So what did you do differently this time?Back in town one day early so I was able to cut today before I leave again tomorrow. Burned through one full tank cutting various limbs and bucking up some 20-24" dead ash - the saw performed great. East to start, idled well, good throttle response and plenty of power. Weather was cool today compared to last time but it made it through the entire tank without any issues with the carb cover in place. I have the H set at 13k max RPM at WOT. I still need to tune in the cut. Also plan to get a carb kit and clean/replace everything just for peace of mind.
Hoping the scored piston holds up . . .
Nothing - ever since I backed the H screw all the way out of the carb the saw has worked fine. Before that the saw appeared to be running fairly lean because no matter how many turns out the H screw was my RPM still maxed out at 13,500 WOT. It was almost by accident that I completely removed the H screw as I was counting how many turns out I was and the screw fell out after about seven turns I put it back in the carb and turned it all the way in, backed it out 1-1/8 turns and started the saw. I could then get the saw to 4-stroke and the RPMs to drop. I could also lean it out as I turned it in and get back to that 13,500 number. I set it at 13,000 at WOT. Went out to cut wood and it did just fine. Initially I cut with the carb cover off because I still had my tach connected to the spark plug wire. Yesterday I took the tach off and put the carb cover back on and was able to cut through a full tank with no issues. I have no idea what happened for sure but my guess is that something was blocking the H circuit in the carb causing the saw to run lean. When I took the screw all the way out the blockage cleared.So what did you do differently this time?
Forget the compression tester and just take a look at the piston through the exhaust port. If t here is any damage you will see it there. I have seen several two strokes with good compression but scored pistons. The compression test is not foolproof.Well either way it sounds like you got it going. Good deal. Id now take this opportunity to check the compression before you consider it to be goog to go.
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Forget the compression tester and just take a look at the piston through the exhaust port. If t here is any damage you will see it there. I have seen several two strokes with good compression but scored pistons. The compression test is not foolproof.
This is normal. Normally they are somehow "different" so that they can't be mixed up.When I came back I pulled the carb. Took everything apart and sprayed it down with carb cleaner. The L screw is nearly twice as long as the H screw and the end has a sharper point (see pics).
Wet plug says that you likely have a leak through the needle and seat. This can be confirmed with a pressure test on the fuel inlet which should hold 7-10 PSI indefinitely. Sounds like you may have more crud floating around inside the carb. Is the sealant around the welch plug(s) flaking off and getting into trouble?Put it all back together. Started with L screw out 1.5 turns. Set idle at 2700 RPMs and WOT RPMs at 12,900. Filled the tank and made 15ish cuts in 16" cherry and ash logs. Saw began to bog again. Hard to keep it idling. Certainly acted like a fuel issue. Checked the tank and was at least 1/2 full (interestingly, no pressure when I removed the cap). Checked the plug and it was wet! Let it dry and and turned the cylinder over a few times with plug out. Put plug back and it fired up immediately but bogged quickly while idling. Repeated this process 3 times with same result - plug was wet every time. Removed the fuel line at the carb and there was no pressure?
But don't they use the same carb on the 7900s? At least that's what my findings were... I could be wrong.I had a 6401 that I put an 84cc big bore kit on. I could not get it to run right. It was always too lean even with the H needle backed nearly out. I got my wire drills (I forget which size now, maybe it was 0.8mm or something like that) but I made the opening in the main jet a few tenths of a millimeter larger. That fixed the problem. H needle adjustability was restored and the saw runs fine. I'm not saying this would solve the problem with your saw since it seems like you have some response when adjusting the H needle. With my saw, it just seemed like the 84cc big bore was demanding too much fuel from the carb that was originally designed for a 64cc saw.