Cross Pop-up Piston in MS390 topend

[blsnelling]

I did retune

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The saw was much richer with the flocked filter.

 

[Terry Syd]

I wear hearing aides, so I have a hard time hearing a lot of things, especially high frequencies. It did sound like it was holding a higher RPM in the wood, so that may be why it sounds 'lean'

I use a small tach that is mounted on the saw to monitor the speed in the cut. That is a much more reliable indicator to me of what the saw is doing. Heck, the difference between 10,000 rpm and 10,200 rpm is only a 2% difference and I don't think I could pick that up even if I had good hearing.

 

[Terry Syd]

I got to thinking about how the Zama twin jets are no longer available and how people will have to start modding the HDs instead. So I decided to purchase a cheap Chinese HD for the 029 off of Ebay. It cost A$17.50 (US$14.00).

All new parts, it won't need a re-build, and I can hack it up with a few ideas I've got. If it turns out a disaster, I can salvage the carb kit out of it.

For those that are afraid of ruining an OEM carb, it is a good option. You can keep your OEM carb as a standby while you play with the Chinese carb.

 

[Terry Syd]

I wish I had an HD-11 carb to look at, I understand they are a good carb for this size engine.

The HD-5 carb I have was made for the 54-64cc saws. I've measured the airbleed/transition holes and compared them to a few other carbs I've got (none of which would be a good match for the HD).

It appears that the first transition hole is .6mm and the second is .5mm. Compared to other carbs I have, I think the second hole could be taken out to .55 or .6mm to get more flow out of the low speed circuit.

When I thought about pulling the welch plug to drill the airbleed holes, I realized I could block the EPA circuit from inside the welch plug area with just a dab of JB Weld - there would be no need to push out the main discharge nozzle and block the circuit from that end. - It may be possible to make the carb mods much easier.

 

[CR888]

I love reading your carb insights Terry. Imagine the collective amount of increased HP is in the hands of those at the wood pile around the globe due to your posting. Its good stuff.

 

[fearofpavement]

So has anyone actually documented compression increases with the domed piston? ie, try a stock piston, measure compression, replace it with a domed piston, check compression. Note any differences...

It's complicated to do these types of checks on this model saw but just wondered if someone has "bothered". I'm curious but not curious enough to do it myself. I have a couple of these domed pistons but haven't tried one yet...

 

[Clemsonfor]

So has anyone actually document compression increases with the domed piston? ie, try a stock piston, measure compression, replace it with a domed piston, check compression. Note any differences...

It's complicated to do these types of checks on this model saw but just wondered if someone has "bothered". I'm curious but not curious enough to do it myself. I have a couple of these domed pistons but haven't tried one yet...

I will do it. I have this postin to install on my 390. But its bot high on my list. I will do before and after comp tests. Unless someone beats me to it.

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[Terry Syd]

I love reading your carb insights Terry

Thanks, I just find stuff like this interesting to explore. - I'll check out this first Chinese carb and if it is a viable carb, then I may purchase a couple more to work on. I can mod one at a time and compare the results to the previous mod. I'm sure I will end up with at least one carb where I went 'too far' - oh well, a spare carb kit...

 

[RI Chevy]

You might be able to get an HD50 carb and try it?
A small list of 440/460 carbs I put together after a call to Walbro tech.

 

[Terry Syd]

Great document, I wish we could get a similar document for ALL the carbs. Heck, tuning could become a 'paint by numbers' affair - just look up the sizes and mod the carb.

I put the HD back on the saw and although I expect I will be able to get as much if not more performance from the HD as compared to the Zama, the idle on the HD just isn't as smooth. It has new diaphragms, it just doesn't want to idle as smoothly. The Zama will idle all day in any position, just ticking over - it is a delight to use.

I think I may have a fix (by inserting a small spring under the pulse diaphragm on the pump diaphragm), but I'll wait until I start working on the Chinese carbs to see if I can find a fix. If so, I think the HD will become a quite nice carb to use.

 

[Terry Syd]

I got off my butt today and went out to work on the HD. Whereas the Zamas have a long tunnel that runs towards the idle port that lines up the bubbles in the idle circuit via capillary action, the HD has a bigger and odd shaped cavity under the welch plug.

The Zama 'freight train' of bubbles ensures a more uniform idle mixture to the idle port. The HD has various size bubbles that shift all over the place. The Zama idle circuit is far superior and allows a better idle (and when you adjust the 'low speed' needle it only adjusts the idle mixture, not the entire low speed mixture).

So I decided to try and redesign the idle circuit on the HD. I found a small stainless steel tube that was approximately the ID size of the Zama circuit. I could only get about 5mm of length in the HD cavity and set it in place with some of the local 'JB Weld' substitute. While I was in there I opened up the forward air bleed/transition hole to .55mm.

After pizz-revving the saw in the backyard, it appears to have a better idle and a better throttle response. I'll have to put it in some wood and give it a hammering to really check it out, but it looks good at this point.

Good thing I'm retired, it took about 4 hours to do.

 

[paragonbuilder]

I got off my butt today and went out to work on the HD. Whereas the Zamas have a long tunnel that runs towards the idle port that lines up the bubbles in the idle circuit via capillary action, the HD has a bigger and odd shaped cavity under the welch plug.

The Zama 'freight train' of bubbles ensures a more uniform idle mixture to the idle port. The HD has various size bubbles that shift all over the place. The Zama idle circuit is far superior and allows a better idle (and when you adjust the 'low speed' needle it only adjusts the idle mixture, not the entire low speed mixture).

So I decided to try and redesign the idle circuit on the HD. I found a small stainless steel tube that was approximately the ID size of the Zama circuit. I could only get about 5mm of length in the HD cavity and set it in place with some of the local 'JB Weld' substitute. While I was in there I opened up the forward air bleed/transition hole to .55mm.

After pizz-revving the saw in the backyard, it appears to have a better idle and a better throttle response. I'll have to put it in some wood and give it a hammering to really check it out, but it looks good at this point.

Good thing I'm retired, it took about 4 hours to do.

Can you post some pics showing what you did Terry?


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[Terry Syd]

Let's see if the concept works first. I had the small stainless steel pipe, so I used that. However, if the pipe hadn't been available I was going to use some insulated wire, rub grease into the insulation, and then pour JB Weld over it. Then pull the wire out. I used that method when I had to create another oil channel to adopt the K095 bar to the 029.

The wire trick may be an even better method as the channel could be made a bit longer.

EDIT: I fired up the saw again this afternoon. I don't think my idle circuit mod did much of anything. I may have to use the wire trick using a smaller diameter wire than the 'pipe'. At least the carb runs good off of idle!

 

[Terry Syd]

I've used the saw a few more times and it runs ok, I just don't like the idle. Maybe I got spoiled by the Zama idle circuit, but this thing works and doesn't die, it just isn't as smooth as the Zama.

I got the cheap Chinese HD today - and its cheap. They eliminated two brass jets to cut down on the price. No biggie on the elimination of the auxiliary high speed jet as I have to block that off anyway. However, they eliminated the auxiliary low speed jet that I open up for more grunt off the bottom.

They also do not have the HD drilling pattern for the air bleed/transition holes.

Those two things are my primary interest in what makes the HD a good carb to mod. Without them, this thing is a dud.

The only good thing they did was to eliminate the EPA circuit of joining the high and low speed circuit off the high speed needle. This carb is drilled directly from the wet side to feed the low speed needle.

So, I got a 'carb kit' for my purchase. I also like the throttle valve it has and I think I can use it to clean up the idle a bit. Just have to try it.

I'll stop by the Stihl shop and see if they have any old HDs in a bucket. I can work with them - and I can easily rebuild one as I now have a carb kit handy...

 

[stretch5881]

Well, I didn't get to try my pig with lipstick in wood. A friend took it home with him while I was repairing his. Today he said he'll buy it. He didn't care about price either. He said that it cut in 10 minutes as much as his old saw did in 2 hours. He also cut the rakers down and it didn't dog a bit.
Thanks to you all for the education.
Now I have to find another one or two.