QT50 Performance Tuning

video posted 1/20/16

Here’s a video showing where I am with tuning my QT50. This one has an MLM exhaust, MLM intake, Mikuni VM18 carburetor, K&N air filter, Boyesen reeds and niche 60cc top end. It’s running a bit rich here. I need to check the needle jet and lower the needle. I’m hoping I have some notches to work with and that it isn’t already at the lowest position. The lower the needle the less rich it runs. You lower the needle by placing the clip more towards the top of the needle so that the needle will sit lower in the carburetor.

I currently have a 20 idle jet and a 95 main jet (btw, stock main jet size is 70 for inquiring minds who want to know). I have a 90 main jet that I can drop down to. I need to review the information that came with the Boyesen reeds. I believe they make the bike run richer. I may have to go back to stock reeds because I’ve read that Boyesen reeds don’t work well with modified engines. Anyhow, I’ll get a chance to work on this soon and update this post on QT50 performance mods.

The good news is that I got it to idle. With a 15 idle jet, it wouldn’t idle without the choke/enricher on. On the Mikuni VM18, you pull the lever up to use the choke/enricher. With a 20 idle jet, it seems to be idling much better.


I haven’t updated this page in a while so here goes.

Despite it dying out on me, this is one of my favorite videos on the qt50. That MLM pipe winds up and then boots you in the ass at about 25 mph and then hold the blank on! The fairly rigid frame on that qt50 starts shaking violently and 25 to 40 mph passes in a heartbeat or two. At the time it died out, it was still accelerating! Damn that was sweet! The Mikuni VM18 carb is almost as lethal as nitrous. Best jetting for VM18 on this setup seems to be 20 idle jet and 80 main jet.

Ok, so I have a bunch of filling in to do on this page. I stripped it down and took out the Boyesen reeds which were designed for the qt50 stock configuration. Turns out Boyesen has another set of reeds designed for modified configurations. I re-installed the stock reed system. This change may have helped a little bit. At this point, I let this project sit on the shelf for months as I had reached my patience limit with it. I took it out again and started messing with the float height and the jetting again. This video immediately above reflects my progress at that point. I got it to run for short bursts and then it would stall out on me at full throttle. I still thought it was a float height issue or jetting issue. After more frustration, I decided I’d rip the frame off again and replace the MLM intake with the stock intake and go back to the stock carb.

Which leads me to . . .

After putting on the stock carb, I still had the same problem. By the way, I have a 102.5 main jet in the stock carb with this set up (60cc top end, mlm pipe, mesh air filter, stock reeds). This information may help others. So I thought to myself, if I have the same problem with two different carbs then the issue may not be carb-related. On a slightly educated guess, I decided to switch out the stock ignition coil with a new one. I thought I had it licked as my qt50 was running fantastically with this change (see video above). Later I developed the same issue again but after longer running periods. I still think the issue is spark related. I think I’m losing spark at high rpms – full throttle (two stroke losing spark at high rpms – full throttle – added again for google searchers).

My thoughts now are to try one or more of the following:

hotter spark plug;
new or different cdi unit;
colder spark plug;
new coil on magneto;
replace crank seals.

I should mention that I took advantage of a treatland sale and purchased the Malossi kit. Not sure if I’ll put it on this ride yet but won’t mess with it until I get all issues ironed out on current set up. I’ll update soon.

3/10/17 Update. Look how wrong you can be. I was thinking that the issue was not the carb and was spark related (see above). I took this qt up to Motor City Riot’s Get Your Shit Running for Spring IV. They told me that my main jet was way too big. I had another main jet on me (about an 87 or so) and put that in. It ran much better for quite some time and then I experienced the same symptoms again. It would quit as if it were running out of gas. Someone suggested that perhaps the float needed adjusted. We had worked for quite some time on another qt that I had brought up so I said that I’d work it out at home later. Davidde told me that I was only 45 minutes from perfection on this qt.

After I got home, I got a lead on a YT60 cylinder for less than $20 shipped. One of the cooling fins was broke but big deal. It was a bargain. So I snapped that up. When it arrived, I took off the crappy Niche cylinder head and discovered that I had the wrong head gasket on the qt. I was using a stock head gasket instead of one made for a 44mm (60cc) piston. Of course, I wondered whether the tiny bits of aluminum that the piston was knocking off the gasket were causing my stall outs. I ordered the right size head gasket and waited for the ebay seller to ship it out.

In the meantime, I decided that I was finally going to replace the right side crank seal. A friend of mine and former Yamaha mechanic had been telling me to do that for quite some time. A new seal helps with starting and can prevent air leaks around the crank. I also replaced the orange ignition coil wire (I had previously used a quick connect on it). Quick connects were also my downfall on another hopper that would stumble on acceleration or after hitting a bump in the road. Turned out that the wiring near the coil was exposed and was shorting out/grounding out and causing the engine to temporarily stall.

I finally received the new head gasket so I slapped that on. Time to ride and see if anything was better. Unfortunately, I still experienced the same stalling out after a run at full throttle. In near desperation, I called my buddy, Clint. I was back to my loss of spark theory. He was unconvinced and maintained that it was still a carb problem. After bouncing thoughts back and forth for a bit, I decided that I’d switch the carb from my Towny and put it on the hopper. If I still experienced the same issue then it couldn’t be the carb. We also speculated that perhaps my main jet was still too rich and maybe it was getting too much gas at full throttle and that it was just drowning out in fuel. It definitely felt rich as it accelerated from a stop and wouldn’t seem to lean out until the rpms caught up.

When I got home later in the day, I decided to go from an 87-88ish jet to an 85. While I was at it, I thought it would be a good idea to also go with a hotter plug. I tried an NGK BP5HS but it wouldn’t start on that. So I pulled another plug out of the mix, an NGK BP4HS. I got her started after a bit of effort. It was just below 30 degrees F outside and I don’t have my choke cable installed. She was still a little rich from a standstill. I took her around the neighborhood and she seemed to be running great. I did a few full throttle runs and even better. I stayed out as long as my hands could take the cold (only one pair of gloves) and she ran great the entire time. I’ll give her a longer test here soon. But things are looking up.

All this time, I was way too high on my jetting. These dudes on mopedarmy.com suggest these ridiculously high main jets and I don’t get it. For example, for a 60cc kit with no other mods, the advice is to use start with a 95 main jet. With my experience, I’m thinking an 80 main jet is too much for just a 60cc kit and nothing else. Following this advice, I put on a 102.5 main jet for my 60cc kit, MLM pipe, mesh air filter setup. Now I’m down to an 85 jet and will probably try an 82.5 jet shortly.

Maybe they are thinking that you have to use a big main jet to keep temperatures down. Gas not only fuels the engine but helps keep it cool. The YT60 cylinder head seems to be helping quite a bit in that regard. I’ll know more with more riding and warmer weather.

The other thing that moped army dudes recommended was a much colder plug due to higher compression of the new 60cc kit. Problem is – if you are recommending these really huge main jets that are dumping fuel into the cylinder chamber, a colder plug is probably going to get overwhelmed and be unable to ignite that type of fuel/air mixture. Maybe everyone’s situation is a little different as no two Yamahoppers are exactly alike and maybe I’ll discover more later but for now I’m going to call a big fat bullshit on some of this advice.

I haven’t even tested compression from 50cc kit to a 60cc kit. I’m betting that a bigger kit may have lower compression. I’ll test and see.

Here’s a cylinder head comparison. Notice how the YT60 head dwarfs the 60cc head and that the 60cc head is the same size as the 50cc stock cylinder head. That Niche kit is a waste of money. “Let’s just rework the stock head so it will handle the bigger piston. We don’t need to give it bigger cooling fins or anything” What a bunch of dorks!

If I need more cooling power later, I’ll have to come up with something. That VM18 carb really kept it cool. I can try that again now that I’m not overestimating the jetting. Stay tuned!

9/30/17 – Ok, it’s been awhile. Turns out, I hadn’t solved my cutting out at full throttle problem like I thought. I finally just took the cylinder off to look at the piston/rings/cylinder. I had a deep scratch going along the cylinder wall on one side. I’m wondering if that small head gasket had something to do with it. I’ve had 60cc kits ship with head gaskets made for 50cc kits. I’m wondering if Niche sent me a correct sized head gasket. The 44mm piston took some material off the smaller head gasket. I bet some of the aluminum got in the cylinder and scratched it.

Since that time, I went ahead and changed the left crank seal as well. I put on a Malossi kit and am still breaking it in. The lesson I learned – CHANGE YOUR CRANK SHAFT SEALS BEFORE YOU PUT ON A BIGGER KIT. It’s a night and day difference. When I was heat cycling the kit after install, it took me forever to get to 275F degrees. And if I didn’t blip the throttle, my bike would actually shed heat at idle. I would watch the temperature gauge drop a few degrees while it idled. I had never experienced that before. In the past, my temperatures would just keep climbing at idle after installing a new kit. I think I recall 350F degrees at idle before I shut if off in the past.

On my break-in rides, I’ve had it shoot up to 358F or so but it would always come back into the 330s later. I did have an SHA carb on but I’ve recently gone back to the VM18 in hopes of seeing cooler temperatures (currently running a 15 idle jet and 85 main jet – will report back soon).

I put a Niche kit on my buddy’s blue hopper, stock carb, stock exhaust and 82 main jet. The highest temperature I saw after replacing both crank seals during break-in was 358F as well. An 80 jet is probably closer to perfect on that bike.

I’ve also learned the importance of changing the charge coil. Hard starting is a thing of the past now.