Engine Idling high on its own cb750k 1977

cb750k 1977

Hey guys my name is Byron .. I'm new here and I am doing my first project but in having some trouble for a few weeks. I don't work On the bike every day I only do it once a week

I bought this bike and it was running ok. Gas was coming out of the float bowl so I decided to clean the carbs. I took them apart and cleaned them. The only things I replaced were the plastic gasket for the float bowl and the gas stop needle. Installed everything on the bike again, the Bike turned on ( I only bench sync the carbs) and I drove it.

Sometimes it did not accelerate as normal when pushing the throttle. It was moving but not as it should. I rechecked the carbs and I adjust the mix screw to 1 and 1/2 from the sit in position. On all of them it seems that it fix it. My friend let me borrow his brothers motion pro carb sync and after it got warm up. I checked that the screws in the tool were out to prevent the engine to suck up the liquid. I try to calibrate the tool with carb #2 but it almost sucked the liquid.

So i decided to buy a vacuum gauge tool to sync the carbs thinking it would be safer but now the bike revs high on its own I can not even try to fix the gas/air mix screw to keep it at 1000 rpms because it revs high on its own. Watching Some YouTube videos i decided to test if it has a vacuum leak I sprayed some carb cleaner to the boot connecting carb #1 to the engine and there was no change on the idling but 30 seconds later I did the same thing again and the revs went high. I retested and the same carb and idling did not change.

I test the other intake boots and same thing happened, sometimes it was revving high sometimes it did not. so I'm not sure if there is a vacuum leak or not ??

A week later after I though maybe syncing the carbs will be the way to go. I warmed up the bike to sync the carbs. I was removing the air screw to connect the adapters for the carb syncing tool ( while the engine was running, idling was about 1000 rpms) and just as I was taking the screw off the idling went high to 3500 rpms. The Screw was still in. I waited for a few seconds but the idling did not change, only After i took the screw off the idling went down.. Then I installed the adapter for the sync tool and the idling went high again Any ideas why?? If I change the intake boots Would that resolve the issue ?? and the issue persists what would be the next step for me to take a look, replace or clean??

Thank you in advance

It sounds like you're on the right track with a vacuum leak. Check that there are crush washers on your vacuum port screws and they are tightened snug when you aren't syncing the carbs. Chances are, if a carb boot is the culprit it will just be one, and not all 4. If you can make the motor surge by spraying fuel on one of the boots, try swapping that boot to another cylinder and test again.


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Thanks a lot.
A have another question how does the vacuum gauge work? I attached the picture of the one I have but I don't understand what all of that means ... Thanks Byron

got the same carb sync gauges ( bank of four ) don't need to worry too much about all the facts n figures on the gauges just get your carbs to run all at the same point on gauge

Concerning the air leakage:

There are little gaskets in the carb spindles that can leak air...needs a full rebuild to get them replaced.
Check if your mixture screws are intace...they are pointy and tend to break and get stuck into the orifices.

Thank you for all the help bros


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Update
Hey guys I bought the carb boots and the engine rev on its own again .. I checked ifor leaks but did not find any with the new carb boot. I started to read and looked at other people builds and saw that the throttle valve that i have is gray. I will attach a picture of it. Other people have those thottle valve so shine ,,, you guys think that maybe the throttle valve is getting stuck??? If that is the case should I sand them with a fine sand paper or there's any way to make them shine again??
Thanks a lot

DO NOT sand the slides. Slides are either bras and chrome plated or aluminum and anodized. Some are anodized silver or gold and others gray. That appears to be what you are looking at.

A the slides actually sticking or is the stiction in the linkage above the carbs. If the slides are sticky, try to ascertain why. The bodies may be distorted or may just be coated in sticky fuel residue. Use acetone to clean them but do so with good ventilation.