Installed a Bing 48 carb on my 036

[Terry Syd]

You mentioned a difference in pop off pressure. POP is something that became popular from the karting forums, but is really overrated. You tune by performance, not something like POP or maximum RPM.

If you pull the metering springs from the early carb (no plugged circuit) and a later EPA carb, it sounds like they will be different springs based upon you observed POP (25 vs 30+). If they are different, you can try the 30+ spring in the early carb to see the difference.

I'd try running the early carb, trying both springs to see which gives you the best powerband - and then compare it to the Bing.

If it is on a par with the Bing, then the guys that have been following this discussion may want to mod their EPA carbs.

 

[drf256]

Neither of these carbs are the carb I was running on my saw in the videos. I'll post a picture of the wet side of the carb that I've got on my 036 in a few minutes.View attachment 37924View attachment 37925

I see a really easy fix

 

[Backtroller]

Holy variety. Is this normal for a model to have so many variations of the "same" Zama carb?

 

[Canadian farm boy]

I see a really easy fix

Just to clarify here( I'm a bit slow) but are you seeing basically what terry said to do?

 

[Canadian farm boy]

I'll clean up my non epa Zama carb and try it out this weekend. I'll try it with both metering springs and I'll also try it without the choke.

Should I do the testing with the v stack or with the metal mesh filter and cover?

The Bing with the v stack was the best in my previous testing

 

[Terry Syd]

Holy variety. Is this normal for a model to have so many variations of the "same" Zama carb?

Now you know why I cautioned "do not assume anything".

I would run the V stack, we're going for performance on this mod.

We may not be finished with the mods on the carb. There is still the issue of the size of the transition holes. We may have to drill one a bit bigger to be able to run more low speed fuel or to clean up the idle.

 

[Canadian farm boy]

Are the transition holes the holes that are seen inside the bore of the carb after the Venturi?
My non epa carb only has 2 of these holes. My other 2 carbs both have 4 holes

 

[Terry Syd]

I'll try it with both metering springs

You will probably find with the stronger spring that the idle will lean out and you will have to tweak the low speed needle to add a bit more fuel. Then adjust the high speed needle again.

The WOT could vary between the springs, just tune for the best powerband.

 

[Terry Syd]

Are the transition holes the holes that are seen inside the bore of the carb after the Venturi?
My non epa carb only has 2 of these holes. My other 2 carbs both have 4 holes

Only two of the holes are transition holes. If Stihlbro was correct when he said there were two holes under the brass jet, then those holes are related to the jet and not the transition circuit.

 

[drf256]

Just to clarify here( I'm a bit slow) but are you seeing basically what terry said to do?

I see that it gives us a dimple likely where the hole should be drilled for the low speed circuit.

I'd have to trace it out.

I have a bad 036 ZAMA here I can play with this weekend. I do also have a Tilly on my 034S.

It would be helpful if someone would take a bandsaw to one.

 

[Terry Syd]

It would be helpful if someone would take a bandsaw to one.

Sometimes when you're trying to figure out these little buggers THAT would help considerably. As it is, if we take our time and work through it with bright lights and WD-40 spray, we can usually figure them out.

Although, sometimes you can end up with the 'oh sh!t' moment of surprise...

I hate that.

 

[drf256]

I may have a bad carb and I do have a portaband.

Probably chopping it into 4 pieces would be enough

 

[Canadian farm boy]

I also measured the Venturi in the Zama carbs last night. The Venturi measured .59". The carb inlets and outlets measured .749"

 

[Terry Syd]

The Venturi measured .59".

A 15mm venturi, well it is what it is. I'm running a 17.45mm venturi on my 64cc 029 and the equivalent of a 19mm venturi on my 50cc Husky 450 (it is a strato).

I got to thinking about the EPA carb and how it might be a BETTER carb to mod. It has that extra jet off of idle, so we have three circuits to work with instead of just two. It also has its own transition circuit (first time I've seen that) so it may not cause any bogging off of idle.

These EPA carbs with the jet off of idle (Walbro and Zama) all use a .25mm jet. It must be a limit in an EPA regulation. On the Zama 3CMs that jet can get drilled out to .4mm to really bump up the fuel flow. The carbs Poleman developed had a heap of grunt off of idle - stop it in the cut, then hit the throttle and the saw pulled like a freight train. Not only did it make the saw more user friendly, but it makes it safer to use. Especially on something like a back-cut where you may stop to check the hinge and then start up again. - After all, we are building WORK saws and not cookie cutters.

Those other carb characteristics are something to check for when you compare the Bing to the Zama, how user friendly is it, good low end torque, is it less prone to bogging, does it have a good clean idle or does it tend to load up.

 

[CR888]

Terry any suggestions on removing the main jet on the twin jet Zama 3CM carb? There is not much room to 'grab' it with pliers. I have ordered a pair of nippers similar to what Rich uses from China for $2 deliverd but it may take a month to arrive. I've drilled the auxilary jet but need to drill under the main to .55mm. This is off a stockish 6400. Interesting carb the 3CM, after the drilling mods there are some other little things I want to look into like trimming throttle plate screw and maybe seat the jet in venturi a little lower.

 

[Terry Syd]

I put a notch in each side of the 'nippers'. It allows you to get a better grip for twisting the idle jet. You need to get the jet turning before you start to pull it out. If you just try to pull it out, you will clip the top of it off. A Dremel cut-off wheel will give you a good notch.

I moved the main discharge nozzle down lower in a couple of my carbs. I didn't notice any difference in jetting, but it did open up the venturi some more. If you do it, you will probably have to mod the groove for the fuel delivery on the nozzle. When you move the nozzle out it may start to cover up the fuel deliver hole in the carb. On one carb I moved it out so much that the groove on the nozzle was outside the carb body. I used JB Weld to seal up around it. I then had to notch the top of the nozzle so that the metering lever would clear it.

When you get the nozzle out, clean up the edge of the venturi. The venturi on the 3CM is well designed, but when they cast the body the molds join at the edge of the ventui and leave a rough edge. That is the area of highest air flow and is the roughest spot in the carb. - Typical manufacturing considerations, a good engineering design stuffed up by middle management cost cutters.

 

[CR888]

Thanks, the turning of the jet explains a lot. Just trying to yank it out messes things up being soft metal. I think I will wait for nipper pliers to arrive and notch each side, that should really grab the jet. The casting 'lip/ring' inside venturi definitely is worth smoothing out. The choke and throttle butterflies have quite chunky brass screws fixing them to shafts, I think they could be shortened and reduced in size. I must say having the carb sit on the lounge room coffee table is a good way to spend some zen time with it, understanding its finer details.

 

[Terry Syd]

a good way to spend some zen time with it,

'Zen time', oh yeah, I've done a fair bit of zen time looking at carbs, porting, pipes, suspension, geometry, yadda, yadda. Sometimes when the conscious mind can't get its head around the consensus reality, it becomes time to listen to the subconscious mind as it deals with the cognitive dissonance. Perhaps that is too philosophical for this thread, let's get back to modding carbs.

 

[drf256]

Terry,

I wish I could understand what you're saying. So way over my head.

So the transition circuit, is that the small orifice under the Welch plug that enters behind the throttle plate?

 

[Terry Syd]

A 'transition circuit' is where the fuel jet for the circuit is brought more slowly into the system. On a carb, you will often find a transition circuit for the low speed circuit. There may be two (or sometimes more with some carbs) holes in the bore of the carb that 'stagger' the fuel flow in as the throttle butterfly opens.

Take a two hole system, as the butterfly moves past the first hole, it begins to suck fuel from the jet, however it can't suck straight fuel from the jet. The transition hole that is still in front of the butterfly allows AIR into the circuit and leans out the fuel mixture. - The butterfly is creating a low pressure behind it on the first transition hole that is sucking BOTH fuel from the jet and also air from the other transition hole. The result is perhaps 50% of the fuel the jet can provide.

As the butterfly moves past the second transition hole, no more air can enter the circuit. Thus the circuit transitions to full fuel flow.

Perhaps someone can post a diagram of the transition circuits in a carb...