Frustrated Beyond Measure

Fast Eddie said:
I posted this in another thread, but as we’ve ended up talking Mikunis here, I thought I’d copy / paste it…

The single Mik vs twin Amals debate is like oil threads, they come up from time to time and are never resolved! IMHO it’s not about right or wrong, it’s different horses for different courses.

Mikunis are good, well made instruments that meter fuel well. They are also very cheap to buy, especially when only buying one!

As such, a single Mik gives good low to mid rpm running. Being a single carb, the ‘hassle’ of synchronising etc is removed (I used inverted commas there because for many, in fact most, synchronising carbs is not really an issue). The single carb is therefore definitely easier to get the bike idling well, getting good off idle throttle response, etc.

But there’s no question that they reduce performance above 5k ish. Comnoz showed it to be the manifold design that’s the bottleneck rather than the carb itself, hence different sizes of carb make little difference.

Its worth bearing in mind that when folk say there’s a loss of ‘top end‘ performance, it doesn’t just mean top speed, it means performance above 5k in all gears.

So, if you don’t ride above 5k anyway, then a single carb will not reduce your performance at all and of course, if replacing worn out or out of synch carbs, you’ll actually notice an improvement in performance.

However, if you’re someone who enjoys the benefits of revving their Commando to 6k, a single carb is gonna impact your fun factor somewhat.

Some people seem to struggle with the cold start on Mikunis as it’s a bit ON/OFF. I’ve got Mikunis on another Brit and I always start it and ride off ASAP not leaving it to idle, that seems to help avoid hassles when the engine temp is between the ON/OFF settings of the cold start system.
Click to expand...
Cheap enrichener.
For a 3-stage enrichener on a VM, look to the snowmobile world. A few more bucks.
Follow the money.
 
concours said:

Ski-Doo 2005 GSX 380 F OEM Parts Diagram for Carburetor GSX 380F | Partzilla.com

Buy OEM Parts for Ski-Doo 2005 GSX 380 F Carburetor GSX 380F Diagram
www.partzilla.com www.partzilla.com

Item 38

Ski-Doo 2005 GSX 380 F OEM Parts Diagram for Engine And Engine Support 380F | Partzilla.com

Buy OEM Parts for Ski-Doo 2005 GSX 380 F Engine And Engine Support 380F Diagram
www.partzilla.com www.partzilla.com
Click to expand...
I thought you were going to show something like one of these remote primer kits https://www.ebay.co.uk/itm/32229389...gGXShiHQWK&var=&widget_ver=artemis&media=COPY
 
Big_Jim59 said:
Larry S, Did you install your manifold with the phenolic blocks? I hear ya on the blue loctite on the two inner cap screws
Click to expand...
I guess the rubber carb mount would serve the same purpose for this installation?
I've never done one, so just guessing here.

I seem to recall someone who switched back to twin Amals saying the biggest motivation for him was the reduced acceleration performance with the single carb setup, but again, I've no personal experience.
 
B+Bogus said:
I seem to recall someone who switched back to twin Amals saying the biggest motivation for him was the reduced acceleration performance with the single carb setup, but again, I've no personal experience.
Click to expand...
Generally speaking, if trying to accelerate as fast as possible you get near the top of the power band and shift in each gear, Roughly about 5500 rpm (depends on many factors). Since the single carb does not perform as well at high rpms you will not accelerate as fast with one. For normal street riding where the police are not noticing you, you probably won't notice the difference in single and dual.

I suppose the real question is why can't a 34mm carb serving two cylinders (one at a time) flow as much fuel/air as two 32mm carbs (one per cylinder) at high RPMs? Maybe it has nothing to do with the carbs but rather the manifold - I think Jim Comstock determined that (may be remembering wrong).
 
Flip up lever

1698324666395.png



1698325687575.png

Frustrated Beyond Measure
 
Last edited:
Last edited:
marshg246 said:
Generally speaking, if trying to accelerate as fast as possible you get near the top of the power band and shift in each gear, Roughly about 5500 rpm (depends on many factors). Since the single carb does not perform as well at high rpms you will not accelerate as fast with one. For normal street riding where the police are not noticing you, you probably won't notice the difference in single and dual.

I suppose the real question is why can't a 34mm carb serving two cylinders (one at a time) flow as much fuel/air as two 32mm carbs (one per cylinder) at high RPMs? Maybe it has nothing to do with the carbs but rather the manifold - I think Jim Comstock determined that (may be remembering wrong).
Click to expand...
I use a few more revs than this when trying to accelerate quickly.
Peak hp for a stock 850 occurs at 5800.
Using 5500 as a shift point doesn't exploit all available power.
Even 5800 is too low as you have to consider what happens next.
On the first to second shift at 5500 the revs drop to 3200. There isn't a lot of hp there!
With the stock cam on the 850 6200 rpm shifts give my bike the greatest acceleration. Power stays fairly flat to 6200 then falls off above that.
Using 6200 as a shift point gives good oomph at the bottom of that next gear.

I think Cook Nielsen got more straight line acceleration from a stock Commando just about than anyone, other than Norman White, who only weighs 37 lbs!
For the 750 Cook used little higher rpm shift points than I do. That must be correct for the 750 as he managed the famous 12.69 second quarter mile, a ridiculously fast quarter for a Commando. His technique was excellent.

Glen
 
This single Mikuni should work out great for me since I rarely turn my Norton past 5K. A single carb should offer better low RPM performance while sacrificing some top speed. Personally, if I was hung up on top speed I would have kept my Honda VTR.
 
worntorn said:
I use a few more revs than this when trying to accelerate quickly.
Peak hp for a stock 850 occurs at 5800.
Using 5500 as a shift point doesn't exploit all available power.
Even 5800 is too low as you have to consider what happens next.
On the first to second shift at 5500 the revs drop to 3200. There isn't a lot of hp there!
With the stock cam on the 850 6200 rpm shifts give my bike the greatest acceleration. Power stays fairly flat to 6200 then falls off above that.
Using 6200 as a shift point gives good oomph at the bottom of that next gear.

I think Cook Nielsen got more straight line acceleration from a stock Commando just about than anyone, other than Norman White, who only weighs 37 lbs!
For the 750 Cook used little higher rpm shift points than I do. That must be correct for the 750 as he managed the famous 12.69 second quarter mile, a ridiculously fast quarter for a Commando. His technique was excellent.

Glen
Click to expand...
No argument, that's why I said: "Roughly 5500 rpm (depends on many factors)" I have no idea what primary drive or front final drive sprocket is in use, what cam is in use, what compression, what ignition timing, what altitude, what humidity level, what gas, what gearset, and so on.

Cool: "Norman White, who only weighs 37 lbs!" My arm weight more that that! :D
 
worntorn said:
I use a few more revs than this when trying to accelerate quickly.
Peak hp for a stock 850 occurs at 5800.
Using 5500 as a shift point doesn't exploit all available power.
Even 5800 is too low as you have to consider what happens next.
On the first to second shift at 5500 the revs drop to 3200. There isn't a lot of hp there!
With the stock cam on the 850 6200 rpm shifts give my bike the greatest acceleration. Power stays fairly flat to 6200 then falls off above that.
Using 6200 as a shift point gives good oomph at the bottom of that next gear.

I think Cook Nielsen got more straight line acceleration from a stock Commando just about than anyone, other than Norman White, who only weighs 37 lbs!
For the 750 Cook used little higher rpm shift points than I do. That must be correct for the 750 as he managed the famous 12.69 second quarter mile, a ridiculously fast quarter for a Commando. His technique was excellent.

Glen
Click to expand...
100% agree Glen.

Also agree that the acceleration and exhaust music between 5 and 6 & a bit is sweet, and addictive, and I ain’t never giving that up for no stinkin’ single carb !!
 
Hey, hold on. I'm the only one here that weighs 37 pounds....
Does Norman really weigh in at 137 pounds road ready? I've got him beat if so.
 
Fast Eddie said:
100% agree Glen.

Also agree that the acceleration and exhaust music between 5 and 6 & a bit is sweet, and addictive, and I ain’t never giving that up for no stinkin’ single carb !!
Click to expand...
You need to ride mine Eddie not all single carbs are equal
Mine starts to really wake up at 6000rpm and will rev beyond 7500 if you let it
 
baz said:
You need to ride mine Eddie not all single carbs are equal
Mine starts to really wake up at 6000rpm and will rev beyond 7500 if you let it
Click to expand...
I'm sure with the right carb(s) or fuel injection, turbo or super charging, case strengthening, crank strengthening, rod strengthening, pushrod strengthening, gearbox strengthening, primary strengthening, valve springs, fairings, and so on, 14000 RPMs would be possible in 4th gear. Hell, throw in water cooling, nitrous and run on methanol, maybe hundreds of horsepower at 15000 RPM would be possible. Why not throw all that away and buy a surplus military jet engine? Then ride on the street lugging along at 4000 RPM!

Sorry, just had to :D
 
baz said:
You need to ride mine Eddie not all single carbs are equal
Mine starts to really wake up at 6000rpm and will rev beyond 7500 if you let it
Click to expand...
Yeah I know Baz. But yours ain’t normal is it !?And certainly ain’t relevant to the discussion about VMs on restrictive manifolds !
 
  • Haha
Reactions: baz
marshg246 said:
I'm sure with the right carb(s) or fuel injection, turbo or super charging, case strengthening, crank strengthening, rod strengthening, pushrod strengthening, gearbox strengthening, primary strengthening, valve springs, fairings, and so on, 14000 RPMs would be possible in 4th gear. Hell, throw in water cooling, nitrous and run on methanol, maybe hundreds of horsepower at 15000 RPM would be possible. Why not throw all that away and buy a surplus military jet engine? Then ride on the street lugging along at 4000 RPM!

Sorry, just had to :D
Click to expand...
Not in my case
It's a stock 750 engine with a big carb
Sorry 😃
 
You must log in or register to reply here.

Similar threads

yooper trooper
Can't get bike running right!!
2 3
Replies
42
Views
1,223
Fast Eddie
Fast Eddie
Big_Jim59
Read My Plugs
2
Replies
26
Views
1,225
acadian
acadian
Big_Jim59
Tuning Issues
2
Replies
23
Views
1,059
Big_Jim59
Big_Jim59
Ekvin
1969 Commando 750 will not start
Replies
15
Views
434
Stevo
Stevo
Big_Jim59
Debby's 1974 850 Norton Commando
Replies
0
Views
115
Big_Jim59
Big_Jim59
Back
Top