Carb rebuild '82 Honda CB900C

[Crazcnuk]

Anyone out there ever rebuild the carbs on a CB900 Custom?

A buddy has one that has a pop all the time when decellerating or cruising at a steady speed.

It has 4-2 turnouts, and the PO had put lawn mower baffles in them. The pipes are very blue at the first bend.

Carb kits for these are about $60ea.

Looking for tips, suggestions, what to look for. I've not rebuilt a Keihin since the 70's.

[cokeman]

Keith: It is pretty simple really. Not much different than our XS carbs. Randackk sells a rebuild kit for aroung $150. Does all 4 carbs, all the O rings gaskets etc that are required. Includes air cut off valves which is probably your buddy's problem. No jets though. I have a cb900c in the garage right now that I am resurecting.

Darrell

[Crazcnuk]

Do you think the pipes would cause it to require rejetting?

I talked to the PO on the weekend, and he said the pipes were blue when he got the bike. I assume the orignal installer of the pipes didn't retune for them.

[rpgoerlich]

I had a CB900F I just sold yesterday. Was an easy job to clean and set them up. I got 4 kits from Georgefix off of Ebay for around $75 If I remember right(been a year). I think the jets and needles were different between the C & the F. Carbkitcapital also had the gaskets and float valves.

He needs to check the valve clearances and adjust if necessary while he's in there...

[Crazcnuk]

Most of those sites don't list the CB900C, and when they do the kits are around $60ea.

The site Cokeman mentioned has them for $195/4 but no float needles or seats, jets are included.

It looks to me like what I should try and find are air valve kits and an accelerator pump kit.

[3Phase]

Most of those sites don't list the CB900C, and when they do the kits are around $60ea.

The site Cokeman mentioned has them for $195/4 but no float needles or seats, jets are included.

It looks to me like what I should try and find are air valve kits and an accelerator pump kit.

Craz, I can't help you with parts or jetting but I just finished rebuilding a set of carburetors for a '81 CB900C. They're relatively easy to work on unless you have to break the set. Good luck finding the correct parts. Some of the jets and the float valve seats are pressed in place. It's easy to ruin the O.E.M. seats and jets and the replacements are expensive. They may or may not work.

One website for the Honda DOHC Customs: CB750C covers '79-'83 Honda DOHC Customs and has PDF manuals for download if you register.

There is a thread in the CB1100F forum about bypassing the air cut diaphragms in the Keihin VB43A [VB43B in Canada] CV carburetors with little rubber plugs.

In a nutshell, the air cut diaphragms are not necessary but make merry with the low speed/idle fuel circuit to stop popping on deceleration. Leave the diaphragms in place but plug the vacuum signal port from the carb throat with a small piece of rubber.

The accelerator pump is, to put it mildly, a complete headache to clean. The pump diaphragm and the o-rings on the pipes between the carbs are easy enough to replace but the micro-holes in the carb body casting, the pump cover and the pump jets in all four carbs have to be thoroughly cleaned -- and kept clean -- or the pump won't work right. No dirt or rusty fuel or you'll be pulling the carbs and cleaning them again.

If you need them, here are the 1980 to 1982 Honda CB900C Keihin VB43 Carburetor Specifications



The Keihin VB42, VB43 and VB52 all look similar. Make sure you have the right carburetors, VB43s for the CB900C, before you get too involved. DAMHIK.

Regards,

Scott

[Crazcnuk]

From reading on the randakk site, he doesn't recommend changing the jets or float needles and seats unless the old ones are damaged (or your tuning, in the case of jets).

He also says you can just block the air control valves. Does this not make the bike pop a lot more?

[3Phase]

From reading on the randakk site, he doesn't recommend changing the jets or float needles and seats unless the old ones are damaged (or your tuning, in the case of jets).

It's the same as any other old, special-purpose, carburetor. Most parts aren't available anywhere except from junked carburetors. Any available parts and literature are probably collectors' items, museum pieces, or third-party aftermarket products with the bar sinister rampant.

He also says you can just block the air control valves. Does this not make the bike pop a lot more?

Plug the air-cut valve control port and richen the idle mixture screws a little to decrease the popping.

If you look closely at Mike Nixon's sample carburetor cleaning photo from his book at Randakks kit page, you can see the two brass pilot air jets on the left side of the Keihin carburetor that's being spray cleaned.

The air-cut valve cuts one of the two pilot air bleeds out of the pilot air circuit during deceleration. That cuts the amount of air in the pilot circuit in half, richening the mixture and decreasing popping and backfiring.

Plugging the vacuum circuit to the air-cut valve diaphragms permanently eliminates that feature along with the problems associated with them: erratic idle and low speed performance, backfiring and popping, poor fuel economy, the inability to synchronize the carbs and the, currently, ~$16.00 -- $20.00 US each price tag.


Regards,

Scott

[Crazcnuk]

The problems you mention, though, I assume are not an issue if all 4 valves are working properly?

Or are they so problematic, that it's best to get rid of them (by blocking) once and for all?

This CB900 runs fine, the carbs synched ok last fall, but the pipes are very blue at the first bend and it pops on decel (sounds like #3 or #4 cylinder). It doesn't like to start if it sits for a few days.

It also lacked some top end, but then the PO told us that the baffles we could see in the 4 into 2 turnouts were actually lawn mower baffles he had installed to make it quieter! We have taken them out, but haven't had a chance to see how the top end is without the backpressure.

The CB900C is actually rated slightly quicker and slightly faster than the xs11, but his won't touch my XS.

[3Phase]

The problems you mention, though, I assume are not an issue if all 4 valves are working properly?

Or are they so problematic, that it's best to get rid of them (by blocking) once and for all?

If all four air-cut valves are working properly the carburetor will work as-designed. They were designed and set to run lean, hence the need for the air-cut valves to stop the lean popping and backfiring. There are quite a few threads about the air-cut valves on the site I linked -- to summarize: the diaphragms suck.

I've only personally laid hands on two sets of the Keihin VB series carburetors. Out of those eight only one of them had a good air-cut valve diaphragm, the rest had holes in them. It's not a scientific sample but that's my experience with them. I think they suck.

From what I could tell after the fact, small amounts of fuel pooled next to the diaphragms behind the metal cover plates, the rubber softened and pinholes grew. Some of the valves were rusted in place and couldn't have worked even if the diaphragms weren't full of holes. I don't know how water could have gotten in there during normal operation but, obviously, it did. The design sucks pond water.

This CB900 runs fine, the carbs synched ok last fall, but the pipes are very blue at the first bend and it pops on decel (sounds like #3 or #4 cylinder). It doesn't like to start if it sits for a few days.

It also lacked some top end, but then the PO told us that the baffles we could see in the 4 into 2 turnouts were actually lawn mower baffles he had installed to make it quieter! We have taken them out, but haven't had a chance to see how the top end is without the backpressure.

The CB900C is actually rated slightly quicker and slightly faster than the xs11, but his won't touch my XS.

Yes, it goes like stink and that 10 speed transmission is kind of nice, too. I know it never wheelied when I rode it because everyone knows you can't wheelie bikes with shaft drives. <looking at the ceiling and whistling innocently>

The stock 4 into 4 pipes are heavy and they're getting hard to find. Those lawn mower baffles, if they worked, are good bit of improvised engineering, I love it!
Check out the cb750c.com and cb1100f.net sites. There are a lot of people out there that have gone through exactly what you're going through now trying to tune their bikes with aftermarket exhausts and a lot more.


Regards,

Scott

[Crazcnuk]

Well, I asn't impressed with the baffles. First, he put them in to make it quieter because it was WAY too loud?

It isn't any louder than any other 4-2 turnouts! It's quiter than my xs...

2nd, the baffles had about 1/2" ID inlet, which means all the flow of the CB had to go through 2 1/2" holes, and that seems a tad small to me, although if you look a the tips on Gold Wings, I don't know...

When we pulled them out we didn't notice (the 4 of us that were there) any appreciable increasein noise level.

Problem is for the last 2 years we were religously watching the 'baffles' as it looked like they may come loose and we'd lose one.

[3Phase]

Problem is for the last 2 years we were religously watching the 'baffles' as it looked like they may come loose and we'd lose one.

It's still a decent hack and it lasted a year and a half longer than my XS.

Did you lose any low and mid-range power when you pulled the baffles?


Regards,

Scott

[Crazcnuk]

I assume so, but it's not my bike and the guy that owns it drives like a little old lady, so it's hard to tell.

You can see it in this pic:

[3Phase]

Good looking bikes. Those are yours and Derwat's XSs bracing the CB900?.

My buddy dropped his 900 going up his driveway on the day he got it. <laughing> It put a big dent in the tank, bent the handbars, the forks and broke some of the fancy geegaws. Then, he wanted me to fix it for him. <laughing harder>



Regards,

Scott

[Crazcnuk]

Yes, derwat's is closest in the picture.

Ryan picked the CB900 up for $400. I found that tank on Ebay for $20 buy it now. He painted the sidecovers with touch up paint, and had the seat recoverd.

He managed to most the blue off the pipes for the show.

He's dumped it twice, at low speed on gravel. Broke 2 windshields. The one in the picture is cracked too. it's his first bike. The first time he rode a motorcycle he borrowed a Mean Streak 1500 and drove it for about 800kms.