Funky issue when trying to go fast...

OK guys I have a weird issue that I think I need to bounce off the group before I get runaway ideas and end up buying blinker oil and muffler bearings...

I have a 750 that I have been tinkering away on for a while and it has some upgrades and stuff. I will list the components below but describe the problem here first. When I was coming home yesterday I decided to see how fast I could get the bike up to for top speed. I wanted to see where it would flatten out the torque curve and of course its just a curiosity.

So I get to going in fourth gear and the bike revs up past about 95 and almost to 100 and I feel the revs being capped off. Almost like fuel starvation or a limiter. I tried about three times to spin the bike up but it just gets up there and after its running 5krpm for a little with throttle on it climbs in power and speed then kinda sits there. I have tons of low mid and top end power and the bike is perfect up to there but when I really pour it on for a bit it just feels like the fuel is starving in the bowls or something.

Fresh top end including 40 over Emgo pistons and rebuilt stock non combat head.
Adjusted valves in spec
Stock plugs look ok I think b7es or something like that (gotta look again)
Power Arc ignition
Dual 32mm JS PWK
Emgo filter
Pingel fuel filter 2 into 2
Half tank of premium gas with stock petcocks standard and reserve both open
Comstock breather
Rotella 15-40 oil


Other than that the bike is running perfect. I have about 1500 miles on the top end and I broke it in with the usual methods varying rpm tons of oil changes etc... I am really suspecting the fuel flow to be the issue but I wanted to see if anyone else with a similar setup has found this to be it.

Thanks

Your description of symptom suggests NOT "a lot of top end power". If it did 115 then fell back, maybe fuel, but this sounds like just no more beans. Main jets rich enough?

What sprockets are you running ? What revs are you pulling at 100 MPH ?

The carbs are a variable which owners used to standard Amals probably won't be able to comment on. Performance is on a par with a single SU and I assume that it's meant to be better than that ?

If the cam isn't losing a lobe then I'd be inclined to check valve timing. It could've been out from new.

I suppose one other question is whether you're a stoutly-built chap. Two-up and with camping gear, a standard Commando won't want to pull much past the ton.

It sounds like your bike is under geared. If you take a couple of teeth off the back sprocket, it might still accelerate just as fast, but be quicker in top speed. It can be very deceptive when the motor is not required to pull hard. Sometimes the motor can spin up at the same rate through the gears regardless of the overall gearing. The giveaway is usually lack of any lag after the gear changes. It depends a bit on whether you are using a close box or the standard one. If you increase the overall gearing, first might become a bit too high, and Mr Policeman will love you.

acotrel said:

It sounds like your bike is under geared. If you take a couple of teeth off the back sprocket, it might still accelerate just as fast, but be quicker in top speed. It can be very deceptive when the motor is not required to pull hard. Sometimes the motor can spin up at the same rate through the gears regardless of the overall gearing. The giveaway is usually lack of any lag after the gear changes. It depends a bit on whether you are using a close box or the standard one. If you increase the overall gearing, first might become a bit too high, and Mr Policeman will love you.

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Giving 'advice' to folks here when you don't actually own, or ride, a Commando can truly lead them up the garden path. ?!

As mentioned above by 79 x 100, it probably all depends on what sprocket you have on the gearbox.
Lotsa Commandos were sold new with a 19 tooth on the gearbox.
This gives good acceleration and go, but you run into redline just about 100 mph in top (4th).

Lots of threads here about changing to a 21 or 22 tooth sprocket, or even 23 (or 24?) and the pluses and minuses of this.
My old 850 would touch 100 mph in 3rd with a 22 tooth fitted, just, if you wound it out. And in flat country went well.
If you live in hilly territory, or always ride with a pillion, it may be too much..

The other possibility is how new are the valve springs. ?
Old or tired valve springs will take quite a few revs off the top end.
The english supplied springs generally didn't last forever - especially if you use them hard - the race bikes were recommended to replace them every few races....

well it really feels like the bikes getting starved for fuel to me. I have the valves in spec and did check before and after and they are still in spec. They didn't seem to sound bad at all and the bikes been running so so good too.

It just feels like I have tons of power at the moment then holding on to the hard throttle and it boggs a bit then comes back when I let off a little.

I have a 19 tooth sprocket in the front and I believe the rear is stock. I am 210lb and the bike is a standard roadster... I am not set up with a side bags or anything so not hauling bowling balls or anything. Sounds like you guys don't think float bowl starvation or low fuel pressure would be it then... It is revving about 5500rpm at that speed if I remember right.

Actually I am very happy with the gearing itself as it is but I was just doin what dudes do and wanted to see what I could wring out of the bike during my ride home. I really didn't expect the sort of immediate flat spot and more of expected a over the curve type feeling when topping it out. I just never topped it out before and wanted to do it.

I really don't have business over 80mph on that thing anyways being old brakes and all!

19 tooth - that says it all.
Did you look at the tacho ?
The needle would have been seeing red, and the engine had simply run out of puff - and cam.

Some folks report the valves float about then, depending on what valve springs were fitted, and how long they've been there.
That kinda sounds like what you say...

Gidday all
hello Icetea
mate, check simple things first.

No blockages in fuel tank breather. open tank lid, put a tin under each side fuel line, measure time taken to flow say 1 pint, close lid and time again for 1 pint. you may get quite a suprise. it may be reduced to a mere bit of a dribble. easy fixed. more or bigger holes.

what i found once was a small air leak on right side where carby meets manifold. run ok it seemed, did not miss or get hot around town seemed good. then a
6 hr run in 38 Centigrade heat and burned inlet valve.

gearing is a factor too. 100mph with 19 T sprocket seems about right. you dont seem to have engine troubles .

Again, wat revs at 100 mph

hope this helps
bradley

then holding on to the hard throttle and it boggs a bit then comes back when I let off a little.

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classic symptom of the main jets being too rich

Mine loves to cruise at 80. I have done the ton tucked and low in forth gear and also in third where it hits just before redline.

With the RGM belt drive the 20 tooth sprocket is a good compromise. If i went to a 19, the ton and beyond would be even more available in forth but no longer in third unless I choose to test the beehive springs and BSA lifters capabilities.

Prevailing winds, grade,and posture are all facters. Mirrors also have a profound effect on this milestone.

Willy nilly going out and tickling a 100 mph sort of tells you that you're there.

When mine was relatively new I had it up to the ton on I95 between Baltimore and DC on early Sunday morning and I felt the same thing, it just ran out about the ton. Scared me shitless and never did it again. 750S 19 tooth sprocket. I almost felt like if the wind resistance wasn't there it would go faster. I also was only about 160 lbs then. I think 210 is pushing it. I think you're just at the limit, but it wouldn't hurt to check the fuel flow over time. I remember I couldn't get it to redline in 4th.

Looks like I may have been bitten by the bug. I am now stuck trying to get just a little more out of the bike heh

:twisted:

Any and every Commando should be able to exceed the ton, especially factory low geared ratio, so yours is definitely sickly. Sure sounds like carb source fuel starvation. JSM carbs have a reported issue of fuel flow and solution, so ping Schmidt and try search here on this thread a few weeks ago. Also make sure last batch of gas ain't bad with some water in it.

In Third . :wink:

Warp Factor SIX . Scotty .

NOW , unfortunately , this isnt the page instructing you to remove the Air Cleaner Assemblies ( pictured ) .
Run the engine untill up to temperature , get up to near maximum speed . & Apply Some CHOKE .

If the Speed Increases , The Jetting is LEAN .

If the Speed Decreases , The jetting is RICH .

( on these , the jet hight is varyable via a knurled knob under float bowl . :lol: 8) .

This assumes dual progressive choke mechanism .



( local turkeys now at the ' test strip ' dyno jetted there RS3100 l e a n & melted a piston. Oh dear . Cheeky sods ' No , We do it this way nowadays . ' :roll: :lol: cant say I didnt tell em .
H.R.D. is right at the top of the test strip . serve em right for useing That Name for their silly Ford toy cars . ) The Sucker NEEDS to Get Up & Settle , to Estabish Full R.P.M. ' Tune ' .

ANYWAY .

19 T gives 115 odd , thereabouts @ 7.000 rpm , In TOP .

Obviously the old girl should do 7.000 in third. But we dont necessarilly Want To . WE want to ESTABLISH the POWERBAND . Therefore . . .

' We ' run backed off a notch , say 7/8 throttle ( put a mark to line up , if yr worried , beforehand . ) And from say 60 , letter run out , in third .
You should feel a bit of a ' peak ' then flattens off / drops off . A few glares at Tacho , if its not bouncy , will tell you what rpm's its schnarling up to .

possably , on the stop , being eager , itll run past this ' peak ' . But So what . We Wanna Know where its cammed for . thereabouts . as they say . SO .

we can try this in second . IN ANY GEAR , it needs a CLEAR ROAD . so NOT suberbia or the rush hour . or you could get a bumper or tow ball in the teeth .
NOW , running er up that way in Third , Dropping into top , Throttle ON ( Jetting established as not bad or pretty good ) She Should run up to there , in top.
If it runs past it . Its undergeared .
If it wont . Youre too Fat .
The Gas is no good.
or something .

Id dive in for some 100 Octane , or race fuel , if youre going to play that game . Some of this modern gas ISNT the Olde ' Distillate ' , at alll at all at all .

Matt Spencer said:

19 T gives 115 odd , thereabouts @ 7.000 rpm , In TOP .

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Matt, in your ramblings, you appear to have misread Appendix A in the Workshop Manual.
It shows, quite clearly in tabular form, that with a 19t sprocket, 6500 rpm is 99 mph in top, and 7000 is 107.
This corresponds to a 4.84:1 top gear ratio.
115 mph would be 7500 rpm - which is, quite clearly, beyond the red zone marked on the tacho.

Oh, and with a 22t sprocket, 7000 rpm would give 124 mph, if it will rev that hard with that gearing.

But then, you don't own a Commando, do you.
Nor never ridden one ??

Yeah, when I had mine up there, the speedo read 120, but I couldn't even get to red line on the tach, so I was probably just pushing 100.

DogT said:

Yeah, when I had mine up there, the speedo read 120, but I couldn't even get to red line on the tach, so I was probably just pushing 100.

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At other speeds, my speedo is spot on, is there an error curve on the top end? Anyone GPS cross check it?

concours said:

DogT said:

Yeah, when I had mine up there, the speedo read 120, but I couldn't even get to red line on the tach, so I was probably just pushing 100.

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At other speeds, my speedo is spot on, is there an error curve on the top end? Anyone GPS cross check it?

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I have gps'ed mine. I was only 1 to 1.5 mph off at any point. It was after a fresh rebuild by Nisonger's.

The ancient Smiths gauges are generally more accurate to actual speed to the exact mph than the modern digital kind set to show couple mph faster than actual to stifle law suits claiming the speedo told em wrong. Ancient Smits clocks are more accurate than GPS d/t the fact GPS only measures horizontal travel so climbing and descending slopes don't register true surface speed which can show a handful of mph slower speed than wheels are turning. The faster ya go over the ton the more these differences and delays show up. Takes about 3 sec on level surface for the mechanical and GPS to agree and notice how sharp turns can throw off GPS too and tire spin mess with the Smiths. I've been through a few worn out AAU's to tell ya blipping throttle only sets rpm higher and does not shock system back down to idle timing return. Being a Commando I'd expect both air leaks and AAU issues at same time adding up. Check carb balance tube too. I know doing the ton is illegal and kind of dangerous but as I know I'm going to Hell might as well built up points for favor hanging out with the she Devil.