Timing numbers for my saw.

[3000 FPS]

Well I have been taking a few of these less expensive saws apart and doing some porting, but only widening the ports. Mastermind and a few others have convinced me to get a degree wheel and time one of them. So what I have is a Poulan 260 42cc open transfer clamshell. Here are the numbers as they are right now. I did raise the transfer alittle for the blowdown.

Exhaust opens ATDC 103*
Duration it is open 62*
Transfer open ATDC 123*
Blow down 20*
Intake opens ABDC 106*
Duration it is open 38*

The intake does not seem to be open for a very long duration and I was thinking of lowering the port. There seems to be plenty of time from when the exhaust closes to when the intake opens. Looking for suggestions.

 

[mdavlee]

You're reading it wrong. If the exhaust opens at 103° the duration is 154°. You double the exhaust and then subtract it from 360. The intake is probably 74° where it opens ant 148° duration.

 

[3000 FPS]

You're reading it wrong. If the exhaust opens at 103° the duration is 154°. You double the exhaust and then subtract it from 360. The intake is probably 74° where it opens ant 148° duration.

Ok so you want the duration to be from the time it opens going down to when it comes back up and closes. But the intake I do not know where you get 74 from.

 

[3000 FPS]

Ok so I went back and double checked the numbers and calculated properly as suggested.

Exhaust ATDC 103*
Duration 154*
Transfer ATDC 123*
Blow Down 20*
Intake ABDC 106*
Duration 148*

I double checked the intake opening after bottom dead center and it is 106*. I marked that as zero and then rotated the crankshaft clock wise until the piston came back down and closed the intake and that came out to 148*. So with that calculation in mind the intake is staying open alot longer than I first thought. Any other suggestions please.

 

[mdavlee]

Most people read the intake in the other 1/4 of the wheel to get a number like I did. The 148 is the same as most of us read 74. Most of the time I like to go to 154-160 on the intake and the same on exhaust with 20 or so degrees of blowdown. If you could add compression then you can raise the exhaust more to gain more rpms and topend to go along with more torque from the compression increase.

 

[3000 FPS]

Most people read the intake in the other 1/4 of the wheel to get a number like I did. The 148 is the same as most of us read 74. Most of the time I like to go to 154-160 on the intake and the same on exhaust with 20 or so degrees of blowdown. If you could add compression then you can raise the exhaust more to gain more rpms and topend to go along with more torque from the compression increase.

Ok I understand now how you got the 74*. Thanks for the correction it really made the timing calculations click in my mind. Since I am not going to raise the compression I am going to leave the exhaust alone but I may drop the bottom of the intake a couple of degrees.

 

[Chris-PA]

Cool thread - this is basically the same engine as is in my Craftsman 42cc, and every other non-strato Wild Thing family saw, so I'll be curious to see what you come up with.

 

[3000 FPS]

Cool thread - this is basically the same engine as is in my Craftsman 42cc, and every other non-strato Wild Thing family saw, so I'll be curious to see what you come up with.

When I get done with it and put it into wood I will post the results and then describe what modifications that I performed on the saw.

 

[3000 FPS]

Here is a couple of pics of my timing wheel and how I attached it. This is was done with the advice of fellow members here.

View attachment 246120View attachment 246121

 

[50blues]

Nice setup. Thats about identical to how i have been doing it. Widen the heck out of it move the intake to around 160 and polish the exhaust. By the time you get done just smoothing out the exhaust it will probably gain a couple and that should be good.

For now untill you want to see what a little more will do.

 

[Chris-PA]

Damn, that looks like fun!

Must. Not. Get. Degree. Wheel. My engines run fine, my engines run fine, my engines run fine..........

 

[3000 FPS]

Damn, that looks like fun!

Must. Not. Get. Degree. Wheel. My engines run fine, my engines run fine, my engines run fine..........

Oh come on it is just another step forward to having fun with these things. I know some here make a living doing this but it is just a hobby for me. I already have a full time job and do not plan on starting another. Remember the Water Boy. You can do it.

 

[3000 FPS]

Just a quick update. I decided to advance the ignition timing by 6*. This is the set up I used to determine where it was. I just took a really small burr and cut out about a 1/16 of an inch on the flywheel to move it.

View attachment 246939

 

[3000 FPS]

So here is the final product and here are the final numbers for the modifications I did to the saw.
Exhaust. 103
duration 154
Transfer 123
Blow down 20
Intake 77
Duration 154

Ignition timing advanced 6 degrees.

The tops of the transfer ports were raised by very little but I pointed them more towards the intake port. I also opened the bottom of the transfer ports only on one side. I opened the side that faces the intake because of the shape of them. I also cut the transfer ports alittle deeper and wider. The Intake and exhaust ports were widened out and the intake port bottom was dropped, but the exhaust I did not change any timing. Muffler mod, but kept the spark screen. I tached it out of the wood running rich at 12800.
The saw runs very good in the wood and the throttle is very responsive. All in all I learned a few things and took a saw that was junk and turned it into something that will be alot of fun to use. I would do it all again and will.

View attachment 247456View attachment 247457

 

[mdavlee]

Glad it has worked out for you. How does it run compared to before?

 

[3000 FPS]

Glad it has worked out for you. How does it run compared to before?

It did not run when I got it and since I had to tear it down I did the modifications right away. I have another one of these that is completely stock that I have not run yet and will now have to compare the two.
I think that once it is broken in and I am not running it on the rich side it may come up another 200 rpms putting it at round 13000. The saw feels strong but a comparison would be good.
I will have to post what I find between stock and modified.

 

[Graham99]

Nice thread mate, it's good to see somebody take a lower specification saw and use it as an opportunity to learn and improve performance. I have a 4620 that I want to play with, so I think it is time to get a degree wheel.

Graham

 

[3000 FPS]

So today I decided to try alittle comparison. I have a completely stock Poulan 295, 46cc with the stock bar and chain. The 295 comes with a 20" bar .325 and the Poulan 260 comes with
18" 3/8 lo pro. The 260 I changed to a 16" bar and stayed with 3/8 lo pro chisel chain.

So I had my wife time me cutting into a piece of elm 11" across that is very dry and hard. The
42cc Poulan 260 beat my 295 by 2 seconds through the cut. Needless to say I was very happy about that.

This shows me the modifications I made are working very good for this saw. But what I really like is that it is the whole package that is better. It is the bar and chain are better, the muffler mod is better, the porting of the saw is better and the ignition timing is better. Put it all together and this is a great little saw. So next I will be doing the 295.

 

[Chris-PA]

Nice job, and the saw looks great. Simple engines, but they run well when opened up. I saved your numbers - I may port mine eventually, but it runs so well with just a muffler mod I really don't want to change it!

The changes you did seem pretty close to what I did on the PP315 engine I put in the 2775, including the timing advance. It also runs quite nicely.

 

[3000 FPS]

Nice job, and the saw looks great. Simple engines, but they run well when opened up. I saved your numbers - I may port mine eventually, but it runs so well with just a muffler mod I really don't want to change it!

The changes you did seem pretty close to what I did on the PP315 engine I put in the 2775, including the timing advance. It also runs quite nicely.

Thanks. I was not sure how opening up the bottom of the transfer would affect it, but I am very happy with the results. You there is just not alot of info on porting these small saws. Even in the wild thing thread there is alot of talk but not specifics.