Pros and cons of different carbs? Keihin vs Mikuni vs Amals?

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robs ss said:
Thanks for that
Would be even better if you told us about your bike (750/850, anything done to engine (mods)?)
Cheers
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I used BrianK's settings on an 850, fullauto head and PW3 cam, standard comp ratio. The settings are spot on, fuel economy is up there with a single Mikuni and performance is great.
 
wavey_davey said:
Reading this post I'm reminded of an article in an early /mid seventies Motorcycle Mechanics magazine on Mick Hemmings tuning advice for Commandos , based on his proddy racer . IIRC there was a commando engine on the cover ? Anyway , he recommended and sold modified inlet manifolds that included a length of car radiator hose . Presumably this benefitted heat and vibration insulation as well as allowing inlet tract length of your choice (within reason )?
Given the number of modified components around these days compared to back then , surprised someone doesn't still make them given relative simplicity ? "Just about every carb that followed the amal Mk1 was rubber mounted .
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As far as I know they are available
Or they were about 15 years ago when I bought a set
But they are easy to make from the standard inlet manifolds if you have a lathe
 
gortnipper said:
Please elaborate
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That high mileage FCR tune did not work well on my current Norton motor configuration. It was too lean everywhere. The motor ran hot. Starts and warm up were not great because it was too lean a needle position, and not enough low speed fuel, and too much low speed air. (Low speed fuel and low speed air are fully adjustable separately on the FCRs.) It misfired and took a few minutes to get to a smooth idle without throttle input. Acceleration was mediocre and top end not that great. Mileage was good in the low 60's, but only 2.5 miles better than the richer tune I run now. Plugs were colorless ashen white, not that that is necessarily bad with today's pump gas, but they are light tan now and the motor does not run hotter than it should.

The high mileage tune did work with "standard compression" Commando pistons, Norton rods, Norton lifters, 2S cam, shaved Fred Barlow ported head. Although, I left the mains at 152. With the "higher compression" pistons, JS2SS cam, longer rods, BSA lifters, and same Fred Barlow head I tried that tune with 142 mains and that was limp. With the 152 mains and needle in the 3rd position from the top with a needle shim, it was ridable around town between traffic lights under 35mph, but not as smooth running as it is now, mid-range was weak and did not pull hard, and it was never really good right off idle without slipping the clutch a little. I put up with that for a 200 miles.

The motor is great now for a little 750 twin Norton since getting away from the high mileage tune spec. Starts easy, goes into a smooth idle after a minute or two of holding the idle high with the throttle, and pulls strong everywhere.

That said my engine is not canted forward. It is a Norton engine though. So, my results may or may not be relevant for Commando owners. I would still watch the plugs and pay close attention to off idle, mid-range pull, and top end performance if I had a "high compression" big cam canted forward engine given what I experienced.

Your motor would more than likely work with that tune. I would be leery of 140 mains in a 920 though unless FF or others are getting away with it and know a smaller main is an improvement from the 152 mains. The high milage tune would/could certainly work on a stock engine and many of the mildly improved motors people here have in their Commandos. I'd watch for any hiccups on stronger builds myself. I also would try the tune that comes with the kit FCRs before trying the leaner tune on a "high compression" engine with a PW3 or larger cam.

I doubt the above drivel will go over well here, and I don't expect anyone to believe what I say. Afterall, I don't do track days, have old Norton race stories, sell Norton parts, restore vintage British motorcycles to sell, rebuild and port Norton heads, work on Nortons for a living, or most importantly still own a Commando. I can tell when a tune works on the motorcycles I've owned though. YMMV

Everybody should do what they've gotta do and find out what works for themselves. That is what I do. I probably wouldn't listen to me either. lol
 
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robs ss said:
Thanks for that
Would be even better if you told us about your bike (750/850, anything done to engine (mods)?)
Cheers
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Yes, please tell us about your engine Brian. That is much leaner than I’d have expected.

Mine was perfect on the Dyno as an 850, 10.5:1 CR, JS#1 cam.

I had to fit a richer needle compared to how supplied (no doubt due to my nine standard engine bits). The as supplied 152 mains were cock on.

I’ve not had it on the Dyno yet in it’s current 920 guise, the 152s felt good. I tried richer (160) but that was too rich. Now trying 155s this weekend.
 
Fast Eddie said:
Yes, please tell us about your engine Brian. That is much leaner than I’d have expected.

Mine was perfect on the Dyno as an 850, 10.5:1 CR, JS#1 cam.

I had to fit a richer needle compared to how supplied (no doubt due to my nine standard engine bits). The as supplied 152 mains were cock on.

I’ve not had it on the Dyno yet in it’s current 920 guise, the 152s felt good. I tried richer (160) but that was too rich. Now trying 155s this weekend.
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Did the 850 dyno perfect with the high mileage tune or with the standard FCR jetting and setup?

I found the standard FCR jetting and what I believe to be close to the standard slow fuel (1 1/8th turns out) and slow air (1 1/2 turns out) adjustments as well as the needle in the 5th clip position down from the top to be more than satisfactory with the beefier than stock built 750 Norton engine, but not a Commando engine orientation.

Good luck with the 920.
 
Schwany said:
Did the 850 dyno perfect with the high mileage tune or with the standard FCR jetting and setup?

I found the standard FCR jetting and what I believe to be close to the standard slow fuel (1 1/8th turns out) and slow air (1 1/2 turns out) adjustments as well as the needle in the 5th clip position down from the top to be more than satisfactory with the beefier than stock built 750 Norton engine, but not a Commando engine orientation.

Good luck with the 920.
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I mean standard as they came from Matt, which I assume is his settings for a standard motor.
 
Fast Eddie said:
I mean standard as they came from Matt, which I assume is his settings for a standard motor.
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Thanks

Agree about coming with standard settings, although they do have a sticker on the side that says Motorcycle Race Use Only, so they might be set up a little on the rich side for a bone stock 750/850 that was ridden mostly around town or at the speed limit by a mature citizen. I would not know though. Not mature enough. What did they put the twist throttles on there for if not to twist them? ;)
 
Schwany said:
Thanks

Agree about coming with standard settings, although they do have a sticker on the side that says Motorcycle Race Use Only, so they might be set up a little on the rich side for a bone stock 750/850 that was ridden mostly around town or at the speed limit by a mature citizen. I would not know though. Not mature enough. What did they put the twist throttles on there for if not to twist them? ;)
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Settings out of the box may have changed, i have had my FCR's for 10 years, but out of the box mine were crazy rich for street use. 35MPG (Her Majesty's large gallons) at first fit. The only time i have run out of fuel on my Interstate was the first test ride with FCR's.:eek:
 
SteveA said:
Not going anywhere near FCRs......one I can't afford them, two, they are not eligible for classic racing.
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Not sure about Eupope but the Keihin Smoothbore CRS carbs are legal for vintage racing in the US. Being smooth bore the CRS carbs produce just as much HP as the FCRs at WOT.
 
ntst8 said:
Settings out of the box may have changed, i have had my FCR's for 10 years, but out of the box mine were crazy rich for street use. 35MPG (Her Majesty's large gallons) at first fit. The only time i have run out of fuel on my Interstate was the first test ride with FCR's.:eek:
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Rough day running out of fuel. 35mpg would be a disappointment.

With my previous engine configuration and the high mileage tune best day staying close to the speed limit was 63 MPG and averaged about 60 riding a little quicker. It's definitely down from that now with the new motor configuration.

OCEMR needle, 152 main, other hard parts jetting in my FCR set is as BrianK listed.

I do have a tuner's advantage in that my 2 into 1 exhaust is tunable. That does have something to do with why mine works with a richer tune. It actually does run really well.

I did some things with the intake manifolds and carburetors that probably help make it all work as well, but they are irrelevant to Commando owners.
 
jseng1 said:
Not sure about Eupope but the Keihin Smoothbore CRS carbs are legal for vintage racing in the US. Being smooth bore the CRS carbs produce just as much HP as the FCRs at WOT.
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Yeah, don't the FCRs put an old Norton in Formula 1 in the USA? Not exactly a competitive class unless everyone riding in Formula 1 is an over weight barely in shape 70 something having some weekend warrior fun. Too Harsh? I hope they don't get pushed up into open unlimited because of an accelerator pump. That would be nuts.
 
My Commando came with a single Mikuni conversion. I had nothing but problems with it. Hard starting was the main irritant. I even sent it out to a carb expert to have it gone through. Never did work right for me. Finally, I replaced it with a pair of Amal Premiers. Best move I ever made. The bike starts easy, never needs choke (didn’t even hook it up), and idles and runs perfectly. It even produced more power that I didn’t even know I had. Plus, it looks stock. I’ll never go back. Just my .02 cents worth.
 
Bonwit said:
My Commando came with a single Mikuni conversion. I had nothing but problems with it. Hard starting was the main irritant. I even sent it out to a carb expert to have it gone through. Never did work right for me. Finally, I replaced it with a pair of Amal Premiers. Best move I ever made. The bike starts easy, never needs choke (didn’t even hook it up), and idles and runs perfectly. It even produced more power that I didn’t even know I had. Plus, it looks stock. I’ll never go back. Just my .02 cents
Hard starting can be caused alot by perished /cracked manifold rubbers which is common even with Mk 11 Amals
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robs ss said:
Thanks for that
Would be even better if you told us about your bike (750/850, anything done to engine (mods)?)
Cheers
Click to expand...
Sorry, stock 1973 850 (one of first 500 from s/n) I have big-arse K&N oiled-paper filters on the FCRs and some aftermarket pipes (nothing radical, peashooter replicas from I don't remember where although maybe somewhat less restrictive), first oversize (20 thou I think) pistons and rings, Trispark ignition, that's about it, otherwise fairly stock. Folks here know my bike from all the help I've sought AND RECEIVED over lo these many moons so anyone who remembers better may chime in.

I don't thrash the thing but I've never had a hint of overheating/overstress. Plugs, not that you can tell much from them these days (other than STOP NOW) look good to me.

I hope this post isn't dated the day before I post about melting down my engine and needing rebuild advice....(if I think it, it can't happen...)
 
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BrianK said:
Sorry, stock 1973 850 (one of first 500 from s/n) I have big-arse K&N oiled-paper filters on the FCRs and some aftermarket pipes (nothing radical, peashooter replicas from I don't remember where although maybe somewhat less restrictive), first oversize (20 thou I think) pistons and rings, Trispark ignition, that's about it, otherwise fairly stock. Folks here know my bike from all the help I've sought AND RECEIVED over lo these many moons so anyone who remembers better may chime in.

I don't thrash the thing but I've never had a hint of overheating/overstress. Plugs, not that you can tell much from them these days (other than STOP NOW) look good to me.

I hope this post isn't dated the day before I post about melting down my engine and needing rebuild advice....(if I think it, it can't happen...)
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Thanks Brian - based on your info I will have a further play with mine, which are a little rich (better than the alternative!)
Cheers
Rob
 
jseng1 said:
Not sure about Eupope but the Keihin Smoothbore CRS carbs are legal for vintage racing in the US. Being smooth bore the CRS carbs produce just as much HP as the FCRs at WOT.
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Same in Europe Jim.

Is that claim of yours backed up by back to back Dyno testing, or is it an assumption?
 
jseng1 said:
Not sure about Eupope but the Keihin Smoothbore CRS carbs are legal for vintage racing in the US. Being smooth bore the CRS carbs produce just as much HP as the FCRs at WOT.
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Main issue here is no Flat slides on Pre '73 classics, so I would be wrong regarding round slide carbs!

Fast Eddie's pictures show a Flat slide version.

And I already run a pair of 36mms on a Shorts Stroke 750, in a vertical mounting with a very long inlet manifold. I might be reluctant to go to a 33mm carb on a shorter straight inlet without a lot of evidence of good HP.

Nor am I sure out of the box jetting would be final jetting.

Still can't afford them! :oops:
 
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Fast Eddie said:
Same in Europe Jim.

Is that claim of yours backed up by back to back Dyno testing, or is it an assumption?
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Look at the smooth bore Keihin in the photo. No obstuction - just a round smooth hole with optimum flow. Its the same with both the CRS and the FCR. The only way to improve flow is to get rid of the needle. Both also come with velocity stacks etc. The CRS is available in 31 33 35 37 39mm etc. The FCR starts at 35mm. The increase in flow of the both the CRS and FCR smooth bores gives an increase in WOT performance over the regular Amals. Its a noticable performance increase. The FCR accelerator pump gives a partial throttle roll on surge advantage but it also increases fuel consumption.


Pros and cons of different carbs? Keihin vs Mikuni vs Amals?
 
I'm like fastEd, I'd like to see some OBJECTIVE Dyno tests re all this. Everybody has a story about how they did [whatever] and they could "feel the power increase." I've done tests on the dyno and on the track where someone said they could feel a power surge and it turned out the change reduced power/slowed ET/trap speed but the vehicle 'felt faster' per the butt dyno.
 
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