Maxes-out around 55mph. 81 CB750 DOHC

[Tune-A-Fish]

If even to cure my own curiosity, are you describing "speed" or "power" does it run up through the gears with good "power" (normal) and then you upshift and it falls on it's face kinda sorta like dragging the back brake?

 

[Joseph New York]

If even to cure my own curiosity, are you describing "speed" or "power" does it run up through the gears with good "power" (normal) and then you upshift and it falls on it's face kinda sorta like dragging the back brake?

Nothing that dramatic or that noticeable is happening. The side effect is mostly just in the speedo. The power seems to be all there in the engine, and by power I mean it has no issues pushing the RPMs up as heigh as I want.

A metaphor that could be used would be that it's as if each of my gears aren't much bigger than its previous, so I don't get much more speed when I upshift. It's like getting 10mph extra per gear. When I'm in 5th gear, it feels that the engine could pull through at least 2 more gears.


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[barnett468]

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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OK your clutch is most likely not slipping then but you can also do the same test in 4th gear going around 2000 rpm.

My question was....does it now turn more rpm at 60 mph then it did before the crash?

If it does, you clutch is slipping or you are not getting 5th gear.

 

[Joseph New York]

OK your clutch is not slipping its that simple.

You can also do the same test in 4th gear going around 20000 rpm.

My question was....does it now turn more rpm at 60 mph then it did before the crash?

If it does, you clutch is slipping or you are not getting 5th gear.

I wish I knew that answer about RPMs at 60mph prior but I only had the bike a week before the wreck and never took it over 40. I only know that the guy I bought it from got it over a 100mph. And it was my old man.

Yeah I don't think it's slipping, and I don't think I'm getting my 5th gear.

Should I be able to switch to my 5th gear with the engine off? It won't shift past 3.

Ill crack open the side panel and watch what goes on when shifting as soon as it stops raining.


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[barnett468]

I wish I knew that answer about RPMs at 60mph prior but I only had the bike a week before the wreck and never took it over 40. I only know that the guy I bought it from got it over a 100mph. And it was my old man.

Yeah I don't think it's slipping, and I don't think I'm getting my 5th gear.

Should I be able to switch to my 5th gear with the engine off? It won't shift past 3.

Ill crack open the side panel and watch what goes on when shifting as soon as it stops raining.


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No, your bike will not shift unless the engine is running and it is moving.

Simply start in first gear then count how many times it shifts.

If it shifts 4 more times and the rpm drops every time then you have 5th gear and your problem is that the bike is geared too low in which case you can install a bigger front sprocket.

 

[barnett468]

.
Did it have this problem before the accident? . . yes . no . I don't know


What is the rpm in 5th gear at 50 or 60 mph?

 

[Joseph New York]

.
Did it have this problem before the accident? . . yes . no . I don't know


What is the rpm in 5th gear at 50 or 60 mph?

No prob before the accident and apparently the previous owner had it do the ton. (Although the original speedo only went to 85 hmmm)

I don't have a tach right now to tell ya the RPMs in 5th. Getting it Sunday and will put together a report of MPH and RPMs per gear then. My guess is around 55MPG/8kRPM/5th. (Original tach redlined at 10k)



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[barnett468]

No prob before the accident and apparently the previous owner had it do the ton. (Although the original speedo only went to 85 hmmm)

I don't have a tach right now to tell ya the RPMs in 5th. Getting it Sunday and will put together a report of MPH and RPMs per gear then. My guess is around 55MPG/8kRPM/5th. (Original tach redlined at 10k)



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You said you only did 40 before the accident so how would you know there is a problem at 55 then?

 

[1fasgsxr]

count the number of teeth on your rear sprocket and front if its not too much work to get the cover off...I think it may help to know this info..

 

[coyote13]

CB750 with a speedo that only goes to 85......wut?

 

[Sonreir]

DOT law from the 80s

All speedos were capped for a while.

 

[coyote13]

Had no idea. I'll be on my way now ;D

 

[Sonreir]

I kinda like it. Plausible deniability.

My old XJ750 used to bury the needle without much trouble.

Get pulled over.

Cop: "You know how fast you were going?"
Me: "Nope. Speedo doesn't go that high."

 

[Joseph New York]

You said you only did 40 before the accident so how would you know there is a problem at 55 then?

I was in a lower gear going over 40 when I wrecked. The fastest I've pushed it happened to be when I was side swiped.

Regardless, the 750 shouldn't be maxing out at 55mph, unless your suggesting that it could be geared for in-town riding??

My CB 500 and 650 both push 80 with ease, so something had to be done.

I'll get ya a bunch of info Sunday when I have a tach.

Thanks barnett


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[barnett468]

yup 85 mph . . absolutely ridiculous idea . . on some models the needle would continue past 85 until it hit the stop used for 0 lol

 

[Joseph New York]

count the number of teeth on your rear sprocket and front if its not too much work to get the cover off...I think it may help to know this info..

Going to do this for sure. I'm going to post pics/vids of everything Sunday.

I just installed clip-ons and haven't received my shorter clutch cable yet FYI. Clutch is disengaging fine but I thought I'd mention this. I do on the other hand get a false neutral going into 3rd sometimes...


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[alzcbz]

It sounds, to me, like you are physically shifting the shift lever, but not actually advancing the shift mechanism.
That pic of the shift lever shows it took a hit. You may have bent something in the shifting mechanism or worse, in the trans.

Put the bike on it's center stand or somehow prop the rear tire off the ground.
With the motor off, put the bike in first gear. You should be able to shift through the gears while spinning the rear tire. The transmission needs to be spinning to properly engage the dogs on the gears.
Are you able to actually get to fifth gear?

 

[teazer]

What we collectively cannot tell from your description is whether the clutch is slipping allowing revs to rise without a proportionate change in speed, OR is it failing to shift up past 2nd or third.

alzcbz suggested a way to try to tell if it is shifting through all the gears. As he explained, it is often difficult to impossible to go through the gears with the motor stopped unless the wheel is turned to allow the dogs to engage.

If it is failing to upshift, it may be a bent shift mechanism or it is also possible that a shift fork inside the trans is bent.

 

[irk miller]

I'm going with the theory of not shifting the transmission. Otherwise, he'd be bogging down or stalling when he gets up in gear at such slow speeds. The drum stopper and spindle are spring actuated and it doesn't take much to knock the spindle off the plate.


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[barnett468]

based on his description in his replies to my questions there is no way in hell the clutch is slipping . . thhe engine is running o all 4 culs . . its either not going into 5 or itt is geard way too low.