Seems like I'm not getting the power / speed I should be getting

[Munk]

Hello!

I have a Trailmaster Mid XRX-R with the governor removed, 22mm Mikuni (clone) carb and loop header. This kart has a 6" (?) driven torque converter, 16" tires, and 46T sprocket. Acceleration seems good, I can get up to 20-25mph in what seems like 2-3 seconds, but on a flat road it tops out at 33-34mph no matter how long I give it. It won't go above that unless I'm on a downslope.

Does this seem right? Stock, this thing is rated for 31mph, and so I (perhaps incorrectly) assumed the extra RPMs of having no governor would net me more than 2-3 mph.

 

[mckutzy]

What is the engine??
What is the front sprocket tooth count?

 

[mark20]

sounds like the valves are floating, did you upgrade the valve springs? as stock ones tend to float around 5000rpms. try adding some 18lb valve springs and see if it improves.

assumeing its a 196cc clone.

 

[Munk]

Yes it is a 196cc engine, and I believe it's a 13T top sprocket. Stock springs are still on there as a bit of a safety measure (since I haven't upgraded the other parts that would probably need to go along with the spring upgrade) so I was sort of counting on the float to keep things safe until/if I upgrade the other components.

If 33-34 mph sounds about right for this build then that's fine, it is what it is. Just want to make sure I'm performing optimally for what I currently have done so far.

 

[Munk]

Also not sure if related, but tuning the carb is proving difficult. Things are running of course, no major issues that I'm aware of (besides needing to feather the throttle to get the electronic ignition to work, that didn't use to happen before the upgrades) but adjusting the air mixture screw seems to do nothing...no higher idle, etc. Adjusting the idle screw works normally, just no change with air screw. I can close it all the way and it idles fine, or open it 5+ turns and still no change. ��

Any ideas on what might be causing that?

 

[Brianator]

13/46 is not ideal, that's a 3.54:1 ratio so my guess is you have alot of belt slip going on, get it up to a 5.4:1 minimum (6:1 is better) and you should see a big improvement in that department. As for the carb I'm thinking the pilot jet need to be cleaned, it should have one hole running strait through for fuel but a bunch of small holes around the "tube", this is where the air gets into the fuel for that circuit. Also if you look at the mouth of the carb, you should see some ports, one of those is a air port that feeds air to the pilot circuit, something could've got in there? Some of them have an "air jet" threaded in (some are pressed in and not removable) so you can control how much air gets into the pilot circuit so you may need to remove it (if equppped and threaded) to clean the port behind it.

 

[Joe-405]

If your threading the air mixture all the way in. It usually means it’s too big.

 

[Munk]

13/46 is not ideal, that's a 3.54:1 ratio so my guess is you have alot of belt slip going on, get it up to a 5.4:1 minimum (6:1 is better) and you should see a big improvement in that department.

Does this account for the torque converter (I understand the general principle behind a TC, but aren't exactly sure how to factor it in)? It looks like a 75T - 80T sprocket gets me close to 6:1, but that seems like it would significantly reduce top speed and increase torque.

 

[Brianator]

It does indeed. Chances are the TC isn't fully shifting into "high gear" because of it's torque sensing system, with a better gear ratio you could probably still get to the same speed because the TC is working as it should but the difference is it will be much more drivable overall and it could possibly go faster, depends on how the TC is handling the poor gear ratio. If you have (or can borrow) a GoPro, set it up so you can see what the TC is actually doing. What hole in the driven is the spring in? You can change the front sprocket also but I wouldn't go smaller that a 10 tooth. Check out this awesome live app @ app.kartcalc.net, go to "quick calc". it should help you figure things out, the "top speed" I believe is pretty theoretical because I can't get even halfway to the top speed it states I should be able to

 

[Munk]

Hey, I finally got around to getting a video. Is the belt supposed to be this loose?
https://youtu.be/Cx5OKjC4xgo

Here it is under load:
https://youtu.be/WeStQMK-XVQ

Also, I discovered that the "air" screw is in actuality a fuel screw, meaning opening it up adds more fuel (not more air). Spark plugs looked like I was running rich, which makes sense now. I'll need to make adjustments and check plug several times to see if I can work that part out.

 

[landuse]

The belt actually looks good to me. There are belt charts for different distances etween the driven and the driver. That will tell you what you need

https://www.gokartsupply.com/tcbelts.htm

 

[Munk]

It does indeed. Chances are the TC isn't fully shifting into "high gear" because of it's torque sensing system, with a better gear ratio you could probably still get to the same speed because the TC is working as it should but the difference is it will be much more drivable overall and it could possibly go faster, depends on how the TC is handling the poor gear ratio. If you have (or can borrow) a GoPro, set it up so you can see what the TC is actually doing. What hole in the driven is the spring in? You can change the front sprocket also but I wouldn't go smaller that a 10 tooth. Check out this awesome live app @ app.kartcalc.net, go to "quick calc". it should help you figure things out, the "top speed" I believe is pretty theoretical because I can't get even halfway to the top speed it states I should be able to

Okay, I think I understand. Because the ratio is 3.5:1 it takes much more forced to spin the wheels, potentially causing belt slippage. By getting a sprocket with more teeth, I would reduce the sheer force required to turn the wheels, potentially eliminating slippage -- which is how I might be able to preserve (and maybe even increase) top end despite going with a larger axel sprocket.

I guess before making any plans, I need to confirm if there is indeed belt slippage. Is that something I could easily see, hear, or smell? I don't think I've ever smelled burnt rubber after topping out, nor have I noticed a squealing sound.

This also pauses my idea of simply getting larger rear tires to increase top speed, since that would make slippage worse (if there is any).

 

[Brianator]

By default the CVT belt is always slipping a bit at least but HOW much is the question... if the primary (driver) clutch and belt get pretty hot pretty quick that's a good indication theres too much slippage. I would personally start with the 75 tooth rear sprocket you spoke of earlier as that would get you to 5.77:1, from there you could go down by changing the front sprocket. 6.25 with a 12t, 6.82 with a 11t and 7.5 with a 10t. There may be alot of experimenting until you find what works best, you could even start with rear sprocket smaller than 75 if you first make sure that with a 10t you'll still end up with something over 6:1. Pain in the arse I know but if it was easy everyone could (and would) do it! Lol

 

[Munk]

That is a pain in the arse, and not just for the reasons you mentioned. It looks like I only have about a half inch of clearance between the bottom of the current 46T sprocket and a scrape/chain guard that's welded to the frame. I also don't think I have more than 1 - 1.5" from the 46T and the 13T sprocket. Adding anything much bigger seems like it'd require extensive secondary modifications.

Would a 10T sprocket make much of a difference over the 13T? An 8T??

https://photos.app.goo.gl/f8xf9cuLQxvgRJgx6
https://photos.app.goo.gl/CQEUWQmaF4VEAV7A7

 

[Brianator]

Oh boy that is a pain alright! Lol. Is the motor movable? A 10t would get you to 4.6:1 which is indeed better and worth a shot! I'd avoid anything under a 10t though, for one they wear really quickly and secondly they're REALLY hard on the chain itself. Time to find a friend with the tools and know how to fabricate.... drop that chain guard a bit (I wouldn't delete it) to fit a larger sprocket IF you can gain some clearance with the motor, might be possible to lift it on 1x1" square tube steel as long as you have the room. It's tough trying to help without being there to see with my own eyes.

 

[landuse]

Like brianator mentioned, raise the engine on square tubing. This will give you more sprocket room

 

[ol'joe]

Just a few tips to consider along the way;

When checking out carbs, the ideal tool for cleaning out orifices is a bit of "tag wire", and you can buy a small roll of it or it is what is used to hold tags on a variety of small parts. It is flexible enough that you can trace the route of almost any hole in a carb, plus clean out the smaller ones of obstructions.

Also, the very simplest thing that sometimes limits top speed is a plugged air induction system. Folks who put oil on the foam in the air filter often find it will plug with fine dust that sticks in the oil. When the plug looks like it has been running rich, always good to check the air filter for plugging.

Lastly, v-belts get slick when overheated and overheated when overloaded. Sometimes "belt dressing" from a spray can, applied after the belt cools off, does wonderful things and stops slippage.

When I was young, people were suspicious of belt drives and belt idler clutches because there was a LOT of poor performance from them, but that was because the belts sold by hardware stores were "fractional horsepower" belts, made for loads of less than one horsepower.

Some ignorant souls still ship those light duty belts for more heavy applications, so it often pays to buy v-belts from industrial sources. My trail scooter will climb VERY steep grades with a total load of nearly 600 pounds, powered by an idler clutch running on a single 1/2" v-belt.

Does wheelies, too! For me, belt drive clutches rule.....Joe