365 special / 372 big bore

[AUSSIE1]

Old thread but for those that may still be interested.

The 52mm kits can make serious HP and torque.

Squish permitted you can leave the base gasket out. It will NOT hurt upper RPM. In fact a standard 372 will be in envy. It will increase the intake duration. I dont widen the inlet, i just flair it out into the cylinder. Keep velocity up. The exhaust can be raised. It works out to be about 1mm. Widen the exhaust. Alot of material comes out of here. Leaving the upper transfers at this stage will have a saw that will scream. I raise my upper transfers a little which will bring the RPM down a bit but make it so much more torquey. It will hold in the cut so much better. The lower transfers get the most amount of work. Drop the trans wall at the cyl 5mm below the base surface. Sharpen the transfer wall at the cylinder. Use a base gasket as a template to cut back the trans so it matches the cases. Do not open up/enlarge the transfers at all. Only neaten them if required. Have a look in there. If there is any inconsistency or it doesnt match the other side, take your time and get the matching. No sharp corners. Flow hates them. Good velocity and entrance to these is key. I angle the uppers more toward the inlet.

Its been along time. If i can find some numbers ill post them including blowdown.

Also the Stihl meteor 064 piston machined into a popup is a very good option for the 52mm bore.

 

[AUSSIE1]

I found some numbers. Too some people these may be conservative. It hasnt been from my experience for a work saw. In Australia all our wood is hardwood and harder hardwood. Torque is the aim. Yet i find with the right setup, youll still have speed in the cut.

 

[trappermike]

Nice port work,but the intake port is way too big to survive any but short races.What size carb do you use,that's what matters,even in work saws.

 

[Yotaismygame]

Is that 101 119 79?

 

[trappermike]

What is the point of having a giant intake port and a tiny carb behind it,all you guys do the same thing,big porting,stock tiny carb. You all are afraid of carbs like 99% of people,but they are very simple when your fear and dought go away.
Plus putting bigger carbs on saws is not possible or easy. Thats why I encourged boring carbs(I gave you all the specs),they are all afraid of the little mysteries and curses of carbs, bigger,but no one listens to me.Sad fact is almost no one knows how they really work,yet so simple.
Your cyl. has great race ports,but the next thing I came against was then the carbs could never be stable at high speed. Many companies tried to make hot 2-strokes in sleds.bikes etc. But almost all models "Lean-siezed" And none of them ever figured it out.
Your work is good,but your great intake port isn't gonna be satisfied by a tiny stock 16mm carb venturi.
I would use a smaller intake port for engine life,but bore the carb- more air and fuel,more power.
Virtually all diaphragm carbs lean out on a hot 2-stroke, it was the "lean-sieze"problem they (Yamaha,Mc- Culloch etc.) could never solve. Then Mikuni float carbs were perfect,and still are now. I thought of putting a Mikuni on my 090 hotsaw,but too big for saws, physically and keep in the rules.
I did try nitro-methanol but can't recommend it.
I'm not sure of your port timing,only give it to me in total duration per port.

 

[trappermike]

Your cyl.,and portings skills are very nice,it takes skill and practice to really use a dremel.

 

[Yotaismygame]

I used your specs when boring a 064 carb.

 

[AUSSIE1]

Nice port work,but the intake port is way too big to survive any but short races.What size carb do you use,that's what matters,even in work saws.

Sorry Mike, but alot of what you say is wrong. I dont want to get into an argument and what works for you, good, but works for me, brilliant.
The inlet isnt big actually. I dont open them up as such. I just flair them out at the cylinder. I dont do races hence the mild numbers. I go for torque which this saw has plenty of. It wont stall in the cut. Australian hardwood. ”Race ports”? Really? No they are not. I go for velocity. Im not about big ports. Like i said previously, dont enlarge unless its the exhaust. Just like the inlet, i dont enlarge the transfers. They are only cleaned up, angled and raised. The flywheel side trans need a little opening on the outer wall to match the opposite side.


Some of us have been quite aware of ‘carb size’. Not having fitted a larger carb doesnt come down to being skeered! I have bored the OEM carb. I ended up with a pinhole leak which i sealed with nail polish.

Mike, your post reads like you know all and everyone else is stupid. I dont claim to be the porter that ends all. I have had great success in building my 371/2’s into very torquey strong work saws with the aftermarket big bore kits in our very hard hardwood.

Regards.

 

[trappermike]

Sorry if I seem like such an assh==le,but I'm just getting really old and not internet savy. I've been into 2-stroke mods since 1972 so I've seen a lot. I just get frustrated seeing people perpetuating old myths or trying to suck people in to doing mods for them. I've never asked a penny from anyone here.
But I did help a Auzzie to build a great hotsaw there. You wanna run him?
Fact is my opinions are free(despite how demeaning I unintentionally sound),and honest to the best of abilities. I do get angry about certain old perpetuated myths. For every post I make there is someone who contradicts it,I guess that's the way it is. Not in real life has anyone told me I'm crazy or wrong.
Sorry if I offended anyone.