Topic: 1981 CB650C - CV carbs rich at Wide Open Throttle

[mzbk2l]

I've read through numerous other posts on this site looking for ideas about my problem, but have not found any definitive solutions yet.

( http://www.sohc4.us/forums/index.php?topic=9582.0 and http://www.sohc4.us/forums/index.php?topic=12142.0 were the closest to my problem)

My situation:  This bike has (as far as I can tell) a stock engine, stock exhaust, and stock carbs.  It starts fine, idles fine, revs fine all the way to the redline in neutral, and rides fine until I get to about 1/2 throttle.  As soon as I hit 1/2 throttle in any gear, it goes so rich that it falls on its face.  I can ride for a month with light throttle and the plugs look fine, but one attempted WOT (Wide Open Throttle) pass will leave them black and dripping with gas.

While my problem may not get solved here, I am willing to bet that I have taken more troubleshooting steps than almost anyone else here has, so I'll share them in hopes of helping someone else.

History:    While living in Michigan, I purchased the bike in 1996 from a previous owner who had let it sit for 4-5 years, so the tank was rusty, the carbs were a mess, and it didn't run.  I purchased a manual, cleaned up the tank, and cleaned (but did not rebuild) the carbs.  That was enough to get it running, and it has run in roughly the same condition ever since.  (Very peppy at idle, takeoff, and part-throttle acceleration, but choking and dying at WOT.)

My initial troubleshooting was to apply the choke while the bike was struggling at higher rpm.  That bogged down the engine so hard it nearly threw me over the handlebars.  Second step was to shut off the fuel petcock.  After about 10 seconds, the bike took off!  Obviously it was running too rich.... now, what to do about it?

The most obvious step was to check for a clogged air filter.  I tried a new air filter, a foam air filter, and no air filter, with and without the air filter cover, and with and without the side cover. 
Result:  MINOR improvement with no air filter or cover, but it would still readily foul the plugs with only a single WOT run.

Second step:  Check the jets and the float height.  Honda said the bike took 120's, and that's what it had in it.  The floats are plastic and non-adjustable, so I bought new float and inlet needle kits for all four carbs.  Around this same time, I purchased new rubber tubes from carb to engine, new t-fittings and hoses for all carb vents, and a new petcock.
Result:  No difference.

Next:  Checked spark quality, ignition timing (electronic igniton), replaced plugs and wires again. 
Result:  No difference.

After this I moved to Arizona.  Roughly the same elevation (1,000 - 1,200 feet), but much warmer weather. 
Result:  No difference.

By this time, I had around 6,000 miles on it, and one day on my way home from work, a rod bolt let go and the rod gave me some extra crankcase ventilation.  I found a new engine (complete minus carbs) from an '81 CB650C that was on its way to be sold when it fell off the trailer and got dragged by one of the straps.  Now I have a new engine in the bike with my original carbs, and guess what? 
Result:  No difference.

After I put in the new engine and had the same problem, I bought and installed four carb rebuild kits that included alll new main jets, metering rods (needles), slow jets, pilot screws... I forget what all, but basically EVERY metered part in the carb.  Now I KNOW the main jets are correct, but
Result:  No difference.

Again, I ignored the problem for a few years, until it started to bug me again, and I got to wondering if the spark could be too weak to ignite the mixture at higher rpms.  At that time, I purchased replacement ignitors (even though they should either be good or bad, not weak), new Dyna coils, new plug wires, and new plugs. 
Result:  No difference.  (Well, it did start easier when it was cold, but that's not a big deal in Arizona.)     

OK, at this point, I have:
- Original carb bodies, slide pistons, and carb covers (basically, slide piston cylinders).
- Replaced all jets, metering rods (needles), floats, needles and seats, carb gaskets, intake tubes, etc.
- Replaced the entire ignition system.
- Replaced the engine.

Obviously, the ONLY thing on the bike that remains constant through all of this is the carb bodies and the slides.  What's left to do?  I haven't found a decent set of replacement CV carbs on ebay yet for a price I'm willing to pay, so here's what I tried:

I took a few plastic washers (actually automobile oil drain plug gaskets) and used one in each carb to shim the springs above the slides so that there was more resistance against the slides pulling the needles out . 
Result:  Biggest difference yet!  Now I can get almost 3/4 throttle in every gear, and the bike has about 12 mph more top end than it's ever had.

The thing that puzzles me is that the metering rods should only control part-throttle mixture, and the main jets alone should determine WOT mixture, so this must be a main jet issue.  Why would the bike not operate properly with factory main jets in it?  Apparently I'm not the only one experiencing this issue, as I've found several other unsolved posts with similar issues here.

My next step:  I bought a Dyna-Jet tuning kit that I'm going to play with when I get home.  I'm giving up on stock size jets and I'll try other sizes.  I had hoped to actually FIX the problem, which would mean that the stock bike would work with stock jets (including mains), but I've given up on that and I'm going to tune it until it works.

I've done a lot more than I mentioned here, but those are the big troubleshooting steps that I would bet get recommended the most.  If you can think of anything I missed, please let me know....

[eurban]

Long shot but . . . I assume that you are not having any charging issues as the CB650s are quite prone.   Do you have a superbright halogen bulb in there or something?  Low voltage would hurt spark performance particularly at high rpms. . . . . .Overall you have been pretty thurough in your tinkering and it would seem quite likley that your carbs have some issue(s).  Here's what I can think off . . .Parts missing or improperly installed in the carbs.  You and perhaps the PO have been thru them and it may be that something is simply absent or not put in right.  Can the vacuum slides go in backwords?.  Is the needle jet (the bushing that the needle slides thru) upside down  or not seated  properly?  Are the slide springs the correct ones for the carbs?  Etc, Etc.  Don't assume that the carbs were together properly when you got them!   On CV carbs, there is no direct connection between the throttle cable and the slides.  Even though you open the throttle all the way(WOT), the slides position is still being determined by the vacuum aplied to it. The rate that it raises and the height that it acheives will be affected by the springs above it and the vacuum applied.  Certainly when the bike is running all out one would think that the needle has completely left the jet yet yet I wouldn't be so sure thats what is happening in your case as your outright performance is limited.

[mzbk2l]

Good points, and good questions - thanks!

If I recall correctly, the charging system is working well.  I know I've checked it with a voltmeter, and the system voltage is 1-1.5 volts higher with the engine running than it is with the engine off.  (I checked on that when I was wondering about the spark quality.)

According to the manuals and all the parts blowups I've been able to find, the carbs are complete, and all of the parts are installed correctly.  While it is certainly possible that the PO was in the carbs, it's not very likely; he wasn't really mechanical, the bike only had 6,500 miles, and EVERYTHING else on it was factory original (even the tires)!

I'm reasonably certain that the carbs are assembled correctly; I'm certainly not perfect, but I've been an auto and diesel mechanic for over 15 years, and I don't get many rejections on my work.  (Mostly Fuel Injected vehicles, and mostly automatic transmission work, so I'm on new ground with the carbs, but I'm very familiar with machinery in general.)  I did use exploded parts diagrams and a service manual when I reassembled the carbs, and I did them one at time so I could compare my reassembly to a (supposedly) factory assembly.  The problem that existed before I rebuilt the carbs remained the same after I rebuilt them, so I did not introduce any new issues through an assembly error.

I have wondered myself if the slide springs are correct, but unfortunately they are discontinued everywhere I've checked.  Honda doesn't have them... Some of the online suppliers show them as available, but every time I order them, I get the big "Discontinued" email back.

It runs sooo good at idle, off-idle, and part-throttle, that I don't know what to do other than begin playing with the main jets.

Thanks for taking the time to read this, and offer your thoughts; sounds like you're pretty familiar with these things!

[750goes]

do these carbs have the needle and clips in them for adjusting the needle height....

do these carbs have an accelerator pump

just thought I would throw that out there

is the timing too advanced or too retarded ??

[mzbk2l]

Thanks, 750goes,

These needles are non-adjustable, as are the floats.  (One of the reasons I wanted to try out the jet kit; it comes with adjustable needles.)

The carbs to have an accelerator pump, and it is working fine; they are very peppy on acceleration, and will throw you back in the seat nicely as long as you don't crank it all the way to WOT.

The timing is dead-on, but I have tried it a little advanced and a little retarded as well.

I'll report back when I try some different jets; if those fix it up, I'm sure gonna wonder why Honda put in 120s and it apparently ran fine that way when it was new....

[750goes]

If the plastic shims worked for you - get some more slightly THINNER or THICKER ones to put in...whatever you think will do the trick..........
If this makes a BIG difference again I would question the rebuild kits...

What are the CARB types and is there any model number stamped on them??? it may be worthwhile knowing what the manufacturer standard settings were, or if there was ever a stuff up in their production...

Intriguing Dr Watson ??

[750goes]

Found this information - thought it may help - can you confirm your engine series and carb type for the bigger jets - or could it be that you have older engine with bigger carb jets ??

still very interesting 

http://www.motorcycleproject.com/motorcycle/text/specs.html

[Ibsen]

Have you replaced the needle jets as well, or only the jet needles?

[mzbk2l]

Thanks for the replies 750goes and Ibsen,

Both engines I've had in the bike are  (as far as I can tell from every resource I've used) original 1981 Honda CB650 engines.  I bought the bike with somewhere around 7,000 miles on it from the original owner, in very nice shape with the exception of the rust in the tank and the 5-year-old fuel in the carbs.  The replacement engine I put in it came from a 13,000-mile 1981 CB650 Custom that a used motorcycle shop was moving to their shop when it fell off the trailer and was totalled.

The carbs are the correct VB44A Keihin carbs, and I first cleaned all of the factory Honda jets, and then replaced all of the jets (needle jets, needles, slow jet, etc.) with Keyster parts.  No difference.

I'm working in Iraq right now, so I can't go out to the bike and try anything, but I just ordered the Dyno-jet kit and I've been sitting over here pondering possible causes and solutions.

Thought I'd pick your brains to see if anyone has any different ideas before I go home and try the jets and adjustable needles....

[Ibsen]

Sorry to say so, but my experience with Keyster kits is that they are crap. The ones I bought for my Kawasaki, witch have the same type of Keihin CV carbs as the 650,  created the same problem as your carbs. And the reason was needle jets and jet needles that was useless. When I finally found stock replacements and measured them to compare, the Keyster parts was way off in size. The jet needle diameter was too small and the taper was wrong, and the needle jet was too large.
I have heard that Honda GL1000 and 1200 owners have had the same problem with Keyster parts.
I guess there is a reason why these kits are cheap to buy.

And I'm not the only one who believes this:

http://www.dansmc.com/carbs.htm

Scroll down and read the last two lines.

[750goes]

I think you need to go down in size on the main jet - try a #90 and drop the float to 12.5 (same as the 1980 650)..
that will give you a good yardstick to start with.

then if all else fails, more shims....tighten up the space for flowing fuel between the jet and needle....

keep us posted

[Ibsen]

The 80 model had Keihin PD carbs, and correctly they had 90 main jets stock. The 1981 model in question here have got Keihin CV carbs with 120 main jets stock. And a drop from 120 to 90 seems slightly dramatic. In fact, normaly the 650s was jetted lean from the factory, so I still don't think the main jet size is the problem.

.

[750goes]

Ibsen,

I bow to your knowledge, but if the bike is running really rich at WOT, then he needs to make a dramatic change and see whether this change has the desired effect......... I think from the post he was going to get more jets anyway....so start at the furthest end of the spectrum and work backwards to find the right solution.......
if the #90 doesn't work then drill out these #90 jets in increments until you get to your required running at WOT, then buy a set that you know will work - saves buying lots of different jets......

hope this helps explain my not so logical approach.

[eurban]

Assuming that the bikes other systems are up to snuff and that it hasn't been significantly modified from stock (exhaust, and intake mods for example). . . . .If the carbs are not some how damaged, worn or improperly assembled & adjusted, then there is no reason that with stock spec jets, needles etc that it shouldn't run well.  If the Keyster parts are not a match for the stock parts then this could be the problem, although the problem seemed to exist prior to their installment.  Assume for the moment that the Keyster parts are OK, then it really would make more sense to figure out what is wrong then trying to correct the problem by changing the main jet to a size smaller than stock.  You would be treating a symptom rather than the disease.  Also there is really not much correlation between jetting on the earlier slider type carbs and the CV carbs (as the huge stock spec difference of a 90 slider jet size  vs a stock 120 CV jet size would indicate) so changing to the specs of another type of carb isn't a good idea. The CVs are a totally different animal!   Probably the easiest solution here would be to aquire a working set of CB650 CV carbs and slapping them on.  You may be better off spending your money there instead of buying parts  to "custom tune" the set you have now. 

[750goes]

If you want to see a change, then start off with 110's, see if that helps the problem--- irrespective of where you start, make a start........it will only take a 5 minute ride and a plug chop to find out if things are better.....also there was a mention of plastic shims........they could do the same thing by getting the needle lower in the jet even if kept the same size.....just to reduce the volume of fuel coming through.....

maybe the carbs are screwed....when the previous owner let them sit for a few years it may have corroded the passages, slides, whatever, and they are worn out.......

not attempting to compare the two styles of carbs involved..

[mzbk2l]

Thanks for all the suggestions and debate, guys.  This is interesting stuff.

I agree that the bike SHOULD run properly with the original factory parts.  That has been my goal with all of the steps I've taken so far.  Since nearly everything (engine, air filter, needles, jets, floats, etc.) has been changed with no effect on the problem, I kinda feel like I've eliminated those things as culprits.

For example, even if the Keyster parts are junk (which I have no doubt could be true in many cases), they made NO DIFFERENCE from the stock parts.  I would think if the jet sizes were different, I would have noticed some change in my symptoms.

I do watch ebay occasionally for a different set of carbs to try, but they alway seem to go for over $100.  It was a toss up as to whether to try another set of carbs (with possibly the same problem) to see if that fixed the problem, or spend the $100 on a jet kit so I could do some different tuning to get an idea of what is going on.  For now, I bought the jet kit, but the next (final?) step will be to try a different set of VB44A carbs.

A couple of notes:  The floats are non-adjustable plastic on these carbs, so no tuning capabilites there.
I did not measure the needle jets, but when I took out the originals and installed the Keysters, there was no change in my symptoms.
The factory (and the Keyster) needles are non-adjustable.  The Dyna-jet kit is supposed to have adjustable needles, so I can play with that setting and see what kind of difference it makes.

Finally:  Keep in mind that I did find at least 3 other posts on this site describing nearly this EXACT same problem, and all of them with 1981 CB650C bikes.  That's almost enough to call it a trend.....

[750goes]

I agree, I looked at a few others and it seemed reasonably consistent - maybe more experienced members can assist with this topic ........ my brain needs a rest....little of it though there is 

[Ibsen]

750goes, I can follow your logic, and it is a reasonable one as well.
The difference between the CV carbs and piston carbs is that the slides in the piston carbs are controlled by the engine intake vacum and not by the trottle grip. On the CV carbs the trottle grip open and close the butterfly valves. To get the slides to open fully on the CV carbs, you will  have to acchieve a certain engine speed to create enough vacum in the carb ventury to lift the vacum slides all the way up. If the jet needle and needle jet that are controlling the mixture in the mid range are providing a mixture that is way too rich, the engine will start to bog down and you will never reach the point where the engine is spinning fast enough to open the slides fully, and you will not be able rev the engine to the red line either. Normally when this problem occure, you can rev it all the way to redline in 1st and 2nd gear, but the higher gear you are in, the worse the problem gets. I had the exact same problem with a set of CV carbs a couple of years back. And the Keyster parts I put in was no better than the worn out parts I took out. Then when I compared the Keyster parts with the NOS parts that I bought, I found that Keyster parts had the same sizes as the worn out parts!
 But as you said, the main jets can also be the problem if the PO drilled out the stock 120 jets to a larger size, or if the Keyster main jets in fact are larger than the #120 stamped on them.

mzbk2l, I would  check the main jets and try to find out if they are the correct size according to the stamp on them. Also check that the choke butterflies are fully open and not blocking the carb venturi when the choke is off, and that all the air jets and their passages are clean and open (The air jets are located under the air jet cover and you have to take out the vacum piston to see it, and the intake holes for them are the holes you can see on the edge of the air filter side of the carbs.). Have you ever syncrinized the carbs btw? Not that it will solve the problem wth the rich mixture, but at least the cylinders will run even.

An article about CV carbs:

http://kz400.com/CB650/CV%20Carbs.pdf

[cb650]

Try a new set of stock jets or 1 size smaller.  Also a new set of plugs.  Even if they have only been run a little.  I had a set that ran good until about 6k then it was like a revlimiter. 
Also have you checked the air gap on the pulse generator?




                        Terry

[750goes]

Throw another one in there - a hotter set of plugs

thanks ibsen - good article - 

[mzbk2l]

More good suggestions; thanks - unfortunately these are mostly things I've checked already....

The carbs have been COMPLETELY disassembled a number of times, I've used compressed air, carb cleaner, and various brushes to make sure EVERY passage in each carb is clean - several times.     

I've checked the pulse generator gap, removed and reinstalled the pulse generator, played with the timing, played with the gap......

It gets new plugs every time I foul out a set; I used to clean them and try to use them again, but then I couldn't tell if my troubleshooting was going in the wrong direction or if my fouled plugs were making a difference.  I've tried NGK, Champion, and at least one or two other brands.  I think I've also tried other heat ranges, but I don't remember for sure (this has been an eight year process).

I have also verified that the choke blades are fully open, and I did have the carbs sync'd at a shop one time when I gave up and tried to have it fixed by a "professional."  They charged me $90 to put an air filter in it (I had a brand new one at home, which was out in an attempt to get more air in to offset the rich condition) and tell me that I had the wrong plug wires on it.  (I had switched from a new set of plug wires and caps from Honda to the Dyna coils and the recommended supression core wires - again, no difference.)

One other thing to note, on the subject of CV carbs not being directly related to throttle position; (and by the way - Ibsen, you're right - 1st gear revs the highest, and each successive gear is progressively worse.  5th gear won't go much over 5,500 rpm)  If I just crank it to WOT, it will bog down almost immediately, but if I'm cruising on the freeway at 55 or so, I can slowly "creep" up on the throttle and almost feel the exact speed at which it suddenly goes rich.  I can back off just a hair and it runs fine,  and then get in it just a touch and it will start to bog and choke.

Hopefully the jets in the Dynojet kit are more accurately sized, and I can start to get a handle on the problem when I see what effect different jets and needle heights have on the condition.

[racing33]

 Very interesting post here. At least to me as I also own a 81 cb650c that has the same EXACT problems. The only thing that has helped mine is to run it without a proper air filter. With the proper air filter it runs great until about 5000rpm and then she starts to load up. I have tried a lot of the same things that have been tried by mzbk2l.
Thanks to Ibsen for the very informative article about the cv style of carbs. This article gave me a couple of areas to look into as well as some great general information.
Who ever finds the cure for this problem has to post it here for the rest of us to use. Other than this problem my bike is a fun bike to ride.
Bruce

[Ibsen]

Mike Nixon, I believe he's a member of this forum, have written an excellent book about tuning and servicing the CV carbs used on the CBX, and the DOHC CB750, 900 and 1100. It can also be used for servicing the CB650 carbs:

http://www.motorcycleproject.com/motorcycle/text/booknook.html#1

And this Honda GL site, Randakk's Cycle Shakk, also have a lot of info about the Keihin CV carbs even if they might be a bit different from the 650 carbs:

http://www.randakks.com/TechTips.htm

[cb650]

Has someone put a shim under the needle?    I have found 2 sets of carbs like this.  And you dont know if PO has drilled the jets.




                     Terry

[750goes]

What sort of brushes did you use to clean the carbs with and or passages ??