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Hey fellas!
I'm a new, proud owner of a 1976 Honda cb750K cafe. I've been waiting for 2 years to get one and I finally did it! Like any old bike though, these need a little work to get running like a top. The issue I'm having is getting the bike to return to idle quickly. It also sticks at a high idle sometimes at stoplights and flicking the throttle doesn't bring it down, only slowly letting the clutch out to grab it does. Anyways, I synced the carbs yesterday and the powerband is smoother with a little increase in power. However, I think I may have a low compression motor as I took a readings with a crappy compression gauge that read around 80-90 psi for all cylinders. I suspect the gauge is bad since it didn't have threads but a rubber nipple end. Plus, it seems really low in that it has somewhat decent power. Compression for my year is 153-187psi right?
Thanks!
(By the way, what size threading are the spark plug holes?)
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Administrator
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Welcome to the forum. I am not sure what thread the spark plugs have, but your engine (K model) is not a low compression engine. Honda did drop the compression on the 77-78 F models. I would check for frayed cables, or sticking parts on the carbs.
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)
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Could it be improperly jetted? I hear that can cause the idle not to snap back quickly.
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Administrator
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Has the air box been changed, or any signs of the jets being changed?
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)
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It has aftermarket pod-style filters on it. Not sure if the original owner jetted it proper though. Any idea what size? 125?
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So I talked to the original owner again and the main jets have been changed. Don't know what to though. It's still returning slowly to idle but I noticed that its doing it only when its warm. It also sometimes idles really high at stoplights and I can't bring it down. Again, this only happens when its warm. When its cold, the throttle revs down quickly. I have also checked the throttle linkage and cables and everything is good, no binding or hangups.
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Administrator
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Can you turn down the idle knob when warm? I know people will use the idle knob when the bike is cold.
If you drop the bowls or at least 1, the number should be stamped onto the bottom of the jet.
The ride IS the adventure. The destination is just to get gas!
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Administrator
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I just had a thought  Check your timing advancer. It might be hanging up keeping the timing advanced when you let off the throttle.
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)
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Ok, so I pulled the float bowl from carb 1 and found the jet to be 120. I feel like the bike is running too rich due to the jet size.(Wet, black plugs) But since I have aftermarket exhaust and filters I feel like it shouldn't be. Isn't one of the common problems of a rich mixture is that it runs well when cold?
@Re-run I can't get the idle below 1000rpm when its warm(which is odd) and I haven't tried adjusting it when cold.
@TOOLS Where is the timing advancer located? I've already set the points and adjusted the timing with a light.
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Administrator
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It's on the end of the crankshaft behind the points plate. I think the stock jets are 105. Maybe you should try a 110.
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)
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M12-1.25 is the thread size. My 750 had a compression numbers in the 60-70 range. I'm going to overhaul it.
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Hi longshanks, I know this is an old post, but I just did a search and it came up.
I have a 76 F, which I turn the idle screw up until it gets warm, and after a few minutes it warms up and I have to turn it down for normal idle, otherwise it revs up to 3000 at idle.
I dont mibd mind doing that. Seems to work better than the choke.
But, like you, when it gets to operating temperature, the throttle wont return. Makes for annoying gear up changes and more than annoying stopping at lights.
Sounds very similar to yours and I kinda think it isnt cables or throttle return springs, but may more likely be a vacuum issue in the carbs.....just wondering if you ever found out.
Cheers
Pete
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I have the same problem on a 81 cb750. When the idle is up to high it opens the throttle valves. When you give it throttle and let off the valves are still open because the idle is set to high and it is pulling fuel from the jets behind the valve. I must have a clogged idle passage as well (two holes right in front of the throttle valve) because when i turn the idle down in stalls out. So the bike either hangs the idle to high or doesnt idle at all  . If anyone has a good way to clean the idle passage i'd love to hear it. Guitar string doesnt work... Would love to Ultrasonic clean but can't find anywhere that is not way to expensive. Seems like your issue would be fixed by using the choke until warm and keeping the idle down at normal.
1981 CB750C
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