Any Go Kart Gurus here? I have a need for more speed but can't get more!

[cabmandone]

My obsession of late is making my Manco Carbide 265CC go kart faster. The kart has a Subaru Robin 9 hp engine. I have made an intake for it as well as a header (yeah... I have lost my mind). I have drilled the main jet to a larger size. The plug was showing very lean when I first ran it, which it would after increasing air flow. I now have a nice brown color on the plug which to me would indicate a pretty good burn. I have completely removed the governor linkage and engine internals. I'm certain I'm getting WOT. The fuel comes from a tank mounted well above the engine and it is gravity fed to the engine.

The kart is rated at around 675 lbs IIRC. The tires are 20x11x8. It has a series 40 Driver and a 7.5" Driven torque converter. I have played around with spring tension on the torque converter. The sprocket on the jackshaft the Torque converter is on has 12 teeth which goes to a 24 tooth sprocket on the rear jackshaft. From there it goes to another sprocket with 18 teeth down to the axle with a 60 tooth sprocket. There is also a reverse gearbox on the same jackshaft as the 24 and 18 sprocket at the rear.

My problem is, the engine isn't winding up under load. With the tires in the air the thing will scream. Once I drop the tires on ground it hits a point like it's still being governed but it can't be because the governor is totally gone. On smooth pavement it seems to wind up better as I can get 34 mph out of it. If I go off road, it drops to about 25mph which seems extreme. On the road or off, I can tell when the engine has hit all it's going to do.

Beyond the modifications to the engine intake/exhaust/carb I have checked the brakes to make sure they aren't tight. I have checked for tow in to see if it was plowing and adjusted to reduce drag. I'm just lost on where to go from here!

He it is in all its glory!

[cabmandone]

OOPS! posted the same pic twice!

[Captain Ahab Jr]

Get rid of some weight, remove the spare seat, front/rear bumpers and other stuff.

That will help make it go

[RKDinOKC]

Valves and timing.

[Charles Freeborn]

One word: Nitrous....

[1990C4S]

Reduce the rotating mass....flywheel and rims/tires.

[javadog]

You're trying to make a difference of a horse or two in a very heavy vehicle.

You could lose some weight, but not enough to make a huge difference.

Yank that engine and replace it with a decent motorcycle engine. 600cc, minimum. Bigger would be better. Some re-engineering would be required but pushing that much weight, you need a serious HP increase to make much of a difference.

[cabmandone]

Originally Posted by peteremsley

That looks like a lot of fun! I assume its a centrifugal clutch? If so, is it perhaps not adjusted properly? Biting at too low RPMs and bogging it?

I should have added, I have my idle rpm's set at just below the cut in for the Series 40 driver with the springs that are in it. It has the yellow springs that are supposed to come in around 1400-1800 rpm IIRC. The engine is supposed to generate 9 hp at 4000 rpm.
Here's the power curve

[cabmandone]

Originally Posted by RKDinOKC

Valves and timing.

More I should have added! The valve clearance has been checked. I haven't gotten into the ignition timing as this thing has a C.O.P. which I think predetermines the ignition timing.

[aschen]

if it winds out well on pavment and not off road, and you want it to be better at the latter (maybe at the expense of the former).......a shorter overall gear would help. You can try a smaller drive sproket or a larger driven sprocket if available.

Otherwise I say a junkyard turbo from a small engine and megasquirt EFI

[cabmandone]

Originally Posted by 1990C4S

Reduce the rotating mass....flywheel and rims/tires.

I have been thinking about narrower tires to reduce rolling resistance, but but keeping the same circumference as I would think I'd want to keep roughly the same circumference because that figures into the gearing. I could go to smaller circumference tires but that would cost me on the top end if I'm thinking right.

[cabmandone]

Something else I'm working on is getting a functional tachometer on it to see exactly what RPM i'm hitting. I have read where the EX27 can run upwards of 5000 rpm safely with the valve springs that are in the engine. I bought a digital tach that uses a wire wrapped around the plug wire (5 wraps) and set it to 1P2r (1 spark for every 2 revolutions) as that's what I have found should be the setting, but I'm still not getting what I think is an accurate RPM reading. I'm topping out at 3200 rpm @ WOT and not building as distance increases.

There is a very noticeable difference in how the engine winds from no load to load. It's like I'm hitting a wall and it won't build more RPM. I'm thinking gearing has a lot to do with it.

[cabmandone]

Originally Posted by peteremsley

that does sound like its over geared... Will it reach 5k with the wheels in the air?

I'm not certain exactly what it reaches without a load. I'm having a friend bring his tach from his Kart over this evening to compare notes. He has two tachs on his, a digital and an analog that is supposed to be for 4,6 or 8 cylinder engines. Both of the tachs read about the same RPM so I have to believe he's getting an accurate reading from both even though the analog technically shouldn't work right on a single cylinder engine.

[cabmandone]

What got me on this quest was getting my ass handed to me by my friend on his two seater Helix 150CC. He has a vehicle weight advantage of about 200 pounds but that gets reduce slightly when you consider he goes about 275 and I weigh in at around 175.

My friend said he was going to have to put a horn on his "buggy" to get me out of the way so the faster Kart could get by. I can't have that nonsense going on!

[1990C4S]

Originally Posted by cabmando

I have been thinking about narrower tires to reduce rolling resistance, but but keeping the same circumference as I would think I'd want to keep roughly the same circumference because that figures into the gearing. I could go to smaller circumference tires but that would cost me on the top end if I'm thinking right.

I've seen dyno tests where the aftermarket rims had way too much mass and reduced HP.

I was thinking more about aluminum lightweight rims and a narrower tire, same diameter.

[Scott Douglas]

My guess would be you're still running too lean and it's running out of gas at the top end.

[unclebilly]

Looks like Rotax kart engines are falling out of flavour. This would be a. Easy 28 HP for an Evo 125cc 2T.

[cabmandone]

Originally Posted by Scott Douglas

My guess would be you're still running too lean and it's running out of gas at the top end.

I have been thinking that as well. My problem is, with each run my plug looks really good!
I think I'll go out and put a plug in it, take it for a ride, do some WOT runs and see how it looks when I get back in. I'll post a pic when I'm done.

[cabmandone]

Originally Posted by unclebilly

Looks like Rotax kart engines are falling out of flavour. This would be a. Easy 28 HP for an Evo 125cc 2T.

My problem is, an engine swap is too easy. I'm not convinced I'm getting all that can be had from the engine I have on it. That's where my mind is now really. I want get it to its maximum capability, then if I blow it up, I'll find a wrecked 350-500cc 4 wheeler or a larger snowmobile and drop engine and trans in for some real fun!

[Tervuren]

It seems to me with difference road/offroad, that you are hitting a friction = torque barrier rather.

Shorter gearing or shorter tires if you aren't getting high enough in RPM would probably be the solution.