Topic: Symptoms of bad air cutoff diaphrams

[cb650guy]

Hello all,

I finall got my 1980 CB650 to idle over the weekend.  It still sounds a little rough with the occasional backfire.  The throttle response is below par and the bike can't rev past 5K rpm.  When I tore down the carburetor I examined the air cutoff valves and they seemed OK to me, the hobbyist.  How would I know if my air cutoff valves are to be blame for my new problems?  I am reluctant to go by 4 new air cutoff valves at 15-20 dollars per pop without knowing for sure that they are the culprit.  Especially when I found no cracks or holes in the diaphragm.

Any advice?

Adam

[KingCustomCycles.com]

I believe the valves were installed to eliminate after firing on deceleration.  If your bike pops when decelerating, you might look at new valves.  It sounds like you have bigger problems to solve before springing for those valves though.

[cb650guy]

Ok.  That sounds reasonable. 

What can explain the poor performance at high rpm?  The bike cannot get about 5K rpm.  If I open the throttle completely the bike will die.  Can it be a carbureation issue?  I was thinking the battery could be weak, but the same thing happens when I hook the bike up to a car battery.  Is there a way to test the coils for integrity?

Its a CB650 Custom btw.

Adam

[TwoTired]

Will the bike rev up when the air box cover is removed?

Agree with KCC, the air cut off valves enrich the mixture on decell, so the fuel in the mufflers can't pop off.

[cb650guy]

I haven't tried removing the air box cover, but that sounds like a solid idea.  Will try when I get home.  If that is indeed the problem how would you fix it?


Adam

[TwoTired]

I'm just relaying what I've read from others trying to solve a problem as you describe.
I don't have a CB650, and don't really know the root cause.  Only the "band aid" fix others have used.
 Is your air filter clean as new?

I would think that the air box cover "mod" is to adjust the carb inlet tract pressure for whatever jetting or fuel flow issues exist in the carbs.

Have you checked your spark plug deposits yet?  If the "cover mod" makes it better, I would expect that they would now be quite sooty at or above the 5000RPM throttle setting.

Do you have the CV carbs or mechanical slide carbs?

Just trying to help...  I probably should just back off the CB650 posts, as I haven't actually worked on one.

Cheers,

[cb650guy]

The air filter is pretty clean, yes. 

I have the mechanical slides.  Thanks for the post!  I will keep you updated.

[TwoTired]

The air filter is pretty clean, yes. 

Just a warning.  You really can't tell by just looking at it.  The air filter pores are far tinier than you can see with the naked eye.  Paper fibers can collapse and make them smaller as well as become plugged by particles.  It aqll adds up to restriction and deeper pressures in the carb throat which is what draws fuel from the carbs.

The filter must be clean as new.  And certainly, any carb tuning should be done with a known clean as new air filter.

Cheers,

[Tdinova]

Did you check the spark plug caps for ohms? They should be 5000ohms.

[cb650guy]

I hadn't considered checking the spark plug caps.  So I should remove the cap from the wire and from the plug and measure from one end to the other?

[KingCustomCycles.com]

Did you check the spark plug caps for ohms? They should be 5000ohms.

never have checked this on any of the 35 or so Hondas I have owned,  I would describe this problem as "unlikely"

[cb650PK]

Are all 4 cyl. running?

[cb650guy]

Yes, all 4 cylinders are running and there is not any backfiring on decelleration.

[cb650PK]

How many turns out on the fuel mix screws?

[cb650guy]

I have it set right now at 2 turns out.  I believe the factory spec is 1.5 but I was told on older bikes to b ring it out another half turn. 

Does screwing it IN richen or lean the mixture?

[cb650guy]

Will the bike rev up when the air box cover is removed?

The bike does not rev any better without the air box cover.  Also, when I did the opposite of what you suggested and covered the air intake to inhibit air flow the rpms shot up.  What could this imply?

[TwoTired]

The bike does not rev any better without the air box cover.  Also, when I did the opposite of what you suggested and covered the air intake to inhibit air flow the rpms shot up.  What could this imply?

...that the carbs are too lean.  You sure you cleaned the pilot circuit completely?

Wait, you mean it revved over 5000?  ...or are you mixing symptoms?

Check the spark plug deposits yet?

Cheers,

[cb650guy]

No, it did not rev over 5k. 

The bike is at my parents house so I can check it tomorrow evening. 

Just to be clear, I will list all symptoms:

1) Can't rev bike over 5K.
2) Idles poorly, wont idle below 2K.
3) I just started a new thread for this, cylinders 1 and 4 were glowing red hot when I let it idle at 2.5K rpm for 5-10 minutes. (I immediately shut down the bike)

If you are correct and the bike is running too lean how do I correct it?  I carefully cleaned the pilot circuit by removing the pressed in pilot jets and dipping the carburetors in Berrymans for 24 hours then blowing compressed air everywhere I could.  In the 2.5K range (the range where the cylinders overheated) what carburetor circuit is 'active'  Is it the pilot circuit?  If it is, could I adjust the idle air mixture screw?

Thanks,
Adam

[Brown Bomber]

Did you check the spark plug caps for ohms? They should be 5000ohms.

never have checked this on any of the 35 or so Hondas I have owned,  I would describe this problem as "unlikely"

I've only owned 2 Honda's and both had bad resistor caps. I broke open the ones that gave poor readings, and the springs on the resistors were rusted. My 500T had one bad cap, and my 750K2 had two, that's half.

Definitely worth checking. 

[TwoTired]

1) Can't rev bike over 5K.
2) Idles poorly, wont idle below 2K.
3) I just started a new thread for this, cylinders 1 and 4 were glowing red hot when I let it idle at 2.5K rpm for 5-10 minutes. (I immediately shut down the bike)

If you are correct and the bike is running too lean how do I correct it?  I carefully cleaned the pilot circuit by removing the pressed in pilot jets and dipping the carburetors in Berrymans for 24 hours then blowing compressed air everywhere I could.  In the 2.5K range (the range where the cylinders overheated) what carburetor circuit is 'active'  Is it the pilot circuit?  If it is, could I adjust the idle air mixture screw?

It sure sounds like the pilot circuits aren't flowing properly.  This circuit provides the base line fuel for idle and also provides some fuel at higher throttle settings where the Throttle valve and main add their contribution.

Covering the intake is like adding choke and forces more fuel from the throttle valve (slide needles) and the main.

When you pulled the pilot jets, did you verify seeing light through them after cleaning?

Is your fuel supply clean?  Cleaned carbs won't stay that way if fed lumpy fuel.  Is your gas tank internally clean?  Is your fuel filter compromised?  Did you flush the fuel lines before sending gas to the cleaned carbs?

Did you vacuum sync the carbs after cleaning?

Cheers,
 

[cb650guy]

I will answer each question in turn:

When I removed the slow jets I could see light coming through the port as well as the jet.

The fuel supply is coming through an inline filter first.  I drained some into a clear gas and it looked clean.

The gas tank certainly has some rust.  It is not flaky and I have checked to make sure the gas is clean.

I didn't flush the gas line because I believe the fuel to be clean.

I have not synced the carburetors because I cannot get the idle correct.  I have been told that to do a sync everything else must be in order first.  Is this true?  Can I sync them at a higher rpm?  I did do a bench sync before I put the carburetors back on the bike.

[TwoTired]

Can you tell us the numbers on the carbs?

Best to get it idling before vacuum carb sync.

The symptoms still sound like inoperative pilot circuits, or some king of fuel restriction.  Maybe you could drop a carb bowl and look inside for non-gas.

If the carb couplers aren't leaking, I'd say you have to go into the carbs again.
You did check the cut off diaphragms for cracks/splits, yes?

I'm about out of suggestions, sorry.

[cb650guy]

The carbs are either PD50A or PD50B.  Is that what you need to know?

The carburetor joints are not leaking.  If they were leaking how would that affect things?  Yes, the cut off diaphragms did not have any splits that I could tell.

When I get home tonight I will check the plugs for deposits.


Adam

[cb650guy]

Ok, I checked the plugs and they were all brown/black.  They all had oily deposits on them however.  Also, one of the pipes is shooting out a flame when it backfires.