Topic: 81 CB650C won't rev under load

[Jbarth13]

Hi all,

Had my bike for a couple of months now and still can't get it to rev well under real load.  In first and second it revs out ok, but in third it won't go past 6500 or so, and in 4th and 5th the same is true.  This means my top speed is limited to about 65 (barely) on flat ground.  If I roll on much throttle at all it starts to sound a lot like its hitting a rev limiter of some sort.  I've never driven the bike running properly, but it seems to have adequate low end torque.  It just hits a wall right at 6500 or so.

The setup of the bike is as follows:

4 into 1 with a V and H exhaust
Stock jetting (as far as I know)
Air Filter held in by fender washers (no restrictive cover)

I have cleaned the carbs, checked the ignition advancer, and done the air filter washer trick.  At a bit of a loss as to what to check next.  Perhaps the carbs need to be gone through again? 

Thanks,
JB

[calj737]

One thing to consider is clutch slippage. Your symptoms could be a number of things, but if you feel your carbs are right, then at least consider looking elsewhere to eliminate some possibilities.

[trueblue]

Check your pulser gaps, can make a huge difference

[cb650]

also new plugs.

[nevernoluck]

Seems I had the same problem on my 1980, it's completely stock and different carbs I believe but as was mentioned above it was clutch slippage at high rpm but just like you it was fine on the bottom end, worth a check

If it ain't that Ill let the more SOHC savvy guys tell you what else to check

[trueblue]

Seems I had the same problem on my 1980, it's completely stock and different carbs I believe but as was mentioned above it was clutch slippage at high rpm but just like you it was fine on the bottom end, worth a check

If it ain't that Ill let the more SOHC savvy guys tell you what else to check

A slipping clutch won't stop the engine from revving, in fact quite the opposite.  The engine will rev and the bike won't speed up. 

[calj737]

Just re-read his original post. As always, Trueblue, you're spot on. Can't be the clutch...

[w1sa]

What has been said above.........and before ruling things in or out, perform a more complete service/tune-up, to at least include valve clearances, fresh air filter,  carb bowl fuel levels and full fuel flow check, tank cap vent clear/clean etc (tried running with the cap seal open?), ignition advance check by strobe/timing light etc..........before or after which you could do a plug chop or two.

I would run the bike (once at full operating temp) at the troublesome rev limit, in top gear, for several minutes, attempting to coax the speed up......at which time do a plug chop and (after allowing the engine to cool a bit) check the plug(s) condition................If for example, the plugs are sooty, your (likely) running far too rich, suffering poor combustion conditions and/or your ignition is not performing..............If the plugs are clean/ light coloured, your likely running too lean (open air boxes don't help) or suffering fuel starvation due inadequate fuel flow/delivery at those continuous revs.

[trueblue]

W1sa the cv carbed 650's airbox was too restrictive from factory. This affected top end performance, the use of washers to hold the air cleaner in is a well documented mod that seems to fix the problem. The other thing you can do is cut a big hole in the centre of the air cleaner cover. This achieves the same result.

This does sound to me like an ignition issue. The other thing that should be added is set your timing with a strobe but set it at full advance. That is the engine at 3500 rpm and set it to the || marks. This is where it will be running most of the time. Don't panic about where the idle falls as long as it is close to the F mark it will be fine.

[Jbarth13]

Thanks for all the suggestions.

I have already changed the plugs in the bike, with no real improvement.  It may be slightly rich right now, but not bad, and I'm pretty doubtful that the mixture is the issue.

I've already done the air filter mod.  Saw a slight improvement, but the problem still persists.

I've tried riding with the gas cap off.  Slightly smoother, but rev limiter problem persists.

I've hooked the bike to a timing light and saw that the advancer does advance as rpm increases.

The rev limiter sound begins and the bike can be slowly coaxed up to 7 or 7.5k (so it isn't a completely hard limit) but it just falls flat on its face.  It won't maintain 70 on the highway in 5th.  Heck, 65 can be a struggle.  BUT absolutely no problem getting to 55 or 60.  Plenty of power.  In low gears it will also rev past 6500 as well (although is sounds crappy) but as you shift up it becomes more and more noticeable.

I will check the pulse gaps this weekend when I have access to feeler gauges.  Anything else I should check?

[nevernoluck]

Seems I had the same problem on my 1980, it's completely stock and different carbs I believe but as was mentioned above it was clutch slippage at high rpm but just like you it was fine on the bottom end, worth a check

If it ain't that Ill let the more SOHC savvy guys tell you what else to check

A slipping clutch won't stop the engine from revving, in fact quite the opposite.  The engine will rev and the bike won't speed up. 

Yep guess I should have read a little closer into what he was saying, I was equating it to my bike issue of just not going anywhere at high revs.........I'll stay quiet now

[RJ CB450]

my SOHC 650, it was carb sync that did the most.  Your carbs synced?  Running on all four cylinders for spark?  Personally, I think the airbox restriction stuff is over rated.  Mine revs up fine, maybe only a difference at the very top in but still cruises highway without flogging.  How about the needle?  It working properly and correct jets?

[Csrgti]

Sounds like fuel starvation to me. To what extent were the carbs "cleaned" ?
Were they completely removed , were they dismantled , were all the jets cleaned with nozzle brushes, or did you just drop them in carb cleaner overnight ? Did you replace any of the jets? What notch is the needle set at ? Was it running the same way before you cleaned the carbs? Have you set your float height ?

[MiGhost]

I will check the pulse gaps this weekend when I have access to feeler gauges.  Anything else I should check?

When checking the pulse gaps. Make sure you use only non-magnetic (Brass) feeler gauges. In a pinch you can use a common business card. It should pull smoothly through the gap.

[Jbarth13]

Thanks for the tip MiGhost!

To answer Csrgti's questions: The carbs were removed and separated, the pilots were removed and swapped to new ones.  Didn't touch the main jets, which are not stock size due to 4:1 and V and H pipes.  Carbs were synced after rebuild.  I know that the previous owner mentioned having a similar issue, but when I got the bike the float bowl gaskets and battery were so far gone it wasnt really possible/safe to ride it.  I just saw it idle briefly and didnt rev it up much.  Given that the problem will actually occur at a very specific rev range and sounds similar to a rev limiter, I'm inclined to think its an ignition issue.  Could be wrong of course.  Does anyone know recommended jet size given the exhaust?

[Jbarth13]

Alrighty, so the gap on the pulse generators is now within spec and the problem persists.  Other ignition related ideas before I start looking at it as a potential carb issue?

[trueblue]

Sounds like you have covered all the bases as far as the ignition goes.  I would suggest your next step to be a plug chop to see what is going on in regards to fuel mixture.  Try putting your air filter cover back on to see if that makes any difference, either better or worse would be a result to let you know you are on the right track.

[cb650]

Just for the hell of it try it with out the air filter.  Easy enough to do.  And new plugs.  I fouled a set and acted just like a rev limiter untill I put in new.

[Jbarth13]

Just tried it without the air filter, no apparent difference.  Plugs in it are brand new.  Less than 1k on them.  I just leaned it slightly because of excessive exhaust popping.  I'll go for a ride and then pull the plugs. 

Also just posted a thread trying to find out the proper main jets given the 4-1 and exhaust setup. Figured putting it in its own thread would be more helpful for those with the same setup.

[Csrgti]

Personally I'd check the gas flow out of the petcock and into the float bowls.
If it was restricted you may have enough fuel flow when the bike isn't under load, but they'd be starving when under load.

Put a container under your petcock remove the main fuel line(s) and turn the petcock on.
You should have a steady flow in the on and the reserve position.
If so hook the fuel line back up and test for fuel going into the carbs.

Turn the petcock off, and drain the fuel bowls into a container .
Leave the drain screws out , and turn the petcock on.
Gas should drain out of all the carbs at the same steady rate.
If not you have a flow restriction.

[RJ CB450]

Personally I'd check the gas flow out of the petcock and into the float bowls.
If it was restricted you may have enough fuel flow when the bike isn't under load, but they'd be starving when under load.

Put a container under your petcock remove the main fuel line(s) and turn the petcock on.
You should have a steady flow in the on and the reserve position.
If so hook the fuel line back up and test for fuel going into the carbs.

Turn the petcock off, and drain the fuel bowls into a container .
Leave the drain screws out , and turn the petcock on.
Gas should drain out of all the carbs at the same steady rate.
If not you have a flow restriction.

Good call!  Inline fuel filters are a big problem for this as well.  Filter is good when freshly rebuilt to ensure all the gunk is cleaned, but I have heard multiple people having problems with em for regular riding.

Can it be coaxed to higher RPM?  IE what happens under light throttle to slowly get higher rpm in the higher gears?

[iron_worker]

X2.

Tank could be filled with rusty #$%* which plugs up the petcock, lines, filters, and carbs. 

IW