Poulan 3800 porting questions

[Reesedlightning]

Recently picked up a Poulan 3800 in awesome condition. But what a friggin turd saw. Has decent low end but super slow. It does have the plated piston and aluminum cylinder and they’re still in great shape. It is an open port cyl.
I’d like to breathe some pep into this thing but I think I need some direction with numbers and such.
I tore it down and got the stock numbers.
With base gasket:
Squish .047
Ex: 98 ATDC
Transf: 120 ATDC
Int: 64 BTDC

Yanked the base gasket and got:
Squish: .037
Ex: 100
Transf: 122
Int: 66

Ideally I’d like another ten thou off the squish but I don’t have a lathe so I suppose that’s that.

As for target numbers, im thinking (maybe):
98
120
70

I’m new to porting so I’m at still trying to get the hang of what numbers work with what saw. I’ve done a husky 262 and a 353 that turned out pretty fun but this saw sort of has me stumped and I couldn’t find much info on hotrodding these older poulans. Any ideas or advice is greatly welcomed and maybe this thread will benefit other members.

 

[Reesedlightning]

Did some grinding this evening after work.
Started by just putting down a piece of 120 sand paper on a flat surface and spinning the cylinder with even pressure. I was able to get the squish down to .033.
I then started widening and shaping the exhaust port. It was very ovular and had some pretty rough tapers into a square port. I opened up the ends and did some blending. I haven’t put a finish on it yet and I think I may still work the end of the port. Currently the exhaust is at my target of 98°
I then put a quick texture on the intake and made a couple quick passes over the bottom of the port and that brought it to 68°. I thought about going more but if I end up taking more off the base, it may put me at my target of 70°.
I did notice there are some discrepancies in the matchup of the plastic manifold block that could be worked so I might get into that tomorrow. I will probably button her up after that and run it to see how she does. The transfers are still at 122° leaving me with 24° of blow down which might be a bit much. Idk. I guess I’ll find out lol.
Here are a couple pictures of the ports roughed in. Keep in mind I still have some blending and finish work to do.

 

[Reesedlightning]

Well she’s back together at
98
120-121
68
.033 squish
It’s got much better pop to it and the throttle is much more responsive. I’m going to put it in some wood tomorrow and see. I gave it 3 heat cycles this evening.

I’m planning on running it for a bit until I line up a machine shop to lathe the cylinder down so I can get the squish to .18 or .20 and then I’m going to bring the numbers to
98
128
72

I think that with more intake duration it’s going to bump up the compression and snappiness and having 20° of blowdown is enough to keep torque but still let the transfers open long enough to let the charge into the cylinder. We’ll see.

Anyone have any input on this?

 

[Reesedlightning]

Update:
Pulled the thing apart again, brought it to the machine shop, got it on the Bridgeport and decked the base.

Got it back on the saw with a squish of .021. Took some 220 grit and massaged the base down to .020.

I started doing more reading so I could figure out what numbers I was going to go with for the ports. Seems like my intake was very anemic so I attacked that first.
I ended up with:

100
121
75
Try to do timing advance and took .020 off the key and it sheared so I put a new key in with no timing advance. She certainly sounds better and the throttle response is right there. I’m going to get it into some wood either tonight or tomorrow and see how she does.

 

[Sierra_rider]

Update:
Pulled the thing apart again, brought it to the machine shop, got it on the Bridgeport and decked the base.

Got it back on the saw with a squish of .021. Took some 220 grit and massaged the base down to .020.

I started doing more reading so I could figure out what numbers I was going to go with for the ports. Seems like my intake was very anemic so I attacked that first.
I ended up with:

100
121
75
Try to do timing advance and took .020 off the key and it sheared so I put a new key in with no timing advance. She certainly sounds better and the throttle response is right there. I’m going to get it into some wood either tonight or tomorrow and see how she does.

Those are pretty safe numbers IMO and should work out fine. I'd consider even more intake duration, but go too much and you risk ending up with a saw that doesn't pick up rpm quick. I've been experimenting with different numbers lately. High 70's on intake and I still have a snappy/quick revving saw.

It's hard if you don't have a lathe...IMO it really opens your options with what you can do. What I've found to work for me, is I'll mill the squish band and then correct the squish by decking the cylinder. That way I can bump the compression up more than if I just did a gasket delete or only decked the cylinder. I'll then run a bit more exhaust duration than some people might, allows me to end up with a saw that can pull high rpm in the cut, yet not fall flat on its face when leaned on.

Some people are worried about losing velocity, but I try to open up the transfers where possible. I aim for the port to smoothly taper off in size until it gets to the upper port opening. I figure that smooth taper w/the most constriction at the actual port window should keep the velocity up. Where the uppers are aimed is important too.

 

[Reesedlightning]

Thanks Sierra. All of what you said makes perfect sense to me. Maybe I can get my machinist to cut the squish band and deck the cylinder more.
I agree on the intake. I think I went conservative because the original intake was so high I wasn’t sure if they did that from the factory for a reason.
Something I am so struggling with his trying to find the relationship between what kind of numbers I should use versus the stroke of the piston. I know those two factors have an effect on how much much you can grind.
I wasn’t sure about blow down either. I watched a lot of Ironhorse and tinman saw on YouTube and they seem to reference wanting at least 20° of blow down or way more in some cases. I recently ported a Jonsered 2051 with a 345xp cylinder on it and at 29° blow down it was a slug. I had to adjust it to about 21 and it really picked up to be a fun saw. So all of that has me second-guessing what the heck numbers I should choose for transfers. I did however get them aimed towards the combustion chamber and towards the intake

 

[Reesedlightning]

Well I just tested it out and it’s much less of a slug but it’s not holding rpm very well in the cut. It also sounds slow. Didn’t get a tach on it yet but initially I’m thinking more intake and maybe more exhaust? I’ll stew on it and try reading more

 

[Sierra_rider]

Well I just tested it out and it’s much less of a slug but it’s not holding rpm very well in the cut. It also sounds slow. Didn’t get a tach on it yet but initially I’m thinking more intake and maybe more exhaust? I’ll stew on it and try reading more

Maybe cut the squish and compensate with decking the base. Then you can get the intake down to 77ish range and exhaust up to 102ish...maybe grind a degree or 2 off the exhaust to get it back to 100-101. 101 is probably a pretty safe number for a 60cc saw...taller than that and it may not pull well in the cut. Heck, you might even leave the exhaust at 102 and test it out before you grind anymore.

I wouldn't touch the transfers anymore...I've found 25ish degrees of blowdown to be pretty good...although some saws like even less blowdown.

 

[Reesedlightning]

Roger that. That’s the advice I needed to hear. I’ll line up my guy with the lathe and report back with how all that goes.
I truly appreciate you taking the time to reply to the thread and giving your input

 

[Sierra_rider]

Roger that. That’s the advice I needed to hear. I’ll line up my guy with the lathe and report back with how all that goes.
I truly appreciate you taking the time to reply to the thread and giving your input

Yeah, no problem man. Just be careful that you don't drop the cylinder so low that you freeport the exhaust at TDC.

 

[Reesedlightning]

Quick update. I got the cylinder back from the machine shop for the second dang time and it looks awesome. Got it on the saw. My squish is .016 which is a bit tight but oh well. My new numbers are
102
123-124
78
The motoseal is setting up as I type this so we’ll see. I can tell just from a couple pulls on the cord it’s got way more compression. I’ll get a gauge on it after I heat cycle it once and re-tune it.

 

[Reesedlightning]

Realizing that I never updated this thread. She’s at 180psi. Pulls a 20” full comp through oak decently but nowhere near where I thought it would be at. Not really stoked with the power so I put her on the shelf and went back to the book. Doing more learning and porting on other projects in the mean time.
I picked a copy of the two stroke tuners handbook and it’s already connecting a lot of dots for me and it’s helped me go back to other builds and make them way stronger. I’m going to put in more time on the foredom then circle back and re-visit the 3800. When I get there I’ll update the thread!

 

[Bubster]

I know this is an older thread, but I was wondering if a 4000 top end would be a direct bolt up on my PP 375 (3800) ? Any needed mods to top cover? Need a 4000 carb ? Thanks.

 

[Reesedlightning]

Yes sir. I believe it is a direct bolt up.

 

[retiredfarmer]

I think I will look for the book you mentioned. Not sure I will do anything but I am sure it will make an interesting read