hello overtone I have a 1977 cb750k that ive had for a while now and have finally got it almost where I want it the only thing now is I can’t get the bike to go above 75. I know it’s not necessary to go much faster but I live off a dangerous road and sometimes I like to get away from other drivers if I can. It starts and idles flawlessly and even revs up and returns back to idle very smoothly. I’ve got a 38 slow jet and a 130 main in it. I also had a 125 in it last week but still had the same top speed. I’ve had this bike up to 115 mph before but at the time I did have round top carbs on it I now have the later key hole version. Could it be carbs? Or could it be cylinders? Anything helps thanks for your time
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Low top speed
- Thread starter Oldtonew
- Start date
BAHNST
CB750 Addict
Any sign of clutch slip? Any brake dragging-you can feel the front disc for heat after a long ride and spin the back wheel after things heat up to temp (again after a ride)
madmtnmotors
CB750 Addict
What motivated you to put huge 125 mains and then step up to massive 130 mains? Stock mains were 115's. Have you checked your plugs? I'll bet you find them black from carbon being overly rich. Check your compression while you're at it and let us know what you find.
You may have answered your own question. If I'm reading this right the problem started after the carbs were changed. Why did you change carbs? It's possible the carbs you have now are having trouble lifting the slides. Also as mentioned, you have some large jets in there. Has there been major mods to the engine that require that much fuel?
I came across some information that said the 77-78 models used the key hole style carbs and I have two bikes being one 77 and a 78 and I believe the original carbs had a 130 in them as well which is what led me to trying the bigger main jets. The key hole carbs I have are also in better condition but no major mods to the engine are done so you think I should go smaller with the main?
Put the stock jets in and give it a try. Also make sure the slides are moving properly and opening all the way.
What are you referring to? Is that the compression? If it is, that is not good. The proper way to test compression is all the plugs out and the throttle wide open while cranking the engine. If it is that bad you might want to do a leak down test to narrow down what is going on.
Yes that’s compression the odometer says 56k on it so I’m assuming it’s definitely due for some rings which still baffles me slightly because it doesn’t skip a beat it runs great just doesn’t want to go very fast.
Did you perform the compression test like I mentioned?
I'm still thinking jets should be put back to stock if you have the stock air box. That is a known problem right now. Compression can be checked after you change the known problem. And with only 56k on the odometer, I would bet valves play a bigger role in the compression than rings.
I will do it that way when I get home from work today.
I have pods on the carbs if I had the air box I’d put it on and try it but I’d have to find one and you’re right it could be valves it’ll probably end up getting a valve job and piston rings after I take the motor out. I’ll have to do another compression check like loose chain mentioned and go from there.
I must have forgotten the pods; Did you mention those? In that case, what do the plugs look like? If they're black, bring the jets down a size. If they're clean'ish, you're lean. Next is valves.
The valves on these bikes play a big role in compression because they get tight over time (leading to inadequate sealing). At 56k, check them anyway. You can do the valves with the engine in the bike. It would be an easier task, THEN check compression.
Okay yeah I will give that a look and no it’s my mistake I failed to mention the pods but I put brand new spark plugs in before taking it for a decently long ride and they looked pretty good afterwards. So valve service first and then check compression
Okay so the compression for all four was 120 -130 psi after doing it the way you told me and I appreciate that. That’s a huge relief but after looking at the plugs 1 and 4 had a healthier brown color to them and 2 and 3 were definitely over fueled. Should I go down on main jet size and see what that does? I had a 125 and top speed was 75ish and then I went to a 130 and it’s was the same speed.
The coils along with the wires and boots and plugs are all less than a month old but maybe it could be timing ?
Why not put the other carbs back on? You said the problem started after the carb change. If that clears it up then you know where to look.
Yeah, interesting only 2 and 3 look bad. That would usually point to coils. Like Loose Chain said, if it ran well with the other carbs, I'd say put them back on and run it. But if you're hell bent on using the current carbs, verify they do have the same jets as 1 and 4, then look at your timing.
Now that I think about it, make sure coil number two connections on the coil are clean. Maybe your spade connectors need some attention.
Now that I think about it, make sure coil number two connections on the coil are clean. Maybe your spade connectors need some attention.
