Saw carb tuning help

noah9988

Active Member
Joined
Jul 3, 2021
Messages
228
I finally got around to getting a saw carb hooked up to my bike. The problem is that it doesn't run right, and I can't make heads or tails of anything. First of all, it's hard to start or doesn't like starting at all. I turn the choke on for a few seconds, turn it off and it starts. However, it doesn't like to stay running, and if it does it idles very high (4-5k rpm). Now in my opinion this seems like it doesn't have enough fuel or has an air leak. Judging by the spark plug and the amount of smoke coming out of the exhaust, it says it's rich. After idling for a while it dies again, and is hard to start. If I give it throttle it just dies, again seeming like it needs more fuel, but I can get it to rev up sometimes when it's just sitting and I've never seen so much smoke come out of it before. If I actually try to ride it, it dies with throttle.

This is the carb I purchased: https://www.amazon.com/dp/B08MQCVX2Q?psc=1&ref=ppx_yo2_dt_b_product_details

I have the fuel line hooked up to the bigger of the two ports, I assume this is correct because I don't think it would run otherwise.

The two screws which I'm assuming are the fuel or air adjustments make some difference but at this point I don't even know which one is the high and low speed adjustment, or which way to screw them to make it leaner/richer, that's how little sense it makes.

It's 40f out, I'm sweating, and my legs are tired from pedaling the thing so much trying to getting this stupid thing to run right. I've never messed with these carbs before so I don't know if I'm doing something wrong or what. @Karl Snarl please save me. 🙏
 

Mossy

Well-Known Member
Joined
Jan 7, 2022
Messages
2,652
I finally got around to getting a saw carb hooked up to my bike. The problem is that it doesn't run right, and I can't make heads or tails of anything. First of all, it's hard to start or doesn't like starting at all. I turn the choke on for a few seconds, turn it off and it starts. However, it doesn't like to stay running, and if it does it idles very high (4-5k rpm). Now in my opinion this seems like it doesn't have enough fuel or has an air leak. Judging by the spark plug and the amount of smoke coming out of the exhaust, it says it's rich. After idling for a while it dies again, and is hard to start. If I give it throttle it just dies, again seeming like it needs more fuel, but I can get it to rev up sometimes when it's just sitting and I've never seen so much smoke come out of it before. If I actually try to ride it, it dies with throttle.

This is the carb I purchased: https://www.amazon.com/dp/B08MQCVX2Q?psc=1&ref=ppx_yo2_dt_b_product_details

I have the fuel line hooked up to the bigger of the two ports, I assume this is correct because I don't think it would run otherwise.

The two screws which I'm assuming are the fuel or air adjustments make some difference but at this point I don't even know which one is the high and low speed adjustment, or which way to screw them to make it leaner/richer, that's how little sense it makes.

It's 40f out, I'm sweating, and my legs are tired from pedaling the thing so much trying to getting this stupid thing to run right. I've never messed with these carbs before so I don't know if I'm doing something wrong or what. @Karl Snarl please save me. 🙏
The smaller nipple is your return line to the tank... Probably why it's flooding ...
 

Karl Snarl

Well-Known Member
Joined
Sep 20, 2019
Messages
5,691
I finally got around to getting a saw carb hooked up to my bike. The problem is that it doesn't run right, and I can't make heads or tails of anything. First of all, it's hard to start or doesn't like starting at all. I turn the choke on for a few seconds, turn it off and it starts. However, it doesn't like to stay running, and if it does it idles very high (4-5k rpm). Now in my opinion this seems like it doesn't have enough fuel or has an air leak. Judging by the spark plug and the amount of smoke coming out of the exhaust, it says it's rich. After idling for a while it dies again, and is hard to start. If I give it throttle it just dies, again seeming like it needs more fuel, but I can get it to rev up sometimes when it's just sitting and I've never seen so much smoke come out of it before. If I actually try to ride it, it dies with throttle.

This is the carb I purchased: https://www.amazon.com/dp/B08MQCVX2Q?psc=1&ref=ppx_yo2_dt_b_product_details

I have the fuel line hooked up to the bigger of the two ports, I assume this is correct because I don't think it would run otherwise.

The two screws which I'm assuming are the fuel or air adjustments make some difference but at this point I don't even know which one is the high and low speed adjustment, or which way to screw them to make it leaner/richer, that's how little sense it makes.

It's 40f out, I'm sweating, and my legs are tired from pedaling the thing so much trying to getting this stupid thing to run right. I've never messed with these carbs before so I don't know if I'm doing something wrong or what. @Karl Snarl please save me. 🙏
Ok do you have reeds or just a piston port with adaptor? The reeds will cause you some issues early on, they need to break in. It will act lean low down, and super rich up top. What you do is, set your needles 1 1/4 our on low side, 1 turn out on high side. Then try riding a little if you can. If you are thrifty you use some of that small gas hose to put on the adjust needles, so you have a knob to reach down and turn by hand.

If you are piston port, start the needles the same, and slowly work up the low side till it idles. Now let it idle till warm, will probably start to die off, so you slowly 1/8 at a time, turn up the low side till it will idle on its own. Now remember the engine need to be warm. I liek to use a heatgun to speed things up before trying to start. That saves me like 5 -10 mins of trying to keep it idling. Now once you get a stable idle, or just it idling a little higher than it should, hammer the gas, did it die out? add an 1/8th to the high side, try again. When the exhaust note sounds a little wet and it won't rev high, you're getting close. You want it so it runs a little bit rough at full throttle when clutch is pulled in, where it won't reach max revs. If you are ringing it out with out a load, you are way, way to lean.

I use the rwj-4b walbro clone on my 100cc nitro, an I set as about 2 turns on the low and about 1 3/4 on the high side with reeds and reg gas mix,
and for my heavy nitro mix I'm about 2 3/4 on the low side and about 2 1/4 on the high side. That's with nitro though.

Also if you are using reeds, they love to leak right at the intake port. An if you drilled and tapped a reed housing, you need to add a gasket to those bolt holes. I fought a leak from my housing forever, then I added a homemade rubber gasket inside and outside and it solved my issue of not starting and running super rich, then I had to fight the timing with the nitro fuel. and yada yada. Anyway, try the 1 1/4 out L 1 out H and work form there. When you set the low side, you have hand on throttle, ( warm engine tht can idle, even if really high) take and turn in till it revs to the moon then slowly starts to fade, go back the other way, no turn it out till i starts to stumble and die, then hit the middle of those to extremes and add 1/16th-1/8 a turn. Then you rev it, and set the high so it sounds just a little rough. An you go ride and fine tune after tht.
 

Karl Snarl

Well-Known Member
Joined
Sep 20, 2019
Messages
5,691
Oh and one thing you need to do if you are running reeds, you need to wait a few seconds for the engine to clear the charge through so you get an accurate idea of what's going on. The reeds at lower rpms are slower to respond to the throttle because of all the extra space the housing and track add and the stiffness of new pedals.
 

noah9988

Active Member
Joined
Jul 3, 2021
Messages
228
Ok do you have reeds or just a piston port with adaptor? The reeds will cause you some issues early on, they need to break in. It will act lean low down, and super rich up top. What you do is, set your needles 1 1/4 our on low side, 1 turn out on high side. Then try riding a little if you can. If you are thrifty you use some of that small gas hose to put on the adjust needles, so you have a knob to reach down and turn by hand.

If you are piston port, start the needles the same, and slowly work up the low side till it idles. Now let it idle till warm, will probably start to die off, so you slowly 1/8 at a time, turn up the low side till it will idle on its own. Now remember the engine need to be warm. I liek to use a heatgun to speed things up before trying to start. That saves me like 5 -10 mins of trying to keep it idling. Now once you get a stable idle, or just it idling a little higher than it should, hammer the gas, did it die out? add an 1/8th to the high side, try again. When the exhaust note sounds a little wet and it won't rev high, you're getting close. You want it so it runs a little bit rough at full throttle when clutch is pulled in, where it won't reach max revs. If you are ringing it out with out a load, you are way, way to lean.

I use the rwj-4b walbro clone on my 100cc nitro, an I set as about 2 turns on the low and about 1 3/4 on the high side with reeds and reg gas mix,
and for my heavy nitro mix I'm about 2 3/4 on the low side and about 2 1/4 on the high side. That's with nitro though.

Also if you are using reeds, they love to leak right at the intake port. An if you drilled and tapped a reed housing, you need to add a gasket to those bolt holes. I fought a leak from my housing forever, then I added a homemade rubber gasket inside and outside and it solved my issue of not starting and running super rich, then I had to fight the timing with the nitro fuel. and yada yada. Anyway, try the 1 1/4 out L 1 out H and work form there. When you set the low side, you have hand on throttle, ( warm engine tht can idle, even if really high) take and turn in till it revs to the moon then slowly starts to fade, go back the other way, no turn it out till i starts to stumble and die, then hit the middle of those to extremes and add 1/16th-1/8 a turn. Then you rev it, and set the high so it sounds just a little rough. An you go ride and fine tune after tht.
Yes I'm using reeds. Do you know which is the high and low on my carb? I think low is the one closer to the throttle
 

Mossy

Well-Known Member
Joined
Jan 7, 2022
Messages
2,652
Ok do you have reeds or just a piston port with adaptor? The reeds will cause you some issues early on, they need to break in. It will act lean low down, and super rich up top. What you do is, set your needles 1 1/4 our on low side, 1 turn out on high side. Then try riding a little if you can. If you are thrifty you use some of that small gas hose to put on the adjust needles, so you have a knob to reach down and turn by hand.

If you are piston port, start the needles the same, and slowly work up the low side till it idles. Now let it idle till warm, will probably start to die off, so you slowly 1/8 at a time, turn up the low side till it will idle on its own. Now remember the engine need to be warm. I liek to use a heatgun to speed things up before trying to start. That saves me like 5 -10 mins of trying to keep it idling. Now once you get a stable idle, or just it idling a little higher than it should, hammer the gas, did it die out? add an 1/8th to the high side, try again. When the exhaust note sounds a little wet and it won't rev high, you're getting close. You want it so it runs a little bit rough at full throttle when clutch is pulled in, where it won't reach max revs. If you are ringing it out with out a load, you are way, way to lean.

I use the rwj-4b walbro clone on my 100cc nitro, an I set as about 2 turns on the low and about 1 3/4 on the high side with reeds and reg gas mix,
and for my heavy nitro mix I'm about 2 3/4 on the low side and about 2 1/4 on the high side. That's with nitro though.

Also if you are using reeds, they love to leak right at the intake port. An if you drilled and tapped a reed housing, you need to add a gasket to those bolt holes. I fought a leak from my housing forever, then I added a homemade rubber gasket inside and outside and it solved my issue of not starting and running super rich, then I had to fight the timing with the nitro fuel. and yada yada. Anyway, try the 1 1/4 out L 1 out H and work form there. When you set the low side, you have hand on throttle, ( warm engine tht can idle, even if really high) take and turn in till it revs to the moon then slowly starts to fade, go back the other way, no turn it out till i starts to stumble and die, then hit the middle of those to extremes and add 1/16th-1/8 a turn. Then you rev it, and set the high so it sounds just a little rough. An you go ride and fine tune after tht.
"I have the fuel line hooked up to the bigger of the two ports, I assume this is correct because I don't think it would run otherwise."... Return needs to go back to the tank ?
 

Karl Snarl

Well-Known Member
Joined
Sep 20, 2019
Messages
5,691
Yes I'm using reeds. Do you know which is the high and low on my carb? I think low is the one closer to the throttle
Yes low needle is closest to the engine and high needle is closest to filter. Use a propane/mapp torch not lit to check for air leaks. Really helps pinpoint them dead on with a directed flow.
"I have the fuel line hooked up to the bigger of the two ports, I assume this is correct because I don't think it would run otherwise."... Return needs to go back to the tank ?
I don't remember right off hand, but yeah if it's backwards, it won't run at all. How I check, I pull spark plug, and spin the back tire, have one line dangling into a pop bottle, other line hooked to petcok, if it doesn't suck the gas up, I switch them. Then do it again to make sure it wasn't a weird vapor lock. Soon as I have fuel squirting from the return line to the beat of the piston, I shove it in the tank.
 

noah9988

Active Member
Joined
Jul 3, 2021
Messages
228
Yes low needle is closest to the engine and high needle is closest to filter. Use a propane/mapp torch not lit to check for air leaks. Really helps pinpoint them dead on with a directed flow.

I don't remember right off hand, but yeah if it's backwards, it won't run at all. How I check, I pull spark plug, and spin the back tire, have one line dangling into a pop bottle, other line hooked to petcok, if it doesn't suck the gas up, I switch them. Then do it again to make sure it wasn't a weird vapor lock. Soon as I have fuel squirting from the return line to the beat of the piston, I shove it in the tank.
I don't have a return line... would that affect it?
 

noah9988

Active Member
Joined
Jul 3, 2021
Messages
228
I did get it to run better but it's still doing the same things. Idle high, dies for no reason at "idle", won't rev under load, still super rich. I'll mess with it more another day.
 
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