Tuned pipe problems/diagnostics

[Boertjie]

Hi guys. I'm in the process of building a Dolmar 85cc bigbore race saw and got stuck on the tuned pipe. Slapping on a pipe from a different saw (73CC) it ran well with lots of torque. Free running revs are 12800 and dropping to 11200 in the wood. I pulled the cylinder and remapped the porting done to design a pipe for it with 2 stroke wizzard. Pipe build and fitted the free running went up to 14500 rpm and now doing 12800 in the wood.
As soon as I hit 12000 free running ans above the machine sounds like its hitting its rev limiter and develop a serious miss. Which all goes away once its under load in the wood.
I know the problem is the pipe but where to look? Do I need to increase its header lenght? Increase its diameter?
Why does it behave like it does with this particular pipe thars designed for it?
Donor chassis a Dolmar PS7900, Husky 272 unlimited coil, Tilly 296 carb from a Dolmar 133, NWP big bore cylinder, timing advanced 30 degrees

 

[mdavlee]

Could be too much timing for it with the pipe sized right. What's your exhaust and blowdown?

 

[Boertjie]

Could be too much timing for it with the pipe sized right. What's your exhaust and blowdown?

Exhaust timing is 204 degrees and transfers 134. Thats new timing after porting

 

[mdavlee]

The numbers sound about right. Did you make an adjustable belly into the pipe?

 

[Boertjie]

Nope. Heard about that method but not sure how to convert it into an actual practical solution.

 

[mdavlee]

You make the 2 biggest sections longer where they can overlap. Just tack them together and test different lengths until you get what you want and then you can cut them off where they're supposed to be and weld it.

 

[Boertjie]

Thanks. Sounds easy enough. Taken you are refering to the straight piece that sits between the two cones?

 

[mdavlee]

Yes the flat between the cones.

 

[Boertjie]

Thanks. Will try that for sure in the morning.

 

[spencerpaving]

What size stinger dia?

 

[Boertjie]

15mm / .593" ,and 120mm / 4.7" long as per Wizzard

 

[spencerpaving]

You might be too rich...on a saw that size I use a inside stinger dia 18mm-20mm depending wether it's gas or alky...I tried a pipe designed for high rpm but it had no torque....redesigned for 1000rpm lower big diff but still freerev the same

 

[trappermike]

I don't believe your miss has anything to do with pipe,they can't cause a miss. It sounds like your saw is running slightly rich at top speed causing the classic stuttering sound(4-stroking),but this is a good safe mixture setting that will give good power and save the motor from over-lean damage,and as noted the miss goes away under load,I say you have it set ok.
But the stinger on your pipe is too small,I've been building expansion chambers for 2-strokes for 30 years
and have never seen one that small on an 85cc motor,it will choke the motor too much and can reflect too much heat back at the piston causing siezures.Years ago Yamaha had that problem on a model of snowmobile and cooked many motors before they figured it out.... I reccomend a 3/4" I.D.stinger,6-8" long.

 

[Rx7man]

Does it have a limited coil? if it's not a fuel mixture problem it could be a limited coil. On my piped saw I found the tuning procedure was completely different.. rather than richening until it 4strokes, You start pig rich and lean it until it just doesn't (in the wood).. the reason for this is the pipe (especially with a steep baffle cone) acts as a rev limiter and makes it impossible to tell what the mixture is like at the top end.

 

[jnl50201]

Good stuff here. I am going to pipe a saw one day. Hope you fixed your problem.

Sent from my Z750C using Tapatalk

 

[Rx7man]

One more thought.. is it a governed carb? I don't know the saw specifically, but some have a richening circuit that come in at a certain airflow (read: RPM)... it makes it mandatory to tune them in the wood.. Many saws have died from those carbs getting set too lean to try and get more RPM out of them than the governor would allow

 

[Wagnerwerks]

Pretty sure the 133 carbs were not governed....

 

[Rx7man]

I didn't think they were, but wanted to be sure... Especially with a tuned pipe with a steep baffle angle you can lean it out all you want and never get more revs out of it

 

[Pinechunker]

I'd go with limited coil. Sold those saws for years. No limit with the carb and usually the coil will begin to kick in around 13,200 in my experience

 

[trappermike]

It also sounds like you may have an ignition miss at high speed. Before your saw maxed at about 12,800,now its running up to 14,500 where it seems to be missing,but you're ok when revs pull down in the wood,this suggests the ignition may be weak above 12,800 or so... check the electronic ignition is grounded well etc.,and try closing the plug gap to .015". A narrower plug gap can compensate for a weak ignition at max revs-try it,see what happens. Check the wire of the spark plug cap actually contacts the spring clip for the plug.