Makita 6400 Vacuum Leak - Maybe

[rta01]

Have a "Home Depot" Makita 6400 I had for several years and upgraded it to a 7900. Replaced seals and bearings (one bearing had come apart was main reason I rebuilt.) Ran pretty good, but was having a little trouble dialing in the rpms. Decided to check for air leak. Blocked the intake with rubber between carb and boot. Removed the muffler and blocked with rubber backed by a metal plate.

Here's where I'm stumped. Using a mityvac run to the impulse nipple, it will not build any pressure or vacuum. So tried a different approach. Plugged the impulse nipple and ran the MityVac to the spark plug hole. Can build pressure that way, but it leaks down. 15 seconds or so to go from 10-15 psi to 0. I've sprayed soapy water everywhere - both seals, between carb and boot, between boot and cylinder, all around cylinder base, at the decomp valve, as much of the case joint as I can see - and get no bubbles.

I'm stumped. Biggest question is why the no pressure through the impulse nipple and build pressure through the spark plug. What am I missing.

 

[ballisticdoughnut]

Could be leaking at case gasket?

 

[lone wolf]

Have a "Home Depot" Makita 6400 I had for several years and upgraded it to a 7900. Replaced seals and bearings (one bearing had come apart was main reason I rebuilt.) Ran pretty good, but was having a little trouble dialing in the rpms. Decided to check for air leak. Blocked the intake with rubber between carb and boot. Removed the muffler and blocked with rubber backed by a metal plate.

Here's where I'm stumped. Using a mityvac run to the impulse nipple, it will not build any pressure or vacuum. So tried a different approach. Plugged the impulse nipple and ran the MityVac to the spark plug hole. Can build pressure that way, but it leaks down. 15 seconds or so to go from 10-15 psi to 0. I've sprayed soapy water everywhere - both seals, between carb and boot, between boot and cylinder, all around cylinder base, at the decomp valve, as much of the case joint as I can see - and get no bubbles.

I'm stumped. Biggest question is why the no pressure through the impulse nipple and build pressure through the spark plug. What am I missing.

So dunk it in water while pumping the Mitivac and find the leak. Works everytime.

 

[rta01]

So dunk it in water while pumping the Mitivac and find the leak. Works everytime.

Was tempted to, but a little concerned about what water might do to the coil.

 

[lone wolf]

Was tempted to, but a little concerned about what water might do to the coil.

Nothing will happen to the coil dunk it while pumping pressure not vac.

 

[lone wolf]

Is the flywheel and clutch off?

 

[rta01]

Is the flywheel and clutch off?

Clutch yes, flywheel, no. Had it off and when it wasn't bubbling with the spray test I put it back on. I'll take it back off before I dunk.

 

[rta01]

Well. That was embarrassing. At first no bubbles from the dunk. Dunking in 5 gal bucket nose down. Finally lowered it all the way in and bubbles came out of carb intake. I thought that can't happen. Thought about it a while and checked my carb blockoff, and yes it wasn't completely covered. Got that fixed and still wouldn't hold pressure, but I could hear the air escaping. Finally found my muffler block wasn't solid. Bottom line, leaks were at the carb and muffler block. Got it under 7 psi now and it's been good for at least 5 minutes before coming in for this post.

I think my initial air leak fears were for nothing. Still need to check vacuum, though.