Frustrated Beyond Measure

baz said:
Not in my case
It's a stock 750 engine with a big carb
Sorry 😃
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Honest question - no shade - just interested: OK, stock 750, not Combat. What is the high point of your power curve and how did a bigger carb with no other change allow that power-curve top end and how often on the street do you come out of the bottom three gears at 7500 RPM or whatever the top end of you power curve is?
 
marshg246 said:
Honest question - no shade - just interested: OK, stock 750, not Combat. What is the high point of your power curve and how did a bigger carb with no other change allow that power-curve top end and how often on the street do you come out of the bottom three gears at 7500 RPM or whatever the top end of you power curve is?
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I'm guessing peak power is just over 6000 rpm
When I first fitted the mikuni TM 40 I was shocked at how much it improved performance over the standard setup especially in the 4-6500rpm range
One thing I hadn't noticed first of all was it was like running into a wall just under 7000 rpm
I tried going up on the main jet and it made no difference
I thought it was the inlet manifold so I made a new one
This gave a general improvement but it still wouldn't rev above 7000
So I went up a size on the needle jet and this was it
I revved it to 7500 and it was still revving on!
But not wanting to ventilate my crank cases I backed off
I tried this a couple of times to make sure all was well
So in answer I never hit 7500 on the street but I'm very often above 6000 in the lower gears
 
baz said:
I'm guessing peak power is just over 6000 rpm
When I first fitted the mikuni TM 40 I was shocked at how much it improved performance over the standard setup especially in the 4-6500rpm range
One thing I hadn't noticed first of all was it was like running into a wall just under 7000 rpm
I tried going up on the main jet and it made no difference
I thought it was the inlet manifold so I made a new one
This gave a general improvement but it still wouldn't rev above 7000
So I went up a size on the needle jet and this was it
I revved it to 7500 and it was still revving on!
But not wanting to ventilate my crank cases I backed off
I tried this a couple of times to make sure all was well
So in answer I never hit 7500 on the street but I'm very often above 6000 in the lower gears
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Thanks, weird though.

The more or less general consensus is that the real problem with single carbs is the intake not flowing as well with lots of explanations as to why. Another is that people fit a too large single carb not realizing that it's only feeding one cylinder at a time. You seem to have disproven both so, any idea how neither of those is the case? Or, what is the case?

Then there is fitting a larger needle jet. All AMAL MK1 and MKII and the Mikuni's that people normally fit would cause me to call BS since the needle/needle jet not involved past above 3/4 throttle. I knew nothing about the TM 40 so I read their manual and tuning info. Tuning might as well have been written by AMAL for the MK1. I see how the midrange is improved and of course an accelerator pump would be helpful for pickup so lets say tentative BS - must be either a coincidence and you did something else, or you're not needing to go above 3/4 throttle or what?

Can't argue with success, so I really want to understand. Even though I'm completely comfortable with sync'ing dual carbs I make recommendations to others and many would find life easier with a single carb no matter which manufacturer or model.
 
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marshg246 said:
Thanks, weird though.

The more or less general consensus is that the real problem with single carbs is the intake not flowing as well with lots of explanations as to why. Another is that people fit a too large single carb not realizing that it's only feeding one cylinder at a time. You seem to have disproven both so, any idea how neither of those is the case? Or, what is the case?

Then there is fitting a larger needle jet. All AMAL MK1 and MKII and the Mikuni's that people normally fit would cause me to call BS since the needle/needle jet not involved past above 3/4 throttle. I knew nothing about the TM 40 so I read their manual and tuning info. Tuning might as well have been written by AMAL for the MK1. I see how the midrange is improved and of course an accelerator pump would be helpful for pickup so lets say tentative BS - must be either a coincidence and you did something else, or you're not needing to go above 3/4 throttle or what?

Can't argue with success, so I really want to understand. Even those I'm completely comfortable with sync'ing dual carbs I make recommendations to others and many would find life easier with a single carb not matter which manufacturer or model.
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The previous owner of my bike had cut the gusset out of the frame that usually hinders larger carbs
I have had a single MK1 32 mm a CV 32 mikuni
A VM 34 mikuni and a VM 36 mikuni
Plus the standard twin mk1s
The TM is light years away from the other single setups I've tried
I believe the flat slide mixes the fuel better even though the gas flow through a 1-2 is hideous
The accelerator pump gives you a dab when you need it
Just makes the bike more rideable for me at least
 

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I got my carb kit. I am taking my time. I pulled the new carb apart and checked the jetting. It came with a few jets so I opted for the 270 main and the 25 pilot. Needle is set in the middle of 5 positions. I made notes of exactly what's in it but those notes are out in the shop. I got the carb fitted working to make sure it was truly seated in the rubber mount. I played with optimum cable routing and managed to get enough clearance for a solid click from the slide slamming home. The air cleaner is a K&N type with double the amount of filter of the afore mentioned pancake filter. I got it sprayed with air filter oil. I am currently working on the fuel lines but I can take my time. It's raining. It's raining hard. In the Texas vernacular "It's raining like cow piss on a flat rock!" It has been bone dry all summer and now this! It's supposed to do this all weekend.
 
People like to tell me that twin 36mm carbs don't work either!

Apparently, 'they are too big', in the same way that 'a Rickman frame is too stiff!'

On an 850 with an ex works short stroke head and big valves I have used twin 36mm Mk1 Concentrics, but I changed these for 34mm MkII Concentrics, and then quickly had these bored to 36mm to get the top end back! (added, if unintended, bonus being that the 34mm MkII is a smaller body than the 36mm MkII)

On my 750 short stroke with a standard Fullauto (Australia) head I have used 34mm Mikunis and 36mm Mikunis, I won't say the 34s didn't work, but it's the bigger carbs that deliver best top end for me with no lack of pickup or acceleration.

Apparently '750 short strokes lack torque', something I haven't noticed. Of other myths like, 'you will need twin discs to stop it', 'a 90 wide front tyre will lock up under braking' and 'a 110 rear tyre turns faster than a 130', I will accept the potential of a 110 rear tyre, though I would choose a 120 if someone made them.
 
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Big_Jim59 said:
.................. It's raining. It's raining hard. In the Texas vernacular "It's raining like cow piss on a flat rock!" It has been bone dry all summer and now this! It's supposed to do this all weekend.
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I live in Normandy, it's raining. It often is. Such that the cows tend to piss on wet mud! Even in summer.

What we probably do have in common with Texas is the ratio of cows to humans!
 
SteveA said:
People like to tell me that twin 36mm carbs don't work either!

Apparently, 'they are too big', in the same way that 'a Rickman frame is too stiff!'

On an 850 with an ex works shorts stroke head and big valves I have used twin 36mm Mk1 Concentrics, but I changed these for 34mm MkII Concentrics, and then quickly had these bored to 36mm to get the top end back! (added, if unintended, bonus being that the 34mm MkII is a smaller body than the 36mm MkII)

On my 750 short stroke with a standard Fullauto (Australia) head I have used 34mm Mikunis and 36mm Mikunis, I won't say the 34s didn't work, but it's the bigger carbs that deliver best top end for me with no lack of pickup or acceleration.

Apparently '750 short strokes also lack torque, something I also haven't noticed. Of other myths like, 'you will need twin discs to stop it', 'a 90 wide front tyre will lock up under braking' and 'a 110 rear tyre turns faster than a 130', I will accept the potential of a 110 rear tyre, though I would choose a 120 if someone made them.
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Ducati were making 40mm carbs work pretty well back in 1972....
I was once told my twin disc T140 was 'overbraked'. Didn't know such a condition existed!
I do know a racer who has stuck to a 110 rear after trying all the others, but I think it's more to do with it working better for him on road circuits, even though the opposition is running 130 cantilevers.
 
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B+Bogus said:
I was once told my twin disc T140 was 'overbraked'. Didn't know such a condition existed!
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Agreed, no such thing. But a guy needed some work on his dual Amal MK2 equipped late T140 with dual front discs. I took it out for a test ride. My right hand forgot it was not on a Norton - I don't like stoppies at my age! Needless to say, it has all the braking you need with some to spare :)
 
I think BMW holds the title for over braking when they introduced boosted brakes a few years back - begs the question why when modern un boosted systems on motorcycles can be locked up with two fingers.
 
Richard Tool said:
I think BMW holds the title for over braking when they introduced boosted brakes a few years back - begs the question why when modern un boosted systems on motorcycles can be locked up with two fingers.
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Yup. That was ONE of the deal-breakers when I test rode an RT. That, and the disposable final drive units. The brakes reminded me of the late 60's large american sedans, overboosted brakes.
 
B+Bogus said:
Ducati were making 40mm carbs work pretty well back in 1972....
I was once told my twin disc T140 was 'overbraked'. Didn't know such a condition existed!
I do know a racer who has stuck to a 110 rear after trying all the others, but I think it's more to do with it working better for him on road circuits, even though the opposition is running 130 cantilevers.
Click to expand...
You can have too much rear brake, a big danger in a corner
 
Richard Tool said:
I think BMW holds the title for over braking when they introduced boosted brakes a few years back - begs the question why when modern un boosted systems on motorcycles can be locked up with two fingers.
Click to expand...
Back when I had moved on from the bike shop I had dropped by for a visit and the owner threw me the keys to a new K100RS and said "try that out." You have to understand that my daily ride was a 1976 BMW R90/6 with many cosmetic improvements. It had a front disk but it was operated by a master cylinder located under the tank (I guess they were afraid it was going to get wet) and that was actuated by a cable from the lever. To say that it was wooden and had no feel is an understatement. I hopped on the K bike, roared out of the parking lot and upon approaching the first corner I grabbed a handful of front brake as I always did. I found myself face to face with the front fender and practically standing on my head. I actually did it twice more before I got the hang of two finger braking.

In other news my single Mikuni is installed. It started second kick on the enricher. I had set the air screw at 2.5 turns out and had to bring it in about .75 turns to get a good tick over. The rain is supposed to let up next week so I will be able to road test it then.
 
Big_Jim59 said:
In other news my single Mikuni is installed. It started second kick on the enricher. I had set the air screw at 2.5 turns out and had to bring it in about .75 turns to get a good tick over. The rain is supposed to let up next week so I will be able to road test it then.
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I’ve got mine set at 1.5 turns and it runs well there. 34mm
 
SteveA said:
I live in Normandy, it's raining. It often is. Such that the cows tend to piss on wet mud! Even in summer.

What we probably do have in common with Texas is the ratio of cows to humans!
Click to expand...
The "flat rock" in indicative of the types of pasture we have available and the type of vegetation found there. In most parts of Texas it takes 5 to 15 acres to support a cow and her calf and that depends on the rain fall. Compared to Normandy I am sure we would look like semi-desert.
 
SteveA said:
People like to tell me that twin 36mm carbs don't work either!

Apparently, 'they are too big', in the same way that 'a Rickman frame is too stiff!'
Click to expand...
I had a 2005 Honda VTR that was one of the most fun bikes I have ever owned. It was 1000cc and sported a pair if 40MM Mikunis. Let me tell you, it worked very well! It also had an enormous air box under the tank that limited the amount of fuel it could carry. It makes me wonder if the lack of top end performance for single carb Nortons can't be attributed to the tiny air filters supplied with such items?
 
Big_Jim59 said:
I had a 2005 Honda VTR that was one of the most fun bikes I have ever owned. It was 1000cc and sported a pair if 40MM Mikunis. Let me tell you, it worked very well! It also had an enormous air box under the tank that limited the amount of fuel it could carry. It makes me wonder if the lack of top end performance for single carb Nortons can't be attributed to the tiny air filters supplied with such items?
Click to expand...
The limiting factor is the manifold. You could remove the filter altogether, but the limiting factor is still the manifold.
 
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