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

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!

http://forums.pelicanparts.com/uploa...1508519502.jpg

OOPS! posted the same pic twice!
http://forums.pelicanparts.com/uploa...1508519633.jpg

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

That will help make it go

Valves and timing.

One word: Nitrous....

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

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.

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

http://forums.pelicanparts.com/uploa...1508521055.jpg

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.

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

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.

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.

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. :confused:

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! :D

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.

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

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

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.

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!

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.

By the way, your problem is gearing. I just went through this on my brother's kart. These engines make power in the top end but don't offer much bottom end. You need to gear it lower to take advantage of the power in the higher revs.

O2 sensor and a voltmeter...

Okay! So I put a new plug in and drove it across the field about 1/8 mile for a warm up. Ran WOT on the way back. There's no lean stumble when opening it up which would seem to indicate the idle circuit is good but the plug looks like it's lean.http://forums.pelicanparts.com/uploa...1508535332.jpg

Yep. According to the engine chart I posted, it's making 9 hp at 4000 RPM. My problem is, I don't think I'm getting to the 4000 rpm range. I'm still waiting for my friend to bring me his digital tach so I can pop it on and compare to what mine is showing. On the WOT run my tach indicated I wasn't getting over 3250 running across the field. Regretfully I live on a small 2 lane hwy so I don't want to put it out on the road on a Friday (State Hwy Patrol runs it more often on the weekend) so I'm going to drive through a field about 1/2 mile to 1 mile country road and have at it! I'm going to bring some tools and play around. My "neighbors" will probably think I have lost it. At this point I'm not sure if they wouldn't be right. :D

You say that... but now I'm wishing I had bought a handheld AFR rather than my mounted unit in my 911. I could have clipped it to the exhaust and checked AFR's... dang it!

Can you fatten the main circuit easily?
If so, I'd fatten until you know it's too fat then lean a little bit from that.

FWIW, my 912 would pull redline but had a 'reluctance to accelerate' in top gear. Dyno test showed it was leaning out at the top end. Fattened it ran like a scalded dog with a real push in the back effect.

That's the trouble with this type of carb, in order to go richer on the main, I have to drill it. There is no adjustment screw for the main jet. I can however add some on the idle side which will make it richer throughout. I'm going to start there but my concern is I'll get too rich on the idle side which will affect my low end response.

At very least you could fatten the idle side to diagnose the high end issue, understanding idle will suck during testing, and if it turns out the high end leaning is the culprit, go back and drill it, then lean the idle back to where it was.

Go do this right now- quickly! before it gets dark!!:D

I gave the idle screw 1/4 turn out and went after the main. I'm at a #57 bit at this point which I think is a .0430 bit. Still no more RPM's :mad:
Plug doesn't look bad. I'm gonna hit it tomorrow morning again!

The answer is shorter / higher numerically final drive ratio. What ever is the easiest to do - which is chain/ sprocket side of things. Trying either 1 tooth less on small drive sprocket - or add 4 teeth on the large driven sprocket.

Further, even with more power and or reducing weight - the final drive remains too tall to do what you want.

Here's where I get lost. As stated this uses a Comet series 40 driver pulley. It goes back to a Driven pulley on a jackshaft that also has a 12 tooth sprocket. From there a chain connects to a sprocket on another jackshaft that has a 24 tooth sprocket. On that same jackshaft there is an 18 tooth sprocket which then goes to the axle which is a 60 tooth sprocket. So where do I start putting the sprockets?? I'd like to simplify things but regretfully I think in order to do so I'd have to get rid of my reverse gearbox and I don't really want to lose reverse as I do take this thing down inside a woods and it's nice to be able to back away from a mess I got myself into or almost got myself into.

robin is a very good golf cart motor but not a go-cart racer
I don't think there is a lot of hop up stuff for it

the honda clone [china] single 390 or 420 has a fair bit of up grades out there
or go nuts with a built 460cc version
they start with a big tork and hp advantage before the ad on's
and some claim way over 25 hp with cams springs forged rods ect
that is the hot single in golf cart hop ups
and not a lot bigger or heaver then the robin

the next is the B&S V twins because of lawnmower racing
there are lots of stuff to make power with them

Holy schit does that look fun. Folks that live in the country get to do things us us urban critters don't.

that plug don't have much gap !
a bit bigger spark [gap] mite help the hi-end

I shot in the middle of the recommended range. I think the range was .24 to .28 so I went .26. I now have a collection of plugs from doing runs, making adjustments, checking the burn and then dropping in a clean plug for verification. I think I'll gap one over the high end a bit and see if it makes a difference.

I might have to swallow my pride and take my brother up on the 18 hp. horizontal shaft Briggs he took off a mower. I just can't help but feel like there's something in gearing that I'm trying to overcome. I'm thinking of going down 3 inches on tire width from an eleven to an eight and going with more of a turf tire as this isn't really a mudder. I would think that would reduce rolling friction somewhat considerably by getting rid of 6 inches overall of tire. Not to mention it might help the thing fit in the back of my truck!