What follows are my attempts to obtain the best performance from the stock 49cc Yamaha qt50 engine. Anything goes – only rule is keep the 49cc engine.
Day 1 – 49cc engine is on my blue qt50. Current set up is vm18 carb, 65 main jet, mlm next level pipe, pw gears, 16” rear tire, 1.0mm spacer. I ordered a puch 40mm head. So I can compare temps between stock head and puch head.
Took a ride with current setup as listed above. Stock cylinder head is horrible on cooling. Idling at stop signs doesn’t really bring the temperature down much. Had a high temp of 387F and I wasn’t even pushing it hard. Could’ve easily seized it with an extended full throttle run. Makes me wonder how anyone could really go fast with a stock head. You’d seize before you got to 40. I’ll up my main jet. Waiting on puch head to arrive. Top speed without an extended full throttle run of 34-35. Mlm next level pipe is not too impressive.
Day 2 – bumped up the main jet slightly to a 67.5 and it took a bit longer for the engine to get heat soaked. I could do longer full throttle runs. Didn’t have gps on today. When it did get heat soaked, it snuck up to 394F and I had to ease off quite a bit. I’ve had the stock cylinder slightly over 400 before. Don’t try that with those niche kits. I can bump the main jet again and see if that does the trick. Also went down to 14” tire. Still waiting on a real cylinder head to arrive.
Because I opened up the reed block slightly, a 2.5 increase in jet size is significant. So it has a 67.5 main jet in it now but a 70 jet is way too rich. I can bump up the pilot jet instead though.
Day 3 – I put a bigger pilot jet in the blue Qt. Ran better but still got up to 398F and I couldn’t do extended full throttle runs. Puch head arrives Monday. I’ll be interested to see the difference in head temps as compared to stock head.
New cylinder head. And cylinder got sanded down after 1.5mm of spacers added to open up all ports fully except intake at BDC.
Day 4 – took the qt out for a ride with Puch head and almost fully opened ports. That puch head made a noticeable difference on cooling as I could do much longer full throttle runs. Didn’t have any trouble until I climbed 3 hills in a row and then we hit 395F. I can probably jet higher after getting the piston to clear the ports. 35-36 is about limit with next level pipe and 49cc. I’ll put the circuit pipe on next and we should jump to over 40. There’s a lot more flow now. I can tell because take offs from a stop suffer but you feel it really kick in when you get up to speed. I cleaned up the transfers on a Honda cylinder like the pic above. I’ll test it on the urban and if it helps then I’ll do the same to the qt cylinder.
Day 5 – So I went from 34-35 with next level pipe to 39.7 with circuit pipe. Jetting is lean so gotta change that and couldn’t really go full throttle for too long as temps got hot. Will change jet and try again.
Day 6 – I let her eat today. 41.6mph. Maybe I’ll shoot for Pete Rose’s number. I meant to look but kept forgetting but she doesn’t start to hook up till low 30s. The puch head keeps her cool. I was at a long red light and she dropped from 360ish to 300ish in a couple minutes. Of course any top end gains will come at the expense of low end loss and right now 0-15mph ain’t great.
Day 7 – I opened up the side transfers on the cylinder skirt. This was helpful on the urban express project.
