Old Briggs 5HP - Idle RPM climbing

[creia]

1972 Briggs 5HP Model 130202- horizontal shaft
Recently rebuilt, including carb (Pulsa-jet)
Starts and runs fine with good throttle response
The problem I am having is the idle RPM climbs (significantly) steadily as the engine warms up. I have it dialed in to idle at about 1700-1750 RPM, however, after about 5 minutes it is idling around 2100 which is too high for the application it will be used for (mini-bike with centrifical ctutch). If I adjust it back down to 1750 RPM, the next time I start it up it will be way too low of RPM (around 1300-1400), and I'll have to adjust it back up again . I am thinking this is somehow heat related- maybe something somewhere expanding with heat and causing an air leak?
I welcome any and all ideas! After all of these years this is a first for me!
Michael

[bgsengine]

or, alternatively, thermal expansion causing a leak (which was compensated for in cold adjustment) to stop leaking.... It'd be hard to tell which one, I might consider checking exhaust temperatures (if you got a thermometer to do that) or, at start-up use the ol' carb cleaner spray around suspect areas trick, if no leaks then, try it again when the engine gets hot and warmed up. I'd normally suspect carburetion assuming everything else is in spec after rebuild (leakdown test cold vs hot, which is also difficult to achieve effectively) and if you have a spare coil handy, try that (could be an intermittent misfire that you could only pick up on a DSO, it'd happen too fast to be visible, and not often enough to cause a real performance issue) and lastly the problem might go away after some run time (new parts broken in, etc.) I remember having that problem show up every so often, but more frequently on winter engines (logsplitters, snowblowers) - we rarely ever serviced things like mini-bikes or whatnot with centrifugal clutches so I don't have a lot of experience that way, our usual solution was to dial down cold idle (immediately after start-up) to just enough for it to stay running (and make sure it will accelerate on snap throttle) and whatever the idle was when it was hot was not normally a big issue (Since most stuff we serviced with those engines typically only ever operated at full throttle anyhow) but I was always curious as to why they did that, but had so few of them I never really had a chance to experiment with them (most were paying jobs that the incentive was get it to run for its purpose and ship it, if it idled a tad fast , no biggie)

[creia]

Thank you Brian,
Nothing showed up spraying the carb cleaner checking for air leaks. The only way I have to check exhaust temp is to measure the temp on the muffler itself using an infrared "point and shoot" gun- it was 600 degrees at its hottest point (near the very end of the muffler) which seems pretty normal to me. Yes, unlike a stationary piece of equipment where a high/climbing idle would not be a problem, it's kind of a big deal on a mini-bike with a clutch, as the higher idle RPM will just start to engage the clutch (which is not good for the clutch-overheating) when the bike is stopped and cause it to creep forward.
Michael

[bgsengine]

Yeah and like I said, just a theory but trying a different coil, or perhaps just add a gasket (or new gaskets - they could sometimes maybe have porosity to them?) or maybe pop the flywheel off, and check/replace key and when putting it back on , try and shift the flywheel forwards a little further as far as keyway allows (if there is any slack at all) - a degree or two if ignition timing change could make a difference, and you can sometimes "stack up" on tolerances in one direction or the other to shift your timing a tad without major modifications... (which is why sometimes just disturbing your bolts by loosening and then tightening can fix a weird issue, or seem to)

Again, since I never definitively diagnosed one of those few engines that did that to me, I am just shotgunning ideas here...

[Arkie]

As a last resort attempt I would probably completely change out the Pulsa jet carb.
and yes some of them old pulsa jets are rare as hens teeth and expensive, but I try to keep couple laid back. I've also seen some of them Pulsa jets carbs have a really critical jet adjustment between cold and hot for a good idle and run. (choke completely different on some and even the bolt mounting hole spacing)
I know you also try to keep things original, but I've installed the updraft type Briggs carb's on the 3.5 and 5hp and stand alone gas tank on some of the briggs that came to me with the pulsa jet and it's gas tank missing.

Summary: think about trying another carb.

[creia]

Thank you Arkie!
I will be trying another Pulsa Jet carb this weekend.
Michael

[Arkie]

Report????????????

[creia]

Report????????????

I ended up swapping out the carb with another one and no more climbing idle RPM.
Unfortunately, I was never able to determine why the other carb was acting that way- nothing definitive or conclusive turned up during my forensics.
Sometimes it just seems like some of these carbs are just a "dud". Thank you for following the the thread...
Michael