Building a 42cc Poulan

[Chris-PA]

Marc (diggers_dad) was kind enough to send me a box of Poulan parts, and I decided to collect some of the spares into complete saws. A lot of the parts came from a 1995 A/V Craftsman chassis, and I noticed it had a "turbo clean" scoop on it, which I had never seen before:

There was also a nice chrome bore engine from 2004 - I assume it was from a PP262 since the A/V spring mount was not tapped. Since I have one of these that is stock excpt for a muffler mod, I decided to port it.

I noticed this one had a considerably lower exhaust port compared to other older jugs, which I found odd. I raised it some to match what I had on those other engines. The lower transfers were also shaped differently, and I opened them up a little:

 

[Chris-PA]

After looking at the top of the jug I noticed another difference - the spark plug boss is extended farther out than on older engines:

It looks like the combustion chamber is bigger too, but it is hard to tell. I was concerned about lower compression, so I decided to leave the op of the exhaust port where it was. The timing came out to be almost exactly the same as a stock older PP260 engine, but the ports are now at 60% of the bore diameter. Here is the timing:

Then I decided to put a larger carb on. I used one from the 46cc Poulans like my 2775, which is 34/64" vs. 28/64". It was a bolt on except for the lack of a purge bulb port and the position of the inlet:

I had to rotate the inlet fitting, and I plugged the purge bulb outlet into the tank with Yamabond:

 

[Chris-PA]

The timing was advanced 5[SUP]o[/SUP]:

I want to try making a pipe muffler outlet for this one but didn't have time, so I combined some modified pieces of the older muffler style with the shield/deflector from the new style:

Handles on:

 

[Chris-PA]

It runs very well - not as radical as the other one I built from some of these pieces (the multi-colored one below), but it definitely has some grunt! Here's a family shot of my 42cc Poulans - my original one is in the middle:

 

[Dan_IN_MN]

Nice work! It's nice to have cheep saws to play around with!

 

[MechanicMatt]

I don't know why, but I too love working on these little buggers. If you need any parts for those "av" models let me know. I actually like building the attached rear handle saws. So I have a few bits laying around for the AV models.

 

[Farley9n]

Nice Work!

Isn't it fun to play with the little poulans! I have a few also that are not as they appear!:hmm3grin2orange: Chrome bore is the way to go......You do nice work.....Bob

 

[Chris-PA]

nice work and a great write up.....one question, doesn't raising the exhaust port reduce the compression?? or is there something going on here that I don't understand....it wouldn't be the first time...:msp_confused::msp_confused:

Thanks! Yes, raising the exhaust port does reduce the compression.

I wanted to raise the exhaust a bit more for longer duration and higher rpm, but then I saw what I thought was a larger combustion chamber, and then I thought about how they had lowered the port. I'm assuming they lowered the port to compensate for the larger combustion chamber, and so I hesitated to raise it any more. I ended up at about the same timing as an older PP260 engine. I do not have a good compression gauge, but I suspect this is not the highest compression engine. It still runs very strong.

I was doing some noodling with it today, and I also got out my ported 46cc Poulan 2775. Even given the displacement difference, none of these 42cc engines has quite the edge that thing has, and I suspect the difference is compression. The purple and gold one has the most radical porting and it goes pretty good, but the 2775 is nuts. Still this new saw will pull its 18" bar buried full while noodling or bucking, and it does not bog easily. I had it running hard noodling some large twisted white ash rounds and it just kept on pulling for a long time.

 

[zogger]

Nice *clean* job man! When you gonna do a two piece head?

 

[Chris-PA]

I was thinking about this lowered port/reduced compression thing a bit more, as it was bugging me as to why they would want to reduce the compression (I'm still speculating about that of course, I'm just to cheap to blow $40 on a gauge). I suspect that thaht they may have intentionally kept the compression lower on these models to make it easier for the average homeowner to start. They later ports are lower, and that seems to have coincided with the later more restrictive mufflers (are they cat mufflers), which may have been an attempt to increase torque to compensate for the restrictive muffler. But that would have boosted compression, so maybe the larger combustion chamber was to drop it back down again.

Or maybe I'm all wet and it's all just methods to reduce emissions I don't know about.

I don't know why, but I too love working on these little buggers. If you need any parts for those "av" models let me know. I actually like building the attached rear handle saws. So I have a few bits laying around for the AV models.

Thanks for that, I appreciate it. I've come to like the A/V systems quite a bit - they don't feel as direct, but I don't like having my hands tingle at all!

 

[Chris-PA]

I was out in the barn looking at the old and new style 42cc cylinders again tonight and noticed another thing they changed - the transfer volume is less on the newer style. The old transfers runners were about 7.7mm deep, while the new ones are about 6mm deep. That's more than a 20% reduction. The timing is the same.

The combustion chamber volume is definitely larger - they kind of carved out an opening like a partial tube extending from the spark plug hole right across the previous hemispherical chamber. Picture boring in with some kind of cutter from the spark plug opening, although it is about the diameter of the plug boss.

Like I said, I think it has to do with the newer style muffler, which has an outlet hole about 0.26" in diameter.

Edit: The intake port is also raised - a lot, like 1/8".

 

[Chris-PA]

So this project required some more work. I had built two ported 42cc engines, the one for this saw and a non-chrome plated older engine that went into the multi colored saw in the family picture shot above. The older engine was slightly free ported from dropping the exhaust port a little too far - but only maybe 0.015" exposed. It was more radically timed (intake duration 159, exhaust duration 164 with blowdown at 23) and ran really well.

After running a tank through each I decided to look at how the engines were doing. And both of them looked like they were scoring! Not deep, but I decided to pull the engines apart.

The old non-chrome engine had always had a shallow stripe right above the exhaust port, but it now had another forming at the side of the port - very shallow and really hard to feel. There was also carbon on the cylinder walls and piston sides, after only having been run about a tank. The chromed PP262 engine was developing two similar ones, and the piston was wearing below the intake port (but not at the bottom of the skirt). I was not happy! The rings looked great though.

So I smoothed them down with 400 grit, using a 1-3/8" dowel wrapped with some adhesively backed foam I get from work. I was able to completely remove the ones on the chromed engine, and the other is back to only the stripe it had all along. I also widened the intake port on the old engine to better match the exhaust, and I swapped out the bottom end for a much newer one.

When I put them back together I decided to swap the engines. I figured the more radical engine should have the larger carb, as the PP262 engine did not seem to be benefiting from it. I ran the one pictured in this thread (but with the other engine) quite a bit today - several tanks bucking white ash - and I have to say it ran very well. I ran the non-A/V saw a little bit too, but not so much as I didn't want to have my hands go numb. It also ran very well.

I peeked in the spark plug hole afterwards and they both look really nice - no marks or carbon, really really clean. The old aluminum engine bore looked almost new.

I'm trying to figure out what I may have done wrong. On the old aluminum bore engine I paid extra attention to deburring the ports, but if that was a problem I'm not sure how it translated into a couple of score marks. I suspect that on the chromed engine I left a burr along the lower edge of the intake port, which was binding the piston slightly and maybe causing it to dig into the exhaust side.

 

[3000 FPS]

Nice job and it looks like fun. :msp_thumbup:

 

[JakeG]

Missed this thread somehow!

Good to see people tinkering with these.

I'm working on one (as you know), and even though it let loose in someone else's hands.. I'm happy to work on it. I started opening it up a little. Need to pick up some 600-800 grit to polish.

Good luck with the saws and I hope they don't score up like mine did.

 

[Chris-PA]

I have been speculating that these engines have lowered compression intentionally in order to make them easier to start. Here is what the squish looks like on a 1999 vintage 42cc:

There is some slop since the crank is only supported by one bearing in this cutaway, but it measures out to around 0.030".

I have not been able to get any of these engines to run the way my Poulan 2775 does, and that engine certainly feels like it has higher compression than the 42cc engines do. So my hunch has been that compression is the difference. The only other avenue I have to explore is the air filter, but I'm skeptical that will make the difference.

Now I need a hop up piston!

Does anyone know if there is a spark plug with longer reach that will fit otherwise?

 

[LegDeLimber]

just some idle thoughts, but do you have a set of the transfer inserts
from some of the later poulans?

would be curious to see what getting the back drag and turbulence
of the piston side (and wrist pin hole) away from the transfer flow
would do for things.

might get you closer to that seemingly preferred (? and closed) quad port flow...

I'm also wondering about some additive work above the bearings,
to fill in and build a taper into the transfer bottoms.
just looks like an ugly spot for causing more unwanted disturbance.
I'm thinking that those areas are only like that as a concession
to die casting processes and related manufacturing costs.

again i'll say it's just some late night pondering on things.

 

[LegDeLimber]

hmph, pics but no report on how he likes it..

 

[Chris-PA]

just some idle thoughts, but do you have a set of the transfer inserts
from some of the later poulans?

would be curious to see what getting the back drag and turbulence
of the piston side (and wrist pin hole) away from the transfer flow
would do for things.

might get you closer to that seemingly preferred (? and closed) quad port flow...

I'm also wondering about some additive work above the bearings,
to fill in and build a taper into the transfer bottoms.
just looks like an ugly spot for causing more unwanted disturbance.
I'm thinking that those areas are only like that as a concession
to die casting processes and related manufacturing costs.

again i'll say it's just some late night pondering on things.

I have wondered about those plastic inserts, but they won't fit the earlier cylinders. The shape of the lower transfers on these is what I was exploring in this thread: http://www.arboristsite.com/chainsaw/232407.htm The clearly went to some effort to shape that hook into the transfers, and they have changed the entrance shape over the years. I'm guessing that the early ones were shaped to capture the pressure wave ahead of the crankshaft counterweights for more power, but that the later ones were shaped to deflect it for lower emissions.

 

[Chris-PA]

Hi im working on the same engine but think mine is 36cc or 34cc, the saw cost next to nothing..here are a few pics of what I have done to it might work?I only used a dremel....View attachment 287011View attachment 287012View attachment 287013

Nice work and I'm interested in how it runs. I thought about cutting out the piston skirt too but hadn't tried it yet.

I'm very curious what saw that came from? The connecting rod is different from any I've seen on the Poulan 42cc type engine. They usually look like this:

 

[diggers_dad]

Warrior,

I may have failed to mention one little tidbit about those saws. I sent them to you to play with and work on - but expected I would get them back when you were finished with all the mods! :msp_w00t:

Seriously now - it looks like you have spent a LOT of time and effort figuring out all the idiosyncrasies of the little Poulans. I like those saws for some of the same reasons you do. They are inexpensive to pick up used, so if your mods don't work you haven't really lost much. They are easy to work on, even with the clamshell design. Most importantly - they are FUN to run when you get everything right and will absolutely amaze folks who are expecting "box store" saw performance.

Great write up and excellent pics.

dd