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Where to Start-1979 750f Super Sport? Carb/Compression or both?
- Thread starter Hastings
- Start date
Inches, millimeters 
Connella08
CB750 Enthusiast
im not convinced that .001" difference in valve lash is going to cause cylinder 2 to not fire, especially if its on the loose side. he posted valve lash numbers, but was a compression test conducted? I feel like valve lash was identified as the problem with no real information to point to it. if compression numbers were low, then i'd start looking at valve clearances. this sounds like fuel or could even still be ignition.
when you did your first spark test, was the engine cold or hot? if cold, the heat soaking into the electrical components (such as pickup coil and ignition coils) could be causing the ohm reading to sway out of spec.
first things first, do a compression test.
when you did your first spark test, was the engine cold or hot? if cold, the heat soaking into the electrical components (such as pickup coil and ignition coils) could be causing the ohm reading to sway out of spec.
first things first, do a compression test.
Reread your first post. Yes, do a compression test. If the cylinders even have 150 they should run and will probably get better after it runs for a while. You can't avoid the carbs, fully clean them, just spraying them is never enough.
Ok I got the bike shimmed saw some improvement but still looks like a carb issue.
Removed the carbs again and had to use an easy out to remove the recessed nozzels for main jet (couldn't get them out first time I cleaned them)
I replaced them.
I also noticed the accelerator pump not working correctly so I replaced it too.
I bench tested the pump and see fuel squirting out of the nozzels in each carb except #4
Since fuel is getting into the #4 bowl should I split the carbs and check the fuel tubes or do you think its just a clogged nozzel in #4 carb?
Removed the carbs again and had to use an easy out to remove the recessed nozzels for main jet (couldn't get them out first time I cleaned them)
I replaced them.
I also noticed the accelerator pump not working correctly so I replaced it too.
I bench tested the pump and see fuel squirting out of the nozzels in each carb except #4
Since fuel is getting into the #4 bowl should I split the carbs and check the fuel tubes or do you think its just a clogged nozzel in #4 carb?
Ok after pulling the carbs a second time here is what I have now.
Pulled carbs again and cleaned them again
Removed and replaced the pressed in jet tubes (didnt due that the first time)
The bike starts and idles but still had trouble reving up
Its been a long time since I had a bike with a carb
After five or ten minutes of starting, idling and restarting
the bike ran smooth, reved up as it should and I was able to ride it with no issues (well other than no back brake but I ordered a new piston)
Next day had to follow the same process but reved and idled just like it should.
I am assuming its due to the cold weather and it being a carbureted ?
Pulled carbs again and cleaned them again
Removed and replaced the pressed in jet tubes (didnt due that the first time)
The bike starts and idles but still had trouble reving up
Its been a long time since I had a bike with a carb
After five or ten minutes of starting, idling and restarting
the bike ran smooth, reved up as it should and I was able to ride it with no issues (well other than no back brake but I ordered a new piston)
Next day had to follow the same process but reved and idled just like it should.
I am assuming its due to the cold weather and it being a carbureted ?
Yes had to use the choke for several minutes but after that it idled and ran great without it.
It appears to be all stock except for the 4-1 exhaust.
One odd thing at first cylinder #2 exhaust was not hot, confirmed it was firing then after running for a minute or two was good, hasnt happened since.
Day two starting and running, cyl #2 exhaust was hot just like the others but still had to wait several minutes before it would rev on up.
It appears to be all stock except for the 4-1 exhaust.
One odd thing at first cylinder #2 exhaust was not hot, confirmed it was firing then after running for a minute or two was good, hasnt happened since.
Day two starting and running, cyl #2 exhaust was hot just like the others but still had to wait several minutes before it would rev on up.
Connella08
CB750 Enthusiast
carbureted engines dont typically like to cold rev unless they are perfectly clean and perfectly tuned. if the bike runs normally when warmed up, I wouldn't worry about it too much.
Its worth noting that since it has a 4-into-1 exhaust, you should check into the jetting of the carbs and make sure you don't need to adjust the sizes. when you mess with exhaust restriction, it can affect the fuel mixture.
Its worth noting that since it has a 4-into-1 exhaust, you should check into the jetting of the carbs and make sure you don't need to adjust the sizes. when you mess with exhaust restriction, it can affect the fuel mixture.
I wondered about the jets, I bought this bike from the second owner who rode it five miles and put it in storage.
I'm not 100% but when I looked at the jets as I recall they were 98 and 25 or maybe 95 and 28. I assumed they were original.
I'm not 100% but when I looked at the jets as I recall they were 98 and 25 or maybe 95 and 28. I assumed they were original.
Agree with Connella. Don't rev the cold engine. Letting the oil circulate and cylinders heat up is a good idea instead.
Your 4-into-1 exhaust may not need jetting, but the brand is important to know. Dekelvic doesn't (in most cases) require rejetting but Mac does as far as I know.
Your 4-into-1 exhaust may not need jetting, but the brand is important to know. Dekelvic doesn't (in most cases) require rejetting but Mac does as far as I know.
Connella08
CB750 Enthusiast
factory jets should be 68 primary and 102 secondary. the secondary jet is the one in the jet holder, the primary is threaded directly into the carb casting. definitely verify your jetting before you run the engine too lean which could lead to significant engine damage.
Connella08
CB750 Enthusiast
definitely check both. make sure your plugs look like they are burning ok. If they look clean/white but are not new, it is way too lean. if they are lean, check the jets and increase sizes as necessary.
