Topic: 82cb650 boggs down around 4500 rpm.

[harleyguywithahonda]

I bought this bike for my little brother but it just doesn't run right. It's all stock, I have been messing with it for quite a while now and need some other ideas to check, I've done jets checked timing, valves, carb sync. Kind of at a loss, any help is appreciated.

[kerryb]

Does it still have the vacuum operated petcock mounted on the carbs?  You might have a vacuum leak in the diaphragm that could cause partial shutoff starving it for fuel at higher demand.  Many guys bypass that with a straight tube or a simple manual shutoff valve.

[fizzlebottom]

I've done jets ...

Can you elaborate on what this means?

Let us also know the following:

• Did you thoroughly clean all of the passages in the carbs?
• Replace any old rubber bits including the accelerator pump diaphragm?
• Check floats for height and leaks?
• Remove that #$%*ty vacuum petcock?
• Check the carb insulators for cracks?

[harleyguywithahonda]

I've jetted up and down, makes no difference. Even ran with and without the filter.

[harleyguywithahonda]

Kerry b, it's still running the vacuum petcock, I'll check the diaphragm.

[fizzlebottom]

I've jetted up and down, makes no difference. Even ran with and without the filter.

Does it have the original #118 mains in right now or did you throw aftermarket brass in? Lot of people report issues with aftermarket brass.

[harleyguywithahonda]

Fizzlebottom I'm pretty sure it's oem jets, but i know for a fact it's the 118 mains. I have also cleaned these carb like 5 time to make sure I didn't miss anything, all the boots are good no cracks in any of the rubber, the accelerator diaphragm looked good but I'll check it again. I had not taken the vacuum petcock off but I will try that now as well.

[fizzlebottom]

I don't necessarily think the vacuum petcock is the problem unless the diaphragm is torn. Does the bike ever get past the point where it bogs down? Does it happen when you crack the throttle, increase gradually, or both?

[harleyguywithahonda]

Fizzlebottom, it revs out well in neutral, will hit the red line. While under load whether cracking the throttle or rolling into it, it will not go past about 4500-6000 it almost feels like it wants to die. The motor just drops rpms right away when it hits the point it gets worse the higher the gear. First gear will get at least to 6000, second around 5000, so on and so forth. The diaphragm is in fantastic shape, still pliable and no rips.

[BomberMann650]

Starting to sound like a timing issue.

If the cam and crank timing are misaligned, it can cause a flat spot.  So would a failing ignition timing unit.

[seanbarney41]

does cb650 still have mechanical advance?

[scunny]

Yes it does.

[seanbarney41]

make sure the advancer is not stuck

[scottly]

The motor just drops rpms right away when it hits the point it gets worse the higher the gear. First gear will get at least to 6000, second around 5000, so on and so forth.

This sounds to me like a fuel starvation problem, as in only enough fuel is flowing to make a certain amount of power. Have you tried applying the choke when it happens? The problem may not be with the vacuum petcock itself, but possibly a plugged fuel strainer in the tank? Try removing the fuel line from the petcock, and see how much fuel flows when you suck on the petcock vacuum hose. Some of these petcocks had a "P" (for Prime) setting which doesn't require suction, but I don't think the 82 650 had one. Also, the petcock might be easily converted to manual operation, as I did on my 82 Ascot 500.

[fizzlebottom]

The motor just drops rpms right away when it hits the point it gets worse the higher the gear. First gear will get at least to 6000, second around 5000, so on and so forth.

This sounds to me like a fuel starvation problem, as in only enough fuel is flowing to make a certain amount of power. Have you tried applying the choke when it happens? The problem may not be with the vacuum petcock itself, but possibly a plugged fuel strainer in the tank? Try removing the fuel line from the petcock, and see how much fuel flows when you suck on the petcock vacuum hose. Some of these petcocks had a "P" (for Prime) setting which doesn't require suction, but I don't think the 82 650 had one. Also, the petcock might be easily converted to manual operation, as I did on my 82 Ascot 500.

Stock, there are a few items that could possibly clog up along the way. There's a finer strainer inside the tank and a more coarse one in the bowl of the petcock. It is definitely possible for those to give way or get all gummed up if the tank wasn't well cared for.

Removing the vacuum petcock is as simple as unscrewing 2 screws to remove the vacuum petcock bracket, plugging the #2 carb's vacuum port, and cutting some new fuel hose.

[harleyguywithahonda]

Starting to sound like a timing issue.

If the cam and crank timing are misaligned, it can cause a flat spot.  So would a failing ignition timing unit.

Is there somewhere I can get specs to ohm out the ignition?

[harleyguywithahonda]

make sure the advancer is not stuck

When I had the timing light hooked up it seemed to be operating correct but I will check again. I believe I greased the weights too.

[harleyguywithahonda]

The motor just drops rpms right away when it hits the point it gets worse the higher the gear. First gear will get at least to 6000, second around 5000, so on and so forth.

This sounds to me like a fuel starvation problem, as in only enough fuel is flowing to make a certain amount of power. Have you tried applying the choke when it happens? The problem may not be with the vacuum petcock itself, but possibly a plugged fuel strainer in the tank? Try removing the fuel line from the petcock, and see how much fuel flows when you suck on the petcock vacuum hose. Some of these petcocks had a "P" (for Prime) setting which doesn't require suction, but I don't think the 82 650 had one. Also, the petcock might be easily converted to manual operation, as I did on my 82 Ascot 500.

I haven't checked the screens yet I will now though, I have pulled the vacuum diaphragm off and ran without still the same. There is only on and off on my petcock as well.

[2wheels]

I have not read through all the posts so this may have been asked??  Have you checked compression?  Could be the engine just cannot make the necessary power.
edit to add - i guess this would be easy to test, can you go faster downhill and slower up hill

[harleyguywithahonda]

I have not read through all the posts so this may have been asked??  Have you checked compression?  Could be the engine just cannot make the necessary power.
edit to add - i guess this would be easy to test, can you go faster downhill and slower up hill

I can't remember if I have but I will, it doesn't even want to go past 6 on flat ground, so I haven't tried up or down hill

[harleyguywithahonda]

I have not read through all the posts so this may have been asked??  Have you checked compression?  Could be the engine just cannot make the necessary power.
edit to add - i guess this would be easy to test, can you go faster downhill and slower up hill

140 psi across the board

[scottly]

Have you checked the flow out of the petcock yet? Remove the fuel line, place a jar or something to catch the gas, and turn the petcock on. Is there a solid stream of gas, or does it just dribble out?

[harleyguywithahonda]

Have you checked the flow out of the petcock yet? Remove the fuel line, place a jar or something to catch the gas, and turn the petcock on. Is there a solid stream of gas, or does it just dribble out?

It's a solid stream whether on or reserve, I did check the screens as well and they are clean

[2wheels]

One more dumb question and I will stop wasting your time.
I am assuming the 4500 RPM  bogg is in 5th gear.  What is the highest RPM you can get to in 3rd gear?  And what speed is that

[harleyguywithahonda]

So I found a 78 cb650 manual online says I should have 530 plus or minus 50 ohms, my meter is reading 641 and 620 ohms for the coil resistance  on the pulse generator side of the harness and 159 ohms on the other side of the pulser coil plug oh and thats 641 for the yellow wires and 620 for the blue wires