Ported 7900 vent issues

[Dolkitafreak]

I have a ported and carb swapped 7900, running a tilly 296a. Saw pulls a vacuum and goes lean after a couple cuts. I crack the cap, its good for a couple more, then it goes lean again. Fuel line isn't pinching, vent appears okay. Saw only has 3 tanks on it, never had an issue with stock carb.

 

[Lightning Performance]

I have a ported and carb swapped 7900, running a tilly 296a. Saw pulls a vacuum and goes lean after a couple cuts. I crack the cap, its good for a couple more, then it goes lean again. Fuel line isn't pinching, vent appears okay. Saw only has 3 tanks on it, never had an issue with stock carb.

Clean the carb screen. Then vac test it.

 

[Dolkitafreak]

Clean the carb screen. Then vac test it.

Brand new carb, I'll check though. Vac test the carb?

 

[huskihl]

Brand new carb, I'll check though. Vac test the carb?

Are you running an air filter on it when it does it?

 

[Dolkitafreak]

Are you running an air filter on it when it does it?

Does it with and without a filter. When I ran it off a bottle (just has a small hole drilled for venting) it ran fine for a quarter tank or so, when normally I can't make one cut without vapor locking.

 

[huskihl]

Does it with and without a filter. When I ran it off a bottle (just has a small hole drilled for venting) it ran fine for a quarter tank or so, when normally I can't make one cut without vapor locking.

Any chance the filter put a kink in the impulse line on top of the carb?

 

[Dolkitafreak]

Any chance the filter put a kink in the impulse line on top of the carb?

It does it even when I run no filter, I'm stumped.

 

[mgr1]

Check the fuel filter and remove the tank vent for a test. The vent is modified after problems with the first series.

 

[Dolkitafreak]

Check the fuel filter and remove the tank vent for a test. The vent is modified after problems with the first series.

It does the same running no fuel filter. I will try removing the vent and running without it. This saw was bought new a couple months back, everything is the latest version. I have tank vents on hand, I can compare and see if there is a difference.

 

[huskyboy]

The later vents have a sintered white puck that presses in over the duckbill valve. The earlier ones were a grey foam piece that tends to get clogged with dust or deteriorate from age and fall out. You can retrofit the later style puck to the early saws. I would test without the duckbill valve and if that fixes the problem, replace the duckbill valve. Ethanol/age might have deteriorated it from working properly. When removing the puck, I found working it out with a pick around the edges to be the best method. Along with loosening the AV springs to gain enough play in the tank to gain access. It’s on the flywheel side of the tank.

 

[5000+]

Test it, easy enough. Do a vacuum test on an empty tank at the fuel hose. No guess work.

 

[CR888]

Sometimes you need a biga tank vent for a biga carb.

 

[Dolkitafreak]

The later vents have a sintered white puck that presses in over the duckbill valve. The earlier ones were a grey foam piece that tends to get clogged with dust or deteriorate from age and fall out. You can retrofit the later style puck to the early saws. I would test without the duckbill valve and if that fixes the problem, replace the duckbill valve. Ethanol/age might have deteriorated it from working properly. When removing the puck, I found working it out with a pick around the edges to be the best method. Along with loosening the AV springs to gain enough play in the tank to gain access. It’s on the flywheel side of the tank.

This is the new style, saw has 4 tanks through it. I have the duckbill out, everything appears okay but I will vac test later. Never had an issue with these tank vents, this is also the first 64-79 I've carb swapped.

 

[redlight066]

Long shot but I’ve had this issue on two different saws. Pressure test the fuel line itself.

 

[Ketchup]

If you’re cracking the fuel cap and getting more cuts, it’s almost definitely a tank vent issue.

 

[Dolkitafreak]

Been busy the past couple nights! I'll mess with it tonight, as long as the 550xpg that should be waiting at home for me doesn't distract me!

 

[davidwyby]

Been busy the past couple nights! I'll mess with it tonight, as long as the 550xpg that should be waiting at home for me doesn't distract me!

Whahappen?


My 6100 spits at me every time I open the tank. Runs fine.