New Mikuni issues on fresh motor MKIII

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Coco

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Newly, rebuilt motor, brand new 34mm Mikuni on an 850 mkIII.

Bike ran great when carb was first put on but rich. Was going to take the bike on the final engine break in ride and the symptoms I am about to decribe started.

Bike starts to give up or bog down when higher revs happen, just before a shift from 1st to second, second to third. Almost like the carb gets too much fuel and bogs down, revs get choppy and when the throttle is backed off bike runs ok at low revs. Idle on first start up is good and bike runs well and starts well. I don't use the choke either. Bike feels like it stutters and kind of lurches a bit and sounds bad. Revs lower slightly and it will smooth out until you give it more throttle to upshift, sysmptoms start again.

I pulled the plugs and they were very black and sooty. Smaller jet was suggested so I put one in to lean things out and symptoms are still there.

New plugs and a smaller air jet was put in yesterday to see if that works. 35 in there now and a 40 was removed. I also retimed the motor and symptoms are still present.

Help! I want to do the final break in ride and get on with things like riding.
 
Ok, pilot jet size is fine at 35, or even 40.

But, what number is the slide, is it a 2.5?

What position is the needle circlip in, the middle?

Main jet size?

Take bowl off and verify installed correctly with nothing blocking, compressed air through all the holes, something is either blocking air or causing too much gas, way too rich for
black plugs.

Top of carb threads onto body finely, can hear slide click onto idle stop when throttle let go when engine off so cable is fully extended and not hanging up slide?
 
What he said^^^^
My setup for 34 single was 240 main middle clip for big K&N, 11/2" pipes no balance, Toga p-shooters.
 
Any chance the enricher circuit (choke) is still open even partially.
I'd make sure plunger is fully down in its seat.
 
highdesert said:
Ok, pilot jet size is fine at 35, or even 40.

But, what number is the slide, is it a 2.5?

What position is the needle circlip in, the middle?

Main jet size?

Take bowl off and verify installed correctly with nothing blocking, compressed air through all the holes, something is either blocking air or causing too much gas, way too rich for
black plugs.

Top of carb threads onto body finely, can hear slide click onto idle stop when throttle let go when engine off so cable is fully extended and not hanging up slide?
Click to expand...

I'm just about to rip the carb open and see what the main jet is and slide. Slide and throttle are nice and snappy and slide is not hanging up. Lid is on good and theads on nicely. C-clip for needle is on middle position.

I'm thinking something must be blocked in there.
 
rx7171 said:
Any chance the enricher circuit (choke) is still open even partially.
I'd make sure plunger is fully down in its seat.
Click to expand...

Choke plunger is down and fully seated.
 
Coco said:
rx7171 said:
Any chance the enricher circuit (choke) is still open even partially.
I'd make sure plunger is fully down in its seat.
Click to expand...

Choke plunger is down and fully seated.
Click to expand...

My thoughts exactly - the choke. The choke lever must be up for off (no choke). I hope it's this simple.

A funny aside - my buddy Doug MacRae was having a hell of a time with his magneto on his race bike at Mid Ohio last month, so he ran up to the swap meet and bought himself a Boyer and we strapped in a battery and wired the thing up. The bike started perfectly, but wouldn't run properly on one side. He ran an entire race with the bike sputtering on one side. We sat there scratching our heads, thinking there must be something in the ignition (last thing we changed, right?). Suddenly I hear an "Aha! Duh!". Turns out he'd been racing with the choke on - the lever apparently bumped as we were hastily wiring up the Boyer.

His description of how the bike ran sounded a lot like yours.
 
Sorry for off topic, but how do you know which size everything is? When I took my mikuni apart I didn't see any labeling. Is it just a diameter measurement? Be able to help more on topics like this if I knew.
 
Choke is definitely off since I never really use it anyway and just keep on the throttle a bit instead until bike warms up. I just blew out every passage with compressed air and nothing came out except a good stream of compressed air so no clogs. I can see daylight through all jets, needle jet is clear ect.

I'll put everything back together and go for a spin. If symptoms still persist, I'll retime again since I'm not sure what else to do.
 
I forgot to mention the slide is a 2.5 and main jet is a 230.

Could the magnet for the ignition have come loose and shifted causing this? I did not check the magnet while retiming the Tri-spark. I retimed it last night to 28 deg and all seemed fine but its still doing what its doing.
 
Coco said:
I'll put everything back together and go for a spin. If symptoms still persist, I'll retime again since I'm not sure what else to do.
Click to expand...

Don't you still have the Amals then ? :)
 
79x100 said:
Coco said:
I'll put everything back together and go for a spin. If symptoms still persist, I'll retime again since I'm not sure what else to do.
Click to expand...

Don't you still have the Amals then ? :)
Click to expand...

I don't. Bike came with a 36mm Mikuni which I changed to a 34mm.
 
I ran a 25 pilot on mine at sea level.
Started and idled perfect.
 
Coco said:
Choke is definitely off since I never use it anyway. I just blew out every passage with compressed air and nothing came out except a good stream of compressed air so no clogs. I can see daylight through all jets, needle jet is clear ect.

I'll put everything back together and go for a spin. If symptoms still persist, I'll retime again since I'm not sure what else to do.
Click to expand...

Coco
if you can start your bike cold and not use the choke on a mik something is a wrong... you should have to at least open it for initial start and then shut it off - i suspect you have a problem with the setting/jetting here - have you read victory library mikuni tuning guide? big help to get maximum benefit from these carbs

this is my 34mm set up as recommend

recommended by panic/victory libary

pilot 25
slide 3
needle 6DH3
main 260

runs good....
 
pelican said:
Sorry for off topic, but how do you know which size everything is? When I took my mikuni apart I didn't see any labeling. Is it just a diameter measurement? Be able to help more on topics like this if I knew.
Click to expand...

veeeery small inscription on the jets/needle
 
magnfying glass, does your needle say 6DH3?

I also bought and installed the trispark from CNW, and I love it.

But tell us exactly how you have it set. Strobe light at 3000rpm and it shows 28 degrees in the primary window?

battery fully charged, trispark grounded properly to lower holding stud bolt, did you change to resister plugs from CNW, stock dual coil setup,
all wires to coils on securely, using new plugs, what kind, plug wires may be older, cracked inside, not delivering clean spark so rich symptoms.?
 
All the jets and the needle have numbers stamped on them, but they're TINY. If you have the carb down, check the needle number and needle jet number. The Mikunis are handy because there are a lot of jetting choices, but it's also a bit of a PITA - kind of overwhelming! But if it was running well and suddenly isn't, don't discount a possible Boyer problem. We had a nicely restored '74 850 that started running really rough on acceleration. We replaced everything - new twin Amals, you name it. Replaced the Boyer amplifier box - still no luck. Finally we replaced the Boyer rotor/stator, since we'd already replaced everything else - and voila, we were in business again. Who'd think that something with basically no moving parts could go bad?....
 
ptourin said:
Finally we replaced the Boyer rotor/stator, since we'd already replaced everything else - and voila, we were in business again. Who'd think that something with basically no moving parts could go bad?....
Click to expand...

You mean the notorious bad wires on the stator plate? If you run a Boyer it's not the third thing you check, it's the first. Luckily Coco has a Trispark, so that's not the problem.
 
MichaelB said:
I ran a 25 pilot on mine at sea level.
Started and idled perfect.
Click to expand...

I'm 1894 feet above sea level. I'm using a 35 now which leaned things out a bit.
 
highdesert said:
magnfying glass, does your needle say 6DH3?

I also bought and installed the trispark from CNW, and I love it.

But tell us exactly how you have it set. Strobe light at 3000rpm and it shows 28 degrees in the primary window?

battery fully charged, trispark grounded properly to lower holding stud bolt, did you change to resister plugs from CNW, stock dual coil setup,
all wires to coils on securely, using new plugs, what kind, plug wires may be older, cracked inside, not delivering clean spark so rich symptoms.?
Click to expand...

Everything is brand new, New Trispark, new coil, new wires, resistor plugs, new battery that holds a full charge ect. i'm getting plenty of juice and a good spark to the plugs. Lights don't flicker at idle and are quite bright.

Needle is a 6DH3.

Bike was stobed and set to 28 deg at 3000 RPM, 35 air jet, 230 main jet, air jet adjuster screw set at 1 1/2 turns out. Trispark grounded properly. Bike is not running as rich as it was in the beginning since I put a new smaller air jet in. Plugs were dry after installing smaller air jet so I went for a ride and symptoms started. Now I'm having slightly wet plugs so the fuel is not all being burned. Something is up definitely.
 
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