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JoeLee
07-06-2017, 08:23 PM
My cousin called me a few weeks ago. He has an Echo PB-251 blower. He said it would start and run for a few seconds or so and die out.
I told him it sounds like a carb / fuel issue. I told him to replace the carb. He got a new one, put it on and same problem, so I guess it's not the carb. I told him to try replacing the fuel filter and line, he did.... same problem.
So he brings it to me...... the miracle worker !!!!
He was right... it would start but wouldn't stay running. I pulled the carb, I notice the gasket had a small hole in it that is suppose to line up with a hole in the carb. I thought for sure that was the issue. I flipped it around. Same problem, won't stay running. Pulled the muffler and it still wouldn't stay running. So the muff or spark arrestor screen ain't clogged. I did find that if I kept it on 3/4 choke it would run.
That tells me there is an air leak......... only place left to look is crank seals, or base, possible crack in the housing. I didn't pull all the plastic to get in there to take a look. Any thoughts?????

JL................

J Tiers
07-06-2017, 08:49 PM
Generally if the choke is needed, there is a main jet issue, OR an air leak that requires rich mixture to compensate.

Seems like you are headed in the right direction.

Highpower
07-06-2017, 09:12 PM
Have you tried pulling the air vent out of the fuel tank, or running it with the fuel cap loose? Possible clogged vent preventing it from letting fuel get to the carb?

Since you pulled the muffler - did you look in at the piston & ring for possible scoring? (Low compression?)

I just went through this on my Homelite string trimmer. Mine turned out to be a bad check valve in the primer base assembly, allowing the engine to suck air through the primer system. Obviously not the problem you've got with a new carb installed.

JoeLee
07-06-2017, 10:16 PM
Yes, I tried running it with the gas cap loose, also the check valve / vent was replaced.

Piston and rings are fine, good compression. He always ran it rich.

There are no adjustments on the carb other than the idle / throttle stop screw.

It's a new carb and it's doing the same thing as it did with the old carb.

JL...............

lakeside53
07-06-2017, 10:42 PM
Do a vacuum test. Block off the muffler with a rubber sheet between the muffler and the cylinder and make an adapter for the carb port. 5-10 inches of vac is all it takes.

jocat54
07-06-2017, 11:07 PM
I just fixed the same issue with my Echo 250 string trimmer (I think has the same carb)--all the same problems.
I heated up a small screw driver and melted the plastic plug over the mixture screw. (actually stuck it in plastic plug and let it cool down) Once the plug was out adjusted the mixture screw (very little-almost none). It now runs like it should.
The plastic plug is right below the gas line fittings.
Zama carb

tom_d
07-06-2017, 11:11 PM
Do a vacuum test. Block off the muffler with a rubber sheet between the muffler and the cylinder and make an adapter for the carb port. 5-10 inches of vac is all it takes.

Could you please expand a bit as far as what to look for in terms of test results?

JoeLee
07-06-2017, 11:28 PM
I just fixed the same issue with my Echo 250 string trimmer (I think has the same carb)--all the same problems.
I heated up a small screw driver and melted the plastic plug over the mixture screw. (actually stuck it in plastic plug and let it cool down) Once the plug was out adjusted the mixture screw (very little-almost none). It now runs like it should.
The plastic plug is right below the gas line fittings.
Zama carb Do you have a picture of where that mixture screw is located???? That would be my last ditch effort to try and get this to run right.
The mixture screw is supposed to be factory set, same with the original carb.
Even a picture pulled from the web would do, just so I know where it's located.

This is the carb.

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQg8dlSJ_vnZH3dqRRxeME5RJGVOXvHZ uGRDgUdqp6Dlddnb4CB

TNX....... JL.................

Highpower
07-06-2017, 11:37 PM
Below the plate holding the primer bulb in place. Part #9 (http://www.ereplacementparts.com/echo-pb251-p07611001001-p07611999999-hand-held-parts-c-35043_35044_200046_36693.html?page=2)

jocat54
07-06-2017, 11:41 PM
Sorry for bad picture-but the black spot right below the primer bulb is the plastic plug. There is a brass ring around the plastic-it may come out also which is okay.

http://homedepot.ugc.bazaarvoice.com/1999apiwrapper/97811/photo.jpg

JoeLee
07-06-2017, 11:43 PM
Below the plate holding the primer bulb in place. Part #9 (http://www.ereplacementparts.com/echo-pb251-p07611001001-p07611999999-hand-held-parts-c-35043_35044_200046_36693.html?page=2)I did see that on the carb. There wasn't any plastic cap over it. It's a small slotted brass screw. I tried turning it but it wouldn't move either way. It has a shallow slot and started to get chewed up, so I left it alone. It should be pre set.
I could try again if I can find a small enough screwdriver with a thick enough tip to properly fit the slot. The only thing I could fit in there was a small jewelers screwdriver and the blade was too thin for the slot.

JL................

Highpower
07-06-2017, 11:55 PM
Actually I don't think that is an adjustment screw at all. Looks like a high speed jet to me - but I don't own one so.....

And being a new carb, I highly doubt it's "out of adjustment" anyway. Besides, adjusting screws normally have springs over them to hold the adjustment. I don't see any spring listed or shown there. My guess is they put the plug over them (normally) to keep people from trying to resize the jet. If you want to tinker I would suggest doing it on the OLD carb.

$0.02

jocat54
07-07-2017, 12:13 AM
I had to grind an old screw driver to fit the slot.
My trimmer was bought new last summer and never really ran right. First time using it this season and had all the problems--they set them to dang lean thanks to the EPA
The brass ring(plug) holds the screw in.

lakeside53
07-07-2017, 12:14 AM
Could you please expand a bit as far as what to look for in terms of test results?

It should hold that vac. Rotate the crank Wiggle crank etc and see if it leaks down. Put some medium oil on each of the seals and see it it gets better. Replace both if one leaks.

epicfail48
07-07-2017, 03:09 AM
Weird question, but have you checked the spark plug and coil pack? I had a similar problem a while back with a chainsaw, it would start but would cut off before too long. Ended up finding out the spark plug needed badly replaced, it was hit or miss on actually getting a spark. Dunno why it would start and run for a bit then cut out, but the new plug seemed to fix it

Highpower
07-07-2017, 03:54 AM
Weird question, but have you checked the spark plug and coil pack? I had a similar problem a while back with a chainsaw, it would start but would cut off before too long. Ended up finding out the spark plug needed badly replaced, it was hit or miss on actually getting a spark. Dunno why it would start and run for a bit then cut out, but the new plug seemed to fix it

He said it runs OK with the choke on though. Rules out ignition.

JoeLee
07-07-2017, 08:30 AM
Actually I don't think that is an adjustment screw at all. Looks like a high speed jet to me - but I don't own one so.....

And being a new carb, I highly doubt it's "out of adjustment" anyway. Besides, adjusting screws normally have springs over them to hold the adjustment. I don't see any spring listed or shown there. My guess is they put the plug over them (normally) to keep people from trying to resize the jet. If you want to tinker I would suggest doing it on the OLD carb.

$0.02My thoughts also. And how would the screw have moved on the original carb. ?? I don't think think there was anything wrong with the original carb. but that is the first and most likely place to start looking when you have a fuel related problem like this.

JL...............

JoeLee
07-07-2017, 08:30 AM
He said it runs OK with the choke on though. Rules out ignition. Yes it does.

JL.................

RichR
07-07-2017, 08:34 AM
... I did find that if I kept it on 3/4 choke it would run.
That tells me there is an air leak.........

Get yourself a can of WD40 or Gumout. Place the straw in the nozzle. Start spraying anyplace you feel there could be an air leak. If you hit an
air leak, you should get a sudden change in engine RPM or possibly stall it.

JoeLee
07-07-2017, 08:51 AM
From what I could see looking down through all the plastic, on the pull cord side there was a lot of dust and crap caked to the housing, like dust would build up if there was an oil leak on a lawn mower engine. Looked like a velvet coating. It may be either a crack or crank seal. Maybe I'll give it another look.
My guess is it's sucking air somewhere. For dust to cling and build up like that the area has to be getting wet with fuel / oil mix.

JL................

Bellingahmster
07-07-2017, 09:41 AM
I had similar symptoms on a free leaf blower... the cause turned out to be the screws holding the cylinder to the crankcase had backed out causing an air leak at the mating joint. Doesn't affect compression, but plays havoc with mixture!

JoeLee
07-07-2017, 11:42 AM
I had similar symptoms on a free leaf blower... the cause turned out to be the screws holding the cylinder to the crankcase had backed out causing an air leak at the mating joint. Doesn't affect compression, but plays havoc with mixture! Yup,
lots of compression, piston and rings are smooth ans shiny with no scoring or signs of overheating, so it's got to be an air leak on the crank case side.

JL.................

Highpower
07-07-2017, 01:32 PM
Just some more random thoughts....

I check for leaks differently than what was described earlier. Instead of using a vacuum, I pressurize the crankcase to 5 PSI and apply leak detection solution (AKA: soapy water) to the crank seals and gaskets to look for bubbles. Gives a quick visual of a leak and pinpoints it.

Does the primer bulb still have fuel in it when it dies? I keep thinking back to when you talked about the carb gasket being installed incorrectly when the new carb was first installed. Makes me wonder if the impulse port for the fuel pump is open (not clogged) and working.

Ah the joys of being a homeowner and having to do yard work, eh? lol.

JoeLee
07-07-2017, 01:48 PM
Just some more random thoughts....

I check for leaks differently than what was described earlier. Instead of using a vacuum, I pressurize the crankcase to 5 PSI and apply leak detection solution (AKA: soapy water) to the crank seals and gaskets to look for bubbles. Gives a quick visual of a leak and pinpoints it.

Does the primer bulb still have fuel in it when it dies? I keep thinking back to when you talked about the carb gasket being installed incorrectly when the new carb was first installed. Makes me wonder if the impulse port for the fuel pump is open (not clogged) and working.

Ah the joys of being a homeowner and having to do yard work, eh? lol.

The primer bulb is completely full of fuel at all times. I can't remember seeing any full like that.
My string trimmer bulb is usually about half full or so when it's running.

JL.................

Highpower
07-07-2017, 02:11 PM
The primer bulb is completely full of fuel at all times. I can't remember seeing any full like that.

JL.................

:confused: That doesn't sound right..... any chance your cousin got the hoses swapped around?

JoeLee
07-07-2017, 02:26 PM
That was the first thing I noticed. I thought it was odd that the primer bulb was locked solid with fuel.
I did in fact check the lines. The front inlet is the fuel (black) and the rear inlet is for the primer hose (yellow). To verify this when I pushed the primer bulb air would blow out the rear inlet. Besides this thing quit running before anyone started to take it apart. I also verified the hose positions from a picture I found on the parts breakdown.

JL.................