xa or xb cylinder

GYT340

GYT340

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Mar 25, 2006
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15
what is the difference between gpx and sst xa and xb
cylinders

thx
 

The 75 sst snojet 440 had a yamaha motor
I didnt know if the prefix was for the snojet vs gpx
Was there any difference in the motors?
 
GOT YA. YOU MEANT A SST SNO-JET THAT USED YAMAHA MOTOR,S. ACTUALLY DUDE, YAMAHA MOTOR,S WERE USED IN SNO-JETS WERE THE SAME AS WHAT WAS USED RIGHT IN YAMAHA SNOWMOBILES. 1975 SST SNOJET AND 1975 YAMAHA USED SAME MOTOR. 3:16 (tony)
 
Also Sno-Jets used tilitson carbs and Yamaha used Keihin carbs and some Mikuni carbs. Brad
 
I have a 1975 SST F/A pretty much scrappled, and a 1975 GPX with 2 Sno-jet engines. One motor is from an SST, the other from a Thunderjet and are the same. Along with the no oil injection (which I don't miss on a performance sled anyway), Sno-jet was MUCH wiser in choosing clutches and carbs too. They use Mikuni 36mm VM's and Arctic Hex clutches on their version of the 433 F/A. Yamaha clutches are reliable...for the clutch itself....but dangerous for over-revving and lack of finesse. Over rev on such an engine = BOOM!! for pistons and sometimes bearings too. I don't have much to say about Yamaha's choice in carbs either. I had one of those darn Kehins on a 75' GP 433 and flung it into the junk pile in favour of a simpler and bigger Tillotson HD. The thing was just made up of at least 3 times the parts that any Tilly or Mik would have and syncing 2 of them is a quite chore.
The hex clutch works EXTREMELY well compared to the simple Yamaha kidney weight (paper weight) clutches. Keep good long bushings and parts in them and they are STILL one of the hardest hitting units for vintage drag racing.
If you plan on running a different clutch on a pure GPX motor then the shaft has to be machined. However... it's probably worth the effort.

The difference in the CYL's is mostly fin sizes in the heads I believe. I'm 99.9% sure the porting is the same but I have never seen XA jugs inside.
 
1975 was the last year for the F/A engines but some sleds of that year were sold as 76' carry-over models.

1976 was the last year for Yamaha engines in Sno-jets but the best one offered was a 50hp, fan cooled (SD) similar to the Yamaha Exciter engines but again... No oil injection, Mikuni carb and Arctic Hex clutch on them instead of the Yamaha/Keihin units.

For 1977, Kawasaki continued to use a couple Yamaha engines in the lineup for the SST models similar to what the 340 and 440 Exciters used.
 
I MIGHT BE WRONG BUT I THINK I REMEBER THE 75 433 YAMAHA DID HAVE ALITTLE BETTER PORTING THAN THE 74. PROBLEY REALLY NEVER MATTERED. MOST OPT FOR GYT KITS!! 3:16 (tony)
 
My 74 SST is a reed engine. The 74 GP 433 I had was also a reed engine. I believe that 74 was the first year for the reed engines. I don't know if they were used in any racing applications. Also the Early reed engines had the splined crankshafts. Those reed 433s in 74 felt to powerful for those sleds. The engine always felt as if it was going to rip itself out of the chassis. Brad
 
Yeah, the ole GP was extremely unnerving to ride compared to an Exciter. I can't imagine what the previous year models were like with the bogey suspension!
 
I used to have a 75 sst motor, and the cylinders we marked xb. this was form the 75 sst, and the only difference were carbs, oil injection, and heads. sst had the 8 fin heads.
 
There are differences between a XA and a XB. Porting is basicly identical but flip over the cylinders and check out the direction of the transfer ports from the bottom...big difference not only in size of transfer ports on the bottom but direction in which they enter. Hard to explain but if you have a XA and a XB flipped over side by side one will see exactly what I'm talking about!

Biggest difference between a XA engine and XB engine is the heads...smaller combustion chamber in the XB's over the XA's and more compression= more power and the 75 had more power due to the increased compression.

As far as the Yamaha clutch goes that was used on the gpx, they can work just fine. The biggest thing is they get dirty real easy and then dont work well. I fiddled with my gpx last winter with the stock clutch and got it to work well and was easily tuneable.

Also, the GPX carbs were unique and not like a gp carb totally and not to be confused with a gp carb. The GPX carbs had accelerator pumps on them and if you can get them dialed in right they work better than a mikuni in a 500-660 foot race as they have no bog with the accelerator pump shot of gas. I know of 2 brothers from the Traverse City area who raced gpx's for years and they only used the stock 74 carbs and they were the guys to beat at the races. So they can work awesome, its just impossible anymore to find the correct carb fuel pump diaphrams for them to make them work properly. I have an idea on how to make them work and will take some machining possibly to do it and I will post this summer if I can get them working with my idea.
 
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