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.

DennisMo said:

Thanks for sharing all this!
What sealant do you use when you put the crankcase halves toegther?
Dennis

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Threebond 1184

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.

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!

Outside of replacing the failed ignition, replacing my intake boots, and changing the oil. I have not done any maintenance to the Norton since rebuilding it. I checked the valve clearance and nothing had moved, I attempted to retorque the head and nothing has moved. I have put 4000 thousand miles on it in the last year since rebuild. I ride it almost daily when dry, my commute to work is short 1.75 miles each way, plus the spirted detours, errands, and weekend rides.

If I call it reliable I feel I will jinx myself so I won't go there, but it sure has been running well and this is the longest consistent period I have had it on the road. Should have done this years ago.

I’m about to start re-assembly of my Seeley motor.

It’s a Maney 1007. I’ve swapped out the stock Maney pistons and Carrillo rods for JS parts. The weight difference is truly astonishing!

I fitted a lighter flywheel (std Maney, previous builder had fitted a one-off heavier flywheel) and still the balancer had to lighten the bob weights further. So much so, it has me wondering if I got my calculations right…

Hopefully find out soon. Fingers crossed.

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...

My crank is balanced to 72 % - 65% is almost there - of course it will feel smoother at higher revs. 54% is for idling around town.

acotrel said:

54% is for idling around town.

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…. or mounting a 360 twin in isolastic mounts…

Here's the edited instructions for securing the bronze blocks.

"On final assembly be sure to put some red loctite in the screw hole before installing the set screw to lock the bronze block in place (use heat to remove). Add a drop of loctite where the lifter separators fit into the brass slots so the blocks won't slip if you happen to remove the locating set screws. Check the tightness of the set screws after break in."

maaseyracer said:

A few thoughts from my commute to work today. First off I took the long way in as I do not commute much more than a mile and a half or so each way. What is impressive is how well this build combination holds an idle and settles back to an idle. I have the idle set at ~1000 rpm and to be honest I am not sure where a Norton should idle, but this does not seem obnoxious and feels like where I had it before. City traffic speeds are not an issue mostly 1st and 2nd gear.

Still working out the tune, it is close, I moved the timing from 26 degrees when I got home this evening to 28 degrees. I raised the needle a position (Keihin CR), it will take me a day or two to figure out where I am tuning wise running CA 91 Octane. Can't wait to be done tuning so I can really open it up.

Parking lot photo:

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With E10 gas and a Tri-Spark ignition, I target 30 degrees and accept 29-31. This is after making sure that the scale is correct or making new marks.

I like the cold idle to be 700 which will be between 1000 and 1100 once warmed up, but this is with dual Amal Premier carbs. Any lower and it will stall, especially when cold, and any higher is not needed. I do not have chokes installed. I just did a Combat with electric start and a single Mikuni and it worked out the same except enricher on, start, immediately turn enricher off.

marshg246 said:

With E10 gas and a Tri-Spark ignition, I target 30 degrees and accept 29-31. This is after making sure that the scale is correct or making new marks.

I like the cold idle to be 700 which will be between 1000 and 1100 once warmed up, but this is with dual Amal Premier carbs. Any lower and it will stall, especially when cold, and any higher is not needed. I do not have chokes installed. I just did a Combat with electric start and a single Mikuni and it worked out the same except enricher on, start, immediately turn enricher off.

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The Pazon currently installed will not idle below 1000 with the JS2 cam, the Tri Spark that died would do 1000, current idle is somewhere under 1500, lets call it 1400. I am playing with a homemade transistor switching ignition and it will hold 900 to 1000, at the moment it needs a revision I have not had the time to play with. The Norton Auto advance really is the best curve out there which is what I was using with my homemade setup, woke the mid range up.

What I learned making my own digital ignition is that consistent spark coming from a digital ignition like a Tri Spark cleans up a lot of the sins of the stock ignition. Adding the right advance with a good ignition just wakes it up that much further.

E10 burns quicker than gas without ethanol, which is why I shorten the timing. I have run 30, 26, 27, and 28. 28 feels the best, and 26 feels good off the line, but 28 feels better once going. I am probably upping the compression here shortly, so I will probably be revisiting both cam timing and ignition timing at some point here.

I'd love to do this myself on a Commando one day maybe the next project after the move thanks for sharing the info everyone.

marshg246 said:

With E10 gas and a Tri-Spark ignition, I target 30 degrees and accept 29-31. This is after making sure that the scale is correct or making new marks.

I like the cold idle to be 700 which will be between 1000 and 1100 once warmed up, but this is with dual Amal Premier carbs. Any lower and it will stall, especially when cold, and any higher is not needed. I do not have chokes installed. I just did a Combat with electric start and a single Mikuni and it worked out the same except enricher on, start, immediately turn enricher off.

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I guess you idle so low because you don't have chokes.

With chokes my Commando will idle happily at 1800 or so settling to 1100 to 1200 when warm and no choke.

The only reason to idle at 700 is if your aim is to destoy your cam and followers.

A very very bad idea in my opinion.

I know I'm probably going to get flamed with this comment but Amal did include a choke for a reason.

johnm said:

I guess you idle so low because you don't have chokes.

With chokes my Commando will idle happily at 1800 or so settling to 1100 to 1200 when warm and no choke.

The only reason to idle at 700 is if your aim is to destoy your cam and followers.

A very very bad idea in my opinion.

I know I'm probably going to get flamed with this comment but Amal did include a choke for a reason.

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Greg’s point was that he (like you) aims for an 1100 warm idle.

And that EQUATES TO a 700 ish cold idle.

I do not believe he is advocating letting the bike idle for prolonged periods whilst cold.

I think you’re right in that if you want the bike to idle reliably when cold, chokes help. Personally I never do that, I start and ride.