MS-170 Carb mod - Saw will only start on high throttle

[Mike Kunte]

Hi All!

Did the final two mods on my MS-170, namely the carb mod (Walbro 215, OEM), and the timing advance. I have some questions for you regarding the carb mod....

1. The original carb has a little "snorkel" on the top, which protrudes into a port in the plastic cover. The replacement carb does not. Nobody seems to have addressed this issue in the "mods" for this saw. Thus, I made a little plug out of plastic on the lathe to plug the hole and prevent dirt being sucked into the open hole.

2. The saw's H and L needles interact quite badly with one another, and it took me almost 2 tanks to dial the carb in. Eventually, very, very small tweaks were required. Even so, every now and then the saw just quits, and takes 50 - 60 pulls (and some fuel in the carb inlet) to get it running again.

3. Saw will not start in normal "run" mode. I either have to hold the throttle partially open (very uncomfortable), or do the whole "pull trigger, full choke on, flick up to high idle" thing before it will start again.

Any ideas? Your input is welcomed!

Thanks,

Mike

 

[Vintage Engine Repairs]

Hi Mike, it’s a compensating carb designed to maintain the correct fuel to air ratio even with a dirty air filter. You can swap the top covers over I believe.

Here is some info for you -



As for your other issues it sounds like a fuel supply problem, be this tuning or something else. Did you block the little drilling in the carb with epoxy as part of this mod?

Also if you advanced the timing too much that could be causing your starting issues too.

If the saw ran well before all the mods, it would be a good idea to return one thing back to original at a time to trouble shoot.

warm regards, Tom

 

[Mike Kunte]

Hi Tom!

Thank you for all the info and the comprehensive reply! I should have mentioned that the saw become problematic after the carb mod, and that the timing advance (which was done subsequently) neither helped nor hindered me.
So, I'm thinking I should try and swap the "tops" of the carbs and see if that makes a difference. I'd love to hear from the other guys who also did the mods whether they did this too.

Thanks again for your input!

Regards,

Mike

 

[Vintage Engine Repairs]

Pleasure Mike, hope you get to the bottom of it bud!

 

[SteveSr]

Hi Tom!

Thank you for all the info and the comprehensive reply! I should have mentioned that the saw become problematic after the carb mod, and that the timing advance (which was done subsequently) neither helped nor hindered me.
So, I'm thinking I should try and swap the "tops" of the carbs and see if that makes a difference. I'd love to hear from the other guys who also did the mods whether they did this too.

Thanks again for your input!

Regards,

Mike

Did you check the crankcase for leaks? These are famous for leaking around the clam shell joint.

BTW, the carb top swap probably won't work as the new carb is Walbro and the original is Zama. Compensation is likely NOT your issue anyway.

 

[Mike Kunte]

Thanks for the replies!!

@Tom - Sorry, I forgot to mention that I did remove the brass plug to the centre impulse line, and I did plug the left hole with epoxy.

@Steve - it was running just fine before the carb swap. I'm assuming (for now) that the clamshell in not leaking.

Mike

 

[Vintage Engine Repairs]

Thanks for the replies!!

@Tom - Sorry, I forgot to mention that I did remove the brass plug to the centre impulse line, and I did plug the left hole with epoxy.

@Steve - it was running just fine before the carb swap. I'm assuming (for now) that the clamshell in not leaking.

Mike

All good Mike what’s your experience with tuning chainsaws?

 

[Mike Kunte]

All good Mike what’s your experience with tuning chainsaws?

Hey, Tom!

Been at it for over 20 years. Been hand-tuning all my saws. Also 40 years experience tuning 2-stroke model aeroplane engines. I've set the idle slightly higher than norma (without letting the chain rotate) to help. The carb does not respond as well to tuning like other carbs... Maybe I'm missing something....

 

[J D]

Was the carb you put on new? Used? Rebuilt? If the metering is set too high it can send you round & round in circles tuning. How many turns out did the tuning screws end up at? When it dies & won't start can you smell fuel like it's flooding when you're pulling it over? Does compression feel good? I'd probably start by double check the sealing connections on the carb boot etc (you may be trying to tune around an air leak).

 

[Vintage Engine Repairs]

Hey, Tom!

Been at it for over 20 years. Been hand-tuning all my saws. Also 40 years experience tuning 2-stroke model aeroplane engines. I've set the idle slightly higher than norma (without letting the chain rotate) to help. The carb does not respond as well to tuning like other carbs... Maybe I'm missing something....

Oh okk we can cross that one off the list of possible reasons

 

[Mike Kunte]

Was the carb you put on new? Used? Rebuilt? If the metering is set too high it can send you round & round in circles tuning. How many turns out did the tuning screws end up at? When it dies & won't start can you smell fuel like it's flooding when you're pulling it over? Does compression feel good? I'd probably start by double check the sealing connections on the carb boot etc (you may be trying to tune around an air leak).

Hi, JD!

This was a brand new, expensive OEM (Stihl) W-215. I have a suspicion you may be right about the metering, since this carb is designed for the MS-250. I ended up with the L at about 3/4 turns, and the H around 1 1/4. But, as I said, the tuning was all over the place at first. It seemed all wrong. When I gun the throttle, and it starves a bit, then I open up the L about 1/8 to 1/4 turn. That drops the idle too low, but it's set almost as high as it can be.... so everything just "feels" wrong with all these settings....

As far as the fuel situation is concerned, I feel it's rather starving than flooding. I can pull it happily on full choke after the "burp" without flooding it. This would easily flood all my other saws. Should I open the metering?

The only reason I wanted an adjustable carb was to do the muffler mod, and then be able to richen up the saw to prevent a lean run...

 

[Mike Kunte]

Oh okk we can cross that one off the list of possible reason

Thanks, Tom! I'll keep it pencilled into the list... one can easily become arrogant with tuning, and it can always catch one out....

 

[J D]

Hi, JD!

This was a brand new, expensive OEM (Stihl) W-215. I have a suspicion you may be right about the metering, since this carb is designed for the MS-250. I ended up with the L at about 3/4 turns, and the H around 1 1/4. But, as I said, the tuning was all over the place at first. It seemed all wrong. When I gun the throttle, and it starves a bit, then I open up the L about 1/8 to 1/4 turn. That drops the idle too low, but it's set almost as high as it can be.... so everything just "feels" wrong with all these settings....

As far as the fuel situation is concerned, I feel it's rather starving than flooding. I can pull it happily on full choke after the "burp" without flooding it. This would easily flood all my other saws. Should I open the metering?

The only reason I wanted an adjustable carb was to do the muffler mod, and then be able to richen up the saw to prevent a lean run...

Wouldn't be the first time a new carb was faulty, I'd definitely be checking it over for an air leak around the carb though. If it is just "over carbed" a MM will help but not sure how much. In the past I've richened up those saws by replacing the fixed jets, over the years they got smaller & smaller to keep in line with regulations etc.

 

[SteveSr]

As far as the fuel situation is concerned, I feel it's rather starving than flooding. I can pull it happily on full choke after the "burp" without flooding it. This would easily flood all my other saws. Should I open the metering?

It almost sounds like the new carb is not getting impulse. No impulse = no fuel pump. Check your impulse plumbing and that the port is at the same location on both carbs and is NOT plugged by a factory plug. I think that I remember something about having to plug / unplug a hole on the WT-215 for some applications. This might be one of them.

 

[trains]

I had read that the plug was removed, and the other outlet filled with jb earlier in the thread.
But agree, sounds like poor impulse, so go over the carb again, is there good signal from the intake boot, have come across the intake boot being fitted incorrectly and pushing over the impulse tube upon assembly causing all sorts of issues.

The snorkel you mention in your original post is the air compensator, and the top cover should swap over on the carbs, no need to mess about with plugging the air filter plastic housing.

you could try swapping the metering lever springs over too after you have checked the metering levers height for correct setting, it may be too stiff, but that usually causes other problems.
Could be some debrits caught under the metering lever, check its holding pressure and its pop off pressure, let us know what it is, and we can go from there.

Weird, sounds like its not getting enough fuel, then too much, thus suggesting the metering lever spring, and that part of the carb.

almost worth going back to 1 and 1 settings, if the idle is too high, drop the LA down to acceptable rpm, and start again.
could have been junk in the carb to start with, and its made you chase your tail to where you are now.
clean carb, check metering lever height, go back to basic settings and start again.

 

[Mike Kunte]

Thank you for all your insights. Will give it a bash!

 

[Mike Kunte]

So...........

The tuning faeries arrived and did their overnight magic on the saw whilst I was sleeping.... everything is working perfectly now. Pleased don't ask me how it happened, because I don't know. Anyway, when it's warm, saw starts on first pull every time. Even if I leave the saw for 30 minutes or more, it starts up easily. The next morning it requires only a pull or two on full choke, then one or two on high idle.

From cold, it behaves very much like when it had the stock carb. Once it starts, you just leave the throttle alone for about 30 seconds. Don't blip it, don't gun it - nothing. After about 30 seconds you can start to rev it up.

Anyway, just thought you'd like to know. Thanks for all the suggestions! You are a very helpful bunch!

Mike

 

[4CornersPuddle]

That's good news, Mike. Even though the "tuning faeries" didn't leave a written report, you can be grateful that they at least brought their little faery screwdrivers in the night.
I have an MS 180 and a 018, both with 215 carbs. They both start as you have described your 170.
Enjoy. I use mine quite a lot in apple tree pruning. Using semi lp chisel chain, I find they cut smoothly and precisely. I often use full chisel on them for other cuts when I want more speed. "More speed" is a rather relative term when using such a baby saw, isn't it!?

 

[Mike Kunte]

Hey 4CP!
You are right! Speed is relative. I'm hoping to do a job with the saw this weekend, so I'm eager to see what the mods have done to the overall "feel" of the saw (and the power). I suppose I was expecting the saw to behave just like all my other (stock, unmodified) saws with respect to handling and starting procedures. This is not so. Watch any vid where modified saws are being used (Buckin' Billy Ray comes to mind), and you'll see the non-approved starting method being used, i.e. right hand on the throttle, left hand pulling the starter rope, and bar resting on the wood. Ok, his saws are ported too...

Anyway, the faeries have now done their thing, and we're now "back to normal".

Mike