'85 Marathon Gas Runs Erratic - full bore or nada

[CPOdaddyduke]

Originally Posted by clr.R4tch3t

How is the up and down movement of the crank? My seals were beyond toast due to lack of maintenance and my crank bearings were also smoked, adding to the seal wear. I found splitting the cases and replacing the bearings to be a fairly painless endeavor.

The crank rotation and piston up-n-down movement feels remarkably smooth and tight (i.e., no discernable lateral movement). My only concern at this point is that I may find that the crankshaft surface that mates to the seal may be worn and grooved, but I won't know that until I clean it up and pop out the old seals. Hopefully tomorrow. Keeping fingers crossed.

One thing is for sure - I have gotten quite an education in 2-cycle engine design and operation. After this, I may re-tackle a couple of old chain saws that no longer run but I kept for a rainy day adventure...

[Dabble Inn]

Very interesting thread, I also have an 85 Marathon that I just tore apart due to the crank having excess horizontal play. It still ran fine but the seals had to be leaking as there was gunk all over the case like yours, You might want to make sure your crank bearings are ok....

I don't want to disrupt your thread so I'll start a new one on my rebuild and what other problems I found, BTW Great fabricated clutch puller.

[CPOdaddyduke]

Update:
I still have not yet finished reassembling the engine and getting it back in the cart, but I am confident that the seal problem is fixed and will get me up and runnung.

I have included a picture of the new seal installed on the clutch side of the crank. I am convinced that this engine has been overhauled before (much evidence of a previous disassembly) and at some point has been sitting up for a long period of time (old seals dried up, and gunk on the crank was very hard - guessing that running after a long time set up is what wiped out the clutch side seal - flywheel side seal looked good, but I replaced anyway). -- I forgot to take pics of my jig for replacing the seals, but if anyone is interested, I will gladly provide a mock-up of how I did it.

I wanted to avoid tearing the engine down completely, since I had very good compression; so I was able to get the old seals out and new ones pressed in without breaking open the crankcase. This was a little tedious, but I managed it OK. The crank shaft surfaces were still in great shape (except for dried up gunk on the output shaft side, flywheel side was protected from the elements). I cleaned the shaft up real good and polished with some 800-grit wet paper and was able to get it as smooth as a frog's tummy; again, a bit tedious with the crank still installed, but I managed.

I had to also rebuild the starter/generator (new brushes), just finished that chore, which went without a hitch.

The only "issue" I ran into was that I discovered that the timing rotor inside the flywheel was making contact with the ignition pulsar under the flywheel. Apparently, it had been before, but still seems to work. I tweaked the pulsar mounting to get it to stop touching the nubs on the timing rotor (kept playing with it until I could no longer "feel" it when rocking the flywheel around. The manual calls for a certain clearance between the pulsar and the rotor nubs, but how on earth could you measure this - there is no access hole in the flywheel and the flywheel has to be in place for there to be a clearance to measure.

Hopefully, in the next couple of days, I'll finish reassembling the engine and get it back in the cart.

[Dabble Inn]

Mine had also been making contact with the nubs on the rotor in the flywheel, I blame it on the crankshaft bearings. I sure hope yours are ok for your sake beings you're into it that far

I see no way to measure the gap either, The rotor is removable so maybe there was such a tool/bushing/sleave that you could slip over the crank then slip the rotor over it for a measurement?? I'm guessing of course, but now ya have me scared as I did remove the igniter and just found the little bit of play in the mounting screws/holes. UGHH!! Guess I'll have to adjust it like you did when I put it back together, I wonder how far down the nubs are wore on mine? Are they about the same as yours? Mine did have ok spark

[CPOdaddyduke]

Originally Posted by Dabble Inn

Mine had also been making contact with the nubs on the rotor in the flywheel, ... ...

WoW! Mine looked EXACTLY like yours (I should've taken pics). Your nubs look good and look just like mine (that sounds kinda funny). I could feel the interference when I first put it back together and immediately figured out what was happenning (could feel two close points of interference). Fortunately, there was just enough clearance in the mounting holes for the pulsar that when I loosened the screws, held it back as far as I could while retightening, then after reinstalling the flywheel, I could no longer feel any interference. Proof will be when it runs after re-mounting the engine. I can't think of any way to set the gap as specified in the manual (it's pretty tight, something like 0.03" if I recall correctly)

I am very sure, at least in my case, there the bearings are good. The crank feels very smooth and reasonably tight, with no discernable end play, runout, or axial wiggle.

I have also wondered about tweaking the timing. As far as I can tell from the manual, the only way to check/adjust the timing is with a timing light; then if it is off, you have to remove the flywheel, loosen the pulsar mtg ring screws, rotate the mtg ring a little, retighten, and replace the flywheel, then do it all over again - as many iterations as it takes to get it right (or at least as close as it needs to be to run right)....

I have even wondered if it might be worth the effort to bench mount the engine and wire up enough parts to make it run so all this can be tested before putting it back in the cart?

[kyerik]

I have tweaked the timing on my 91 Marathon. It has the 3PG 2 cycle reversible engine. I rotated the pulsar slightly so that the timing is more advanced in the forward direction. There is more power in the forward direction but also more backfiring, spark knock and dieseling/"run on" on hot days when running it hard. That and it has very little power and is harder to start in reverse since the reverse timing has been retarded. I am likely going to set it back to stock or at least really close to stock.

[Dabble Inn]

CPOdaddyduke; Funny about our nubs lol. and glad they look the same cuz mine ran fine..

kyerik; Good to know about the affects of too far advanced.....

For the timing I read to center the set screws in the adjusting slot on the mounting ring, When I took mine apart I noted it was just a hair advanced of center. I mean like at the most less than a 32nd, I'll be putting it back at that or maybe dead center. But definitely not farther retarded than center, And it ran fine in reverse too.....

I think if you do that there wouldn't be any need to bother bench testing it, Sounds like a lot of extra work

[Dabble Inn]

Gee just thought of an interesting fact, If I remember right the advanced timing in forward is 21* and 18* in reverse. So when running in forward it's going to get a second spark 39* after the first one, That would be 18* after TDC on the down stroke. Hmmm Wonder if that's at the opening of the exhaust port yet? Not sure why but I think I like that lol

[clr.R4tch3t]

Exhaust port opens 95° after TDC on a 2pg motor. Transfer ports open 118° ATDC, transfer ports close 62° ABDC, and exhaust closes 85° ABDC. Total exhaust duration is 170° and transfer time is 124°.

[CPOdaddyduke]

Status Update:
Got the engine back in the cart, all cleaned up and kinda spiffy looking. It does run!!!

BTW, I found one other problem that could have contributed to my original problem - the spark plug terminal on the spark plug wire was rusted almost completely off; the boot was just holding the contact close to the plug. I just got a replacement secondary terminal for a lawn mower engine spark plug wire ($5.00) - seems to have done the trick.

So far, I have not checked any ignition timing, have not made any adjustments on the new carburetor, and have not reinstalled the muffler (open exhaust for now). I also have not put the drive belt back on, want to get the engine running smooth before that.

It does run, although not very smoothly at low RPMs. Sounds kinda like a chain saw - doesn't really want to idle without lots of "hit-n-miss", does rev up fairly nicely when throttle is opened, except there is still a hint of fuel starvation. I am not convinced that the fuel pump is at 100%. I had planned to install a LP electric fuel pump, but one of the fittings fell off of it, so gotta take it back for replacement.

I checked the spark with an automotive spark tester, spark looks VERY GOOD.

It is very loud without the muffler.

I tried an experiment with the muffler - am wondering if anyone else has ever tried this. I read a lot of posts about these mufflers getting clogged up and people "BBQ'ing" them to get all the oil residue out. Well, I tried closing off one end of it and filling it with gasoline and letting it soak for a few days, shaking it up a little each day. After 4 days, I drained all the gasoline out - a black mucky mess came out. I then back-flushed it with water from garden hose to get all the gas out, then used air pressure to blow as much of the water out as possible (bunch more black mucky mess came out). It has been drying out for a day now, will put it back on tomorrow to see what happens. Hopefully, I won't blow myself up. (eventually, muffler will need to be replaced, it leaks like a sieve)

Tomorrow, reinstall muffler, check timing (adjust as necessary), and adjust carburetor for smooth running.

Have I forgotten anything else?