Funky issue when trying to go fast...

Matt Spencer said:

7000 on 19 is 115 near as damn . Depending on tyre & wear .

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In which Universe ??
And on whose Commando ?
If the tyres are worn, its going to be slower.

If you are going to add 10% to the factory published figures, we may as well all tell fishing stories.
And believe anyone who can do 150 mph in 2nd...

A lot of magazine tests of the times said a Commando would only do about 100 mph.
If it had a 19t fitted, as many were stock, thats not very surprising ?

You should be able to find 110LL avgas at your local small airport. Now whether they will sell it to you in jugs, can't say. I have no problem here and it's only about $5.50/G. Better than racing fuel price wise.

If you decide it might be a carb issue, this is from an XS650 forum a few months ago. I can't find the thread but I saved the link to Mikuni tuning. It's the best tuning guide I've seen for VM's. There's a link to reading plugs for lead free gas containing ethanol too. Set the zoom to 110% for the carb guide, it clears up the text a lot.

http://www.amckayltd.com/VM34-36.pdf

http://www.wallaceracing.com/plug-reading-lm.html

Jim Schmidt was investigating fuel flow issues a few weeks ago. Comparing Amals and PWKs. You might ask him what the outcome was.

Unread postby batrider » Sun Jul 21, 2013 7:28 am
Jim Schmidt was investigating fuel flow issues a few weeks ago. Comparing Amals and PWKs. You might ask him what the outcome was.

Click to expand...

fuel-flow-through-32mm-amal-bowl-t16708-15.html

Is there a reported fuel flow rate I need from the petcocks to feed the PWK 32 properly? I'll drain my tank and rate the flow but don't see a measurement to rate it against...

You don't really need to drain the tank, just see that there is more than a dribble coming out when you disconnect the hoses and turn each tap on.
We assume you are not trying this on a near empty tank.

At 100+ mph, it'd give about 25 or 30 mpg ??
So thats a gallon every 15 minutes - say a pint every 2 minutes.
Thats an English pint, of the non-drinking variety.....

Looks like that other thread is getting to the nittygritty of the problem.
Maybe it should have been titled as fuel flow through through PWK floatbowls....

Sprocket Size Overall Gear Ratio 6000 rpm 6500 rpm 7000 rpm
19 4.84 92 mph 99 mph 107 mph
20 4.60 97 mph 105 mph 113 mph
21 4.38 102 mph 110 mph 119 mph
22 4.18 106 mph 115 mph 121 mph
23 3.99 112 mph 121 mph 130 mph


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1. Number of Gearbox Sprocket Teeth = 19 and Top Gear Ratio = 4.84:1 = 15.429 mph/1,000 revs in Top Gear (or 24.857 km/h). It = 108 mph @ 7,000 rpm.

Revs
Mph
Mph
Mph






1,500
23.14
19.07
13.50

2,000
30.86
25.42
18.00

2,500
38.57
31.78
22.50

3,000
46.29
38.13
27.00

3,500
54.00
44.49
31.50

4,000
61.72
50.84
36.00

4,500
69.43
57.19
40.50

5,000
77.15
63.55
45.00

5,500
84.86
69.91
49.50

6,000
92.57
76.26
54.00

6,500
100.29
82.62
58.50

7,000
108.00
88.97
63.00



3. Number of Gearbox Sprocket Teeth = 21 and Top Gear Ratio = 4.38:1 = 17.000 mph/1,000 revs in Top Gear (or 27.286 km/h). It = 119 mph @ 7,000 rpm


Fourth Gear
Third Gear
Second Gear

Revs
Mph
Mph
Mph






1,500
25.50
21.21
15.00

2,000
34.00
28.28
20.00

2,500
42.50
35.35
25.00

3,000
51.00
42.42
30.00

3,500
59.50
49.49
35.00

4,000
68.00
56.56
40.00

4,500
76.50
63.63
45.00

5,000
85.00
70.70
50.00

5,500
93.50
77.77
55.00

6,000
102.00
84.84
60.00

6,500
110.50
91.91
65.00

7,000
119.00
99.00
70.00

http://www.nocnsw.org.au/technical/speed-in-gears

A proper Commando does not need holding WOT to hold the ton so mileage is only in lower 40's to curise this, unless a strong head wind. It takes accelerated tire wearing mean hearted almost constant holding WOT to over the ton when not harshly braking in twisties games for 30-60 miles to get down to mid 30 mpg. Ancient as these things are they are pretty darn efficient but then so are the HD's getting very similar mileage by very similar operation.

Didn't the problem involve air bubbles in the line? Would need to be using clear tubing to see it. I ran into something similar with my single VM. I got around it by using 90deg fittings at the tank.

Issue resolved. Bending the tab in the float bowl to raise the fuel level did the trick. Lots of power at top end and I overcame the 100mph struggle quite easily. No changes to carburation at all other than that.

Cheers,

cool, or rather in the vernacular 'bitchin'

Or maybe that's dating myself sliding down the hills in Santa Barbara.

Very good to solve with just float level which some say the other items function all draw from. Btw did you have to diddle the pilot screw any afterwards. If so what its turns out now?

Well the PWK on the inside has a long adjuster so you can adjust it in the fly when installed. Well it's cool but when I placed the carb back on I moved the aircrew. It happens. So when I ran it down the street I heard a few pops and screwed it back in a little till the pops stopped. The other one which needs a scdrewdeiver was never moved. I suspect I found the original spot with the few adjustments.

They are both out about 1/4.

No pops on deceleration and gradual increase in power through 5500rpm. The bike didn't scream in 4th at over 5k before, now it's just waking up at 5k in 4th!

It really is nice to Fix stuff here and there. Especially when I don't break something else in the process!!!

It really is nice to Fix stuff here and there. Especially when I don't break something else in the process!!!

Click to expand...

A real mood uplifter to read and glad JSM carbs can make it fly. Double good ya didn't create a new mystery to solve. Now must resist breaking the ton so easy so often, to be spotted and ticketed.

Glad you solved the puzzle. Were those carbs bolted on new out of the box? Pre-owned? Previously opened at all?

concours said:

Glad you solved the puzzle. Were those carbs bolted on new out of the box? Pre-owned? Previously opened at all?

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They were new out of the box. I believe the manufacturing process changed or something without Jim knowing it. Such a slight change that you can't really see with the human eye I guess... Weird but oh how much different the performance is when it's fixed. Just bend the tab a little bit and it allows more fuel in the bowl.

iceteanolemon said:

concours said:

Glad you solved the puzzle. Were those carbs bolted on new out of the box? Pre-owned? Previously opened at all?

Click to expand...

They were new out of the box. I believe the manufacturing process changed or something without Jim knowing it. Such a slight change that you can't really see with the human eye I guess... Weird but oh how much different the performance is when it's fixed. Just bend the tab a little bit and it allows more fuel in the bowl.

Click to expand...

Just curious. Usually factory settings work flawlessly. I had to think about "bend the tab" thing, lol I learned on Miks and the Amal method seems lengthy by comparison.
Back to the float level, what was it measured at? Supposed to be? Both carbs? Inquiring minds want to know...

I didn't measure them at all. I was just sitting around scratching my head and whining on the hoards till honor sent that link. I barely read much so all I did was kinda see the picture jsebg posted with a flat 4mm piece of stock sitting under that tab and saying bend it...

I'm sitting in my garage looking at the action on the floats and the tab is bent about 2mm but the shutoff is closer to the swivel so it can't have been too much difference to the actual fuel level.

The crux of it is I have enough fuel now. I can adjust it till the fuel level starts spewing fuel but I only did it in a small increment so as not to go too far and overfill the bowls. I'm know I bent it exactly the same on both carbs but only by feel. I bet the floats are a couple mm higher now...

Cool. Glad you resolved it. Good read, and I like that pic of your bike parked along Soledad Canyon Road. Yeah, that road too often is a speed trap nowadays, commuters avoiding the freeway, fish in a barrel for the tax collectors. The white looks great.