How to reduce fuel consumption with a Mikuni?

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Hi, I am in Invercargill New Zealand and have borrowed an early drum-brake 750 Commando from the bloke who is building one for spares for me. It has a single Mikuni VM carb and starts and runs well, with no pinking or misfires. Despite this, and keeping revs below 3500 (cops in NZ use radar from the opposite direction several miles away) it uses 8 litres of fuel per 100km - about 30 miles per gallon. I hope to get my own bike in a few weeks so don't want to get too involved e.g. jetting. I've looked at the tuning manual but I'm no mechanic and wonder if anyone can advise on the adjustment? Cheers Rob
 
Have you checked for fuel leaks, and for fuel being syphoned out of the myriads of breather tubes those Miks can sometimes have.

If its not your bike, and if its not obviously chugging out black clouds of unburnt fuel, its not advisable to go messing with the jets just yet ?
First real step would be to do a compression test on it, and see that the engine is in tip-top condition first.
And then run this by the owner.....

And, do a dip test of the tank, with known quantities of fuel in it.
Service station pumps don't always dispense what they say they do.
And with a small tank like a roadster (is it a roadster ?) a small shortage can make for a big difference in mpg ??
 
Dang I don't think racing C'do's burn that much fuel. Check float level and online for suggested needle in you size Mukuni and maybe see if brake or something dragging it down too. If only 3500's throttle you sure ain't getting into the main jet zone. One little issue with those unfamiliar with miki enricher levers is which way it goes on or off, though should run misfiring yuky enough its noticed pretty soon. Upper 40's lower 50's mpg is the average to expect.
 
Thanks for the replies. Yes it has a roadster tank, which is a problem even with normal fuel consumption in NZ e.g. Wanaka to Haast road. There aren't any obvious leaks or petrol smells, and the compression and running are otherwise good. It was checked over recently by a local old boy who is a Wiz with Brit bikes, but he is a purist and prefers Amals and points set up (it has Boyer black box). He has given the engine the thumbs up, and it feels very strong especially at low revs - much better than the Buell Ulysses I had for 2 years. I am not lugging it though and keep it in the mid range whenever possible. The plugs are black and sooty so I was wondering about an adjustment rather than jet changes. I may just leave it till I get my own bike but I worry about chucking that much petrol through the bores and engine.
 
plumstead said:
It has a single Mikuni VM carb and starts and runs well, with no pinking or misfires. Despite this, and keeping revs below 3500 (cops in NZ use radar from the opposite direction several miles away) it uses 8 litres of fuel per 100km - I've looked at the tuning manual but I'm no mechanic and wonder if anyone can advise on the adjustment? Cheers Rob

Hi Rob, having been through several Mikuni episodes, and the 1st thing is to check the is that enriching choke lever is raised. If that is OK, and the tip of the plunger is visibly down then the fault is inside. You really can't adjust this externally, i.e. carb on the bike. It really needs to come off, stripped and be cleaned and rebuilt. Its not a difficult job, just be patient and keep everything clean and in an orderly manner. The over-rich consumption can be from different sources - enriching choke plunger stuck, float level, plugged air circuit, etc. You'll need a quality carb cleaner aerosol, Nulon is by far the best. Strip the carb right down and clean every jet and every passage way. Check the float height and float needle. If the process is a bit intimidating, you should take the carb to a motorcycle shop, they should understand it, its a basic device. Reftting the carb to the engine is pretty simple, its just a couple of hose clips.
 
ML said:
plumstead said:
It has a single Mikuni VM carb and starts and runs well, with no pinking or misfires. Despite this, and keeping revs below 3500 (cops in NZ use radar from the opposite direction several miles away) it uses 8 litres of fuel per 100km - I've looked at the tuning manual but I'm no mechanic and wonder if anyone can advise on the adjustment? Cheers Rob

Hi Rob, having been through several Mikuni episodes, and the 1st thing is to check the is that enriching choke lever is raised. If that is OK, and the tip of the plunger is visibly down then the fault is inside. You really can't adjust this externally, i.e. carb on the bike. It really needs to come off, stripped and be cleaned and rebuilt. Its not a difficult job, just be patient and keep everything clean and in an orderly manner. The over-rich consumption can be from different sources - enriching choke plunger stuck, float level, plugged air circuit, etc. You'll need a quality carb cleaner aerosol, Nulon is by far the best. Strip the carb right down and clean every jet and every passage way. Check the float height and float needle. If the process is a bit intimidating, you should take the carb to a motorcycle shop, they should understand it, its a basic device. Reftting the carb to the engine is pretty simple, its just a couple of hose clips.

Word.
 
Check the needle clip position, leaner of course is with needle in lowered position. I love my Amals but my buddy and me have had better results power response and mileage and maintenance wise, as many others, form the modern Japanese single square bowl look rather than the two more pleasing to look at rounded balls bottoms.

It was an old fart to me, Kiwi slutty slurring slang slinger that taught me about why-how pirates became associated with ear rings and had nothing at'all to do with cosmeticx. My Trixie Combat real deal Amal's came from famous NZ bike shop when globalist bankster exchange rates were sticking it to em. I think yoose NZ dudes should hook up with the Icelanders for some action to get your full worth going against the rest of the mundane world.
 
plumstead said:
borrowed ... I'm no mechanic and wonder if anyone can advise on the adjustment?

Why not ask the person who owns the bike before touching it ?
 
Float Level. Gas in float ? Unscrewed jet from vibrations. Bent needle. Clip. Strip er' down.
 
Do you know what the part numbers are for the calibrated parts?

Maim jet, pilot jet, needle jet, needle and slide cutaway?

You should be able to get into the high 40s (MPG) even low 50s with a "feather" edge cruise, assuming that valves and timing are set correctly, and that you have even compression above 120 PSI or a leak down test shows less than 10% leak down.

PM me I am happy to help you with jetting.

Bill.
 
If you shake the float beside your ear you can hear if it has petrol in it. If it is leaking and sinks you will get flooding. The plugs should not be black and sooty. Also it you immerse it in hot water, you might see air bubbling out. I'd also check that there is nothing under the float needle and that it is seating properly. If the float level is wrong, rejetting usually won't fix it. It is the first thing to set to the manufacturer's specification, before tuning the carb elsewhere.
 
If everything is ok with the engine and you're getting poor fuel economy I would start by lowering the jet needle and see the result. Since you're only revving it to 3500RPM you're not on the main jet anyway. You can always get a leaner needle jet or a thicker needle. I have Mikuni carbs on all of my British bikes and love them .They do take some dialing in.
 
A bad float you can hear gas in it for sure if shaken, I had a bad Amal one ,put it up to the lightbulb to see the fuel inside. Gasoline over many years can eat through plastics and seals and rubber bits ,petrify fuel lines too as we know. :shock:
 
I do this for a living.

Need to know what parts you got in your mixer.

Random shots at various user adjustable parameters will only dig your hole deeper. Before you totally foul up your ride pull the carb apart and see what's in it, if that is above your skill level then stop right now and find a local mechanic that can strip your carb. If you can tell me what's in it I will tell you what should be in it; you can choose to purchase the replacement parts anywhere you choose.

Bill.
 
Mikuni carbs are much more versatile than Amals. The range of needles is much greater, and it pays to get the information on the various tapers. I use Mikuni 6DP needles in my 34mm Mk2 Amals with methanol fuel. It is possible to adjust the mixture to the critical performance level, right across the full range of throttle openings. Amal carbs are usually quite crude in comparison- the methanol kits available for them are absurd.
 
I don't agree at'all about random changes not being able to solve tuning issues as have had some shockingly surprising results on a few different engines V8's, 2 tokes and Norton twins, so have at it as if fails then you've gained some knowledge and can always go back to scratch and follow world wide published references on Munki carb tuning stages and fault finding. I will never forget how depressed I was after 5 yr build up on Peel and over a year of timid break it then trying various things that just worsened my mood to go fast till point of shooting out the end of super trap cap and almost got run out from under in mere 1st driveway throttle blip that more pissed me off thinking I was just caught by surprise not holding on much in ongoing depression i'd picked wrong brand of cycle to have any fun with, so got set like on my P!! dragster and snapped throttle again to only end up on seat back instead of tail light lens, then a shit eating grin hit and raised my mood to permanent state i ain't ever been rid of since.
 
None of this stuff is rocket science, you just go at it systematically:

Check your float level, float needle and seat, and check for blocked jets.
The slide cutaway and pilot jet and idle screw set the low running, and a No3 slide cutaway is almost universal.
The needle jet size and needle are critical, if it is too lean the motor will cough at mid throttle openings so lift the needle or if necessary fit a bigger jet.
If too rich the motor will give sluggish response to the throttle. It should accelerate cleanly and rapidly and miss and spit on very cold mornings when the choke is off.
The main jets are not critical unless they are too lean. Check them by full throttle runs on a slightly uphill road and look at the plugs.

The most difficult thing to get right is the needle and needle jet setting for 3/4 throttle, after that it is a soda.
 
Something seriously messed up there. I get 50-55 miles per US gallon with my 850 Commando fitted with a single 36 mm Mikuni. This translates to more than 60 miles per Imperial gallon or about 21 km per liter. Must be a sticky float needle or leak somewhere. Does the Bike run OK? Can't imagine it would if a float needle is sticking.
 
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