ArboristSite.com Sponsors


Tilly HU-51a on Stihl 032 ave

Jungle Jim

Jungle Jim

ArboristSite Member
Joined
Mar 22, 2011
Messages
55
Location
Central KY
Think I've got a carb issue on this saw but need some help.

First some history: when it runs it runs good! It worked fine for years then gradually became fussy to start & keep running. Lots of fooling with high / low / idle adjustments, venting tank, waiting for cool down, etc. It would have spells where it wanted to lock up, break my hand ... other times would pull just fine. Eventually it started to dump a little fuel into the air filter. This got worse and worse until one day all it would do was flood and the saw became useless. At that point I let it sit nearly a year with fuel in it. Fast forward to recent weeks when I actually needed it. [Farm & firewood saw -- cuts / bucks about 3 cords annually (since 1984). It has a new Stens electronic module, always runs premium fuel / Stihl mix.] In spite of the neglect it did grudgingly start and ran a few minutes roughly with much coaxing on the trigger, until it flooded its air filter, then never started again.

I only 'sort-of' know what I'm doing so please bear with me. OK, probably a carb issue so I put in a kit branded Rotary which was a very close match to rebuilt the carb. I do have the Tillotson service manual. Still can't get the saw to run. Depending on the control lever setting, it will either start, then starve OR not start and flood, or maybe pop, then flood. I've adjusted the control lever many times and have become expert at taking the carb apart.

Regarding the control lever, the Tilly manual says to set it level with the floor of the metering chamber. OK, that nets start & starve. It will pop, run fast, and quit and the plug is dry and no gas leaking from the carb. Any setting below the floor of the metering chamber = same result. OK, at a setting 1/32" above the floor of the metering chamber it will flood (wet the air filter) and not even fire. At a setting 1/64" above the floor it will start then starve. There's only 1/64" left to play with between these extremes. Can it be that fussy about its setting? I've put carb kits in this saw before and never had such trouble. Also after a few pulls it is liable to try to lock up, become hard to pull, like hydraulic lock. I am using the settings of 1 turn out on both the high and low screws. I have pressure tested the inlet needle and it will easily hold 10 lbs or so indefinitely. With alternate pressure and suction on a tube to the fuel inlet you can feel things opening and closing.

The carb was disassembled and soaked in fuel mix, then blown out with compressed air. When I first disassembled it the metering lever was set quite high; apparently it ran ok for some time like that or the setting changed somehow. The screen under the needle had a little debris which I cleaned out and the old needle looked just fine. I've tried both the old and the new with the same results. There was a tiny amount of debris under the welch plug. Everything seemed to blow thru OK. Yes I do have the gaskets & diaphragms on correctly and the old ones looked OK. The only thing I didn't fool with is the main jet as there was not one in the kit, but did blow through it. I'm not sure if there's a check valve it there or not. No carb cleaner used, just fresh mix.

One thing I'm not sure about is the butterfly in the venturi. I had it apart and did not remember which way it went: there is a little notch in one side of the plate and it can go either of 2 ways. The notch can line up with the low speed nozzles or be opposite them. I did the former but it's not clear on Tilly's sheet. Could this make a difference, hmmm.

OK, I've considered that there might be some other issues with the saw and have checked things out. Here's the list. Everything from the fuel tank to the carb is OK, venting, filter / hoses clean, no dissolved foam, no water, etc. Using fresh fuel mix in a clean tank. The exhaust system is clean; port, muffler good. The intake boot and impulse hose are good and fit tight. The carb is mounted directly to the intake book with boot sleeve intact but no additional gasket (supplied with kit but interferes with impulse hose fit), no vacuum leaks. The saw has good compression. The spark is good and plug is correct. The flywheel key is OK. If there is any play in the crank bearings I can't feel it. Have not tested the crank seals. The entire starter assembly has been serviced OK. Locking up is not from that and not ignition timing. With the plug out the flywheel will turn freely but you can feel the magnet pull slightly around TDC. The coil gap is the business card setting around .032". The entire engine has been flushed out internally (per HarleyT). The saw will leak bar & chain oil just sitting but it doesn't seem to be getting into the crankcase.

Thoughts anyone? Thanks --Jim
 
Jungle Jim

Jungle Jim

ArboristSite Member
Joined
Mar 22, 2011
Messages
55
Location
Central KY
If you put a few drops of fuel in the saw via the spark plug hole it will start easy and run fine for a few seconds until it starves. It will do this with OR without the carb mounted.
 
pioneerguy600

pioneerguy600

Lost in Space
Staff member
Joined
Dec 23, 2007
Messages
41,341
Location
N.S. Canada
The notch in the throttle plate locates pointing towards the adjustment screws. The carb must not be flowing fuel for some reason , did you remove the adjustment screws and check to see if brake cleaner will come out the fine orifices in the carb throat/venturi when sprayed through the adjustment screw holes?
 

J D

ArboristSite Guru
Joined
Apr 19, 2020
Messages
673
Location
NZ
I second Pioneerguy, there's probably still something blocked/gummed up/not right with the carb or impulse.
Regarding the control lever, the Tilly manual says to set it level with the floor of the metering chamber. OK, that nets start & starve. It will pop, run fast, and quit and the plug is dry and no gas leaking from the carb. Any setting below the floor of the metering chamber = same result. OK, at a setting 1/32" above the floor of the metering chamber it will flood (wet the air filter) and not even fire. At a setting 1/64" above the floor it will start then starve. There's only 1/64" left to play with between these extremes.
Set the metering lever flush as per the manual. Further adjustment can be made with the mixture screws. Sounds like the L might need to be turned out a bit further.
The only thing I didn't fool with is the main jet as there was not one in the kit, but did blow through it.
Main jet has a valve in it that doesn't like being blasted with compressed air
The notch can line up with the low speed nozzles or be opposite them.
The notch should be lined up with the jets
The carb is mounted directly to the intake book with boot sleeve intact but no additional gasket (supplied with kit but interferes with impulse hose fit),
Trim the gasket where it interferes & fit the gasket
 
Jungle Jim

Jungle Jim

ArboristSite Member
Joined
Mar 22, 2011
Messages
55
Location
Central KY
Have you modified the timing? Or does the Stens module work as it should?

No mod to timing, and Yes I think it works as it should but I am going to check it over again, clean & tighten connections. I know Stens needs solid ground connection (it's mounted directly to the coil).

I'm making some assumptions here that because the saw will start & run with a little fuel manually dropped in the cylinder, that the spark & engine itself are OK. This could be wrong of course!

Although I'm not sure if there's such a thing as a 'weak spark' I can see a nice blue arc with my test (plug removed, etc) however this is not a test under compression conditions. I haven't measured anything --but could-- and the plug looks fine. It's not brand new however but not very old either and was known to work OK the last time the saw was running. Don't have a new one on hand.

I can see reasons for suspecting ignition problems even though it seems counteriintuitive.

The only thing I wonder about currently is the flywheel gap to coil. It seems a little wide to me and that a tighter setting could boost the spark. I don't know the Stihl specs for this but I measured it at .032" and it is a little tighter at S end of magnet than N pole which I can easily reset if that is critical. Basically it was set with business card method. Does someone know the actual setting for this or does it matter?

While in this neighborhood, I looked at the flywheel key and it appears intact. Also I cannot detect any play in the flywheel by manipulating it; if there is some it's not huge. I do have a dial indicator if it comes to that.

Thanks --Jim
 
HarleyT

HarleyT

A-Hole!!
Joined
Dec 6, 2014
Messages
18,770
Have you modified the timing? Or does the Stens module work as it should?

No mod to timing, and Yes I think it works as it should but I am going to check it over again, clean & tighten connections. I know Stens needs solid ground connection (it's mounted directly to the coil).

I'm making some assumptions here that because the saw will start & run with a little fuel manually dropped in the cylinder, that the spark & engine itself are OK. This could be wrong of course!

Although I'm not sure if there's such a thing as a 'weak spark' I can see a nice blue arc with my test (plug removed, etc) however this is not a test under compression conditions. I haven't measured anything --but could-- and the plug looks fine. It's not brand new however but not very old either and was known to work OK the last time the saw was running. Don't have a new one on hand.

I can see reasons for suspecting ignition problems even though it seems counteriintuitive.

The only thing I wonder about currently is the flywheel gap to coil. It seems a little wide to me and that a tighter setting could boost the spark. I don't know the Stihl specs for this but I measured it at .032" and it is a little tighter at S end of magnet than N pole which I can easily reset if that is critical. Basically it was set with business card method. Does someone know the actual setting for this or does it matter?

While in this neighborhood, I looked at the flywheel key and it appears intact. Also I cannot detect any play in the flywheel by manipulating it; if there is some it's not huge. I do have a dial indicator if it comes to that.

Thanks --Jim
The Stihl trigger that was used on the 031 and one for the 032 are different. The Oregon module, which I am familiar with, replaces the one for the 032, and if put on the 031, the flywheel timing would need to be altered.
 
Jungle Jim

Jungle Jim

ArboristSite Member
Joined
Mar 22, 2011
Messages
55
Location
Central KY
Thanks. The saw is an 032 av *electronic* OEM and the original module failed. Dealer said they couldn't get one. Someone on AS said the Stens from mowerparts would work. Indeed it worked better than ever and has for a good 10 years or so. I have no reason to suspect it now.

I'm looking into the electricals today. All connections look good but am cleaning it up anyway. What I did find was that the spark plug is out of spec. We have an NGK BPMR7A @ .020". This is not what the manual says but may be an equivalent. Anyway, this is a resistance plug that is 10k ohms new. This one measures 4.47k so it's half what it should be. The plug looks good with just a little light brown deposit and very little wear, nice sharp edges, no sign of overheating, etc. But the resistance is off which I believe makes the coil work harder. Could this be a problem?

I'll measure the coil next.

Thanks --Jim
 
HarleyT

HarleyT

A-Hole!!
Joined
Dec 6, 2014
Messages
18,770
Yeah, I imagine that the Stens unit is the same as the Oregon one. I don't know why I forgot which model we were discussing. I am getting old.
 
Jungle Jim

Jungle Jim

ArboristSite Member
Joined
Mar 22, 2011
Messages
55
Location
Central KY
Me too Harley, I'm 74.

OK, finished the electrical. My coil is 8.75k ohms secondary and 1.2 ohms primary. Switch good, plug wire good. Connections cleaned. Flywheel gap touched up. The only electrical issue I see is that the plug itself may be a little weak although appearances are good. I'll get a new one. This could maybe make it harder to start but I don't think this is my underlying problem. To be cont'd ...
 
Jungle Jim

Jungle Jim

ArboristSite Member
Joined
Mar 22, 2011
Messages
55
Location
Central KY
Updated Info:

Yesterday it pulled over OK, popped on the 3rd or 4th pull, then starved, quit AND became hard to pull, refused to start again. Cylinder looked starved. I can see a starved cylinder would lack much lubrication which could perhaps cause this to some extent but this is hand crunching hard, different from the 'backlash' type from ignition and more like a hydraulic lock. But with the spark plug removed it pulls over nice and easy, every time.

I have a suspicion that this hard pulling is a big clue. So the saw sits overnight with its plug out. Today it is still very hard to pull, not totally locked up but hurts my hand and can't get the revs needed to start because of this resistance. Again removing the plug and it pulls easily.

Did nothing to the carb today except open the Low screw another 1/4 turn. The default setting is 1 turn out but I recall having to run it a little more than that normally, so 1 1/4 turn out. The carb's control lever is still set at a height that gave me a lean condition previously, at 1/64" above the spec, which says flush with the floor. So I was able to pull it over just once, with difficulty, then it locked up and I couldn't pull it over again. Maybe an 800 lb gorilla could. After this Iooking into the cylinder it was dry.

OK, let's leave the plug out, pull it over a few times without ignition switch = still no fuel getting into the firing chamber. OK, let's pull it over a few times with the throttle cocked and full choke as per normal starting procedure. Hmmmm, the carb flooded itself massively, totally wet its air filter and still NO fuel made it to the firing zone. But plenty of fuel hit the crankcase; standing the saw up a good bit ran out the carb throat. Then, of course, with the plug in it became impossible to pull over.

Hypothesis: Fuel is getting to the carb. Fuel is getting to the crankcase but fuel is NOT getting to the top of the piston chamber. And the lock I'm experiencing would be hydro due to the trapped fuel in the crankcase.

Yes, No? How to fix?


The notch should be lined up with the jets
Yes, that's the way I did it.

Trim the gasket where it interferes & fit the gasket
I've tried it both ways, with and without the gasket and it makes no difference. The issue with "fit" is that its thickness prevents the impulse hose from pushing on all the way. Some say it doesn't belong there anyway and the carb seems to fit just fine against the intake boot // boot sleeve. I can see the potential for a vacuum leak here however but it seems OK.

THANKS everyone --Jim [way behind on firewood and ice storm cleanup]
 
Jungle Jim

Jungle Jim

ArboristSite Member
Joined
Mar 22, 2011
Messages
55
Location
Central KY
Harley you were right to suspect ignition but it was the spark plug! The old plug looked very good and had spark so I didn't question it at first. When measured however its resistance was way out of spec. With a new plug the saw started right up and ran well for 4 or 5 minutes until it flooded itself by wetting the air filter. Hopefully I can adjust this out after the saw cools down and drys out. The control lever is set on the high side presently. This is progress! --Jim
 
Jungle Jim

Jungle Jim

ArboristSite Member
Joined
Mar 22, 2011
Messages
55
Location
Central KY
FIXED
Fooled by a spark plug: I assumed because it looked good, wasn't very old, and had spark that it was a good plug. Nope! It's resistance measurement was way off. Apparently that's enough to really mess up electronic ignitions. Always check the simple stuff first, duhhhh.

The carb was at fault too but the rebuild took care of that. I was trying to tune around the underlying problem of ignition. Once that was solved adjusting the rebuilt carb to factory specs worked great. The control lever is flush with the floor of the metering chamber. The H and L screws are 1 turn out per Stihl manual. Just a little more on the H for tweaking.

Surprising how many weird symptoms there were from the combination of plug + carb adjustment. Lessons learned. It starts good now and runs like a champ again; tore through 3 logs of 18-20" white oak without any issues. The hard pulling problem has gone away too. Easy to flood crankcase with spark problem.

Thanks everyone. -Jim
 
Top