I just completed rebuilding the motor and carburetors. At cold idle, the engine idles at 1500 RPM. After the engine warms up, it idles at 3,000 RPM. I did the carb spray cleaner around the carb boots and retightened the band clamps. These are new rubber boots. No improvement. I verified the sync port screws were in position. While at hot idle, I can press down on the #3/#4 carb throttle tab and it returns to 1500 RPM. Once I increase the throttle, it returns back to 3000 RPM idle. My thought is as the motor heats up, the carbs positioning might change slightly causing the throttle plates to bind. I lubed the throttle cable. Has slack and needs replacing. The choke is in full open position. The Honda dealer suggested an intake valve could be not sealing completely when hot. I did sync the carbs. I keep coming back to being able to get the idle down by pressing on the carb throttle lever. Any thoughts?
You need to get the idle down.
What year are the carbs.
Can you take a photo of the top of the carbs with the top covers off?
I am betting the locknut and set screw are wrong.
Did you stick some drill bit or something under the slide to bench sync them?
The slides are just open too far OR you are running pods and you did not change the idle jet.
That will cause the idle to be high from a lean condition.
I have had this problem before. First try adjusting the air/fuel (depending on which type carb you have.) If that does not work, turn up your main idle adjuster screw, and re-sync the carbs bringing the idle down with the sync screws. This has worked for me in the past.
TOOLS
Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."
1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
While at hot idle, I can press down on the #3/#4 carb throttle tab and it returns to 1500 RPM. Once I increase the throttle, it returns back to 3000 RPM idle.
This sounds like a good clue. Can you figure out what's causing this to happen?
carbs are stock for 1979. I rebuilt all four carburetors. Stock jet sizes. I'm running the stock air filter box. Problem only occurs after engine is warmed up.
Two other engine problems noted that may/may not be related to this issue.
Small oil leak from the front left corner of the cam cover or the tach cable connection.
After engine is turned off, I can see a faint trail of smoke from #1 muffler. Since I purchased these mufflers used, the previous bike might have had a problem with #1 cylinder and this is the residual oil burning off - or my #1 cylinder is running too rich - or I need more break-in time.
There's a thread over on the DOHC forum about this. The carb bank was re-assembled just slightly skew, not enough for the throttle plates to bind when cold, but they would when the engine was hot.
The guy just relaxed the screws and retightened and the problem went away.
Yes they did. I noticed as I tightened the two rail plates, at lease one of the plates would stick. I loosened and retightened in different sequences several times until they all operated ok. I think I'll try loosening the rail screws to see if it helps.
There's a thread over on the DOHC forum about this. The carb bank was re-assembled just slightly skew, not enough for the throttle plates to bind when cold, but they would when the engine was hot.
The guy just relaxed the screws and retightened and the problem went away.
My thoughts exactly.
Oh, and don't take your bike to THAT Dealer... Intake valve sticking when hot causing fast idle?!?
81 Honda CB750C - Current Project
67 BSA Spitfire MkIII - Next Up (Full Resto)
81 Honda GL1100 - Bob / Cafe´
80 Suzuki GS750L - Bratstyle
72 Honda CB450K5 - Basket Case
73 Honda CB350F Cafe' (Gone but not forgotten)
Don't wait for opportunity to knock... kick the door down and drag the old harlot in!
I had the same problem with my '74. My problem was not listed among any of the things to look for when troubleshooting though. Mechanic spotted it and showed me....the carbs had fuel in them when stored, the normal varnish build up was there, but also some moisture must've gotten in there, because some parts of the casting in the carb had corrosion (or suffered the effects of electrolysis) and there were fissures in parts of the casting from that which allowed excess fuel to get where it wasn't supposed to after warming up. A new set of carb bodies from ebay solved my problem. (which was IF you could get the idle smoothed out, it was no lower than 1500 RPM, and after warming up it wouldn't idle below 4200!)
(mechanic held a flashlight up to parts of the stripped down carb bodies, and you could see light coming from what SHOULD be solid parts of the carb body.)
So I'll add....
There may be just one more thing to look at. :)
Mark Davis
Palm Beach Gardens, FL
amateur photographer, hot rodder, motorcyclist, adventurer
"Four wheels move the body. Two wheels move the soul."
I had the same problem with my '74. My problem was not listed among any of the things to look for when troubleshooting though. Mechanic spotted it and showed me....the carbs had fuel in them when stored, the normal varnish build up was there, but also some moisture must've gotten in there, because some parts of the casting in the carb had corrosion (or suffered the effects of electrolysis) and there were fissures in parts of the casting from that which allowed excess fuel to get where it wasn't supposed to after warming up. A new set of carb bodies from ebay solved my problem. (which was IF you could get the idle smoothed out, it was no lower than 1500 RPM, and after warming up it wouldn't idle below 4200!)
(mechanic held a flashlight up to parts of the stripped down carb bodies, and you could see light coming from what SHOULD be solid parts of the carb body.)
So I'll add....
There may be just one more thing to look at. :)
Mark Davis
Palm Beach Gardens, FL
amateur photographer, hot rodder, motorcyclist, adventurer
"Four wheels move the body. Two wheels move the soul."
If you reply to this email, your message will be added to the discussion below:
Current Status June 21:
Removed all the carburetors. Inspected all the boots. Look good. Ordered new o-rings for the fuel connecting tubes. I had to cut a slot in all the choke plate screw heads to remove them. Ordered new screws. Disassembled the carburetor pistons. Discovered some gaulling on the surfaces. Removed using 1000 grit paper and polished using a dremmel. Discovered the large seals were not installed consistent. Some had lip facing up and some down. Installed all seals so the lip side laid against the carb housing. Awaiting receipt of the o-rings and screws before I can reassemble and try.
Took the extra time to do some cleaning and polishing of the carbs. I bought a new Dremmel and really love it.
Those rings being upped around might have been the issue that caused your high idle.
Am i seeing Allen head screws everywhere? Those aren't stock and are a common upgrade, so it looks like a Po at least tried to take care of those carbs.
Why did you need to cut into those choke plate screws? Was the Allen head stripped?
Sent from my BlackBerry device
From: RodneyCB750K1979 [via Honda CB750'S]
Sent: Sunday, June 23, 2013 3:31 AM
To: seestheday
Subject: Re: Engine High Idle
Current Status June 21:
Removed all the carburetors. Inspected all the boots. Look good. Ordered new o-rings for the fuel connecting tubes. I had to cut a slot in all the choke plate screw heads to remove them. Ordered new screws. Disassembled the carburetor pistons. Discovered some gaulling on the surfaces. Removed using 1000 grit paper and polished using a dremmel. Discovered the large seals were not installed consistent. Some had lip facing up and some down. Installed all seals so the lip side laid against the carb housing. Awaiting receipt of the o-rings and screws before I can reassemble and try.
Took the extra time to do some cleaning and polishing of the carbs. I bought a new Dremmel and really love it.
Removing the choke plates
If you reply to this email, your message will be added to the discussion below:
I installed the choke plates last month using hex sock head s.s. screws and thread lock. When I went to remove them, both of my allen tools rounded off and the sockets partially rounded out. I tried to find some brass screws but was unable to find any. I ordered some s.s. phillips head screws. This time I won't use Thread Lock but will brat the ends instead.
I have never done anything besides just tighten the choke plate screws. Never have had one come loose.
TOOLS
Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."
1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
The DOHC carb cleaning manual recommends thread lock. I did, but I haven't had to take them out since I put them in. I'm hoping that mine won't cause me troubles when/if I pull them.
They will. Guaranteed. I even heated the screws with a torch and had no effect on Thread Lok. These small screws just don't have large enough heads to apply the necessary torque to break the Thread Lok bond. I suggest if you must use Thread Lok to try slotted screw heads. Once I cut a slot in the screw head, i had no problem loosening the screws using a flat blade screwdriver. I always have problems with small phillips head screws rounding out.
Locked
