Maxes-out around 55mph. 81 CB750 DOHC

Joseph New York

Joseph New York

Active Member
I recently got side-swiped (hit and run) and side-surfed my CB750 going 40mph. Luckily I just got rashed but the bike's stator cap was completely grinded open, sparking and spewing out all it's oil for an entire city block.

Had the bike towed to a garage, put on a new stator cap (stator/timing seemed untouched), synced the carbs, lubed her up and she was back on the streets.

Starts, idles, and rides amaizingly, no bog, no hang ups, but something is def wrong... I can climb RPMs very fast, seems the power is there, but I burn through 5 gears just to get to 50mph. Attempting anything faster and its screaming around 7/8k (no tach). Seems to have less torque the higher I shift.

Perhaps it could be the timing advance? Stator? Sprockets? I don't know where to look first or how to diagnose this. I'm in the scooter lane fellas, please help!
 
So the bike has plenty of power, it's just revving way too high without the accompanying speed?
 
A couple options are make sure all plugs are firing by visually inspecting spark then misting a little water on the head pipes while the engine is at 2000 rpm . . if some steam and some don't the non steamers are suspect

Check idle timing and max timing .

If that is OK then maybe cam timing . . it might have jumped a tooth but I seriously doubt it.
 
Sonreir said:
So the bike has plenty of power, it's just revving way too high without the accompanying speed?
Click to expand...

Hah, yes, I know that sentence doesn't makes sense, but what I'm trying to say is that the RPMs accelerate fine. The RPMs rise and fall in sync with the throttle responsively like before the accident. It's not revving too high (what I meant by not hanging up). The screaming RPMs I'm referring to are when the current gear can't push the speed faster than the current speed. My 5th gear in this case.

Another way to put it, I need 5 additional gears to do the ton ;)


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barnett468 said:
A couple options are make sure all plugs are firing by visually inspecting spark then misting a little water on the head pipes while the engine is at 2000 rpm . . if some steam and some don't the non steamers are suspect

Check idle timing and max timing .

If that is OK then maybe cam timing . . it might have jumped a tooth but I seriously doubt it.
Click to expand...

When she was all back together after the wreck, it had one cylinder not firing, but I remedied it and the last test showed all 4 firing. But it's been a few weeks and I'll check the plugs and take her temperatures with my IR gun to rule it out.

I feel though if was running on less than 4, then it wouldn't go through the gears as fast, no? I've had non-firing cylinders on my other bikes and it would have a hard time pushing the higher gears. Seems to be the opposite effect here.

As to checking the timing, are you recommending I do a strobe test?


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bolla said:
Garage filled wrong oil/slipping clutch -adjustment??
Click to expand...

I hope not. I hate taking to garages for this reason but my hands were tied. Who knows what they put in her, invoices are as bad as doctors notes, and never even mentioned oil. Any ideas on how to look into this?

I haven't mentioned it, but sometimes I'll hit a fake neutral. Also, I can't shift the gears up past 2nd with the engine off. Not sure if that's normal or not.


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Do the revs stay steady with the acceleration? If you rev it all the way up to 8K or so does the bike accelerate steady up to it's max speed for that gear? If you let off the throttle does the bike decel steady as well?
 
1fasgsxr said:
my 1st instinct is to say your clutches are slipping for some reason...
Click to expand...

That's kind of what I'm thinking, too, but I was hoping to get more info.
 
Joseph New York said:
When she was all back together after the wreck, it had one cylinder not firing, but I remedied it and the last test showed all 4 firing. But it's been a few weeks and I'll check the plugs and take her temperatures with my IR gun to rule it out.

I feel though if was running on less than 4, then it wouldn't go through the gears as fast, no? I've had non-firing cylinders on my other bikes and it would have a hard time pushing the higher gears. Seems to be the opposite effect here.

As to checking the timing, are you recommending I do a strobe test?


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Click to expand...
are you saying the engine runs fine and it revs as far as it used to but at 60 mph it is now turning more rpm than it used do? . . if this is the case then your clutch is slipping and you need a new one and nake sure you refil with good oil.
 
barnett468 said:
are you saying the engine runs fine and it revs as far as it used to but at 60 mph it is now turning more rpm than it used do? . . if this is the case then your clutch is slipping and you need a new one and nake sure you refil with good oil.
Click to expand...

Yes and no. The engine is revving in perfect response to throttle, at all speeds. The drivetrain doesn't seem to be responding to the engine. (Is that worded correctly??)

I agree with you that it seems like a problem with my clutch or gears as that seems to what's not performing. It doesn't seem to have a slipping problem though, as it has no problem staying in gear. Just not getting much out of each gear...

Unfortunately the clutch/gears aren't my forte, and I'm not sure how I should check for the problem areas. Im a willing student though!

What's a way I can check to see if I need a new clutch? How do I check to see if my gears are ok? Is it cool that with the bike not running that I can only switch to the 2nd or 3rd gear? Should I pop off the side cover and monitor the gear shifting?

Thx!


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1fasgsxr said:
my 1st instinct is to say your clutches are slipping for some reason...
Click to expand...

I feel that way too as long as you don't mean it's slipping completely off. You saying maybe the clutch friction is slipping?

It does decelerate like someone asked, and it not having problems staying in gear. But the gears seems to have a less effect on my speed than before.


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You didn't answer my questions specifically enough . . it sounds like you need someone knowledgeable to look at it for you.
 
Clutch is simple to test. Drive bike. Take it up to 50 mph or so in top (5th)gear. Maintain constant speed. Turn throttle wide open. If engine speed increases and road speed does not, or engine speed increases disproportionately rapidly compared to road speed, the clutch is toast. Alternatively, the rear sprocket could have humps instead of teeth. Don't laugh, I have seen it!!

It is possible that incompatible oil is now in the engine and causing clutch problems, but this sounds too severe for that. Change the oil to something known not to cause trouble like Rotella and try it. May take a few dozen miles, but likely there has been enough slipping already to have been terminal.
 
Left side was shaved yes? maybe the clutch lever got some rash? if so could it be preventing it from releasing entirely? bent, rashed or twisted???
 
barnett468 said:
You didn't answer my questions specifically enough . . it sounds like you need someone knowledgeable to look at it for you.
Click to expand...
Sorry I'm not being informative. I'll check the plugs & timing ASAP. "Someone knowledgable" just handed the bike over to me without noticing the speed issue, so I'm going to attempt to resolve it with you knowledgable people! If I get nowhere fast, I'll take it back. To give them credit, the garage doesn't take test runs on the highway, just city block and right now it performs fine in town.


jpmobius said:
Clutch is simple to test. Drive bike. Take it up to 50 mph or so in top (5th)gear. Maintain constant speed. Turn throttle wide open. If engine speed increases and road speed does not, or engine speed increases disproportionately rapidly compared to road speed, the clutch is toast. Alternatively, the rear sprocket could have humps instead of teeth. Don't laugh, I have seen it!!

It is possible that incompatible oil is now in the engine and causing clutch problems, but this sounds too severe for that. Change the oil to something known not to cause trouble like Rotella and try it. May take a few dozen miles, but likely there has been enough slipping already to have been terminal.
Click to expand...
With your test, at about 50mph, I pull the throttle and it increases engine speed and ground speed, but not as proportionally as it should. At about 50, to maintain that speed, I'm at around 5 rpms. Turning throttle wide open, it only accelerates to around 55mph. When I don't accelerate I slow up. Your thinking the clutch is toast?


Tune-A-Fish said:
Left side was shaved yes? maybe the clutch lever got some rash? if so could it be preventing it from releasing entirely? bent, rashed or twisted???
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True, I think it's plausible. Here is a pic of the stator cover. You can see the clutch lever scraped the housing...

9bd3c1241cb240835fd9aca0c13c8fca.jpg


Going for another ride tonight when it stops raining. Hopefully I'll find out more info. @barnett468, I'll have wet exhaust wrap that will give me a great steam test...

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You need to answer my question about rpm in detail . . if you know the answer it will resolve the question of the clutch possible niong bsc.

The shop does not need to go more than 50 feet yo tell if the clutch is bad.
 
Go 15 mph in second gear

Pull the clutch in

Rev the engine to 4500 rpm

Let the clutch out instantly/fast

You will instantly know if it is slipping.

If it lifts the front wheel off the ground and/or tries to pull the bars out of your hands, it is not slipping . . its that simple
 
barnett468 said:
Go 15 mph in second gear

Pull the clutch in

Rev the engine to 4500 rpm

Let the clutch out instantly/fast

You will instantly know if it is slipping.

If it lifts the front wheel off the ground and/or tries to pull the bars out of your hands, it is not slipping . . its that simple
Click to expand...

I know for a fact that it will wheelie and pull the bars out of my hands, in that scenario. No doubt. It's why the garage or myself never noticed there was an issue. I went on the highway a few days ago and it's when I noticed. It's not until I'm in the higher gears that It lacks speed.

Can you reiterate the questions you want me to answer in detail about RPMs. I can seem to find it...

Thanks!


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