Mikuni doesn't run anymore on idle

Bonjour à tous

I have this bike since March.
Was running fine at all rev despite high petrol consumption 9-10l / 100 km (26 - 23 MPG) and black spark plugs at idle
Then it starts to ran not smoothly at low rev and low throttle.
Seems to depend on engine temp
Run pretty well when cold, then come to irregular running at 30mph when start to be hot +- 2km

When opening full throttle even starting from low rev it's very fine but not fine at constant low opening throttle (less than 1/4 or 1/8)

Run fine at high rev, no problem to take 100 mph even on light slope. Pretty nice chocolat spark plug color when stopped at full load.

Run well after this high rev run but comes again after +-1 km to irregu'ar running at low rev.
After a stop (a couple of minutes) to check spark plugs it runs well 1km then comes to ¥£$*%+@£# running.
Idle screw is almost tighten at max!?!?
Air scew does not change anything (3 to 4 turns that is too much but less bad setting)
When in bad running phase, iddle is irregular and exhaust give dull noises

When cold, starter system runs fine

Check all carb data and clean by flushing with cleaning liquid then blow comp air in the idle circuit
Cheked with new spark plugs, same result
Inlet rubber connection (carb to head) changed for new as well as inlet paper sealings

Looks like idle air circuit is blocked or not free

Test not yet done : running without air filter

Bike and settings
750 commando roadster
19theeth sprocket
Electronic spark (Boyer, I guess)
Battery loaded
Single VM34 mikuni carb
KN filter (cleaned)
Main jet 250
Pilot jet 25
Needle160H4 at midle (3/5)
Needle jet 159Q2
Throttle valve 2.5
Float level checked
Timing well checked

As far as I know all above carbs data are supposed to give good results, isn't it?

What do I missed? :shock:
Any idea welcome

Merci par avance
Cheers
Bernard

I think the Pilot Jet at 25 is too small. Try replacing it with a 35 or 40. It may also be blocked, use a small screwdriver to remove it and make sure it is clean. With a 35 or 40 Pilot Jet, the Cold Start lever only needs to be on for starting and 1 minute of running. If the Pilot is partially blocked (or too small) the motor runs bad under 2500 RPM.

Also, the Air Screw should only be turned out 1.5 or 2 turns at the most.

"Idle air circuit appears blocked" I once had a VM28 with a bad running idle, had to be up on the needle and idled fast. Finally, I found the idle air circuit partially plugged by a hay seed.

ML said:

I think the Pilot Jet at 25 is too small. Try replacing it with a 35 or 40. It may also be blocked, use a small screwdriver to remove it and make sure it is clean. With a 35 or 40 Pilot Jet, the Cold Start lever only needs to be on for starting and 1 minute of running. If the Pilot is partially blocked (or too small) the motor runs bad under 2500 RPM.

Also, the Air Screw should only be turned out 1.5 or 2 turns at the most.

Click to expand...

Thanks for your replies

Strange to me as I have heard from many, including a brit bike workshop here, that the right pilot jet for 34 mikuni on 750 is 25 or 30.
On top of that black spark plugs means already too rich so too big pilot jet ( or air pilot circuit blocked). Am I wrong?
I did not find any stopper (block) in air pilot circuit.
Is there different sizes for the air jet in air pilot circuit?

For the air screw, I fully agree 1,5 to 2 turns shou'd be max but, pitty, it's (a bit) better with more turns :cry:

Cheers
Bernard

RoadMyster said:

On top of that black spark plugs means already too rich so too big pilot jet ( or air pilot circuit blocked). Am I wrong?

For the air screw, I fully agree 1,5 to 2 turns shou'd be max but, pitty, it's (a bit) better with more turns :cry:

Cheers
Bernard

Click to expand...

Yes, I think you are mistaken. A pilot jet is and "air" jet. So a bigger pilot jet will offer more air to the idle circuit.
The amount of turns on the airscrew is relative. Meaning there is no set amount as long a response is attained.

Also, remember turning the air screw out allows for more air. So, if you are showing rich indication and you get positive results when turning the air screw out, then this makes sense and the next size up pilot jet should be close to ideal. That is, of course, if the circuit is clear.

One more thing, identify that the circuit is clear by shooting carb cleaner through it and witness the mist shooting out the tiny holes when followed by compressed air.

I would either clean really good or replace those fouled spark plugs, they are toast as is

I ran a Mikuni 34 for 20 years, you pilot is too small, go to a 35 or 40

I assume your needle is in the middle position, verify that is "still" is and that the circlip is in place good

but first, trash those plugs

1up3down said:

I would either clean really good or replace those fouled spark plugs, they are toast as is

I ran a Mikuni 34 for 20 years, you pilot is too small, go to a 35 or 40

I assume your needle is in the middle position, verify that is "still" is and that the circlip is in place good

but first, trash those plugs

Click to expand...

I agree. Get rid of the fouled plugs first. Pilot size and circuit cleanliness is critical and takes close inspection and diligent cleaning to be sure something isn't in there. Make sure your float is functioning properly while your in there. Check for fuel in float (leak).

Pilot adjustment is based on how the engine is running in your particular location (elevation). Turn the screw out until highest rpm is obtained. Turn 1/4 turn in from that position and adjust idle speed to proper idle RPM. Number of turns means nothing with one exception. If you have to turn it out 3 or more turns or to the point where the spring no longer retains it, your pilot jet is sized wrong. The manual specification for screw position is for "after rebuild". This is a starting point to get the engine running. The screw is then adjusted for your engine and conditions.

The fact that the bike ran fine before indicates something changed. Most likely dirt in the carb or bad gas.

Don't forget about vacuum leaks. Check manifold mounting nuts, carb mounting, etc.

First, facts and answers:
Plugs have been replaced for new without any improvement
I put cleaner an comp air in the air jet, get out from air screw hole. Put in air screw hole, get out from air jet and/or small hole in the outlet in the carb main flow downstream the needle jet.

Needle in the midle, seems to be rightly installed

Then clarification and question:
Misunderstanding or I am wrong?!?!

Pilot jet (rep 21 on mikuni part list) is in vertical position close to the main jet
Isn't it a fuel jet, larger it is more fuel it gives at idle?

Air jet (rep 28 on mikuni part list) is in horizontal position in one of the three hole on the inlet air side of the carb
Isn't it a air jet? On this one I didn't find size number. This one ,if I understand, larger it is more air it give to the air screw.

http://www.mikuni.com/pdf/vmmanual.pdf

Thanks
Bernard

Yes those are the correct parts 21 pilot jet, 28 air jet. Is there an air jet in the carb? I'm assuming so since you said it ran fine before. There are only a couple of choices for air jet and they are usually in the carb when purchased "if" purchased for 4 stroke application. This gets complicated because if you did not purchase for Norton then the stock Mikuni's do not come with an air jet. This is because these carbs were originally for 2 stroke applications and if bought as a "generic" they will come without an air jet. You will need to clarify because you said it ran fine before.

Make sure you check for vacuum leaks...thinking that might be a possibility if you were running okay before.

Some additional info. I'm assuming you are running a VM34 single carb.

Base line jetting For a VM34;

260 main jet, 35 pilot jet, Q2 needle jet, 6dh4 jet needle, 2.5 cutaway, 2.0 main air jet.

This is different than what you posted.

No two bikes are the same. Since the Mikuni is not the "native" carb for a Norton, carb tuning is hit and miss and requires an understanding of the circuit functions and position of the main needle at different throttle positions. If the main needle is in the center position for example some fuel will be coming out of the main jet at idle depending on vacuum (suction). Pilot jet sizing can relate to the main needle position. A 25 pilot might be adequate depending on main needle position and size. Your main jet is smaller than recommended so that has an impact. The needle numbers relate to the taper and diameter of the needle. This also impacts fuel delivery at different throttle positions. The number you gave is also different than recommended needle. The cutaway is the shape of the throttle slide at the bottom and also impacts the fuel delivery and vacuum.

If it were my bike I would buy the next 2 sizes up of pilot jets and try them. Jets are cheap and the proper way to tune is to have multiple jet's and needles to select to tune your carb for your bike. I just installed a Minuki TM Flat Slide carb on my Norton 850 and I bought 5 pilot jets based on the recommended size. Most recommended 30 so I bought 25, 27.5, 30, 32.5 and 35. This gave me 2 jets on either side of recommended. As it turned out I needed to use the 27.5 jet to get the proper running at idle and through 1/8 throttle.

The adjustments in jet size is much finer and more critical than people think.

Ok thanks,

I thought to air leaks and changed the rubber connection from carb to inlet valve as well as manifold paper seal (properly cut in the middle to avoid one comes above the other).

Yes, there is a pilt air jet

Then, as the inproper running came suddently without any setting modification I will come back to previous settings and will restart investigation from here.

I will revert if I find something

Talk to you

Bernard

RoadMyster said:

Then clarification and question:
Misunderstanding or I am wrong?!?!

Pilot jet (rep 21 on mikuni part list) is in vertical position close to the main jet
Isn't it a fuel jet, larger it is more fuel it gives at idle?

Click to expand...

You are correct. The Pilot jet is a fuel jet not an Air jet.

Bernard,
How did you go about checking the float level? From the sound of it, you are getting rich running on the pilot circuit even though you've got a 25 pilot jet and 3 turns out on the screw? I can say from personal experience with my VM that you can find yourself chasing down solutions for a high fuel level regardless of the height of the tangs on the arm. For me, going with a 25 pilot and 3-4 turns out on the air screw did nothing to resolve the rich running. I started by refitting a 35 pilot and setting the air screw at 1.5 turns out. I then lowered the fuel level till the problem resolved, it didn't take that much. It transformed the way the engine ran at low throttle openings. Crisp response and clean porcelain.

Bob,

Thanks, very possible i didn't go deeper enough in that way.
I just check the distance between arms and seal and arms horizontal when carb head down.

I am going to go back to previous settings those when bike was running fine and try to lower the float level step by step.

All your replies gave me knowledge, experiment and open eyes and minds, many thanks

Bernard

To the original post: Your idle jet is too small and may be fouled, or with the rest of your jetting package your plugs may be fouled.


Although there are several jetting packages that will work, as stated in earlier replies, yours appears to be on both sides of the bell curve, being too lean in the idle and too rich in the mid and high end.

I ship 34mm VMs for stock or near stock 750 Commandos with the following jetting package for sae level to 5000':

Main jet: 230

Idle jet: 40

Needle jet: 159 O-8

Needle: 6DH3

Slide: 2.5

Air jet: 2.0

With this package the only tuning I have helped customers with (rarely) is the occasional movement of the needle clip and a change out of the main jet to a 240 or a 250, but I have not yet been requested to go above a 250. I can see that if someone had more aggressive cams, more open exhaust and a less restrictive air cleaner they would, probably need a thinner needle (or larger needle jet) and/or a larger main jet, but in my experience customers that purchase single carb conversions tend to side on conservative states of engine tune.

RS

Do these carbs not have rubber washer/seal in the seat of the enrichener (choke) circuit? I think there is a good chance gas is getting by this, perhaps, worn seal so your engine is effectively running on the enrichener all the time.

Bernard,
I forgot to mention that it's possible to over shoot the mark when tuning carbs. There's an adage, " Make your changes going rich at two steps at a time. Go lean one step at a time." It applies to jet sizes, needles, and fuel level. It comes from the fact that there's a price to pay for going too lean but no problems from going too rich but wasting sparkplugs.

Biscuit said:

Do these carbs not have rubber washer/seal in the seat of the enrichener (choke) circuit? I think there is a good chance gas is getting by this, perhaps, worn seal so your engine is effectively running on the enrichener all the time.

Click to expand...

Inside the choke bore is a brass piston, the bottom of that piston has a piece of rubber roll crimped that seals the choke circuit when the choke is off; examining both the rubber and the seat is prudent when richness is in evidence, it can be pressure tested with low pressure, nothing above 4-5 psi.

RS

Grrrrr,
I filled in my cart with Roadscholar recommendations (thank you RS), filled in all the boxes, name, address, selected postage to France, got a corresponding price,filled in the card data, clicked on "confirm" and...... "we do not accept payment from foreign countries" :roll: :roll: :roll:

Any advise about better web site for purcahsing jets and needle, welcome 8)

RoadScholar said:

Biscuit said:

Do these carbs not have rubber washer/seal in the seat of the enrichener (choke) circuit? I think there is a good chance gas is getting by this, perhaps, worn seal so your engine is effectively running on the enrichener all the time.

Click to expand...

Inside the choke bore is a brass piston, the bottom of that piston has a piece of rubber roll crimped that seals the choke circuit when the choke is off; examining both the rubber and the seat is prudent when richness is in evidence, it can be pressure tested with low pressure, nothing above 4-5 psi.

RS

Click to expand...

I saw the rubber is marked but seems in good condition (still elastic without crack, so not too old).
Nevertheless this is a track to explore.
I will do all the check this week end and hope the weather will allow me to do road tests