pop-off pressures

[Molecule]

the hi-needle in the Tillotson HU carb of a 6800 doesn't seem to produce a change in operation. In the prior life of this eBay saw, the pison was seized to the point of self-destruction, so pop-off metering may be the cause. I replaced the crank seals, and I rebuilt the carb with the Tilly rebuild kit, which unlike the Dolmar/Makita rebuild kit does not include a new metering spring. Now I'm trying to check the valve seat and pop-off setting.

In some prior threads, Glens has pointed to this site: http://www.aerocorsair.com/id28.htm, which is oriented toward rebuilding larger carbs for small personal aircraft. It recommends setting the pop-off pressure in the 10-12 psi range for larger carbs. (Higher pop-off limiting fuel flow at wide open throttle (?-possibly due to inability of fuel pump to push sufficient high side fluid pressures due to higher vapor pressures of some fuels-?), and the starvation at the jets then causing lean siezure during flight, and possible death of the pilot.)

In their service manuals, Dolmar/Makita *imply a *max pop-off pressure of 0.5 bar, or about 7.5 psi, but I can't tell if they are talking about testing the crankcase seals or the metering valve pop-off setting. That's all I've found re numeric values. Does anyone know if the Tilly/Stihl/Other rebuild manuals, have any numeric specs for testing carb pop-off?

What would be the pop-off for the Tilly HU & HS chainsaw carbs? (A search on "pop-off" yields some discussions on testing, but doesn't indicate the range of desired numeric values.)

 

[Swannie]

If this is a 6800i and the hi-speed needle seems unresponsive ( ie won't lean out) it could be the start injector needle is bad causing an over rich condition. The neighbor's 120si had this problem and the start injector was only available thru Dolmar, (about $30.00). Since this saw also had a primer bulb arrangement for cold starts also, I just used some fuel proof sealant and sealed off the start injector where it seats in the carb.

 

[Molecule]

thanks Swannie --

It is a 6800i (and absent intentional abuse by a prior owner should make a real nice 20" saw--at least that's what my hopes were.) During the carb rebuild, I replaced the old injector valve (whose rubber tip was a little worn down) with a new one. I also pulled the welsh plugs, and cleaned the filters and jets under them. I will add that this carb has the biggest main jet I've ever seen. On the main jet, there was a clogged up screen, and a screen holder insert, but no replacable jet. I cleaned it and looked it over pretty good, e.g. the milling of the jet in the body of the carb was real sharp, straight and clean, so I let it go.

Might be the tip (and/or seat) of the main needle valve is damaged (both looked ok). It would not surprise me if the valves were jammed down by a prior owner (this was one h3lluva bruised and abused saw - worst I've ever seen- not that I've seen that much- but it's about as far as my imagination could go anyway, as far as abuse goes).