Ms390 trouble. How much impulse at impulse line

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Boredrob

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I put a new hi-way motor (complete motor that I sealed and tested) on a broken ms390 a friend brought over. Now I have good compression and spark but no fuel. I can open the carb (HD 18b I rebuilt) and there is no fuel in the top making it through the needle. Metering lever is flush. All gaskets are new. I can pull a vacuum on the fuel line and get fuel up to the carb so that's clear. Everything is clear enough for carb cleaner to spray through. I put my vacuum guage on the impulse line and it goes from -.5 to +.5 psi. It felt weak to me, that's why I checked it. Does anyone know what a normal pressure would be.
 
No, I don't know of one. All my saws are husky. Normally I work with industrial machinery or atv/auto, so electric, diesel, or 4- cycle for the most part

I removed the needle and cranked it a few pulls without it even installed and still have a dry carb. I can blow through the carb inlet and it's clear. I'll sleep on it tonight. I'm tired of screwing with it at the moment. I make good money to work on machinery, I hate getting stumped by a junky 2- cycle I'm fixing for free. If I had a fuel line and impulse line I'd just change them. But honestly there is a 75 percent chance I won't even get the money for parts back so I hate to change more than necessary.
 
I'll double check, but I'm 99.9 percent sure it's correct. Ive never screwed that up, although I have laid the metering diaphragm on the carb before the gasket. I'll certainly check though.

The carb was also dry when i tried to start the new engine....before I rebuilt it. and supposedly it was running when it locked up, then it sat for a few weeks. The old diaphragm in the top (metering side) was pretty brittle and I just knew the rebuild would fix it. So I'm at 130 for the engine. 7 for the rebuild kit, about 50 Hail Mary's for the cursing involved (everything about this saws design is great for function and physical size, not so much for rebuilding) . And half a tube of bio freeze for my shoulder from cranking.
 
IMO it doesn't take much to run the fuel pump.
What happens if you put a small hose against the impulse port on the carb and blow/suck on it? Should hear the fuel pump clicking, and should hold pressure/vacuum, or if you can't hear it click then take the pump cover off and make sure the passages are clear from carb face into the fuel pump cavity. If it's all clear and still doesn't click, the diaphragm might not be broken in enough yet to hear the clicking...not a problem.

With the fuel pump cover on, and if you take the metering diaphragm off and push down on the lever with your finger, can you blow through the fuel nipple on the carb?

The diaphragm opens the needle when there is vacuum on the carb throat. If you're not getting fuel in the metering side, it might not necessarily be your impulse line. The impulse line wont overcome the needle seat and if the metering diaphragm isn't getting sucked on to open the needle, then no fuel will get in there.
And if you cant blow through the fuel inlet to the carb with your finger pressed down on the metering lever, then fuel isn't making it to the back side of the needle. Could be wrong pump diaphragm at that point.
 
The impulse line wont overcome the needle seat...

The last time I put it together I left out the needle altogether just to see. Still no fuel. The fuel inlet and the impulse inlet are clear, I've cleaned them and can also spray through them. I can also blow through the impulse and fuel lines. I have blown through the inlet and pushed the lever down and verified that the needle worked (before leaving out the needle just to be sure) All diaphragms and gaskets are new (and oem walbro) , but the problem was there before the carb kit too, it would only fire on fuel I fed it. The impulse hole in the cylinder is also clear, I stuck a pipe cleaner in it just to be sure no yamabond got on it, but I'm not really sloppy with my sealant either. The cylinder and piston are hi- way. The cylinder does hold pressure too, so seals are good. I don't have a compression checker handy but compression feels very good compared to the 100 or so other saws I've ran.

I will try to hear the fuel pump click. Definately worth a shot. I appreciate the suggestions. I'll repeat... while my job titles list me as a specialist on many industrial machines, I'm no chainsaw specialist. I'm surprised by the number of ways such a simple machine can find to malfunction.
 
See if the carb will pump with you simulating the impulse line by mouth. Just suck in and out lightly for like a minute at a steady rate that should draw some fuel down into the metering chamber.
 
So with the needle out and the fuel pump and cover installed, you can blow through the fuel line inlet? If so, then I'd say the problem has to do with the impulse circuit not pumping fuel.

If the carb is dry, then either the circuit between the tank and metering chamber is blocked(this has to be checked with the cover and fuel pump diaphragm installed. when you touch the lever while blowing intobthe carb and air flows, then that means fuel can make it between the fuel line inlet and metering chamber) or the pump is not drawing fuel into the carb.

Assuming you have checked the impulse line for cracks?
 
So with the needle out and the fuel pump and cover installed, you can blow through the fuel line inlet?
That's correct. I also checked it by blowing through and touching the lever before just taking it out.

Assuming you have checked the impulse line for cracks?

Correct. I blew into the impulse with end plugged and it held pressure. It could be collapsing though. Another concern is that the new engine never ran. So the impulse hole could be too restrictive. 0.5 psi seems less than other saws I've worked on. I wasnt overly impressed with the new cylinders external finishing.
 
I removed the filter and capped the end and blew through the other end. The fuel line holds pressure. I can draw fuel up through the hose pretty easily. I manually worked the impulse and it drew air up through the carb so I believe the carb is fine. I guess I'll take the saw completely apart again and see what may be going on. Maybe my last saw job. I've rebuilt excavators in less time than this P.O.S.
 
I've rebuilt excavators in less time than this P.O.S.

:laugh:

Small engines work on some pretty basic technology. Just gotta work with what ya got and think about why its not doing what its supposed to.

I dont think it'd take all that much impulse psi/vac to run the fuel pump....but, Ive been wrong before.

Got a working saw you can put your gauge on and see what the impulse pressure/vac output is?
 
I do have working saws but all are pro series Huskies. The only one I have took apart recently had the impulse through the intake block (266xp). I may measure my 372 just to see. Half a psi is pretty weak (half pressure and half vacuum so 1 psi total) . I usually dab grease and the impulse blows it a mile. On this saw the impulse blew it away but not really forceful. I've never messed with any saw in this series though so that may mean nothing.
 
Put a shot of fresh fuel/carb spray directly into the carb throat, and try to start.
It should roar for a few seconds until that fuel charge is burned off.
If it doesn't, well, then your problems lie elsewhere.
 
It runs if I put fuel down it. The problem is definately in the carb or impulse. I can crank it 25 times and the top of the carb is completely dry.
 
...also, even a tiny speck of dust (hair, etc.) between the pump diaphragm and the carb body will cause an air leak and the symptoms you describe...
 
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