Hanging throttle/high idle

ChickenKnees

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Edit: originally posted this in SOHC carb thread by mistake - not sure how to remove that post.

Hello all!
I have been racking my brains on a recently purchased 1981 CB750K, with the VB42 carburetor set up; original air box and original breather tubes.

The bike was sold to me in 'as is' condition, with the knowledge that it had a sticking throttle. The real issue is, is that the engine will start and idle great, both cold and at operating temperature.
As soon as the throttle is opened up past 2000rpm and let go, the idle speed stays anywhere from 2500-4000rpm. The only way to bring the bike back to the ~1200rpm idle is to push back on the throttle connecting rods momentarily, thus closing the throttle plates.

I have read and learned a lot about this issue, both on this site and from others via Reddit. The only reasons as to this issue is either throttle valves sticking, or vacuum leaks.

Now, I have had a specialist Honda Technician pull my carbs apart, replace Idle air control valves, check plates, jets, alignment, clean the carbs, lubricate lifters etc etc. I am fully sure this is not a fuel/air or throttle valve issue.

What I have now discovered after 3 dissasemblies is that there is infact a complicated vacuum leak between the carbs 2 & 3, where the rod for the throttle valves run through the carburetor body. This is on both inside sections of the carbs, where the threaded nut (on carb 3) and the main return spring (carb 2) are located. Behind these are a lock washer, a silver color or tin washer, a plastic washer and then the copper tubing with felt gasket material. I am sure this felt has worn away as carb cleaner does leak through and raise the rpm.

What is the solution to fixing this? Are there any kits to replace these and what is the best practice?
I appreciate any tips and tricks into fixing this issue.
 

fg1972

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Hi there, I feel your pain. I think I dismantled my carbs at least another two times after fully rebuilding them and I did replace the felt throttle shaft seals which don’t seem to be an available replacement part however there is a helpful guy (Aaron) that makes them. Send an email to him at dude@dudesgarage.com for more info.


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ChickenKnees

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Thank you FG1972 and Travis for your help!
I managed to track down some felt gaskets, as well as all o-rings and washers for a rebuild. After completing the rebuild I still had the same issue.
None-the-less, I decided not to input anymore time or money into the Keihin Carbs and instead ordered a set of Murray's Mikuni VM34 2 into 4 carb replacements. A little pricey but I think it should add more to the aesthetic of the bike and hopefully let it run (and idle!) more smoothly. I will post a update once they have arrived and are installed. I may have a set of VB42 carbs to sell afterwards for parts if anyone is interested.
Cheers.
 

dirtdigger

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I would have gone with a set of 4 individual carbs. The 4 into 2 will be down on power compared to the 4 individual carbs. The 4 cylinder bike motors like individual carbs, if they ran better with two honda would have done it from the factory.
 

deeveeance

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Thank you FG1972 and Travis for your help!
I managed to track down some felt gaskets, as well as all o-rings and washers for a rebuild. After completing the rebuild I still had the same issue.
None-the-less, I decided not to input anymore time or money into the Keihin Carbs and instead ordered a set of Murray's Mikuni VM34 2 into 4 carb replacements. A little pricey but I think it should add more to the aesthetic of the bike and hopefully let it run (and idle!) more smoothly. I will post a update once they have arrived and are installed. I may have a set of VB42 carbs to sell afterwards for parts if anyone is interested.
Cheers.
Any update on this? I'm having a similar problem with idling and already disassembled the carbs 3 times and getting a bit impatient.
 

Tomos

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It appears that problem is with carb cables or throttle system on a handle bar.Cleaning whole throttle system may help with a new cables installed. After carbs are reassembled bench sinhronization is in order before installing carbs beck on the bike.Then proper adjustments can be done.
Hope this helps,Zoran
 
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