Help diagnose fuel delivery problem - Stihl 051

[OneStaple]

Hey,

Something is wrong with the fuel delivery system in my 051 and I can't figure out what. I purchased this a little while back to fix up and have never run it myself, so it could be any number of possible things.

Here's what I know (hopefully I remember everything):
--After cranking on the engine for a bit, the fuel line between the gas tank and the carb is totally dry. It's not too little fuel, but no fuel at all.
--There is good gas flow through the gas tank filter and elbow connector.
--Both the fuel hose and impulse hose appear to be in good condition and didn't give off any air when I blew hard into them while plugging the other end. Granted, this might not detect a minor crack, but I'd expect at least some fuel flow if the crack/leak was that small.
--I carefully took the cover off the fuel pump on the carb and the seals in there appear to be in decent condition, although I don't have a new one to compare them to. But there's no cracks or anything at all. They almost look newish.
--When I put my finger over the end of the impulse hose and crank the engine, I can't feel any pressure change. I don't know if it's supposed to be strong enough to feel though, or if all the movement I had to use to crank the engine at the same time would have kept me from feeling a sensitive change in pressure.
--I can't see past the nipple where the impulse hose connects on the engine side without quite a bit of work tearing it apart, but I assume it's a hard connection in there (between the engine and the other side of the nipple). Is that something worth looking into?

So, what else can I look at, or what might I have overlooked? I'm not sure what to check next.

Thanks!
--Tyler

 

[Fish]

Well to start, just put a dribble of fuel down the throat of the carb, with the
air filter/cover off, and try to start. It should start then die, when the fuel burns off, if not, then you have more troubles.
Do that, and post back and we can take it from there.

 

[OneStaple]

Ok, I did that, and it definitely is trying to start. It'll catch for like half a second and then die, I guess as it uses the gas up. It's far better than when I was just cranking on it before without putting gas in the carb though.

What should I check next?

--Tyler

 

[harrygrey382]

--There is good gas flow through the gas tank filter and elbow connector

so you pulled the fuel line off the carb and blew into it, and heard bubbles blowing in the tank yeah? I've recently had two 051s with blocked fuel lines...

If you can get your hands on another carb, might be worth it. Even iff only borrowing one...

But having said all that, I reckon it sounds like an air leak. Vacuum test it, or get a dealer to do it. When of the 3 crank seals is probably not holding up so it can't provide an impulse to the pump, I think you should be able to feal the pulse, but not sure. Or you could pull the flywheel and clutch off and have a look at the seals, may be a really bad one to not even start. But often can't see damaged seal.

 

[andrethegiant70]

You're going to need to give us some background...... has this saw run in the past, if so, how long ago? Have you had the head or carb stack apart?....was the saw given to you by your girlfriend's ex-boyfriend's father-in-law because he rebuilt it and then it wouldn't start? and so on... tell us the story please, otherwise we'll have to give you the whole derned song and dance, which takes time, especially for those of us who have this window up in the background and really ought to be doing productive work instead...

:hmm3grin2orange:

 

[Fish]

No, if all else was healthy, it would vroom for several seconds, then die when the fuel was used up, so you have spark/ and/ or other problems.

 

[OneStaple]

Yeah, with gas down in the carb, the chainsaw will sound like it's about to start, but then die. It never holds an idle for a few seconds. I can pull the rope about 1/4 of the way up, it catches and sounds like it's trying to start, but then I'll finish the pull and it will be dead just after I bring my hand back down to give it another pull.

Fuel line definitely isn't blocked. I blew through the part between the elbow joint and carb quite easily. Without the fuel line attached at the elbow joint, fuel came flowing out very easily when I tilted the chainsaw a bit.

andrethegiant70 - you're not going to like this answer, but I purchased it off ebay. Based on what the seller said, it has been run in the past but has been sitting for about the last two years. Came without the clutch and air filter, but otherwise is complete (minus a few other random bolts). By the way, who really wants to be doing other productive things when they can be playing with chainsaws?

I checked for spark last night, and the plug readily gave a nice meaty spark. I'd think that the saw's attempt to start running (it kicks itself over a few times) would confirm this. Unless there's a problem with prolonged periods of giving spark or something.

--Tyler

 

[rxe]

Big air leak? You know on the old 070/090 cases there is a big nut on the bottom to lock the crank? Well, I locked the crank, sorted the clutch, then forgot to put the nut back on. After about 5 minutes of pulling (with no fuel being drawn in at all), I remembered the nut. Put it back in - started first pull. Given the saw is off fleabay, you have no idea at all what it has been through!

 

[Lakeside53]

Intake gaskets...
Impulse line..
Seals...


Just pressure and vac test it...

 

[OneStaple]

rxe (and others) - Do you happen to know if there's any nut like that that I could check for on the crank that would cause an air leak?

And what's the best way to go about doing the vacuum and pressure tests? I don't have any of the equipment, including the special flange that the 051 shop manual says I need to connect to the carb port and seal the muffler port. Would it just be easier to take it to a dealer? What would I expect to pay for that?

I think I might go rip the flywheel and clutch off to take a peak at those seals.

--Tyler

 

[Lakeside53]

No nuts...

You can just slip a piece of rubber beneath the carb and muffler.. then use the impulse line to puff and suck.

Peaking at seals? na....

The oil pump drive also accesses the crankcase -another place to leak, and there are two seals on the clutch side.

 

[OneStaple]

Ok, a piece of rubber makes sense and is easy enough to come by. What about the pressure/vacuum testers? Is there somewhere I could pick one up cheap (and preferably easily...do auto stores carry things like this?)?

Interesting side note - when I just pulled the fuel line off, it had fuel in it. So that cranking when I was putting gas straight into the carb was enough to pull fuel into the fuel line (I did it a number of times, not just a couple). Interesting that my cranking without the gas earlier wasn't enough to get any in. Any thoughts on this development? I'd assume there's still some vacuum leak somewhere since it wasn't enough to get the chainsaw to actually run.

Anything else I can be looking at while I wait to get a vacuum tester? The oil pump drive?

--Tyler

 

[rxe]

Ok, a piece of rubber makes sense and is easy enough to come by. What about the pressure/vacuum testers? Is there somewhere I could pick one up cheap (and preferably easily...do auto stores carry things like this?)?

Piece of rubber and a bike pump does the job. If you are looking for a big leak, you don't need a gauge - you'll hear it. Use the bike pump carefully - a decent one can generate a lot of pressure. Gauges are needed for small leaks.

 

[Erick]

Have you run a compression check on this saw???
(compression gauge available at any auto parts store)

Is the piston scored.... did you pull the muffler cover to check???

More info please.

sounds like your either real low on compression and can't generate a pulse or you have a major air leak.

You can get a mighty vac pump (sears, harbor freight, nappy), a bike pump (anywhere), and a fuel pressure gauge (reads pressure and vacuum) and with some hose, a few T's and unions, and a little creative thinking you can make your own pressure/vacuum tester. Cut a piece of bicycle inertube to use as the rubber to slip between the carb and muffler and follow the procedure in your service manual.

How much "mix" (yes it should have oil in it) are you putting down the carb??? It sounds like it wants to run. Try about ½ to 1 teaspoon of mix and see what you get. Should roar to life and then die, but keep your thumb on the kill switch and kill it if it rev's sky high.

Good luck let us know what you find.

 

[OneStaple]

Ok, some updates...

I did a compression test and got about 90-100 psi (which seems low to me). I don't know of that'd be affected by the relatively small quantity of gas/oil (yes, I used a mix) that's gone through there in the recent past (only what I dripped in).

No visible scoring on the piston. I haven't worked directly with pistons much before to know what's bad, but it was pretty clean with only some really light marks in the direction of travel. Not scratches, but more just from rubbing back and forth many times. Still almost looked polished.

I managed to pick up a Mity-Vac vacuum pump off craigslist today for $20 off a little old lady. I cut some rubber from an inner tube to seal the muffler and carb ports and stuck those under the respective connections. When I connected the vacuum pump to the impulse line, I realized I'm an idiot :bang: (well, but my being an idiot still doesn't explain everything, so read the full explanation below and not just the part where I'm an idiot). There is a crack in the impulse line. Somehow I overlooked it before even though I checked. This got me excited, because that's a relatively easy fix, but I did the vacuum test with a spare piece of tube that I had laying around anyway, and basically I couldn't get any vacuum to be produced. The connections were good up to the impulse connection, because they produced vacuum on my finger, but once it was connected to the impulse port, nothing, even if I pumped the vacuum pump quickly. Wouldn't take any pressure either. Piston was TDC. Only thing I can think of there was that I didn't create a good seal with the rubber inner tubes (one caught on the screw and tore as I took it out at the end, but it should have been fine through testing). So either the rubber inner tube seals were bad, or I have a major leak.

However, I was excited to try with a new vacuum line, hoping that that was my entire problem. I tried pouring gas in (at least 1/2 tsp...enough to flood the engine at one point), but got nothing more than before. A few coughs like it was trying to start, and I could pull it relatively easily even without the decompression button, but it wouldn't roar to life. Granted, I didn't have an actual impulse tube on there, but the tube I was using seemed to have decently thick walls (thicker walls and slightly smaller ID than the actual impulse tube, but a different type of rubber...I believe it was a fuel line off an old model airplane).

So what now? Something to do with the compression? Retry the vacuum/pressure tests?

Thanks,
--Tyler

 

[Lakeside53]

90-100 - either you have the wrong type of gauge or your saw is dead...

Your gauge needs a schrader valve right at the the tip where it screws into the plug hole..


Forget vac if you can't get pressure. Pump pressure as best you can and go around the saw with spray bottle of soapy water - that will tell you where its leaking.

 

[OneStaple]

Hmmm...the compression gauge I have is one my dad had laying around for cars. I've included some pictures of it and the connections. There's no schrader valve right at the end, but then that's just an extension tube of sorts. If you look at the 4th picture, it looks like there's a shrader valve buried in there (at the joint half way up the tube). Does that mean the saw is pretty dead?

I can retry the pressure test tomorrow with a spray bottle of soapy water, as you mentioned. There's no problem with using the impulse nipple to connect the vacuum/pressure gauge to, right?

--Tyler

 

[OneStaple]

So a few developments, the main one being that the saw runs! As to why, I only have guesses.

Some parts came in today (air filter, clutch...neither of which really mattered for getting it running earlier) and I picked up a few others (mainly, new spark plug and a tube to act as the impulse tube). When I replaced the spark plug, the new one came with a significantly different gap (0.5 mm, also written on the box). The copy of the repair manual I have says a number of gaps, including 0.15-0.25 mm, 0.15-0.30 mm, 0.2-0.3 mm, and 0.5 mm. I had made sure it was at about 0.25 mm before, which I guess now was wrong.

For the impulse hose, I had tried it with a different tube to act as the impulse tube last night, but I put one one with slightly stiffer walls today. I don't know if that made a difference or not.

Finally, since it was mostly together, I decided to give it one more shot with the new plug and impulse tube before tearing it apart again for another pressure/vacuum test. After a bunch of pulls, it started to catch and then finally roared to life. It didn't die either, but just kept going and then started getting faster. I had to kill it with the switch. I did it again a bit later (without adding more gas in the carb) and it came alive on the first couple pulls, again going for about 15 seconds before I killed it with the switch.

I'm pretty sure I'll have to tweak the carb settings and everything, but I'm just excited that it runs and seems to pull adequate fuel from the tank.

Since it didn't do so well earlier on the compression or vacuum tests, is there any value in trying those again at this point (compression wouldn't be too hard to do)?

And thank you all very much for your help and advice!! It has been greatly appreciated!

--Tyler

 

[Erick]

Glad to hear you got it to come to life.

Now for the bad news. That is a shrader valve in pic 4 which means (ass-u-me-ing you gave the saw a few good pulls when you did the compression test) your saw needs new rings. The good news is rings are a pretty easy job even if it's not a cheap one (relatively speaking).

Now for bad news number 2.... the fact that you couldn't get the saw to hold vacuum or pressure at all, and the fact you said that after the saw started and ran, it began to creep up higher tells me you've got a pretty good air leak. Don't run the saw anymore until you find it, you can/will run it lean and fry the piston and cylinder. Do like Andy said and try to pump air into the saw while checking it with a spray bottle (dish soap and water). Spray all around the base of the cylinder and at both crank seals, check the intake boot, and the impulse fitting it's self where it goes into the case. Make sure the spark plug is tight and the decomp valve is not pushed in and check it with the soap for leaks. Check the cases themselves around the gasket that seals the two halves together. There is a leak somewhere and you have to find it.

Now before you get knee deep into the saw looking for the air leak, I would suggest that you run the compression check one more time just to be sure. I have seen some pretty good mechanics (myself included) make some pretty silly/stupid mistakes. Make sure all of your connections are tight and that the decompression valve is not pushed in (I've seen that one before) and give the saw 4 or 5 good strong pulls. If your compression readings are still low (and I think they will be) your gonna have to re-ring it.

Good news is fix that air leak and re-ring it and she should be one heck of a lot of fun, and you'll have the satisfaction of knowing you fixed it yourself. and Enjoy!

 

[Erick]

OK I just went back and looked again (skimmed over it the first time, sorry) and noticed that your shrader valve is in the middle of the hose on your compression gauge. The length of hose between the valve and the cylinder is causing you to get a lower reading than it should be, by how much I don't know but your gauge is not going to give you and accurate pressure reading on a saw. You really need to get a gauge with the valve at the end of the hose like Andy said. Your compression is likely enough to get the saw running right now, probably not gonna have the giddy up it should with fresh rings but it should run. Most important thing right now is to find that air leak. Get it fixed and you can worry about compression then. Run your pressure/vacuum test again and make sure the decomp valve is not pushed in. I've actually done that one myself, push in the valve pull on the saw, no start, tear it down to run pressure/vacuum test, nothing, realize you left the decomp button down, kick self in the arse, run test again everything fine you get the idea. Good luck