[RP McMurphy]

Hey all,

So to preface this, I have done some reading on successful carb settings with the BS29 that comes with the AVL bikes. I get the gist of carb tuning, and what settings will affect which part of the throttle opening.

Biggest problem right now is a bit extreme-- getting engine backfires through the carburetor (and occasionally the kickstarter), sometimes severe enough to pop the carb right off it's rubber mounting flange. That's pretty alarming, particularly when it happens on the street 

Before I go to much further, here's my current setup--

BS 29 Carburetor, stock engine, with a K&N cone filter and free-flowing exhaust-- all the emissions skunkworks have been removed or blanked-off by the P.O.
Main Jet: 122.5
Vacuum Slide Needle: Clip on the richest notch, with the spring resting on the washer above the clip, then a shim washer, then the plastic adjuster.
Idle Jet: 17.5
Fuel-Screw: About one full-turn out.

It's worth noting that I've seen some hairline cracks on the rubber mounting boot. I just tried blocking them off with some black electrical tape (yeah yeah, I know), but I'm not sure if that helped.

Having a weird time starting-- bike usually starts on the first kick, but then doesn't want to start back up after I've shut it off. It doesn't seem to want to run at all without the enrichment knob at least halfway out, and has a very poor idle, like the engine is straining to keep going. This even happens with the "choke" (enricher) partway on.

Other than that, seems to run okay. Plug was sooty (rich) the last time I pulled it, which is probably for the best, all things considered. I did notice that the power bogged at WOT during one ride, but I believe that may have had something to do with leaving the enricher on.

So all that said... anyone have any tips or carb settings that might help? I'm pretty well stumped here.

[DanB]

I'm betting that your carb intake (rubber boot) is letting air in some where. Replace it; electrical tape won't do. Also, do you still have the PAV setup?  There's a air line from the top of the carb intake. If that's off or cracked, it'll give you the problems you're seeing.

You say you have a cone filter. Have you supported the carb with a hanger?  The intake can't handle the weight of a vibrating unsupported carb.

Hope these help! 

[RP McMurphy]

Hey Dan,

Yes the carb is unsupported, no I don't have an airbox or bracket to keep it from vibrating.

Any idea where I should pick up a new boot from? Does CMW stock the correct boot for the AVL?

Can anyone with a similar setup recommend a baseline jetting from which to start? That would helpa lot.

[Kevin Mahoney]

We have virtually everything for the AVL

[1 Thump]

I'm betting that your carb intake (rubber boot) is letting air in some where. Replace it; electrical tape won't do. Also, do you still have the PAV setup?  There's a air line from the top of the carb intake. If that's off or cracked, it'll give you the problems you're seeing.

You say you have a cone filter. Have you supported the carb with a hanger?  The intake can't handle the weight of a vibrating unsupported carb.

Hope these help! 

Yep, air leak explains whats happening. Easy fix. Call CMW.

[tooseevee]

Hey Dan,
Yes the carb is unsupported, no I don't have an airbox or bracket to keep it from vibrating.
Any idea where I should pick up a new boot from? Does CMW stock the correct boot for the AVL?
Can anyone with a similar setup recommend a baseline jetting from which to start? That would help a lot.

           First of all you must cure the micro-air leaks in your manifold. NField Gear has everything you need. Go there and go to the carb parts section and look.

           Your main jet and pilot seem too big. That's why your plug is black. You want a #110 Mikuni main jet.

            You want a #15 Mikuni Pilot Jet. That's what it came with stock.

            The needle should be a 4DHL38 and I think you're too rich in the richest groove. Start in the middle groove. And the tiny Mikuni shims go under the E-clip to give an adjustment between two grooves. I did away with the evil white plastic thing, not needed, but that's up to you. I don't know exactly where you have that plastic ring so I don't really know if you truly have the needle at its richest. I did away with it totally when I was running that carb.

             Turn the mixture screw in all the way (Carefully!). Then turn it out 1 1/2 turns. It's a fuel screw so CW will lean it, CCW will richen it.

             This should get it started and running if the diaphragm and manifold are OK. It will never idle right if you have intake leaks. Get some new NGK plugs.

              PS: I don't have that carb on my bike any more.       

[DanB]

Hi RP.

I definitely would go to CMW. Give them a call. If you have the PAV still (thing hang off the cylinder on the left exhaust side), consider getting rid of it. You'll need to order the blanking plug from CMW as well.

Jetting: I was close to your setting with free flow in and out. I wouldn't change anything until you get the new boot and then do plug chops. I had a 17.5 pilot, 120 / 122.5 main( temp dependent), shim under middle groove for needle. Like 2CV, I ditched that carb and got a TM32.

Don't forget to make a bracket or hanger to support the carb.

[RP McMurphy]

Hey guys, thanks a million for the responses.

I got stuck the other night, had to clean the plug off on my t-shirt in the dark 

This the rubber boot I should get? Or do I have to call CMW to dig up the right part?

http://www.nfieldgear.com/mikuni-carburetor-intake-manifold-rubber/

I'm also thinking of just saying to heck with it and picking up a TM32 or the Keihin-style flat carb from CMW. Expensive, but meh.

Thanks again.

[tooseevee]

Hey guys, thanks a million for the responses.
I got stuck the other night, had to clean the plug off on my t-shirt in the dark 
Thanks again.

            You see, that shows again you're too rich. It might be all needle height, but with mine (K&N cone, wide open exhaust, all PAV stuff gone) I tried the 17.5 Pilot and it Did Not Like It. Went back to the #15, E-clip in the middle with one Mikuni shim under it and it was fine. 1 1/2 turns out on the mixture screw.

              But they're all different, aren't they?
 
              The blurb for that manifold says Iron Head, but maybe it fits the AVL. I don't find a manifold that says AVL with BS-29. To me the write-ups online are insufficient and incomplete in many cases. The website is also hard to navigate and the menus pop up or disappear without you wanting them to just moving the cursor around. It's so maddening now I avoid it. Click on Shop Accessories on the home page and you get Shopatron. What the hell is Shopatron?

            Click on NField Gear you get a big Not Found in Red, but you can still get in. It's very weird.

             

[DanB]

Just a final thought.... The plug is so sooty due to the fact you're running with the choke pulled. I thought I read that somewhere. If that's not the case, follow 2CVs recommendations. 

I recommend just calling NField gear. They are very helpful.

[flyboy]

I wouldn't change any jetting until you replace the rubber manifold. Was it running ok at some point after the free-flow setup?
The mainfold sucking air explains why you need to add fuel with the enricher on, in order to run. As previously said, also explains why the plug if fouling.
Replace the manifold and support the carb. If you can swing it, order the Alloy Manifold from Ace and you'll never replace it again. Also a good match for the TM32 you'll end up buying.

[RP McMurphy]

Okay, so the saga continues...

I replaced my intake manifold (the rubber carb boot), including the o-ring that runs along the inside. Didn't change out the stock paper filter, mostly because I didn't know it was in there 

That didn't miraculously solve the problem, but turning the idle-speed adjuster nearly all the way up sure did. Is that normal? Oh, I also flushed the K&N cone with water and dried it out over night. I know I know, I need to re-oil it and clean it with their unobtainium solution, but I was on a tight schedule.

Okay so long story short, bike idles just fine with the choke off now. But there's a new problem. Because of course there is. Now I'm getting near-stalls when I try to wring the throttle out somewhere around 1/2 to 3/4 of the way to WOT.

I just tried it with the white collar removed entirely, and now I get pops and splutters through the carb just revving it in the driveway. Doesn't sound healthy. I'm guessing both these are signs of a lean mid-range, but someone want to verify that? It's like hitting a virtual wall in the rev-range, past which the engine just wants to stall. Snap the throttle closed and it comes right back to life.

Friend mentioned it could be float-height... any thoughts?

[tooseevee]

Okay, so the saga continues...

I replaced my intake manifold (the rubber carb boot), including the o-ring that runs along the inside. Didn't change out the stock paper filter, mostly because I didn't know it was in there 

That didn't miraculously solve the problem, but turning the idle-speed adjuster nearly all the way up sure did. Is that normal? Oh, I also flushed the K&N cone with water and dried it out over night. I know I know, I need to re-oil it and clean it with their unobtainium solution, but I was on a tight schedule.

Okay so long story short, bike idles just fine with the choke off now. But there's a new problem. Because of course there is. Now I'm getting near-stalls when I try to wring the throttle out somewhere around 1/2 to 3/4 of the way to WOT.

I just tried it with the white collar removed entirely, and now I get pops and splutters through the carb just revving it in the driveway. Doesn't sound healthy. I'm guessing both these are signs of a lean mid-range, but someone want to verify that? It's like hitting a virtual wall in the rev-range, past which the engine just wants to stall. Snap the throttle closed and it comes right back to life.
Friend mentioned it could be float-height... any thoughts?

            You're doing too many things at once!  And they're not "Problems". They're just steps on the way to dialing in a Mikuni CV carb; many steps.

            You've got to be sure of your homework on how CV carbs work in general and then do more homework on how your little Mikuni CV works so you understand the steps and that you gotta do things in order. ONE thing at a time. It's not easy, but you've gotta do it. Or you'll never be happy. And if you get a TM 32 later you'll hafta do it ALL all over again. 

              E.G., you took the plastic collar off. Where was it? Under? Over? Was it effecting needle height? Does it matter? Did you change the E-clip position when you removed the plastic collar? It sounds like it went to shit with the collar off which means you must have changed the needle height.

               It just all gets too dicumbobulated and confusing if you don't slow down and do specific things in a specific order and understand what you're doing. I'm not that good at this any more. It can go to hundreds of postings and get really, really confusing. I'd LOVE to be there and help, but I'm afraid of it here. Lots of more wicked smahter  people (and  younger) than me are on here.

          (I hope you don't take me wrong) Good luck       
           (No. Leave your float height alone. For now).

[1 Thump]

RP: These CV carbs are pretty forgiving. You have to get in the ball park and that should rid most, if not all your problems.

Two questions:

1. Did the bike ever run right? If yes, what was the jetting at that time?

2. You should look at the paper filter. It could be clogged up and contributing or worsening the problem.

The ball park/ estimated jetting is a known for these carbs for various altitudes and mods (filter/exhaust). So, why dont we begin with basics; check the air filter first. Then check for exhaust leaks. 

Errrrrrr....wait you said you have a cone filter, so my guess is the paper filter is defunct....or not?

[DanB]

E.G., you took the plastic collar off. Where was it? Under? Over? Was it effecting needle height? Does it matter? Did you change the E-clip position when you removed the plastic collar? It sounds like it went to shit with the collar off which means you must have changed the needle height.

I agree. Sounds like u took the collar off the the needle dropped right down to lean the sh@t out of it. If it's popping and acting like it's hitting a wall, think lean.  Id also suggest some research on tuning Mikuni carbs via intraweb Magic. Won't cover your specific cv carb but will get u close either.

Oh, one lat thing. As 2CV points out. Change 1 thing at a time and don't touch the float.