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heimannm said:
The fuel caps were equipped with a spring under the duckbill valve and the intent was that excess pressure in the fuel tank would "pop off".

View attachment 734431

It is possible that your O-ring repair doesn't allow any pressure to escape and so the pressure in the tank becomes excessive forcing the metering needle to unseat. Next time you are running the saw try opening the cap and venting the pressure, see if that reduces the tendency to flood the carburetor.

Mark
Click to expand...

That makes sense, thank you .
I didn't know about the spring , kinda of reminds me of the anti back fire kit I used to install in old holley carburetors so the power valve diaphragm wouldn't be shot in the event of the backfire.
Your problem, the saw starts out super lean , you run it and then its erratic and floods? Sounds like (metallurgy) install headers on an old cast iron block ,cold engine tap tap tap ,warms up, headers seal. Just my 2$. Which could be the piston also swelling with heat. Wish I could be more help.
 
heimannm said:
Now - a problem of my own, or really a problem with Kevin's (2 broke2ride) 7-10.

I decided to give it one last attempt, stripped it down to the short block and rechecked both pressure and vacuum. There is a slight pressure leak through the compression release, and it was still burping out through the automatic oil pump. I replaced the automatic oil pump with a used one from the attic and that cured the pressure leak there so there was just a blip, blip, blip of bubbles coming from the vent port of the compression release. I pumped the vacuum up to 20" Hg (out of the water now just in case you were wondering...) and it would very slowly bleed down, less than 1" per minute. I did put some heavy oil around the seals and it didn't change at all. I am not sure but I suspect the vacuum leak may have been through the compression release port as well. I had replaced both crankshaft seals when I worked on it last time and I believe they are still quite effective.

I didn't want to take any chances so I replaced both gaskets on either side of each of the insulators between the cylinder and the fuel tank/airbox as well as between the airbox and the carburetor and even changed out the insulators just in case one had a crack or chip that I could not detect. I went so far as to scrape the mounting surfaces on the block and the air box to insure there were no irregularities affecting the ability of the gaskets to seal.

The saw will start, run up at full throttle, but will not operate with the throttle at idle position. I fired up my 7-10 to make sure all was O.K. then switched the carburetor into Kevin's. The only way to get it to idle was to run the idle speed screw all the way in (mine had the idle speed control in the carburetor) and lean it was down. I don't think it was actually running on the idle circuit but the throttle is open far enough to pull through the main jet as I could turn the idle adjust screw all the way in with either carburetor and the saw keeps running, until it dies...then I had to choke it one pull and it would restart and repeat the scenario. As the saw continued to warm up the idle became even more erratic, seeming to lean out and speed up, then appeared to get rich and flood out

On the previous attempt I did install new points, a different condenser, and even changed the flywheel as Poge had suggested with no effect whatsoever. When I tested it Saturday morning it would certainly run at full throttle and cut pretty well, but would never operate reliably at idle.

The only thing I have not changed is the piston itself. Looking at the IPL's I see than serial numbers with the 10 prefix had a different part number (69030 as opposed to 85239 in all the other late model 70 cc saws) which is a thick ring piston.

Kevin - did the piston you changed out look like the one on the left or the one on the right???

View attachment 734445

View attachment 734446

View attachment 734447

Makes me wonder if the porting is different based on the piston and we have the wrong piston in yours?

Mark
Click to expand...
Mark, you may be onto something. I'm sure I still have the old piston. I will check when I get home tonight. I'm pretty sure it had thick rings as I was elated to see thin rings on the piston you supplied.
 
I had these pics of the old piston in my phone, not sure if it helps.
a31696819d398d8f751072580b8bb86c.jpg
01b164027e8a938dfe1f2bd700b1a302.jpg
 
heimannm said:
Makes me wonder if the porting is different based on the piston and we have the wrong piston in yours?

Mark
Click to expand...

IMG_0173 (640x480).jpg​

I had some time to root through IPL's and do some general searching and comparing this afternoon and was able to find both thin and wide ring pics of what was supposed to be the 85239 piston pictured on the left. I also found a pic of what was supposed to be a 69030 with the same windowed design as the 85239 which was suggested to just be a part number update. I also found three different assembly numbers for the shortblock/cylinder assembly, but presumably for external differences vs. any transfer port design changes if indeed the only differences between the 85239 and 69030 pistons are the part numbers and the ring sets. If true, that then poses the question of what does that 2" version of a 10-10 style piston actually go to? The other applications I could locate for a 2" piston in the 10 series was for the early 10 series up through the LG-6 which lists a 69412 piston with the 87166 (thick) ring set and indeed has the 10-10 style 'window' design as the one pictured on the right. If that's the piston currently in the saw and the cylinder is in fact a 7-10 cylinder, I wouldn't expect it to run as intended simply due to the difference in piston design. And if an 85239 piston is simply a re-numbered 69030, that would most likely eliminate the possibility of significant transfer differences along the way being a reason for any erratic performance between the two. The only possibility there (extremely unlikely if even possible) would be the cylinder itself being an early 10 cylinder with an 85239 piston. Of course this is all based on the 'wrong piston' theory and assumes the piston in the saw is the one on the right which is a thick ring piston anyway and doesn't align with Kevin's delight to have received a thin ring piston from you in the first place.

And to Brian's pulse theory..., the wrong piston my well function at higher rpm's but lack the ability (or partially block) the pulse passage with it's broader skirt design just enough to create the mystery problem at lower rpm's due to insufficient primary compression in the case not being able to drive the pump diaphragm adequately, though it doesn't take much to drive it. A little grease on the pulse passage while pulling it over slowly would tell the tale there.

Ok. I'll go now. LOL

:popcorn2:
 
The piston on the right above is 85240 which was used in the early model (suffix A and B) 6-10A saws. Later models (C, D, E, and F) all used 85239. It was remembering that piston that made me follow up on the early 7-10 models. The 6-10 IPL also shows the piston with the open skirts but as you can see, the earlier piston (85240) is quite different and make me wonder what the 69030 really looks like. The illustrations in the IPL's are not totally accurate, but the piston that Kevin pulled from the saw did look to have the open skirts like the 85239.

The impulse passage is clean and open as I checked it several times. I did replace both insulators (between the cylinder and the air box, and between the air box and the carburetor as well as replace the gaskets just to make very sure everything was hunky dory.

Kevin - was there anything else you might have changed while working on the saw, even something insignificant?

Mark

And a P.S., when I opened up my later model 7-10 with the pop up compression release it had the thick rings, I remember because I broke one trying to reinstall the piston not realizing it was fit with locating pins...the IPL's would all have you believe that saw should have the 85239 with thin rings although the 1974 IPL lists both thin and thick rings even with the 85239 piston listed.
 
Mark, currently searching for the old piston without much luck.
The only other things I changed where drilling the oil tank to use three screws to mount the oil pump and I replaced the the insulator under the carb due to me breaking the original and replaced the trigger, again I broke the original.
 
It isnt looking good for finding this old piston. I dont get it. I even kept a box full of the old gaskets but the piston is nowhere to be found. Its bothering me because I'm a hoarder and I never throw anything away but it was also over a year ago that I did that so who knows....
 
Sorry to intrude on this all important discussion,but does anyone know what the bar mount pattern is on a 380A?I've been told that I can sub out a modified Husky bar,but which one of those do I use.Tim was kind enough to come by after the GTG & help me out with a couple of saws,the 380A being one of them.I'm very close to it being a runner now,just gotta rebuild the flatback yet.
Ed
 
I have several saws but just picked up my first Mac 10-10A Runs like a champ!
I do have a question about it though! It didn't come with a bar. I know it takes the D176 mount pattern but those bars tend to be fairly expensive. Was wondering what you guys have used on them and what is compatible? I really want to get this ole beast cuttin' soon. Thanks Guys!
 
Kevin hayes said:
i go for the bigger power myself even if its not needed just cause i want to

Sent from my VS995 using Tapatalk
Click to expand...
Either one of you ever ran a SP125C? That's big power and surprisingly very manageable power. Not a hard to handle beast that one would think. Very easy saw to run but will break you down to carry.

Brian
 
Brian Thacker said:
Either one of you ever ran a SP125C? That's big power and surprisingly very manageable power. Not a hard to handle beast that one would think. Very easy saw to run but will break you down to carry.

Brian
Click to expand...
no sir i have not yet had the privilege and hadnt found one i can afford but do have that 795

Sent from my VS995 using Tapatalk
 

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