I need to pick your brains... this might take a while...

If this is off topic, i apologize in advance.

I am trying to jet a motor. its a four stroke twin with a single port intake manifold. 750 cc engine. I am using a 38 mm mikuni vm carb. this carb is not stock. Trust me, its not stock :naughty:

If any of this sounds remotely ignorant, please let me know. I am racing lawn mowers. we are required by the classes to use a mower engine no larger than 22 hp, and I have managed to acquire an 18 hp opposed twin briggs flathead.

I built a custom intake, and have attached a single carb. my big challenge is to get the pilot jetting correct, and the needle/jet correct for my application.

Does anyone have a clue what Im talking about, or is everyone sitting back giggling? :naughty:

Wow, that's just going to be a lot of trial-and-error.


if the engines are back firing then the pilots are to lean, and you nned to richin it up (for example: 45 pilot to a 50) so bigger the number, more fuel gets put out. what are the main problem with the engine?

I don't know a whole lot about the subject, but at first glance, it appears that your carburetor is undersized. My WR250 has a 39mm carburetor which leads to the following:

The bike's rev limiter kicks in at 13000 RPM. Since it's a four stroke, every other revolution it takes a gulp of air from the carb. So it uses 250cc X 6500 "inhales"/min= 1625000 cc/min of air or 1625 liters of air per minute. All of this air has to go through that 39 mm hole that is known as the carb throat.

Obviously your 750cc lawnmower engine won't be running 13000 RPM, but say it runs 1/2 that, say about 6000 RPM. Since it's 3 times bigger, its air appetite would be 1625*3/2 or 2437.5 liters of air each minute. Shoving that much air through the same or similar carburetor would lead to a much higher flow velocity through the carb. That, in turn, would mean much higher vacuum levels, which probably would suck much more fuel up through the jets, and it would probably pretty far off of the designed mixture. I wonder if you could get jets small enough to offset this?

If not, you might be forced to make at least a dual intake running two carbs, or try to find a much larger carburetor to do the job. It would take a 54mm carburetor to have twice the cross sectional intake area, but I doubt that they make one that large. Using the above information, the increase in cross sectional are would need to be 2437.5/1625=1.5 times as large, so that would mean 46.5mm for the carb inlet. Again, there may not be a carb that large, so a carb larger than 38mm with smaller jets could probably do the trick if you wanted to stay with the single intake.

Again, I'm no expert, you'd want to talk to JD about it. Just some thoughts.

well, thanks for the info guys... Im working on it so far. It hasnt run but on the test stand, and it seems to be a bit rich. might be due to the idle jet.

Since I have a 750 cc twin, and low rpms, would the large amount of vacuum provide the atomization required to use a 2 stroke main jet tube? the carb seems to respond well to a blip of the throttle.

No backfiring, no flooding, yet. still playing with it.

oh, I found out that its a 36 mm vm mikuni.

How do you keep the idle fuel screw *(not the slide adjustment screw)* from being too loose in its bore? Thing doesnt want to stay in one position.

Put a spring on it. Also found out mine is a 37mm not a 39mm. My wife found that I overstated by a couple of mm too. :naughty:

it wont take a spring.... its the small screw for the idle mixture, not the idle speed adjustment. seems to run a bit better when unscrewed. I cant get it to stay at the 1 1/2 turns that it likes.

This things running pretty good, but a tad rich. still playing.

There should be a spring on both the air screw and idle speed screw. The pilot jet in a 36mm VM Mikuni is usually near a #35 - #40. What is yours?

The standard 36mm VM jetting is-

#35 pilot jet

Needle jet (nozzle) Q-5 type#159 (the nozzle unscrews from the bottom)

Needle 6FJ6

2.5 slide

#310 main jet

Is the carb from a dirt bike or snowmobile or a new aftermarket carb?


Glad to have you back James.

I got the carb out of a pile, so theres no guaranty of what it was, what was wrong with it, or if it will work in the future. The slide is a 3.0... but I am confused in the names of your screws... or maybe i dont have the right one... probably dont have the right one.

Time to hit the motorcycle shops.

Since this carb came off of a 2 stroke by the nozzle, should I buy a new four stroke nozzle? and if so, how hard would it be to install in the carb? I dont see any way to remove the nozzle... :naughty: yet...

I will see if I can find some/all of these parts at a local shop. If not available, is there an online site to purchase them? :naughty:

Duh... :naughty::naughty: I found the site that Im on. Boy, do i feel like an idiot.

You guys are pretty good, I'll give ya that.

Wheres the "dork" smiley?

ok, we have load test! carbs a bit lean on the leanest settings. coming out of the turns, it backfires hard, but by feathering it, i can make it run. need to richen it up.

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