81 SRX motor questions

N

nysrx

Member
Joined
Nov 25, 2003
Messages
475
Age
66
Location
New York
These 81 motors had a crank issue. When they were recalled by Yamaha they replaced the bottom end and installed Comet clutches. Now did this resolve the issue with these motors or did they still have problems with them afterwards. If so what can be done to make these motors reliable. I have 5 81 motors that have no vin# stamped on them and a Comet clutch installed. So I would imagine they have been redone under warranty at one time or other. I have 1 original motor like new with the vin # still on the lower case. What caused all the problems with these motors. I would like to build one strong reliable motor. Thanks
 

the problem with those motors was more from the exhaust port location and size. I had one that ran over 5,500 miles without a problem after we had Forest Lake Motorsports weld some bridges into the exhaust ports and do some custom porting.
I do remember something about the center crank bearing and oil.
Mills
 
I have heard a lot of theorys on these motors. One was carbs, another was oiling, cranks that crystalized etc. One of the problems my friend that races one saw was length of the crank on the PTO side. His theory is that at high RPM the crank is flexing slightly causing a harmonic to resonate back though the crank. He cut his off, turned it down and reinstalled the clutch.

Brian
 
THIS WAS A VERY HIGH PERFORMANCE MOTOR. THERE WERE 21 RECALLS FOR THIS SNOWMOBILE FROM THE COMET UPDATE TO INGINTION TIMING. THESE SLEDS WERE MADE TO DRAG RACE ON THE LAKES. THIS ENGINE MADE NO HP UNTIL 6000 RPM. THAN IT CAME ON LIKE HITTING A BRICK WALL. HP WAS 88 AT 9250. YAMAHA FINALLY RECALLED THEM AND DESTROYED THE MOTORS. ALOT WERE NOT TURNED IN. MANY SURVIED. DEALER ONCE TOLD ME THE BAD ONES BLEW UP RIGHT AWAY, GOODS ONES RAN GREAT. I HAD A GOOD ONE, STILL LIVES TODAY. 94 MAX HAS ONE WITH ORIGINAL # MATCH TO CHASSIS. ANYBODY THEORY ON THIS ENGINE IS GOOD. ONE THING FOR SHURE, YAMAHA WAS KILLED WITH WARRANTIE ISSUES ON THEM AND THATS BASISLY WHY 1982 SRX WAS NEVER RELEASED. CHUCK, I DON,T KNOW IF A 81 SRX WOULD WORK IN A VMAX CASE. DON,T REALLY KNOW WHY YOU WANT TO DO THAT. MY GUESS THE VMAX CRANK HAD SOME PRETTY GOOD CHANGES FOR REALIBILY. 3:16 (yammiie tony)
 
8m6-8r9 and 8u9 base are the same.
you can put 8m6 cyls. on 8u9 base and everything fits and work with proper carburation
 
I asked my friend about putting a 540 crank in this motor. He claims that you don't have to do that. I've heard of guys who did, but they didn't give details about if anything had to be turned down, clearanced or what not. My buddy did say that you can run Vmax rods in this motor. They are a little beefier than the stock SRX rod. He runs a 81 SRX full mod sled. Approximately 120 HP, machine work by Delaughter Racing Products. He also pilots Dave's 660cc Unlimited Mod. Dave built that motor originally back in the early 80's. It's a custom built 3 cylinder made out of two 81 SRX motors.

Brian
 
The 81 SRX & the Vmax crankcases are different, the bearing diameter on the Vmax cranks are larger in size so the cranks will not interchange , but you can put the SRX cylinders and pistons on the Vmax cases and vise versa, the vmax also has better rods that have the side beams highly polished to resist cracking and I think they may be magnafluxed from the factory the SRX only has 1 nipple to run the fuel pump and the Vmax has 2 .There are a few other outside differences also but very little . I have both sleds and have compared them to each other . The thing to remember about the 81's is if they haven't blown up by now then there is a good chance they won't.
 
It's my understanding that the 83 VMAX crank will fit this motor, and is stronger and shorter than the original SRX crank.

Brian
 
Just to set the record straight on these two engines cranks & crank cases . The 81 SRX crankshaft has a outside bearing diameters as follows , the PTO bearing has a outside diameter of 78 mm and the center and mag bearing diameter of 68mm,and inside diameters of 32mm, where as the 83 up Vmax has a outside bearing diameter of , PTO has 80 mm and the center and mag diameter of 72 mm with an inside diameter of 35 mm, so if the vmax 's pto's diameter is 2 mm larger and centers and mag's is 4 mm larger then how can the vmax crank fit in the SRX case ? You can't change bearings because the inside diameters of the Vmax is 3 mm larger ,then the only way to do this is to use the whole Vmax crankshaft and crank case . I hope this will clear this matter up.
:2strokes:
 
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