Author Topic: 81 CB650 carb alternatives (Read 18594 times)

strevellyan · « **on:** June 20, 2012, 05:51:24 pm »

Hello,

So I am new to this forum and motorcycles in general, but I recently bought my first motorcycle (81 cb650c) and I am rebuilding it and making some modifications, which I am completely new to. Everything seemed pretty much all there except the carbs where dirty and leaky. I rebuilt the carbs and there was an extreme hesitation in the mid range (3500-5500) rpms. I tried everything and out of frustration I finally had a mechanic look at and rebuild them. He extracted the pressed in pilot jets and threaded in replacements. After all was said and done my bike is at least rideable now but still hesitates in the mid range, though not as bad, but bad enough to be annoying. He said that the seals and linkage might just be worn out.

I have read that this is a fairly common occurrence in the CV carbs for 81 CB650s. I have been trying to ascertain what crabs will work as an alternative. I have read that certain cb750 and cb550 carbs can works but with little specificity as to which years.

Ideally I would like to get away from the 81 CB650 carbs (VB44A) with pressed in pilot jets, which are harder to find anyway. Beining that there is an abundance of cb750 and 550 carb racks on ebay, I thought it was worth exploring.

I would greatly appreciate any input and insight on the subject!

thanks
Scott

Gordon · « **Reply #1 on:** June 20, 2012, 05:59:30 pm »

Welcome!

It will be much easier and cause far fewer headaches to get the bike running well with the stock carbs than with carbs from another bike. I know that's not what you want to hear, but it's the truth. Pressed in jets can be pulled out and re-inserted by a home mechanic without having to tap the carb bodies to accept threaded jets.

edit: okay, so that's not necessarily the "truth", but just my opinion.

strevellyan · « **Reply #2 on:** June 20, 2012, 06:17:08 pm »

Thanks for your response, I already have new pilot jets tapped and screwed in. As I mentioned, I have rebuilt the carbs myself and had them rebuilt. I can't seem to get rid of the mid range hesitation (neither could two different mechanics), which is why I was looking to replace the carbs altogether.

Do you have any other ideas for what could cause that hesitation?

Scott

davesee · « **Reply #3 on:** June 21, 2012, 02:50:42 am »

there is a post around about alternative carbs...even the '80pd's work with different boots, and i think that there is a kawi kz alternative. hopefully it'll come up in a search.

another alternative is to find an 80 head and carbs. this would be a straightforward swap, as would another set of 81's.

gordon speaks the truth, though. there are lots of 81 82 650's running those carbs without issue, so they should work for you. there's a fly in the ointment somewhere.

on another note, is that your bike in your profile pic...i think we did similar mods. can you shoot us a big pic?

fmctm1sw · « **Reply #4 on:** June 21, 2012, 04:32:53 am »

Make sure your accelerator pump is working..

trueblue · « **Reply #5 on:** June 21, 2012, 04:58:10 am »

http://forums.sohc4.net/index.php?topic=40287.0
Check this out

strevellyan · « **Reply #6 on:** June 21, 2012, 11:02:13 am »

Thank you for your feedback,

Davesee, I'll look into it a little bit more. I have invested soo much time into figuring out what is wrong with these carbs I would almost pay for new ones just to torch mine. But I do agree that there has to be something wrong. Do the 82' carbs have pressed in pilots?

fmctm1sw, the accelerator pump was missing when I got the bike, but I have noticed in other forums that people bypass them sometimes when they are not working. Would that be affecting the mid range power?

trueblue, I saw that post, but I feel like those carbs are even harder to find that the 650's. Is their somewhere better than ebay to look.

strevellyan · « **Reply #7 on:** June 21, 2012, 11:06:52 am »

here are some pics of the bike, almost done.

strevellyan · « **Reply #8 on:** June 21, 2012, 11:07:41 am »

fmctm1sw · « **Reply #9 on:** June 21, 2012, 03:16:31 pm »

Quote from: strevellyan on June 21, 2012, 11:02:13 am

fmctm1sw, the accelerator pump was missing when I got the bike, but I have noticed in other forums that people bypass them sometimes when they are not working. Would that be affecting the mid range power?

Folks are bypassing the vacuum petcock, not the accelerator pump (AFAIK). As mentioned earlier, there are a couple long threads about these carbs. Good luck!

davesee · « **Reply #10 on:** June 21, 2012, 04:16:59 pm »

i don't have any experience bypassing the pump, nor have i really heard of that. i believe the pump allows for a leaner jetting. i also think that this could be the source of your problem. as i understand it (and my understanding is limited), the accelerator pump dumps fuel in a quick burst when there is a sudden vacuum (like a twist of the throttle.

now, i have the 80 pd carbs, and i may misunderstand the whole thing...but it seem like a lag would result.

keep at it, man. your bikes looks sick!

cgswss · « **Reply #11 on:** June 21, 2012, 06:36:17 pm »

I have an '81 my self- and I too have been frustrated by the carbs. As to replacing them, I have been looking for a reasonable set of '82 carbs. While I haven't done it yet, It is my understanding that the '82s will bolt right on. Also the '82s have removable jets and they have the same carb spacing.

IF you don't have the accelerator pump hooked up, this is certainly a reason for the bike to give you a lean surge at low RPMs when you open the throttle.

I went to look at a used '81 for sale, and the owner had replaced the carbs. I wish I knew what the carbs were from- but they had the newer small pods on top. he said they were basically just a bolt on except he had to reverse the choke bracket- He didn't know the source of the carbs, but was told they were from a late model Honda.

If it helps you any, I have mine (with the original carbs) working very well. I threw away the vacuum on/off and got a used but working diaphragm for the accelerator pump so it works fined. Mine pulls fine from 1000 rpm to 10,000. My only problem at this point is that I have a very tiny gas leak I can't find. I can Smell the gas, and I can feel a dampness on the float bowl of the #1 carb when its on the side stand. I'm sure I'll find it when I tank the tank off the next time. I'm waiting for some fancy new plug wires right now, when they get here I'm sure I'l fix the leak at the same time. But the good thing is the bike will run great with the stock carbs!

trueblue · « **Reply #12 on:** June 22, 2012, 01:21:16 am »

Quote from: strevellyan on June 21, 2012, 11:02:13 am

trueblue, I saw that post, but I feel like those carbs are even harder to find that the 650's. Is their somewhere better than ebay to look.

I never checked out ebay, but my local bike wrecker has at least 2 sets, and he is only a relatively small operation, so they are fairly common.

strevellyan · « **Reply #13 on:** June 22, 2012, 07:45:03 pm »

So I was mistaken, I meant that the petcock vacum is bypassed/removed. The accelerator pump is in working order, I replaced the diaphragm/rod and spring.

tomkimberly · « **Reply #14 on:** June 22, 2012, 11:55:43 pm »

Quote from: strevellyan on June 21, 2012, 11:02:13 am

Thank you for your feedback,

Do the 82' carbs have pressed in pilots?

You want the VB44B or VB44C carbs, both of these have screw in pilot jets, the VB44A carbs do not. Look at the boss on top of the right carb for the markings.

Tom

davesee · « **Reply #15 on:** June 23, 2012, 05:40:37 pm »

suspect petcock might be the cause of some troubles.

strevellyan · « **Reply #16 on:** June 25, 2012, 11:00:45 pm »

Yeah, I was looking at some 82 cb650 carbs that I believe are a match, but with screw in pilots. I have found one set on ebay. Does anyone know of any other places to buy used or rebuilt carbs?

thanks,
Scott

lsemory88 · « **Reply #17 on:** December 24, 2014, 03:15:03 pm »

scott,
don't know if you'll see this...but was wondering if you could send me some pics of your seat. have a 650 custom as well and at a loss about what to do for a seat

harisuluv · « **Reply #18 on:** December 24, 2014, 04:05:20 pm »

Forgive me if I missed it, but I gathered from the pictures that you have the stock airbox. What kind of filter is in there?

Have you checked for vacuum leaks (boots on both sides)

I see no mention of vacuum syncing the carbs at all, this is even more critical for CV carbs.

I see no mention of the air screw setting at all at present or of altering made any changes.

I am inclined to disagree with your mechanic, "work out seals or linkage" sounds kind of like the verbal equivalent of a shrug.

This business about pressing out the pressed in jets and retapping. I have my doubts on execution.

I will reaffirm that getting alternative carbs from a different model aren't going to make your problems go away, just give you more problems. It is a very tempting thought to be able to just buy your way to a fix, but that's not very realistic. Mid range hesitation indicates a needle jet issue, or as previously stated, out of sync, IMO.

hellraiserDLX · « **Reply #19 on:** December 25, 2014, 08:34:16 am »

I would probably listen to the man that builds some very nice carb racks above me ^^^ but that's just me

750cafe · « **Reply #20 on:** December 25, 2014, 10:20:24 am »

Quote from: strevellyan on June 21, 2012, 11:06:52 am

here are some pics of the bike, almost done.

Is there something making it impossible to get any side view shots of that bike?

Eric

faux fiddy · « **Reply #21 on:** December 26, 2014, 01:10:53 am »

Quote from: harisuluv on December 24, 2014, 04:05:20 pm

It is a very tempting thought to be able to just buy your way to a fix, but that's not very realistic. Mid range hesitation indicates a needle jet issue, or as previously stated, out of sync, IMO.

Me too on finding some non-cv stuff.

And many needles are a one size fits all thing and could be at issue, not just up or down on the clip but the shape. Different manufactures of new sets can behave differently.

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Author Topic: 81 CB650 carb alternatives (Read 18594 times)