Dyno run (2017)

Status
Not open for further replies.
SteveA said:
Anybody else interested in the air jet info might want to read this that I have received from a classic race only forum.....Makes interesting reading for me:

'Technically, the air jet is only really functional at high engine speeds and large throttle openings. It has the effect of changing the slope of the fuel curve - the carbs tend to richen up as rpm increases for the same throttle opening. The air jet can compensate for this - a small jet will make the jetting richer at the higher rpm (bigger jet make things leaner), though the changes with air jet size are not linear. There will be different combinations of air jet and main jet that produce the same apparent mixture strength, though peak power may not be the same (as a result of different fuel droplet size): this can be useful as a cooling effect on some engine.

Typical 4 stroke applications can be confusing: 26-28mm carbs often like a 0.5, 30-34mm around 1.0 and 36-38mm back to 0.5'

No mention of the 2.0 size these carbs are often supplied with for Commando application! And somewhat in line with Victory Library VM manual reccommendations for air jets.
Click to expand...

Great stuff, Steve!
 
Short answer to your earlier question Ken: Nothing.

If you don’t go above 3k rpm, twin carbs will do nothing for yer.
 
Steve,
I suppose if your motor is in a Commando frame you will have set the isolastics fairly tight for track use, wonder what head steady you use ?
 
oldmikew said:
Steve,
I suppose if your motor is in a Commando frame you will have set the isolastics fairly tight for track use, wonder what head steady you use ?
Click to expand...

There is no wiggle in my back wheel, but it is smooth as a baby's bum. Head steady by Jim Comstock, available from CNW or Jim. The best design I have found and really makes a difference.
 
Doug MacRae uses a Commando frame for racing and has tricked up the isolastics. Kenny Cummins uses a Seeley frame. It would be OK for road use as long as you did not want to ride it slowly because the crank balance factor is usually very different when there are no isolastics.
 
acotrel said:
Doug MacRae uses a Commando frame for racing and has tricked up the isolastics. Kenny Cummins uses a Seeley frame. It would be OK for road use as long as you did not want to ride it slowly because the crank balance factor is usually very different when there are no isolastics.
Click to expand...
Yes the balance factor is very different for a commando frame, they have been raced mind, but would have thought the wheel base was really too long, speaking as someone who had both front and back wheels break away and then sliding across the road without coming off , would say that is not necessarily a bad thing. What however I was thinking about was that the commando head moves arond and whilst the carbs are not getting 'vibration' they might in the case of float bowl trouble appreciate some damping.. Why i asked Steve what head steady he was using for they seem to vary in the movements they permit
 
Last edited:
oldmikew said:
Yes the balance factor is very different for a commando frame, they have been raced mind, but would have thought the wheel base was really too long, speaking as someone who had both front and back wheels break away and then sliding across the road without coming off , would say that is not necessarily a bad thing. What however I was thinking about was that the commando head moves arond and whilst the carbs are not getting 'vibration' they might in the case of float bowl trouble appreciate some damping.. Why i asked Steve what head steady he was using for they seem to vary in the movements they permit
Click to expand...

My carbs are additionaly secured to reduce movement!

I have raced a Commando, never noticed any carb issues in the whole season I did!
 
Steve,
My thoughts about this were set in train by wondering what effect the rubber connectors between the airbox and the carb mouth has? presumably as they are ,or used to be very stiff, there must some damping of movement of the carbs themselves which possibly had a stabalizing effect that my airbox free examples mounted on flexible hosing lacked ! But this particular base you have well and truly covered
 
Usually carburettors are more affected by vibration when they are secured away from the motor, or touch the frame
 
oldmikew said:
Steve,
My thoughts about this were set in train by wondering what effect the rubber connectors between the airbox and the carb mouth has? presumably as they are ,or used to be very stiff, there must some damping of movement of the carbs themselves which possibly had a stabalizing effect that my airbox free examples mounted on flexible hosing lacked ! But this particular base you have well and truly covered
Click to expand...

Commando Amal Mk1s are rigidly bolted to the engine. Mk2s are on rubber sleeves, as are Mikunis. Left to their own devices they will wobble around, typically giving the rubber sleeves a short life!

Alan is right to suggest you don't want them touching the frame, but with bellmouths and an open inlet, you don't want them wagging around either, so they need support to prevent too much movement, but not hard mounting to the frame, I hold them together with strips of old inner tube! and support teach one with a piece looped around the frame! Not that you can see them in this photo!
 

Attachments

  • Dyno run (2017)
    22222038_10155504477956628_1791390815939147409_n.webp
    100.4 KB · Views: 415
SteveA said:
Commando Amal Mk1s are rigidly bolted to the engine. Mk2s are on rubber sleeves, as are Mikunis. Left to their own devices they will wobble around, typically giving the rubber sleeves a short life!

Alan is right to suggest you don't want them touching the frame, but with bellmouths and an open inlet, you don't want them wagging around either, so they need support to prevent too much movement, but not hard mounting to the frame, I hold them together with strips of old inner tube! and support teach one with a piece looped around the frame! Not that you can see them in this photo!
Click to expand...
 
Yes that is so , but I mounted my mk1s on home made manifolds connected by radiator hose and jubilee clips, advantages being a heat break and a steeper downdraught angle. Not as radical as the mk1 angle used on the F750 mind.. I did think of having an extended rubber or soft bell mouth , but to have worked it would have needed to be a multiple of the 'tuned' inlet manifold length.. so that is a couple of feet.. certainly if attached to a rigid point would provide some damping but wherever it had finished up on the bike it would, apart from looking monstrous , the outlet would be in a low pressure point. There is a lot of dead air inside a fairing and inits wake. So the ideas was shelved .. Ah those wasted hours of the 70s.

Inner tubes do seem a good way to go to prevent spurious movements, and thanks for posting the photo.. Nice Kit.. Amal tell me their MK 2 is ok up to 35degree downdraught . But you seem to be running that with your Mikunis , thought they needed less
 
oldmikew said:
.....

Inner tubes do seem a good way to go to prevent spurious movements, and thanks for posting the photo.. Nice Kit.. Amal tell me their MK 2 is ok up to 35degree downdraught . But you seem to be running that with your Mikunis , thought they needed less
Click to expand...

It is less when I am sat on it
 
When I bought my MK2 Amals from Mick Hemmings they were kitted for methanol. The mounting rubbers did not last very long - cracked in a very short time. I replaced them without specifying which fuel I was using and have not had any more trouble. If your motor is shaking and you have rubber mounts, the more you steady the carburettors away from the motor, the more subject to vibration they become. The situation is worse with the standard non-flexible set-up. Then the carburettors cop everything the motor sends their way.
 
What I mentioned about the carburettors touching the frame is very important. If you look around your carbs and can see any spots on the frame which have been hammered, you need to fix that. The vibration can cause flooding and with carburation, the slightest bit too rich and the engine becomes sluggish, even though it might not be very obvious to the rider. It is like the difference which occurs when you turn into a slight head-wind. If you know your race circuit very well, you can detect it - you just don't get there as quick. I don't use rubber strips under the bellmouths to support the carbs - the mounts by themselves give enough support.
 
So what horrendous anomalies have you observed so far ken

I find a std 850 34mm single carb comes on quite nicely at 3k starts to go flat about 5k and dead at 6k ,I don't want to go flat out in a straight line and putter down the highways about 65mph, but in the country on twisty road using the large selection of gears available to me 3 to 5 1/2 is plenty for me, still manage to scare the crap out of my self on enough occasions
 
Status
Not open for further replies.

Similar threads

Joachim
  • Locked
Dyno runs before and after engine mods
2 3 4
Replies
69
Views
4,960
SteveA
SteveA
E
  • Locked
850 Dyno Run
Replies
13
Views
2,487
Fullauto
F
iwilson
Dominator dyno run
2 3
Replies
56
Views
6,287
iwilson
iwilson
B
  • Locked
Norton 750 Commando Free diagnostic dyno run
Replies
5
Views
1,214
cjandme
C
Doug MacRae
  • Locked
Short Stroke 750 Dyno Run
Replies
12
Views
1,769
X-file
X
Back
Top