208cc Briggs Stratton running poorly at high altitude

[ripcord]

Surging usually indicates a lean fuel air mixture. At high altitudes, carbureted engines run richer, not leaner, so high altitude jets are SMALLER than the normal ones. Installing one is going to put you in the wrong direction if you're running lean.

I would look for vacuum leaks, cracked plastic inlet tubes, bad carb mounting gaskets, loose carb mounting bolts, dirty carb passages, low float bowl settings, an improperly installed or damaged governor spring or anything that would allow excess air into the engine or anything that would restrict the flow of fuel such as a dirty fuel filter or bad gas cap vent.

 

[GraphiteAE]

Surging usually indicates a lean fuel air mixture. At high altitudes, carbureted engines run richer, not leaner, so high altitude jets are SMALLER than the normal ones. Installing one is going to put you in the wrong direction if you're running lean.

I would look for vacuum leaks, cracked plastic inlet tubes, bad carb mounting gaskets, loose carb mounting bolts, dirty carb passages, low float bowl settings, an improperly installed or damaged governor spring or anything that would allow excess air into the engine or anything that would restrict the flow of fuel such as a dirty fuel filter or bad gas cap vent.

The mesh air filter definitely wasn’t clean but it wasn’t filthy either. I have less than 8 hours use on this thing. Would that be enough to run lean?

 

[ripcord]

The mesh air filter definitely wasn’t clean but it wasn’t filthy either. I have less than 8 hours use on this thing. Would that be enough to run lean?

A dirty air filter would make it richer, not lean. Does it smooth out if you give it a little choke?

 

[GraphiteAE]

A dirty air filter would make it richer, not lean. Does it smooth out if you give it a little choke?

No actually it wants to die with the close most of the time

 

[MaximumMini]

Look at the spark plug.
Is the insulator lighter and ashy colored or darker and possibly oily?

 

[MaximumMini]

I just had an ongoing problem similar to this with my JD riding mower.
Surging badly.
It was the carburetor 100%.
Cleaned the carb asbestos I could and it still surged.

Replaced it and it runs better than new.
Could be a number of things...but a dirty or malfunctioning carburetor will absolutely cause surging.
Oh....did anyone mention that you should check and or adjust your valves. That can cause surging too.

 

[grasscutter]

Will that fix my idle problem?

That's a good question. I don't see an idle circuit anywhere on the drawing. Usually there will be a high altitude pilot jet for the idle circuit. Your carburetor offers a high altitude main jet. Maybe there is no separate idle circuit and all engine speeds run off the main jet. I've never heard of that.

 

[mk378]

That is a Honda clone carburetor. It has an idle circuit with the idle jet being a plastic piece inserted at the top as @K.Aoi said. There were some old Briggs designs with no idle circuit. It should be noted that in any case an OPE engine was never intended to idle like a luxury car. Idling them down is strictly a functional thing to keep it running so you don't have to restart it.

It's common for the idle circuit to be clogged. This will cause severe surging and possible stalling when attempting to run at idle. The test is to pull on the throttle linkage rod to hold the throttle plate closed against the stop screw. (Do this with a warmed up engine and no load). It should run smooth and at a decently slow rpm. If it stalls, try getting it to run smooth and slow by holding the linkage almost against the stop screw. If that works adjust the screw.

If it always stalls instead of running slow, remove the idle jet and clean it. It is underneath the head of the stop screw, so the screw has to be taken all the way out first. Technically that can be done with the carb in place, but it is better to remove carb and spray cleaner through the passages leading to and from the idle jet while it is out.

 

[GraphiteAE]

That is a Honda clone carburetor. It has an idle circuit with the idle jet being a plastic piece inserted at the top as @K.Aoi said. There were some old Briggs designs with no idle circuit. It should be noted that in any case an OPE engine was never intended to idle like a luxury car. Idling them down is strictly a functional thing to keep it running so you don't have to restart it.

It's common for the idle circuit to be clogged. This will cause severe surging and possible stalling when attempting to run at idle. The test is to pull on the throttle linkage rod to hold the throttle plate closed against the stop screw. (Do this with a warmed up engine and no load). It should run smooth and at a decently slow rpm. If it stalls, try getting it to run smooth and slow by holding the linkage almost against the stop screw. If that works adjust the screw.

If it always stalls instead of running slow, remove the idle jet and clean it. It is underneath the head of the stop screw, so the screw has to be taken all the way out first. Technically that can be done with the carb in place, but it is better to remove carb and spray cleaner through the passages leading to and from the idle jet while it is out.

I can't seem to find the rubber boot anywhere, or anything that might be an idle circuit

Can you see it here? https://www.partstree.com/models/13...tratton-horizontal-engine/carburetor-group-1/

 

[The_Eric]

There is an adjustment screw for the idle speed - if it will be anywhere, it will be under the head of the idle speed adjustment. Remove the screw and pry the (usually) black plastic screw looking thing out. The orifice that is in it is tiny, but serviceable. This is almost always the problem.

 

[The_Eric]

I can't seem to find the rubber boot anywhere, or anything that might be an idle circuit

Can you see it here? https://www.partstree.com/models/13...tratton-horizontal-engine/carburetor-group-1/

Can you post a pic of the carb?