Topic: Engine Missing While at Cruising Speed (1981 Honda CB650C)

[grcamna2]

The CV carbs dont have adjustable needles.  I have seen them shimmed but think that creates other problems.   
Viper do you have the carb vent tube hooked to the airbox?
Unless the jetting is way off the CV's will do a good job compensating.

Are you talking about the hoses that originate from the t-fittings that go between 1/2 and 3/4 and route back towards the airbox? 

Dave...

Those 2 T fitting hoses from 1-2 &3-4...are just supposed to curl back around the backs of the carbs, but not connect to the airbox...they're just 2 vent-to-the -atmosphere hoses.

[cb650]

Viper yes those are what I was asking about.
Scunny in 82 the CV's went with screw in idle jets.  Dont know the #'s but essentially the same carb as 81.

[fmctm1sw]

wooooooooooooooooaaaaaaaaaaaaaaahhhhhh there, back up the train. did you say the idle jet isn't removable ?
how did you clean them then. can't you pull them out. ??

Lol.  Carb cleaner and compressed air.  I know they are clear. I've forced carb cleaner through them.  They are pressed in and can't be removed without major modifications.  Think drill, extactor, tap/die, etc.

The way I did mine was like this.  I removed the mixture screw and connected a tube there.  Then, blow into the tube and cover the idle jet passage.  You'll be able to hear a change in the pitch of the sound IF air is passing through the idle jet.  Found one of mine clogged that way...

[grcamna2]

Viper,
    How's it doing so far ?

[F16Viper68]

Viper,
    How's it doing so far ?

Fixed one issue but some issues still exist.   

I fixed the issue where it wouldn't accelerate past 5k RPM's.  I think it was being caused by a stuck auto advancer.

The issue now is the bike is hesitating when I crack the throttle open.  If you give a moderate twist of the throttle, it hesitates and then takes off like a rocket is stuck up its ass.  It also hesitates if you just crack the throttle a bit.

I was able to finally figure out how to get all of the carbs sync'ed up (#3 is a PITA).  I tried to adjust the idle mixture on the carbs using the service manual instructions but I can't get the idle speed to drop as I back out the idle mixture screws.  You're supposed to stop when it drops 50 RPM.  Well my idle just keeps going up and up.  I think I was backed out six turns and it wouldn't drop.   

For giggles I tried riding it with the screws at 2.5 turns out (way too lean), 3 turns (better but hesitates), and 4 turns (same results).  I pulled the plugs and they are bright white indicating a lean condition. 

I'm scratching my head at this point because no matter what I do it's running lean.  The only thing I can think is the idle circuit.  But I know it's passing air and I've force carb cleaner through the circuit on all four carbs.     
 
A while back you asked what type of air filter I was running.  I'm running something I bought at Advance Auto but I can't for the life of me remember who made it.  Have there been issues with aftermarket air filters?

Dave...

[grcamna2]

Install a Honda O.E. air filter and then recheck you're jetting w/ a plug color check...you may have to richen up you're jets.

[F16Viper68]

Install a Honda O.E. air filter and then recheck you're jetting w/ a plug color check...you may have to richen up you're jets.

I didn't think the main jets came into play at idle speed? 

Dave...

[fmctm1sw]

Install a Honda O.E. air filter and then recheck you're jetting w/ a plug color check...you may have to richen up you're jets.

I didn't think the main jets came into play at idle speed? 

Dave...

He was probably talking about riding it in general.  I wish you were closer, we could put our heads together.  I didn't read back through the whole thread but is it idling properly?  Are all the pipes getting hot?

[F16Viper68]

Install a Honda O.E. air filter and then recheck you're jetting w/ a plug color check...you may have to richen up you're jets.

I didn't think the main jets came into play at idle speed? 

Dave...

He was probably talking about riding it in general.  I wish you were closer, we could put our heads together.  I didn't read back through the whole thread but is it idling properly?  Are all the pipes getting hot?

It's idling just fine now and all the pipes are getting  hot.  I pulled the plugs one at a time to ensure all four cylinders are firing.  Plus I discovered if a cylinder isn't firing the Motion-Pro sync gauge slams to the top.   Today I was able to get all cylinders sync'ed.

I've heard of rich/lean issues stemming from air flow issues so I'm sure the air filter could affect the mixture.  To be safe I've ordered a Honda OEM air filter and 125/130 jets.   

I have a question for the collective. If you screw the idle mixture screws out (counter-clockwise) does that make the mixture richer or leaner?

Do you still need a caliper piston?

Dave... 

[bender01]

Out will richen.  = more air!

[Spanner 1]

OP dude you have had 108 folks replying to you..... and you don't know/care about your air screws for your bike.. how much hand-holding do you need ??

[fmctm1sw]

Do you still need a caliper piston?

Dave...

In short, maybe.  I gambled on a complete system off ebay.  The master cylinder was frozen and both of the pistons were rusted.  I was able to get one out and clean it up to the point I think it will work.  So I got both calipers on it and a master cylinder from a small four right now.  I haven't had a chance to bleed it yet.

[F16Viper68]

Do you still need a caliper piston?

Dave...

In short, maybe.  I gambled on a complete system off ebay.  The master cylinder was frozen and both of the pistons were rusted.  I was able to get one out and clean it up to the point I think it will work.  So I got both calipers on it and a master cylinder from a small four right now.  I haven't had a chance to bleed it yet.

Well let me know if it doesn't work out of you.  I have one that only has some minor pitting and might work.  I'll send it over if you need it.

Dave...

[cb650]

FYI for everyone the vent hoses are supposed to go to the airbox originally.
Scunny yours is a 79 right?   79/80 you can remove the press in idle jets eaisy enough.  The 81 CV's would be a problem though.

[F16Viper68]

FYI for everyone the vent hoses are supposed to go to the airbox originally.
Scunny yours is a 79 right?   79/80 you can remove the press in idle jets eaisy enough.  The 81 CV's would be a problem though.

Yeah, like I said before they can be removed but you have to use an extractor, then tap out the tubes, and install the screw in type that are in the 82 carbs. 

I know those passages are clear but I'm beginning to wonder how clear they are.  I know the actual jet hole is clear but what about past the hole? 

But the bogging issue I'm experiencing does seem to point towards the accelerator pump.  It's been replaced and it does pump fuel but maybe it's not adjusted correctly.  I think I'm going to pull the rack (sigh) and test it.

I have a Honda OEM air filter coming and 125 and 130 main jets but I think I'm shooting in the dark there. 

Dave...

[fmctm1sw]

But the bogging issue I'm experiencing does seem to point towards the accelerator pump.  It's been replaced and it does pump fuel but maybe it's not adjusted correctly.  I think I'm going to pull the rack (sigh) and test it.
Dave...

If you do that, use that tube technique (I would say "blowing technique" but I l know where that would get me around here  I mentioned earlier, trust me on that.  On a related note, I don't know if my accelerator pump is working either.  How are you verifying it's pumping fuel?

[F16Viper68]

But the bogging issue I'm experiencing does seem to point towards the accelerator pump.  It's been replaced and it does pump fuel but maybe it's not adjusted correctly.  I think I'm going to pull the rack (sigh) and test it.
Dave...

If you do that, use that tube technique (I would say "blowing technique" but I l know where that would get me around here  I mentioned earlier, trust me on that.  On a related note, I don't know if my accelerator pump is working either.  How are you verifying it's pumping fuel?

Yeah, I've done the tube thing and they are flowing.  Because every time I'm working on the carbs with the #2 float bowl off and I activate it the little #$%* sprays me.     But now I'm going to take a close look and make sure it's spray out the accelerator pump holes in each carb and it's gapped correctly. 

Dave...

[lucky]

Today I was able to take my first ride on a Honda CB650C I've been working on for the past three months (link in signature).  I noticed while riding that the engine was missing/stumbling when I held a steady speed (2.5k RPM to 3k RPM).  The engine pulls very smooth (strong) when you twist the throttle but as soon as you steady out it starts to stumble a bit.

I've done a lot to this bike but I'll list some of the stuff I've done that could affect the engine performance:

+ New spark plugs
+ Adjusted valves (all but one was tight)
+ Set timing statically and confirmed dynamically
+ Rebuilt the carbs (idle mixture set to 2.5 turns out, sync'ed)
+ Changed the oil
+ New air filter
+ Compression 150 psi across all four cylinders
+ New intake boots/insulators
+ Checked for vacuum leaks (none...now)
+ Verified spark at all four cylinders
+ In my opinion is idles a bit rough and occasionally backfires

Any ideas on where to start or things to look at?  New wires? Coils?

Dave...

Start at the bottom of the list .
"In my opinion is idles a bit rough and occasionally backfires"
I would bet money one of the idle jets is clogged up.
Take them out and make sure you can see daylight through them.
Don't just squirt them with carb cleaner. Look through them.

[lucky]

My 750 ran a little rough between 2 to 4 grand also. I turned my petcock off and checked to see if the bike smoothed out. It did, so I adjusted my float heights (one float needed replaced wouldn't float) and cleaned the jets up, synced them and it runs as smooth as a babies behind now. Give it a try,

I'll give that a try but I replaced the needle floats when I rebuilt the carbs and polished the seats.  The float heights on the CV carbs aren't adjustable.

Dave...

If they were all metal float needles and you polished the seats it WILL leak.
No way you could get the angle correct.
Now you will need to put in new seats and this time put in rubber tip float needles.

[grasscutter]

+1 Yes, to verify proper operation of accel pump.

-1 on polishing the needles?  No. 

+1 on replacing them with rubber tipped.

-1 about the rich / lean suggestion.  WRONG!
More air makes things more lean.  Screwing them out gives them more air.
Tightening the screw is richer and loosening it is leaner. 
This is standard on all idle air screws on every carb known to man.

Pilot jets of incorrect size will absolutely play a part in a 'bog' when cracking the throttle wide.
They handle all starting, idling and initial power delivery.
If nothing seems to be happening when adjusting idle air screws, you have the wrong idle jet (not likely),
or the pilot system still has an obstruction (likely).
(your diagnosis of a slight backfire also confirms this)

Are you soaking the carbs and related parts in a Chem Dip overnight?

+1 on looking through each jet, not just spraying cleaning fluid. (Not wise to poke little strands of wire through them though!)

[w1sa]

.......................................-1 about the rich / lean suggestion.  WRONG!
More air makes things more lean.  Screwing them out gives them more air.
Tightening the screw is richer and loosening it is leaner. 
This is standard on all idle air screws on every carb known to man..........................................

Which is not to be confused with 'Idle Mixture Screws' (IMS), which I think these 650 carbs have...............an air screw regulates air volume to the mixture chambers/tubes..........an IMS regulates mixture volume to the carb throat for delivery to the cylinder(s).

So, for these carbs the IMS should be screwed OUT for RICHer,  and  IN  for  LEANer

[F16Viper68]

+1 Yes, to verify proper operation of accel pump.

-1 on polishing the needles?  No. 

+1 on replacing them with rubber tipped.

-1 about the rich / lean suggestion.  WRONG!
More air makes things more lean.  Screwing them out gives them more air.
Tightening the screw is richer and loosening it is leaner. 
This is standard on all idle air screws on every carb known to man.

Pilot jets of incorrect size will absolutely play a part in a 'bog' when cracking the throttle wide.
They handle all starting, idling and initial power delivery.
If nothing seems to be happening when adjusting idle air screws, you have the wrong idle jet (not likely),
or the pilot system still has an obstruction (likely).
(your diagnosis of a slight backfire also confirms this)

Are you soaking the carbs and related parts in a Chem Dip overnight?

+1 on looking through each jet, not just spraying cleaning fluid. (Not wise to poke little strands of wire through them though!)

Yeah the needles were replaced with rubber tips needles.

Ok thanks for straightening me out with the idle mixture screws.  I started mulling it over and it seemed I had rich/lean thing backwards in my head. 

Are pilot jets the same as slow speed jets?  If so, they are not removable on these carbs.  Carb cleaner and compressed air flows through them and I've visually verified they are open.  Multiple times.

Oh something happens when I back out (lean) the idle mixture screws...the idle speed keep climbing and climbing and climbing 

I did soak these carbs overnight. 

[bender01]

Out will richen.  = more air!

Out for rich in for lean,
Sorry for that!

[grcamna2]

But the bogging issue I'm experiencing does seem to point towards the accelerator pump.  It's been replaced and it does pump fuel but maybe it's not adjusted correctly.  I think I'm going to pull the rack (sigh) and test it.
Dave...

If you do that, use that tube technique (I would say "blowing technique" but I l know where that would get me around here  I mentioned earlier, trust me on that.  On a related note, I don't know if my accelerator pump is working either.  How are you verifying it's pumping fuel?

Yeah, I've done the tube thing and they are flowing.  Because every time I'm working on the carbs with the #2 float bowl off and I activate it the little #$%* sprays me.     But now I'm going to take a close look and make sure it's spray out the accelerator pump holes in each carb and it's gapped correctly. 

Dave...

Dave,
    Checking each carb throat for equal flow through each accel "jet" does make it respond Lots better ...those little accel "squirter" ports in each carb throat can give you a bit of work to clear them completely ..I've used a can of Gumout and just kept back-flushing them, I haven't tried using a tiny little E string or better the smallest wire in a Honda jet cleaner tool(usually for only pilot jets of the smallest size)...but that may help also . I would set the adjustment to have no gap..or at least a lot less than is recommended,they always respond better with less gap .

[grasscutter]

.......................................-1 about the rich / lean suggestion.  WRONG!
More air makes things more lean.  Screwing them out gives them more air.
Tightening the screw is richer and loosening it is leaner. 
This is standard on all idle air screws on every carb known to man..........................................

Which is not to be confused with 'Idle Mixture Screws' (IMS), which I think these 650 carbs have...............an air screw regulates air volume to the mixture chambers/tubes..........an IMS regulates mixture volume to the carb throat for delivery to the cylinder(s).

So, for these carbs the IMS should be screwed OUT for RICHer,  and  IN  for  LEANer

Checked my manual to be sure we were talking about the same thing.

OP has a 1981, which (unless changed by the previous PO), he should have CV carbs.
CV carbs only have a pilot screw adjustment.

They are factory set with limiter caps.  The caps only allow the pilot screws to
be turned clockwise to a leaner setting.  <--this is direct from the manual.

So.. turning the screw in (clockwise.. tighter) makes it leaner. 
The factory set the limiter caps, so that whoever owned the motorcycle
would not be able to turn the pilot screw 'out', counterclockwise,
for a richer setting.

Its not myth that they made the 650 as lean as possible and if you wanted
to adjust anything, you ended up making it even leaner!!!

Float level also plays a HUGE role in a rich / lean condition.
If float level is to high (too much fuel in bowl), the result is 'rich' fuel / air mixture.
Float level too low (not enough fuel in bowl), the result is 'lean' fuel / air mixture.
The floats have to be DEAD ON!