79 cb750f shuts off when stopping after 15-20 minutes of driving

[Ryanlonger]

I just recently purchased a 79 cb750f. It's a beautiful bike I picked up for a steal of a deal after one of the valve shims broke on the previous owner.
After fixing the shim I've taken the bike out for a few short rides. I'm now having an issue once the bike heats up where when I come to a stop light it shuts off at idle. It seems to be okay in neutral but once I put it in gear and am not moving it dies. It rides perfect before it reaches this point and it has only been around 15 degrees (Celsius) when I'm riding so I can't see that the engine would be overheating. I'm thinking it's something electrical that's worn out and doesn't work properly once heated up. Any suggestions or ideas on what to be looking at? Thanks

 

[amc49]

I'd compression check that sucker. Those are well-known for valves never getting set and then compression suffers and a sudden tendency to die as soon as throttle dropped off is a symptom of that. VERY rare to get shim damage in them as well, don't suppose you've bent a valve slightly do you? That can come easily with shim damage.

Low compression is the most common reason why these don't run well later in life, then the carbs get yanked because of the male testosterone thing as in 'it must be the carbs', it's ingrained in our DNA...........Of course the carbs CAN be a problem but often BOTH issues are present. For sure, carb work till the end of the universe does not touch compression issues and why so many after loads of work still never run right. It's also a male testosterone thing to never think about a 35-40 year old engine as having other than the highest compression. Too much effort to check and more fun to tear down carbs that then never go back together right ever again. Ergo, all the problem bikes out there. Look great, run, uh, not so well..............

The engines were designed to run on low lead fuel available at the time, the valves definitely do not like the later pure 100% unleaded we now have to use and ethanol makes it even worse. Unlike most other engines out there these generally close up valve clearances with wear other than open them up. Why? The cams wear faster at the bearing holes to then let the valves not have as much clearance as they seem to measure out to have. The valves themselves recede pretty fast too. Evidence? .002" of carefully measured clearance will often act like zero, the engine will burn valves that are that tight and you think they are fine since the OEM spec includes that number as OK. No. NO.

Looking for around 170 psi there...................

 

[Ryanlonger]

Thanks for the reply! The valve looked fine from the top end, had the lifter body off and it went back in and floated nice. Compression could be the issue but I thought if anything compression would be better once the engine heats up and all the metal expands a bit. I checked the valve clearances and there all within spec but I think at least 2 of them were at 0.002" so I might try changing those out to a smaller shim.
The carbs were supposedly cleaned last year but that doesn't always mean anything. I took the bike out for a highway run last night and even after coming off and stopping a few times it seemed fine. Once I get back into the city and was hitting a lot of lights it happened again. Seems to only be when the engines at its hottest. I'll have to read up on how to do a compression check.