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2020/12/16 18:51:35
dillonm695

Ryobi 2-stroke gas trimmer - runs until hot then dies

Hi all, thanks for reading my post and even more thanks if you can help me to a solution.
 
I have a 20yr old Ryobi 2-stroke trimmer, 725r which is a 31cc MTD engine and MTD carb.  It will start up just fine, but after a few minutes of running it starts to lose power like it is starving for fuel, and then it dies.  Once it is at normal operating temperature (hot) this happens.  When stone cold, it starts up fine, runs fine.  I can get about twenty minutes of work done, and then it dies.  Once it gets hot and dies, when I push on the primer bulb, the bulb stays compressed and is very slow to release.  This seems to me like an internal problem with the carburator.
 
I just replaced the original carb because I could not get it tuned right so it would both start easily when cold, idle, and run with power.  (I think I'm going to go get that carb back from the trashcan...).  With the old carb, once I got it started and running and normal operating temperature, it would run fine as long as I needed, and was easy to start when hot.  When I installed the new carb, I also installed new fuel line and fuel filter.
 
So, did I get a bad carburator, and should I request a refund from Jacks, or is there something else to try?
 
Thanks again,
Max
10 comments Leave a comment
dillonm695
Correction: the original carb was a Zama carb, the replacement is MTD.
 
A little more info.  A few years ago, the trimmer started getting hard to start when hot, and didn't want to idle, and some of the fuel lines started leaking.  I replace the fuel lines, but still had trouble, so I looked for a rebuild kit for the carb and didn't find one.  I took the carb apart and cleaned it, and carefully put it back together, but without any instructions to guide me and no idea of any adjustments.  The high speed and low speed jets were not touched, they were under the EPA locks such that only a small amount of adjustment was possible.
 
No luck, still hard running when hot. 
 
I cut off the plastic EPA locks on the two jet screws, and then turned them out, counting the turns so I could put them back in the same position.  Now I have learned that a better method is to tighten them down, counting the turns required to tighten them, and use that number / method to reset them once removed.  With the jets out, I took it apart again, and cleaned it all again, and now the running was improved, but I could never get the jets adjusted right so it would start easily when cold or hot.  I could adjust for an easy cold start, and have decent power, but forget it if I had to re-start when hot.  I could adjust so it would start easily when hot, and idle well, and have plenty of power, but forget about cold starting.
 
Maybe I should have come here first to get advice on adjusting these two screws so the trimmer will start cold or hot.
2020/12/16 20:31:33
AVB
Which 725R do you have? Need the model starting with 41 and PN of the carburetor you ordered to see if the carburetor was the right one. There are 21+ versions and not all use the same carburetor. I looked at just three of them randomly and all three had different carburetors.
2020/12/17 08:27:36
dillonm695
The PPN of my trimmer is: 41BD725A034
The carburetor I ordered and installed is: 791-182875
2020/12/17 09:01:57
AVB
dillonm695
The PPN of my trimmer is: 41BD725A034
The carburetor I ordered and installed is: 791-182875

Okay the carburetor PN is the right per the above model number. What strange is you removed the plastic limiter caps. The images I find of the carb is a Walbro that doesn't have limiter caps.

 
Curious what is the compression numbers? If it is right at 100 psi then that may be the problem as that is the bare minimum for a 2 cycle to even try to run right.
 
2020/12/17 09:50:28
dillonm695
That image you posted makes them hard to see.  At the bottom right of the carb in your photo are two sliver cylinders, conjoined, and they are where the hi and lo jets (and the tamper-proof caps) are located.
 
Compression:  I have a compression tester, may need to do that test.  What kills me is that with the previous carb, it would run OK when hot, if I got the hi/lo jets right, and run for 30 minutes or more and be easy to re-start while hot.  When it was stone cold, it was a real pain to start, unless I futzed with the jets, and then I'd have to spend another 30 minutes futzing once it warmed up and wouldn't run.
 
Could the compression be OK when cold, get worse when hot?  So the machine stalls like fuel starvation (going off a cliff, one minute it is running, 5 seconds later it is dead)?  I see that symptom as a fuel issue, but I'm asking here because I don't know the solution.
2020/12/17 11:53:44
dillonm695
This image has a red arrow pointing to the location of the jet adjustment screws.
 
2020/12/17 15:34:50
dillonm695
Well I just found the instructions for my old Zama carburetor (a C1U model), so perhaps I will try to resurrect that with a rebuild kit and put that back on.
 
 
 
 
2020/12/17 15:56:36
dillonm695
So I'm thinking that the big clues for the problem with this MTD carb is the way it dies, as if starving for fuel, and then the primer bulb stays sucked down when pressed, when the engine is hot.
 
How does the primer bulb circuit work?  Does it act to suck air out of the carb, or does it push fuel into the carb?  I guess I could disconnect the lines and experiment with a can of fuel to figure out which way it pushes the fuel.
2020/12/20 13:15:23
dillonm695
AVB, I tried to measure the compression, but my tester doesn't seal well against the engine so I couldn't get a decent reading.  With the screw-in adapter, highest reading was about 20 psi.  Using the rubber tip that one holds tight against the head to make a seal, I got it up to 60 psi.  Verified that there was good spark.
Engine wouldn't start now.  Primer bulb stays flat after pushing on it.  Engine wouldn't fire up.
 
I put the old Zama carb back on, and now that I know which jet is which, I adjusted the high speed jet per the Zama instructions I found.  Engine starts right up.  Warmed it up, and adjusted the jets a bit to get a decent idle and decent power, and used the blower attachment to blow leaves off the drive and street.  Worked around the yard for a couple hours, blowing leaves and then mulching them with the mower, and the engine starts up everytime with no issues.  Primer bulb doesn't stay flat when depressed.
 
So I'm going to contact Jacks to see if they will refund this MTD carburetor that doesn't work.
2020/12/23 19:18:58
dillonm695
Just to close out this thread, Jacks did me right!  They took my request for a refund, asked a follow-up question, and then after a week or two they issued the refund.  Thanks Jacks!  This is great customer service and I'll be sure to keep ordering my parts here.
 
The old Zama carb is still working fine, engine starts up when cold using full choke, and stays running strong once warmed up.  Power is down a little, I probably need to adjust the carb jets a little more, but the engine keeps running fine.
2021/02/01 17:56:32

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