Mikuni Carb puzzler

[Buffalo Bill]

Well I have been working on my 550 project. I tried syncing the carbs but, I found that turning the adjustment screws did not do much to change the vacuum readings?
Currently the bike will idle but if I twist up the throttle to 3-4000 rpm the engine will stay there for a 5-6 seconds before slowly dropping down to idle.
The #3 & 4 plugs look clean to whitish, the #1 & 2 plugs look dark to black.
I tried testing for a vacuum leak by spraying Ether onto the intake boots, but had no reaction.
I guess I'll have to give in and do a valve clearance test.
Any clues Boys?

[first timer]

have any pictures of the boots? I have found time and time again if the boots are old and rubber is pealing from the mtl plates they are shot. Even if you can't test with ether or carb cleaner or what ever. That trick only works if you can get the bike to idle low like 1k.

The 550's are very sensative to air leaks

[ryonker]

I did my valves first and they were all in need of adjustment. Did you replace the o-rings on the intake boots? Mine were shot--hardened just like spaggetti noodles. Sounds like you are a bit lean on 3 & 4 and rich on 1 & 2.

Rick

[GQROD]

If it's hanging up before the idle drops you have a leak or you are running too lean perhaps due to mixture.

Do plug chops at idle and see how lean, adjust your mixture screws until it does not hang up, if it still hangs up then your leak is either the valves needing adjustment or boots or o rings to the manifold.

It's good practice to do the valve adjustment first and get that out of the way then be methodical and check everything else. Took me forever and tons of plug chops, but it was the only way to fix it.

Lastly check your choke cable and adjustment, sounds wierd but if this happens when the engine is cold it could just be that it's not getting enough fuel during warm up.

Hope this helps.

[Marine36]

Hanging throttle... I have just been through that. Obviously make sure all cables are free and not damaged. Make sure you have good boots and orings.
then go to BassCliff's website find the info on highest idle adjustment and do that. My 1100g would not idle at 1k or slowly creep up in idle if set higher and when the throttle was goosed rpms would hang high. When I enriched pilot circuit in carbs (air adjust screw) problems went away.

[ryonker]

GQROD is right I think.

"Took me forever and tons of plug chops, but it was the only way to fix it."

I just decided to buy a colortune so I wouldn't have to do lots of plug chops. Either way, you will eventually get to the right mixture.



Rick

[jetta90]

Yes for sure........as mentioned upthread, check your valve adjustment. Even if they turn out to be adjusted within spec it is something that needs to be completed. Then you can go from there.
At this point, the valve lash is unknown on your engine. It could lead to some real uglies down the road.

[GQROD]

GQROD is right I think.

"Took me forever and tons of plug chops, but it was the only way to fix it."

I just decided to buy a colortune so I wouldn't have to do lots of plug chops. Either way, you will eventually get to the right mixture.



Rick

I also bought a color tune because i wanted to make my life easier, it definetly is worth the money because it makes sure you are not chasing ghosts, it pin pointed that i was out of the range of idle mixture adjustability and i wound up going with one number higher on my pilot jets. Stock air box and stock exhaust.

Then i did plug chops again after i color tuned it and found my mixture was a light grey to light tan. All your cylinders may not be the exact same color when you get it running right, but if the color of the plug is correct then you know you are not so lean you will burn a hole in your piston or blow a head gasket.

Plug chops are the bible for leading you in the right direction, the color tune will just confirm what you have already done to adjust the engine and wether or not it has made a difference.

Propane actually was better for me in determining if i had a leak, none of my boots or seals were leaking however i still had a hanging idle, that's when i read my plugs and realized they were dangerously lean!!

It has now been over six months that the bike starts up right away with half choke ( new seals and new choke cable ) and thanks to melodicmetalgod for providing the correct pilot air jets it has run perfectly, idles at 1000 when warm and about 800 RPM when cold.

When tuning just don't give up and be methodical. Hope this helps.

[76FJ55]

Just to verify: you are trying to sync the cards using the screws on the throttle shaft, not the Idle mixture screws in the carb bodies, correct?

[MelodicMetalGod]

Sounds very similar to how my bike was when I first rec'd it. Ultimately, besides the cleaning the carbs throughly, the only issue was poor seals at the intake boots. The originals looked fine to me at the time, boots and o-rings. It wasn't until I finally broke down and bought a set of boots and rings from LeeGS550E. Once I had the new rings in hand I realized just how flattened (and ineffective) the original rings were. The second I installed them, no high idle.

Best of luck!