VMAX Downdraft Carb conversion

[BLIGHT]

Hey guys,

Some of you may know I've been playing around with different alternative carb setups for the GL1000F. Well, frankly I haven't had a lot of success, but after many many hours of tinkering I have come up with a pretty cool alternative!

First I tried two pairs of Ninja 500 CV side-draft carbs and had poor performance for several reasons. Then I tried four Mikuni TM34 flatslides and they actually worked very well after extensive rejetting, but there were definite throttle pull effort issues (four springs) and there was also a tendency for throttle 'hang' at closed throttle at high rpm (like when downshifting)... high vacuum was sucking the slides hard against their bodies and not allowing the return to the bottom stops. Pretty exciting when riding, eh?

So, I had this idea to use the downdraft 4 carb setup from the Venture/VMax Yamaha 1200. It's a Mikuni CV design and is really easy to tune once installed on the GL... especially the slide needles and pilot screws. Oh, and throttle butterflies won't hang due to vacuum...

I used four standard Mikuni carb flange adapters, and just slid them over to line up with the GL intake ports perfectly before tightening the screws. Carb port spacing ends up being about 5" on center, so I had to modify all the linkages. It's a LOT of work, and unless you are REALLY nuts AND handy in the workshop, don't attempt this conversion.... please!

That said, I tried several different pilot and main jetting combinations and even had to stack the needles to get decent throttle response. Well, it starts easily and runs well now. I'm getting 35 mpg with a pretty heavy throttle hand.

It looks pretty cool too I think! I also found some K&N dual port oval filters that mount up easily. I tried some individual pods and they worked well too. Not so much luck with the velocity stacks... I think it would need larger mains for those, but I'm pretty sick of jetting exercises!!!

Here's a few crappy (phone) pics: I'll post up some more 'professional' ones later on...











Cheers,
BLIGHT

[Oldboy]

Cool work, love the out of the box thinking....nice stable too.

[Bert]

Looks very different in a good way.

[past in time tony]

WOW! Nice! Great work, way to hang in there and get some carbs working that really look cool too. How do they sound? I have webbers on my 75 and they have such a cool sound, even with air cleaners. Great lineup in the garage, u have great taste in bikes.

[twowings]

Nice rack! of carburetors that is! Got to congratulate you on your persistence in pursuing the goal. Looks good and looks cool.

[BLIGHT]

Thanks guys! It was a lot of work and frustration, but worth it in the end.

[Gowing]

Love your wing, what a job,
.was wondering what side covers are those,?
Thanks dave

[BLIGHT]

Love your wing, what a job,
.was wondering what side covers are those,?
Thanks dave

Thanks Dave, the side covers are from a later model GL1200 I believe... I cut off the original mounting tabs on the frame and on the covers, and trimmed the back edges some to fit, and then held them on with small extension springs so I can remove them when I need to.

Not too tough, and makes the bike a lot lighter looking visually, especially with them blacked out.
BLIGHT

[chrisgumuski]

Looks really nice. can you post your jetting specs?

[gregforesi]

A man after my own heart. I like it. Getting the plug wires in looks like it took a bit of head-scratching too.

[brokentoe]

Very nice work. We likes it.

[ericheath]

I'm not sure if Mikuni has the same jet numbers as Keihin, ( I read somewhere they have different numbers ) but I am curious what sized jets you used. I cannot seem to find the right combo for my bike, 1200 with 1000 heads, cams, and 77 Keihin carbs. I wish I had a set of the Vmax. I am at 140 main with 70 air, 70 primary with a 140 air. It has an open air filter above the plenum. It accelerates awesome, although I can't give it full throttle right away. That makes me think rich, but giving it three gears, holding 8 grand for five or ten seconds and then shutting it down and coasting to a stop shows pretty white plugs. It also bumbles when holding speed at 50 mph. I have tried one washer under the needle. How do your knees fit? Any interference?

[BLIGHT]

I'm not sure if Mikuni has the same jet numbers as Keihin, ( I read somewhere they have different numbers ) but I am curious what sized jets you used. I cannot seem to find the right combo for my bike, 1200 with 1000 heads, cams, and 77 Keihin carbs. I wish I had a set of the Vmax. I am at 140 main with 70 air, 70 primary with a 140 air. It has an open air filter above the plenum. It accelerates awesome, although I can't give it full throttle right away. That makes me think rich, but giving it three gears, holding 8 grand for five or ten seconds and then shutting it down and coasting to a stop shows pretty white plugs. It also bumbles when holding speed at 50 mph. I have tried one washer under the needle. How do your knees fit? Any interference?

Hi, the Mikuni jets are different than Keihin. The Mikuni BD34 CV VMAX & Venture carbs are virtually identical, but the VMAX has way larger jets and a different needle taper. Needle jet is larger too I think. The main jet is in a holder and feeds fuel to the needle jet and needle also in the holder. The main oddly enough also feeds fuel to the pilot circuit. (The pilot jet is in the same holder next to the main)

I think I finally settled on Mikuni 125 mains and 25 pilots, and left the air jets stock. The K&N filter is very free flowing (lots of area) so I ended up raising the needles about 2mm to get rid of a stumble at 1/4 throttle. I have turned out the pilot screws about 2 turns... maybe 2 1/4.... I forget now. The pilot screws are super easy to adjust... so is changing the needle height with the diaphragm covers right out there in the open.

Oh, and with the shorter seat, my knees do contact the air-cleaners. It's not as bad with the velocity stacks or individual cone shaped pod filters. I ride it like a cowboy on a horse with my knees stuck out. Looks a little racier that way too....
BLIGHT

[BLIGHT]

Here's a few better pics with the Yamaha Venture/MAX CV downdraft carbs and velocity stacks... stacks look pretty cool but needs some additional main jetting work: (BTW, Those are the ribbed Venture diaphragm covers in these pics... the MAX covers I'm using now are smooth polished aluminum)







Think I'll stick with the K&N oval filters for now and just ride it!!!

BLIGHT

[HOTT]

Wow ! Nice.