Topic: Runaway RPMs after Carb Rebuild - CB550

[Texas Ty]

Howdy SOCHeads,

I rebuilt the carbs on my '76 550. Finally got everything reassembled after re-jetting for UNI filters (105 mains). Upon cranking the bike up the RPM rapidly increases and I've got to shut the engine off to keep it from redlining. Any ideas what could cause this? My suspicion would be needle adjustment but wanted some input before I start digging away at the carbs again.

Thanks in advance,
Ty

[ekpent]

Yes it sounds like the slides are set to high. Can you lower them with the idle adjustment.?

[Texas Ty]

Negative. The idle screw is unable to adjust them to the necessary level. Do you think the jet needle needs to be lowered a notch or two?

Cheers,
Ty

[ekpent]

I am a 750 Guy mostly, different carbs. Sure a 550 person will chime in soon who knows more than I on that model on how to lower the slides and get your idle adjustment back.

[Texas Ty]

Copy that and thanks. Another thing I suppose could be a leak in the boots causing air to get in?

I finally got the idle to settle for a minute but as soon as I gave it some gas, even after letting off the throttle, the vacuum caught and the RPMs took off again. Would jet needle adjustment even explain this or does it sound like the whole slide needs to move? Kinda thinking out loud here as I'm out of ideas.

Ty

[TwoTired]

Almost certainly you've screwed all the individual slide adjusters to position the slides too far from the carb throat "floor".

Did you synchronize with a drill bit?  This is the usual cause for the symptoms you describe.  You now have a do-over ahead of you.

You did synchronize the carbs, didn't you?


People who recommend the drill bit method should have their foreheads thumped. imo

[Texas Ty]

Ha! I sure did use the drill bit method. Looks like I'll be biting the bullet on the gauges or trying to find some to borrow...

Is 1/16" just far too large an opening for idle, or is it just the imprecision of that method that allows the vacuum to get out of hand and cause the unwanted revving?

Thanks TwoTired.
Ty

[TwoTired]

The non PD carbs do not have a master carb, so you get to assign one.  Pick a carb, any carb, back out the idle knob so that it can't hold any slides open.  Adjust the individual slider for the "master" carb so that the slide can rest on the bottom of the carb throat floor.  Lock down its adjuster and never move it again thereafter.  Now adjust all the other carbs to the master slides position for a mechanical sync of all carbs.  I use light (light bulb) so they all rest on the floor blocking light.  You could even use a drill bit if you must.  Use the idle knob to position the master slide open just enough for the bit to slide under it tightly.  Then just all the others so the drill bit has the same slip fit as the master.

You will still need to vacuum sync after the engine tappets are properly set and the engine is running.  Just don't mess with the master carbs' slider adjust.  Match the other carbs vacuum to whatever the master has.

[Texas Ty]

Thank you for the info TwoTired. Will try in the morning and report back.

[Duanob]

Did you put the return spring back on? Just curious, high idle is one thing for vac leaks and out of sync carbs but redlining is a bit of an extreme condition.

[Texas Ty]

Yessir return spring is back on. About to head to the shop and re-sync.

[Stev-o]

Ty - I have sync gauges you can borrow, I synced my 550 last weekend...

[Bailgang]

OK this is going to sound stupid but I have to ask. When you bench synced the carbs with a drill bit did you back off the idle speed screw once you were done? I bench synced the carbs on my 550 once with a drill bit but forgot to back off the idle speed screw once I was done, when I fired up the engine the revs shot right up, not necessarily to redline but high enough that it had me scrambling to flip the kill switch.

[Mr Freeze]

The needle position isn't going to affect the idle and make it runaway, do what TwoTired suggested as far as syncing and also check for air leaks around the carb boots.

[Texas Ty]

Got called in to work today so didn't have a chance to work on the bike. Gonna go back tomorrow. I have a suspicion that I've made the simple mistake Bailgang pointed out. It's funny what you can overlook and freak out about after 10+ hours in the garage.

Stev-o, I'd love to borrow those gauges. You're in Austin, correct?


Routinely amazed at how helpful the members of this forum are.

Thanks y'all
Ty

[Stev-o]

Stev-o, I'd love to borrow those gauges. You're in Austin, correct?

Yes, live out by Lake Travis but work in Austin.   

[KeithB]

Just did the same thing to the same bike and had the same problem.
"Bench synch" didn't work out so well.
Took the time to vacuum synch and all good now.
Double double check all boots are tight and no leaks!
Don't ask......

[Texas Ty]

Stev-o kindly lent me his sync gauges today, so I'm going to vacuum sync first thing in the morning. Redoing a bench sync with TwoTired's advice helped a lot. Excited to see how i can get it with the gauges.