75 338 gpx and 78 440 exciter setups
- Thread starter 338gpx
- Start date
steelydan
New member
I ran a red comet spring on the primary and stock secondary spring on mine and dropped 2 teeth on the top gear. Mine ran 62mph with that setup. I weigh around 250lbs.
RJH
New member
Just to start…a GPX tuned to near optimum performance will not beat a T Jet tuned to optimum performance…in a drag race. However – on a 5 lap ice oval – a GPX can gain ground back and win.
Naturally – not all T-jets are tuned to optimum performance.
It’s just a weight thing.
My GPX had stock clutches so I can’t help you there. The carbs -need to be perfect – being rich 1/8 of a turn can take you from first to last….serious. For that sled….I just put in on the stand and pinned it - if it ran clean to the factory recommended rpm – it won.
You need springs in the back from a GP – wound up tight – even add a hockey stick piece to make it higher. On the front skid arm – drill that shock. Weld on an extended shock bracket mount on the front skid arm. The shock is attached too close to the arm – resulting in very little weight transfer. I’d say a good 3” from the bar.
The stock engine is just fine – engage about 5000. It can win against any motor out there…..only less weight will beat you.
If you are allowed rag tracks – get one – biggest hold-up is the track on that sled. Keep the skis off the ground – you need good clutching to do this. They should lift about 2” then just feather the track on the run.
Naturally – not all T-jets are tuned to optimum performance.
It’s just a weight thing.
My GPX had stock clutches so I can’t help you there. The carbs -need to be perfect – being rich 1/8 of a turn can take you from first to last….serious. For that sled….I just put in on the stand and pinned it - if it ran clean to the factory recommended rpm – it won.
You need springs in the back from a GP – wound up tight – even add a hockey stick piece to make it higher. On the front skid arm – drill that shock. Weld on an extended shock bracket mount on the front skid arm. The shock is attached too close to the arm – resulting in very little weight transfer. I’d say a good 3” from the bar.
The stock engine is just fine – engage about 5000. It can win against any motor out there…..only less weight will beat you.
If you are allowed rag tracks – get one – biggest hold-up is the track on that sled. Keep the skis off the ground – you need good clutching to do this. They should lift about 2” then just feather the track on the run.
RJH
New member
You have to mark the spines on the primary – and always put it back on the same spines. Yours are probably broken in – but new – I spent ½ a day with rubbing compound running it back and forth – it must slide easy.
I then used the GYT kit clutch – I’m sure one of the vintage guys can tell you what color primary and secondary springs they were…I don’t remember.
You need to grind the primary cam to a smooth radius – no humps. I used the 440 secondary helix on the 340.
This is the wild part – (probably why my clutching today is so light) you wind the secondary as almost as tight as you can get it. At a point…you will think – what the hell…...then you do one more twist and the thing goes …………10 mph faster off the start.
You can relate that theory to new clutching. The engine would always be 2 rpm's around the factory recommended rpm without the tight clutching – but just would not go fast as it was shifting off the power band. You need to find the power band of that engine.
My 340 usually always won back then (when competition was very stiff) and we got 2nd or 3rd in 400 and top 3 in 440 ..then they gave me a 440.
I then used the GYT kit clutch – I’m sure one of the vintage guys can tell you what color primary and secondary springs they were…I don’t remember.
You need to grind the primary cam to a smooth radius – no humps. I used the 440 secondary helix on the 340.
This is the wild part – (probably why my clutching today is so light) you wind the secondary as almost as tight as you can get it. At a point…you will think – what the hell…...then you do one more twist and the thing goes …………10 mph faster off the start.
You can relate that theory to new clutching. The engine would always be 2 rpm's around the factory recommended rpm without the tight clutching – but just would not go fast as it was shifting off the power band. You need to find the power band of that engine.
My 340 usually always won back then (when competition was very stiff) and we got 2nd or 3rd in 400 and top 3 in 440 ..then they gave me a 440.
