Topic: Way high RPM after carbs rebuild

[Can550]

It's a 1974 cb550k with 8500 miles. Stock air box and filter

Carbs were pretty gummed up, removed, took them apart and had them ultrasonically cleaned (were reassembled by the the guy who cleaned). Not sure if he bench synced.

I put these back and bin fires up right away with choke. RPM ranges 1800-2200 but as soon as choke is released, it jumps to 7000-7500. While twisting throttle, it moves up but then comes back to 7000-7500 mark.

Pulled the filter out of the air box, placed my hand, lots of suction and engine shut off right away.

Removed throttle cables, no difference. Idle screw has no impact either. Not sure what to do. Would hate to pull the carbs out, wish something could be done while these are on the bike

Appreciate any help.
Thanks



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[TwoTired]

This symptom usually means the carb slides were adjusted so they can't reach the bottom of travel.

Gonna have to pick one and make sure the slide can hit bottom with the idle knob back out.  Then vacuum sync the carbs to the one you selected to reach bottom of travel.

[Can550]

Thanks twotired
I recall seeing cylindrical part in all 4 carbs was at about 1/4" open. I have installed the carbs already. Can I slide them down while carbs are still inbike ?


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[TwoTired]

Seems possible.  I haven't done it that way.  The issue is seeing when the slide has bottomed.  Might be able to "feel" it with the sync adjuster tension/resistance.

Give it a try.  Worst thing would be you have to take them off to do it.

I suppose the other issue would be if the slides were installed backwards.   The slide bottoms should have a cutaway on the filter side.  If reversed, you can also get RPM runaway.  This shouldn't happen if the parts for each carb were segregated from the rest during tear down.  But, if slide from the right got switched with slide from the left...uh oh.

Cheers,

[firebane]

Bench sync. Will do wonders.

[evinrude7]

Bench sync. Will do wonders.

+1

[arncruz]

check the idle adjustment screw.

[Can550]

Once again thanks twotired. I recall seeing a +1/4" gap at the bottom of sliders. I have a friend coming over with his sync tool to vacuum sync. Hoping that he will be able to fix the problem.

I relate very much to your diagnosis as the feeling is if throttle is twisted to 7000 rpm position whereas it is not.

Sliders are up high as much they would be with twisted throttle.

Useless adjustment screw is all the way out (not touching anywhere).



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[firebane]

Useless adjustment screw is all the way out (not touching anywhere).

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The screw doesn't work cause your slides are too high. You need to do a bench sync which consists of screwing the idle screw in about 1/8 to 1/4 then use the slide screws to lower them as far as they go.

If you don't do this no adjustments you make will matter and this is just a small part of the bench sync. Which can be done without removing carbs.

[Can550]

Thanks Firebane
I shall try doing so over the weekend and shall report back.

On the same note, I'll remove the four top covers of all carbs, release the bolts from bent tab, loosen all four bolts to drop the sliders to the bottom, tighten them back,screw in idle adjust screw by 1/8" and then fire her up to perform vacuum sync
??



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[TwoTired]

Thanks Firebane
I shall try doing so over the weekend and shall report back.

On the same note, I'll remove the four top covers of all carbs, release the bolts from bent tab, loosen all four bolts to drop the sliders to the

No.  You should be able the get the slides to the bottom just by using the sync adjusters.  That's probably why you have the problem to begin with. All four adjusters were "chased" for sync, eventually skewing them all too far from the bottom.
You have to pick a carb to bottom its slide (with the idle knob backed out).  Then use the idle knob to raise it for an idle speed.  Then use that carb as a master and vacuum sync the other three to your asigned master.

[Can550]

Many thanks TwoTired and all others.

I (with some guess work) managed to drop the rpm. As advised by you, used sync/ adjustment screws. Now stuck as no one and nowhere to find help for vacuum sync.

Since I don't have vacuum sync tool

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[TwoTired]

Seem you have two options.  Buy a carb sync manometer set.  Or pay someone to do it for you at a shop...if the shop will work on a bike this old.

Where are you located?

[Can550]

I'm in Brampton Ontario


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[TwoTired]

I was going to say bring the bike over to get it synced.  But, California is a hefty trip. (Dual meaning, that.)

[Can550]

Lol yes
On the same note, what is fair charge for a shop to sync these. I am going to contact few around. Not sure if it is safe to ride to shop ?


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[Can550]

I was going to say bring the bike over to get it synced.  But, California is a hefty trip. (Dual meaning, that.)

Much appreciated the offer/ thought. I wish it was not that far


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[flybox1]

Lol yes
On the same note, what is fair charge for a shop to sync these. I am going to contact few around. Not sure if it is safe to ride to shop ?


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1 hr labor...~$70-100

....or you could buy your own for the same price, learn to use it, and be able to properly care for your bike at any time 

[Can550]

Great idea
With help of this board and members like you in particular, I'm confident one can learn to use it properly

I'm considering to take my carbs off and do the 1/8" bit method bench sync for now. Kind of hard to do so while still in the bike


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[firebane]

Great idea
With help of this board and members like you in particular, I'm confident one can learn to use it properly

I'm considering to take my carbs off and do the 1/8" bit method bench sync for now. Kind of hard to do so while still in the bike


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You can do the 1/8 trick in the bike as I just did this myself.

[flybox1]

Great idea
With help of this board and members like you in particular, I'm confident one can learn to use it properly

I'm considering to take my carbs off and do the 1/8" bit method bench sync for now. Kind of hard to do so while still in the bike


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It can be done with the carbs on the bike, but for the sake of simplicity and accuracy, i'd suggest doing it on the bench.
Do it right, once....and never have to do a bench sync again 

[TwoTired]

Any particular reason you don't want to buy a carb sync tool?  Hard to imagine having a vintage bike and not working on it yourself.  They need frequent maintenance.

Even if you get the slides perfectly aligned across the 4, unless all four cylinders have exactly the same draw and efficiency, you won't achieve a proper vacuum sync, which is what the motor cares about.  Old motors don't usually have exactly equal cylinder efficiency among the four, but can still run wonderfully.

Driving the bike with unsynchronized carbs won't hurt anything mechanical.  But, it may cause you to emote expletives and require a high idle.

A "bench sync" is just a starting point.  It is rare that it translates to a proper vacuum sync.

Cheers,

[Can550]

I'm terribly sorry being so noob. If both sides of carbs are closed with boots on them?

I thought drill bit is inserted under the slide


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[Can550]

Any particular reason you don't want to buy a carb sync tool?  Hard to imagine having a vintage bike and not working on it yourself.  They need frequent maintenance.

Even if you get the slides perfectly aligned across the 4, unless all four cylinders have exactly the same draw and efficiency, you won't achieve a proper vacuum sync, which is what the motor cares about.  Old motors don't usually have exactly equal cylinder efficiency among the four, but can still run wonderfully.

Driving the bike with unsynchronized carbs won't hurt anything mechanical.  But, it may cause you to emote expletives and require a high idle.

A "bench sync" is just a starting point.  It is rare that it translates to a proper vacuum sync.

Cheers,

Hi TwoTired
No hesitation on buying a sync tool. Actually I would like to know which one should I?

It is just so that auto stores near me had only single gauge. Will be looking at more options next week. Just thought of bench syncing them meantime (to be ready for vacuum sync)


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[Can550]

What is the opinion on

Motion Pro 08-0411 Syncpro Carburetor Tuner


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