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High Speed Bogging/Stalling: thoughts?

 
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tenders
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Joined: 03 Dec 2018
Posts: 137

Buddy International 50 Italia

PostPosted: Wed Nov 27, 2019 3:39 pm    Post subject: High Speed Bogging/Stalling: thoughts? Reply with quote

Friend's 2014 Buddy 50, 4300 miles, Prima BBK, stock airbox, stock exhaust, stock idle jet, various main jets. I've adopted it for troubleshooting, and I'm stumped.

The engine runs fine up to 40mph, but after 10-20 seconds at about 45mph, near top speed with the BBK, it behaves as if the throttle has been cut off. As the engine slows, a twist or two of the throttle often revives it, but soon after increasing above 40mph again, it cuts out again. Sometimes the throttle has no effect, and the scooter stalls. It starts again unfailingly, although sometimes the kickstarter needs to be applied after several cranks after the electric starter doesn't get it to catch. The engine seems to run indefinitely at 40mph and slower.

My biggest concern is that the engine is seizing at high speed, but I don't think this fits those symptoms. The engine sounds normal, the back wheel doesn't lock up. I'm waiting for a spark plug temperature sensor to arrive to confirm the head isn't getting too hot before I drive it again.

The carb has been disassembled and cleaned. Currently running 88 main jet with the hope of preventing a lean condition (stock is 75, BBK installer upjetted to 85). The fuel has a bit of extra 2T oil premixed, in addition to the oil pump's contribution. The fuel filter has been replaced. The vacuum petcock has been replaced. I've looked carefully at the throttle position sensor and it's working fine. The air filter and spark plug have been replaced. The air box cover has been checked carefully.

The scooter had some issues with high voltage blowing out headlights and panel illumination, and the problem turned out to be a disconnected ground wire at the regulator/rectifier. (Not one of the regulator prongs, but another ground wire connected to the bolt holding the rectifier in place.) The voltage now no longer goes above 14.7V at any speed, as designed. But as a result I've swapped out (and then restored the OEM) a bunch of electrical parts, none of which solve the high speed bogging/stalling issue: the issue is NOT the coil, CDI, stator, regulator, or battery.

My current thinking is that, barring an overheating head, I've somehow missed something in the carb. But could this be a symptom of a clogged exhaust, carboned-up head, or improperly-installed BBK?

TIA!

EDIT: The carb may have been the problem. Took the carb off again and did four things:
* (again) reamed out the passage into which the needle valve protrudes with a tiny pipe-cleaner carb tool,
* did the same for the main jet passage (not the jet itself, but the carb body passage through which it sprays),
* installed the retaining clip on the needle valve so that the "cloverleaf" side of the clip faces the needle valve, ie, is farthest away from the carb edge, and
* adjusted the float so that its bottom edge dangles 5/8" below the level of the flange when the carb is held in operating position. This is a key measurement I have not seen anywhere else that I took from a brand new carb when I replaced my own last March.

Of these, I suspect the mis-set float valve level was the issue, and prevented the needle valve from letting enough fuel into the carb to sustain the BBK's fuel demand at high speeds. I wonder if the PO's BBK installer did not have the setting available and didn't set it right when he upjetted the carb. It may have been wrong long before the BBK was installed - wouldn't have mattered for a stock Buddy configuration that doesn't go much faster than 40. Again, for future fiddlers looking for this stat on Google: The Keihin PB16 carburetor's float bottom should be 5/8" below the flange when held in operating position!
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k1dude
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Joined: 08 Jun 2008
Posts: 2423
Location: Northern California
'08 Orange Buddy 125

PostPosted: Mon Dec 02, 2019 9:03 am    Post subject: Reply with quote

Likely a bad stator.
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tenders
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Joined: 03 Dec 2018
Posts: 137

Buddy International 50 Italia

PostPosted: Mon Dec 02, 2019 11:03 am    Post subject: Reply with quote

It wasn’t the stator, I replaced that (with high hopes).

It was a carb issue - either the float height setting (my best guess), some gunk removed after multiple cleanings, or the way the carb had been reassembled.

Running like a champ now and going back to my friend, to everyone’s relief.
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