Topic: 82 CB650SC Carb Vacuum Leak Question

[janich4644]

Hi All,

I've been lurking the forum for a while now and I've found a lot of great information.

However, I have a vacuum leak on my 82 CB650SC that I've had a hard time pinpointing. When started, the bike will rev and hang around 3-4k RPM and even higher if the choke is OPEN. I've been doing the carb cleaner spray test on all of the boots and carbs. I didn't seem to find anything for a while until I took the tank off to get better accuracy with my spray. I found if I sprayed between carb 3&4 that the idle would change drastically. between the two carbs is a short stubby tube (fuel rail?) and a longer thinner tube (accelerator pump rail?). here's an image of the two split apart. https://imgur.com/C6IkZtJ
Could I be missing a price that fits between these two carbs?

Could the throttle shaft seals also be in question? It seemed like that area was what caused the large change in idle.  And if there was an issue with the seal or if there was one missing, would it really cause that bad of a vacuum leak? it does seem like there's a little play in the throttle shaft.

My question is, could I be missing a price that fits between these two carbs? If not, am I right to question the throttle shaft seals as being the culprit?

Also, I found this piece of rubber hose https://imgur.com/ilCu6uO that was in my bucket of left overs (mostly screws I was replacing) after I did my rebuild. It doesn't seem to fit between 3&4, but does seem like it could fit between 2&3. Does anyone recognize it?

Thanks,
Justin

[kerryb]

There should be 3 tubes (pipes) connecting each pair of carbs (VB 44c).  If you have a left over tube, it should probably be reinstalled.

[janich4644]

There should be 3 tubes (pipes) connecting each pair of carbs (VB 44c).  If you have a left over tube, it should probably be reinstalled.

Very interesting. I've never seen that diagram before, nor have I see those air tubes 

So looking at this diagram https://images.cmsnl.com/img/partslists/honda-cb650sc-nighthawk-1982-c-usa-carburetor-assy_bighu0219e0c18_6f23.gif it looks like there is one between 1/2 and 3/4 and hoses that meet at a T joint in the middle. but then what does the middle T joint plug into, if anything?

[kerryb]

That picture comes from the Honda factory service manual.  I think we need to start with verifying exactly which carb model you have.  It is stamped on the side of the carb bodies.  My carbs from a stock cb650sc are model number VB44c.  The double-tee carbs are shown in the manual are model VB44a.  What do you have, and are you still using the vacuum diaphragm fuel valve?

[Mr. Mike]

Well I’m no 650 guru, but by chance does the T follow through with a tube to the air box?
The CB650 information on this site shows two tubes extending from the rack, but I cannot find information on just where they go. One might be fuel...but the other? I’m going on memory about the airbox as a friend has an 81 CB650, and we’ve been around the block with his carbs.

[janich4644]

I looked at the stamp on my carbs and they are definitely the VB44c model.

I picked up some felt at the store and am going to attempt to make some DYI gaskets. fingers crossed that they do the trick.

as far as the missing air tubes, I found them in a box of parts the previous owner had... completely broken. from looking around online, I think they just allow air to escape back toward the air box. Once I have the felt gaskets in place on the throttle shafts, I'm going to attempt to get it running again and see if I can't just go without those. they're a bit hard to find and what I have found are expensive ($70 for the pair). If they just let air escape, I don't really seem the need to replace them yet.

[janich4644]

So I replaced the felt gaskets in question, remounted the carbs and it still seems to leak a bit from that area. Not as bad. However The issue still remains. It only idles at a reasonable RPM will full choke and revs between 3-5k off choke. This is with the idle adjustment all the way out. Am I potentially fighting something that isn't a vacuum leak? Could something else be causing this idle issue?

[irish44395]

did you bench sync the carbs before you put them back on the bike??
Pete

[janich4644]

did you bench sync the carbs before you put them back on the bike??
Pete

I did not. I'll have to look up some videos. Any helpful tips?

[Hutch]

[janich4644]

Thanks Hutch!

Could the high revs at idle be related to either the vacuum pump or the fuel accelerator pump?

[Hutch]

Neither, odds are its the carb boots... common problem.  I spent a week chasing my tail with a similar problem.  Finally listened to advise and replaced the boots, problem solved.  Best price I found was here https://www.servicehonda.com/oemparts/a/hon/506f8f82f8700229747b976d/cylinder-head

[janich4644]

Neither, odds are its the carb boots... common problem.  I spent a week chasing my tail with a similar problem.  Finally listened to advise and replaced the boots, problem solved.  Best price I found was here https://www.servicehonda.com/oemparts/a/hon/506f8f82f8700229747b976d/cylinder-head

That is a good price. And I thought about that, just I'm just not getting a change of engine speed when I spray the boots with carb cleaner. But maybe I'll try again tomorrow and see what results I get.

[Hutch]

Neither, odds are its the carb boots... common problem.  I spent a week chasing my tail with a similar problem.  Finally listened to advise and replaced the boots, problem solved.  Best price I found was here https://www.servicehonda.com/oemparts/a/hon/506f8f82f8700229747b976d/cylinder-head

That is a good price. And I thought about that, just I'm just not getting a change of engine speed when I spray the boots with carb cleaner. But maybe I'll try again tomorrow and see what results I get.

I experienced the same, no change in revs when spraying cleaner.  I exhausted all other options before changing the boots.

Sent from my Moto G (5) Plus using Tapatalk

[janich4644]

Neither, odds are its the carb boots... common problem.  I spent a week chasing my tail with a similar problem.  Finally listened to advise and replaced the boots, problem solved.  Best price I found was here https://www.servicehonda.com/oemparts/a/hon/506f8f82f8700229747b976d/cylinder-head

That is a good price. And I thought about that, just I'm just not getting a change of engine speed when I spray the boots with carb cleaner. But maybe I'll try again tomorrow and see what results I get.

I experienced the same, no change in revs when spraying cleaner.  I exhausted all other options before changing the boots.

Sent from my Moto G (5) Plus using Tapatalk

I think you convinced me to give it a try. Calling my local Honda dealer today to see what prices they can get me.

I did bypass the vacuum pump with no change in results.

I'll keep you updated with what happens once I get the boots replaced. Fingers crossed.

[janich4644]

I got the new carb boots put on the bike yesterday. It started and was idling at 1500 with seemed like a good sign. I decided to take it around the block for a test ride. It was very laggy with throttle under load. I don't know if that's because I didn't vacuum sync the carbs yet or not. I did get it back around the block and as soon as I rolled up to my home it started screaming at me at 5k rpm again.

I'm a little distraught the new boots didn't fix the high revs, but it's strange to me that at first it seemed to run better. I don't know what else to do besides take the carbs off AGAIN and completely go through them. I also think I need to replace the felt seals on the throttle shafts with o-rings to get that vacuum leak sealed off once and for all.

I just don't get it. I have checked the throttle plates and they all close uniformly. Any other ideas are welcome.

[Hutch]

I would start by backing out the set screw.  Also check the throttle cables, make sure the throttle is working correctly.  Adjust at the throttle and carbs as needed, the throttle is supposed to have a bit of slack.  Finally, make sure the boots are fitted securely and in the right direction (check arrow on carb boot) on both the engine and carb sides.  I always use ratchet straps to ensure a good fit.

[Hutch]

I forgot to mention, do yourself a favor an melt the caps off of the mixture screws so that you can properly adjust. 

http://www.hondacb650.com/viewtopic.php?t=11695

Be sure your tank is clean.  I had to clean my carbs twice, because some fine rust particles made there way to the carbs and clogged the jets. 

Also, this... http://honda-cb750-s.456789.n3.nabble.com/file/n4046333/Honda_Carb_Manual_revD.pdf