Lightweight pistons and rods in P11

[Owl Russell]

Does anyone have feedback on fitting lightweight pistons and rods in a P11?
Especially regarding engine vibration.
Regards Ali Russell

 

[Schwany]

I've never combined the stock rods, which are lighter than popular aftermarket rods, with light weight short skirt pistons, but I have done Carrillo long rods and light weight short skirt pistons as well as MAP Cycle long rods and light weight short skirt pistons in a P11 engine.

The difference between those two setups and stock is a big improvement at high RPM. A well balanced rotating assembly be it stock or modified is about the same vibration wise in a solid mount Norton frame for typical riding. The pulse at idle felt in the bars is slightly reduced with the lighter pistons and long rods, but the vibration is never going away in a Norton twin engine in a P11 frame.

How a Norton engine feels in a Commando specific frame with isolastic engine mounts is not the same as it is in a P11 solid mount.

If you can find somebody that really knows Norton engine cranks and have them rebalance your rotating assembly, that is about as smooth as it will ever be.

Mine is smoother than stock, but I noticed the other day while I had the idle turned up to around 2000 RPM intentionally, I could not read the lettering on my plug wires because the wires were vibrating so fast. Probably time to tighten some fasteners.

 

[Owl Russell]

Thanks for coming back.
The vibration on mine is bad at higher revs, difficult to focus on number plates at 60 mph + , also keep shedding parts as I ride, carb drain plugs, horn etc.
Suspect this could be better!
Do you have any ideas what sort of balance factor would be appropriate ?
Thanks Ali

 

[Schwany]

Mine is currently 75 on the balance factor for a Molnar crank in the P11 engine. My stock crank was balanced at 67 and was much better than 52 or whatever they are right out of the factory. This subject has been covered a few times on this website and advice is all over the map.

Do a search, it'll be entertaining.

 

[mdt-son]

The vibration on mine is bad at higher revs, difficult to focus on number plates at 60 mph + , also keep shedding parts as I ride, carb drain plugs, horn etc.
Suspect this could be better!

Of course it was better when new. The cause is probably non-standard pistons and a crankshaft not re-balanced. Poorly balanced carbs and a 4CA points plate will contribute too.
I test-rode a freshly restored P11 once. It vibrated terribly.

AMC's 650 cranks were dynamically balanced at the factory which made the bike exceptionally smooth. I don't think AMC or NV did this exercise on the Atlas cranks. Dynamic balancing is claimed to be rewarding.

Please note the balance factor employed by the factory is correct for the factory specification only (i.e., BHP dished pistons, BHP piston pins, etc.). If you deviate from this, you have to rely on forerunners.

I trust the HYCAM group in London will advise on the optimum balance factor for your engine and the expected outcome of dynamic balancing.

- Knut

 

[Schwany]

I trusted the person that designed the Molnar crank for the balance factor he uses for solid mounted Norton engines.

Gene Romero's machinist did the lightening and dynamic balancing work on the my stock P11 crank when I removed the dished pistons and went +.020 with the Commando pistons to get some more compression for the SS cam. Many other things were also done at that time. Restoration was the last thing on my mind. I was trying to get the most out of the engine. I've accomplished that with what I have now. Only about $12KUS and 32 years later.

Of course it was better when new. Yikes!! That is a statement I couldn't make for any new motorcycle I've owned and worked on. All of them were better when I got done with them. There were some teething issues with the changes I made to the P11 over the last 5 years, but I got there in the end as well. A Commando would be a nicer ride, but it's not bad at all for a P11.

 

[Junglebiker]

I balanced mine at 62% with JS Racing long Carillos and short skirt pistons. I did that based on opinions I read here on this forum. Haven't had it running yet! Next time I am in the US long term I'll finally get it running. I need to re-examine my carburetion choices and I suspect I will end up going to a single carb. I don't need high RPM's, I'm a low end-to-mid range torque junky. Also, I suspect that my magneto drive chain needs replacing.

 

[Schwany]

I balanced mine at 62% with JS Racing long Carillos and short skirt pistons. I did that based on opinions I read here on this forum. Haven't had it running yet! Next time I am in the US long term I'll finally get it running. I need to re-examine my carburetion choices and I suspect I will end up going to a single carb. I don't need high RPM's, I'm a low end-to-mid range torque junky. Also, I suspect that my magneto drive chain needs replacing.

Unsolicited useless advice cometh from me. I'm just trying to fit in where I don't belong. lol

Not to put too fine a point on it, but you can have all that torque and midrange using two carburetors with a mild cam. A single 34mm Mikuni would be easier than keeping two synced up, unless you got the 33mm CRS's or FCR's JS is selling for use on a Norton engine. They stay synced unless you start doing things to them that do not need to be done. Neither of those options are cheap though.

You should like the JS Wiseco Carrillo long rod combo in the 750. I'll check back here when I see your name pop up to see how your modified P11 project is working out. It should be a some fun once it's done.

 

[Junglebiker]

I've currently got a pair of clone carbs on there--I've had decent results using clone carbs on other bikes, but the more I think about the effort and expense I've put into my P11, the more unhappy I am about having gone with the cheap carbs. Also, I'm using a pair of Triumph intake stubs, the ones with the slight curve to them which makes the carbs sit level. The problem I have with them is that there is no way to tighten up the nuts in between the intake stubs. I can get them hand tight and maybe a tiny bit more, but there's no room for a tool and so they are not tight enough that I am confident that there is no vacuum leak there. Going with a 2-into-1 intake from MikuniOz (or a US supplier if I can find one) and a single carb (and I have many carbs which I think would work well on this application) just seems like a better and better idea.

 

[Schwany]

Hmmm, use allen head bolts and make a custom short end allen wrench. Still fiddly to tight up though.

Map Cycle has a single Mikuni carburetor Atlas setup, but I don't know if they will sell the manifold separately.

Add on gibberish: I still have a Mikuni TM I had bored out to 39mm for an ATK 604. Sold that ATK with a Dellorto pumper on it. I used to have days where I wished the big TM would have fit under the P11 downtube, but single flat slide carburetors on a two cylinder head can be a long tuning session without an accelerator pump IMO (right now power junky I am). The 35mm FCRs on the other hand are working very well on my P11 750 engine with the accelerator pump functioning. Basically like fuel injection except for the cold engine startup routine. I can even crawl around like a old man as long as I don't use any gear higher than 2nd. lol