Flywheel weight?

worntorn said:

Does make you wonder about the effectiveness of that porting job.
The stock MK3 850 (-blackcaps) with 32 mm rh4 head will romp thru 6500 rpm in the first 3 gears.
I shift mine at 6200 +- when trying for maximum acceleration.
What did Norman mean by " you know this is an 850?"
That is is untuneable as compared to a 750?

I recall that Jim Comstock fitted a PW3 to an otherwise stock 850, can't recall the model year/ head type. It made a couple of extra bhp at 6500 rpm but had a big drop in midrange bhp (6?)vs same bike with stock cam.
The owner really liked the high rpm powerband and didn't mind the midrange loss as that accentuated the " on the cam" part of the rev range. Kind of like a 2 stroke, I guess. They can be fun.

It worked well at the top and so should yours!

Glen

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SteveA said:

What you don't say:

Is this in every gear or just top?

What gearbox sprocket do you have fitted.

And......the simple solution......fit an STS Fullauto! I'm sure AN will have them back in stock soon!

Simple is

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SteveA said:

What you don't say:

Is this in every gear or just top?

What gearbox sprocket do you have fitted.

And......the simple solution......fit an STS Fullauto! I'm sure AN will have them back in stock soon!

Simple isn't cheap!

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Steve/All
Gearbox sprocket is 19t but i have a Norvil belt primary which Norman worked out to raise the gearing to a correct level, At 4000 its doing 72/74 mph and at 5000 90mph, i have not really thrashed it in third so maybe that s the way to go.....Norman did say that the 850 is slower than the 750 though, My suspicion is with the bored out inlets not creating enough velocity of fuel/air entering the head .....when weather and work allow i will see what it makes in third......Colin




when the weather improves and work permits

colin davie said:

Steve/All
Gearbox sprocket is 19t but i have a Norvil belt primary which Norman worked out to raise the gearing to a correct level, At 4000 its doing 72/74 mph and at 5000 90mph, i have not really thrashed it in third so maybe that s the way to go.....Norman did say that the 850 is slower than the 750 though, My suspicion is with the bored out inlets not creating enough velocity of fuel/air entering the head .....when weather and work allow i will see what it makes in third......Colin




when the weather improves and work permits

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Bigger ports mean less vacuum, so need more taper on the needles. Fuel enters the inlet tract because of the pressure differential above and below the jets. With big ports the differential is less, so to get the fuel/air mixture at optimum, bigger jetting is needed. Bigger ports suit higher rev ranges at the expense of lower revs. At high revs there is more vacuum. 500cc Manx Nortons have a huge venturi - a 500 cc single with smaller venturi can beat them in tight circuits, if a similar frame and gearbox is used.
The Manx was developed for the IOM TT.

The low Commando crank balance factor can stop the motor from revving high. My motor wants to rev way over 7,000 RPM in every gear, even in third gear, and the overall gearing is very high. I have never had it anywhere, where I could wind it right out in top. Winton Raceway is only 3 kilometers in length. On the longest straight, it is in top about 3/4 down it.
The gearing is very unusual, my bike does not respond similarly to most when I alter the gearing. It is actually quite ridiculous.

I suggest the flywheel weight does not matter when you jet , gear and use an exhaust system to suit it, and the conditions in which you ride,
I just learned to live with the 850 crank - I did not accept that it could never be good.

colin davie said:

Steve/All
Gearbox sprocket is 19t but i have a Norvil belt primary which Norman worked out to raise the gearing to a correct level, At 4000 its doing 72/74 mph and at 5000 90mph, i have not really thrashed it in third so maybe that s the way to go.....Norman did say that the 850 is slower than the 750 though, My suspicion is with the bored out inlets not creating enough velocity of fuel/air entering the head .....when weather and work allow i will see what it makes in third......Colin




when the weather improves and work permits

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The port job may or may not be recoverable. More importantly, it may not be economically recoverable. Hence my Fullauto comment.

I realise something else you don't mention is compression ratio. An increase in compression may help the cam work. But I suspect Norman White would have discussed that with you.

And if Norman installed the cam I would expect it to be timed properly.

Port size and port shape are different things. I had an ex works short stroke head with rather large valves, the motor revved mostly to 6800. The ports were the work of John Baker and at the manifold face they measured 34mm. Everyone says this cannot work, but with 36mm carbs and 10.5:1 compression, they did.

SteveA said:

The port job may or may not be recoverable. More importantly, it may not be economically recoverable. Hence my Fullauto comment.

I realise something else you don't mention is compression ratio. An increase in compression may help the cam work. But I suspect Norman White would have discussed that with you.

And if Norman installed the cam I would expect it to be timed properly.

Port size and port shape are different things. I had an ex works short stroke head with rather large valves, the motor revved mostly to 6800. The ports were the work of John Baker and at the manifold face they measured 34mm. Everyone says this cannot work, but with 36mm carbs and 10.5:1 compression, they did.

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Good point, well made.

Big ports CAN work if done properly, and they CAN yield results at high rpm. Steve Manley’s stage 3 heads have 36mm ports for example.

But they do need to be correctly done. And they will almost certainly need bigger valves. And they do need a suitable cam. And that cam will need a suitable CR. Etc.

@ Colin, there is more than one way to skin a cat, with many ways delivering similar end results. But mixing and matching different aspects From different sources seldom yields better results. Better to stick with one source IMO. In your shoes, as you already have close ties with Norman, I’d simply follow his advice to the letter.

colin davie said:

Steve/All
Gearbox sprocket is 19t but i have a Norvil belt primary which Norman worked out to raise the gearing to a correct level, At 4000 its doing 72/74 mph and at 5000 90mph, i have not really thrashed it in third so maybe that s the way to go.....Norman did say that the 850 is slower than the 750 though, My suspicion is with the bored out inlets not creating enough velocity of fuel/air entering the head .....when weather and work allow i will see what it makes in third......Colin




when the weather improves and work permits

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I wouldn't worry at all about the engine at 850cc capacity being somehow slower than 750cc. I don't believe it's true and the period timed tests of the day show that it was the other way around, up until the change to very restrictive silencers on later model 850s. I suspect that silencer change created the " 850slow, 750 fast" myth. In a way, it wasn't a myth since after a certain date if you bought an 850 Commando and kept the exhaust stock, you had a very slow bike that any 750 or early 850 could wallop.

In any case, with effective tuning on both, the bigger engine has more potential.
That's why there are 920 and 1007 cc versions of these things.
If the power levels somehow went down as the engine capacity went up, we would be busy sleeving these engines down to the smallest size possible!



Glen

Could be as simple as a wrongly sized main jet, so runs well on needle jet up to 3/4 throttle and then falls away.

kommando said:

Could be as simple as a wrongly sized main jet, so runs well on needle jet up to 3/4 throttle and then falls away.

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I mentioned that earlier and Colin replied that he runs 260s but has tried everything from 220 to 300.

Glen

worntorn said:

I mentioned that earlier and Colin replied that he runs 260s but has tried everything from 220 to 300.

Glen

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Yes he did Glen, but he sort of inferred that changing the main jet size made no difference. That is something I find 'suspicious'.

Even if it made matters worse, changing main jets should have an effect when the throttle is wide open, but not below 3/4 open throttle.

Silly observation, if the throttles were not being lifted to full open due to an incorrectly set cable or cables, might that present the symptoms described?

Fast Eddie said:

Good point, well made.

Big ports CAN work if done properly, and they CAN yield results at high rpm. Steve Manley’s stage 3 heads have 36mm ports for example.

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36mm, you know, I didn't know that!

Has the cam been checked to see that it is still giving full lift? Some PW3 s wore out very early.
Did the engine pull better at top when the cam first went in or has it always lacked top end with the PW3?

Glen

SteveA said:

36mm, you know, I didn't know that!

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And 5mm O/S inlet valves.

Thanks for all your input,,,,,,

If you drop the main jets out completely then the engine will still run well up to 3/4 throttle and at full throttle 8 stroke. If you drop the mains and it does not 8 stoke at full throttle then something is amiss, if it 8 stokes at full throttle then even 300 is too small.

With my bike, the main jets are always slightly too rich. It you are going the burn valves or pistons, it will usually happen when you are using full throttle. However I run the needle jets as lean as possible right down the needles. With lean needles, you tend to feed the throttle on rather than whack it open. I do not believe this stuff about high compression helping the cam to work. It might make the effect of the cam more pronounced, but the timings have the same effect at any compression ratio. - They change the power band and provide better cylinder filling. As you raise the compression ratio, you do similar to leaning-off the jetting or further advancing the ignition timing. Doing any of those things will improve performance of many motors. The risk is in going too far, when you do them without being aware of the symptoms. Some guys do not know when their motor is running too lean. If my motor misses. I usually go straight back into the pits and fix it.
Combustion conditions depend on three things - fuel mixture, compression ratio, and ignition advance. Regardless of compression ratio, you can usually end up in the same place by juggling the other two variables. Methanol fuel makes it much easier because the jets flow twice as much, the tuning errors are halved. - 'there is a fine line between pleasure and pain'.
Poms are better than Australians at this stuff.

With my bike, the exhaust system is far too loud. I think that is the reason some superbikes use Exup. It is supposed to improve torque, but I suggest it might be so they can use a better cam without falling foul of the noise regs. It must take a bit off the top end, but the middle would be fatter.

kommando said:

If you drop the mains out completely then the engine will still run well up to 3/4 throttle and at full throttle 8 stroke. If you drop the mains and it does not 8 stoke at full throttle then something is amiss, if it 8 stokes at full throttle then even 300 is too small.

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To set the main jets, a high speed run down a long road is usually needed, then check the plugs. Difficult to do on public roads or even a short road race circuit. If I do not get a miss while riding flat-out, I usually do not worry.

colin davie said:

I hear all this talk of red lining and wonder why my 850 mk2a with close ratio box, pea shooters and PW3 cam fitted by Norman White struggles to get past 6000rpm ,this is with clip ons and rearsets...........?

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When I first installed a cam in a 750 with two S's on the drive side exhaust lobe, I did not have adequate crank case ventilation since the cam did not support a timed breather. That really messed with the top end. No amount of carburetor tuning made a difference. Probably has nothing to do with it though, because I don't think I could reach 6000 RPM when crankcase ventilation was poor. Maybe check whatever PCV (breather) plumbing you are running anyway?

Fast Eddie said:

Good point, well made.

Big ports CAN work if done properly, and they CAN yield results at high rpm. Steve Manley’s stage 3 heads have 36mm ports for example.

But they do need to be correctly done. And they will almost certainly need bigger valves. And they do need a suitable cam. And that cam will need a suitable CR. Etc.

@ Colin, there is more than one way to skin a cat, with many ways delivering similar end results. But mixing and matching different aspects From different sources seldom yields better results. Better to stick with one source IMO. In your shoes, as you already have close ties with Norman, I’d simply follow his advice to the letter.

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Please take with a grain of salt - I have only minor personal knowledge of porting...

Around 1970 I was talking to the Triumph 500 tuner for the 1966/67 Daytona 200 winning bikes. A Triumph 500 with twin carbs produces pitiful HP but they handled excellent and had excellent riders. He told me that they tried a few things for more power. One was larger, straighter, polished intake ports - made the bike slower. Then they tried standard ports but polished - still slower than stock. Then they just cleaned them up and made them a little rougher than stock - that made them faster. He said the only other big things they did were "650 racing springs", exhaust porting, and high-compression pistons (I think 13:1). At the same time, there was factory-made a 5-speed gearbox for the 500 sitting on the shelf with me drooling over it. Only two existed and none on the road so it wasn't allowed to be used for the races.