chinese carburetor

[Terry Syd]

Here's another trick. Say you have a saw and you advance the timing. That helps the top end power for cookie cutting, but you lose some power down low where the cylinder charging is higher. The saw needs the extra timing at higher revs because there is less time for the charge to burn with the thinner charge, but down low there is enough time.

OK, so now you start with the stock timing and keep increasing the fuel flow of the low speed circuit/s. Eventually, you get it a tad too rich and you start loosing that low speed torque. Not a problem, in fact, you can use that extra fuel.

The reason you lost the lower RPM torque was because the extra fuel slowed down the flame front. So now you can advance the timing to speed up the flame front. You can get that low end torque back AND get that fast cookie cutting speed.

 

[k_kindig11]

Why not file a bit off the throttle plate on the side OPPOSITE of the idle port. That will lean out the idle and when the engine is idling the backflow up the intake will be reduced over the idle port (part of it can go past the other side of the throttle plate).

You don't need to take the throttle plate out of the carb to give it a bit of a trim. Just stick something in the carb to hold the throttle plate open. The edge of the plate will stick out of the carb and you can trim it. I use a small fine needle file to trim

I guess I just figured it would be easier to mess with the metering arm. I have no experience with altering the throttle plate... No idea how much to take off. So you're saying with carb at WOT, I need to file off of of the part sticking out of carb. That side must be the side opposite the idle port? Or do I need to file off the left or right side of the part sticking out? I guess I'm confused where exactly the idle port is. Looking a the diagram I'm struggling. To put 2 and 2 together. I hae limited knowledge with carbs.

 

[Terry Syd]

The idle port is the small hole in the body of the carb that sits just behind the throttle plate. With the throttle closed you can look into the carb from the engine side of the carb and see a small hole. That's where the fuel comes out at idle.

Just in front of that hole is the throttle plate and you will see a small cutaway on the plate where air comes through at idle. It is the air through that cutaway that blows across the idle port and causes a low pressure to suck the fuel out.

When you file a bit off the side of the plate opposite the cutaway you will allow more air to go through, bypassing the idle port, and it will lean out the idle.

I mentioned about backflow at idle. Backflow is caused by the piston compressing the mixture in the crankcase and the pressure rises before the piston can close the intake port. This causes mixture to travel back out the carb. If it blows across the idle port again, it can pick up more mixture - which then gets sucked in on the next intake stroke. That can cause the engine to 'load up' at idle.

By trimming the plate on the opposite you can lean out the idle mixture and also prevent some of the backflow from blowing back across the idle port.

Go slow on trimming the throttle plate. Take just enough off that you can see a bit more space when you look through the carb. Try it out, and then maybe you will take a bit more off. Just make sure you still have some adjustment on the idle adjustment screw before you trim again. The idle adjustment screw is the screw that holds the idle plate slightly open at idle. You will need to turn down the idle with the idle adjustment screw after each time you have trimmed the throttle plate. Check to see that you can still close the idle plate further before you trim the plate.

 

[Terry Syd]

If the metering arm is adjusted correctly, moving one way or another won't fix your problem. If the arm was up so high that it pressed against the diaphragm, then the valve might leak. Just check that it is adjusted at the correct height for the carb.

If you have a leaky metering valve, it will cause the engine to flood at idle.

However, the symptoms you mention of it idling for a while and then finally loading up and stopping sounds like it may just need a bit of leaning out. Try turning the Low speed needle in a bit and see if that clears the problem. If it does, then there is your answer, you needed to have it run leaner at idle.

Turning in the low speed needle may make the engine less responsive when you hit the throttle. In that case, the throttle plate trimming trick can allow you to run more low speed needle for better response and still have a good idle.

 

[Terry Syd]

Another idea, sometimes the damn EPA vents can cause higher pressure in the tank. You may need some extra POP on the metering valve. Bending the arm down to increase the preload on the spring is a half-ass fix to the problem. It can create other problems if the diaphragm has problems stretching out that far - you will lean out at high revs.

If you need more POP and you don't have a stronger spring, then take an aluminium beer can and scissors and cut a small disc to fit down in the well for the metering spring. When you put the spring back in it will sit higher. That will increase your POP.

 

[XSKIER]

Good info here. Although in a repair type situation, I may suggest using a pressure tester to make sure of the needle and seat's sealing integrity. Once verified, let the tuning begin! I see that watsonr has a tuning kit available with multiples of several different pop-off springs.

 

[nmurph]

Yes, lots of very good info. However, I have lowered the metering arm on Zama carbs several times to overcome the loading-up-at-idle syndrome. It requires a bit more L needle, but it fixes the problem without making the saw slower off idle. Going forward I will be try filing the plate to see how this works.

I like the idea of the beer can under the spring. Won't stretching the spring do the same thing?

 

[Terry Syd]

Yep, lots of ideas. Trying to diagnose a problem over the internet has its difficulties.

"I see that watsonr has a tuning kit available with multiples of several different pop-off springs." Now that is a great idea. He should start a thread on it and show some pictures of the springs.

What I'd really like to see would be for Zama to put replaceable jets in their carb. Then have them offer a jet selection kit. Crikey, you could tune up a great carb in an afternoon. This drilling of fixed jets routine is a drag.

I've done the 'stretched spring' thing and don't do it anymore. If you get it right, it's fine. Get it too long and your back to cutting the spring. Then once you've stretched a spring, you can't 'unstretch' it. Shimming the spring allows you to reverse the mod and preserve your springs.

 

[Terry Syd]

I mentioned the EPA valves. Trying to dial in your carburettor when the pressure in the tank keeps changing can be headache. One time the tank is pressurized the next time there is a vacuum.

Unfortunately on my saws the EPA vents got 'lost'. I don't know where they are so I had to make up fully vented tanks.

 

[Terry Syd]

nmurph, you sound like you're a shop owner who has to deal with carb problems all the time. Instead of cutting a piece from a beer can to use as a shim, you can purchase a box of washers to have on hand. I measured the spring OD on a Zama spring and it was 3.14mm. The well in the carb appears to be 4mm wide. So a washer of 3.5mm OD should work.

Here's a link to some washers 3.5mm OD, a packet of 25 for $5 - that saves a lot of labor time.

http://www.scalehardware.com/produc...-11/1-4-mm-machined-washer-brass-qty-25-p-252

 

[Terry Syd]

nmurph, I noticed that you had a 84cc Dolmar/Makita. As I recall, the Dolmar has a Zama twin jet carb (C3M). If you want to try some jetting changes that will give the saw some torque, you can try the jetting I'm using on a 372BB.

Every Zama that I've looked at has used a .25mm jet for both jets (.25/.25). I eventually moved to a .35mm for both jets (.35/.35). There is one problem with that jetting combination, the low speed needle only has 1/4 turn of adjustment and it is very touchy to set.

If you only go to .30mm on the low speed jet you will have just over 1/2 a turn of idle mixture adjustment. A 1/2 turn to adjustment is a lot easier to work with (twice the movement). You could start with a .30/.35 combination, perhaps a .30/.40 combination would work even better.

However, I've just tried a low speed needle out of a C3M S5G from a 044 that has a slower taper (Zama part # 0031032). With that needle and the .35 low speed jet I can get 1/2 turn of adjustment. I'm sure there must be even slower taper needles out there, so if you have a few Zamas you can look at (or anybody else out there) and find a Low speed needle with a slower taper than the 044 carb, I'd appreciate the heads-up on the needle.

 

[k_kindig11]

Turning in the low speed needle may make the engine less responsive when you hit the throttle. In that case, the throttle plate trimming trick can allow you to run more low speed needle for better response and still have a good idle.

Lots of good info. Thanks for taking the time to share and explain. I feel like the L screw is where it needs to be. I lose response if I turn it in any further. You explained the idle port and trimming the plate wonderfully. I understand completely and will have to give that a try. Thanks again!

 

[one.man.band]

Here's another trick. Say you have a saw and you advance the timing. That helps the top end power for cookie cutting, but you lose some power down low where the cylinder charging is higher. The saw needs the extra timing at higher revs because there is less time for the charge to burn with the thinner charge, but down low there is enough time.

OK, so now you start with the stock timing and keep increasing the fuel flow of the low speed circuit/s. Eventually, you get it a tad too rich and you start loosing that low speed torque. Not a problem, in fact, you can use that extra fuel.

The reason you lost the lower RPM torque was because the extra fuel slowed down the flame front. So now you can advance the timing to speed up the flame front. You can get that low end torque back and get that fast cooking cutting speed.

terry...agree with paragraphs 2 and 3, just think that paragraph 1, could possibly be trouble. (ie. lean mix with advanced timing).

most modern 2ST which have CDI microprocessor controlled curves, which i have read about so far, (cannot find enough info for saw microprocessor cdi curves)..... start off with a retarded spark for ease of starting, then have a quick advance curve up to a couple of thou rpm before peak torque. From peak torque on up in rpm... have a curve retarding downward as rpms increase (almost falling of a cliff), to the max motor rpm. interesting note, is that the amount of spark retard at max rpm is even less degrees before TDC than at starting.

interesting thread.
-joe

 

[Terry Syd]

The first paragraph refers to a person's observations about how their saw might respond to an advance in timing. Some saws may not respond that way, but I expect a large percentage would.

I've tried to find some information on advance curves for the magneto systems on saws and I haven't found anything like an advance curve you'd find on a modern CDI system. All the 'curves' I've found have just been simple retard systems for starting and then going to full advance, usually by around 2,000 rpm.

The early CDI systems on bikes were set up like the saw magneto systems, just a set ignition timing. However, the newer systems can vary all over the place. I remember a CDI system for a KTM that had a huge spike in timing in the 'black hole' of power just before the engine came on the pipe. That timing was quickly retarded as the engine did come on the pipe and the cylinder charging increased, then it started advancing again as it came off max torque, then quickly retarded as it hit max revs - it was all over the place! The result was a surprisingly useable powerband.

As far as our saw magnetos, if we are stuck with a set ignition timing, then tuning the fuel mixture is the best we can do to vary the rise time in the chamber.

 

[one.man.band]

was searching for the same info a while back about the basic magneto system. could not find anything even listing the specs for voltage, rise time, spark energy, etc.

found some interesting info on a Aprilla site about changing CDI curves with dyno results.

did not mean to derail a good thread. sorry.

-joe

 

[Terry Syd]

I didn't start this thread, it just turned into a technical discussion of carbs. In fact, the issues of carburetion and ignition timing work together, so bringing up some more discussion on ignition timing helps fill in some gaps about tuning the carbs.

When I was building bike engines I would try changing the ignition timing along the way to get a feel for what is happening in the engine. The combination of the change in carb metering and ignition timing would give me a hint as to where my next change should be. I'm doing the same thing as I work with chainsaws.

To me building a work saw is kinda like building a strong enduro bike engine, I want it to pull like a freight train down low and scream like a jet on top end, quick throttle response and stone cold reliability.

Anybody know of any low speed needles with a very slow taper?

 

[Terry Syd]

In case anyone is interested in drilling out a carb, here is an example of a set of pin vices and micro drills on Ebay - http://www.ebay.com.au/itm/Pin-Vice...Crafts_JewelleryMaking_GL&hash=item27d50a8342

You definitely need a pin vice to hold the drills. The smaller drills are not drills but reamers. Since there is already a hole in the jet, you just ream it out - CAREFULLY. I've got two different packets of the drills one is .3mm - 1.6, the other is a 61-80 set. Since the numbering system is different, the size of the drills are different and you have more options for drill selection with both packets. You could use either packet if you didn't want to get both.

A set of micro drills is a nice addition to your tool box. You will find other uses than drilling out carbs.

Clean off the carb with contact cleaner, or whatever, to remove any lubricant from the jet. Use a micrometre or callipers to measure the drill before you use it (the drills are small, they can get mixed up).

You've all seen drill presses and how the rigid structure ensures that a vertical hole is drilled - well, you are the 'rigid' structure for the drill. I use a chair with arm rests so that my elbows are supported. I remove the lid and pump diaphragm so that the carb sits flat on the edge of the table. I can lean over the carb at a comfortable height and slowly twist the drill. I use a lamp next to the carb for good light and I wear a 2X magnification reading glasses. The first pass of a .3mm drill through a .25mm jet will take some time, relax and be patient. If you are going to drill a .25mm jet out to .35, then run a .30 reamer through first and then go to the .35 reamer.

Sure wish Zama made replaceable jets...

 

[SawTroll]

Zama, owned by Stihl, made in China.

I don't like made in China, but these carbs aren't so bad.

I totally agree, nothing wrong with Zama carbs, if they are the right ones!


There are other Chinese carbs, that are much more problematic!

 

[Terry Syd]

Niko, that's a problem with picking up 'Zamas' off the internet. What we want to use is the 'twin jet' constructed carb. Unfortunately, as I found out, even an original early Zama may not be a twin jet. Then there are the cheap 'Zamas' that are obvious copies of Walbros with Zama cast on the lid. Those are easy to pick as the discharge nozzle comes in at a different angle.

If I was going to order a few carbs, I'd get just one first and confirm that it was a twin jet, then I would put in the balance of the order.

I'm still working through a couple of carbs on two saws. I haven't been able to test them for more than three weeks. Hopefully, next week I may get a chance to see where I'm at in this process.

If someone was going to work up a carb for their saw, a good approach would be to use two carbs. They could make a mod to the first carb and check it was beneficial. Then make the same mod to the second carb and hold it in reserve. Then open up a jet to another size and check it out. By always having a sample of the previous mod you could always put on the carb with the previous mod to compare it to. When you go too far, you'll know it and can use the reserve carb.

If you can pick up cheap Chinese copies of twin jet Zamas for $20, it's good insurance.

Hopefully, a few guys will get interested in the idea of modding the carbs and post their experience. After a few results, we might have some guidelines for drilling. Something like a 30/30 for 65cc, 30/35 for 70cc, 30/40 for 80cc - or whatever.

 

[Terry Syd]

One last idea, there are some saws that won't be able to use the Zama carb. I took a look at a couple of Walbros and found spots where I could drill a second jet on the carb so that it functioned like the Torque jet on the Zama.

I may do that to the Walbro that came on my project 029. I'll use a small drill and drill through from the metering chamber. The hole will be exposed after about 1/4-1/3 throttle. I'll then take a small dab of GB Weld (GBW) on my finger and push it into the hole from the carburettor bore side. Clean up the bore, put a piece of tape over the hole and turn the carb upside down for the GBW to set.

I can then drill out the GBW in increments to see what size 'jet' works best. When I go too big on the jet, I can drill out all the old GBW, reapply new GBW and go back to the required size.