Funky issue when trying to go fast...

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.

Click to expand...

Mine showed 120, (22T up front) the last ten took a mile... Closed course race track only, of course. :wink:

" I almost felt like if the wind resistance wasn't there it would go faster."
The wind resistance is the LION'S SHARE of the of the resistance to speed, the wheels rolling resistance is a small part of the equation. Hence fairings on race bikes. Did you tuck in at all? Left hand on the top yoke/bar?
3,000 lbs. of lumber on a flatbed behind your 1/2 ton pickup, or a 3,000 camper behind it... night and day.

iceteanolemon said:

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

Click to expand...

pulls strong past 5K RPM in 3rd gear?

Rohan said:

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

Click to expand...

Right, everyone follow Rohan's lead, as we all know he never talks about any motorcycle he does not own or ride....... which is why he is so very seldom seen posting here at all.....

concours said:

iceteanolemon said:

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

Click to expand...

pulls strong past 5K RPM in 3rd gear?

Click to expand...

Just took it out for a quick spin. Got it warm and full of gas and in 3rd I have tons of power at 5500rpm. At sustained high throttle in 4th it just seems to give after a while and sit there till I back off the throttle and then I have power again.

I didn't get a chance to try to run it up too fast though as I need to be out somewhere else for that!

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.

My experienc are the engine are runing to lean. Go up one or maybe two nummber on main jet.

A surge of power after a bit of WOT back off implies over rich main jet.

Kvinnhering said:

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.

My experienc are the engine are runing to lean. Go up one or maybe two nummber on main jet.

Click to expand...

Yes.

hobot said:

A surge of power after a bit of WOT back off implies over rich main jet.

Click to expand...

No. The more power at less than full throttle is the needle jet/jet needle at the proper ratio, once the slide is completely open, the vacuum signal decreases dramatically, the main jet sizing is inadequate to flow the needed fuel at those low vacuum conditions.

Opps, my bad, yep less vacuum at WOT bore would not pull as hard on a lean jet as just off WOT draw. Hope just simple JSM carb bowl flow issue as its sure fun to run up the ton now and then pretty darn quick in the time/space/mood available. Lore has it that tire makers had to come up with a new speed rated 19" tire as first Cdo's could sustain 120 mph, or so the lore goes.

Improved power after backing off from full throttle is a classic sign of "weak" mixture and too small of a main jet, according to carb tuning guides. Make sure float levels are right, both petcocks are open, consider removing fuel filters to ensure best fuel flow into carbs. However, it is safer for the motor to err on the side of richness. If power drops with larger jets, then try with smaller jets. For safety from law enforcement, try this on an uphill grade rather than top speed on a flat road or a dyno if available.
Ergo, since "90% of carb problems are electrical" then I am probably wrong again.
Welcome to the humbling experience of tuning carbs. I will see what the majority vote is and please let us know how things turn out.

beng said:

Rohan said:

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

Click to expand...

Right, everyone follow Rohan's lead, as we all know he never talks about any motorcycle he does not own or ride....... which is why he is so very seldom seen posting here at all.....

Click to expand...

I may not ride my Nortons much Ben, but at least I know whats in them. And when someone starts prattling on about ripping a few teeth off the rear sprocket, we know they haven't even looked at the 'rear sprocket' on an older Commando - it is of course part of the brake drum, so going smaller isn't really a sensible comments at all, is it. ?

There are (way too) many other similar comments here too about machinery which the comments indicate obviously haven't not even seen the bike in question Several of the prime 'offenders', if we can call them that, have never owned or ridden a Commando or indeed any Norton in their life. ? - Should we let such nonsense pass by uncommented or unchallenged ?? Armchair experts and bench racers are welcome, but getting it totally wrong should at least get a rebuke ?!

We have not seen you talking too much about riding Nortons either, have we ?
Your race Norton(s) even appear to be together and going.....

I know this isn't applicable to it-l's bike, he has different carbs, but for all the Mikuni guys, this would be the balls. I had two sets of these years ago, great for atmospheric variation.

Adjustable main jet bowls for VM's

Copied from Hitchcocks Amal tuning:
If at full throttle the engine runs 'heavily', the main jet is too large. If at full throttle by slightly closing the throttle or air valve, the engine seems to have better power, the main jet is too small. With a correct sized main jet, the engine at full throttle should run evenly and regularly with maximum power. If you are testing for speed work, ensure that the main jet size is sufficient for the mixture to be rich enough to keep the engine cool, and to verify this, examine the sparking plug after taking a fast run, de-clutching and stopping the engine quickly. If the plug body at its end has a cool appearance, the mixture is correct, if sooty the mixture is rich, if however there are signs of intense heat, the mixture is too weak, and a larger main jet is required
I don't know squat about Dual 32mm JS PWK
but the data and process should be the same as the amals. Did you do a plug chop at the ton?.... or as close to it as the bike would get you there?
Things you need to know and then report back. Use avgas if you can chase it down. It still has lead in 100 low lead. blue fuel.
it's better to decipher the plugs than with pump gas.
Cheers,
Thomas
CNN

Yah I want to try the plug chop. I just think the only place I can actually do it I would have to bring a plug wrench out there to do it at. I also have a place in town selling the performance gas over 100 octane. I guess this is good as time as any to try that stuff. I think it's 7 bucks a gallon though?!?

I don't have any issue with carburation when immediate hard throttle through the gears though. Jut at top of fourth maxing speed... Today running it up in 3rd it screams up past 5500 and I don't quite run it to 6000 but its really in a strong part of the powerband when I shift. Then in fourth it's strong all the way up and then the plateau... Ugh.

Its gentler on the engine to try it in 3rd than in top.
If it won't do it in 3rd, it sure won't do it in top...

So have you disconnected the fuel lines, and measured the flow out of the fuel pipes.
If something is blocked or clogged, could be as simple as that ?

Amal carbs also had those adjustable mainjets.
This one for a monobloc (I think).
Bike it goes on isn't complete yet to try it.
Not sure if its an Amal part or aftermarket either.
It screws in down the bottom, in place of the mainjet and basenut.

http://postimg.org/image/ljvi2y565/

Rohan said:

Its gentler on the engine to try it in 3rd than in top.
If it won't do it in 3rd, it sure won't do it in top...

So have you disconnected the fuel lines, and measured the flow out of the fuel pipes.
If something is blocked or clogged, could be as simple as that ?

Amal carbs also had those adjustable mainjets.
This one for a monobloc (I think).
Bike it goes on isn't complete yet to try it.
Not sure if its an Amal part or aftermarket either.
It screws in down the bottom, in place of the mainjet and basenut.

http://postimg.org/image/ljvi2y565/

Click to expand...

Amal Mk1's had fixed numbers: 230, 240, 260 based on application. Not sure of PWK's but should be similar.
Flow out of fuel pipes???? With your mains X 2 orifice size is the maximum amount of fuel that the carbs will let in to the induction intake. This will be your bottleneck for fuel delivery.
My thoughts.
CNN

You'd be surprised how many fuel taps and filter are clogged up with rust, debris, paint flakes etc, and can't even flow enough fuel for normal riding sometimes.
As well as reports that ethanol is swelling up the sealing material in the fuel taps, and restricting the fuel flow.
Always worth checking - eliminate the bleedin obvious....

Rohan said:

You'd be surprised how many fuel taps and filter are clogged up with rust, debris, paint flakes etc, and can't even flow enough fuel for normal riding sometimes.
As well as reports that ethanol is swelling up the sealing material in the fuel taps, and restricting the fuel flow.
Always worth checking - eliminate the bleedin obvious....

Click to expand...

True
CNN

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 ??
Rohan
Posts: 2820
Joined: Thu Aug 26, 2010 5:30 pm
Private message

BALLS . pissy little git .

7000 on 19 is 115 near as damn . Depending on tyre & wear . 23 T gives near that in third . which is fine in the rain with a 4.00 19 rear . 11 miles in 8 minute in a cloudburst . ( a big cloud :lol: )
aint dawdling , but restrained velocity in the suberban areas , unlike most cityslickers .

Power peak of the Std. Cam is 6.000 which is near as damn 100 or 99 if youre persnickety .
The wotsit Bros one pulled to redline two up , uphill , back in the day .

Any Triumph that wont pull a ton in thirds a bit off . incudeing 5 speed T150s . Swiftly & with style .

Rightio . To see if its carburated right , pull of into the gravle on the side of the highway . 2nd gear at about 40 mph , give it a blip . Let in the clutch holding the throttle on . It should shoot up a 50 ft. rooster tail .
( Id wonder what those peculiar vibes were - wind onshore , deposited over first two lanes of traffic . ) Only dont tell anyone I said so .

A thing that catches out the Ex whizz bang riders used to Vacum Diaphram Carbs , is that you need to ' milk ' the throttle & co-ordinate gear shifts / brakeing & THROTTLE POSITION . The CARB doesnt do it for you

In Fact whacking the thing on the stop at low revs , say 2.000 rpm's is going toi make the rings very uncomfortable . Also it isnt neccesarilly neccessary
to have the throttle ' on the stop ' when redlineng . Particularly in the swervery , its seldom possable .

Though rideing on dirt roads requires a differant technique . Particularly with revs up in high gears . A degree of sensitivity and circumspection .

Or on youre head .