High Speed Jet (Fixed Jet)

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A few weeks back there was a discussion about the high speed jet, called the fixed jet in a Stihl IPL, and its role in tuning the carburetor.

A little research shows that the US bound MS660s are equipped with a Walbro WJ-69 carb. The IPLs that I have show a .64 jet as standard, with a .62 jet optional for high altitude operation. After porting my 660 I found that the H needle needed to be 1.25 turns out to get the saw to four-stroke. I figured a new jet was needed. I ordered the 1122 121 5607 .74 fixed jet from the dealer. This jet is found in the WJ-76 carbs found on saws not headed for North America.

When I disassemled the carb I found a somewhat unpleasant surprise. The saw had been shipped with a .62 jet. EPA strikes again.

I thought I would share a quick how-to for replacing this jet.


1. Remove the carb and orient the top toward you.

<a href="http://www.flickr.com/photos/41385771@N03/5196813864/" title="top by zweitakt250, on Flickr"><img src="http://farm5.static.flickr.com/4087/5196813864_739d8513e4_b.jpg" width="1024" height="819" alt="top" /></a>


2. Remove the top cover. This will expose the fixed jet. It is the slotted brass part below the lever.

<a href="http://www.flickr.com/photos/41385771@N03/5196214377/" title="62 by zweitakt250, on Flickr"><img src="http://farm5.static.flickr.com/4086/5196214377_24bd00603b_b.jpg" width="1024" height="819" alt="62" /></a>


3. Remove the fixed jet. Be sure to use the correct size screwdriver as brass is soft and unforgiving.

<a href="http://www.flickr.com/photos/41385771@N03/5196215083/" title="driver by zweitakt250, on Flickr"><img src="http://farm5.static.flickr.com/4071/5196215083_bef930bb14_b.jpg" width="1024" height="819" alt="driver" /></a>

4. This is the new jet to install. The jet number is stamped on top.

<a href="http://www.flickr.com/photos/41385771@N03/5196815358/" title="new by zweitakt250, on Flickr"><img src="http://farm5.static.flickr.com/4131/5196815358_83c24c1d4e_o.jpg" width="762" height="610" alt="new" /></a>



5. The new jet in place. Reassemble the saw then tune as usual.

<a href="http://www.flickr.com/photos/41385771@N03/5196816190/" title="74 by zweitakt250, on Flickr"><img src="http://farm5.static.flickr.com/4083/5196816190_86042fa91f_b.jpg" width="1024" height="819" alt="74" /></a>





After the .74 jet was installed my 660 now four-strokes with the H needle 7/8 of a turn out. That is how it should be.


Thanks for looking!


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What difference does it make if it's at 7/8 or 1 1/4 turns out?

blsnelling said:

What difference does it make if it's at 7/8 or 1 1/4 turns out?

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You were part of that discussion Brad, do you not remember?


It puts the needle in a position relative to the seat that allows the needle to have a proportional effect on the mixture. As the needle moves farther away from the seat a non-linear change in the mixture is a result. That is why the non-EPA saws have the .74 jet.


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Tzed250 said:

You were part of that discussion Brad, do you not remember?


It puts the needle in a position relative to the seat that allows the needle to have a proportional effect on the mixture. As the needle moves farther away from the seat a non-linear change in the mixture is a result. That is why the non-EPA saws have the .74 jet.


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Click to expand...

Now we have the reason with the pics BTW, how did you get such clear macro pictures?

blsnelling said:

Now we have the reason with the pics BTW, how did you get such clear macro pictures?

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The pics were made using my D2Xs and 50/1.8 with a Canon 6T close-up lens.
On camera flash bounced off of the ceiling. Thanks!


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Tzed250 said:

You were part of that discussion Brad, do you not remember?


It puts the needle in a position relative to the seat that allows the needle to have a proportional effect on the mixture. As the needle moves farther away from the seat a non-linear change in the mixture is a result. That is why the non-EPA saws have the .74 jet.


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Click to expand...

I'm not sure what all you just said but it sounded good anyway.

Tzed250 said:

You were part of that discussion Brad, do you not remember?


It puts the needle in a position relative to the seat that allows the needle to have a proportional effect on the mixture. As the needle moves farther away from the seat a non-linear change in the mixture is a result. That is why the non-EPA saws have the .74 jet.


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Click to expand...

It actually makes perfect sense as the taper in the seat gets larger. Thanks for the info.

Nice info.
Thanks.



Lee

How much does the jet cost you? I would'nt think too much. I'm wanting to open mine up now to see which mine has, .64 or.62.

Can you feel any gain in power, just from changing that jet?

Dang, got me wanting to check mine too.

thanks for the info.

Anthony_Va. said:

How much does the jet cost you? I would'nt think too much. I'm wanting to open mine up now to see which mine has, .64 or.62.

Can you feel any gain in power, just from changing that jet?

Click to expand...

The jet was $5.99.

No power gains. The correct mixture is still the same, it is just delivered with the needle in a different spot. The carbs are engineered to work with the needle @ ~1 turn.

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Tzed250 said:

3. Remove the fixed jet. Be sure to use the correct size screwdriver as brass is soft and unforgiving.

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Nice pic and description.

Probably old news to most, but having a set of gunsmith screwdrivers really makes carb work easy.

Here's the one I have.

Actually it's 2. I lent my set to a buddy and he broke some of the bits, so now I have spares.

Tzed250 said:

The jet was $5.99.

No power gains. The correct mixture is still the same, it is just delivered with the needle in a different spot. The carbs are engineered to work with the needle @ ~1 turn.

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Click to expand...

So the real gain is "tunability", right? Thats enough for me to want one too.

Anthony_Va. said:

So the real gain is "tunability", right? Thats enough for me to want one too.

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Yep. I just went thru this a few weeks ago on an 044 project.

Great Thread!!! Thanks Tzed

Anthony_Va. said:

So the real gain is "tunability", right? Thats enough for me to want one too.

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Exactly, just remember, you can tune a saw...but you can't tune a fish!


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Modifiedmark said:

I'm not sure what all you just said but it sounded good anyway.

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Exactly. Straight over my head

Thanks for the great write up and detailed pictures!

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You're welcome! I enjoyed doing it

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Tzed250 said:

Exactly, just remember, you can tune a saw...but you can't tune a fish!


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LOL tunability good one, great thread thanks for pics and your time

74 is a big jump from 64, the 066's down here have the 34 lo and 64 hi and they're not leaned out at that size unless the needles damn near closed, would have thought 74 would be easy to bog-down rich and fumy and hard to lean out for a revvy top-end without bending the fuel-lever in all directions, a 66 or 68 jet would be better than a 74 unless the saws got a souped-up engine

pgg said:

74 is a big jump from 64, the 066's down here have the 34 lo and 64 hi and they're not leaned out at that size unless the needles damn near closed, would have thought 74 would be easy to bog-down rich and fumy and hard to lean out for a revvy top-end without bending the fuel-lever in all directions, a 66 or 68 jet would be better than a 74 unless the saws got a souped-up engine

Click to expand...

Souped up....I guess you could call it that:

http://www.arboristsite.com/showthread.php?t=153849