Carbtard

[NateSaw]

20+ years on fuel injection.. I'm a fool with these tiny carbataters. Digging into the on on my bosses 362 mtronic, that I've already ported. This thing screams, but seems/feels like it could use fuel at wot... Likely, I got too used to the overly enriched, blubbering conventional carbed saws I've done thus far. I'm enamored with mtronic. It makes sense... But I'm still a fool on these carbs...

 

[NateSaw]

This is the solenoid bore. I already went up a size on the seat. The hole on the left leads to the nozzle. Brass one on right leads to idle and transition? I drilled that up from 0.35mm, to 0.4mm. Probably a mistake. Looks as though the main comes hogged already. I drifted the nozzle out, and the hole in the side of that matched the (likely over 1mm) hole in the chamber. Take me to class fellas. Anyt threads on here you recommend, etc. I'm all ears! As always, thanks in advance for your hard earned expertise!

 

[Dolkitafreak]

View attachment 395864
This is the solenoid bore. I already went up a size on the seat. The hole on the left leads to the nozzle. Brass one on right leads to idle and transition? I drilled that up from 0.35mm, to 0.4mm. Probably a mistake. Looks as though the main comes hogged already. I drifted the nozzle out, and the hole in the side of that matched the (likely over 1mm) hole in the chamber. Take me to class fellas. Anyt threads on here you recommend, etc. I'm all ears! As always, thanks in advance for your hard earned expertise!

You drilled the seat?

 

[Ketchup]

View attachment 395864
This is the solenoid bore. I already went up a size on the seat. The hole on the left leads to the nozzle. Brass one on right leads to idle and transition? I drilled that up from 0.35mm, to 0.4mm. Probably a mistake. Looks as though the main comes hogged already. I drifted the nozzle out, and the hole in the side of that matched the (likely over 1mm) hole in the chamber. Take me to class fellas. Anyt threads on here you recommend, etc. I'm all ears! As always, thanks in advance for your hard earned expertise!

Interesting. You may be starting in the wrong place. What does the metering side look like? You should be able to punch out the main nozzle and add fuel by drilling out the hole on the side of the nozzle.

 

[NateSaw]

I'll try to drive out the nozzle again tonight and get a picture but it seemed to me that the hole in the side of the nozzle was as large as the whole in the solenoid chamber for the nozzle.

 

[NateSaw]

You drilled the seat?

Yes. From 0.6mm, to 0.77mm. And polished with hardwood tooth pick per Doc's prescription.

 

[NateSaw]

I ran this saw a bunch today. A few things I noticed since drilling out the low circuit. 1)This is hands down, consistently the easiest starting mtronic I've run. Both cold and hot. No decomp, I can load up the recoil, and just pull past one compression stroke and it pops off.
2) It has amazing throttle response, and idle recovery.

As far as it running lean, I don't think it ever was. I think I was just paranoid. It's a screamer. When I do another 362 m-tronic, I'll aim for 103* exh, or later. This one could use a bit more torque. However, it's only 2 tanks in on fresh rings so more will be had.

 

[NateSaw]

Fwiw, I brought it in as follows :
102*exh
124*-129* primaries
128*-138* secondaries
I left the intake and strato's alone.
Took 0.016" from base to land exh timing
Took 0.019" from band to land 0.018" squish
Advanced ign timing 7*
Wcs foam filter with airbox mod
Hogged factory muff port

 

[Dolkitafreak]

Yes. From 0.6mm, to 0.77mm. And polished with hardwood tooth pick per Doc's prescription.

The polishing is what I was concerned about. Sounds like you took necessary steps to get it sealing, good work.

 

[NateSaw]

The polishing is what I was concerned about. Sounds like you took necessary steps to get it sealing, good work.

I've good coaches on here. I didn't think to do it. Went back in after Doc's cautions.

 

[drf256]

I spoke to Nate about this carb. Was trying to get some clarity here.

It seems that the solenoid well fills from the metering side from the top. The brass jet on the right feeds the Welch plug well. The larger hole on the left feeds the nozzle area.

He already punched the nozzle out and found the single jet hole to be over 1mm, which kinda seems oddly big. It was a single orifice, so maybe the 362 just isn’t limited by the nozzle the way the 261 and 462 are (462 has 2 holes).

BTW Nate, you can take more out of the band than that. In certain saws, one wants to take just enough to keep the factory transfers at a certain height if they happen to be perfect (ie Deet’s hybrid). Otherwise you’re better taking some more out of the band and increasing compression, especially on an M-tron saw. The MT can fatten the saw up in long cuts better if heat builds, a non-MT saw can’t and starts to lean out. You can always raise the exhaust back up.

 

[NateSaw]

I spoke to Nate about this carb. Was trying to get some clarity here.

It seems that the solenoid well fills from the metering side from the top. The brass jet on the right feeds the Welch plug well. The larger hole on the left feeds the nozzle area.

He already punched the nozzle out and found the single jet hole to be over 1mm, which kinda seems oddly big. It was a single orifice, so maybe the 362 just isn’t limited by the nozzle the way the 261 and 462 are (462 has 2 holes).

BTW Nate, you can take more out of the band than that. In certain saws, one wants to take just enough to keep the factory transfers at a certain height if they happen to be perfect (ie Deet’s hybrid). Otherwise you’re better taking some more out of the band and increasing compression, especially on an M-tron saw. The MT can fatten the saw up in long cuts better if heat builds, a non-MT saw can’t and starts to lean out. You can always raise the exhaust back up.

You definitely can see my goal hear was to remove as little from the uppers as possible. I think what I did here would be considered "very mild porting"

 

[NateSaw]

Is there a preferred squish to cc ratio? Squish to bore diameter? In my novice search for a pattern to all this porting magic... Say it's the first time you've seen a model... How do you guys come up with your plan?

 

[Ketchup]

On bigger saws I leave a little more squish. IE. 0.016” on a 2511, 0.024” on a 395xp. Mostly because the con rod is much larger and it’s expansion is greater at operating temperature.

But how much you take out of the band matters for other reasons. The more you take, the smaller the combustion chamber gets and typically you get more compression. Exhaust port height also effects compression.

Also, the squish area ratio can effect performance. A rough standard is 40% but people deviate here for specific characteristics.

There’s a lot of interaction of why and how. A simple place to start is simply removing enough band to get your exhaust height a few degrees below where you want it.

 

[NateSaw]

On bigger saws I leave a little more squish. IE. 0.016” on a 2511, 0.024” on a 395xp. Mostly because the con rod is much larger and it’s expansion is greater at operating temperature.

But how much you take out of the band matters for other reasons. The more you take, the smaller the combustion chamber gets and typically you get more compression. Exhaust port height also effects compression.

Also, the squish area ratio can effect performance. A rough standard is 40% but people deviate here for specific characteristics.

There’s a lot of interaction of why and how. A simple place to start is simply removing enough band to get your exhaust height a few degrees below where you want it.

The area ratio. Thank you!

 

[NateSaw]

I picture later exh closing the cylinder sooner, thereby raising compressed volume. I've also often thought about the chamber... Even though making IT smaller, and the band a larger percentage of the bore raises compression, I wonder about chamber filling. For example :
Say this 362 for instance... Provided mtronic does a good job of enrichment, would not trading a bit of compression, for more volume of combustible mix in the chamber, have an intersecting line of performance gain coupled with a more stable operating temperature? These are the questions ponder daily, that most of you have already discovered the answers to

 

[huskihl]

I picture later exh closing the cylinder sooner, thereby raising compressed volume. I've also often thought about the chamber... Even though making IT smaller, and the band a larger percentage of the bore raises compression, I wonder about chamber filling. For example :
Say this 362 for instance... Provided mtronic does a good job of enrichment, would not trading a bit of compression, for more volume of combustible mix in the chamber, have an intersecting line of performance gain coupled with a more stable operating temperature? These are the questions ponder daily, that most of you have already discovered the answers to

The amount of fuel and air charge trapped as the exhaust closes is determined before the size of the combustion chamber comes in to play at all. The amount that you can get on top of the piston is from timing numbers, shape of the ports, area/volume of the ports, and case volume. Lower case volume ends up pushing a higher percentage of that charge up through the transfers quicker. More case volume slows the reaction time of the piston to pull it through the intake and push it through the transfers.

 

[NateSaw]

The amount of fuel and air charge trapped as the exhaust closes is determined before the size of the combustion chamber comes in to play at all. The amount that you can get on top of the piston is from timing numbers, shape of the ports, area/volume of the ports, and case volume. Lower case volume ends up pushing a higher percentage of that charge up through the transfers quicker. More case volume slows the reaction time of the piston to pull it through the intake and push it through the transfers.

You've helped me see about case volume. That's what inspired me to keep the uppers tight and limit the shaping of the lowers on this saw. The side feed saws, well, I'm afraid to do anything but FILL the lowers to RAISE case compression. I suppose what I picture as far as the chamber volume, is say once all the stars are aligned for getting the most fuel/air up top, when does reducing the size of the chamber reach the point of diminishing returns?

 

[huskihl]

You've helped me see about case volume. That's what inspired me to keep the uppers tight and limit the shaping of the lowers on this saw. The side feed saws, well, I'm afraid to do anything but FILL the lowers to RAISE case compression. I suppose what I picture as far as the chamber volume, is say once all the stars are aligned for getting the most fuel/air up top, when does reducing the size of the chamber reach the point of diminishing returns?

Lesser chamber volume is going to add compression.

Depends on where you want the power band at and the size of the saw.

Compression helps RPMs in the cut when you need more torque to cut faster in larger wood (work saw).

Excessive compression hurts rpm when you need more speed and less torque (race saw).

I’m sure everyone has a little bit different opinion on it, but somewhere in the neighborhood of 250 psi on very small saws and 180 on very large saws is pretty close to optimal in my mind

 

[NateSaw]

Badass.

Lesser chamber volume is going to add compression.

Depends on where you want the power band at and the size of the saw.

Compression helps RPMs in the cut when you need more torque to cut faster in larger wood (work saw).

Excessive compression hurts rpm when you need more speed and less torque (race saw).

I’m sure everyone has a little bit different opinion on it, but somewhere in the neighborhood of 250 psi on very small saws and 180 on very large saws is pretty close to optimal in my mind

Well, I like torque. Race saws are cool and fun to watch. And that clone 660 class looks affordable and may be something I'll play with one day. But I like grunt. I like being able to pry on the saw in the cut. I'll be aiming for high compression and mid to late exh timing.