what are the characteristics of the 4S cam

Is it streetable? will it need an increased idle? Other factors are: Mick Hemmings big valve conversion and 920 kit. What can I expect? Also, fed by a 34 mm Mikuni.

You can expect needing one more carb...i would expect.

The characteristics of the cam are a power curve that comes in strong at 4,500 ish according to common opinion. But it is still perfectly good below that, especially on a big motor. This can be lowered by having a radius ground on the followers. Mr Comstock can advise on and execute this for you. In my personal opinion, it’s a good cam and goes well with the package you have, or most of it.

However...NONE of the items in your list go with a single carb !

I have a Maney race cam in my 920, which I am informed is basically very similar to a 4S. I have no complaints at all, mine is also running 11:1 CR and a stage 3 Maney head. According to convention it should be a horrid street bike. It is most certainly not! It has more torque than a stock 850 everywhere, but it has far more power in the 4,000 to 6,000 range, which is where I use it most. It will rev higher but, as far as I can feel, there is no real advantage. It starts and ticks over perfectly.

Your Hemming head will (I think) be milder than a stage 3 Maney head, so will likely have more low down and less high up, meaning, even better for the street.

I do have Matts 35mm FCRs feeding mine though and will say that your 34mm Mikuni WILL NOT feed your motor adequately above 4,000 or 5,000 rpm.

I had a 36mm Mikuni on mine when the motor was stock and it clearly and obviously restricted it at 5,000. You have bigger cylinders to fill. And have all that work aimed precisely at improving cylinder filling, and have a carb / manifold that is incapable of even meeting the cylinder filling requirements of a stock engine.

This isn‘t a Mikuni bashing rant, they’re great carbs, and single carbs are great for some people’s riding style. But you have a clear miss match with your set up.

I completely agree. The 34 is a big restricter and there should be a very noticeable improvement with a 36. However, this is what is on now and will have to do for the time being. I installed a CNW E-start so now it is short on available space. Matt assures me that a 36 will just squeeze in. This is not my bike. It starts up in one RRRP but runs a bit rough while ideling. Just off idle and it smooths out very well. This is just sitting on the HF lift table and only run long enough to set the 2 detents for the E-start. My main question is.... the rough idle due to the cam or carb? Will this bike idle around 1200?

The rough idle is most likely a partially blocked pilot jet IMHO.

It could be carb settings to of course, not sure where you’d start for base settings with your set up though.

But you’ve not yet mentioned ign type / timing or plug type / condition etc?

It could be cam timing I guess, but I’m not sure how far out it would have to be to effect idle, I suspect quite a lot.

I assume you’ve got the tappets set correctly for the 4S cam ?

I my opinion the idle issue is not because the cam, or carb, is incapable of good idle.

Stick a colortune on it and look for clues.

kommando said:

Stick a colortune on it and look for clues.

Click to expand...

PS only use this in the dark, don't ask me how I know.

The ignition is the standard Boyer. I have not checked timing yet.. Fresh plugs of course, BP7ES. I have an Innovate AFR gauge, perhaps the owner will want to continue on with tuning. My job was to install the E-start and rebuild the front master cylinder. I would like to get a color tune, have never used one.

Sounds like you’ve done what you were asked to do.

Please don’t advise this guy to fit a 36mm Mik, it will make little, if any discernible difference.

Twin Amals at the least, or save up for the FCRs if he wants to liberate the inherent performance of his motor.

With a stock motor I have used a single 34 mik and 36 mik I couldn't tell any difference at all

baz said:

With a stock motor I have used a single 34 mik and 36 mik I couldn't tell any difference at all

Click to expand...

Cos the manifold is the bottleneck according to some.

Fast Eddie said:

Cos the manifold is the bottleneck according to some.

Click to expand...

Not with mine because the bike was transformed with a 40mm TM mik

I was referring to your 34-36mm comparison. Or did you do something different with the manifold with them?

No with the same manifold I went from a 32cv to a 34vm to a 36vm to a 40tm
There was no noticeable difference (to me) between the 34 and 36 but the 40 TM was a revelation
All on the same manifold

baz said:

No with the same manifold I went from a 32cv to a 34vm to a 36vm to a 40tm
There was no noticeable difference (to me) between the 34 and 36 but the 40 TM was a revelation
All on the same manifold

Click to expand...

Can you buy a TM 40 set up for a Commando from anywhere ?

trident sam said:

Can you buy a TM 40 set up for a Commando from anywhere ?

Click to expand...

No you can't to my knowledge ,most people don't want to notch the gusset in the frame to fit one
Someone said RGM were going to bring out a 36mm TM kit but I never heard anything more about it

OK Thanks

baz said:

No with the same manifold I went from a 32cv to a 34vm to a 36vm to a 40tm
There was no noticeable difference (to me) between the 34 and 36 but the 40 TM was a revelation
All on the same manifold

Click to expand...

Interesting !

Fast Eddie said:

Interesting !

Click to expand...

Sorry to the original poster of this thread coz it's it's getting derailed somewhat
The issue I had with the tm40 initially was it'd go a bit flat when revved above 6000rpm it would go a bit flat but if you held on it'd get fully on the main jet and pick up at 7000rpm I called it a day when it hit 7500 rpm
I was thinking this was the manifold restricting it so I made a new one with a 40mm inlet but it made no difference
So I went up a size on the needle jet,this sorted it out so that it'd rev clean from zero to 7500rpm and really picks up above 5000rpm

I have a 4S in my 920 , idles beautifully , twin FCR's