Modding a 7900

[Lakeside53]

I thought it was "Put out or get out!"

we're on the edge here:hmm3grin2orange:

 

[litefoot]

Wonder if Talon has any pissing smilies?

 

[drmiller100]

I like fish!!!

could be you like fish.

chicken would be more of a relative though.

 

[drmiller100]

You like bottom feeders?

:hmm3grin2orange:

i like chasing chickens back into the coop.

 

[drmiller100]

Dr... I think you're done here. At this point, even if you concede that even tho you do know something about 2 stroke engines as they apply to watercraft or things that run on snow, but are totally lost when it comes to chainsaws, you've lost their (the ones that DO know about chainsaws) respect. They're talking amongst themselves about FISHING if you haven't noticed. There's yer sign. Time to take your leave before it really gets ugly. If you want your saw to run, send it to dneiger on this forum. He will do you right and give you a saw that runs well, that is if you haven't already messed it up.

good luck in your future endeavors,
Ian


Ian 0 Alcohol 2... It's been a rough season for Ian folks... and fishing is out of the question.

ummm, go back. I opened the fishing season. The confidence of some of the folks on this list is fairly impressive.

What is more impressive is the knowledge of some of the folks that quit posting on this thread when the guppies started circling.

that didn't sound right. Early on I got some great advice from some really knowledgable, helpful folks.
Thanks!

Then the guppies showed up, and the benevelont sharks wandered off........

 

[Freakingstang]

read this thread. you can see what I've done so far.

Ok, since you don't understand, how would you have set the carb up before you did a search. I saw another post where you mentioned burbling. lol.

The rev limiter has nothing to do with the tuning of this saw from a performance perspective.


Bye

 

[sawinredneck]

ummm, go back. I opened the fishing season. The confidence of some of the folks on this list is fairly impressive.

What is more impressive is the knowledge of some of the folks that quit posting on this thread when the guppies started circling.

that didn't sound right. Early on I got some great advice from some really knowledgable, helpful folks.
Thanks!

Then the guppies showed up, and the benevelont sharks wandered off........

Well here is where the problem lies, you admit to modding all of these things on thissaw, and want to do more. But you dont feel confident with adjusting the carb! That is where the pitfall began, initial adjustment is about as basic as one can get, if you are not confident with that, then, just maybe, you need to go back to the basics, then work you're way up. It took me a while before I had enough ballzz to even mod a muffler, and then it took me getting p'oed at a saw that I was unhappy with before I tore into one.
Some of you're theories are interesting. You seem to have a decent grasp on two stroke theory, but you completly miss the boat on th basics?

So............ Here is my sugestions, ask a bunc of stupid questions, wear out the search function, listen and read, and ASK!!!! There is more info on this site than you can imagine, BUT, if you come on with the end all beat all atittude, you will be done shortly.
Andy

 

[04ultra]

I like fish too...








.

 

[sawinredneck]

I like fish too...








.

Yep, Jeff is the man!!!!!!!!!!!!!!!

 

[Freakingstang]

uhmmm Tuna, uhmmmm

 

[spacemule]

His posts give me the impression of a boy about 15.

 

[drmiller100]

Well here is where the problem lies, you admit to modding all of these things on thissaw, and want to do more. But you dont feel confident with adjusting the carb!
Andy

actually, what i posted was to ask if reading the wash on the piston wasn't a valid way to verify jetting.

I never got an answer. Search didn't help.

What could be simpler then to jet by ear under load, then pop the plug and verify?????

you guys DO know how to read wash, right??????

 

[sawinredneck]

If you really want to pull the cylinder each time you adjust the carb, sounds wonderful

But why not read the plug instead?
Andy

 

[sawinredneck]

A week ago I got a new 7900. Very impressed so far.

My helpers set it down and a log rolled over it bending the compression release and breaking the plastic on the cover, so she's not a virgin no more. My helpers also took the rakers down some. Like there is little raker left.

So today I tore it down. Did a few things to see if I could get a few free ponies. Part of my goal is to keep it looking completely stock to mess with the loggers just a little.

Muffler. Removed muffler, looked it over closely. Took a 6 inch socket extension, set it down through muffler inlet, and beat the back plate inside the muffler down to increase flow a bunch. Then used old wood chisel to remove spot welds on half of that little exhaust outlet shroud, removed the screen, and used a hole saw and my drill press to take outlet from 9/16 to 7/8. this more then doubles the outlet size.

Inlet. took plastic outer carb cover housing off. Used hole saw to drill out separator a bunch. Then used dremel to remove two of the cover louvers out. Kind of hated to do that, but figure I can buy another cover. Inlet in particular looked really restrictive to me. ground off the nubs so I can adjust main circuit more if need be.

Porting. I cleaned up bottom of jug to get more flow into bottom of transfers. Hogged out bottom of cylinder jug to increase surface area. cleaned up ports in the sides of the piston. spent some time measuring transfer ports size.

I didn't measure port timing, or change timing, or really change much of anything that way.

So all this adds up to probably a littel power. If I want much more power, I think it needs wider transfers, maybe wider exhaust port.

I was a lot surprised at the hook on the tops of the transfers, and how HARD they are pointed back at the intake side. I'm all for not runnign intake out the exhaust, but that sure seemed excessive.

Also interesting was the combustion chamber offset hard to the intake side. Makes a HUGE squish area on the one side. What is the goal with that? better heat dissipation????

also interesting was that the whole thing came apart with the supplied torx wrench except for the exhaust screen and one clamp on the intake.

Put it all back together, adjusted mixture. she sounds a little more serious now, but not obnoxiously different.

there sure seems to be a LOT of meat in the jug. I noticed the jug was cast by Mahle, which makes quality stuff. I've got a buddy who is really good at sleds. i might order a jug in and make him grind the hell out of it. A jug and piston is what, a 100 bucks????

Nope, don't see a question about "wash" there?

 

[Freakingstang]

So you've got all of that purty weld splatter on your muffler outlet. What happens if some of it drops back inside the muffler and goes back into the engine???

So I set the mixture by running it in a log for a few seconds. then tweak, then run, then tweak.

I am not good enough to understand what people learn from running a saw for 5 seconds at a time WOT with no load, but willing to learn.

for sure my saw makes more power at some rpm much lower then when it hits the rev limiter. to me, this means something inside is limiting it somehow.

my guess is transfers. what do you think?

Funny, you said noithing about "wash" or checking the plug in this post regarding setting the carb up......

 

[Freakingstang]

And then this was your explanation...I don't see anything about checking a plug or looking for wash or wasps... LOL

So you've got all of that purty weld splatter on your muffler outlet. What happens if some of it drops back inside the muffler and goes back into the engine???

First off, it is weld, it isn't going anywhere i could have ground it smooth for the carmera but... When is it going to drop backinto the engine? while it is running and blowing exhaust out the hole? I hate negative vaccums. there is a screen on it, just not in the pic. the muffler was done a year ago and has seen many hours of use with no ill effects.

So I set the mixture by running it in a log for a few seconds. then tweak, then run, then tweak.

what are you tweaking for? sound? or hitting the limiter? you don't want it on the limiter.

I am not good enough to understand what people learn from running a saw for 5 seconds at a time WOT with no load, but willing to learn.

they are listen for a burble. they call it "four stroking" it is how you set it by ear. You "tweek" it until it burbles and it is good to go. The burble will, or should if set correctly, clean up as soon as it is in the wood. Do a search on here for "four stroking"

for sure my saw makes more power at some rpm much lower then when it hits the rev limiter. to me, this means something inside is limiting it somehow.

The limiter is an act of EPA. not a tuning tool.

you will never make peak power on the limiter in this saw. Go back to the last post i wrote, i'm not doing it again. you can't make max power with too little fuel. Lean is mean, but too little will sieze. it is not like a limiter on an MSD or other RPM limiting device on a car, bike, quad, boat, etc. Those all drop spark while there is still fuel present. This coil has no idea how much is there and doesn't care. it will not rev past 13.5K period. Even if the high speed jet could be screwed completely in (and in theory still run at infinite rpm on no air and fuel mix)

my guess is transfers. what do you think?

yes, the transfers are the limiting factor on this saw.



Let me ask you this. how would your power be, or how would you tune it if there was no rev limiter

 

[drmiller100]

If you really want to pull the cylinder each time you adjust the carb, sounds wonderful

But why not read the plug instead?
Andy

ummmm, cuz the plug doesn't tell much.

no bore scope???????????????

 

[Freakingstang]

ummmm, cuz the plug doesn't tell much.

no bore scope???????????????

The plug tells a lot, you just have to have a lathe to get the results of the plug....

 

[04ultra]

ummmm, cuz the plug doesn't tell much.

no bore scope???????????????

Plug chop tells alot from what I see.....






.

 

[drmiller100]

oops. wrong thread.

so you guys jumped me for not knowing how to jet a saw. I told you how I did it.

then, you folks made fun of my method.

to be fair, I do verify with wash.

so, tell me a better way.