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Old 03-31-2014, 10:47 AM   #1
685 OP
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Carbie fun

So my bro's come and gone for the spring. We had the 90/6 mongrel running pretty good by the end of his visit. He did some tinkering on the bike, and I had to redo some of the stuff he did, but that's to be expected--I've lived with this thing for the past 6 months and know some of its quirks.

The bike is bored to 1050 cc, and has dual plugs, an Mikuni VM38 carbs. Center pull, not CV's. They need rebuilding and some deep tuning, I think. Which I'm willing to do, eventually.

But I've been thinking about experimenting with the other main Japanese carb company's offering in this range. It's the constant velocity, CVK 40mm. Mine are late model (after 08) used ones from KLR650 bikes--650 cc thumpers. But the carb with an accelerator pump was also used on Harleys. So the range should be about okay, but even the lean jetting of the KLR may be a little fat.

I've got the carbs, and the KLR cables. On the KLR, it's a push pull cable setup, on the BMW it's only going to be a pull cable. And the carb throttle cables will be adapted to fit up to the cable splitter located under the starter relay and voltage regulator.

Here's a pic of the test fitting:


image hosting 12mb

I'll have to do some magic with the air filter snorkels, they are way smaller than the intake of the CVK. And the CVK needs a fairly clean airway in, there's a vent that operates the CV diaphragm.

I know that this carb may be too large for the displacement, even with tight jetting. But on the KLR, it starts so easy and is so flexible, it's worth a try. On the KLR, at least, I've been really happy with the carb. If it's a crappy match, I can get the $200 invested out on Ebay. I haven't changed anything on the Mikuni's & the cables so if they're the better carb, I can go back to them.

More to come. . .
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Old 04-03-2014, 05:56 PM   #2
685 OP
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Started it today.

I'm still short a choke plunger and spring for the left side, but I managed to get the old dog started today with the new carbs.

Been a fun week, I've cut down 4 cables, two are choke, two are throttle. And soldered new cable ends on to mate up with BMW controls. That went amazingly well, btw. And I'll expound on the process if anyone is interest.

Today I got it all together (except for one of the chokes.) And not having a choke isn't gonna harm anything--just plug the enricher port with my "bird" finger, & hit the starter. Took a little grinding the starter, but it started, now starts fine with the right hand choke engaged & the left one blocked. Idles very nice--steady. Starts a lot leaner, I think--not much burnt hydrocarbon smell. And the choke can be turned off almost immediately, like my KLR. Here's a pic of the "circus color" right side carb. I don't mind the black powdercoating, but the red trim is kind of--garish.


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I managed to get the snorkels on, getting the rubber boot onto the carb was the hardest part--sort of like fitting a tiny, tiny tire. But with that mounted to the carb, the whole thing fits together like it was OEM.

Since I don't have the choke parts, I can't really ride it to see how it performs. As soon as the parts come, I'm going to take it out for some test riding and evaluating the jetting.

It all went together so well, looks proper and stock. I'm a happy feller tonight.
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Old 04-03-2014, 07:56 PM   #3
globalt38
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Quote:
Originally Posted by 685 View Post
I don't mind the black powdercoating, but the red trim is kind of--garish.

I kinda like the red "highlights"
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Old 04-03-2014, 10:11 PM   #4
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I'm with you 685. That red ain't right. Natural aluminum (steel?) would look real nice though

Waiting for the ride report. Good luck.
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Old 04-04-2014, 02:57 PM   #5
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Can you rework them and put them into a "left and right" carb?
It would be cool if it works good!
Are they smaller than stock bings?
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Old 04-04-2014, 03:39 PM   #6
685 OP
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No, they're identical. Linkage connection on the right side of the carb, choke linkage on the left. So, like the Mikunis, there's really no left or right carb.

I believe that the original carbs on an R90/6 was a 32 mm, these are 40 mm. They are the same sized Kehien CVK as used on the 883 cc Sportster, the 650 cc KLR, and others. I think I'm going to be able to get away with the much larger flow of the 40 mm because the bike is built out to 1050 cc and it has dual plug heads. By my figuring, it's right in the airflow middle of the 883 twin and the 650 single. I'm hoping that the carbs don't have too much airflow volume for the BMW engine--that's my only concern.

As pictured, they fit in the VM38 intake boots just fine. The carb length from the intake to the outlet is right for me to use the rubber boot and the stock snorkel up to the airbox. It all fits together like it's supposed to and won't look cobbled.

I got everything hooked up. The pretty black & red carb leaks like a sieve out the float bowl. Took the bowl off and put some gas in so I could actually see where the leak is. Has a pinhole leak at lower left in the picture:


upload img

I could seal it up with JB Weld, but probably won't.

I don't ever think I'd buy a powdercoated carb again. The coating on the bowl is fubar, the guy that did the original work is sending me out another bowl and o-ring bowl seal. Just a speed bump. Dead in the water waiting on the choke anyway, but it's disappointing.

There are, btw, other CVK sizes--from about 28mm all the way up to 40mm with increases in size in increments of 2mm. I don't really know if there's a performance benefit (or penalty) yet. But the CVK is a very easy carb for cold starts & it's easier to tune than the VM38.
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Old 04-04-2014, 05:07 PM   #7
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Looking forward to the outcome. My biggest gripe about airheads + bing CVs is the cold starting. On the three I've had (R100, R80ST, R80G/S) the warmup cycle is tedious. There is a point after starting when it doesn't want any choke but is not warm enough to idle without stalling unless some throttle is applied. Maybe it's the delay in warming the oil, airheads seem to take a long time to get the oil temp up. After about 10 minutes the idle comes up to near normal (I set mine on the high side of 1100).

Anyway good luck and keep us in the loop.
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Old 04-04-2014, 05:31 PM   #8
685 OP
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Originally Posted by acap650 View Post
Looking forward to the outcome. My biggest gripe about airheads + bing CVs is the cold starting. On the three I've had (R100, R80ST, R80G/S) the warmup cycle is tedious. There is a point after starting when it doesn't want any choke but is not warm enough to idle without stalling unless some throttle is applied. Maybe it's the delay in warming the oil, airheads seem to take a long time to get the oil temp up. After about 10 minutes the idle comes up to near normal (I set mine on the high side of 1100).

Anyway good luck and keep us in the loop.
I will. I'm fairly excited. My very limited experience was that it was ready to come off choke and would idle after about 15 seconds. That's the starting profile of my 08 KLR, too, so it's promising.
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Old 04-04-2014, 06:01 PM   #9
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Ritmo Sereno have used those carbs on their bikes. I'm guessin' therefore that they'll work. Unsure of their setup though.
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Old 04-05-2014, 03:14 AM   #10
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they look like the carbs I had on my Ural and they were standard fitment!
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Old 04-07-2014, 04:26 PM   #11
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Thumb Update

I JB Welded the leaky float bowl--that works pretty well, btw. An undamaged one is in the mail, I hope. But at least I can start riding the bike with the new carbs. The final piece, a choke plunger & spring, arrived in today's mail. So a slapped that sucker in.

Starting. It starts at a touch of the starter button and with choke on idles between 1800 and 2000 rpm. The choke can be reduced immediately so that the idle drops to about 1500, and the bike idles smoothly and doesn't want to stall.

Out on the road. It runs smooth from idle, no hesitation, even off idle. And the new carbs provide at least as much power as the Mikunis. Slowing down to stop at a light, they don't hold any extra rpms as you slow and then stop. Jetting seems pretty close, but more road testing will tell the tale.

The bike, an R90/6 with a 1050 overbore kit, is feeling modern and settled. And it has a shit load of power. It's suddenly a very sweet running airhead. I'm happier with this conversion than I could ever have anticipated--maybe not for the keep it correct/restoration guys, but for those of us that have bikes that are long past any reasonable expectation of restoration, it's a perfect match.

If one has VM38's or Bing 38's, getting the CVK 40 to mate up is a piece of cake. The KLR throttle & choke cables have to be adapted to fit into the BMW throttle junction under the tank, and I have the choke control that's mounted on the side of the airbox. After I sourced the cable ends and the ferrules from Flanders.com, and some silver solder and flux--it was about an hour of cutting and soldering.

These are KLR parts. Both sides use the same carb. On the KLR, they were push pull throttle, and my left side one (the one that's not powdercoated) included both throttle cables, a choke cable and the choke plunger and spring. That side was $100 shipped and from an 09 model. The powdercoated one came with no cables at all and was $80 shipped and was from an 09, too. A choke cable with plunger and spring was sourced from Ebay for $16 shipped. Cable adaptation was about another $30. So I'm in for about $226. And I've got low mileage, late model carbs with fuel tight float valve mechanisms that don't spill gas on one's boots (got tired of smelling like gas all the time.)

For me, a better carb than the Mikunis, which seem like an old, old design, like from the 60's

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Old 04-07-2014, 04:40 PM   #12
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Any thought to taking this to the local dealer to have the bike hooked up to a gas analyzer to check your mixture?

I'd think about any brand shop would have one.
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Old 04-07-2014, 04:43 PM   #13
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Originally Posted by SOLO LOBO View Post
Any thought to taking this to the local dealer to have the bike hooked up to a gas analyzer to check your mixture?

I'd think about any brand shop would have one.
Probably not a bad idea. I have a friend with an a/f analyser, and I have a spare set of header pipes--maybe just weld in a bung and hook 'er up.
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Old 04-08-2014, 11:39 AM   #14
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I put a CVK on my 1972 Sportster. It is a wonderful carburettor. I too considered putting a set on my R90. I'm glad to see you got it sorted. I experienced that same perfect starting from my CVK. Have you noticed an improvement in fuel economy? That's usually the other outcome from installing these.
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Old 04-08-2014, 12:17 PM   #15
innathyzit
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Quote:
Originally Posted by acap650 View Post
Looking forward to the outcome. My biggest gripe about airheads + bing CVs is the cold starting. On the three I've had (R100, R80ST, R80G/S) the warmup cycle is tedious. There is a point after starting when it doesn't want any choke but is not warm enough to idle without stalling unless some throttle is applied. Maybe it's the delay in warming the oil, airheads seem to take a long time to get the oil temp up. After about 10 minutes the idle comes up to near normal (I set mine on the high side of 1100).

Anyway good luck and keep us in the loop.
I am new to airhead ownership and thought that something was wrong, but you have described my starting procedure issues perfectly so all is well then.

Was working on my GS the other day and looking at my KLR and wondering if anyone had ever put a cvk on an airhead.

It would be easier to make them handle deep water too as the Bings aren't too flash in that area and my KLR with the "T" mod is like a submarine.

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