Topic: CB650 won't rev past 6k in final gear. Any ideas?

[theenemy]

Hey guys,

Firstly let me apologize for asking a question that may already have been answered. I bought this 1981 cb650 standard and have been doing a lot of research for the various problems I am having.  One of my main reasons for buying such an old bike was to familiarize myself with the mechanics and operations of motorcycles.

The problem I am currently having is that when I am in final gear the bike revs up to 6k (about 75mph) and starts to hesitate.  I dont feel like it is sputtering just hesitating and not giving me anymore.  The carbs have been cleaned at least 4 times as well as synced but I may just breakdown and have them professionally cleaned.  I pulled the plugs and it seems like I am running lean.  I just don't know how I can rev 1st - 4th at WOT without hesitation and cant with final gear.  1981 had CV carbs and as I said before I am not an expert in anyway.

Thanks!

[Schnell]

Your bike may not be developing enough horsepower (for some reason) to push the bike faster in top gear.

[theenemy]

What could cause something like that?  I can take 4th to redline shift to 5th which seamlessly keeps building my speed and then once I get to 6k it just stops.  1st - 4th feel strong, I see no loss of power through those gears at any throttle position.

[crazypj]

Sounds like you have a slipping clutch?
 Bike should be geared for about 125mph, 6,000 should be more than 75, probably closet to 90?
 my 550 was doing 80mph around 5,500rpm
Clutch springs were a bit weak when new

[theenemy]

I was wondering about that... I read that the 650 is easily capable of over 100 mph.  The clutch and clutch cable were just recently replaced, I am pretty certain the speedo is reading correctly because on the highway cars were going past at what I assumed was 80 mph.  I'll have to look further into that.  Would the clutch slip so much that at a certain point it would stop increasing speed?  I still feel as though my fuel delivery is being compromised somewhere as well.  Thanks for pointing out the clutch though.

[crazypj]

Air resistance goes up with square of speed, I've seen a lot of bikes that will easily do 80mph in 3rd, 4th, 5th (or 6th), but won't go any faster
If rpm is going up without increase in speed it can't really be anything else.
Did you fit new clutch springs?
 Even when they are in spec they are weak, i've always recommended changing springs way before plates

[theenemy]

That makes sense, I'll have to replace with some better clutch springs and see if that helps.  Thanks crazypj.  I feel like in final gear the rpms should still climb even if my speed doesn't though.  I believe you are right about my clutch but I still feel a hesitation that I think equates to fuel delivery.

[Schnell]

If the clutch were slipping, the revs would go up with no increase in speed. That's not happening, right?

[theenemy]

If the clutch were slipping, the revs would go up with no increase in speed. That's not happening, right?

Exactly, my RPMs and speed remain the same at around 6k.  I do feel that I should have more power output at 6k though. The bike revs great through all gears but the final one.

[Schnell]

First gear has lots of power but little speed. Top gear sacrifices power for speed. There's enough power to pull redline in the lower gears but not the top one.

[theenemy]

First gear has lots of power but little speed. Top gear sacrifices power for speed. There's enough power to pull redline in the lower gears but not the top one.

Which just brings me back to square one.  Could I be looking at a timing or valve issue?

[Schnell]

How's the compression? Could be tired rings?

[chewbacca5000]

How's the compression? Could be tired rings?

+1 it might not be getting good compression on all 4, or 1 cylinder might not be firing.

[kghost]

Stock air box set up?

Stock paper filter?

No pods?

[crazypj]

Stock air box set up?

Stock paper filter?

No pods?

 I was just about to ask the same question, sounds like it could be real lean

[cb650]

Try running with out your air filter once.  The cv carbs sometimes get starved for air.

[w1sa]

From what's been described so far, it seems like you might be starving for fuel under sustained open throttle/engine load......make sure the fuel flows to the carbs, (unimpeded high even flow), with the cap secured....(at 6k rpm cruise in top with good tune,you're burning probably 6 litres/1-1/2gal an hour)...sustained load always requires full fuel delivery rate to the float bowls.....otherwise, at high throttle openings while accelerating thru the lower gears, you can lower the float bowl fuel level and as you hit top gear you run out of sufficient vacuum to drag the fuel from the lower depths of the float bowls. CV carbs are even worse...they depend on vacuum (healthy firing engine) to hold the main needle open for even more fuel and more revs.

[theenemy]

I am using the stock air box with filter but without the restrictive air scoop.   I will do a compression test sometime within the near future.  Thanks for all the help, hopefully I can work this out soon.

[theenemy]

From what's been described so far, it seems like you might be starving for fuel under sustained open throttle/engine load......make sure the fuel flows to the carbs, (unimpeded high even flow), with the cap secured....(at 6k rpm cruise in top with good tune,you're burning probably 6 litres/1-1/2gal an hour)...sustained load always requires full fuel delivery rate to the float bowls.....otherwise, at high throttle openings while accelerating thru the lower gears, you can lower the float bowl fuel level and as you hit top gear you run out of sufficient vacuum to drag the fuel from the lower depths of the float bowls. CV carbs are even worse...they depend on vacuum (healthy firing engine) to hold the main needle open for even more fuel and more revs.

What can I do to make sure I am getting unrestricted flow?

[w1sa]

On the delivery side, you can measure the fuel flow rate to the carbs by opening the drain screws or simply leave the bowls off and collecting the drained fuel.  I think you might need a fuel flow several times higher than 6k rpm cruise consumption requires (maybe more)...look to see perhaps 300 to 400 mls per minute.

Flow can be restricted for a number of reasons....vacuum lock in the tank cap (the vent is blocked).....Fuel tap/filter is clogged with debris....fuel line is not clear....float valve(s) sticking...incorrect float heights/settings

You can therefore isolate any restrictions by elimination, and by proving the good points of flow in the fuel circuit between the tank  and the carb bowls.

[theenemy]

On the delivery side, you can measure the fuel flow rate to the carbs by opening the drain screws or simply leave the bowls off and collecting the drained fuel.  I think you might need a fuel flow several times higher than 6k rpm cruise consumption requires (maybe more)...look to see perhaps 300 to 400 mls per minute.

Flow can be restricted for a number of reasons....vacuum lock in the tank cap (the vent is blocked).....Fuel tap/filter is clogged with debris....fuel line is not clear....float valve(s) sticking...incorrect float heights/settings

You can therefore isolate any restrictions by elimination, and by proving the good points of flow in the fuel circuit between the tank  and the carb bowls.

This is really helpful, I will start looking into my fuel flow ASAP.  I suspect my 4th cylinder carb. I have noticed the header pipe isn't getting as hot as my other pipes and when I checked for fuel out of the drain screw I noticed I didn't get near as much as the other carbs.

[iron_worker]

Sounds like your carbs need a good cleaning. You're probably leaning out near WOT.

Could be related to low fuel levels, clogged emulsion tubes, etc etc.

IW

[crazypj]

It may be easier to get a funnel and 5 gal gas can then remove fuel tap drain tank and check filters

[F16Viper68]

I know this thread is old but just in case someone is searching and stumbles across it I'm going to toss my two cents in.  I went through something very similar with my 650 and traced it to a rusted auto advancer.