The ton plus 10, what could be the problem?

Tonight I had the bike out for a spin and did 110 on a long straight section in 3rd gear. I'm running a 22 tooth front and when I shifted to 4th I expected to accelerate just a bit but she wouldn't hold it and slowed to 105 at around 5300 rpm.
I have a combat head and a web cam 312a with radiused lifters. The cam is advanced 5 degrees which made a good bit of difference. I probably shouldn't expect any different with this rather mild set up. Cruise at 70 mph is just over 3500 rpm which is nice. I really like the low rpm cruise and it pulls nicely from a bit under 3000. Any thoughts on how to get a few more suds?

Have you still got Combat CR?

It won’t have the same Combat top end, as you have de tuned it. Probably for much better overall effect, but you’ve sacrificed a bit of topmend.

And, it is over geared if you want to pull max in top.

The compression is what a combat would be with a .040 head gasket. It is pretty high.

That would make it approx half a point lower than stock then I guess?

As you say, still pretty high then.

But your cam has given you a far more streetable motor, at the expense of some top end.

But whatever the engine tune, the symptoms you describe are clearly of an over geared motor. You need to decide between a nice relaxed motor at cruising speed, or one that will pull out in top!

Yes, I am getting used to that idea. I just keep reading about faster Nortons and wondering if I might be able to get the best of both worlds. Given the choice though I will keep what I have.

IMHO, that’s a sensible choice.

All depends on what you use it for most and what you want out of it.

motorson said:

Yes, I am getting used to that idea. I just keep reading about faster Nortons and wondering if I might be able to get the best of both worlds. Given the choice though I will keep what I have.

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Don't you have speed laws where you are, today's law and speed limits as well the fun police stopping us having fun with high speed runs, here in Aussie land anything over 40 ks over the speed limit and they take your ride or car off you for 30 days and if it happens 3 times you loose your ride forever, bloody hoon laws, to many young ones getting killed from high speed crashes on powerfull machines has stuffed it up for us older experience riders.

As for having a hot motor they run better with stock gearing for high speed runs, my hot 850 with lots of work done to the motor revs freely past 8,000 rpms but would I want to do that without destroying my motor and would I want to run high speed on highways with speed cameras, police with radar guns hiding behine trees, mobil speed devices etc etc, I like to keep my licence and my hard earned cash as well as my bikes, I still ride my bikes hard but you have to be carefull where you do it and be smart about it and when you get caught except you have done wrong, I had 3 month sespension over a 1 1/2 years go for lost of points and it nearly killed me without riding my bikes, the fun police are out to get you.

Ashley

I pity the fellow that hasn't got a couple of places he can run up to the ton, and over, for a few miles if he wants, without any danger to others on open roads.

That's a sad, sad, state of affairs...

grandpaul said:

I pity the fellow that hasn't got a couple of places he can run up to the ton, and over, for a few miles if he wants, without any danger to others on open roads.

That's a sad, sad, state of affairs...

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I agree

grandpaul said:

I pity the fellow that hasn't got a couple of places he can run up to the ton, and over, for a few miles if he wants, without any danger to others on open roads.

That's a sad, sad, state of affairs...

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It is and getting worst, my state Queensland, Australia our GOVT is broke and in big det, speed cameras and traffic infringements is a big money earner for our GOVT, of course they say its for road safety but we know its revenue raising and when the kids are on school holidays the fines double as well as licence points and you only have to be 1km over the speed limit to get a ticket if caught there is no leeway at all and to make things even worst they are lowing the speed limits on some of the best riding roads around to catch us rotten bikies out.

Ashley

Wow.
Nothing like living in a ‘free’ country eh?!

My 850 with 22T has shown me 120 in 4th several times. I think the speedo is accurate (I know it's spot-on at 70mph & below).
YMMV

You might try standard Combat gearing which was a 21 tooth gearbox sprocket..

Had a look at the book of original road tests.
In September 1972 Cycle Guide tested a Combat 750 and got 109.5 mph as a highest one way speed, which is not a true top speed as wind is helping. True top speed would be the two way average.

Another test of the Combat by Modern Cycle in January 1973 lists " maximum speed as geared 108 mph"

I can't find evidence that the 850s were slower than the 750s.
On the contrary, the 850s seem to be good for an extra few mph of top end, then they introduced the super quiet exhaust on the mk2a. Top speed as well as acceleration
were greatly reduced by those mufflers.

Another note- the testers are mostly around the 120 lb mark!

So if your speedo is accurate and you are doing a true 110 or thereabouts, nothing wrong with that.
If you are twice the weight and , more importantly, twice the width of those scrawny young testers, then a true 105 would be expected. Speedos generally give a somewhat optimistic reading, but not always.
They can be accurate at 70 and wildly out at 100. I have a friend who repairs Smith's magnetic and Chronometric speedos. He uses a calibration device which shows true speed against what is happening on the speedo.
That is where I learned that accurate at 70 only means accurate at 70.


Glen

My book of all of the Commando road tests show Combats top end around 108
with the first 850 of 1973 around 106
and progressively more stifled 850 barely topping out over 100-103

The first 850 tested in the Norton Commando Ultimate Portfolio was tested by Cycle Guide. They list an " actual top speed 116 mph"
Most of the 750s on test list a little lower number, 110-114, when given as "actual" rather than "estimated"

Of interest-
In June 73 Motorcycle Mechanics took a new 850 Interstate to the Santa Pod dragstrip.
They reported " In a straight race race against a 750 the 850 was a lot quicker over the first 1/8 mile and still pulling away slightly on the last part of the strip"

This 850 has a level exhaust with silencers that look a lot like those found on the 750s.
So there's no evidence of sluggish 850 s until the black caps were fitted. On the contrary, the 850 seems to give a small bump in performance when both bikes are geared the same.

Glen

motorson said:

Tonight I had the bike out for a spin and did 110 on a long straight section in 3rd gear. I'm running a 22 tooth front and when I shifted to 4th I expected to accelerate just a bit but she wouldn't hold it and slowed to 105 at around 5300 rpm.
I have a combat head and a web cam 312a with radiused lifters. The cam is advanced 5 degrees which made a good bit of difference. I probably shouldn't expect any different with this rather mild set up. Cruise at 70 mph is just over 3500 rpm which is nice. I really like the low rpm cruise and it pulls nicely from a bit under 3000. Any thoughts on how to get a few more suds?

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Interesting result, thanks for sharing. First a question about your seat-of-the-pants intuition from the 3rd gear roll-on. Did you feel that the real yank was done in the lower 6000 rpm range, even though the engine continued increasing in speed or did it pull smartly all the way to 7000 rpm and beyond? I ask because I’m trying to get a feel for where you might perceive the HP peak to be.

When you mention that the cam is advanced 5 degrees I’m wondering how it is actually positioned? Is it 5 degrees advanced from the normal 102-103 IN centerline or 5 degrees from straight-up or what? Advancing 5 degrees from either point would emphasize mid-range grunt while clipping top end a bit, which might contribute to why it sagged a bit in 4th gear, and why I asked at the outset where you think the HP peak might be. Regardless, probably a real nice set-up for drivability with the 22 tooth countershaft.

On a slightly different note, the 110 mph in 3rd gear (5.06 oa ratio) would suggest you were really “wringing it’s neck” there (7300 rpm?), and the shift to 4th (4.19 oa ratio) would result in a rpm drop of ~ 20% (6000 rpm), that ultimately sagged down to your reported 5300 rpm. I mention the foregoing in an attempt to tie my comment at the outset regarding cam timing to your end result. I think it is interesting that the final engine speed in 4th gear (5300 rpm) occurred very near the torque/HP crossover point for a stock 850 (and I know your 312A cam puts you in a much better place than stock). Given the potentially over-geared situation you seem to be in, your cam timing would certainly emphasize 4th gear performance nearer the torque peak than the HP peak, and the cross-over may be the sweet spot.

"I think it is interesting that the final engine speed in 4th gear (5300 rpm) occurred very near the torque/HP crossover point for a stock 850 (and I know your 312A cam puts you in a much better place than stock)"

Isn't this the same for all engines, all vehicles ?( capable of at least 5252 rpm)

Torque and horsepower must crossover, in other words, be equal at 5252 rpm.
That's my ( limited) understanding of the simple equations used to compute HP from tq or tq from HP.

Glen

1up3down said:

You might try standard Combat gearing which was a 21 tooth gearbox sprocket..
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no sir, the stock Combats came with a 19 tooth in the interest of quarter mile acceleration

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Possibly the US market ' combats ' had 19 tooth sprockets but the UK version had a 21 tooth .