Please Help w/ Mikuni Carb Settings

I've got a Mikuni conversion and lately have had a problem with poor acceleration. The bike idles well, but under acceleration, it pops and misses, almost like I forgot to turn the fuel on. I checked the plugs, and they look good. No signs of fouling. I fiddled with the mixture screw (the small screw on the right, not the idle) and it seemed to improve the situation, though I need to experiment some more. I'm out on the road right now with no access to my manuals. Can someone give me a good starting point for the setting of this screw, and how best to locate the sweet spot? Currently, I'm turned out 2 1/4 turns. Am I close?

Thanks

John

My air screw is 1 1/2 out.

As stated, the standard mixture setting is about 1 and 1 half turns out from fully seated.

Your needle should be in the middle notch, easy to check it.

It is possible you are not getting enough fuel, how old are your petcocks?

If you take the fuel line off do the petcocks flow a nice steady line of gas, they may be gummed up and blocking.

Also check the carb mounts to manifold and then to the head, any air leak could cause your symptoms.

Turning the air screw is just one of many adjustments to find that balance. There is no set number of turn for no 2 Norton breath exactly alike. You say your idle is good. Do you have good response off idle when popping the throttle? If not, turn the air screw in or out to get the best response then adjust the idle with the throttle stop screw.
As for as you symptoms are concerned, start with raising the needle one notch (move the clip one notch down). Sounds like you are lean in the mid range. This should help. Run and repeat is needed.

These carbs are very needle and needle jet dependant. Once you get it right, you will not look back.
It would be helpful to know what you have there. Needle, needle jet, main and pilot.

Keep us posted.

Does this happen when the motor is fairly cold, i.e. just off choke and ready to run? if so the Pilot Jet is too small. A .35 or .40 is about right for a Commando VM34.

Mick

1up3down said:

As stated, the standard mixture setting is about 1 and 1 half turns out from fully seated.

Your needle should be in the middle notch, easy to check it.

It is possible you are not getting enough fuel, how old are your petcocks?

If you take the fuel line off do the petcocks flow a nice steady line of gas, they may be gummed up and blocking.

Also check the carb mounts to manifold and then to the head, any air leak could cause your symptoms.

Click to expand...

Good advice. The petcocks are new, but I'll double check for flow. At first I thought the problem was plumbing, as it seemed to run fine the first 200 miles I rode it today. As I got low on fuel and switched to reserve (which I only did once today), is when the problem occurred again. It's done it in the past, but usually only after running some miles. I figured it might be a low fuel level situation. Well, upon re-fueling, it still missed. Later, after letting the motor cool and then riding, it still acted up. After checking the plugs, I fiddled with the mixture (right screw), and it seemed to run a lot better. I'm going to pull the bowl and check for debris.

1up3down said:

As stated, the standard mixture setting is about 1 and 1 half turns out from fully seated.

Your needle should be in the middle notch, easy to check it.

Click to expand...

I recall awhile back, I leaned the needle clip one notch, because I thought it was running too rich. Maybe I should go back to the center position.

pvisseriii said:

Turning the air screw is just one of many adjustments to find that balance. There is no set number of turn for no 2 Norton breath exactly alike. You say your idle is good. Do you have good response off idle when popping the throttle? If not, turn the air screw in or out to get the best response then adjust the idle with the throttle stop screw.
As for as you symptoms are concerned, start with raising the needle one notch (move the clip one notch down). Sounds like you are lean in the mid range. This should help. Run and repeat is needed.

These carbs are very needle and needle jet dependant. Once you get it right, you will not look back.
It would be helpful to know what you have there. Needle, needle jet, main and pilot.

Keep us posted.

Click to expand...

Which direction leans the mixture? In or out?

ML said:

Does this happen when the motor is fairly cold, i.e. just off choke and ready to run? if so the Pilot Jet is too small. A .35 or .40 is about right for a Commando VM34.

Mick

Click to expand...

No, actually it seems to run better when cold.

One other thing. Starting the engine with the choke (in moderate temps) never seems to work. I usually have to turn in the idle screw one turn, and then it usually starts right up. Idle seems normal, but then after the engine warms up, the idle is way too fast, and I have to lower it. This doesn't seem right, which causes me to suspect that my carb settings are screwed up.

Thanks

If the bike ran fine for 200 miles, I can't see that there is anything wrong with the tune of the carbies.
However, when you went to reserve, the problem started.
I think I would be removing the tank, lines, carbies, air filter, etc.,
and cleaning everything. Is it by chance a Fibreglass tank, or a rusty Steel.?
Do you run a fuel filter on each line, it might be time for new ones.
Clean that carby out meticulously, especialy the, Starter Air Inlet which is at 3 o'clock looking from the rear, and
the two tiny holes in the bottom rear of the inside of the carby where the bottom of the slide would sit if it wasn't Cutaway.
They are part of the cold start system, if they are blocked they won't work.
This should get it starting and idleing properly again if it used to be allright.
The main issue is that it seems to lean out when you have been riding a while and I think it is a problem caused by
a restriction of some sort, that could start at the Taps (petcocks) or any where below them.
If you have a restriction to fuel flow, then turning the Air mix screw might help, but it will probably push the problem
on to somewhere else.
If it used to run well, go back to those settings after you have cleaned everything up... And start again.
Mikuni's don't change once they are right, they will also run on different settings as to another similar engine.
The mixture will lean off as the Air screw is turned out (anti clockwise). Richer... clockwise.
It's all good fun...
AC.

Bonwit said:

Which direction leans the mixture? In or out?

Click to expand...

Turning the screw out leans the pilot mixture.
Pete's right, you need to find out what jets you've got before you can make any sense of what's going on. If you got your kit from Sudco it's going to be way rich. Rocky Point seems to send them out better jetted for Commandos.
pilot jet
air jet
slide
needle & jet, height
main
float height
Until you write it all down you're in the dark, and so are we.

AussieCombat said:

If the bike ran fine for 200 miles, I can't see that there is anything wrong with the tune of the carbies.
However, when you went to reserve, the problem started.
I think I would be removing the tank, lines, carbies, air filter, etc.,
and cleaning everything. Is it by chance a Fibreglass tank, or a rusty Steel.?

Click to expand...

Thanks everyone for all the replies. I was thinking fuel flow too. I just pulled the fuel lines and fuel flow is excellent, so it's not the petcocks. It's a steel tank, and looks pretty clean. No fuel filters, other than the finger strainers. I checked the bowl. Fuel there is clean, no sediment found. I did return the clip to the center position, for what that's worth.

Just to clarify, the only starting problem centers around having to raise the idle to get it to fire. When I do that, she starts and idles like a top. It's just that after riding to the first stop sign, I have to reach down and dial it back to a more normal idle, because by then it's too fast. This just doesn't seem like the way it's supposed to work, but I didn't know if that had anything to do with my acceleration problem.

John

bpatton said:

Bonwit said:

Which direction leans the mixture? In or out?

Click to expand...

Turning the screw out leans the pilot mixture.
Pete's right, you need to find out what jets you've got before you can make any sense of what's going on. If you got your kit from Sudco it's going to be way rich. Rocky Point seems to send them out better jetted for Commandos.

Click to expand...

The previous owner installed the carb kit, and I don't know where he got it. I have worked with Bill at RockyPoint, going over the jets installed trying to track down what I though was a rich condition (poor fuel economy). I've replaced the slide and one jet which had no marking on it per his recommendation, but I can't remember off hand which one. I'm going to ride it in the morning to see if anything I've done has helped. When I get home, I'll get into it deeper and look for some obstruction.

G'Day Bonwit,
Don't worry about poor fuel economy, Lean engines run hotter.
You are right, it's not how it's suposed to work.
On a Mikuni the... Cold start system, is not related to any other part of the carburation process, and has
minimul affect on anything other than cold starts.
By tuning the idle up, you are actually decreaseing it's affectiveness.
They work best on very little throtle.
Mikuni's don't actually 'choke' the amount of air going into the carb, as per Amals, but rather they have
a seperate Fuel/Air system, for cold starts.
If the throtle is open too far, air won't be sucked in through the " Starter air inlet".
That's why you must check the Air inlet passage.
True, it won't effect your lean problem after starting.
Also, some engines don't need to have the 'choke' lever pushed right down, or fully open, to work efficiently.
The cable adaptor can be a good addition to English bikes as then you can easily adjust to desired opening.
Once the engine has started and been run for a while then they can be started normally.
I hope I explained this properly and is some help. Good luck.
Get it cleaned, then ride it again, then start on one thing at a time.
AC.

Bonwit said:

pvisseriii said:

Turning the air screw is just one of many adjustments to find that balance. There is no set number of turn for no 2 Norton breath exactly alike. You say your idle is good. Do you have good response off idle when popping the throttle? If not, turn the air screw in or out to get the best response then adjust the idle with the throttle stop screw.
As for as you symptoms are concerned, start with raising the needle one notch (move the clip one notch down). Sounds like you are lean in the mid range. This should help. Run and repeat is needed.

These carbs are very needle and needle jet dependant. Once you get it right, you will not look back.
It would be helpful to know what you have there. Needle, needle jet, main and pilot.

Keep us posted.

Click to expand...

Which direction leans the mixture? In or out?

Click to expand...

"out to lean"
You may need a larger pilot if running on an 859. Say 45 to 50. What pilot jet do youhave now? Turning the air screw out allow air into the pilot curcuit therefore leaning. So, if yours is turn out 2 1/2 turns, you my be at the limit of your current pilot jet.
Again, the mid range is the needle. Lower the clip(raising the needle). What notch is it in now?

Please let us know if you find a solution. I have been having a simalar problem, sometimes after riding for a while at freeway speeds of around 70 to 80 my bike will start to cut out if I accelerate. It comes back but I don't like this. Normaly it is very responsive from start to high speeds but once in a while will cut out on hard acceleration. Clean new gas lines, new BAP petcocks steel tank looks good and the filters seem clear. Last night I rode with both petcocks open just to see if it was the main one not flowing enoughe at high speeds & it still cut out onece going about 80 when I grabed some more throttle. Seems like gas staved condition but it seems to be flowing plenty. Any thoughts?

I believe the single Mikuni is a great option for the Norton but should not be considered as a performance upgrade. I ran a 36mmVM on a 750 with great success well knowing that although the bike would start and run reliably, would never match the performance of a duel carb setup.

I switch to a 34TM incorporating jetting from the VM and was please with the increased top end performance with economy.

A 34mm VM on a 750 will only go so fast and on an 850 even less. Nortons like to breath and any restriction, intake or exhaust, will effect top end.

I have enjoyed the benefits if single carbs, but for me it was time to put the heart back in the bike and I have transform my machine into a very fast, very quick, yet very reliable road bike. Search my posts to follow my transformation history.

gtsun, you need to enrich en your needle/needle jet circuitry. At least start by raising the needle. Middle notch should be considered as a starting and NOT the standers. The 5 notches are there for a reason, use them.

Also the air screw 1 1/2 turns out is a starting point, not a standard. As with any variable adjustment, when near the end point, top or bottom notch on the needle or 1/4 turn from the bottom or 2 1/2 to 3 turns out with the air screw, jetting will have to be changed or the limits of the venturi needs assessed.

Thanks. Also didn't mean to hyjack your thread, just sounds like we are both having the same trouble. I was wondering if it was just that the single carb couldn't keep up with the 850's needs at the times I was running it hard and fast. I'm not to concerned beacouse I want to go to the dual Amals anyway.

Hey gtsun, Here in Oz we are lucky to still have reasonable quality petrol, but as you guys don't, I wonder
if this isn't the start of many carby problems. Those tiny jets and inlets ( Starter air inlet, and Air correction inlet ) that are hard
to see if not pointed out might be getting gummed up. Especially if the bike isn't ridden regularly, or put to bed for Winter.
Both guys have said the same thing... That the bike normally runs OK.
If these carbies were new additions, I would say they are lean at middle to high openings, but if they are not new, then there must
be a reason for them to lean off as they are.
Next down the line is the Needle and Seat, it lets ALL the fuel into that carby and just like a dirty Tap or gauze filter,
it must be clean. Has the Float level been checked, They don't often move but stranger things have happened.
Low float level can give difficult starting, and a general leanness right through.
The Air correction jet is at 6 o'clock loking from the rear. It helps out at very high openings by allowing air through
the carby body to those holes in the side of the Needle Jet, which then immulsifies the fuel that comes up from the Main.
If the Air correction is not clear or just the wrong size, it can be compensated with Main jet sizes, but won't be as efficient.
The trend in the early days of Mikuni addaption to four stroke motors was to remove this jet completely, this was a missunderstanding.
It should be a # 1.0 or there abouts, but again is another changeable tuning system. This jet lets air in, so bigger is Leaner.
Have fun and be carefull out there.
I wonder if this is a spark problem. he he he . sorry.
AC.

Just to "throw a wrench into the works"... how is the battery voltage reading ? Below 12 V. can also cause similar symptoms.