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Racing an 150sx in MX2 and wanting to be competitive!

3K views 7 replies 2 participants last post by  Axzon 
#1 · (Edited)
We are racing a 2013 150sx in MX2. Top speed has always been good, but the bottom end has been missing. That is not a surprise as it is where a 250f performs better than a 150 2str. We have also had an issue with passing the 4 strokers once we are sitting behind them and an opportunity opens. My boy has learnt to read the tracks and figure out where he can use the screaming 2 stroke motor (and a lighter bike) to his advantage. He is doing well. But even then...

So we did bite the bullet and fitted a VHM head. Today we had a chance to do some testing. We have not changed any jetting or other carbi settings, but the bike was a different animal! Our Boysen reeds still needs replacing and we have new ones coming, but the virus takes its toll on how long before they are here. Once they come we will be close to where we want to be!

The most enjoyable part for me today was to listen to the exhaust sound. We have a HGS pipe setup and the sound has always been good, but today the difference was that at no stage did it sound like the motor was under any strain. Even a small turn of the throttle was straight away rewarded by a surge of power and a change of the exhaust sound!

We are soo looking forward to next w/e's racing!!

Anyone else have/had a VHM head on their 2 stroker? If so any comments??



Not sure why the pic wants to lie down? But nothing I do seems to be able to fix it. So I will leave it as it is. Bike is still to have a wash and a polish of pipe after the practice/testing.
 
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#3 ·
Our first race meeting with this head is this Sunday. As it is at a local club day we expect to be up mainly against the same group of MX2 riders we have been racing at these events monthly. Last meeting there was only one rider (on a Yamaha 250f) that beat us in the final race (Race 3 is the only points race and the one where the classes always race separated/split starts). So we will be able to gauge the improvement!
 
#4 ·
So how did our race day with the new VHM head go? Well, there was two parts:

Part 1. No changes made to bike. Was still set up as at practice at home with the VHM head including gearing, carbi etc.
Vinny went out in practice and struggled. But as the track was natural terrain and his class (MX2) was the first ones out and the grass was still damp from the night, we did not want to make any changes based on that.
So in Race 1 he went out with bike as it was. But the struggles continued. Bike did not want to do what he was expecting. It would push straight in corners and he dropped the bike 4 times and finished last. This was not what we had expected.

Part 2.1. Back in the pits we decided to "dumb down" the power the head was providing. But I had no intentions of reducing the compression or opening up the motor. So I changed the gearing. We had a 13 front and a 51 rear fitted when we came. I changed that to 14/48. My logic was that this would tame the snappy power a bit. We also fitted a wheel with a 100 wide tyre (original one was 110 wide) and dropped the pressure to 8 psi. Vinny had a short practice on a paddock and came back telling that he liked it!
Race 2 went much better. After some initial figuring out how he could now ride the bike, he started to catch up with the rest of the riders and passed over half of them to finish in 5'th.

Part 2.2. Before Race 3 I added one more tooth at the back, so we now had a 49 sprocket at the rear as I felt that he had lost uphill power. I also took the tyre up to 10psi as I was worried the tube would rip. Start went well and after some minor tussles Vinny was in 3'rd. He was hassling the rider in 2'nd and showing his front wheel on occasions. In the last lap Vinny passed to finish in 2'nd. We were very happy about that as the winner of the race is a fast 4str rider with a few years more experience than Vinny. And we still have much to learn about the setup with this head!

But this proves what a difference the VHM head makes! We still have heaps of power on tap by just changing the gearing back to what it was initially. I recon that gearing would be close to perfect in a Sand MX race as in that state the bike gave no consideration to what surface the rear wheel was on, it just provided the same explosive power each time the throttle was turned! And with the New Zealand TT coming up in 2 weeks we will have a bike that can reach some incredible top speeds with more gearing changes!

https://www.youtube.com/watch?v=vjEUHQ9NfG0
 
#5 ·
In just over a week we will ride in the 2021 NZ TT. In 2020 Vinny did OK and got a 6'th overall in the MX3 class.(Racing to a 7, 7, 7, 7, and 6'th). At that stage our 2013 150sx was standard with no "go fast" bits. We will do MX3 this year too, but aim a little higher! In the attached video you see Vinny having to come from behind. Big reason for that is that MX3 caters for all cc's of bikes, so we were up against 450cc bikes who left Vinny behind at the start. Report of this years race to follow!

Here the 2020 NZTT racing. (Vinny is #333):
 
#7 ·
Things did not go as planned sadly. Day 1 (two races, 2'nd of them added here below) was good and it looked like we would reach our goal to be 5'th (or higher) as Vinny's racing was fast. Sadly there was also a crash where he was taken out from behind in a corner. The crash damaged the r/h side radiator and it started leaking. But it was not a fast leak, so for Race 2 we just filled up the radiator and he was back out.

3 classes were racing together in his races: MX3, Classic and Enduro (those 4 Enduro boys were fast!!).

Race 2 looked promising! Vinny was sitting in 5'th for quite a few laps until a crash in a corner. He got up fast, but dropped down to 9'th and finished in 8'th. But when his class was separated from the others we found that he was in 5'th after Day 1!

Day 2 started with a look lap before the first race. At slow speed Vinny's front brake came loose (a bolt was suddenly missing... and that after doing the "full bolt check" the day before). This resulted in a warped brake disk and bent bracket. Race 3 was a DNS. For Race 4 and 5 we dismantled the front brake and Vinny went out to have fun (do wheelies, rip the burms and stay out of the way for the rest of the riders). He placed 13'th overall in his class.

For next race meeting his bike will be a fraction heavier as result of Locktight being fitted to most bolts!

Race 2:
 
#8 ·
On Sunday just been we raced natural terrain MX. Some minor issues with carb setup that we almost sorted (but riding also had to be adjusted to achieve this). Will look in to that later this week. But the proudest moment was when Vinny beat the land owners son (who was riding a late model Yamaha 250 2stroke) with help of the 150sx and his riding skills!
 
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