Mikuni atomizer pop off

Bradley71

Member
I been reading some,and a lot say 1lb of pressure drop maximum.how do they get 1 lb.best i can get is 3lb of drop..pop off set at 8 dropping to 5 and holding strong.what am i doing wrong.do i need to use shims under the spring.im using a spring kit from competition carbs..🤷🏻‍♂️Somebody help me out here.the needle and seat is from speed parts
 
From what I've seen, that's about as good as it gets. When it pops at 8 your gonna lose more than a lb before it can get sealed back up. Holding at 5 after popping is good. Some will leak down slowly after popping and still function properly as long as they hold tight coming up from 0 to their pop off number. It will have good fuel control working like that. I'm assuming alky with 8 pop, how big is the seat?
 

Bradley71

Member
From what I've seen, that's about as good as it gets. When it pops at 8 your gonna lose more than a lb before it can get sealed back up. Holding at 5 after popping is good. Some will leak down slowly after popping and still function properly as long as they hold tight coming up from 0 to their pop off number. It will have good fuel control working like that. I'm assuming alky with 8 pop, how big is the seat?
Ya know i really dont know.i got it off ebay from speed parts.its been a good needle and seat bout a yr old.im glad someone said 5 would be fine.bc i would have thought ive been doing it wrong for the last 10yrs😂
 

Pete_Muller

Moderator
Bradley,

Don't sweat that drop. Even something like that type of pressure tester you use (or *how* you use it) can effect how much it drops before it seals back up. As long as it *does* seal, you'll be in good shape. (y)

PM
 

frc71

Member
Bradley - if your Mikuni popoff "pops" at 8psi and hold pressure at 5 psi, then the carb will work fine.
 

Bradley71

Member
Bradley,

Don't sweat that drop. Even something like that type of pressure tester you use (or *how* you use it) can effect how much it drops before it seals back up. As long as it *does* seal, you'll be in good shape. (y)

PM
Well i got to reading those ec tricks lol it said maximum amount of drop to be 1 lb.i thought yeah sure how the heck do ya even get that.
 

Bradley71

Member
Heres where i seen that😂
 

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It figures that came from EC, he was a absolute perfectionist on everything he did. He was probably testing with the pump stack off, and he did use wd40, both will help the needle seal. But as been stated several times now , your fine with what you've got. In the 26 years I worked with EC I saw first hand his attention to detail and the skills he had lead to some very fast racing equipment.
 
The reason I asked about the needle and seat is the Mikuni's seat is .080 which is marginal for alky. Most have changed over to a bigger Grose jet with higher fuel flow. If your 100 cc your probably fine , don't put it on 135 or 150 without more seat flow though. There's a couple options to get more.
 

Bradley71

Member
The reason I asked about the needle and seat is the Mikuni's seat is .080 which is marginal for alky. Most have changed over to a bigger Grose jet with higher fuel flow. If your 100 cc your probably fine , don't put it on 135 or 150 without more seat flow though. There's a couple options to get more.
I havent seen alot of different needles and seats for them.i had some grose jets but people told me they are a pain to deal with so i stayed clear of them..
 

Bradley71

Member
The reason I asked about the needle and seat is the Mikuni's seat is .080 which is marginal for alky. Most have changed over to a bigger Grose jet with higher fuel flow. If your 100 cc your probably fine , don't put it on 135 or 150 without more seat flow though. There's a couple options to get more.
Hey jon where can we find these other options of needles and seats at.that would be useful information.
 
Sure , one is a seat Carol has at EC dist. I use it if it's going on a 100 cc engine. It has a .090 size seat , which will help , and uses a tillotson needle. When I build a Mik for a big engine , 135 , 150 , I've had Mark Bergfelt of UAS fame install a seat he sourced from a lawn and garden carb. It was really big. Not sure if he still has them. The Grose jets are a pain , big fuel flow , and then they quit sealing.
 

Pete_Muller

Moderator
I'll add my experience with Grose jets: I have *never* had any success with them.

My "guess" is that it's due to the nature of the fuel flow being more "on or off" with a Grose jet, whereas fuel flow with a needle and seat is more related to how much the needles is being moved off its seat.

Plus, I would always prefer a rubber-tip needle (Viton) on a brass seat, as it just seems like vibration would be less of an issue than with a glass ball on a metal seat.

PM
 

rainman

Site Supporter
I'll add my experience with Grose jets: I have *never* had any success with them.

My "guess" is that it's due to the nature of the fuel flow being more "on or off" with a Grose jet, whereas fuel flow with a needle and seat is more related to how much the needles is being moved off its seat.

Plus, I would always prefer a rubber-tip needle (Viton) on a brass seat, as it just seems like vibration would be less of an issue than with a glass ball on a metal seat.

PM
I am not an expert at all but for some reason my best carb ever has been a Tillotson EC built for a friend of mine who built an open engine and is a Grose Jet carb, and has given me less trouble than the needle ones. Might have been lucky or maybe this particular carb is really good. Anyway this is one carb I do not want to change into a needle type. Problem is last time i did have the spared\s needed to replace the Grose jet but it is gonna be hard to find them in the future.
 

Pete_Muller

Moderator
I'm the first to say: "If it's working, don't mess with it!" :)

I just "personally" had my 2-cycle Open engine carb work quite a bit better with a rubber tipped needle.

PM
 

rainman

Site Supporter
Rainman , check with Kermit for a ball jet replacement, he's using them in his billet twin carb setup for the 250.
Last time you did a great job. though I was lucky to have the ball jet. i have to check and see if I have any left. Will that ball jet be the same size as mine? I get my Sudam built by Kermit so would be a good option for me if he has them. Thanks a lot, John.
 
Rainman, not sure on the sizes. They should be pretty close to what you have. You probably have a .112 , that was a popular size for the open Briggs flathead opens back in the day and we kept a few around.
 
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