Dual BNi38 Carburetors - Wont idle.

[nitrosport_5]

Ive got 2 BNi 38 Carbs on my engine. Ive got the tuning pretty much dialed in although Down low there is a bit of a bog, and i cant get it to idle. If i adjust the idle screw in far enough to where the engine will idle, if i rev it to 3k it will stick there and not want to idle back down. Back the idle screw back out a ways. and i can rev to 3k and it will come right back down. I tried turning the low speed screw in little bits at a time, and it seemed to not help much.

What adjustments do i need to make to change down low tuning? It runs great once i get up over 2500-3000rpm. Ive been reading up on things and it sounds like the low speed needle may be what i need to adjust, but it is almost all the way in at the moment. At this point do i need to drop a jet size? raise the pop off pressure?

Thanks in advance guys

 

[LouDoo]

Is this even a SeaDoo? If so what year and model? You really need to adjust the idle with the ski in the water.

Lou

 

[smc0922]

According to his profile looks like he modded a snowmobile...

Either way buddy if you are turning the idle mixture needle IN, you are leaning the mixture and making it not idle. Sound like you need to set the initial turns out and see where you are there.

 

[racerxxx]

Nitro,

That thing should have Mikuni 44's round slides on it in a sled. Personally I'd do what it takes to fit them, I can't see them taking up that much more space downwards due to the ski's linkage is on the bottom. Do you have the carbs bolted directly to the 670 or are you using an adapter? What pump are you using--the sled fuel pump or the Seadoo fuel pump? Nice project by the way!

Edit: post a pic of your carb

Here's what he's working on:

 

[nitrosport_5]

Nitro,

That thing should have Mikuni 44's round slides on it in a sled. Personally I'd do what it takes to fit them, I can't see them taking up that much more space downwards due to the ski's linkage is on the bottom. Do you have the carbs bolted directly to the 670 or are you using an adapter? What pump are you using--the sled fuel pump or the Seadoo fuel pump? Nice project by the way!

Edit: post a pic of your carb

I didnt innitially say it wasnt a seadoo because it should not matter..i dont think? These carburetors are set up exactly as they are on a seadoo. I believe these were on a 717.

I cannot fit the roundslide carbs. I have tried everything possible... i too wish i could use them. ive still got em on hand. even with a shorty intake they wont fit. Only way to fit them is put the engine up really high in the chassis, and that would make the center of gravity high, and no room for my radiator and fuel tank.

I am using an intake from a 717 seadoo i beleive. It is just the complete setup behind the rotary valve. I am using the stock seadoo fuel pump that is attached to the side of the mag side carb.

Ive got the thing running really good with these carburetors, just cant get the idle nailed down. It also is hard to start, have to crank it for a good 15 seconds before it sputters and starts to fire. These carbs are a bit different than i have worked with before, and that is why i joined here... to hopefully get someone that can point me in the right direction of getting them tuned up. Hell ill even become a supporter if you guys are willing to help me. Been fighting these things for a while.


Carb Setup

Clearance between trans and carb. With the roundslides they wont fit down in the space. even if they did, there is no way a filter would fit on them.

intake setup. this is a spare i have in my parts pile.

Here is an updated pic. the Odyssey i put a 570 Yamaha exciter sled engine in a couple years ago. been the most reliable atv ive owned.

 

[Minnetonka4me]

Those things are dope!

Do you have the primer hooked up? It doesnt pop right off when its cold and you prime it?

When you have this "bog" have you tried to hit the primer to see if it helps or hurts?

 

[nitrosport_5]

Those things are dope!

Do you have the primer hooked up? It doesnt pop right off when its cold and you prime it?

When you have this "bog" have you tried to hit the primer to see if it helps or hurts?

Yes the primer is hooked up. When i start it cold with the primer after it has been sitting for a day, it will pop right off, run for a sec and then die. prime it again and it will pop right off again and usually continue to run. as long as i hold the throttle a little bit because i dont have the idle figured out. Once it is fully warmed up, it takes a little bit of cranking before it will start. This is not using the primer. If i gave it a small shot it would fire right off.

I have not tried the primer when there is the bog. Will try that tomorrow. A little late here. dont want the neighbors to hate me any worse haha.

 

[bell99man]

Sounds really lean at idle from your answers. This might be due to the smaller throat on these carbs if it were originally designed for 44's but I am definitely no expert here. If this is the case, maybe a larger low speed jet.
The other possibility is your tank location and distance from the carbs. I think these things were setup to be about level with the tank and the flow is provided by a closed low pressure loop. Again, not an expert but some things to look at. Good luck and yeah, that thing looks fun man.

 

[SabrToothSqrl]

Man that's gotta be loud, how are you cooling it?

 

[Minnetonka4me]

Good point on the fuel flow..Some machines have restrictors on the incoming line.

I would also agree on the Lean vs. rich...after your last response. The primer test will tell you if thats the issue. If it takes off when you hit the primer you are too lean on the low end. 2 turns is the absolute max you can turn them out, and its always better to change jets and have the adjusters as close to closed as possible.

 

[nitrosport_5]

Good point on the fuel flow..Some machines have restrictors on the incoming line.

I would also agree on the Lean vs. rich...after your last response. The primer test will tell you if thats the issue. If it takes off when you hit the primer you are too lean on the low end. 2 turns is the absolute max you can turn them out, and its always better to change jets and have the adjusters as close to closed as possible.

I was thinking it was too rich down low, and once it revved it cleaned out making it not want to come back down to idle. but what you guys are saying makes sense. i will go out and play with it here in a little bit.

Man that's gotta be loud, how are you cooling it?

Sure is loud. Modified a 500CC dirtbike muffler for a larger pipe.. its more or less just for decoration because it does not seem to do a hell of a lot. i did not want the huge snowmobile can hanging out the back.

as for cooling, i am using the factory water pump, and a Honda Civic aftermarket radiator with a custom fan shroud. when i am riding it doesnt get much over 160. i think it is right about where the thermostat opens up.

If you want to see all the pictures i have from day one, the photobucket album link is here
http://s23.photobucket.com/albums/b354/nitrosport_5/Alabama Pilot/

 

[nitrosport_5]

I am finally happy to report that i figured out what was causing me all this greif.

i got the carbs dialed in so they were running perfect, but wouldnt idle. I was puzzled. One day randomly surfing youtube there was a guy trying to find a vaccum leak on an ol ford. What was the symptoms.. wouldnt idle!. SO i decided to tear apart my intake system again and inspect each and every gasket and sealing surface with all but a darn magnifying glass. Turned out that there was a bit of sandblast remnants between the rubber o-ring that is between the rotary valve cover, and the piece the carbs bolt to. Cleaned all of the crap out of there, made sure everyting looked good, lapped the flat surfaces where the gaskets contact, and put it back together. Fired it up and it even sounds different than it ever has. Almost sounds like it has a miss to it. Put my hand over the clutch side carb and it would just keep idling. put my hand over the mag carb and it would kill it. looked down into the throats of the carb and they are out of sync. got them close and it is running great, just need to get it out and jet it.Also have a sync tool on the way

Lessen learned here? check the condition of both sides of the little o-ring seals... ill know better next time. LOL

 

[Jetskigoodies]

I was thinking you might have had to do a pop off test, it seemed like the pop off would have been low, so you would be running lean. Looks like you found your problem though