Carburation, valves or compression?

[Kike_tfs]

Good afternoon everyone. They would help me a lot if you try to help me with my problem. I have a 1982 CB750 KZ, RC01.
The motorcycle was stopped for 25 years when I bought it. I restored everything I could myself, but I did not open the engine.
I cleaned the carburetion myself, but the bike has the problem that it starts well, but once it warms up the idle speed changes and goes up little by little, that is, as it warms up I have to adjust the idle speed.
The bike has been in 2 different workshops. The first carburation was cleaned, adjusted with a vacuum gauge and the problem persists.
The second workshop in which he has been has gone further and in addition to cleaning the entire system again, they have adjusted the valves and have mistakenly changed the lowjet but the motorcycle continues to have the same problem, and now it fails at low revs .
I have measured the compression and it is not very good. When cold they give approximately 116 psi each cylinder and when hot 130 psi.

It does not seem to have vacuum leaks, which also, if it were so, would always give the same failure both when hot and cold.

Does anyone know what I'm missing?

 

[SenseAmidMadness]

How far does the idle go up?

Did you measure compression with the throttle held wide open?

Vacuum leaks often get worse with the engine hot.

 

[Kike_tfs]

If I regulate at idle when cold, when it warms up well it can go up to 2500/3000 rpm.

Compression measured both hot and cold with open throttle

 

[zhill22]

quick question. What type of air filters are you using? Are you using the OEM airbox or are you using pod filters?

 

[Kike_tfs]

Original Airbox...

 

[SenseAmidMadness]

You should, in this order:

Remove the carburetors and swap back to the original jets
Make sure the carburetors are thoroughly clean again
Thoroughly examine all of the rubber parts that hold the carburetors in place and make sure the band clamps and screws and bolts holding them are tight
Make sure your throttle and choke cables are hooked up correctly

Rising idle like that sounds like a vacuum leak on the engine side of the carburetors to me, because those get worse as the engine temperature rises. You might have multiple issues, though. Did the shop that did the valve adjustment tell you what valve specification they were aiming for?

 

[Kike_tfs]

They have tried spraying the carbs on the outside with flammable spray to look for vacuum leaks but found nothing. The mechanic told me that the valve adjustment he made was the one he read in the motorcycle's workshop manual.

 

[SenseAmidMadness]

So firstly, the valve clearance in the factory service manual is too tight for long-term use. The recommendation is that it should be closer to .12mm or .13mm. The tighter valve clearance specification listed in the manual, which has a minimum of .075mm, can lead to burnt exhaust valves over time.

It sounds like your carburetors have an internal problem. You may have a ripped accelerator or cutoff diaphragm, a stuck choke butterfly, a clogged air passage, or something else. You need to open them up and go through them again and make sure that the pilot/starter jet is the correct size in all four carburetors. You also need to make sure that everything is clean, every rubber part is intact and in good condition, and that all sliding and moving surfaces move freely.

 

[Kike_tfs]

A detail that I forgot to name... I have a 4-in-1 handmade exhaust, I don't know if it could influence something.

 

[Kike_tfs]

Update:
I have gotten other second hand carburettors, recently repaired with a restoration kit.
They are well assembled and fit with no apparent vacuum leaks, and I still have the same problem...
It is clear that I have a problem with cylinder liners or valves...

 

[Kike_tfs]

Sometimes I think it would be better to sell it, but it's so pretty....

 

[SenseAmidMadness]

Vacuum leaks aren't usually from the carbs themselves. On this bike the biggest potential for leaks are the 8 rubber boots holding the carbs in place and the 12 clamps that hold the ends of those boots tightly, as well as where the boots enter the airbox. All of those are potential leak points. It is very easy when you remove and reinstall the carb set to pinch or otherwise misalign any one of the rubber tubes and thus allow a vacuum leak. You should also make sure you don't have a hole in your airbox and that your air filter is firmly seated in place inside it.

Have you ever checked the carburetor synchronization? How about the screws in the intake ports where you insert tubes to check for carburetor synchronization? Those are also a potential vacuum leak point. One of them could be missing!

Have you checked that your ignition timing is not too far advanced?

Is your choke cable hooked up correctly? Are your throttle cables adjusted to the spec listed in the factory service manual?


This has a cause. You can find it. Something in this machine is not the same as when it left the factory, and if you find it you can fix it.