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!
 
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.
 
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
 
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?
 
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.
 
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.
 
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.
 
Yeah, I forgot on some you can see the advance and weights without pulling the points or electronic ign plate
 
Sorry for the late reply I was away for work, oh OK I'll check if the mechanical advance works properly then! and probably give it a nice good clean and lube anyway!
I'll let you know, I'm just away from the bike right now and can't test this right now...
 
Sorry I had so much work in the past months I didn't had any chance to go back to the workshop to work on the bike before, but I just did.

So I pulled the carbs out again to check the accelerator pump, I noticed the the accelarator pump return spring (on the outside) wasn't correct so I though this was the issue, I replaced it with the correct one (much stiffer) but it didn't fix the problem :(

I also pull the advance mechanism out to check and clean it with some WD40, but it didn't changed anything either. It was also pretty clean to begin with.
I've check for vaccum leaks by spraying some carb cleaner all around at idle and couldn't find any vaccum leaks again...

I have no idea what to check now.. :( I spent a lot of time trying to restore this bike as much original as possible but still can't fix this...

The previous owner was doing some racing, in the 80's with the bike probably (since it sat for 20 years untouched) and did many mods like : removal of starter, center stand, adding a steering damper, etc. So my guess is that he was a serious racer back in the 80s.

IMG_20211230_170413.jpg


I also noticed that the engine has been opened, and probably either rebuilt, or prepared in some ways, since I can see traces of blue gasket on the bottom end. So maybe he did a kind of engine modification on either the pistons, cylinders, cams, cranks.... which would explain why it idles so high ? I have kept the stock jetting...

Sign that the engine has definitely been opened:

IMG_20211230_170419.jpg


Also, when I got the bike, I guess that he was doing some work on it, it only had 1 shock, no cover on the points.... the story I got was that the poor guy became sick decades ago and couldn't used the bike anymore and it just sat for 20+ years.

Here is a video that clearly shows the problem when the bike is warm: https://1drv.ms/v/s!Auu8yAfGtqmfga9TQGWygSn-pYRg1g?e=gfCxxK

It idles perfectly fine until you give it some throttle then it gets stuck a 3500 rpm


IMG_20211230_174247.jpg
 
Excess RTV is not good. A bit can come off into the oil stream and then block flow to a component. Someone got carried away with it.

I had a '79 CB750F, but the changes to DOHC made it so my experiences wouldn't be much help. Certainly check that the high idle ramp on the choke mechanism isn't catching.
 
Yeah the guy sure got carried away with RTV, hope it is not the same on the inside...

Yeah the DOHC are quite different. Choke ins't catching I have checked it as well.
 
A possible cause of the hanging idle is weakened springs on the mechanical advance. It is not uncommon to have to cut one loop (coil) off a spring and bend the next coil up to make a new hook on the end. You can run the engine with a timing light on the timing marks to see how soon the advance kicks in and when it comes back out. A little research on the sohc forum should reveal rpm for advance to come on.
 
Back
Top Bottom