Help building,tuning a Strato, Husqvarna 445

[Terry Syd]

Compression

No, unfortunately I don't have a compression guage. If you think you might need a new ring, how about squirting some oil down the sparkplug hole and see if that helps seal the cylinder. If the compression noticably improves then maybe a new ring is in order.

If one of the piston manufacturers starts making a domed piston with a full width intake skirt for these stratos, they may fine an eager market with tuners. With the full width skirt the intake port could be opened up (although the strato function would no longer operate). If the 346 piston skirt is long enough, it might be an option for widening the intake port.

As a clamshell design we can't remove the cylinder gasket to improve the compression. So if I could get a domed piston for the thing I would have already ordered it.

I had a bit of trouble getting mine going after the porting. The engine wouldn't run under load. I had set the coil at the factory .03mm gap. The specifications say .03mm plus or minus .01mm.

I isolated the problem back to the coil and then tried tightening the gap - the engine wouldn't start. I then opened it up to .035 and she came to life.

The specifications are misleading in that it appears the engine should run if the gap is .02-.04mm. It looks like you may have to fine the sweet spot in that range.

I am going to play around with that gap. I'll try different settings and then with the sparkplug pulled out and laying on top of the cylinder watch the spark. I'll see what gap will give me the fatest spark at the lowest RPM.

 

[Terry Syd]

Carburetion

Rick, I tried running with the Strato function disabled and found out that this engine needs the extra air through the strato ports. This strato design is more than just putting a layer of air over the mixture in the transfers which gets blown out during blow down. On my engine there is considerable mixing of the two charges. Essentially the first mixture into the cylinder does get mixed and would be very lean, some of that lean mixture may then get shortcircuited or blown out, however the rich mixture behind it is what keeps the engine running.

Another thing I noticed was if I leaned down the low speed adjustment, I got a hick-up as the strato valve opened. That strato valve is taking a big gulp of air.

I tried taking out the venturi to 11.5mm. I didn't want to go any further until I checked that the High speed screw had enough adjustment to go further. I'm glad I did as at 11.5mm I have run out of adjustment. I ran out at about 2.5 turns out on the screw. If I keep turning the screw out, I can't get the engine to four-stroke. I think the fuel ratio is probably alright at this point, but I will have to get the saw into some wood and check the plug.

I was surprised when I looked at the surface of the venturi. There was a casting ridge all the way around the middle of it. Even if a person didn't enlarge the venturi the carb would flow better just by smoothing out the ridge.

Since you are running the 445 you won't run out of adjustment as quickly as I did by enlarging the venturi.

I'll take a look tonight and see if there is some main jet I can drill out to increase the fuel flow. If not, then I will have to get the other carb.

After richening up the low speed the idle was running a bit ragged. It was a little too rich at idle. I fine tuned the idle by shifting the throttle plate a tad. There is a cutaway in the plate that lines up with the first idle circuit hole. By shifting the cutaway off to the side I slowed the speed of the air across the idle circuit hole so there was less pressure drop. There is only so much adjustment that can be made before the throttle plate will jam in the bore under movement, so if anyone does this adjustment, check the movement of the butterfly to make sure it still has full and free movement.

If someone did run out of adjustment with the plate, then perhaps adding a thin layer of solder on that side of the hole would act as a substitute to decrease the air flow.

Again, does anybody have any ideas about swapping out the throttle shafts on these carbs?

 

[Husq445]

Here are some pictures of my porting, would like to here what anyone thinks. Thanks for your info also Terry. I have not been able to find anything on the carb shafts yet.

 

[Husq445]

Built my own dome piston, tig welded with 4043 aluminum, not quite finished in the pics. I needed a new ring before I started but wanted to compare psi on the same half worn out ring for comparison,

120psi stock piston
175psi dome


Cannot stop modding this saw!

Any way here are the pics of the piston and a shot of the tranfer port openings too.

 

[Terry Syd]

Compression +

Wow, love the work on that piston crown, now that has got to generate some torque.

Looking at your pictures, the 445 is different in several ways to the 450. The exhaust port on yours is square, while the 450 is a trapezoid shape. Also, your cylinder porting of the exhaust ended up more square than mine, I ended up with a more oval shape that had to be blended into the square outlet. Your porting looks good.

Your transfers are also different. On the 450, the rib that you put a knife edge on doesn't exist. The seperation of the two ports doesn't begin until past the base gasket surface.

I took a look at my carb and I can't see how I can modify it to flow more fuel (at least with my available tools). I'll see if I can pick up one of the 13.5mm venturi carbs and give it a go.

When I finally get the carburetion sorted out, I'll try tuning the intake length with a 'boost bottle'. When I had the cylinder off, I filled in a spot next to the intake with some Liquid Metal so that I could drill and tap a pipe into the intake port. I'll then put a length of hose on the pipe with a plug in the end. By clamping the hose at different points I can change the volume of the 'boost bottle', which will change the tuned length of the intake tract. I'll wait a few months and see if the liquid metal will retain its strength and adhesion after being subjected to the heat of the engine. If it retains its integrity, then I'll drill it out and try tuning the intake.

I wish I could get a piston like yours...

 

[Terry Syd]

Carb fixed

I tried raising the height of the lever arm that opens the fuel valve. The carb now works well. It appears the factory may have set the level lower to reduce emissions.

I lifted it up about 2mm. If you look from the side, the lever is just under the surface of the carb body.

As soon as I fired it up I could tell it was running a lot richer. I started turning the screws in to clean it up and when the engine got warm it would no longer idle, the idle circuit was pulling in too much fuel. I raised the idle adjustment and continued adjusting the screws.

I ended up with two turns out on the low speed screw and 1.5 turns out on the high speed screw.

I now feel confident I can bore the carb out a bit more and still be able to adjust the fuel ratio. I'll try taking the carb out to 12mm tonight.

 

[Terry Syd]

12mm

The carb is now out to 12mm and running fine. The low speed screw is 2.5 turns out and the high speed screw is 3 turns out. I'll get some testing done and perhaps some side by side comparisions with other saws.

Taking the venturi out to 12mm is a difficult job, however getting it out to 11.5mm is fairly easy, just time consuming. If you do take it out, remember to keep the choke point in the venturi at the same level of cross section as the outlet for the high speed nozzle. That is where the speed will be highest and the 'vacumn' greatest at pulling fuel out of the carb. You should put some tape over the idle and low speed holes so that they can't be damaged or have foreign matter drop into them.

I really feel that this engine could use the 13.5mm carb. Perhaps you may want to try that and avoid the hassle and work of a partial fix. They sell on the internet for $56. I'd like to give one a try, but I have to order from Australia. You could be the guinea pig and find out of the shafts can be swapped. I'll get you a link for one if you are interested.

I'll eventually try and get some pictures, but I moved recently and I still can't find the cable for my camera (Gawd, I hate moving)

 

[Husq445]

Borrow a compression gauge if you can. Curious what the stock comp should be.

 

[Husq445]

Sure send the link. see what I can do.

Rick

 

[Terry Syd]

Link

Here is one of the links I found - http://www.m-and-d.com/ZA-C1M-EL28.html

When I go down to the shop (I live in the hills) I'll see if the shop will check the compression when they test the saw. If I have the same combustion chamber as you (likely) then with the small displacement increase in the 450 I am probably sitting around 130 psi.

I want to eventually bump up the compression ratio on this engine. Usually when I tune an engine I am a compression ratio freak - I try to get the highest compression ratio the engine can reliably run for its intended purpose.

On this engine I will try to increase the cylinder filling as much as possible (which will also increase the fuel flow through the engine for cooling). With the lower compression ratio and the relatively low exhaust port I will have a good volume inside the cylinder which I may be able to fill up with mixture.

As I mentioned in a previous post, the boost bottle may be able to tune the intake resonance. I've done this before and when you get the volume of the intake tract right for the desired RPM there is a noticeable increase in torque when the returning wave packs extra mixture into the crankcase as the port closes. On a saw, with the limited power range, the intake tuning may be less of a hassle than on a motorcycle.

I'll try and get all the beathing issues out of the way before I bump up the compression ratio.

 

[Terry Syd]

Defect

Rick, I found a manufacturing defect on my 450 that may be common to your 445. On the coil, the blue wire to the kill switch was sticking straight up from the coil. It was coming out of an opaque insulator that kept the wire vertical.

The wire was getting cut by one of the baffles in the top cover. It was about halfway through when I found it. I took the starter cover off to take a look at the way the top cover sat on the wire. I noticed that the opaque insulator was blocking the second baffle that directed air to the front of the engine.

I soldered up the wire and then I cut the insulator off. The connector for the wire was actually suppose to attach from the side, not from the top.

I put the wire back on the coil the way it was designed to fit and there was no longer any blockage of the air to the front of the engine.

You might want to check your saw and see if you have the same defect.

EDIT: Upon further reflection it appears that the defect was one of assembly rather than manufacture, probably caused by myself when I reassembled the saw after porting. However, I will leave the insulator off the wire as it does tend to impede air flow to the front of the cylinder.

Did you ever order the EL28? If you did, I'd like to hear what problems/successes you had with it.

 

[Terry Syd]

Piston trick

I went back into the jug to do a mod to the piston. While I was there I opened up the exhaust another millimeter to 27.65mm. That gave me a port that is 62.8% of the bore, that is enough for me as it is getting very close to the holes for the muffler bolts.

The mod I made to the piston crown is to increase the port area during the intial blowdown. I first tried this mod on a roatry-valve 350 I had back in the 70s and didn't notice any difference in power, not surprising the amount of Gawd-awful amount of power it had in the first place. I tried it again on some other high performance two-strokes and still nothing to write home about.

However, on a working saw with the very low exhaust window and the short period for blowdown before the transfers open, I think this mod could work. The saw piston also has a fair amount of material above the ring to work with relative to the size of the port. Doing this mod on a cylinder that has an 85mm bore the scallops in the piston barely make a change to the port area, however with the small bore and small exhaust port window on the 450 I was pleasantly surprised at the increase in area.

What I am trying to do is to curve the top of the piston to the curvature of the exhaust port. That way when the port opens, it opens across the entire port window and not just in the center. Further, since the opening is an arc, it will have more area than if the port was cut straight across the top of the port.

The arc in the exhaust port is there to accomodate the movement of the rings back into the groove on the piston. However, that arc does create a slower opening of the port until the flat-topped piston can move down far enough to open up the full width of the port.

On the 450 there was only 3mm of piston travel between the opening of the exhaust and the opening of the transfers. With my wider exhaust port it took nearly all of that distance before the full width of the port was exposed. Now the full width is exposed almost immediately.

If you do this mod, you will have to take your time. You will have to shine a light into the spark plug hole while you measure up what needs to be taken off. Take very small amounts off with a small flat file.

You want to retain the symetry of flow in the way the exhaust enters the port so that it doesn't create currents in the exhaust flow that affect one side of the cylinder over the other. To do this, I kept the initial opening of the port in the center and then just slowly let the window expand from the center. The port is fully open across the width in about .2mm.

The extra port area during the initial critical blow down period should drop the cylinder pressure much faster to allow the torque peak at a higher RPM. I consider this mod a good alternative to raising the exhaust port.

 

[Husq445]

Thanks for all the great info. I just recieved my ring today, have not had a chance to run this saw much, was waiting for the ring. Want more run time with the dome piston, before another mod. So carb will have to wait for right now.

Went to a husky dealer today 2 hours away to get the new saw that I wanted. Had a hard time trying to find a 372xp, even harder time trying to find the 372xpw that I wanted(got it though):chainsawguy:.

While I was there I asked that while he was setting my saw up,(I do not have a tach yet) he check the rpms on my 445, still set a little fat and turning 13,800 at wot. To my dismay he only set the 372 to 11000 rpms.:bang: Should have taken it home and done it myself.

By scalloping the piston you lower the compression though, right? Like a raised exhaust port.

I really noticed with the dome that I did, that the torque has incressed massively am able to push on bar harder without slowing down as much.


Rick

 

[Terry Syd]

Scalloped piston

There is very little material taken off the piston for the two little scallops. The compression ratio won't be affected that much. If I had lowered the entire crown next to the port or raised the port, then the timing would have changed and there would also be a more substantial drop in compression. I also want to work with the stock timing figures as I am also after torque.

Yeah, I like higher compression ratios for the extra torque and the efficiency it gives an engine. After I get these breathing issues out of the way, I will see if someone local can weld up a bit of a dome for me to trim to size. I better get a compression guage before I do that.

I'm not trying to build a racer, just a good running work saw for cleaning up around here and cutting some firewood. I have a bad back and I don't want to wrestle with a heavier saw. If I can make this little 50cc saw perform like a 60cc saw I'll be more than happy.

I think I will go ahead and order the carb. I've run out of other ideas to try at this time.

 

[Terry Syd]

El28

Rick, I was uncertain if the carb would be compatable, then I remembered the 'Beg for Manuals' thread. I asked for a copy of the 570/575 manual so I could look at the exploded diagram of the carb. It appears to be identical to the ones on the 445/450, albeit with a bigger venturi and throttle plate.

I'm going to order one.

 

[Terry Syd]

Compression

I finally picked up a compression gauge and the reading for the 450 was 135 psi. It's about what I thought it would be. I'm looking forward to having a dome piston.

Looking at some of the stock compression pressures for other saws, I see that the high range is around 160-180 psi. I'll probably shoot for a compression pressure in that range.

Zama no longer manufactures the EL28 carb, instead they offer an 'airhead' carb that is suppose to be a direct replacement for the EL28. The airhead carb is a one unit carb that has the strato valve attached to the carb. I suppose that may save some extra hardware on a 570/575, but I don't know how it is going to affect the installation on a 450. Hopefully any problems can be cured with a hacksaw. The carb is on the way.

 

[Husq445]

Let me know what you find with the carb. Are you going to have a dome piston built or can you find a dome piston and modify the skirt? BTW how old is your 450?

 

[Terry Syd]

New saw

The saw's new, I got it because of the new property I bought.

I'll let you know how the carb work goes. I've now got access to lumber yard and I can do some controlled cutting there. So I'll try some cuts with the modified stock carb (12mm) and then with the new 13.5mm venturi.

I'll have a domed piston made up. I'll use your information to give to the welder. All I have access to is oxy/acetalene. I mentioned my compression pressures, but I should note that I live at little over 2,000 feet of altitude. That drops the compression by 5%.

Recomputed, the compression pressure at sea level would be a little over 140 psi. Still pretty weak compared to other saws. I expect this is part of the 'homeowner' tuning from the factory, it allows the homeowner to use old low lead fuel left over from last year.

I'll take a different approach to the dome design. I am trying to trap as much mixture in the cylinder as possible in order to bring up the torque. I'll shape the backside of the dome using the transfer ports. With the X-torq design I can remove the side covers and see the angle on the port edge closest to the exhaust. I'll cut that angle into the dome so that the dome doesn't disrupt the transfer flow into the cylinder and cause turbulence and dilution of the mixture.

I would also expect that the saw has slightly retarded timing from the factory (again as a precaution to detonation or overheating). However, when you bump up the compression you often have to retard the ignition timing. So, the increase in compression may bring the timing closer to optimum.

 

[Terry Syd]

More info

I figured out a way to keep the throttle open with my toe while I tested the compression. The highest reading was a tad over 140 psi, so with the recalibration for altitude a stock 450 is about 150 psi.

Rick, I've been doing more research on these diaphragm carbs and have determined that the metering lever spring is the primary tuning element. In particular the 'pop-off' pressure for the metering valve that allows fuel into the metering chamber.

I've also discovered that the C1M-H58 (13.5mm venturi) is likely the better choice for these saws. It is a factory installation on a Homelite 45cc saw. The C1M-H57 (12.5mm venturi) is a factory installation on a Homelite 38cc saw.

Both of those carbs have the same nozzle assembly and L & H speed screws. If someone started with a H57 and decided they wanted more airflow, they could take out the venturi to 13.5 mm and essentially make it a H58.

I've also learned that the EL-28 has a jet in the idle circuit to limit the fuel flow at idle. That shouldn't be a problem with my 50cc strato as the carb is off a 73cc strato. The 'airhead' assembly can be removed from the EL28 by simply unbolting it.

The EL28 has a different nozzle assembly than the H57/58. I'll take a look at it when I get it and determine what the difference is. I expect that it may be to allow more flow for the larger engine. This could work out for me as I may take the EL28 venturi out some more. The C1S variant of the Zama carburetors is a 'mirror image' of the C1M carb - the adjustment screws are on the other side. I note that Zama lists a 14mm venturi variant of the C1S carb, so the EL28 may get some further work to it.

 

[Terry Syd]

El28

Rick, I received the EL28 today and immediately set about swapping the shafts. I made a few minor mods to the carb body to set up the springs on the linkage and then started putting it back together. It took me a while before the light came on - the choke valve rotates the opposite direction to the stock 445/450 choke valve.

I'm checking into the H58 and see which way the choke rotates. If it is the same as the EL28, then this is a more complicated mod than most people would want to get involved with. I'm going to have to build up some liquid metal in the front of the carb to make a stop for the choke valve and then reshape the choke valve to clear the main jet outlet.

I also discovered that there is no 'jet' to limit the fuel flow at idle. All Zama did was cut a small divet out of the other side of the throttle valve to bleed air past the throttle valve (I usually just drill a hole in the throttle valve, that way I can solder it up if I make it too large and then re-drill it with a smaller drill).

The EL28 also does not have an inlet for the primer pump. I'll find a bit of small pipe and put one in.

I tried picking any difference to the pop-off pressure between the stock carb and the EL28. Unfortunately, my finger tip isn't that well calibrated, I wasn't sure if the EL28 had just a bit more tension on the lever arm. I would have to pull the springs and compare them side by side.

The carb venturi looks great, nicely shaped (better than the stocker) and of course there's much more area than the stock 11mm.