750 Combat top speed problem (2015)

daveh said:

......Also, are the rings a good snug fit in each of their grooves?......

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This is worth looking at. The ring grooves wore out way before the piston clearance became excessive on both of my pistons. This was at around 25K miles (also a Combat). Still the original bore now at 38K.

Could be a combo but don't think rings are your main issue as my Combat 2 yr ago had cracked carb boots unknown sucking in THE Gravel and gradually smoked more and more over a season till began to foul plugs/stall just to ride off from drive way but prior to that had plenty of power and response while leaving fumes to those following.

Update: have new +.020 Hepolite rings installed after cleaning up the pistons. Barrel is on the crankcase. Pistons are a bit loose with 0.008 clearance at the top of bore. Bottom of bore is .004". Good enough for me. I've got to get the new flame ring head gasket with head installed (always fun getting the pushrods in place) and button this up over the coming weekend. Club ride on Mt Hamilton the following weekend.

David your family may not care or understand but we do so encouraging progress to read - stolen form rest of life and hope that allows making some lasting memories other than more annoying mysteries.

At 80 mph on a level road, my 850 would be about half throttle. The needle jets would be the dominant part of your jetting at that point. You're not really nearing top speed at 80 mph unless you're going up a big hill. Mark your throttle to get a better idea of where you are in your jetting. What kind of gas mileage are you getting? Maybe a dirty air filter? Did it used to run better? Cam timing being off a tooth somewhere could probably do this.

I just got several things sorted on my Combat and it easily did 100 mph in third gear before I ran out of room. My timing was at first too far advanced. Then I retarded it and it quit misfiring. I didn't put a light on it but noticed it lacked a bit of power on top end. The timing light showed that I had retarded it too far. (about 3 degrees too far.) When it was missing on the top end I started messing with float level and jetting. Those are both set just right now at .080 below deck on the float and 130 main jets which is stock on a Combat.

Now that I timed it just right. (20 degrees before top dead center at 2000 rpm for a Lucas Rita.) It just wants to go fast. I also have a copper head gasket that is .050 inches thick. And, I run 93 octaine non-ethanol fuel.

Cheers, Dan.

My 750, (non combat) stopped accelerating at just over 80mph too. I would twist more throttle and it would to bog down instead of go faster. As I released throttle, it seemed to respond again just below full throttle positions. I was running 240 main jets at the time, and using 93 octane (no ethanol) I figured it was either too rich or too lean,... I had both 220 and 260 main jets in the parts bin and a jet key to make a quick swap if needed.

I switched to 220 main jets and the bike doesn't bog down above 80mph now. There was no need to try the 260's. I also did a timing case breather modification too when I put the bike back on the road with the 220 mains. I'm not sure if that factored into the result too, but it works fine now.

I ran it up to 90mph on the day I had it out testing and the bike has a tiny amount of weave at that speed, so I don't think I want to hit 100, The engine felt like it would go more but the rider wasn't willing to...

I'm not sure what size sprocket anyone is running to be hitting 100 mph in 3rd gear, but I can't imagine doing that on my bike with a 20 tooth sprocket (originally my bike came with a 19) What are the RPM's at 100mph in 3rd gear?? Probably way more than I want to see on my bike's tach....

o0norton0o said:

I'm not sure what size sprocket anyone is running to be hitting 100 mph in 3rd gear, but I can't imagine doing that on my bike with a 20 tooth sprocket (originally my bike came with a 19) What are the RPM's at 100mph in 3rd gear?? Probably way more than I want to see on my bike's tach....

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Calculating from the numbers in the factory manual, 3rd gear with a 22t would be just touching redline at ~100 mph.
I thought I had it printed on a chart somewhere, and it may have been shown here recently too...
If your back tire is worn much, it won't (quite) do it.
Combats have more chance of doing this than most other Commandos too,
since the power tails off much after 6000 rpms on the stock cam.

Not that we have ever seen a proper dyno / torque chart of a stock 750, or 850 for that matter. (?).

With a RGM belt drive at 1.96:1 and a 20t sprocket, I hit 100 at just under 7200. It is something that I have done a couple times and not something I practice.

The "Norton Commando Technical" link on this web site says a 21 tooth sprocket (Which is what I am running.) will go 119 mph at 7000 rpm. My old 1970 Fastback did 110 mph at 6000 rpm with a 22 tooth front sprocket but would not pull the next 1000 rpm due to a lack of power. It was running badly at that speed and backfiring a lot so I feel sure that a good tune up would have made the difference.

I may have over reved my motor just a bit to do 100 in third but I only did it for a second and could only look at one gauge at a time!

I would love to be running the 22t sprocket but with a Combat engine torque is not the emphasis. Mine shows 60 mph at 3500 so that translates into the 119-120 at redline.

Just one comment about tire size and speedometer reading on a Commando. Since the speedometer is driven off of the rear wheel it will show the same speed at the same rpm no mater what tire or wheel you have on it. What changes is the correctness of the reading.
Dan.

motorson said:

Just one comment about tire size and speedometer reading on a Commando. Since the speedometer is driven off of the rear wheel it will show the same speed at the same rpm no matter what tire or wheel you have on it. What changes is the correctness of the reading.

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Quite so.
For folks wanting correct speedo readings, or worried about this, it is possible to get someone familar with Smiths to
calibrate a magnetic speedo to read correctly for the wheel/tire fitted.

Probably not something you should try at home, although apparently it is an easy adjustment.
With the assistance of something that can spin it at suitable revs, to be able to set it....

Presumeably, these new Smiths electronic speedos could be adjusted by software to read correctly ??

Hi.
Just to add: a GPS speedo will give an accurate speed reading regardless of drivetrain configurations.
Ta.

A proper healthy mechanical Smith is more accurate than digital spdo on moderns and faster responding than GPS and more accurate than GPS if on slopes. Smiths are accurate to 1 mph. Can search this up, ping Smith directly or compare and test this for yourself. What sucks is the lamp mount and cable routing not to mention whimpy rear drive. All Cdos should pull over the ton though some just take a bit longer is all.

I found using a GPS for a speedo that they often don't work in road tunnels, road cuttings and amidst the skyscrapers.
All places where speed cameras seem to proliferate, hereabouts.....

Accurate to 1 mph eh ?
My needles seem to hover around more than that.
Maybe your roads are smoother ....

(And regardless of your tire size, or how much it was worn ?).

Both my Combat speedo clocks worked perfect with steady needle at any speed but had numerous intervals of rear drive or cable breaking or wearing out which showed up as twitchy needle or no reading till something other than the clock itself attended too. Do search up references on Smiths accuracy or contact yourself as this has legal implications if not guaranteed to read stable correctly when working right. I found GPS lagged behind car speedo on Ozarks steep slopes by a few mph til leveling off.

Speedos as supplied are legally only required to be accurate to within 5%.

Heck, a mm of tire wear could chew up some of that.
With a few mm to go....

British road bike tests of old used to note speedo accuracy.
They were often quite close at 30 mph, but usually read 4 or 5 mph under at 60 mph.
(So were really only doing 55 or 56 mph)

Rohan said:

I found using a GPS for a speedo that they often don't work in road tunnels, road cuttings and amidst the skyscrapers.
All places where speed cameras seem to proliferate, hereabouts.....

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Hi rohan.
Correct but these are also locations where 100+mph runs are not often conducted.
Ta.

Smith brags on 1 mph accuracy and some the repair detail articles back this up. Can not help it if others have been missing out on all a Cdo can be and do. Not uncommon for Moderns to read ~2 mph faster than road speed d/t fudging for legal suits over speeding tickets.