1978 CB750 F2 SOHC - Carb issue

tahitianrider

Been Around the Block
Hi,
I am in the process of restoring a 78 CB750 Super Sport, the last SOHC model of the 750 fours.
The bike sat in a barn for about 20+ years but was in good condition. I completely rebuilt the carbs: tore them apart, did ultrasonic cleaning on every carb body and parts and rebuild everything with rebuild kits (new seals, jets, etc.).
The bike restarted right away! I then synchronized the carb using vacuum gauges. They are perfectly synchronized!

However I am have a issue I can't resolve: when the bike is warm, after a 5min ride. It does not want to go back to idle, as soon as you rev it up, in stays up around 3500 rpm! it doesn't go back to the 1200 rpm idle, unless you put in it first gear and gently let go the clutch while braking to try to stall the engine.
I was almost sure this was a carb sync issue, I had the same problem on a FJ600 and solved by re-sychronizing the carbs. But it is not, I did another carb sync with the bike very warm and the readings are still perfectly the same on each carb!
There is no vacuum leak either, I spray some carb cleaner around every carb and intake boot while the bike was running and it did not rev up.

Is there another thing to check on the carbs? maybe the accelerator pump is sticking?
Could this be an issue with incorrect valve clearances?
Thanks for the help!
 

Maritime

Over 10,000 Posts
When you set the idle speed, was the bike fully warmed up? It sounds to me like you set it to idle when cold and after a warm up its too high. Reset it after a full warm up and see if it stays. The old honda should need choke to start cold and should need it until fully warmed. If you dont need choke and it idles right away your set wrong.
 

tahitianrider

Been Around the Block
When you set the idle speed, was the bike fully warmed up? It sounds to me like you set it to idle when cold and after a warm up its too high. Reset it after a full warm up and see if it stays. The old honda should need choke to start cold and should need it until fully warmed. If you dont need choke and it idles right away your set wrong.
Yes the bike was fully warmed up. If I try to reduce the idle speed while it is warm, or hanging at 3500 rpms, it stalls. The bike won't start without the choke when cold either
 

Maritime

Over 10,000 Posts
Yes the bike was fully warmed up. If I try to reduce the idle speed while it is warm, or hanging at 3500 rpms, it stalls. The bike won't start without the choke when cold either
OK, so that rules stuff out. You mention the accelerator pump. that could be sticking open. Check valves but not sure they would cause a high idle if out of spec. It's got to be a fueling issue. Float height could be an issue. Check they are in spec. To high and there is too much fuel at idle in the bowls?
 

tahitianrider

Been Around the Block
OK, so that rules stuff out. You mention the accelerator pump. that could be sticking open. Check valves but not sure they would cause a high idle if out of spec. It's got to be a fueling issue. Float height could be an issue. Check they are in spec. To high and there is too much fuel at idle in the bowls?
I'll check te accelerator pump and float height. I did make sure that float height was right though when rebuilding the carbs.
 

Maritime

Over 10,000 Posts
Pull the points, remove the plate and twist the advance. It should move freely and snap back when released. If it is sticky and stays moved, remove it, clean and lube it good until it moves freely and put it back after.
 

CarbsAndCylinders

Careful With That Axe Eugene
I forget how it is set up on your bike but if you can run the engine with the points cover off and see the mechanical advance, then raise the rpm, watch the weights move and make sure they move back to the original position after you let the engine return to idle (or it's high idle). You should raise the rpm until the weights advance the timing completely. Otherwise, you have to do what Maritime said.
 

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