High Speed Jet (Fixed Jet)

[Terry Syd]

Yeah, some of the bike carbs could get fairly complex. I knew my way around carbs like Mikunis. I used to make my own jets, slide cutaways, turn needles, etc. I didn't measure my collection of brass jets in numbers of pieces, it was more like in pounds. When I first looked at a diaphragm carb I wondered how the simple thing could work as well as it did.

Working with the POP was the big 'ah ha!' moment for me.

 

[Bill Becker]

One look at the tuning possibilities for a Keihin FCR and it will be made clear how simple these little pumper carbs are.

Ditto!!! on that. I have some modern Yamaha and KTM 4 stroke dirt bikes with Keihin FCR flat slide carburetors. They have far more adjustment possibilities than the all position diaphragm carburetors on chain saws. They also are designed to run at different throttle positions, chain saws are designed to run at 2 speeds: idle and full throttle. Interesting point on these jets is that there are different sizes for the same model saw depending on which country it is sold in, when was it made and what style exhaust is on it. For instance I've seen a 10 year old 046 with a factory installed dual port muffler have a .70 jet, a 7 year old 046 with a factory installed DPM have .64, a 2 year old MS460 with a solid front muffler have a .68, a new MS460 with a solid front muffler have a .62.

Of all the saws I've owned and run the ones with the best performance (idling, throttle response and full throttle cutting) were the 066, 020T and 048 I had 16 years ago. I still have the 066, best running saw I have today, it will idle for 5 minutes and rev to full throttle with no hesitation.

 

[David (saltas)]

I love my bike carbies

Fortunately a chainsaw only has to idle when it starts, and run WOT that is why they get away with being so primitive

The throttle on a chainsaw only needs to be a two position switch

You may want more than that from your saw, but It can be as simple as I suggest , show me were the user manual instructs you to cut with anything less than WOT.

 

[one.man.band]

all just different ways to get to the same proper fuel/air ratio at WOT. in my mind, that is the only thing that matters anyway.

just a thought: maybe the only thing that is EPA related is the # of turns out recommendation? walbro says anything from 1 to 4 turns out, and stihl says 1. but stihl puts their carb on.

 

[Tzed250]

For all of the nay sayers that wanted to poo-poo on this thread about the need for the larger fixed jet, I just found this interesting tidbit...

RBtree -> I was a bit out when I thought adding a Magnum front to an 066 muffler only doubled the area...

The standard muffler on a USA post 1998 066 or MS660 has an exit hole of 0.43 inch diameter. Prior to 1998, this was 0.63 inch. In 1998 they also made changes to the cylinder porting to reduce emissions (not sure what they did).

The USA Magnum muffler front has a internal rectangular hole (with rounded ends) of 1/2 x 1 9/16 inches feeding an external slot of 5/16 x 1 1/2.


So, area of outlet:

Prior 1998 : 0.311 square inches
Post 1998 : 0.145 square inches

Add a Magnum muffler front to either : add 0.469 inches... (and it might be a tad more than this).

Yes, the saw winds up pretty good with a magnum front and you have to remove and reset the limiter caps to richen the carb.

The internal box with all the smaller holes in it is the same for all the 066 mufflers.

That Post 1998 size sure seems puny in comparison to the other numbers!


NOTE: anyone messing with an earlier 066 muffler needs to make sure they have a main carb jet of at least .72mm. The "red eye" 066's had a main jet of .66mm (because the ignition limited the max rpm). Stihl published a tech note to say that using the .66 jet and turning up the rpm will result in piston seizure... as the saw couldn't get enough gas at high rpm.

 

[295 tramp]

WOW!
I just read the whole thread,i have to say you guys have the knowledge of trained engineers.
Most of you i can tell you have that racing background, as to shave a 1000th of a second here and there.
I have to say this is a great post and the responce here was very educational.
Thankyou:msp_thumbsup:
Randy

ps
I diddn't look at the date when this had post
i'm trying to catch up, so much to read and learn