are the possibilities? Partially clogged main jet, intake vacuum leak, fuel contamination, fuel level low in the bowl. Do you have a PCV system? If so, check it for leaks and proper valve operation.

However, there was one other thing you said: If by tuning you mean adjusting the timing, there is a directive from Universal about power timing at idle but the preferred method by far is to power time at greater RPM with the advance fully engaged. My earlier power timing efforts were at idle but checking it carefully a few years ago with a timing light showed it was grossly advanced. Getting it right made a huge difference in performance.

I've got my timing light along. When I started this "playing" around the roll pin was showing up a good 10 degrees to the left of the mark (too retarded?) on the flywheel cover. It was adjusted to be as centered as possible using that reference. I'll start there this evening.

[GregH]

Originally Posted by roadnsky

Can you give us more detail on what you did?

Have you cleaned or rebuilt the carb in the recent past?

No since spring 2018, it has been working fine and now this year I'm trying to tweak things.

The engine was running rich last year. Start in 2sec on choke, choke off right away. Plugs very dirty end of season.

Backed off idle mix 1/4 turn this spring and played with distributor to get the roll pin at zero mark and from there a little adjust for idle rpm.