Stihl 028 Carb Pressure Test

[nos351wcm]

Greetings,
STIHL 028 AV Super S/N 216688915
The carb has no numbers on it. Been replaced I'm assuming.
Last ran today, Friday May 22, sea level, 70 degrees with light rain.

Replaced air filter and spark plug with NGK BMR7A
The owner mixes own fuel 50:1. Fuel is <month old

Saw will idle. if the throttle is cracked to WOT, saw will bog and die. If the throttle is slowly depressed from idle to WOT, engine will go to WOT, but almost sound like the timing is not advancing. Im not sure how to describe the sound it makes.

But, when i pressure test the carb, i can put upwards of 30PSI of pressure into the fuel inlet, and the needle will not unseat. This sounds wrong to me, leading me to believe the issue is carb related. if I manually depress the metering needle, the pressure will bleed off to zero.

This carb does have a one way check valve which i tested using the suck/blow method with a fuel line. Seemed to be ok. The metering diaphragm didnt make a crunch sound when moved back and forth and was very soft like it should be. The fuel pump diaphragm flaps also didnt seem too stiff.

Again, there are no numbers on the carb, so I believe it to be a repalcement.
Thank you for looking.
-dan

[bgsengine]

Greetings,
STIHL 028 S/N 216688915
The carb has no numbers on it. Been replaced I'm assuming.
Last ran today, Friday May 22, sea level, 70 degrees with light rain.

Replaced air filter and spark plug with NGK BMR7A
The owner mixes own fuel 50:1. Fuel is <month old

Saw will idle. if the throttle is cracked to WOT, saw will bog and die.

Its starving for fuel. typically when there's a crankcase pressure or vacuum leak that keeps fuel pump from pumping fully..

If the throttle is slowly depressed from idle to WOT, engine will go to WOT, but almost sound like the timing is not advancing. Im not sure how to describe the sound it makes.

But, when i pressure test the carb, i can put upwards of 30PSI of pressure into the fuel inlet, and the needle will not unseat. This sounds wrong to me,

You are correct - sounds like metering lever is set too low. Not sure what carburetor is supposed to be in it without a carburetor base model (if it is Zama, it'd be cast into the body usually centered in between the throttle and choke plates on the adjustment screw side (often in the recessed area over the venturi) But as a general rule of thumb, you need the metering lever to sit at about level..

leading me to believe the issue is carb related. if I manually depress the metering needle, the pressure will bleed off to zero.

This carb does have a one way check valve which i tested using the suck/blow method with a fuel line. Seemed to be ok. The metering diaphragm didnt make a crunch sound when moved back and forth and was very soft like it should be. The fuel pump diaphragm flaps also didnt seem too stiff.

Again, there are no numbers on the carb, so I believe it to be a repalcement.
Thank you for looking.
-dan

It may well be , but usually the OEM carbs, it is very hard to see the faint (etched) model numbers, and with Zama the base model is often tricky to find without completely cleaning the carburetor 100% all around.. Then again it might be a Bing carburetor.. Ive only ever seen two of them.

[nos351wcm]

I contacted a stihl dealer earlier. They can get a rebuild kit which includes the diaphragms, gaskets, needle valve, spring... can not remember if the welch plugs are included. I do have the zama Z metering lever tool if it is a zama. The other Stihls I've worked on all had Zama carbs. I will look into readjusting the lever and much closer for the carb model numbers.
I have yet to contact the owner to relay the info i have for him.

Thank you for your response.

[KE4AVB]

The 028 originally came with a Walbro WT-16

[nos351wcm]

The 028 originally came with a Walbro WT-16

Great to know. When I called the dealer the parts associate listed the components that come in the kit and it included the metering needle, spring, and lever. The Zama carb kits I've dealt with just came with the metering diaphragm and gasket, and the fuel pump diaphragm and gasket.

I feel like walbro kits come with more components than the Zama kits.
just my .02

Thanks for looking

[bgsengine]

I feel like walbro kits come with more components than the Zama kits.
just my .02

Thanks for looking

Depends on the kit - Walbro kits may or may not include metering springs (assuming you get the K-kits as opposed to the D- kits which are just diaphragm and gasket) and likewise with Zama kits - GND- kits are just diaphragm and gasket, while the RB- kits are rebuild kits and may or may not include metering springs (just like walbro)

Rube of thumb: a REBUILD kit (Zama RB-xxx or Walbro Kx-xxx) will generally include metering levers, welch plugs, sometimes even check valves and diaphragms (as well as sometimes even purge bulbs) and may or may not include metering spring (depending on how many different springs were used in a given base model series) , while the DIAPHRAGM kit (Walbro Dx-xxx or Zama GND-xx) will generally just be diaphragms and gaskets only.. Long ago (back in the late 1980's to early 1990's) I can recall Walbro repair kits that had as many as 8 different color-coded metering springs, you'd use only one of them, depending on which carburetor model you were servicing (kit had a instruction sheet and chart in it too) - but by the late 1990's They'd eliminated the springs from kits and were consolidating kits (Example: 1980 kits might be K1-WA, K2-WA, K3-WA, K4-WA - as many as 10 different kits, while these days there's just 3 or 4, the most common being K10-WAT and K20-WAT) Back then - the most common carburetors were Walbro and Tillotson.. Zama had not made any big gains into the market yet at the time- they were not really common and maybe 3 or 4 models (C1, C1S, etc)

[KE4AVB]

As you know BGS I haven't been at this as long as you so I haven't seen any Walbro kits with springs just Zama kits. But are right Walbro only a few multi model kits where Zama has at lest a couple hundred different kits now.

This would explain why I have 20 different K kits on hand when compare to the 31 RB kits that I stock on a regular basis. And with Stihl stopping Zama from selling the RB kits that fit only Stihl equipment we have no choice but to buy from the local dealers instead of the Zama distributor. The problem here lays with Stihl as they don't provide the dealers with a cross reference to the Stihl PN. Although I do have a cross reference, the local dealers still can't get the full RB kits. Even when they can find the PN on their computer the whole carburetor is just about as cheap. This is because the kit costs have jumped 3-5 times in price. Plus even if the local can find the kit they don't stock them so they want to hit me with $20 shipping charge making a in stock cheaper than a kit.

Stihl really have shot themselves in the foot on this along dealer charging nearly $100 per hour for repairs many loyal Stihl owners are switching over to brands like Echo and Husqvarna. So this year I have only seen three Stihl trimmers and normally I would have seen over 30 units.

I know all too well that you must determine which carburetor is being used as we never know which carburetor was put on the equipment as Stihl is constant swapping carburetors. I got one trimmer model that done had at 6 different carburetor supersedes.

[Arkie]

IMHO:
It's running lean.
You can get a clone carb for around $12 with free shipping to use as a test for that Stihl.

Some carb kits costs around $30 and might be wrong one if you cannot see any ID on your carb. The dealers IPL will probably show a Walbro WT-16B carb, but Stihl seems to throw most anything at them now-days.

Appears your needle spring is little to strong for the pop off to be lower and if it pops off that high and does not hold 3-5 psi good it's also leaking back when it pops and will cause erratic engine operation.
Someone may have stretched the spring or the lever is set wrong, but $12 clone will get you in the ball park so as to make sure your are not whipping on a dead horse or whipping in the wrong area.

China is getting little better at cloning carbs, but ethanol fuel will do them in fast.

[nos351wcm]

OEM carb is discontinued. Not surprised honestly. I found an inexpensive carburetor on ebay for around $20(?). Definitely a chinese copy. I will post the results when the carb arrives. Fingers crossed.

IMHO:
China is getting little better at cloning carbs, but ethanol fuel will do them in fast.

>>I agree

Depends on the kit - Walbro kits may or may not include metering springs (assuming you get the K-kits as opposed to the D- kits which are just diaphragm and gasket) and likewise with Zama kits - GND- kits are just diaphragm and gasket, while the RB- kits are rebuild kits and may or may not include metering springs (just like walbro)

Rube of thumb: a REBUILD kit (Zama RB-xxx or Walbro Kx-xxx) will generally include metering levers, welch plugs, sometimes even check valves and diaphragms (as well as sometimes even purge bulbs) and may or may not include metering spring (depending on how many different springs were used in a given base model series) , while the DIAPHRAGM kit (Walbro Dx-xxx or Zama GND-xx) will generally just be diaphragms and gaskets only.. Long ago (back in the late 1980's to early 1990's) I can recall Walbro repair kits that had as many as 8 different color-coded metering springs, you'd use only one of them, depending on which carburetor model you were servicing (kit had a instruction sheet and chart in it too) - but by the late 1990's They'd eliminated the springs from kits and were consolidating kits (Example: 1980 kits might be K1-WA, K2-WA, K3-WA, K4-WA - as many as 10 different kits, while these days there's just 3 or 4, the most common being K10-WAT and K20-WAT) Back then - the most common carburetors were Walbro and Tillotson.. Zama had not made any big gains into the market yet at the time- they were not really common and maybe 3 or 4 models (C1, C1S, etc)

WOW ! They really "slimmed" those kits down.


This 028 seems like a great saw. Worth keeping it running. Certainly feels like it at least.

[Arkie]

If you get er going with the clone and want a OEM carb PM me and I'll steer you to a guy that may have a non-clone or know of a lead on one.

He don't like China carb clones on Stihl saws and may have something.
He may even have parts for your carb.
The 028 when all is correct is a great saw. I have about 3 of them, one is several years old, maybe 30 and still goes like the energizer bunny, only needed chain and a carb kit and only reason it needed the carb kit is I let it rest too long between uses.

Give me a heads up notice here later if you have PM'ed me and I'll watch my junk mailbox?