JS Motorsport Pistons/Rods First Impressions

maaseyracer said:
On Jim's site he says, "Installation of the JS Motorsport lightweight pistons and rods will automatically change a 750/850 stock Commando wet balance factor from 52% to around 65% with no modifications to the crank."

Did you keep the 65% balance?

The second engine I built is lower compression and had the crank rebalanced when the journals were ground. I am curious to see how it compares.
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The stock crank was rebalanced to 62% late 80's or early 90's (shite memory on when), so ended up closer to 75% maybe. Don't know the actual numbers when it was all said and done with the JS parts. I read another of your posts here and note that you left the crank as is. Great result for you.

My engine with the JS parts in it ended up smooth running a low RPM, a little worse at HWY cruise, and about the same at high traffic ticket RPM. The JS2 cam is nice though. I'm surprised more people don't use it in a street bike if they are going to use a JS cam.

The motor has a 3 lbs lighter Molnar crank balanced to 75% in it now, MAP mid-compression pistons and long rods, and still using the JS2. Also geared way up compared to stock gearing I was using. I'm happy with the performance. I would think your other lower compression motor will work fine. I don't own a Commando. Solid mount P11, hence the higher balance factor. And I can feel if the motor is well balanced everywhere.

The set screws for the lifter blocks won't loosen up if the Blue loctite was fresh and well shaken. I'd still check them once in a while though. If you are ever out riding and hear a never heard before ticking sound, pull over and check those set screws. I did not, and it was a dumb ass mistake running up the freeway another 8 miles at 80mph. Good thing money grows on trees. lol
 
maaseyracer said:
On Jim's site he says, "Installation of the JS Motorsport lightweight pistons and rods will automatically change a 750/850 stock Commando wet balance factor from 52% to around 65% with no modifications to the crank."

Did you keep the 65% balance?

The second engine I built is lower compression and had the crank rebalanced when the journals were ground. I am curious to see how it compares.
Click to expand...
Nice review.
Did you finish the second engine?
If so, what's it like?
 
Thanks for sharing all this!
What sealant do you use when you put the crankcase halves toegther?
Dennis
 
gortnipper said:
Personally, I would not set it that low. Oil pressure to the head at idle is VERY low anyway, and doesnt really start to move a lot until >2k rpm, so I try and keep my idle in the 1250 range at least, which sees my needle move off the stop.
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My concern is more that there is no splash lube on the cam at idle, or indeed as you say until above 2K, maybe 2,5K. Which to me means that though with good intent, the extra 250 rpm isn't achieving much.

So I prefer to blip the throttle and avoid letting it tickover much at all. Yes, mine is a race bike with a vertical engine, but I would do the came with a road bike, habit I suppose, But a sound one I think.
 
I built my race bike engine in 2014 with JSM rods and pistons from the start. It’s a 77mm bore short stroke with a Maney crank rigidly mounted in a Rickman frame. Steve balanced the crank for me to JSM supplied figures, with a slightly higher factor than Jim uses. But I love the thing. Definitely smoother than I anticipated.

I asked Jim before I bought stuff from him if he thought he would still be in business in a few years, well, here we are 10 years on.
 
Smooth running usually has to do with balance factor - are the JS rods and pistons a bit lighter ? Also longer rods also give longer rock-over time at TDC. Some time ago I considered Jim's 12 to 1 pistons, however I did not want to ask questions while not being ready to buy. I believe his 12 to 1 pistons have slight domes. That would be much better than the Triumph type 12 to 1 pistons, which cause poor combustion and coking on the crown of the piston on the side which is away from the plug.
I did not even attempt to race with my 850 engine, until I had raised the balance factor. I achieved 72% without drilling the cast iron flywheel. But the threaded steel plug in the flywheel counter-weight is not a good answer.
 
In Victoria, Bob Rosenthal was the only person I can remember who ever road-raced an Atlas. He used a balance factor of 79% which is what my mate used in his Triumph 750. That high balance factor was common in British race bikes of the 50s and 60s. It makes a difference to performance.
Light cranks in a British twin are stupidity - the performance is never as good. Have a ride on a Triumph Saint - it was designed for traffic officers to ride. Japanese four-stroke twins work on an entirely different principle, it is a common mistake to believe they are better. When I was racing my Triton 500, other guys were trying to race Honda 450s - they were never anywhere near me.
 
MichaelB said:
Nice review.
Did you finish the second engine?
If so, what's it like?
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Engine has been long done and was completed when mine was. My friend is not as quick as I am to assemble the rest of his bike. Hopefully I will know by this summer how it goes. Mine is still running flawless, I have a few revisions I will post when I get the time to install them. The motorcycle is daily ridden in fair weather which is days like today here. Perfect commute machine. Did great on the All Brits ride last year as well.

DennisMo said:
Thanks for sharing all this!
What sealant do you use when you put the crankcase halves toegther?
Dennis
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I used Hondabond, but Yama bond and permatex Motoseal are probably just as good.
 
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maaseyracer said:
Engine has been long done and was completed when mine was. My friend is not as quick as I am to assemble the rest of his bike. Hopefully I will know by this summer how it goes. Mine is still running flawless, I have a few revisions I will post when I get the time to install them. The motorcycle is daily ridden in fair weather which is days like today here. Perfect commute machine. Did great on the All Brits ride last year as well.



I used Hondabond, but Yama bond and permatex Motoseal are probably just as good.
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Well yes, because Hondabond and Yamabond are Threebond, in Honda and Yamaha packaging!
 
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