Mikuni carb repalcement

[cntymnty]

I've been chasing my tail trying to get the original Mikuni 38 mm carb to work correctly in my restored 78 390. For a new replacement it looks like the Mikuni VM 38-9 is what I want. Everyone agree?

 

[suprize]

lectron?

 

[grouty]

Lectron ...

 

[stormer254]

Lectron and Smart carbs are both very good but you should be able to make a Mikuni work All my Autos carburate well with the only change from stock setup being the main jet and that is usually only a size or two!

 

[grouty]

Just perfect ...

 

[suprize]

big brother has banned all EX EU communications???

 

[justintendo]

the problem is paying money for a new mikuni..if you are doing that you are better off paying more and getting an upgrade for the leccy. the mikuni can be tuned well.

maybe you can tell us exactly what the problem is with yours you have now? unless its corroded or the slide bore is worn out there is no need for a new one.

 

[GaryM]

New lectron is quoted to me by the maker at about $450 complete. Seems high but that was quote. Mikuni $99

 

[Darrel78]

I’m downsizing my bikes/spares. I’ve a used 38mm lectron with the cable and throttle that was mounted on my WXE360 for $250 plus the shipping.

 

[street2dirt]

The Lectron is worth the $$$ just for the time saved not messing with jetting. Plus, temp or altitude changes don't affect it. You can sell your extra carbs & parts and recoup some cash..... Stop chasing your tail...

 

[Crashaholic]

I agree with justintendo. Share with us the problem(s) you're having and maybe someone can offer a simple and cheap resolution.

 

[cntymnty]

I appreciate the push for the Lectron, but for a variety of reasons, I'm staying with the a Mikuni. That said, I've got the mid range and WOT dialed, its the idle circuit I'm having issues with. From idle to about a quarter throttle, its rich and blubbers, as soon as I open it up, it cleans up and runs great. The plug is a nice tan, so I know the overall jetting is good. I've gone from the 45 idle jet that Husky calls for to a 40, then a 35, and now to a 30, adjusting and playing with the air screw each time...and this is at various altitudes from 700 feet to 3500 feet. Each time I drop a size, it improves, but it's still an issue. On a 390, I should be able to motor around a track just above idle and it run clean. It was suggested I file some of the slide cutaway down to get more air in for the idle circuit. Husky calls for a 2.0 slide. Somebody has already file the slide cutaway to about a 4.0 ! So, instead of chasing the tail even more, I figured I would just buy a new Mikuni 38 and start from scratch. After spending thousands on the purchase price of the bike and resto, a hundred dollars for a new carb is nothing.

 

[grouty]

Sounds like you have answered your own question. If the slide should be a 2.0, and it now has a homebrewed 4.0, then why not just buy a new slide or a spare from your other bikes. I would guess the fix is about 25$

 

[Crashaholic]

I've got the mid range and WOT dialed, its the idle circuit I'm having issues with. From idle to about a quarter throttle, its rich and blubbers, as soon as I open it up, it cleans up and runs great.

Husky calls for a 2.0 slide. Somebody has already file the slide cutaway to about a 4.0

I've gone from the 45 idle jet that Husky calls for to a 40, then a 35, and now to a 30,

Excellent post describing the situation. It appears that the carb has been leaned out quite a bit the in lower rpm range without any positive change. The mods to the slide are even a step in leaning it out even more. I just went through the same low rpm blubbering problem on a 70 400 and it was due to air leaks at the intake and base gasket. Like your motor mine ran great from 1/4 to WOT. Ironically it had even passed and leak down test after a rebuild. Goes to show that when you think one thing is okay, like gaskets and seals, things break.

 

[cntymnty]

Crashaholic, wouldn't an air leak cause a lean condition vs a rich condition at low RPM? I haven't done a leak down test, but the motor was completely rebuilt by John at Vintage Husky. When I put my intake back on, I used a new rubber boot, new gaskets and applied aircraft gasket sealer on every gasket surface. I've had the issue since I first fired the bike up. I called SUDCO (mikuni carb dealer in California) to replace the butchered slide ( I don't have any spares). Naturally, my original carb is obsolete and any slide I order would be a slightly different dimension (Bob Villa, This Old House) Even for a rebuild kit, I would have to take the carb apart and give SUDCO the dimensions of every part I wanted to update....The tail chase...A new carb is starting to look better, eh?

 

[husqyhamm]

Airjet??
What reeds you running?

 

[GaryM]

That price quote is for newly made Lectron Carb right from the Factory here in Texas. I can still get 30 old lectron for that 250 price
but it does not have the adjustable jet etc. Of course I was surprised by that price too !!

What lectrons are you purchasing for 250 ?? new ?

 

[Crashaholic]

wouldn't an air leak cause a lean condition vs a rich condition at low RPM?

I may have misunderstood your post.

My experience with air leaks is that they screw with throttle response from idle to 1/4 throttle. When reading your post I associated the term blubbering with an engine that is cycling between fuel starvation and then suddenly getting enough fuel to raise engine rpm only to return to the starving condition. This happens as the rider works the throttle between idle and 1/4 throttle in an attempt to keep rpm steady while under load. If the motor were getting to much fuel at 1/8 throttle I would think that when under load it would die.

Another thing that comes to mind is a plugged idle circuit, that is to say the circuit outside of the idle jet itself. Doesn't take much.

From what you said about the rebuild it certainly sounds like the vacuum system is tight but......

What do think?

 

[Kartwheel68]

I appreciate the push for the Lectron, but for a variety of reasons, I'm staying with the a Mikuni. That said, I've got the mid range and WOT dialed, its the idle circuit I'm having issues with. From idle to about a quarter throttle, its rich and blubbers, as soon as I open it up, it cleans up and runs great. The plug is a nice tan, so I know the overall jetting is good. I've gone from the 45 idle jet that Husky calls for to a 40, then a 35, and now to a 30, adjusting and playing with the air screw each time...and this is at various altitudes from 700 feet to 3500 feet. Each time I drop a size, it improves, but it's still an issue. On a 390, I should be able to motor around a track just above idle and it run clean. It was suggested I file some of the slide cutaway down to get more air in for the idle circuit. Husky calls for a 2.0 slide. Somebody has already file the slide cutaway to about a 4.0 ! So, instead of chasing the tail even more, I figured I would just buy a new Mikuni 38 and start from scratch. After spending thousands on the purchase price of the bike and resto, a hundred dollars for a new carb is nothing.

The range of throttle you are having problems with is not adjusted with the pilot. The pilot jet ONLY functions when the throttle is completely closed (actually it still meters fuel above that but it no longer is useful as a tuning tool above idle), as soon as you crack the slide open even the slightest bit you are now on the needle jet/needle/slide cutaway. You should go back to a 45 pilot, try a 3.0 slide and get 2 sizes leaner needle jets, that will clean up your idle-1/4 blubber, or try your home filed 4.0 with leaner needle jets.

 

[suprize]

old carb, bits worn, get a new one and start again. the lack of response to jetting changes is a dead giveaway. Lectron is the rolls royce solution, new mikuni is the top of the range f250. do it and get back to us when youve fitted it