Single carb conversion thread...

[TeeCat]

That carb will never tune right on an XS650 with a Q5 needle jet in place. That's the heart of the carburetor, and if you could get running with that you'd be lucky to get 30 mpg. That's why you're smoking--as soon as the slide lifts you're getting a massive fuel dump through the NJ. One more time--use a 159-P6 or maybe, in your single-carb application, 159-P8 needle jet and a 6F9 needle; the one in that carb (a combination I've never seen before) is entirely unsuitable. And get that main air correction jet out of your intake bell if you haven't already done it. I'm done on this thread.

Okay, these are on my shopping list! And it's good to know exactly why I need to do this, too... i.e., why I'm smoking. Glad you offered the fact that you had never seen this carb ship this way too... interesting. And I have not removed the main air correction jet, so I'll do that as well. Off to study the schematic/read more, and I'll have to order these parts, because there's no sense doing more til I have them, I suppose.

Thanks muchly and cheers, griz.

TC

 

[5twins]

The air jet should be in a hole at the bottom of the intake bell. It's the only removable jet in that area, the others are pressed in .....

 

[TeeCat]

twins, yup, I see it on the schematic... thanks, sir!

Tuning notes to self (and whosoever else is following/curious):

Target baseline setup for single Mikuni VM 36mm RS carb:

- air screw currently @2.5 turns out
- throttle stop @3/16 drill bit
- pilot jet: 25 (current)
- main jet: 200 (current)
- 159-P6 or 159-P8 needle jet/6F9 needle (en route)
- main air correction jet in intake bell (removed since initial pilot circuit session)

 

[TeeCat]

twins, I was unable to see the photo at work, but thank you for that... saw it once home. The air jet is out now. Saturday, I had bought a little set of precision screwdrivers - needed one to get at my air screw in this tight spot - and as it happens, just breathing on the largest of them with a grinder had it fitting the slot in the air jet perfectly. So that's off my list. Thanks for the pic!

Other parts are on the way.

 

[joe-wiseguy]

well i can tell you that i am no carb guru. it is a sure thing that there are a few guys here that are by watching this and other posts.... i am definetly learning a thing or two by watching this and other posts. i am a welder, fabricator and am gleening more info on tuning as i watch. i am sure i will be getting more from my own two rides with my intake and single carb setup. i have many using this setup that are haveing the time of their lives with it but none have contacted me back with this kind of insight into dialing in the vm36mm with it. i will watch and thanks to you all. cant wait to get more from my own bikes.

 

[Travis]

.... you may have to operate the throttle if not warmed up completely while you adjust the air-fuel screw to get the bike to run at a peak..... running this screw in will lean it out, while running the screw out will richin it. rule of thumb is that if you run the screw out to 3 or more then you should compensate with a richer or higher number pilot jet. now if you turn it in and it runs better then go a lower pilot.

example...... say you are running a 22.5 pilot. it runs but is popping some when you release the throttle or it doesnt want to idle down when the throttle is realeassed..... both are signs of lean fuel mix. so you start running the air-fuel screw out and get to say 3.5 turns and it is running better.... well you have now taken it through the next pilot size with the screw.... so at this setting you are running the equal to a 25 pilot..... so what do you do.?? you install a 27.5 which is the next jet up from your current settings and turn in the air-fuel screw back to @ one turn out and see how it runs. if you have to take it out to 3.5 again to get better response then you repeat the steps above. you should be able to get it close and perhaps have to screw back in the air fuel and find good power and running in all ranges. ....

Guys correct me if I'm wrong, but isn't this all backwards? Like already mentioned, they are air screws on the VMs. Some bikes have fuel screws, some have air screws, some have both screws (two separate screws in each carb body).

So here's my overly simplified explanation: When you turn the air screw in, you are restricting air, and when you turn it out, you are giving it more air. If you turn it way out and it runs better, that means it needs more air/less fuel, so you go down on the pilot size. If you turn it way in and it runs better, that means in needs less air/more fuel so you go up on the pilot size. Pretty much everything joe said above makes sense for carbs with fuel screws, so that's probably what he was thinking of.

Also, in Joe's example above, he skips pilot sizes. I wouldn't ever do that. Maybe that part was just a typo.

... And weather/time permitting this week, I have to get up the nerve to do my tappets... need to get serious about that if I want a good tune. ....

Definitely do this and set your cam chain tension asap before you mess with anything else.

 

[TeeCat]

Travis, my cam chain's good. Definitely have to do my tappets, though. I have that decompression lever valve cover, though, and I'll have to sort out how that works, exactly.

Also, yes, backing the air screw out leans the mix on this carb. I thing that may have been part of the reason why I was so rich at first.

Hope to have my needle and needle jet before the weekend so I can get something done.

OH! While I'm thinking of it...

Is it necessary to actually remove the carb from the bike again to change the needle and needle jet, or will I have enough room (if you look at the pics) if I leave the carb body on the manifold? I know I'll have to have the top and bowl off, but how does the needle jet come out? It's not threaded, from what I can tell from pics, so is it a "press fit" or just inserted into the carb body beneath the main jet and the fuel retaining cup, iirc?

TC

 

[Tech7]

I'm really hoping this works out for you. I've been watching this for a while, and initially I was surprised you had so many problems with your BS38's. I love mine. I hope for your sake, there isn't an underlying problem that wasn't related to carburetion at all. I guess I'll just keep watching and waiting. Good luck.

 

[TeeCat]

Tech, me too. I cleaned those carbs a million times, replaced boots, replaced needles and seats, adjusted float settings, synched them to death as best I could... but I never completely rebuilt them or had them rebuilt. So there's a possibility that those carbs just had some boogaboo that I never uncovered. Now, we'll find out, I suppose. And hopefully, any other issues I discover can be readily corrected.

Keep your fingers crossed.

 

[5twins]

If you haven't ever set your valves, that may be the problem right there. If you've got some tight ones, that could cause the bike to run bad, die when it gets hot, all sorts of things. Don't worry about the compression release cover, it has nothing to do with setting the valves. The compression release mechanism will come out with the cover. And quit sticking drill bits in the carb throat, lol. That's a sync thing for when you run 2 carbs to match slide openings between them.

 

[CDNTX650]

No offense man, but you go off and order a carb that won't be super easy to tune for a novice and install it and you havent even done the simplest of all tasks like setting Xs valves first?

Stock bs34's and 38's are so simple to tune as long as the carbs arent knackered. On a stock bike i wouldn't even consider getting anything else. CV carbs are so simple. Hell my stock bs38's (bike is a 74) were so corroded and shitty, i scored a clean pair of 34's off of ebay that didn't even need to be cleaned or rebuilt and xs johns needles for 80 bones. Ran like a top...minus the rings and head gasket i had to replace.

 

[TeeCat]

If you haven't ever set your valves, that may be the problem right there. If you've got some tight ones, that could cause the bike to run bad, die when it gets hot, all sorts of things.

Well, twins, this will be the weekend I learn, and do it. I have been afraid of boogering it up without someone looking over my shoulder, but I have to learn how to do it, so it's crunch time, regardless of whether I receive the jets in the next day or two or not. Most every other task I have undertaken on this bike I have managed to do somewhat okay, eventually, so I'll just approach it with a positive attitude and a six of Dogfish Head in the fridge.

TC

 

[5twins]

I think you'll be surprised how easy the valves are to set on one of these. Cam chain and valves need to be done 1st before you do timing and carbs. Start a new thread on valve adjusting if you have any questions. I'm sure you'll get lots of replies and various techniques for doing them.

 

[TeeCat]

twins, that's what everyone seems to think. But remember that I can bend the laws of physics. Just not usually in my favor.

I bought new tappet cover gaskets some time ago, so I'll get those out and do it right, while I'm at it. I have done a lot of reading here and elsewhere on this topic - there are several threads on the topic here... inxs, I think, did a good little guide in Tech - but in my reading I have uncovered three different lash specs for '73 bikes, so I want to make sure I use the most appropriate one. But if I run into troubles I'll start a thread. Planning on Saturday.

I think I may be the first guy here ever to replace a crank seal "in situ" (without splitting the case)... but a valve adjustment makes me nervous... go figure...

TC

 

[5twins]

Use .003" intake and .006" exhaust. That's the accepted setting for all years and is actually the spec on the service data sheet I have for your model. You can view and download the data sheet here if you like .....

http://i1020.photobucket.com/albums/af327/bmwbobz/Service Data/72-73ServiceData.jpg

 

[TeeCat]

Ah... viewing and downloading. Interesting... I never knew that there was a universally accepted spec for all years!

Note to self: Schedule Vulcan mind meld with 5twins...

Thank you, sir!

TC

 

[TeeCat]

Hey guys, quick question:

After sitting on my hands for a week waiting for my needles and needle jet from jetsrus, I learned - only after making an inquiry as to when the parts might have shipped - that they have not; the needle jet is backordered til the 18th. So, in your opinion(s), am I wasting time, gas, and effort trying to do any more dialing in? Should I just move on to something else, like the side cover mod?

I think I know the answer, but...

Aaah, the frustration of waiting for a critical part that you could easily lose in the space of a kitchen table...

TC

 

[TeeCat]

Well, my jets have shipped. I won't have them til this coming week, so in the meantime, I thought I'd go ahead and try to button up the aesthetic bits. I have in my head a scheme for a little support to bear the weight of the carb and intake assembly; I don't want it just hanging from the manifold on clamped fuel hose. The support is based on little threaded intake tubing bracketry from Spectre Engineering. Neat and clean. That will be a quick job.

But today I did the job that was worrying me. You purists - no crying cause I cut a side cover, okay? You aren't half as bugged as I was scared!

Here we go:

A profile shot...

What you see as you're mounting...

Mounts in the stock location with the original rear hook and bolt...

The carb top and throttle cable are protected, but not obstructed...

Easy access to the unfettered choke/starter lever...

Looks a bit like a dealer mod...

So, I'm pretty happy with the cosmetics. Hope dialing in goes as well. There's one little hole in there from the original OHC 650 badge. I have a little bolt in there right now, but I might make a little "handle" with a nice pretty round-head allen machine screw. I didn't even mark the original paint. Might put some thin auto trim around the edge... or leave it. Thoughts?

TC

 

[joe-wiseguy]

the carb itself if installed with a small airpod filter wont need any support but you are using what looks like a heavy but nice looking airfilter system that will need to be supported. your installation looks clean.

 

[pumps]

Looks very clean. Good job.