Removing Mikuni Pilot Circuit Plug

[Sam280Z]

I have triple Mikuni 44phh carbs on my 280Z. The number two cylinder seems to be dead, but only at idle. I want to remove the pilot circuit covers to look inside but i have no idea how to remove them. I do not want to damage the carbs. Anyone know how?

 

Thanks,

Sam

[Tony D]

Before you go there, check the idle mixture adjustment screw against another one.

 

I have seen countless screws with the tip twisted off and lodged in the idle port, causing this problem. People overtorque the screws when 'seating' the screw, and when they back-off to start idle fuel adjustment, the tip remains in the body.

 

REmoving the broken tip is as easy as pulling the carb, inverting it, and pressing carefully with the tip of a screwdriver---you can usually see the tip sticking out of the idle mixture hole!

 

Other than that, anything that would get into those passages should be relatively easy to dislodge using high pressure air, with an appropriately Non-OSHA Approved Rubber-Tipped Blow Gun (for that 'occlusive seal' to really put pressure on the passages and blow what ails-ya out!)

 

As I recall, there are some larger flat-bladed screws that you can remove to open up access to the idle/transition ports for inspection. It's where the machinery in production drilled the holes (but this may be a Weber or Dellorto...)

 

If you mean the simple lead BB's pressed into the body, make sure Wolf Creek Racing can supply you with new ones before you remove them! Because the only way I know of to get 'em out is by drilling them out. And once they're gone...if you can't replace them, it becomes a case of the cure being worse than the original disease!

 

Good Luck.

[Sam280Z]

Thanks for the response. After I posted this i went out and fiddled with them a bit - took out the idle screw and jet while it was running. No change in running without them. When I put them back in, that cylinder picked up and started running again. Must have been a piece of junk in there.

 

I rebuilt the carbs a few years ago and there was a phenomenal amount of encrusted beige powder (kind of the consistency of drywall dust) in the bottoms. My concern is that some of this may have made its way into the pilot circuit passages.

 

Webers and Dellorto have the inspection cover you speak of. Mikunis have a plug that has a slot in it like it may have been screwed in. But every slot on every carb is aligned perfectly; they are peened to the carb body in two places; and they are labeled "L" & "R" with "L' on the left barrel and "R" on the right.

 

I'm not scared of undoing the peening, I just don't know what lies beneath and if I'll be able to reassemble.

[Tony D]

I have a milk crate of old 40PHH's out in the shed, if I have some time today, I'll go knock one apart and take some photos for you.

 

But really, if you soak the bodies in carb cleaner and blow it out with a good compressed air gun with a rubber nozzle, it should dislodge everything in there. Blowing backwards through all the circuits if you can also is a good idea.

 

And for anybody else reading this, no, they are not for sale. And yes, I do mean literally a 'milk crate full'...I'll take photos of it as well...LOL

[Sam280Z]

Thanks Tony, pictures would help a lot. I have blown air though them. The stuff in the bottoms of the carbs when I rebuilt them was almost like concrete.

 

The thing that concerns me is that I have 52.5 pilot jets in and it takes 1.75 turns out on the idle screw for the best/fastest idle (measured with a vac gauge and tach on my timing light). The "How to Modify" book says that if you have less than 2.5 turns out, you probably need a leaner pilot. However, depending on the weather, I can occasionally get a lean pop out of the intake at cruise.

[Tony D]

Crap, I got called out on a job, so I didn't get back there.

Just got back home.

Anyway, my stock engine has 55's for idle jets, 52.5's sounds small to me I'd expect a lean pop with that small an idle jet! Many people undersize those jets. They forget the huge accel pump squirt and nozzle size adjustment to mask idle to main transition. Up to probably 3000rpms, you basically run on the idle jets in a mikuini. That's how a set of 40's on an L24 with an automatic and a 4.11 gearset turned in 28mpg at 65 driving LA to Phoenix (least, that's how I rationalize it!) Pathetic when your "Economy Chase Car" gets worse mpg than the hot 'sports car' you are trailing!

I'll get to the crate tomorrow. No concrete to pour, so I should be able to do it no problem.

 

"Turns Out" is a relative thing. I have seen people who swear 1/2 a turn is 1 turn. Guess the question begs is a turn 360 degrees from seated, or 'one turn of the hand' in which case it's only 180 degrees....

[Sam280Z]

I count turns as 360deg.

I thought 52.5s were small too. but according to the "How to" rule of thumb, that would be too large (I don't believe this is the case). My next size up that I have on hand is 57.5.

 

Thanks again

[Sam280Z]

I put the 57.5s in. It seems a bit better, but may be on the rich side now. It still only takes about the same number of turns out, which leads me to question the advise in the "how to modify" book.