Tuning Makita 7901 big bore

[j.kleijn]

Hi

I just installed a bigger kit plus a HD air filter on my Makita DCS7901. Two piston rings broke, but the third seem to work (I found out , the hard way. that these rings have a top and bottom side!).

I use ASPEN 2-stroke and add some extra oil (1:25) for the first 2-3 tanks. A Dolmar dealer I visited claims these saws should run 1:40; is that true?

My service point in my town advised me to remove the limiter caps on the carb. I openend both the L and the H with 1 turn.

When I started the machine, the rpm in idle seems to me too high. Is it save to adjust the L? No risk making it too lean?

Does the screw above the L/H also play a role?

 

[MCW]

40:1 is fine and what I run in my Dolmars.
Also try to adjust the idle screw first to get the idle rpm down. Only adjust the L screw if it bogs off idle (as a rule). The screw above the L/H screws is the idle screw. Counter clockwise will lower the idle rpm.
I've found that a stock big bore kit on a 7900/7901 etc requires very similar jet settings to a standard 79cc Mahle top end.
If adjusting the idle screw does not make a difference to idle rpm then stop and check for air leaks.

 

[makguy]

There is a risk of running the L side too lean, but at 1 turn out you should be in the ballpark, so I wouldn't worry with minor adjustment.
If the throttle response is good and there's no bog on acceleration, back off the S screw to lower your idle RPM.
However, from your description it sounds like you might be adjusting the carb without a bar and chain on the saw, because unless the clutch is engaging and spinning the chain at idle, or the saw dies at idle, the rpms are probably fine.