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parrisw
Tree Freak
Cool. Will be interesting to find out.
Terry Syd
Addicted to ArboristSite
Update: The FJ2 carb is no longer available and there is no stock left. The only carb in that series that is available is the FJ5. I was willing to take a punt on the FJ2 at $68, but the FJ5 is $265 - that's too rich for me even if I knew the carb worked.
Edit: As long as I'm talking carbs, here's some information some of you guys might be interested in.
When I first adapted the 570 series carb to the 450, I notched the top of the throttle plate to lean out the idle. That allowed me make the Low speed needle a little richer so that when I opened the throttle the transition holes dumped more fuel. The mod cured the lean condition when I cracked the throttle.
I then started dropping the POP to change the fuel delivery curve and make the carb richer at lower revs. The low end power of the saw increased as I dropped the POP.
I finally ended up having to solder in the notch I made in the throttle plate as the lower POP would allow the metering valve to open much sooner. The result of the lower POP was the off idle was now too rich, so I had to go back to a stock throttle plate. I'm now running 1/2 turn out on the Low speed needle.
If you have a stock carb that is too lean off of idle, it seems to me the best approach would be to decease the POP to increase the fuel flow. The lower POP will also change the fuel curve and would probably help many saws to have a broader powerband.
Edit: As long as I'm talking carbs, here's some information some of you guys might be interested in.
When I first adapted the 570 series carb to the 450, I notched the top of the throttle plate to lean out the idle. That allowed me make the Low speed needle a little richer so that when I opened the throttle the transition holes dumped more fuel. The mod cured the lean condition when I cracked the throttle.
I then started dropping the POP to change the fuel delivery curve and make the carb richer at lower revs. The low end power of the saw increased as I dropped the POP.
I finally ended up having to solder in the notch I made in the throttle plate as the lower POP would allow the metering valve to open much sooner. The result of the lower POP was the off idle was now too rich, so I had to go back to a stock throttle plate. I'm now running 1/2 turn out on the Low speed needle.
If you have a stock carb that is too lean off of idle, it seems to me the best approach would be to decease the POP to increase the fuel flow. The lower POP will also change the fuel curve and would probably help many saws to have a broader powerband.
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duramax99
ArboristSite Operative
For anyone wanting to swap a 385/390 carb onto a 372 it can be done and does fit under the stock plastic even with the HD filter setup. Takes a little bit of cutting and tinkering to make it all fit but nothing that can be seen from the outside of the saw.
parrisw
Tree Freak
Cool, you got any pics of the swap?
Terry Syd
Addicted to ArboristSite
Will, just to let you know about those two BB jugs. Both jugs had the same offset of the ports, however the black jug had the ports sitting 5 degrees higher. The black jug had exhaust at 163 and intake at 142, the unpainted jug had 153 and 152. The blowdown was 24-25 degrees.
I decided to go with the black jug and have cleaned it up. Kept the 163 exhaust dropped the intake to 154 (likely will go to 158 when I go back in) and cut the piston crown to bring the transfer duration up to 122.
The jug had slightly higher transfer ports on one side (about .5mm), which shifted the flow in the cylinder quite dramatically when looking at the carbon deposits in the head. I've run into .5mm difference before on jugs - motorcycles. On a motorcycle cylinder using a 70mm stroke, a .5mm difference isn't that much (although worth cleaning up), however on a chainsaw with a 35mm stroke that is a heap of difference.
I kept the transfer down to 122 duration at this point so I can go back in and check the flow pattern. I can still raise (lower the piston crown) the transfers to 124, but I want to make sure I have the pattern right when I do.
Thanks for posting your numbers, it looks like I will end up with virtually the identical numbers you used.
If anyone wants to cut their piston crown to 'raise' the transfers, you can use a ring in the cylinder to mark the port positions. Looking through the exhaust port, I centered the gap of the ring on the center casting mark, then marked the postions of the transfers on the bottom of the ring. Put the ring on top of the piston (you have to compress the ring to fit the crown) and centered the ring gap on the arrow on the crown. The ring has to be flipped over, but since the transfers are mirror image of each side there is no problem.
Once you have the piston marked you need to mount it in the saw and look through the spark plug hole to tweak the marks to the right spot. After that, it is a simple matter of trimming the crown until you get the timing you want.
I decided to go with the black jug and have cleaned it up. Kept the 163 exhaust dropped the intake to 154 (likely will go to 158 when I go back in) and cut the piston crown to bring the transfer duration up to 122.
The jug had slightly higher transfer ports on one side (about .5mm), which shifted the flow in the cylinder quite dramatically when looking at the carbon deposits in the head. I've run into .5mm difference before on jugs - motorcycles. On a motorcycle cylinder using a 70mm stroke, a .5mm difference isn't that much (although worth cleaning up), however on a chainsaw with a 35mm stroke that is a heap of difference.
I kept the transfer down to 122 duration at this point so I can go back in and check the flow pattern. I can still raise (lower the piston crown) the transfers to 124, but I want to make sure I have the pattern right when I do.
Thanks for posting your numbers, it looks like I will end up with virtually the identical numbers you used.
If anyone wants to cut their piston crown to 'raise' the transfers, you can use a ring in the cylinder to mark the port positions. Looking through the exhaust port, I centered the gap of the ring on the center casting mark, then marked the postions of the transfers on the bottom of the ring. Put the ring on top of the piston (you have to compress the ring to fit the crown) and centered the ring gap on the arrow on the crown. The ring has to be flipped over, but since the transfers are mirror image of each side there is no problem.
Once you have the piston marked you need to mount it in the saw and look through the spark plug hole to tweak the marks to the right spot. After that, it is a simple matter of trimming the crown until you get the timing you want.
duramax99
ArboristSite Operative
I'll try to get some pics tonight. As far as cutting its mostly just some trimming of the top cover and carb adapter. I used the 372 carb adapter trimmed off the wing where the impulse line goes through it then "clocked" it so the 390 carb sits flat. I used the 390 air filter adapter and HD filter. Haven't had much of a chance to run it yet but it seems to pull quite a bit harder than before the swap.
parrisw
Tree Freak
Cool sounds good Terry, let us know how it works out.
Cool, thanks, from another BC guy. Where in BC are you?
Is your 372 stock? Ported?
duramax99
ArboristSite Operative
im from clearwater about an hour north of kamloops. The 372 i put the carb on is very mildly ported more like the ports are just cleaned up with a muffler mod. IT feels like it pulls just about as well as my 372 BB thats ported squish tightened up and a muffler mod.
parrisw
Tree Freak
Cool. I'm in Victoria.
duramax99
ArboristSite Operative
heres a few quick pics, nothing pretty but i wanted to make sure it would fit before I clean everything up.
parrisw
Tree Freak
Sweet, thanks. I'll have to find a 390 carb to try.
parrisw
Tree Freak
I don't know. I can go check my 390 carb for ya.
AUSSIE1
Al.
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parrisw
Tree Freak
Nice, thanks AL. Wonder what it'll do for a ported 372.
AUSSIE1
Al.
- Joined
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Dunno Will. I haven't looked into that much. Manifold side is the same 371/2 385/90 395. Filter side can't remember. I did notice differences in the venturi by eye, but this was some time ago, like years. Reserve capacity's will be different amongst other things.
Dunno mate, but I tend to think work to the original carb is all that's needed for the 70cc plus carbs for a neat ported work saw. Doing what Duramax has done is worth the try when little cutting and shutting is needed tho. I tried the 395 carb on the 385 as is as it's just a straight swap apart from the choke which only needed a little tie wire to activate temporarily. No gains there.
parrisw
Tree Freak
Right ya. I've never modded a carb. How do you go about opening up the venturi when the nozzle is sticking down in there?
Terry Syd
Addicted to ArboristSite
I thought the carb on the 385/390 had a bigger bore diameter. The 372 carb is only 17mm with a 20mm bore. The 385 carb had a 19mm venturi, if it had a 20mm bore that would only leave 1mm for the venturi or a mere .5mm on each side. The 385 carb should have a larger bore to accomodate the larger venturi.
We're talking about a 77cc BB with an increase in cutting speed. If the cutting speed is raised 10% that would be equivalent of a 84cc engine. The 385 carb is likely a better choice for the ported BB.
We're talking about a 77cc BB with an increase in cutting speed. If the cutting speed is raised 10% that would be equivalent of a 84cc engine. The 385 carb is likely a better choice for the ported BB.
