Issues with my Mikuni VM34

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Nobody's said anything about a Norton with Delorto's! I've heard they were the hot ticket back in the day?? Only done to mix things up a bit, lol lol.
 
Hortons Norton said:
Nobody's said anything about a Norton with Delorto's! I've heard they were the hot ticket back in the day?? Only done to mix things up a bit, lol lol.
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My other motor had a pair of 36 Dell'Ortos but it was a bit hard to compare because it was a crappy build. I imagine they would work as well as anything else.
 
The single PWK and manifold set up that I bought from JS Motorsports, worked well in helping me get my old Mk3 850 going after the bike had been neglected for so long. Cj
 
concours said:
Nope.
The engine will rev 7000 in first gear with a tiny carb.
Ask that in fourth, it won’t.
It’s the difference between “free revving”, and making an engine’s “full power”.
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I think we’re agreeing aren’t we?

When mine had a single 36mm Mik it totally ran out of steam around 5k. Yes, it would rev higher, but it was a slow, painful and ultimately fruitless process!

I was very surprised when I rode a friend’s MK3 (stock apart from being fitted with peashooters) at how that revved freely past 7k.
 
baz said:
I have an almost new phoenix SU setup on my shelf
If I ever get around to fitting it I'll post my opinion on here if people aren't too sick of same old same old
Cheers
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Don't bother would be my opinion. I fitted a brand new Pheonix SU to my 850. It started easily, idled well, ran well, & as very economical, but was just the same as a single Mikuni or Amal in that it lost all it's performance once it got to 90mph. If on the other hand you like doing LOTS of miles between fill ups fit an Interstate tank & an SU.
 
Matchless said:
Don't bother would be my opinion. I fitted a brand new Pheonix SU to my 850. It started easily, idled well, ran well, & as very economical, but was just the same as a single Mikuni or Amal in that it lost all it's performance once it got to 90mph. If on the other hand you like doing LOTS of miles between fill ups fit an Interstate tank & an SU.
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Sounds like the perfect carb for me. If you get stopped for doing over 90mph here your going to lose you license. something I don't want to do.
 
Matchless said:
Don't bother would be my opinion. I fitted a brand new Pheonix SU to my 850. It started easily, idled well, ran well, & as very economical, but was just the same as a single Mikuni or Amal in that it lost all it's performance once it got to 90mph. If on the other hand you like doing LOTS of miles between fill ups fit an Interstate tank & an SU.
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I rode 1500 miles alongside an iron head Harley Sportster, fitted with an SU carb. Side by side, used 20% less fuel than my XS1100. Started, ran great. Performance? We just didn’t expect any...

The Mik & Keihin CV carbs were a low cost iteration of the SU.
 
Fast Eddie said:
I think we’re agreeing aren’t we?

When mine had a single 36mm Mik it totally ran out of steam around 5k. Yes, it would rev higher, but it was a slow, painful and ultimately fruitless process!

I was very surprised when I rode a friend’s MK3 (stock apart from being fitted with peashooters) at how that revved freely past 7k.
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If you say so. I don’t want to disect words, but...
If I had a nickle for every time a friend/family member called, said: “can you help me? Car won’t turn over. “
So, I arrive equipped to fix starter/battery/etc.
The failed vehicle cranks over SMARTLY.
So, now I’m checking spark/fuel, with out everything I need.
So, I patiently explain the engine cranks/turns over, but does not fire/start.
“Oh, ok. But why won’t it turn over?” Because, inaccuracies that get repeated, get credibility.
So, the words kinda matter. :cool:
RPM is merely how fast the crankshaft is turning. NO INDICATION of power, unless some other info is included. (Like “in 4th gear, WOT”)

The undersize carb will acheive revs, until it hits the wall of power needed to overcome resistance. In the lower gears, like you said, it will rev, just not pull QUITE as hard at WOT.
The whole single/twin carb has been thrashed to death.
My take is, if two carbs scares/annoys you, then fit a single.
If you have mediocre or better skills, then twins are NP. I’ve never ONCE thought “boy, wish I had only one carb behind that 850 twin.” Ever.
 
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concours said:
I rode 1500 miles alongside an iron head Harley Sportster, fitted with an SU carb. Side by side, used 20% less fuel than my XS1100. Started, ran great. Performance? We just didn’t expect any...

The Mik & Keihin CV carbs were a low cost iteration of the SU.
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In fairness just about any bike is more economical than a standard xs1100 although I did once fit a set of fj 1200 carbs on one and got bit more to the gallon maybe 45mpg can't remember
 
Hortons Norton said:
Sounds like the perfect carb for me. If you get stopped for doing over 90mph here your going to lose you license. something I don't want to do.
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baz said:
In fairness just about any bike is more economical than a standard xs1100 although I did once fit a set of fj 1200 carbs on one and got bit more to the gallon maybe 45mpg can't remember
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lol, true, Wiseco 1196cc big bore kit, Yosh cams, and jetting to suit didn’t help either.
3.2 gallon tank (“Special” model)
But, also alongside was a 1977 XS750C, stone stock, and we all cruised 75 mph on the slab. That SU equipped Sporty with the 2.X peanut tank bested us all, every time. My MPG’s were same as the 750 triple.
 
concours said:
lol, true, Wiseco 1196cc big bore kit, Yosh cams, and jetting to suit didn’t help either.
3.2 gallon tank (“Special” model)
But, also alongside was a 1977 XS750C, stone stock, and we all cruised 75 mph on the slab. That SU equipped Sporty with the 2.X peanut tank bested us all, every time. My MPG’s were same as the 750 triple.
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We used to say "an xs11 could get past most things except a petrol station"
Sorry to fullauto for taking this thread even further off subject
 
concours said:
If you say so. I don’t want to disect words, but...
If I had a nickle for every time a friend/family member called, said: “can you help me? Car won’t turn over. “
So, I arrive equipped to fix starter/battery/etc.
The failed vehicle cranks over SMARTLY.
So, now I’m checking spark/fuel, with out everything I need.
So, I patiently explain the engine cranks/turns over, but does not fire/start.
“Oh, ok. But why won’t it turn over?” Because, inaccuracies that get repeated, get credibility.
So, the words kinda matter. :cool:
RPM is merely how fast the crankshaft is turning. NO INDICATION of power, unless some other info is included. (Like “in 4th gear, WOT”)

The undersize carb will acheive revs, until it hits the wall of power needed to overcome resistance. In the lower gears, like you said, it will rev, just not pull QUITE as hard at WOT.
The whole single/twin carb has been thrashed to death.
My take is, if two carbs scares/annoys you, then fit a single.
If you have mediocre or better skills, then twins are NP. I’ve never ONCE thought “boy, wish I had only one carb behind that 850 twin.” Ever.
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Quite right, I was not referring to the ability of the engine to reach the redline in neutral whilst on the centre stand, I’m sure it could, but can’t see why one would.

But under load in ANY of the 4 gears it had, it simply did not produce the power to create acceleration much above 5k.

Ok up to that point, then it become a gasping wheezing asthmatic.
 
The restrictive manifold is part of the problem.
 
Fast Eddie said:
Yes, that’s my understanding too.
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I think in the past JC had mentioned that also.

One reason some FI bikes have large TB's is the butterfly can become part of the restriction so its diameter is increased for that reason as in twin 61's. (mm)
Not quite the same as a carburettor but some of the old wives tales regarding venturi size are just that.

On one of my bikes (from a stock 52rw to 84rwhp) it went from its stock 32PHF's (Which I had considered refurbishing and fitting to the Norton) to Mallossi 40's to 41mm FCR's.
With the FCR's hp went up, torque went up and fuel consumption went down (top end went from 190kmh to over 230 on a sit up bike)

The point of that is the Mikuni VM34 (My 1976 RM125 MX had one as stock) is old and dare say a relic, it still works but there are more modern options that would not only work more efficiently but would give even better drivability........ the fly in that ointment is the outdated manifold.

One sure sign of old age is loosing ones sense of curiosity or want to learn.
 
I like the simplicity of my single carb , have had no issues period ... could care less which brand name as long as it was this trouble free ... I understand there are those that enjoy getting top performance out of these old bikes , that is really cool .... for me though ,less time spent in shed during ride season is a big plus , I’m lucky with having the 2 Italian twins that have modern day performance ... so the Commando is a cool bike from back in the day , not a rip roaring speed machine ready to take on all comers ... the single vs twin carb thing has been beat to death here many times ... again “we all right and we all wrong” , accept this as a personal choice and move on , Jesh ! ..... would like to point out however the Mikuni thread only 4 pages while Amal troubles thread is pushing 8 pages haha!
 
baz said:
We used to say "an xs11 could get past most things except a petrol station"
Sorry to fullauto for taking this thread even further off subject
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Hey! No problem here. Always good to get some other opinions.
 
Craig said:
I like the simplicity of my single carb , have had no issues period ... could care less which brand name as long as it was this trouble free ... I understand there are those that enjoy getting top performance out of these old bikes , that is really cool .... for me though ,less time spent in shed during ride season is a big plus , I’m lucky with having the 2 Italian twins that have modern day performance ... so the Commando is a cool bike from back in the day , not a rip roaring speed machine ready to take on all comers ... the single vs twin carb thing has been beat to death here many times ... again “we all right and we all wrong” , accept this as a personal choice and move on , Jesh ! ..... would like to point out however the Mikuni thread only 4 pages while Amal troubles thread is pushing 8 pages haha!
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Exactly! Which was what prompted me to post this,
 
....and I never need to mess with my twins.
Set up once, they don’t just “go out of tune”.
 
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