exhaust port timing in a saw vs a motocross bike questions?

[mixxer]

As a new to mx 2 strokes I notice that the bikes exhaust port opening times are an average 10-15 degress earlier than a saw. I know that they have power valves to change the timing at rpms, but why can't a saw get away with 84 deg vs 98-101 on a saw. I have been reading the two stoke tuners literature seaching for a comparison and a hint into why. Stock motos a make power at 7-8k rpm. Stock saw power at 8-10k. It is the giant time and duration on the motocross bike that does it?

Nope....
Motocross bikes all have:

1) case or cylinder Reed Valve intake tracts....
2) exhaust valve setups that vary exhaust timing based of rpm
3) expansion chamber / pulse charged tuned exhaust systems
4) digital adjustable timing advance curves that take rpm and throttle position and sometimes gear selection into account...and adjust accordingly...

All adding up to early and wider powerbands than 2strokes that don't have those advantages

 

[Rx7man]

Nope....
Motocross bikes all have:

1) case or cylinder Reed Valve intake tracts....
2) exhaust valve setups that vary exhaust timing based of rpm
3) expansion chamber / pulse charged tuned exhaust systems
4) digital adjustable timing advance curves that take rpm and throttle position and sometimes fear selection into account...and adjust accordingly...

All adding up to early and wider powerbands than 2strokes that don't have those advantages

Great autocorrect.. If only it had fear selection

 

[wood4heat]

A "Motorcycle" Two Stroke at work:

https://racerxonline.com/2018/07/29...llopoto-full-moto-washougal-125-all-star-race

The camera angle SUCKED!!! But I couldn’t stop watching. I raced Washougal many many times on a KX125, KX250, and finally a CR250RT. The track really hasn’t changed. I miss the hell out of racing. Sometimes being a responsible adult sucks.

And MX bikes have many advantages over saws but one I haven’t seen mentioned is $$$. If they could sell as many saws at $5,000+ each as the big 5 sell bikes Im sure the manufacturers would figure out how to build a saw with the same power!

 

[Little Al]

Sort of Chalk& Cheese syndrome Saw whatever state of tune does all it"s work at WOT with usually a variation on a diaphragm carb 2smoke engine bike I can only comment on roadrace/GPbikespower vavlve insy outsie ductedpipe &stinger ,variable slide fully all singing/dancing carb, ecm self adjusting timing on ignition etc.

 

[Huskybill]

As we go up in cc with the moto engine the power band gets wider. I got the specs on porting a husky 2100 from a factory salesman. I tried it on a 1981 Husqvarna cr250. It changed the power band putting the shorter power band at the top. Nothing newer could catch this bike down the straights.

I read a post about how to port suzuki T500 from the aussies. A guy in England tried it and started winning. The T500 is a piston ported engine like our saws are.

 

[trappermike]

First of all,when comparing a bike engine to a saw engine,first compare the size of the carb from one to another,the bike will have a much larger carb,even though it's designed to run at much lower revs.,and it will make much more power. Chainsaws still have pretty primative cylinders,whose porting barely equalls the Japanese porting in the early 60's. Basically a 2-stroke with mundane port timing and a tiny carb,but lightwieght crank and piston to turn lots of revs. Unlike bikes with a wider power band the saw engine needs decent power at a high steady cutting speed,because of serious weight limitations and size limitations. Of course you cannot put a motorcycle size carb or expansion chamber exhaust on a work saw. So motorcycle port timing will not work really well in a saw,you have to compensate for higher rpm and a smaller carb and restrictive exhaust.A bigger carb and free-flowing muffler will help. A little more ign. advance may help,and comp. in the 160 -180 may help specially in smaller motors. In an all-out race 100cc or 122cc saw I may reduce compression to 135 -140 for full rev power.

 

[Huskybill]

All the porting I learned with measuring the 2100 ported husky worked on the bikes with reeds and we’re piston ported with no reeds. The older piston ported bikes came to life when ported. I just raised the exhaust port, widened the exhaust port, lowered the intake port and widened it, shortened the piston skirt and opened up the transfer ports. On the engines with reeds I installed a reed spacer. I never tried a carb spacer on the older saws.

 

[Ted Jenkins]

The camera angle SUCKED!!! But I couldn’t stop watching. I raced Washougal many many times on a KX125, KX250, and finally a CR250RT. The track really hasn’t changed. I miss the hell out of racing. Sometimes being a responsible adult sucks.
And MX bikes have many advantages over saws but one I haven’t seen mentioned is $$$. If they could sell as many saws at $5,000+ each as the big 5 sell bikes Im sure the manufacturers would figure out how to build a saw with the same power!

In the earlier days MX engines were piston port with a few rotary valve. In the sixties I welded up cylinders and manifolds for reed valves. During the seventies the reed induction started to become fine tuned to the delight of many competitors. Like many developments it was an evolution. The ignitions systems started out very weak so a pocket full of spark plugs was essential. To start with Mukuni was not making many carburetors so you had to some how aquire some good Amals which were a nightmare to jet. With the fine tuning of the CDI boxes one could actually go out and ride their bikes.

I looked forward to racing Washougal every year. I would plan on two or three weeks of no work to arrive at Washougal by Wednesday or Thursday for open practice. Most years I would fill up the Toyota with plenty of food and ice hook up my trailer and head for Washington. Washougal being the first stop then off to the BC race and end up at the Fernley race. In those days I did not have very many stroker motors so pulling the hill was a challenge when it got rough. When it hit 90 F and the humidity hit 80 % us Californians had a very distinct advantage. A very great time. Thanks

 

[Huskybill]

I purchased the Clinton go kart / chain saw engines to play with in the future. These are the 2.5 hp panther engines. A smaller cc mikuni carb I’m thinking would work if rejetted. Of course changing the needle jet, the needle and resetting the pilot and main jet. On one husky bike a 390 cr I removed the 38 mm mikuni carb and installed a 40 mm carb. Rejetting the pilot and main jets and going two sizes smaller on the needle and needle jet I got it to work properly. This bike has some hit to it. It wasn’t for a novice rider.

 

[Ted Jenkins]

The key to happiness used to be how well a decent two stroke MXr was tuned by the carburetor. Then there was the selection of the reeds stiffer for more RPMs or soft for more torque. Fatter pipe for broader power or more narrow for an increase in HP. In the days of Amal tuning was crude at best. I had several metering rods that I stuck in a drill to taper or make small adjustments. Then Mukini started making carbs larger than 26 or 28 MM and wow oh wow. Then the D slide and square slide making even more difference. In the days of early Hodaka is was not too uncommon for pumper carbs to be used or full chain saw carbs with great success. I even remember tuners using two chainsaw style carbs with out any known problems other than more adjusting to make life a little more tricky. Thanks

 

[Rx7man]

I purchased the Clinton go kart / chain saw engines to play with in the future. These are the 2.5 hp panther engines. A smaller cc mikuni carb I’m thinking would work if rejetted. Of course changing the needle jet, the needle and resetting the pilot and main jet. On one husky bike a 390 cr I removed the 38 mm mikuni carb and installed a 40 mm carb. Rejetting the pilot and main jets and going two sizes smaller on the needle and needle jet I got it to work properly. This bike has some hit to it. It wasn’t for a novice rider.

When did you race? you might know a couple of my friends that raced in the late 70's, Mike Mack and Peter Panek.. Mike was on the big bikes, Peter was on the 125's

 

[Huskybill]

I didn’t race my kid did I turned wrenches and built bikes