High Revs

[Shipper]

After approx 200 miles (320K) of terrific performance I am dealing with my next issue. Currently it will start every time and idle perfectly from cold after choke. But, after I ride any distance at all, upon return it revs real high at idle. Not being a pro at carb set up - any suggestions are greatly appreciated.

Thanks crew.

 

[Royboy]

How high is "real high"? With both my 650's I normally advance the throttle stop screw a bit when starting out with a cold motor and then turn it back to about a 1200 rpm idle after the motor warms a bit. Pretty common trait with the 650's.

roy

 

[grizld1]

Pretty common trait with any carbureted motor that lacks automatic choke and high idle boost. There are many threads on this issue and you'll find them if you do a few searches. May be normal, maybe not, more information needed. Bottom line: if you're using OE breaker point ignition or an electronic ignition that employs the OE advance governor, the advance rod may need lube and/or the bobweight springs in the governor may need attention. The problem may also be caused by a vacuum leak between carb boot and head, boot and carb, carb and airbox. Plenty of material on all of this, just look.

 

[Shipper]

Pretty common trait with any carbureted motor that lacks automatic choke and high idle boost. There are many threads on this issue and you'll find them if you do a few searches. May be normal, maybe not, more information needed. Bottom line: if you're using OE breaker point ignition or an electronic ignition that employs the OE advance governor, the advance rod may need lube and/or the bobweight springs in the governor may need attention. The problem may also be caused by a vacuum leak between carb boot and head, boot and carb, carb and airbox. Plenty of material on all of this, just look.

Thanks guys, gave me some starting points. One question, if it’s a vacuum leak, wouldn’t it run high on start up? This only happens once I’ve run it and it doesn’t matter how far I go - short or longer distance.

Thx

 

[Shipper]

PS, it’s original own points.

 

[Shipper]

OEM

 

[grizld1]

Expansion due to heat often opens a leak that seals up when the motor cools. Had one like that that drove me batty some years ago; took off the boot, replaced the gasket--no joy. Took off the boot and applied sealant to the gasket--no joy. Took off the boot, replaced the gasket, applied toughass sealant to the gasket--no joy. Took off the boot and did what I should have done to begin with, checked the flange. I don't know how he managed to do it without stripping the threads, but some sorry SOB of a PO had cranked down on the carb mount screws so hard that the edges of the holes were extruded. Dressed the flange carefully with a fine file carefully checked for flatness (too many "flat files" are anything but). No more leak. Same goes for the ignition rod. If you haven't lubed it, do it.

 

[Superjet]

I just went thru this exact same problem. Ended up being throttle shaft seals. Start your bike up and spray carb cleaner at intake boots at both engine and carb sides. Spray throttle shaft ends on both carbs bodies and if you hear idle drop....bingo! I had new carb boots and had clamp ends touching and as soon as the spray hit the ends of the throttle shaft dropped off. Rebuilt using oem seals and new allen bolts (thanks again Robinc). Works great now!

 

[Shipper]

So, ya - tightened down everything I could access, clamps on intakes, clamps to air boxes etc., ran good first time out but wasn’t long before high rev came back at idle. Sounds to me like an air leak I can’t access without tearing down carb/air box/ intake gaskets.

 

[grizld1]

Don't assume, inspect. There are many ways to check for air leaks and many threads detailing them. I used to use bursts of carb cleaner, but I like gggGary's method better--gas from an unlit propane hand torch will cause a change in idle speed when it hits a leak.

 

[Shipper]

Thanks G,

Good advice. Will do some reading tonight. Like the propane idea.

 

[Jim]

Don't assume, inspect. There are many ways to check for air leaks and many threads detailing them. I used to use bursts of carb cleaner, but I like gggGary's method better--gas from an unlit propane hand torch will cause a change in idle speed when it hits a leak.

Add electrical contact cleaner to the list of stuff that works good. It's flammable, so you get an increase in revs when it gets sucked in.... and it evaporates quickly with absolutely no mess or residue.

 

[Shipper]

ORRRRR! Remove and replace the advance weight springs. Reading threads is invaluable. Running like new again - unbelievable. Thanks to all who replied for the valuable tips - as always, much appreciated.

 

[grizld1]

Congrats on the repair! (Hope you also lubed the advance rod.)