I have a 1981 cb750C and I really like it but it just won’t go beyond 7k rpms and I’m confused about it. I adjusted the jets from 68/102 to 72/110 because it’s getting colder outside. But I can’t be sure if the jets are the issue since it did this with the original jets too. I have the stock airbox and 4-1 Mac headers along with an 18 inch cone engine muffler, and I don’t think anything else has been modified on the bike. I checked for leaks in the boots using spray but couldn’t find any. What else should I look into? I’m hoping ancient dad chimes in since he usually has good advice
Your idle seems to be hanging way too high. I suspect a vacuum leak could be the problem. It might not just be the boots; you should try spraying the throttle shaft too. There are felt seals around the shaft that can leak, although it’s not common. I replaced mine while I had the carbs out. If your boots are aftermarket, make sure they are good quality since the cheap ones don’t work well.
This is strange for sure, but I’m not convinced it’s a vacuum leak. It seems to sound flat when it cuts back to 2 cylinders. I wonder if one of the coils is weak or leaking spark, which would affect that pair of cylinders under load. You could check the plug caps and the connections to the wires on both coils. Look for any signs of spark leakage too. I’ve never seen a later 4 cylinder with a bad igniter box, so I’m not sure how it would act.
@Echo
What do you mean by ‘cut back to 2 cylinders’?
Jamie said:
@Echo
What do you mean by ‘cut back to 2 cylinders’?
It means one of the coils for those cylinder pairs wasn’t firing quickly enough for the RPM. A bad condenser can cause this.
@Kai
Exactly, or it might be too weak to fire both properly, making it sound flat. If all 4 were firing when you opened the throttle, it would have screamed over 10,000 rpm. But this is a strange situation, and a condenser could be the issue if there are any.
@Echo
Could a weak battery cause this problem? My battery seems to die quickly even when I use it.
Jamie said:
@Echo
Could a weak battery cause this problem? My battery seems to die quickly even when I use it.
That could definitely be the case. If your battery can’t hold at least 12.6v at rest after being fully charged, it’s likely failing. On older models with points and condensers, a low battery often caused one coil to cut in and out because it didn’t have enough power to fire both. I haven’t worked on the newer ignition systems much, but I would suspect they could be affected too.
If your current battery doesn’t pass the voltage test after sitting for a while, you should get a new AGM battery that can last over 5 years. This way, you’ll rule out one possible issue and ensure you have a solid battery for years to come.
Be sure to check for air leaks around the intake boots. This is a common issue for the cb900s, and it might be similar here.
It could be a fuel starvation problem, maybe there’s a blockage in the fuel pipe, tap, fuel filter, or carburetor. You could test this by running an auxiliary fuel tank and bypassing the tank, tap, and filter.
Or it might be a leak in the intake rubbers between the carbs and head. They could have small splits allowing in air, which you can check by spraying carb cleaner or starter fluid on them to see if the bike revs up.
Lastly, though it’s less likely, a weak spark or failing coil could be the culprit. You can check the resistance with a tester, but these usually don’t fail.
You might want to try removing the air filter and see if that helps.
Luca said:
You might want to try removing the air filter and see if that helps.
That’s a clever idea
Luca said:
You might want to try removing the air filter and see if that helps.
That’s a clever idea
I have a similar issue with my 81 650, but only in gear. It revs fine in neutral, so it might not be exactly the same. But I find that running it without a filter works fine. I still haven’t figured out what that means, but it’s clear it has to do with airflow.
@Luca
Have you looked into whether the jetting has been changed?
Broderick said:
@Luca
Have you looked into whether the jetting has been changed?
From what I know about the bike’s past, I doubt it has been. I didn’t think to check when I had the carbs apart, and I’m not sure how I would even know. I don’t recall seeing a number on them.
Have you tried adjusting the mixture and idle screws to see what happens? Your idle is about three times faster than it should be.
Reagan said:
Have you tried adjusting the mixture and idle screws to see what happens? Your idle is about three times faster than it should be.
When it starts, it idles at 1.2-1.5k, and then it goes up as it heats up.
@Jamie
That sounds like a sign of an air leak.
I had a similar issue when I put in a low-quality accelerator pump I bought online. Once I replaced it, the performance improved a lot.
Also, consider relaying your ignition coils. If you can get Dyna coils, they would be a solid upgrade.
Running on two cylinders seems to make it run rougher than yours, and it barely stays running.
Buddy, you really need to sort that idle out first. I’m guessing it’s a major carb issue along with some vacuum leak. But hey, at least you have a ‘rev limiter’ now.