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

[F16Viper68]

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...

[grcamna2]

Possibly lean jetting?(plug color check) or a bit of stuff got into the emulsion tubes or pilot jets? something still in the tank? check the sparkplug resistor caps also.

[F16Viper68]

Possibly lean jetting?(plug color check) or a bit of stuff got into the emulsion tubes or pilot jets? something still in the tank? check the sparkplug resistor caps also.

It could be running lean, I'll check the plug color.  I'm having trouble figuring out how to set the idle mixture since the idle is varying more than +/- 50 RPM all on it own.   

I really don't think got into the carbs post rebuild but it's a possibility.  I spent a long time restoring that tank with POR-15 and the petcock has a filter on it. 

I've been meaning to research the plug wires on this thing.  I've noticed they come in two parts.  Something I've never dealt with before.

Is it possible a valve is sticking? 

Dave...

[grcamna2]

Dave,
 I worked on a man's CB650 once & I did a few basic things,he asked me to replace his Air filter so I installed a Uni high flow foam filter...he also had the Mac 4 into 2 system on it;just that change of filter was enough to really lean out his entire jetting w/ the higher flow Mac pipes to make it hesitate real bad & it was in the Fall when the weather was cooling down also which makes lean jetting really show it's stuff.I'll never forget how mad that man was(Jim K.),he took off in a real huff & told me he'd never come to me again...,so I learned that lesson.The CB650's seem to be lean running as it is with stock pipes and filter,you may need to try going up a jet size or 2 to richen it up depending on your plug color. I can tell you it ran better when it was fully ridden warmed up.

[Jerry Rxman Griffin aka MuthaF'er]

Does the '81 have the fuel shutoff mechanism? I haven't heard too many good things about those.

[grcamna2]

+1 on what Jerry Rxman Griffen just said; remove that unit....and,..take it apart,clean it out completely and then do some bench testing and reset it so that it will move more fuel through your system....or maybe install a good quality inline fuel petcock.

[F16Viper68]

+1 on what Jerry Rxman Griffen just said; remove that unit....and,..take it apart,clean it out completely and then do some bench testing and reset it so that it will move more fuel through your system....or maybe install a good quality inline fuel petcock.

Oh the vacuum petcock aka "The Son of Satan" was one of the first things I removed.     I figured I was going to have enough problems with this bike. 

Funny you should mention this bike doesn't like being cold.  During my test flight yesterday afternoon I rode it over to a fellow riders house in the neighborhood and was thinking how this thing really needs to be warmed up before riding.  I'll have to remember the old girl is warm blooded.   

One thing I forgot to mention is this bike doesn't have the t-fittings installed between the carbs.  I tried to plug the holes (dumb I know) but I had to remove the plugs because fuel started pouring out of the intakes.  So I don't have the t-fittings between 1 and 2 or 3 and 4.  Also, I'm missing the straight line between 2 and 3.  I've been researching this subject off and on and can't figure out the purpose of these lines.  But it might be the cause of my issues so I'll get the ball rolling on installing them.

Dave...

 

[grcamna2]

Dave,
   What exhaust are you running ? it may have to be jetted up richer.

[F16Viper68]

Dave,
   What exhaust are you running ? it may have to be jetted up richer.

The stock 4 x 4 that comes with the Custom.

[bjatwood]

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,

[grcamna2]

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,

+1 on what Brian said.

[F16Viper68]

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...

[F16Viper68]

Ok, one thing I never checked on this bike (stupid) is are all the spark plug wires setup correctly.  I discovered someone in the past had crossed up the #2 and #3 wire.  Interestingly enough it didn't make a whit of difference on how the bike idles, rides, or accelerates.    What in the world...

Also, today while riding (can't do this on my Wee Strom) I removed the #1 wire and noticed a big loss of power.  Same thing with #4. 

Later tonight I'm going to pull the plugs and see how they are doing. 

Dave...

[grcamna2]

Dave,
          Did you ever pilot an F16 ?

[F16Viper68]

Dave,
          Did you ever pilot an F16 ?

From the backseat, twice.    I was in the AF for 11 years as an F-16 Avionics Tech. 

Dave...

[Pecantree]

Dave
I don't have any tips for you But, I keep picturing you riding down the highway and realizing someone has just absconded with your motor!
 

Good luck
Steve

[sixthwisconsin]

Dave
I don't have any tips for you But, I keep picturing you riding down the highway and realizing someone has just absconded with your motor!
 

Good luck
Steve

HAHAHAHA!!
"Engine Missing While at Cruising Speed (1981 Honda CB650C)"
Thats the first thing that came to my mind but I didn't want to be the first smart arse to say it!!

[bender01]

Try running it in the garage in the dark on to see if there are any spark plug wire problems.

[grcamna2]

Dave,
          Have you ever taken apart one of those electro gyros before?...I imagine those Avionics units are extremely precise and ultra sensitive; a very specialized art indeed !
I truly respect your trade and I realize that you kept lots of people alive by keeping their instruments at peak efficiency; those dash instruments were the only things giving the pilots real world feedback to what was going on.

[grcamna2]

Dave,
          Did you ever pilot an F16 ?

From the backseat, twice.    I was in the AF for 11 years as an F-16 Avionics Tech. 

Dave...

That must have been a HUGE thrill ! I would LOVE to go for a nice "horizontal" flight in one of those babies !

[F16Viper68]

Dave
I don't have any tips for you But, I keep picturing you riding down the highway and realizing someone has just absconded with your motor!
 

Good luck
Steve

It took me a bit to figure out what the heck you were talking about.     If someone "beamed" out my motor I certainly wouldn't be posting here. 

Dave...

[F16Viper68]

Dave,
          Have you ever taken apart one of those electro gyros before?...I imagine those Avionics units are extremely precise and ultra sensitive; a very specialized art indeed !
I truly respect your trade and I realize that you kept lots of people alive by keeping their instruments at peak efficiency; those dash instruments were the only things giving the pilots real world feedback to what was going on.

Nah, luckily the precision internal box stuff was taken care by our backshop folks.  I was on the flightline working on everything from fire control radar systems to digital flight controls to chaff/flare systems.  When I came in in 1990 the Inertial Navigations System (INS) was using mechanical gyros but soon after we converted them to ring laser gyros.  Which were much more accurate and reliable.  I had a great time in the USAF. 

Dave...

[F16Viper68]

Dave,
          Did you ever pilot an F16 ?

From the backseat, twice.    I was in the AF for 11 years as an F-16 Avionics Tech. 

Dave...

That must have been a HUGE thrill ! I would LOVE to go for a nice "horizontal" flight in one of those babies !

Yeah I can't say I've ever done anything close to as exciting as those flights.  Hit Mach 1, vertical climbs to 30k ft, pulling 9g's (ouch), and playing in the clouds.  It did not suck.   

Dave...

[F16Viper68]

Try running it in the garage in the dark on to see if there are any spark plug wire problems.

Thats a great idea.  Thanks!

I pulled the plugs tonight and they certainly are not running lean. 



I'm not ready to call the mixture rich though because this bike has been idling a lot the past few months while I've been working on it.  I have only put 10 miles on it tooling around the neighborhood.  Once I get it registered I'm going to take it for a longer ride and see how they come out. 

I also decided to do another compression check since I had the plugs out.  Not great but at least they are all close. 



Dave...

[grcamna2]

Those plugs sure don't look lean to me...overly rich in fact.
What service repair manual are you using??
 I think those are good compression #'s;was it a warm engine? & did you hold the throttle wide open w/ the choke butterflies wide open ? What idle mixture screw adj. does the book recommend? The floats are(that little ss tang)adjustable & they need to be checked for fuel inside of them...shake them;they may be set too high.
Fresh plugs and a strong ride will give you a more accurate plug reading.
Did you adjust your cam chain? also check your spark advancer mechanism,it has centrifugal weights & springs and needs to return to base timing under a certain rpm, sometimes the weights jamb up a bit or get worn,or the springs need some tension; the pivot should be lubed w/ a thin coat of high temp lube.
It sure would be easier if the bike was in front of US,wouldn't it ?!