Author Topic: Setting up keihin VE54 (32mm CV) carbs on a 836 (Read 16252 times)

cafebob · « **on:** March 22, 2009, 09:22:19 PM »

I am going to tune a set of Keihin carbs off a 85 CB650SC (Nighthawk) for use on my 836 CB750. The 836 motor has a 8:1 compression and a 4-1 exhaust with no muffler.

The spacing on the carbs is identical to that of SOHC 750's and slide right on to the stock boots, which are almost brand new, no cracks and still somewhat soft.

The carbs are bone stock, they have been cleaned and reassembled. The bike starts and idles perfectly at 1200 RPM once it warms up with them on. I can turn the throttle about 1/8-1/4 before the bike will die, i think from fuel starvation.

The carbs have 115 mains and 35 slow jets. They are 32mm bore, CV carbs.

Does anyone have a guess as to what I should bumps the mains up to? I was thinking 125 or 130 on the mains, and maybe 38 on the slow, or just leave it alone. The 130 comes from what others have reported on there 836 motors.

Any pointers are welcome.

CB

754 · « **Reply #1 on:** March 22, 2009, 10:45:51 PM »

How & why did you end up with 8:1 compression?

Are you planning on running a turbo?

Did you try the carbs as is on that motor?

cafebob · « **Reply #2 on:** March 23, 2009, 06:10:21 AM »

8:1 as achieved mostly with with shorter rods, and head selection. yes, it is designed for a turbo. I just need to get the bike tuned to run NA first.

The carbs as is, only give me a nice idle, and I can rev to about 2500-3000 RPM (about 1/8-1/4 throttle) before it stalls.

TheHun · « **Reply #3 on:** March 23, 2009, 06:35:11 AM »

check your plugs...my carbs in the beginning would do this..before a good clean and tune..and the carbs were flooding

Soos · « **Reply #4 on:** March 23, 2009, 06:49:26 AM »

Quote from: cafebob on March 22, 2009, 09:22:19 PM

I am going to tune a set of Keihin carbs off a 85 CB650SC (Nighthawk) for use on my 836 CB750. The 836 motor has a 8:1 compression and a 4-1 exhaust with no muffler.

The spacing on the carbs is identical to that of SOHC 750's and slide right on to the stock boots, which are almost brand new, no cracks and still somewhat soft.

The carbs are bone stock, they have been cleaned and reassembled. The bike starts and idles perfectly at 1200 RPM once it warms up with them on. I can turn the throttle about 1/8-1/4 before the bike will die, i think from fuel starvation.

The carbs have 115 mains and 35 slow jets. They are 32mm bore, CV carbs.

Does anyone have a guess as to what I should bumps the mains up to? I was thinking 125 or 130 on the mains, and maybe 38 on the slow, or just leave it alone. The 130 comes from what others have reported on there 836 motors.

Any pointers are welcome.

CB

Awesome to know the later DOHC 650 carbs are the same as cb750 carbs...
hence the same as SOHC 650 carbs, and 550 carbs as far as spacing....

Someone needs to add this bit of info to the carb FAQ to go along with the other carb measurements already there!

l8r

Soos · « **Reply #5 on:** March 23, 2009, 06:51:34 AM »

Quote from: cafebob on March 23, 2009, 06:10:21 AM

8:1 as achieved mostly with with shorter rods, and head selection. yes, it is designed for a turbo. I just need to get the bike tuned to run NA first.

The carbs as is, only give me a nice idle, and I can rev to about 2500-3000 RPM (about 1/8-1/4 throttle) before it stalls.

Did you clean them first, or just stuff 'em on?
I would guess one of the carbs has a plugged passageway...
But I don't really know, as I have never been into a set of DOHC cb650 carbs(yet!).

Wish I could help more.

l8r

Soos · « **Reply #6 on:** March 23, 2009, 06:53:26 AM »

The thing I would think.....

Since they were set up for 650cc's(or threreabouts) your 836cc's is pulling WAY too much vacuum compared to the 650 they go to does, and therefore you are pulling too much fuel?

A plug check would be necessary IMO.
Probably a DROP in main jet size.

l8r

cafebob · « **Reply #7 on:** March 23, 2009, 08:25:27 AM »

The carbs where completely broken down, soaked in carb dip, and reassembled. I don't think they are plugged up.

I'm not sure that they are pulling that much more vacuum.

this link http://www.ch250.net/techtips/19.htm seems to make me think that larger jets are needed. I might just buy a range and give them a try.

754 · « **Reply #8 on:** March 23, 2009, 09:38:01 AM »

I dont know CV carbs well.

I do remember this from selling S.U. kits for twins.

There was springs to change, and needles. You may not have a lot of choices or availability, to play with on those carbs

cafebob · « **Reply #9 on:** March 23, 2009, 04:35:47 PM »

I put 130 main in them an now the bike hesitates from 2500-4000 then takes off. With the 115 in it couldn't even get above 3000.

I think I need to either try maybe 125 main, or up the slow jet a couple.

How hot should the cam cover get just reving the motor on the stand?

TheHun · « **Reply #10 on:** March 23, 2009, 05:33:52 PM »

Could be so many things...my problem was..and has similar symptoms as yours...i actually had to jet down from 110 to 105

How is your timing...if you set it to the "F" mark..try and setting it about 1/4 past it towards the advance line...this made my bike run smooth as silk

cafebob · « **Reply #11 on:** March 23, 2009, 06:36:57 PM »

Hun is yours a 836?

TheHun · « **Reply #12 on:** March 23, 2009, 06:42:15 PM »

Well..actually I do not know what is done internally on the motor...havent opened her up...it does have carbs off the 750F but its a K bike.

JLeather · « **Reply #13 on:** March 24, 2009, 05:32:37 AM »

cafebob, your problem is not in the jets. You aren't getting the slides to lift. Basically you're only running on the idle circuit and the lowest notch of the main circuit. About 1/8 throttle is when the carbs should start opening the slides, which are vacuum operated. Put the jets back in it you were using that gave you a good idle-1/8 preformance and fix the slide problem before changing any of the jets again.

Now, the slides could simply be dirty and sticking, but I doubt that because usually only one or two will stick and you said you already cleaned them thoroughly. Are you running an airbox of any kind on this bike right now (I'm guessing not)? And what kind of exhaust are you running? CV carbs require a good vaccum signal to raise the slides, and a lot of that vacuum is generated in the airbox. Ask anyone who's tried to put velocity stacks on a DOHC Honda and eliminated the airbox. They suddenly find that the bike falls flat on it's face at 2000 rpms because the slides won't open at the proper rate anymore and it starves for fuel. You can address this by putting in weaker springs in the slides and by opening up the vacuum passages that feed the slides to account for the weaker vacuum signal. Start with the springs. See if anyone makes a dynojet kit for those carbs with weaker springs in it. Don't start with the passages because that is a much more drastic change and it's easy to go to far and end up needing a new set of slides.

AND, once you get it all dialed in N/A on these carbs as soon as you put a turbo on it you'll completely change the actions of the slides again and need to go through this all over. Not trying to dissuade you from the project, it sounds like a great venture. Just saying that it sounds like you aren't very familiar with tuning CV carbs, and if you don't get it tuned in just right you'll go lean and detonate the motor under boost.

The reason it runs the way it does on 130's is because you are feeding too much fuel in the low end, and after a certain point you're drawing enough air to compensate for it. The slides are supposed to feed more fuel as you rev, but by putting the 130's in it you're feeding all that fuel through the lower circuits ending up with a really over-rich idle (the hesitation) and then eventually it evens out. If you get it revving fast enough you will get the slides to open even with the wrong springs and apssages, which is probably why it continues to run at WOT with the larger jets.

Soos · « **Reply #14 on:** March 24, 2009, 06:43:02 AM »

Ahhhh... What Jleather said rings true...(in my mind)
I hadn't thought whether or not you are running with an airbox.

With an airbox, and stock filtration, I would say yes, drop the jets in size.
Without the airbox, Jleather has hit it on the nose from what I have read elsewhere.

You going to use those carbs on your turbo, or running a different carb setup?

l8r

cafebob · « **Reply #15 on:** March 24, 2009, 08:47:48 AM »

No air box, but I may fabricate the turbo manifold and tune it with it on. The carbs have sat for about a year after I cleaned them, so the slides could be sticking. When I cleaned them I did not notice and pin holes in the diaphragms. I am going to go back over them again and see if they are sticking anywhere.

I am running a 4-1 aftermarket exhaust.

The reason I went with these carbs as I have spoke to another member who used them and was very happy with the results on his 836 bike. He said all that was needed was a re-jetting. I looked through my old emails and he gave me the name of the mechanic that did the work for him. Maybe I will call him and see if he can remember what he did to them.

cafebob · « **Reply #16 on:** March 24, 2009, 08:42:49 PM »

update...

took the slides out and made sure they where smooth...maybe they need a little cleaning, lubeing, i couldn't really tell a difference afterward.

Got some 40 slow jets and tried them out. bingo. revs real nice now, good enough to drive at least. so i spent the day mounting the rearsets. all i need to do is make a shift linkage and I will be able to try it out.

right now i have 122 mains, 40 slow. At 5280 feet.

bert96 · « **Reply #17 on:** March 25, 2009, 03:52:58 AM »

I hope your diapragms are good because they are not available from the dealer anymore.

Buber · « **Reply #18 on:** March 25, 2009, 04:45:41 AM »

Quote from: fishhead on March 24, 2009, 05:51:35 AM

Have you checked the diaphrams(rubbers above the slides) for any pin holes in them?

Gentleman, may I point to you that those carbs (VE54) don't have rubber diaphragms? There's aluminium piston that seals quite well, and no rubber. Just unsrcew teh top and you'll see... Or check the assy diagram.

Regards...

Soos · « **Reply #19 on:** March 25, 2009, 06:46:32 AM »

Quote from: Buber on March 25, 2009, 04:45:41 AM

Quote from: fishhead on March 24, 2009, 05:51:35 AM
Have you checked the diaphrams(rubbers above the slides) for any pin holes in them?

Gentleman, may I point to you that those carbs (VE54) don't have rubber diaphragms? There's aluminium piston that seals quite well, and no rubber. Just unsrcew teh top and you'll see... Or check the assy diagram.

Regards...

Thats true for SOHC cb650 carbs.
However, the DOHC cb650 carbs DO have rubber diaphragms instead of the aluminum pistons the earlier SOHC CV carbs have.

l8r

cafebob · « **Reply #20 on:** March 25, 2009, 07:53:39 AM »

hes right there are rubber diaphragms. mine are in good shape.

The over rideing reason I chose these carbs is that they should lend themselves to blow through application nicely.

As long as I apply manifold pressure to the pressurize to carbs and use a rising rate regulator for fuel, they should perform nicely. Tuning will still be a fun adventure, but I shouldn't have to do alot of work on the carbs themselves. The earlier CV carbs (pistons ones) need to be sealed to take the pressure, as do the CR slide type carbs on the 1st gen 750's. This can be a very difficult process. But blow through is the way to go. Draw through has so many downsides that I just had to bite the bullet and set out into new territory (as far as SOHC 750's are concerned)

If these carbs don't work out for me I have heard that KZ series from the late 80's also will fit.

But for now things look good.

JLeather · « **Reply #21 on:** March 25, 2009, 09:32:12 AM »

What are the numerous draw-thru downsides that made you go this route? Just curious since I have an ATP draw-thru kit that I'm putting on a motor in the near future. I know the downsides to drawthru, but none of them were a deal-breaker for me.

I will certainly be curious how this works out for you, though. i seem to remember bouncing a few ideas around with you sometime last year on the subject of turbo SOHCs.

Oh, and if I change my mind and go blow-thru I'd get a set of fuel injectors/bodies off a 90's CBR and do a blow-thru FI setup. Blow-thru is easy with FI because the fuel is already pressurized and the bodies are already sealed.

cafebob · « **Reply #22 on:** March 25, 2009, 10:06:03 AM »

Over all I think it is just simplicity...Blow through is a simpler system.

To start with draw through you have to seal the turbo for pressurization. Not a problem with ATP kits, but if you want to use a modern turbo this will require a tear down and seals added.

New turbos are a big big plus, they are smaller, last longer, spool up faster. Lots of reasons to go with a new turbo.

You are going to be running a single large carb...not the easiest thing to tune on these bikes. I think the feeling is the bendix carb on the ATP kit was kind of crappy. I believe people are using a big mikuni smoothbore. Still its hard to tune from what I have read. I think there is someone on this board that is running a ATP kit and he has lots of tips for tuneing them....

Iceing. Nuff said.

Blow through is much easier to tune IMO. Because the carb is pressurized, a single jet combo will work over a larger range of conditions. Transition from ATM to boost is much smoother. With a draw through system you have to rejet constantly for different boost conditions. A lot of people end up adding an aux fuel addition system when they are under boost.

Finally, I haven't seen any draw through systems out there other than the old ATP kits...Could be because alot of bikes and cars are FI now, but alot aren't and I have seen many carbed KZ's with blow through systems at the track and they do really well.

JLeather · « **Reply #23 on:** March 25, 2009, 01:09:34 PM »

I think I'm gonna have to disagree with you on the simplicity aspect. A draw-thru setup doesn't require hardly any changes for increased boost. The whole point of a draw-thru is you're compressing the A/F mixture. The carb mixes the A/F to the same ratio no matter how much you are compressing it afterwards. i.e. if you set the carb up for a 12.5:1 A/F ratio at 0 boost, it theoretically has the same A/F ratio at 30 pounds of boost because you're compressing an already pre-mixed ratio.

It's true, however, that for excessive boost it doesn't work anymore. There is a limit to the amount of fuel the carb will flow, and after a certain poitn you have to start injecting extra fuel (kinda like adding fuel with nitrous) but that's really well beyond the realm of a streetable bike.

The simplicity advantage of a draw-thru setup is no carb modifications. No sealing the carb, no pressurizing the bowl, no rising-rate FP regulator, etc. Simplicity is, in fact, the only thing going for a draw-thru setup.

The downsides to draw-thru are for starters lack of intercooling. You can't get much past 8 pounds on a draw-thru because heat buildup makes your horsepower gains negligible. You can't intercool a draw-thru setup because fuel will puddle in the intercooler and ignite causing an explosion. You can meth-inject a draw thru to get past 8 pounds, but again not on a street bike.

Another downside to drawthru is inefficiency and lack of finesse. Without a blow-off valve the system has a tendency to want to keep revving for a second after you let off the throttle. The wastegate will un-spool the turbo but you have to use up the existing boost which makes the engine surge for a second before throttling down. The inefficiency is because A) the turbo has to spin through the additional seals which provide more drag, and B) because the turbo has to spin through a A/F mixture instead of just air which drags on the impeller.

It's true that a blow-thru system is more efficient, makes more power (when properly tuned), and is easier to control. BUT, the one thing that a blow-thru is not is simpler. Both building and tuning a blow-thru is more difficult.

I would definitely like to see your blow-thru setup once it's finished. Have you done the plumbing yet, chosen a turbo, an ignition system, etc?

cafebob · « **Reply #24 on:** March 25, 2009, 04:28:12 PM »

Well I agree with most of what of said, I guess to me it seems simpler blowing through modern CV carbs. Mostly because you don't have to really modify them at all. Just give them boost signal and use a rising rate regulator and you are pretty much there. Where as going draw through with modern equipment is a real headache.

For my setup....

I have everything for the turbo system. I have a IHI turbo, small IC off a VW, rising rate regulator, low pressure fuel pump, dyna2000 w/ coils from a vf500 (3 ohm i think). MY fuel tank has an in and out stop cock. I am going to post pics of the bike today, and will see I have plenty of room on the bike. I am going to mount the turbo in the front of the bike, and run pipeing back the to carbs.

My hold up right now is getting the exhaust manifold made. It is a little beyond my fabrication abilities, and most of my quotes have been in the 300-400$ range to have one made, which is outrageous if you ask me. If I had a mandrel bender I might try in myself.

If anyone knows a guy in the colorado area that could help me fab the exaust let me know.

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Author Topic: Setting up keihin VE54 (32mm CV) carbs on a 836 (Read 16252 times)