chainsaw won't start and I'm running out of ideas

[Old2stroke]

Did you test the primer system to see that it was actually moving fuel in the right direction (from the carb, into the tank)? If it is hooked up backwards, the pressure in the tank can force fuel into the carb, bypassing the inlet valve. Also, there are 2 check valves built into the primer bulb, if they have failed the same thing can happen. These primers have nothing to do with the operation of the engine, they are just a convenience to help purge the main fuel line and fill the carb with gas so that you don't have to pull it over on choke so many times to do the same thing. Why not eliminate the primer and see if it cures the problem, just pull the primer line off the carb, plug it with something (my favorite is a small ball bearing) then push the line back on. I agree that it is unlikely that both carbs would have a leaking inlet valve issue but that could be resolved with a simple pressure test.

 

[jklier]

Did you test the primer system to see that it was actually moving fuel in the right direction (from the carb, into the tank)? If it is hooked up backwards, the pressure in the tank can force fuel into the carb, bypassing the inlet valve. Also, there are 2 check valves built into the primer bulb, if they have failed the same thing can happen. These primers have nothing to do with the operation of the engine, they are just a convenience to help purge the main fuel line and fill the carb with gas so that you don't have to pull it over on choke so many times to do the same thing. Why not eliminate the primer and see if it cures the problem, just pull the primer line off the carb, plug it with something (my favorite is a small ball bearing) then push the line back on. I agree that it is unlikely that both carbs would have a leaking inlet valve issue but that could be resolved with a simple pressure test.

When the primer bulb is pushed fuel squirts out of the line that is loose in the tank. I agree with you on eliminating any possibilities. I'll still give it a try with the primer completely out of the loop. I think I've got a fitting somewhere that I can attach a bike pump and gauge to the inlet line.

 

[jklier]

I disconnected the fuel line running between the carb and the primer bulb and plugged the line with a small bearing. In this setup I can start, stop, and restart the saw without apparent flooding. So, the source of the flooding has been the outlet connected to the bulb. I have quadruple checked that I have the bulb plugged in the correct way. Fuel does squirt out the open end when it is pressed. Is it possible that this bulb is faulty and is allowing fuel to flow back up once the tank gets some pressure in it? I'm going to grab another bulb and try. I also have a new gas cap arriving today since the old one is seeping fuel.

 

[jklier]

Well... it appears the new primer bulb I put on last week was faulty. I replaced it with another new bulb and the flooding issue appears to be gone. So, apparently the normal pressure being built up in the tank was pushing fuel back through the priming bulb.
Once I determined I could restart I was still not able to keep it at idle even with adjusting the carb settings. In the meantime, the new gas cap arrived from Amazon and... that appears to have resolved the idling issue. I'm going to let it sit and get completely cold and then try to restart to see if it's normal in that condition. Hopefully, this one has been put to bed. I likely would have given up by now if it wasn't for the suggestion here that led me the right direction, so thank you to everyone!

 

[SteveSr]

I disconnected the fuel line running between the carb and the primer bulb and plugged the line with a small bearing. In this setup I can start, stop, and restart the saw without apparent flooding. So, the source of the flooding has been the outlet connected to the bulb. I have quadruple checked that I have the bulb plugged in the correct way. Fuel does squirt out the open end when it is pressed. Is it possible that this bulb is faulty and is allowing fuel to flow back up once the tank gets some pressure in it? I'm going to grab another bulb and try. I also have a new gas cap arriving today since the old one is seeping fuel.

Another thought... Check valve in the primer might prevent this scenario... But make sure that the primer discharge hose back into the tank DOES NOT get into the fuel where pressure could push fuel back through the purge circuit and into the engine.

 

[jklier]

Well... it appears the new primer bulb I put on last week was faulty. I replaced it with another new bulb and the flooding issue appears to be gone. So, apparently the normal pressure being built up in the tank was pushing fuel back through the priming bulb.
Once I determined I could restart I was still not able to keep it at idle even with adjusting the carb settings. In the meantime, the new gas cap arrived from Amazon and... that appears to have resolved the idling issue. I'm going to let it sit and get completely cold and then try to restart to see if it's normal in that condition. Hopefully, this one has been put to bed. I likely would have given up by now if it wasn't for the suggestion here that led me the right direction, so thank you to everyone!

I just went out and gave it a cold start. Put the choke on for one pull then pulled with the choke off and it fired right up! Idles fine and throttles up strong.

To recap so as to help someone with a similar issue that is reading this thread, this is what I believe to be the series of issues and fixes that I had with this saw.

1. I believe I did start out with an issue with the original carb and/or fuel lines and gas cap.
2. I replaced all of the lines, bulb, and carburetor from a kit the was ordered through Amazon.
3. The carb and lines were fine but the purge bulb was allowing fuel backflow into the carb after shutdown and severely flooding the engine.
4. To test this I removed the purge bulb from the circuit by unplugging the line at the bulb and capping it off. This allowed the engine to shut down without flooding. This indicated that the issue was in the purge system.
5. replacing the purge bulb with another new one eliminated the flooding issue. At this point, the saw would start but not idle properly even with adjusting the carb.
6. replacing the gas cap then took care of the stalling at idle problem

I'm not a small engine mechanic by any means so I welcome any comments from the experts as to my explanation of the problems and fixes. Thank you again for all of the help!

 

[RED-85-Z51]

I just went out and gave it a cold start. Put the choke on for one pull then pulled with the choke off and it fired right up! Idles fine and throttles up strong.

To recap so as to help someone with a similar issue that is reading this thread, this is what I believe to be the series of issues and fixes that I had with this saw.

1. I believe I did start out with an issue with the original carb and/or fuel lines and gas cap.
2. I replaced all of the lines, bulb, and carburetor from a kit the was ordered through Amazon.
3. The carb and lines were fine but the purge bulb was allowing fuel backflow into the carb after shutdown and severely flooding the engine.
4. To test this I removed the purge bulb from the circuit by unplugging the line at the bulb and capping it off. This allowed the engine to shut down without flooding. This indicated that the issue was in the purge system.
5. replacing the purge bulb with another new one eliminated the flooding issue. At this point, the saw would start but not idle properly even with adjusting the carb.
6. replacing the gas cap then took care of the stalling at idle problem

I'm not a small engine mechanic by any means so I welcome any comments from the experts as to my explanation of the problems and fixes. Thank you again for all of the help!

Isnt it fun..sifting thru diagnostic flow charts, so to speak...dealing with not knowing if new parts are faulty..

Sent from my LM-G820 using Tapatalk

 

[SteveSr]

...dealing with not knowing if new parts are faulty..

Best way to avoid that is go with OEM and AVOID the Chinesium!

 

[Bobby Kirbos]

So what is your next project saw?

 

[jklier]

Isnt it fun..sifting thru diagnostic flow charts, so to speak...dealing with not knowing if new parts are faulty..

Sent from my LM-G820 using Tapatalk

It's not the first time I've had bad parts out of the box. It happened recently with my old jeep. I guess I should put that higher on the list.

 

[jklier]

Best way to avoid that is go with OEM and AVOID the Chinesium!

Guess what's stamped on my OEM carb? Yep, Made in China.

 

[jklier]

So what is your next project saw?

Ya know, getting this thing to finally run good again was pretty satisfying. Maybe I'll add this to my long list of hobbies.

 

[sean donato]

If it's a zama carb that shouldn't come as a surprise lol.

 

[SteveSr]

Guess what's stamped on my OEM carb? Yep, Made in China.

Well, China can make a quality product provided they are carefully managed and controlled. It is mostly when they are left to their own devices that it becomes a race to the bottom with the aftermarket junk.

 

[jklier]

Well, China can make a quality product provided they are carefully managed and controlled. It is mostly when they are left to their own devices that it becomes a race to the bottom with the aftermarket junk.

I agree. 2 years ago I put a Chinese made carb on my 20 year old mower. It still starts on the first pull every time.

 

[jp233]

True about China. It is simply hit or miss, but buying the BRANDED units seems to mean that there is some quality control involved. Recently I rebuilt the carb on my Stihl HL100 hedge trimmer and guess what. Carb said Made in China (Zama unit), the bowl-side cover said Stihl.

Good to know the primer bulb was the problem, and cheap and easy to replace. Sure not fun to get to that point, though.

It's difficult when throwing parts at a simple device and still doesn't correct the issue. I'm dealing with a Troy Bilt 4-choke TB4BP backpack blower that starts first pull even when dead cold and on half choke, idles perfectly, but bogs on initial throttle unless you very gingerly open it up, and won't stay at WOT - misses, chokes, wants to die. If you back off the WOT just a tad, it runs fine. Just doesn't seem like full RPM though either but maybe because it's spinning an impeller vs a simple trimmer head. I've replaced OEM MTD parts - spark plug, air filter, carb. Adjusted the valves, changed oil. It might be the fuel lines, but they look newer was told they were replaced about 2 years ago (this carb has 2 lines, one main and one return from the bulb built into the carb) although the vent gets into the fuel level when it's topped up - no avoiding it because of tank shape, similar to 4-choke trimmers too. Tried a known-good fuel cap from my other similar trimmer.

It has run the dang same before carb, after carb, before plug, after plug, before valves, after valves before filter, after filter - everything. There are no Lo/Hi adjust screws, just idle. So it's bugging me..... only thing I can think is the ignition module is bad or some incorrect spacing or rust on flywheel, but this thing is 10 years old and was really clean all over did not appear used and abused. So driving me nuts but I'm out of simple parts - other than the ignition module I guess which will be a pain to remove the whole impeller housing to get to it.