New minibike build, and a few others..

[Raywelder]

So I went with the standard bore size, punching out to the next size was gonna be ridiculous money wise.

Make sure you get a large slide carb for this thing. It will help with the throttle response so much while helping to produce more power.What is the stock hp?

Believe it or not, I am considering going back to the stock carb. I have a mikuni on it, about the size of the intake on it.

Its way too rich. It BARELY runs.
Not that I can't change the jets out in it. But I'm thinking it will be easier to modify the stock carb.

The mikuni is so far off, as far as tuning goes. Even if I get the jets somewhat right, I don't even know where to start on changing out the needles.

I'm waiting until I get my motor built back up anyways.
I'm looking to find a nice stock one for it, for several reasons.

Stock one has a fuel pump on it, The tank is just barely above the carb now, Which makes routing the house a pain because air seems to get trapped in the line when it gets low.

Lawn mower racers are using the stock carb, with all the same mods or more than I have on mine!!!

I also seen in my repair manual that some of the carbs that were made have an adjustable Main jet, which I didnt know was possible. I thought they were all fixed jets, (like the ones I have now) But I think it will hard to find one with that though.

 

[sexyvicta]

a tecumseh carb should bolt on and they are easy to find with adjustable jets

 

[newrider3]

A friend and I have both actually done dual carb setups on Briggs opposed twins using two Tecumseh bowl carbs from 6.5hp or bigger engines. They run surprisingly well, the only hard part is synching a throttle cable to two independent throttles.

 

[exenos]

The reason that lawnmower racing uses the stock carb is because its either on or off no real in between, which is where your engine will be most of the time. A slide crab provides better throttle response and better fuel economy than a butterfly. As for the jetting I would suggest going down about ten sizes if you say its so rich that its barely running. Jets at $2 apiece at mfg supply and here's a link to the mikuni carb tuning http://www.mikuni.com/fs-tuning_guide.html Flat slide
www.mikuni.com/pdf/vmmanual.pdf Round slide

Its fairly obvious whether its a round or flat, just look in the carb barrel and see
Even if you have a flat slide the round slide manual is a good read, it explains EVERYTHING about the slide carbs and how they work.
Here's the mfg supply link for the jets http://www.mfgsupply.com/snowmobile/snowcarb/snowcarbjets/snowcarbmainjets.html

 

[Raywelder]

The reason that lawnmower racing uses the stock carb is because its either on or off no real in between, which is where your engine will be most of the time. A slide crab provides better throttle response and better fuel economy than a butterfly. As for the jetting I would suggest going down about ten sizes if you say its so rich that its barely running. Jets at $2 apiece at mfg supply and here's a link to the mikuni carb tuning http://www.mikuni.com/fs-tuning_guide.html Flat slide
www.mikuni.com/pdf/vmmanual.pdf Round slide

Its fairly obvious whether its a round or flat, just look in the carb barrel and see
Even if you have a flat slide the round slide manual is a good read, it explains EVERYTHING about the slide carbs and how they work.
Here's the mfg supply link for the jets http://www.mfgsupply.com/snowmobile/snowcarb/snowcarbjets/snowcarbmainjets.html

Its also a rule for most classes in Mower racing, so they're kinda stuck with em. I put the Stock carb back on it, It needs cleaned up and tweaking, but I think I am gonna try it out for now, At least for a while, So I can at least get an idea what it feels like with Stock carb. The engine Revs so much high and faster with the stocker. I went ahead and bought a rebuild kit for it (13 dollars for everything :arf and an adjustable main jet kit for it. Gonna Tinker with that and see how I like it.


I did move down in jets already! Problem is, even with the pilot jet and main jets dialed in, Where do you even start with the needle? The carb I happen to have has a Fat, straighter profile needle in it with a blunt end. Needles arent as cheap as jets.

Like I said for now, The Stock carb, with a few mods, will get me set up running decent for now. Then we will see what happens from there!

 

[Raywelder]

A friend and I have both actually done dual carb setups on Briggs opposed twins using two Tecumseh bowl carbs from 6.5hp or bigger engines. They run surprisingly well, the only hard part is synching a throttle cable to two independent throttles.

Upon my research tonight, I found a thread on a super mod class Opposed mower with Dual carbs on it. Guy says it runs up to 7500 RPMS and hes blown the rods on a few motors with it ( all without pressure lube systems though)

7500 is crazy considering 5-6 is already pushing it.

 

[newrider3]

Problem is, even with the pilot jet and main jets dialed in, Where do you even start with the needle? The carb I happen to have has a Fat, straighter profile needle in it with a blunt end. Needles arent as cheap as jets.

Generally you raise or lower the needle, rather than changing it out. The pilot jet basically controls fuel just off idle, when you're at less than 1/4 throttle. The sizing of the main jet determines WOT fuel delivery, when the needle is fully out of the jet. Everything in between is controlled by the timing of the needle. Rich sputtering or loading up in this mid range indicates you need to lower the needle to lessen fuel delivery, so you'll want to raise the clip a notch or two.

If it is barely running at idle, you need to play around with the pilot jet and air screw. A correct setting paired with a correct idle setting should give you a good response when you whack the throttle, and it should return to idle immediately after you release it.

 

[Raywelder]

I rebuilt the stocker. Rebuild kit 12.00, Kit for adjustable main jet 15.00

It runs surprisingly well! It runs rich enough I think after the cam it should still be fine.
Got all the parts in. I plan on taking the block in for machine work tomorrow.

 

[exenos]

What are you doing to the block? Boring it out?

 

[Raywelder]

I got .02 over pistons, because the of the wear on the block, I had to move up a size.

I had the guy do a 3 angle valve job too.

Came out real nice. I was planning on putting it all together when I realized I overlooked something.

The crank bearings... They were toast. The crank is fine, but the bottom of the bearings are wore in pretty deep.

Ordered replacements, but thats gonna hold me back on the motor.
I realized that my gasket kit didnt come with valve seals either.

I did powdercoat the Block, I disassembled the whole bike, sandblasted the frame, I;m working on making everything on the frame pretty now.

Satin black with a extra heavy coat of durable clear in case my carb ever leaks, hopefully if that happens it wont screw it up too bad.

 

[Doc Sprocket]

Dang, that's pretty!!! Would you consider polishing the paint off the ends of the cooling fins? That'd look dynamite!

 

[Raywelder]

Dang, that's pretty!!! Would you consider polishing the paint off the ends of the cooling fins? That'd look dynamite!

I was thinking that, But most of it will be covered by shroud, So i dont think it'll be worth the time.


I got the bearings in today, punched out the old ones and pressed in the new ones. And while staking in the one on the outer cover... I slipped and scratched the bearing It only took a month to get!

It wont slow me down too much, I got the one in the block changed so I will be able to assemble the rest, I just wont run it until I fix that bearing.

 

[Raywelder]

Getting 'er all back together slowly. Alot of time went into fitting the cam and getting the valve clearances right.
Painted the frame and cover (Pics 2-4)

Made some valve covers (Pic 5)

 

[exenos]

Are you going to have this dyno'd or just use the but dyno to see if the engine mods made a diference?

I think some clear valve covers would look awesome on this.

 

[Raywelder]

Are you going to have this dyno'd or just use the but dyno to see if the engine mods made a diference?

I think some clear valve covers would look awesome on this.

I never Dyno'd the motor before I put the mods in so its not like I have a base line, however that was the whole point of putting it together enough to drive before with the stock motor so I have idea of how it runs.

 

[exenos]

Well, you know its 18hp@3600 rpm right? You can get an estimate of 26flbs of torque.
You could use that as a starting point

 

[Raywelder]

Well, you know its 18hp@3600 rpm right? You can get an estimate of 26flbs of torque.
You could use that as a starting point

True, that would be a good starting point, Assuming if it was putting out 18hp

I had no plans of putting it on the dyno, But it would be interesting to see the results. I wonder what that would cost.

 

[exenos]

I think that a chassis dyno for a car is $100 for three runs. If you call around and tell a few dyno shops what your doing they might give a discount since its a small engine and wouldn't need a huge amount of setup. And if you only want a ball park you wouldn't need a full 3 runs, just one. The dyno chart would help you figure out what the best rpm is to shift at (where the torque and hp curves cross I think)

 

[Raywelder]

Pics

 

[B.M.800]

That looks sweet.