Immediately Jumps to 6K RPM at Startup (After Carb Cleaning)

Hi guys,
I just cleaned the carbs, replaced the carb holders and fuel hoses, then put them back onto the bike. I set the idle to approximately 2.5-3 turns out as per the manual instructions. The bike started right up and immediately jumped to 6k RPM and slowly increased. I cut the choke, that didn't effect the idle rate. I turned off the bike, checked the throttle cable to see if it was getting abnormally pulled, and it seemed normal. Started the bike again, jumped to 6K so I turned the idle screw from 2.5 to completely closed, the bike still held steady with no effect. I'm kind of stuck to what is causing the bike to not respond to the choke or the idle screw. Any suggestions?
Thanks,
Remy

 

Welcome Remy,
Did you "bench sync" the carbs before you re-installed? There could be issues with the sync adjustment screws or something could be binding in the linkage.

 

I had that when I put my carbs back together one of the three or so tiems I pulled them apart last year. Did you do a bench sync on the carbs? In my case I'm pretty sure one or two butterflies WAY out of adjustment pegged the RPMs when I started her up (I had NOT adjusted them on the bench - who am I foolin'? I don't HAVE a bench ).

Any other ideas?

Oh, to be sure it's not your throttle cable, you could disconnect it at the carb bank and make sure its fully closed. The adjustment on the handlebar side is PRETTY twitchy if I remember correctly.

 

Re: Immediately Jumps to 6K RPM at Startup (After Carb Clean

Check the carb boots themselves for cracks, and also where they seal onto the carb bodies, for airtightness.

What you need to do is run the motor, and spray WD40 or carb cleaner onto the boots. If you have air leakage, the revs will zoom.

It's gonna be nervewracking though, if your revs are shooting up like that anyway... you need to eyeball them at least, take them out of the equation if they're perished. Some people here have managed to patch them up.

It could also be the little rubber seals on the butterfly shaft - they are in the body of the carb, one on either side of each butterfly. That was my problem for exactly the same symptoms.

Search for the ''throttle shaft seals'' threads here. You can get replacements, but they're quite flimsy compared to OE, and don't inspire confidence. Some people have used quad seals and HTV instead, which is messy but do-able.

Good luck.

 

yeah I had that problem once, just adjusting at the handle bar fixed it, I had screwed the cable nut out too far and it caused the idle to race.

 

That or the bottom adjustment is set too high. Cable not properly in the lower holder or caught up somewhere. Go back and check the obvious again.

 

Re: Immediately Jumps to 6K RPM at Startup (After Carb Clean

So I just removed the carbs and bench synced them using feeler gauges to match the "smile". Then I reinstalled them without connecting the throttle cable to see if that was the cause. I have the fuel tank removed, but placed higher than the carbs on a stool, with an extra long fuel hose connecting it. Started the bike, immediately displayed the same behavior of jumping to 6K RPM. So it is definitely not the throttle cable that is the problem. Also it doesn't seem to be bench syncing either. It idled at a normal rate prior to cleaning the carbs. Any other ideas?

 

Re: Immediately Jumps to 6K RPM at Startup (After Carb Clean

I just had that problem. The cause in my case was that I had let some gas get into the crankcase, it was being sucked back into the airbox via the vent tube, and was promptly being drawn into the carbs causing the engine to race up to 4000.

Extra rich mixture FTW!

 

Did you get the jets in the correct spot. Pilot up front and main in the middle. Also did the emulsion tubes get fully seated. They can be installed so they are not fully seated.

 

Re: Immediately Jumps to 6K RPM at Startup (After Carb Clean

You talked about mixture screw, how about the idle screw.

When I bench synced my carbs I eyeballed them. Could just barely see a light under them. Idled at 2K rpm until I backed the idle stop screw down.

 

Re: Immediately Jumps to 6K RPM at Startup (After Carb Clean

Got It! Removed the carbs again. Re-Bench Synced again. Reinstalled everything again. Started up at 6k again! Then I turned the idle screw back AT LEAST 6 or 7 times, when it decreased from 6K to 5K. Kept opening the screw until it idled perfectly at 1200-1500 RPM. Must have turned it ten times at least. Either way, starts and runs perfectly now. It's a little odd that I needed to turn the screw that much, but at least it is stable and ridable around the block so I can warm up everything at an even pace to change out the fluids. Thanks to all for their patience with my newbie self. This forum is awesome.

 

Remy ...

12-1500 rpm is not Idle.
That's actually Off-idle and too high.
Lube the Spring and threads of the Idle adjuster and bring the Idle down to 1000 rpm for now.
Set-up the Pilot Mixture Screws to give you the best 1k Idle.

When you get 1,000 rpm's Dialed-in ... we'll tweak it sick!

 

Re: Immediately Jumps to 6K RPM at Startup (After Carb Clean

Copy that. Tomorrow when I'm changing the oil and drive shaft fluid, I'll bring it down. Once I have it all prepped and fresh, then I'm going to have to start figuring out how to get it synchronized and idling perfect. I'll search that though prior to taking up any more space on this board. Thanks again for everything.

 

Hey, man ...

Take MORE space!

Use the "Signature" feature to profile your bike ... like "thumbs"
________________________________
Yr Mod Miles Stock YICS Y/N? What your fixing.

 

Cool it turned out to be something simple. For the idle speed you need to know the idle as different engines have different idle speeds. Mine is set to exactly 1050 as per the XJCD EFI specs. The little 550s rev higher at idle than the 650 and 750s.

If it's hunting round 1200 to 1500 it needs to be sync'd so it is smoooooth and does not hunt (as much).
Then colourtune the hunt away for good.

Purrrrrrrr!

You think it's idling perfectly now just wait till it is spot on .

See the FAQ for a couple of methods to sync 'em and search "Rick's free YCIS tool for all" thread for his string and MMO YCIS tool.

Rick, have you thought of a name for your string YCIS tool?

 

Yeah the 550 is a little higher. According to the Clymer manual and the factory manual it is 1200rpm for idle.

 

No. I haven't named it.

I haven't had much ... just sit and think ... time.
It's prime riding time,

 

Re: Immediately Jumps to 6K RPM at Startup (After Carb Clean

Samsr, can you tell me a bit more about the underlined statement above? Every time I've put the tubes in, they just fall into place and a small push with your finger "pops" them down into their seat......is there anything else special that I'm missing? There isn't any front/rear directional index or anything, is there?

 

They sometimes need a bit of persuasion to get back-in to the fully seated position.

The washer and Main Jet will hold it in position ... just fine!