High Altitude set up for Mikuni (2018)

lloyds-son

lloyds-son

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Jul 17, 2011
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Just started after years of slumber 1973 850 Roadster, new pistons, 40 over reborn, valves, guides,etc. yahoo it starts and runs, bad news running very rich with single Mikuni carb here in mountain states - high altitude. Looking for advice on solving this issue. Thanks.
 
Not answering your question but for some reason, Amals seem more tolerant of changes in altitude.
May have something to do with the area of float bowl or dimensions/design of emulsion tube/needle.
 
You only say it's starts and runs rich? Do you mean it's just rich on the pilot circuit or throughout the whole range?
If it's just on the pilot and you have the air adjusting screw more than 2 turns out you need a smaller pilot jet
Other circuits of the carburettor will need a simular approach,it's a lot easier if you mark your twist grip
 
Normally, you shouldn't have to make any obvious changes at anything under 5000 feet. What altitude are you?
 
About 6000 feet. I haven’t opened up the carb to inspect. Might be in need of cleaning
 
Rocky Point Cycle can provide you with a solution for your altitude.

Mikunis do a better job of mixing air and fuel in a narrower range than Amals or Bings, which tolerate altitude and ambient temperature better (as stated above), but do a mediocre of mixing over the same broader range.
 
RoadScholar said:
Rocky Point Cycle can provide you with a solution for your altitude.

Mikunis do a better job of mixing air and fuel in a narrower range than Amals or Bings, which tolerate altitude and ambient temperature better (as stated above), but do a mediocre of mixing over the same broader range.
Click to expand...

It's interesting, isn't it? I wonder why, in terms of their respective design traits? There are so many slight differences.
 
I live at 6000 feet altitude here in New Mexico and my single Mikuni 34 for my 850 is jetted
stock except for the idle circuit jet which I dropped to a 35 from the CNW supplied stock 40.
I check the plugs regularly and they are clean, I doubt you have a jetting problem.
If it has sat for a long time first put in brand new plugs then take the carb float off and clean it out
of any scummy gas accumulated there. You may also want to take your two gas tank petcocks out
and insure the filters are not clogged up. If your slide is in the middle notch it is fine for your altitude. I have ridden with stock jetting in excess of 10,000 feet altitude for short periods.

Can you tell us your Mikuni settings such as what size pilot jet, what notch is your slide clip in, what size main jet?
 
Thanks, I have new petcock, I opened up the carb, everything looked clean but I cleaned the passages with very thin gauge wire, cleaned up the floats and checked that they do in fact float. Needle is in the middle setting, spark plugs get black and sooty within minutes. I don’t know what the jet size is for the main and pilot jets. I can pull them out and inspect, what sizes should they be?
 
How many turns out is your pilot air adjusting screw?
Are you sure the choke is turning off?
 
Check the enrichener plunger seal is in good shape, sealing, to start with.

Then ride it.
Report back.
Get the numbers from all four peices of brass, post them here.
 
1up3down said:
I live at 6000 feet altitude here in New Mexico and my single Mikuni 34 for my 850 is jetted
stock except for the idle circuit jet which I dropped to a 35 from the CNW supplied stock 40.
I check the plugs regularly and they are clean, I doubt you have a jetting problem.
If it has sat for a long time first put in brand new plugs then take the carb float off and clean it out
of any scummy gas accumulated there. You may also want to take your two gas tank petcocks out
and insure the filters are not clogged up. If your slide is in the middle notch it is fine for your altitude. I have ridden with stock jetting in excess of 10,000 feet altitude for short periods.

Can you tell us your Mikuni settings such as what size pilot jet, what notch is your slide clip in, what size main jet?
Click to expand...


Stock? I think not. The factory supplied VM 34, spigot carb comes with 260 main jet (way too rich), a 159 Q-2 needle jet (way too rich), a 35 pilot (could be right) and a 6DH4 needle (close)

The "stock" jetting package could be OK if you could run your Norton at air pressures equivalent to 60-90 feet under water, far from 6000 feet above sea level.

To run acceptably at 6000 feet I suggest a 220 or a 230 main jet, a 159 O-6 needle jet, a 35 or 30 pilot jet and a 6DH3 needle with clip in position 3, maybe 2, as a good starting point. I would also consider/experiment with setting the full advance timing up 2 or 3 degrees, in stages.
 
baz said:
How many turns out is your pilot air adjusting screw?
Are you sure the choke is turning off?
Click to expand...
For the the first start of the new engine it was at 1 turn out, I’ve opened it to 2 turns and tried turning it in to 1-1/2 and out to 2-1/2 and these different setting are not affecting the richness issue.
 
you say your plugs get black and sooty fast, is it possible you have the choke lever down instead of up?
do not mean this is an insult but I have run into it before, also if you use the choke for starting do you raise the lever to turn it off quite soon like within 15 seconds to not overload and then use the throttle to complete the warmup?
 
concours said:
Check the enrichener plunger seal is in good shape, sealing, to start with.

Then ride it.
Report back.
Get the numbers from all four peices of brass, post them here.
Click to expand...
RoadScholar said:
Stock? I think not. The factory supplied VM 34, spigot carb comes with 260 main jet (way too rich), a 159 Q-2 needle jet (way too rich), a 35 pilot (could be right) and a 6DH4 needle (close)

The "stock" jetting package could be OK if you could run your Norton at air pressures equivalent to 60-90 feet under water, far from 6000 feet above sea level.

To run acceptably at 6000 feet I suggest a 220 or a 230 main jet, a 159 O-6 needle jet, a 35 or 30 pilot jet and a 6DH3 needle with clip in position 3, maybe 2, as a good starting point. I would also consider/experiment with setting the full advance timing up 2 or 3 degrees, in stages.
Click to expand...
I’ve got 3 of the brass items: 220 main jet, 30 pilot jet, needle is 6DH3
 
lloyds-son said:
I’ve got 3 of the brass items: 220 main jet, 30 pilot jet, needle is 6DH3
Click to expand...
Have you verified all passages are clear, including the air passage from the air horn to the main well? And to the pilot circuit?
When you remove the needle jet, is the time verify air flow to it. I’ve seen them plugged with insects, hayseeds (no air filter) and bits of foam (old deteriorating foam filter)
 
1up3down said:
you say your plugs get black and sooty fast, is it possible you have the choke lever down instead of up?
do not mean this is an insult but I have run into it before, also if you use the choke for starting do you raise the lever to turn it off quite soon like within 15 seconds to not overload and then use the throttle to complete the warmup?
Click to expand...
Good advice. This is my first experience with the Mikuni. I am not sure which direction is on and which is off howeve after removing the check mechanism I see the metal stop may not have been mounted properly. Perhaps this is my issue. I’ll be back working on the bike in a couple days
 
Once you get the Mikuni set up you'll be surprised how well it works. Mine is jetted for sea level but I have ridden it as high as 6,500 ft. It got noticeably richer at about the 5,500 mark, but never fouled the plugs even at the top. I did this same ride numerous times with twin Amals and I couldn't tell any difference in the level of richness but the Amals did deliver a bit more power.
I now live east of Phoenix at about 1800 feet and it works great here. Once the temps drop a bit I plan to ride it to the top of Mt. Lemmon at 8000ft. I will be carrying two extra sets of plugs just in case. I don't anticipate any issues.
 
lloyds-son said:
I’ve got 3 of the brass items: 220 main jet, 30 pilot jet, needle is 6DH3
Click to expand...

The main, pilot and the needle are good starting points, and could "win-the-day", but the needle jet, if as supplied from Mikuni, is a 159 Q-2 which is way too large; start with a 159 O-6.

Choke (enrichener) is on when the part of the choke lever where you put your thumb and fore finger is down; after warm-up the same part of the lever should be up.

We really need to know what calibrated parts are installed in your carb. You say that you have 220/30/6DH3, did you remove them or where they supplied with the carb? Is the carb new or used? If used then the cleaning advice you have in this thread is good stuff.
 
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