Need carb help from an experienced tuner please

[modman82]

I have a modified 787 in an 96 XP that is in the process of being broken in(Ports matched and polished, lightened flywheels, bored to 82.5 with Wiseco pistons, light mods, etc). I'm trying to tune the carbs(rebuilt to stock specs) to get enough throttle response to break it in properly. I have read and understand most of Mikuni's carb tuning info but have a few questions I hope someone can help with.

I got it idling good tied to the trailer in the water at about 1530 rpm's with the low speed adjusters way out to 1 1/2 to 2 turns from bottoming. This is the only way I can get it to rev smoothly off idle (More fuel demands being that the motor is modified?) I know it's still in the process of being broken in but want it to run decent enough to get to all the throttle positons to do the heat cycles it needs to break in(32:1 premixed, oil pump delete). The plugs look a little rich but OK in both cylinders. Do I need to go to a bigger low speed jet then try leaning the low speed needles in until it's closer to spec? My pop off is at 33 on both carbs... When I do rev it past half throttle with the low speed adjusters open so much it seems to bog then want to clean out making me think that a bigger high speed jet is needed to... If I have left out any info just please ask.

Thank you so much

 

[LouDoo]

I'm no carb. expert but from what you're saying I think you need larger jets, the LS adjustment is only effective to about 3000rpm. The guy you really need to talk to is Dr Honda. If he doesn't see this and respond by tomorrow morning you might want to PM him.

Lou

 

[Dr Honda]

I'm no carb. expert but from what you're saying I think you need larger jets, the LS adjustment is only effective to about 3000rpm. The guy you really need to talk to is Dr Honda. If he doesn't see this and respond by tomorrow morning you might want to PM him.

Lou

He already sent me 8 PM's last night. But... I'll be nice, and read over it, once I get a chance.

 

[LouDoo]

Well, I did tell him to wait until tomorrow morning, I probably shouldn't offer your services so freely, but then again what are friends for?

Lou

 

[modman82]

Guys I'm sorry as I never got any confirmation that the PM's went through. I appreciate all the help you can give me.

 

[Dr Honda]

A few questions first....

1) are you running the stock air box?

2) are you running the stock exhaust?

3) are all the tests above on the trailer? (was there any riding?)

 

[modman82]

Yes to the stock air box and exhaust. The flame arrestor is well seated, but has had some of the fine mesh screens removed. All of the tests have been done on a trailer so far. I haven't actually riden it yet to put a real load on it as I feel it will die and not perform correctly.

 

[Dr Honda]

Well... with the stock box, and pipe... you really don't need to re-jet. But... that depends on how much of the mesh you removed. Even with all of it gone... it should run on stock jetting.

The bore size, shouldn't have an effect on the jetting... other than it can actually go rich, since there will be a larger vac produced.

If you have a buddy with a boat, or a PWC... go for a ride. I know on the 800 engine... it is a little more touchy as far running rich when cold... and generally, it needs to "Clear out" before it will run right.


Oh... I would also try to put it back to 1 turn out on the low screw... then adjust the idle, in the water, so it's stable.

Here's the reason.....

The low speed screw is truly a "Trim" for the fuel when the throttle valve is closed. (and vac is high) The fuel it controls is only on the large, idle port, below the butterfly. once you start to open the throttle... the "By-pass Ports" will be exposed... and they are controlled by the pilot jet. So... opening the low screw ONLY effects the idle fuel. By having it open that far... you will need to have more air needing to come in... and in turn, are probably exposing the bypass ports. (adding more fuel) So...it's a circle.

 

[modman82]

I appreciate your input. The stock airbox is off(as that's the only way to get to the adjusters) but I have drilled 4 1" or so holes in the lid as one of my basic mods. I read that the stock airbox flows just as much air as many aftermarket pods/filters by dooing this and offers far superior water protection. I haven't tried leaning the motor out yet by turning the LS adjusters in until I get an un-stable idle either... I think I really need to do this first to see where they stand to determine if I need a larger or smaller pilot jet. I have only messed with it enough to get a decent throttle response and to rev cleanly... That only happened at 1 1/2 to 2 turns out. It is idling good with the tach showing a little over 1500 in the water(tied to the trailer).

 

[Dr Honda]

Well... since you removed mesh, and cut hols... the air box is not stock... and you should be running a lower pop-off. Set it to the low end of spec.

 

[modman82]

Lower than the 33 I'm already at? The stock specs are 23 to 43 so I'd have to change to a different needle valve and spring? The whole "pop off" tuning portion is getting a little easier to understand the more I read... I do have the chart that Mikuni makes showing the different combinations, but don't really have the lean hesitation when I open the throttle off idle. I got it to idle well and the plugs look good and it doesn't want to die with those LS needles open so much feeding it plenty of fuel. That tells me the pop-off doesn't need adjustment?

Now if I go back to the ramp and lean it in to try to find the sweet spot even more (and find that the LS needles are closer to spec), should I leave it alone and then concentrate on the high speed portion?(because at that throttle position it feels like it's not getting enough fuel and running rough and dying) I DO have it idling good now with them being so far out and the plugs look good but a little wet. Please remember to that this is a fresh rebuild and that I'm only trying to get it running better to break it in and not doing any fine tuning at this point. Thanks again from a guy trying to learn about the 4 different tuning parts of a Mikuni!

 

[SabrToothSqrl]

After you get done with all this, race a bone stock one. I'm curious to know what all this effort brings. Reliability should be a factor as well... Is the MPEM going to be modified with a new rev limit? New prop?

 

[modman82]

Me too as i've just done mods that didn't break the bank and only hope for the best in reliability once I get it tuned right... I don't plan on getting a rev limiter to spin it up any higher, but if it performs and tunes well(and bumps the stock limiter) I may just have to get one. If anything, I'd keep the stock prop and get that R&D nozzle set-up too.