Help building,tuning a Strato, Husqvarna 445

[Husq445]

Been watching this site and hunting around for info on building my own saw, so far so good. Building a Husqvarna 445. Successfully did a muffler mod, this is a trick on these saws as they have a pot metal type alloy so I tig welded an aluminum pipe in 3/4" ID. (Show pics later if anyone interested) I am a specialized welder by trade so this was the easy part. I then widened the exhaust to within about .150 of the skirts may have lowered the port a little too. not much. I then reassembled and ran. The gains were signifigant, not unreal. Knowing this is a strato engine didn't want to mess with intake or transfers.......at first! Tore down a couple of weeks later lowered the intake some, but cannot go much wider than factory. Maybe .100 Tore the transfer runner covers off the block. smoothed out some of the corners and widened the port on one side towards the intake side alittle. Having hard time tuning with the general rules of tuning a 2 stroke. Setting so it 4 strokes at WOT seems like its running an ungodly RPM, nothing like factory.Gains cannot be that great on a 46cc saw? Any help tuning a strato would be great. Rick

 

[troutfisher]

Post some pics please! I've never messed with a strato motor, so I'm not much help with tuning. Do you have a tach?

 

[Husq445]

No tach yet, still looking. all the pics I have are of the muff mod that I am about to load. Never took any while tore down. Maybe I will just to get opinion. Rick

 

[Husq445]

Some pics of the muffler mod. A little dawg change in these pics too. Plasma cut out of stainless steel.

 

[troutfisher]

nice looking weld! does the chainbrake still work?

 

[Husq445]

Thank you. Yes it works. Had to trim the guard a little, but all functions well.
Rick

 

[troutfisher]

I would want to see RPM's on a tach to know if it's over-reving. It can be tuff to judge by ear on a modded saw.

 

[Husq445]

What about standard tuning of a two stoke. So that they 4 stoke at WOT and clean up just into the cut. I had thought that I heard or read somewhere that standard 2 stroke tuning did not apply on a strato? Yeah a tach would be great but I would like to be able to read this by ear a little better?
Rick

 

[Husq445]

If it helps any I am about a quarter of a turn out from stock on the L screw and about half to 3/4 turn on the H screw. Idle up about a quarter turn also. This sound like a ballpark adjustments?
Rick

 

[blsnelling]

There's really no such thing as ball park adjustmenst once the saw's modded. I wouldn't be surprised to see 1500 RPMs over stock with what you have done. But it is what it is. Does the saw have a rev limited coil? The saw can sound like it's 4-stroking to the untrained ear, and actually be lean and bouncing off of the limiter.

 

[Husq445]

I'm assuming all limited coils are blue? Mine is black.

 

[blsnelling]

I'm assuming all limited coils are blue? Mine is black.

As far as I know.

 

[Husq445]

Thank you

Got it tuned in now. Have about 6 tanks of gas through it now, runnin awesome! Cannot believe how much is to be gained by this small CC saw. Seemed to be even better after a few tanks of gas, maybe my imagination? Burning some fuel now, not quite twice as much as before. Seems to cut twice as fast though. Next a bigger saw and a port job soon follow!!!! Thanks to all, this site is great. Addicted now, thanks a lot! Rick

 

[Terry Syd]

450

Rick, glad to see someone else modding the 445/450s. I just finished porting my 450. The width of the modded 450 exhaust port is limited by the muffler mounting screw holes. I'll measure it tonight, but it looks to be a bit over 60% of bore, that's up from 47% of bore.

I have a theory on why there is some confusion on porting the stratos. In the conventional 2-stroke saws the factory could put a restrictive muffler on the jug and limit both the sound and the emissions through back pressure. In the stato/X-torq that Husky used it looks like they put the restriction on the inside of the engine.

Whereas a muffler mod on the older saws worked wonders, it doesn't have the same significant impact on power with the strato.

When I looked at the lower part of the jug where the entrance to the transfers were, I thought it was just some casting flaws that needed to be cleaned up. As I started to 'clean it up' I kept filing and filing. It turns out that the entrance to the transfers was only about 50% of the area of the transfer port itself. This appears to be intentional to keep the transfer port velocity lower so that there would be less chance of mixture loss. It also allows the owner to clean it up and get the full flow of the transfer ports.

The next problem with the entrance with the transfers was the loop around the supporting web between the two cylinder bolts. On the 450 it appeared to be too small to support both fully opened transfer ports. I didn't want to weaken the web by cutting it down. What I did was vent the web with a port so that mixture could go directly to the transfer entrances. The web retains its integrity and the mixture can still flow.

I then looked at the carburetor. The 450 has a small 11mm venturi. I am ordering another Zama C1M carb with a 13.5mm venturi. As you mentioned the width of the intake port is very much limited by the width of the piston skirt. My port is only 15mm wide and I am not going to widen it any more, there just isn't any skirt to work with.

Now here's another point about the internal limiting by the factory. If you put a smaller carburetor on a strato then there will be less flow into the crankcase before the strato ports open. In other words, they probably wanted to keep a stronger depression ('vacumn') in the crankcase in order to get the strato ports working harder (for the EPA).

By putting on a bigger carb, the strato function won't work as well as there will be less 'vacumn' in the crankcase. However, I expect that the larger carburetor will allow the engine to develop more power, especially after porting.

 

[Terry Syd]

60%

I just measured the exhaust port and the width is 26.7mm, which with the 44mm bore puts the exhaust window at an honest 60% of the bore.

I also moved the bottom of the exhaust port down. Previously the piston was about 1mm lower than the bottom of the exhaust port at BDC. I cut the port so that it now about 1mm lower than the piston top at BDC. This allowed me a bit more port area and still have a good radius on the bottom of the port.

On my engine the exhaust port and the heat shield, gasket and muffler did not line up. The exhaust port was wider at the top than the gaskets, etc. However, it was smaller than the gasket about halfway down the port and about 2mm smaller along the floor of the port.

I opened up the shield, gasket and muffler so that it matched the port on the top. I then opened up the bottom of the exhaust port so that it matched the externals.

I then took the floor of the port back into the cylinder to the existing 'pressure drop ridge' (for wont of a better description). I've read that the common 'step' in the exhaust port is to create a rapid drop in exhaust pressure as soon as possible after the exhaust port opening. So instead of a taper to the exit of the exhaust port, I have a full volume port up to the ridge.

The 450 has the same muffler as the 445. I did a muffler mod on the original port to 85% of the exhaust port opening. Now I will have to recalculate and open up the muffler some more. I expect that I may use some 'fish gills' on both sides of the muffler.

Rick, the 445 probably has the same carb as the 450, which means you could also go to the larger 13.5mm venturi. The throttle bore would be 16mm (up from a stock 14mm), which you could then lightly taper it into the 15mm internal diameter intake manifold.

If you wanted to check and see if there may be some advantage to a larger carb, you could disconnect the strato linkage and do some cutting. Whereas the carb has an 11mm venturi, the strato valve has a 16mm bore. When the strato butterfly is opened to the transfers, the flow through the carb probably drops way off. Since this is a piston ported intake, that gulp of fresh air is taking place halfway through the intake cycle at a point of very high demand. That big gulp at the halfway point no doubt affects the remaining 'vacumn' in the crankcase for the other half of the intake cycle.

I haven't tried disconnecting the strato butterfly yet and running the engine. However, I don't expect much. if any, necessary change in the jetting. If you give it a go, how about reporting back to us.

 

[Terry Syd]

C1m-el28

Can anyone give me some advice about swapping carburetors on chainsaws. I'm looking to purchase a C1M-EL28 which is the 13.5mm venturi carb. It is used on the Husky 570, 575 and 576XP. I can pick one up off the internet for US$56.00.

The various external linkages are different, but I'm wondering if the shafts from my present EL-37B can be swapped into the EL28 so that the various linkages for the 450 can be used.

 

[Husq445]

I actually had my carb off over the weekend and can tell you swapping shafts wont work, the machined flat for the butterflies would be to small.
On mine all that I did was remove the shafts and filed them down on the opposite side of the flat, that shaft alone has to restrict a lot. Big shaft crossing a little hole. I stopped only when I thought it would loose rigidity. Shortened the screws next. Filed the heads flat and tips almost flush with shaft. When I finished, with both butterflies open, looks like only resistance would be the butterflies themselves.
Put it back together and tested. you can definitely tell at wide open throttle.
As far as putting the other carb on. Seems like a lot of carb. I am no expert, but if that is the factory carb off a 575 seems like thats going to be too much. the venturi is not a whole lot bigger but is the jetting going to be to much?
Rick

 

[Terry Syd]

Butterflys

I've got my carb in front of me and see the flat you refer to. There is about 1mm of space on either side of the butterfly before the flat goes to full shaft diameter. Since the larger butterfly is 16mm rather than 14, it looks like Zama may have used the same shaft cutting for their carburetors. In any case, I could always file the flat a bit further if I ran into problems. My main concern is getting all the hardware of the 450 on the carb.

The other alternative is to enlarge the venturi. I could easily take it out to 12mm, and using an egg shaped venturi diameter to avoid some of the internal passages, I could enlarge it further. If I took it out to 12mm that would be a 9% increase in the area of the venturi.

Perhaps Brad or some of the other old hands could comment about carb sizes for saws. I wonder what size venturi the high revving 346XP and NE are using.

EDIT: Rick, I just remembered that your engine was 46cc and not the 50cc of the 450. You might get by with just taking out the venturi to 12mm. However, the 450 has a 9% larger displacement, so I will need more flow than you in order to get the full potential of the engine.

 

[Terry Syd]

346XP carb

I did some research and found that the 346XP has a Zama C3-EL32 carburetor which has a 15mm venturi. I expect from looking at the charts at the Zama website that the throttle bore is 17.5mm. In any case, the venturi is the choke point in the carb and it is 15mm.

With that in mind, the 450 should be fine with a 13.5mm venturi. I'm not porting for speed (although this thing really revs now), I want to retain the torque for all the Ozzie hardwood. The 13.5 venturi looks like about right for the 450 after porting and a muffler mod.

The question still arises, can the shafts from the 450 carb be used in the 570/575/576XP carb? Anybody have any ideas or experience in this area?

 

[Husq445]

Terry, was just curious, have you checked the compression on your 450.

I was just wondering what these should run for compression, how much lower than a pro saw, wondering if I need to change my ring or if the compression guage I am using is not optimal for this little 2 stroke.

Using a brand new Mac Tools compression guage, automotive style, with 2 piece hose. Has shrader valve in end though.




Thanks Rick