Lark-o
ArboristSite Operative
Awesome. I have my fair share of time invested with 4 barrel performance automotive carbs and float carbs on wheelers and snowmobiles but I find myself with a lot of questions about diaphragm carbs.
chinese carburetor
- Thread starter bluedxj
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Terry Syd
Addicted to ArboristSite
Your not alone with your questions about diaphragm carbs. I spent a heap of time researching everything I could find. What I found was EVERYTHING that was suppose to be authoritive had errors in it. The whole forum on EC Birt's webpage is full of BS advice. I ended up having to verify by empirical testing much of the available information.
Here's something I didn't expect to find, the pump diaphragm is only needed during idle. There's still lots I haven't figured out, like why there are different size metering orifices for the metering valve or why some carbs have the same POP, but have different spring rates installed. I have some theories, but don't have the resources to test them.
Thankfully, I have figured out enough that we should be able to work on these Zama twin jet carbs like we used to work on the Mikuni carbs for bikes.
Here's something I didn't expect to find, the pump diaphragm is only needed during idle. There's still lots I haven't figured out, like why there are different size metering orifices for the metering valve or why some carbs have the same POP, but have different spring rates installed. I have some theories, but don't have the resources to test them.
Thankfully, I have figured out enough that we should be able to work on these Zama twin jet carbs like we used to work on the Mikuni carbs for bikes.
bluedxj
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good to know. i bought a 51.
Well, I got the carb. and it is really hard to adjust, when I installed it there was zero fuel being pulled up. I took it back out and pulled the meter side and the needle was stuck, I freed it up and it either loads up in the low end or dies out in the high end in a cut. I raised the tab to allow more fuel in and now it floods, I guess I'm missing something. I can't believe that the high needle affects the low so much.
XSKIER
Addicted to ArboristSite
Mikuni SBN carbs are a lot like OPE carbs. They're pretty easy to understand. I've never had a positive result my changing the metering lever height. I've only been able to change pop-off by the needle/spring combo.
http://www.mikuni.com/pdf/sbn_manual.pdf
http://www.mikuni.com/pdf/sbn_manual.pdf
Terry Syd
Addicted to ArboristSite
KZ1000, your symptoms sound just like the time I got mixed up on which was the High and Low needles! Crikey, I couldn't figure out what was going on LOL.
The only time I noticed a significance with the metering level was on a saw where the lever was way off. It was bent down far enough that the diaphragm had difficulty stretching that far and the engine leaned out. Brought the lever back up to where is was supposed to be and I had to lean out the needles.
Too bad there isn't a manual for these diaphragm carbs....
The only time I noticed a significance with the metering level was on a saw where the lever was way off. It was bent down far enough that the diaphragm had difficulty stretching that far and the engine leaned out. Brought the lever back up to where is was supposed to be and I had to lean out the needles.
Too bad there isn't a manual for these diaphragm carbs....
LegDeLimber
Addicted to ArboristSite
For some reason I can't post a freestanding comment but seems I can post as a reply w/qoute ??
anyways, about the spring or lever question.
Seems like lightening the spring would affect the low speed flow more as that's when you don't have much vacuum to pull the diaphragm.
and seems like raising the metering lever would tend to show up more as the revs climb
from a stronger pulse to move the diaphragm and push the lever through a longer swing
that raising it would allow.
Has anyone here tried these changes (individually ) and noted what results and if I'm anywhere
close to correct in my thinking?
Terry Syd
Addicted to ArboristSite
I'm having some trouble myself in posting - anyway, you can see the cut-and-paste I did from the Mikuni PDF. Some good stuff in there. I can now see what they are doing with changing the size of the metering orifice.
The one way valve in the high speed circuit does the same thing the one way valve does on our discharge nozzles.
The Mikuni is fairly basic, the circuits are similar/identical to many of the saw carbs out there.
The one way valve in the high speed circuit does the same thing the one way valve does on our discharge nozzles.
The Mikuni is fairly basic, the circuits are similar/identical to many of the saw carbs out there.
LegDeLimber
Addicted to ArboristSite
Terry Syd: Did your older posts survive the site changes? Your comments about porting and strato-motors
was a lot of info that I hope wasn't lost.
was a lot of info that I hope wasn't lost.
Terry Syd
Addicted to ArboristSite
I just checked the search function and I found the early thread where I was struggling trying to get my head around saw engines and the strato design - a trip back through memory lane - http://www.arboristsite.com/community/threads/help-building-tuning-a-strato-husqvarna-445.107976/
The early stuff in that thread is probably only good to help someone keep from making the same mistakes.
The early stuff in that thread is probably only good to help someone keep from making the same mistakes.
Terry Syd
Addicted to ArboristSite
That flow diagram would be better if the flow rates were like the other Mikuni flow diagrams that show the relative flows of the different jets. We don't really tune much for such things a 1/4 throttle.
Terry Syd
Addicted to ArboristSite
If you take a look at the low speed circuit you will see a circuit like the 'idle/transition' circuit in the Zama 'twin jet' (I call it that to designate it differently from the 'Walbro' type circuit).
The low speed jet provides fuel for both the idle and transition. The air/fuel emulsion for the idle circuit is controlled by the combination of the low speed jet and the size of the transition holes. A bigger jet changes the emulsion ratio. The biggest thing to note is that the fuel at idle is ultimately controlled by the low speed adjuster - you can always tune the idle back where you want it. However, by increasing the size of the low speed jet you can increase the total flow through the low speed circuit.
The low speed jet provides fuel for both the idle and transition. The air/fuel emulsion for the idle circuit is controlled by the combination of the low speed jet and the size of the transition holes. A bigger jet changes the emulsion ratio. The biggest thing to note is that the fuel at idle is ultimately controlled by the low speed adjuster - you can always tune the idle back where you want it. However, by increasing the size of the low speed jet you can increase the total flow through the low speed circuit.
LegDeLimber
Addicted to ArboristSite
good to see that thread is still up. I've got a 450 that needs a bit of massaging.
Current issue is a walbro wla-1-1 on a pb-500h echo. using the mix screws I can drown it at idle
but still cant fatten up the top end at all.
...Thus my personal interest (at the moment) in the fuel flow changes from pop-off setting vs metering lever height.
I need some more juice at the top. I ran the low so rich it was gurgling
in an attempt to get some more for the top.
But that wasn't enough to cover it.
If I have any success with this
{ reed valved ! Insulator Asy Part Number: P021014991 }
and stratoed critter,
I may try to start a thread for it rather than further derail this one.
= = =
seems this works for posting? trying once more.
why is it working under "more options" and not "post reply" though
but that's a question for another section, right?
Don't like using this unsecured of a brower to post though.
Current issue is a walbro wla-1-1 on a pb-500h echo. using the mix screws I can drown it at idle
but still cant fatten up the top end at all.
...Thus my personal interest (at the moment) in the fuel flow changes from pop-off setting vs metering lever height.
I need some more juice at the top. I ran the low so rich it was gurgling
in an attempt to get some more for the top.
But that wasn't enough to cover it.
If I have any success with this
{ reed valved ! Insulator Asy Part Number: P021014991 }
and stratoed critter,
I may try to start a thread for it rather than further derail this one.
= = =
seems this works for posting? trying once more.
why is it working under "more options" and not "post reply" though
but that's a question for another section, right?
Don't like using this unsecured of a brower to post though.
Terry Syd
Addicted to ArboristSite
Your going to have to do a heap of work on that 450. The biggest hurdle I ran into was enlarging the transfer ducts. It takes a lot of time and effort. Funny thing, after I had finally got the saw to work the way I wanted, Husky came out with the 550 which has the same idea I came up with for the transfers. My suggestion is to work with the 550 - it also has a lot better air filtration. In order to get the 450 to breath I had to use the mesh filter and it sucks all sorts of fine dust past the mesh (did I mention I cut wood in Australia?)
There is a considerable difference between the various carbs. If you post some pictures of the inside of the carb's metering side I might be able to decipher what kind of circuits there are and what you can do to modify them.
There is a considerable difference between the various carbs. If you post some pictures of the inside of the carb's metering side I might be able to decipher what kind of circuits there are and what you can do to modify them.
LegDeLimber
Addicted to ArboristSite
Ditto that on the variations !!
I think I can get some pics, just need to sort out the lighting for those tiny holes.
Since it's a backpack blower, Wouldn't you say that It should be posted in the
" Hand Held Equipment and Tools" section
in order to keep things proper ?
and I'm guilty of forgetting to look in there myself.
I think I can get some pics, just need to sort out the lighting for those tiny holes.
Since it's a backpack blower, Wouldn't you say that It should be posted in the
" Hand Held Equipment and Tools" section
in order to keep things proper ?
and I'm guilty of forgetting to look in there myself.
Terry Syd
Addicted to ArboristSite
Mate, we're discussing carburetion. Your carb may be the same or similar to someone else's carb. I don't know exactly what will work on your carb, but I may be able to point out some things and after discussing some tuning principles, you may be in a position to tweak your carb. Your success (or failure) can be built on by the next guy.
roncoinc
Official Stihl Disser
To comment on the fuel pump not needed except at idle..
I had a saw in last week that would start on choke and run full and close to full throttle,would not run at idle or much below WOT.
Pulled zama and sure nuff,,fuel pump gasket with NO fuel pump !! LOL !!
dunno WHO did that !!
I had a saw in last week that would start on choke and run full and close to full throttle,would not run at idle or much below WOT.
Pulled zama and sure nuff,,fuel pump gasket with NO fuel pump !! LOL !!
dunno WHO did that !!
