Carburettor Testing (Get it Going)

[4000fixed]

Carb settings saw running

The factory uses a digital tach, I use a digital tach. Lately I've found on a new 455 Ranchers that they will run fairly poor under a load when set to 12,400 rpm no load, and run super at 12,900 to 13,200 rpm no load. 30 years of tuning saws and I still can't tell 600 to 800 rpm by ear. Guys, try testing yourself. Set idle or high side by ear, then put a tach on it. You might be surprised. We actually bet each other in the shop to see who can get closer by ear to the actual setting that the tach shows. Some saws aren't as touchy on the torque as the 455 Rancher. Just an example. Besides a screw driver, my tach is the most used tool in the saw shop.:greenchainsaw:
4000fixed

 

[4000fixed]

Carb settings saw running

The factory uses a digital tach, I use a digital tach. Lately I've found on a new 455 Ranchers that they will run fairly poor under a load when set to 12,400 rpm no load, and run super at 12,900 to 13,200 rpm no load. 30 years of tuning saws and I still can't tell 600 to 800 rpm by ear. Guys, try testing yourself. Set idle or high side by ear, then put a tach on it. You might be surprised. We actually bet each other in the shop to see who can get closer by ear to the actual setting that the tach shows. Some saws aren't as touchy on the torque as the 455 Rancher. Just an example. Besides a screw driver, my tach is the most used tool in the saw shop.:greenchainsaw:
4000fixed

 

[Ikeholt]

The best carb tip that I've read, and I don't remember who wrote it, is to simmer the carb on a stove top in water and lemon juice. It works great. Disassembling the carb when warm saves gaskets and makes the whole job easier.

 

[AUSSIE1]

The factory uses a digital tach, I use a digital tach. Lately I've found on a new 455 Ranchers that they will run fairly poor under a load when set to 12,400 rpm no load, and run super at 12,900 to 13,200 rpm no load. 30 years of tuning saws and I still can't tell 600 to 800 rpm by ear. Guys, try testing yourself. Set idle or high side by ear, then put a tach on it. You might be surprised. We actually bet each other in the shop to see who can get closer by ear to the actual setting that the tach shows. Some saws aren't as touchy on the torque as the 455 Rancher. Just an example. Besides a screw driver, my tach is the most used tool in the saw shop.:greenchainsaw:
4000fixed

Fine for an unmodified saw in possession of a tach.

 

[Rustynutz]

As far as nitrile rubber go's I think the tip of the metering needle and the check valve or valve's use it, and I've not seen a carb kit that includes the check valve? and I still recommend only fitting the part that's U/S not stuffing them full of unnecessary bits and bobs,

Like your hot lemon dip, sounds yummy (its all good stuff,keep it coming)

yea them plugs are expensive got mine from fleebay £1.75 (haaa that's better) they come with a flexible connector and a tube with a mirror so you can get into tight spots, takes all the guess work out of setting up your motor, a good tacho's not a cheap bit of kit and you still cant actually tell if your too lean with it just the RPM that's OK if you got the original manufacturers spec or years of experience. and anyway you get to look inside so you can see if its got any electrical faults too, I just found them a great diagnostic tool. and well worth the money.
R....

 

[redunshee]

Thanks Guys
I've seen some great carb explanations here, but most seem to require lots of expensive vacuum gauges and equipment, I wanted it to be simple things most people have got laying around. and lets face it most of us just want it running not set up carburetta repair facility.
The other thing I forgot to mention is to only fix the part that's broken, say if its a brittle diaphragm just fit that, I know you get all kinda bits in the repair kit but that don't mean you must fit them, and in fact what usually happens is you make it worse and inject more faults, making repair much harder.
fit what's needed and keep the other bits for spares. I'm sure you will find them useful in the future.

Remember the old adage: if it ant broke don't fix it

Good points but I make it a habit of replacing the metering needle when rebuilding. I've tried to use old ones and more times than not the carb doesn't run correctly. It just seems easier to replace while carb is apart. I do like your tips on checking for leakage. I do have an expensive pressure and vacuum tester but sans that, you 've provided a couple great " do it on the cheap" tips. Congrats.
Bob

 

[GlenM]

Good points but I make it a habit of replacing the metering needle when rebuilding. I've tried to use old ones and more times than not the carb doesn't run correctly. It just seems easier to replace while carb is apart. I do like your tips on checking for leakage. I do have an expensive pressure and vacuum tester but sans that, you 've provided a couple great " do it on the cheap" tips. Congrats.
Bob

I'm with you on the metering needle, I buy the walbro 82-75 in paks of 10, it fits a lot of the zama carbs as well....
and oftentimes only the metering diaphragm needs to be replaced.....
fwiw

 

[Rustynutz]

Thanks for all your comments guys

I tend to agree with your remarks on the metering needles and if you fit a new diaphragm that will all probably need adjustment anyway's, my point is more around pulling welsh plugs and causing internal leaking and potential damage if its not necessary.

coming back to my last post, can you get check valves ??? or is it a scrapper if there gone?

I'm working on a Mini Mac with a Walbro MDC carb, got the K1-MDC kit (no Check Valve in there) and looking around think there a bit like rocking horse sh*t so if that go's its a bin job for the Mac (Probably not first to go in the bin) having seen what a nightmare to work on they are!

R...

 

[GlenM]

coming back to my last post, can you get check valves ??? or is it a scrapper if there gone?

The idle check valves on zama c1u-pxxx carbs are not serviceable. My experience has been if fuel leaks out the idle progression holes when you're priming, it's bad and has to be trashed. I'm thinking it's caused by too strong a blast with carb spray and too high pressure air.
The main nozzle with it's check valve can be pressed out and a new one inserted.
fwiw

 

[Rustynutz]

But can you actually buy them???
I've never seen anyone selling them over here!

 

[roncoinc]

Thanks for all your comments guys

I tend to agree with your remarks on the metering needles and if you fit a new diaphragm that will all probably need adjustment anyway's, my point is more around pulling welsh plugs and causing internal leaking and potential damage if its not necessary.

coming back to my last post, can you get check valves ??? or is it a scrapper if there gone?

I'm working on a Mini Mac with a Walbro MDC carb, got the K1-MDC kit (no Check Valve in there) and looking around think there a bit like rocking horse sh*t so if that go's its a bin job for the Mac (Probably not first to go in the bin) having seen what a nightmare to work on they are!

R...

As posted earlier, with a USC pulling welsh plugs may not need to be done..
Any carb that has rubber in it has those parts in the rebuild kit.
if the carb needs work ALL the rubber and gaskets should be replaced anyway.
if the metering diaphram is bad the fuel pump and ALL associated parts should be replaced also.. DONT cheap out on parts and labor,do it RIGHT the FIRST time if your going to do it at all..

On the mini mac, BIN it is !!

 

[kam]

Small Engine Diaphragm Carburettor Tests
QUICK TEST
Prove metering system works
Put 10psi on fuel inlet and see it hold there, (will creep down slowly) put tube on diaphragm vent hole and blow, watch inlet pressure drop
If it holds and drops that’s working



So I just installed my second new carb kit on my 036 PRO
Zama C3A-S39A...checked the metering lever height is level with the carb body.

On pressure testing the fuel inlet..12 psi drops to 0 in
10 secs, a steady but fairly quick drop.

When I submerged the pressurized carb in water ,bubbles
come out the idle jets.

Is that too big of a leak?

 

[roncoinc]

Small Engine Diaphragm Carburettor Tests
QUICK TEST
Prove metering system works
Put 10psi on fuel inlet and see it hold there, (will creep down slowly) put tube on diaphragm vent hole and blow, watch inlet pressure drop
If it holds and drops that’s working



So I just installed my second new carb kit on my 036 PRO
Zama C3A-S39A...checked the metering lever height is level with the carb body.

On pressure testing the fuel inlet..12 psi drops to 0 in
10 secs, a steady but fairly quick drop.

When I submerged the pressurized carb in water ,bubbles
come out the idle jets.

Is that too big of a leak?

Depends on how you have the screws adjusted.
a running saw needs something to flow thru those holes..
final test is putting it on saw and seeing how the jets respond to adjustment.
Why a second kit ?

 

[heimannm]

So I just installed my second new carb kit on my 036 PRO
Zama C3A-S39A...checked the metering lever height is level with the carb body.

On pressure testing the fuel inlet..12 psi drops to 0 in
10 secs, a steady but fairly quick drop.

When I submerged the pressurized carb in water ,bubbles
come out the idle jets.

Is that too big of a leak?

12 PSI drop in 10 seconds is too much, something is wrong with the needle & seat or with the adjustment of the metering lever level.

Mark

 

[maico490]

coming back to my last post, can you get check valves ??? or is it a scrapper if there gone?

You can get Zama main jet check valves from Rowena: http://rowenacarbs.co.uk/ I got some recently. They were in stock and only £3.36+Vat. Here's the link to a post I made on replacing them:
http://www.arboristsite.com/showthread.php?t=145468
Both strimmers are running fine now.

 

[kam]

Why a second kit ?

After I installed the first kit minus the welch plug, I couldn't
get the saw to run. In disassembling the carb several times, I
lost the spring and new welch plug, so had to order another.


-----------------------------

12 PSI drop in 10 seconds is too much, something is wrong with the needle & seat or with the adjustment of the metering lever level.

Mark

Lever is level...needle is new....I cannot see the seat clearly, but I sprayed with WD 40 numerous times and I can see daylight through it. I do not see major damage to the seat, but my eyes are not good enough to see minor damage deep
inside. The old needle did not looked damaged, but has been replaced.


I still haven't replaced the welch plug. I don't have the proper tool to set it. The carb was very clean when I got the parts in a box saw, so I'm hoping it's not the problem.

 

[roncoinc]

After I installed the first kit minus the welch plug, I couldn't
get the saw to run. In disassembling the carb several times, I
lost the spring and new welch plug, so had to order another.---------------------------
Lever is level...needle is new....I cannot see the seat clearly, but I sprayed with WD 40 numerous times and I can see daylight through it. I do not see major damage to the seat, but my eyes are not good enough to see minor damage deep
inside. The old needle did not looked damaged, but has been replaced.

I still haven't replaced the welch plug. I don't have the proper tool to set it. The carb was very clean when I got the parts in a box saw, so I'm hoping it's not the problem.

LOst the spring !!?? LOl !!
i never done THAT before :jester: well ,not TO many times !
are you sure the spring IS the correct one ??
gaskets in order ??
gasket,metering diaphram,metal cover.
fuel pump,gasket,metal cover.
thats the order.
anything dirty under a welch plug usually blocks fuel flow..not let more through..

 

[kam]

LOst the spring !!?? LOl !!
i never done THAT before :jester: well ,not TO many times !
are you sure the spring IS the correct one ??
gaskets in order ??
gasket,metering diaphram,metal cover.
fuel pump,gasket,metal cover.
thats the order.
anything dirty under a welch plug usually blocks fuel flow..not let more through..

It's the right kit RB 31. and parts are identical to original , all the holes and flaps in the right places and in the right order.

I've done a dozen or so carbs before, they all worked without
any pressure testing (didn't have the knowledge or tester back then anyhow).

 

[kam]

"When I submerged the pressurized carb in water ,bubbles
come out the idle jets."


It's not the needle and seat that's leaking. The bubbles continue even when I apply gentle pressure on the needle with my finger.

I understand metering jet, but I'm not clear on the function of the "check valve", so I don't see a path for the leak.


Does anyone know a US source for small Zama parts?

 

[roncoinc]

"When I submerged the pressurized carb in water ,bubbles
come out the idle jets."


It's not the needle and seat that's leaking. The bubbles continue even when I apply gentle pressure on the needle with my finger.

I understand metering jet, but I'm not clear on the function of the "check valve", so I don't see a path for the leak.


Does anyone know a US source for small Zama parts?

http://www.divshare.com/folder/687780-930