Backfire/runs rough at high RPM

[maurice43]

Briggs 14.5 hp OHV I/C engine model 287707 on a riding type mower.
It broke an intake push rod and I replaced it and then adjusted the valves to the recommenced clearances.
Gapped and replaced the spark plug.
Changed the oil and cleaned air filter.
It sat a few years with old gas in it and so the float needle valve was sticking causing flooding issue.
I removed the carb, cleaned with carb spray. Sprayed all the jets and passages. No flooding issue after that.
I did not turn any adjustment screws on the carb.
Now the problem:
It starts right off with no problems.
It idles at low RPM smoothly and at full choke smoothly but as soon it reaches half to full throttle it starts "hunting" and backfiring then within a few seconds dies.
It will start right up again with minimal choke no problem.
No smoke from exhaust.
Ideas?
Questions?
Please ask as I need to get this running for a family friend that has medical issues and cant push mow their yard.
Thank you!

 

[bertsmobile1]

Maurice,
Please do a time line on your post as it is hard to follow.
Did it sit for years before or after the pushrod job ?
Was the pushrod job done before or afteer the carb job ?
This is important so we have some sort of an idea where to start

 

[maurice43]

About three years ago it it broke a pushrod and then I just pushed it behind the shed. I did all the repairs in the last few days.

 

[bertsmobile1]

First thoughts
1) carb fuel passageways still partially blocked
2) not enough fuel getting to carb
3) air leak between the carb venturi and the inlet valve
4) blown or leaking head gasket

 

[maurice43]

1) carb fuel passageways still partially blocked:
I will remove carb and clean more thoroughly.

2) not enough fuel getting to carb:
Will blow out lines,change filter(should have done this in the first place)

3) air leak between the carb venturi and the inlet valve:
What is the inlet valve?
I did spray around the outside of the carb at gasket mating surfaces to see if RPM changed because of leak but didnt find any.

4) blown or leaking head gasket:
Will do a leak down test to see if I can verify.

 

[maurice43]

1) carb fuel passageways still partially blocked:
I will remove carb and clean more thoroughly.
Results: Removed carb and resprayed all passages. Letting soak overnight in cleaner.

2) not enough fuel getting to carb:
Will blow out lines,change filter(should have done this in the first place)
Results: Lines are clear,filter is clean.

3) air leak between the carb venturi and the inlet valve:
What is the inlet valve?
I did spray around the outside of the carb at gasket mating surfaces to see if RPM changed because of leak but didn't find any.
Results: Didn't find any leaks. I sprayed cleaner around venturi and inlet valve mating surface with no change in RPM.

4) blown or leaking head gasket:
Will do a leak down test to see if I can verify.
Resultsid a leak down test. About 15 to 20 lbs. With piston at TDC. No air leak at carb, exhaust or oil fill(crankcase) but I did hear air deep in the engine with the OHV cover off. Air getting past rings? Will do a compression test tomorrow.

 

[Mikel1]

On 3 he's referring to intake valve.
What % leak do you have?

 

[bertsmobile1]

On 3 he's referring to intake valve.
What % leak do you have?

No a leakig intake valve can only get the gas from one source, the carb.
However the throttle vales & choke pivot pins, can get flogged and leak air as can the gaskets, mounting flange & manifold .
If an inlet valve leaks then you end up with less fuel in the cylinder which is the same as running on part throttle.
If an inlet valve guide seal leaks the you can get dillution of the charge but it has never been a problem on mower engines.
A motorcycle engine doing 9000 rpm is a slightlu different story but a mower running 3600 is a bit different seals rarely need replacing unless the engine has been cooked.

 

[Mikel1]

No a leakig intake valve can only get the gas from one source, the carb.
However the throttle vales & choke pivot pins, can get flogged and leak air as can the gaskets, mounting flange & manifold .
If an inlet valve leaks then you end up with less fuel in the cylinder which is the same as running on part throttle.
If an inlet valve guide seal leaks the you can get dillution of the charge but it has never been a problem on mower engines.
A motorcycle engine doing 9000 rpm is a slightlu different story but a mower running 3600 is a bit different seals rarely need replacing unless the engine has been cooked.

Sorry, your terminology is difficult to understand some times, not as bad as KK though. I thought you were referring to gasket at intake to cylinder head.

 

[bertsmobile1]

4) blown or leaking head gasket:
Will do a leak down test to see if I can verify.
Resultsid a leak down test. About 15 to 20 lbs. With piston at TDC. No air leak at carb, exhaust or oil fill(crankcase) but I did hear air deep in the engine with the OHV cover off. Air getting past rings? Will do a compression test tomorrow.

I hope you are saying that the drop was 15 to 20 lbs after 10 minutes .
If that was the test pressure then it is way too low.

Air leaking into the crankcase can be rings or a gasket blown at the pushrood tunnel which is exactly where most intek engines blow.