Subaru SP170 - Stalls when throttle opened quickly

[The_Hamster]

Subaru SP170DT1150 on a Rigid air compressor. Has a Mikuni carb on it, with idle mixture screw. Fairly sophisticated for a small engine carb. From what I've gathered it is very similar to the Subaru EX17 engine.

This engine has had a multitude of issues lately. It got a brand new carb installed this last summer. Then a piece of plastic jammed the intake valve open (no idea how it got in there) and the head had to be removed to get the plastic out.

On reassembly the engine will not start without carb cleaner or some form of fuel shot into the intake. Once running it seems fine. It is on an air compressor and it will charge the air tanks and then go back to idle just fine. However when the compressor calls for the engine to spool up it pops the throttle to WOT and the engine instantly dies. If you slowly open the throttle it is fine. But sudden jerking kills it, and the compressor opens it with a spring loaded device to WOT instantly.

The carb is clean as a whistle, it is virtually brand new with only a few hours on it. I put some copper through all the jets and shot carb cleaner around, still no go. I found a spec online stating that the idle mixture screw is supposed to be 1.25 turns out. When I drilled the plug out that covered it I found it to be fully seated. I turned it out to 1.25 after completely removing it and cleaning behind it with carb cleaner and compressed air.

The only part of this carb I have not removed is the emulsion tube. I can't find a spec for the float height anywhere online as of yet.

The difficulty in starting from cold (and hot) and death when the throttle opens indicates to me that it is running lean. The intake gaskets are new, and the air cleaner is installed. Anywhere else I should be looking to get this running properly?

I'm open to suggestions at this point.

 

[eljefino]

Sounds like opening up the main jet will fix it.

If you still have the old carb you can practice on its jet. You can also check its emulsion tube to see if it's identical to the new one or not, even though it's a means-to-an-end and could look radically different and still work similarly. Having just idle and full-bore should be very simple to make work.

But it's super easy to mod your main jet.

Get the mini drill bit kit from amazon ($6) and just fit them as go/ no-go until you find your size, then go up one at a time, testing afterwards.

Does this have a centrifugal clutch? Can you bump the base idle up until just below the clutch catches?

 

[CT8]

First check for a leak between the carb and the engine. Fuel then dig into the parts.

 

[The_Eric]

If you decide to start drilling, remember that a little bit goes a VERY long ways. Many times increasing the diameter by .002" to .003" will increase the area of the jet by 10%.

Some simple math is in order here for sure- first figure out the approximate diameter of the jet, then do your math for area of a circle (pi x r(squared)), then figure out how much more area you think you want (I'd start at less than 10%) and work it out from there.

 

[chad8]

Do not ever drill jets especially when chasing a problem like this. You will NEVER get it correct and it will not fix the problem.
You either have an air leak or a bad fuel line or gasket somewhere in the fuel system that is not sealing.
I did not see a fuel pump in the diagram , but i did not study it very hard either.
That is why it won't pull fuel on startup. Not enough suction for fuel .
Same reason it is stalling.
Slight chance that the in tank filter is blocked too or the line is cracked letting in too much air. .
http://www.ereplacementparts.com/subaru-robin-sp170-engine-parts-c-133327_183514_183515.html

Spray carb cleaner around the intake and gaskets when idling to see if it revs higher. Same with fuel line and under clamps. .
That will be the source of the leak.

If not , find the gasket(s) responsible.

 

[The_Hamster]

There is indeed no fuel pump, gravity fed and the carb has a fuel filter setup on the side of it that blocks the idle mixture screw while the fuel line is attached. Which is unfortunate.

I'll do more searching for air leaks and report back.

Thanks;

 

[The_Eric]

Originally Posted By: chad8
Do not ever drill jets especially when chasing a problem like this. You will NEVER get it correct and it will not fix the problem.
You either have an air leak or a bad fuel line or gasket somewhere in the fuel system that is not sealing.
I did not see a fuel pump in the diagram , but i did not study it very hard either.
That is why it won't pull fuel on startup. Not enough suction for fuel .
Same reason it is stalling.
Slight chance that the in tank filter is blocked too or the line is cracked letting in too much air. .
http://www.ereplacementparts.com/subaru-robin-sp170-engine-parts-c-133327_183514_183515.html

Spray carb cleaner around the intake and gaskets when idling to see if it revs higher. Same with fuel line and under clamps. .
That will be the source of the leak.

If not , find the gasket(s) responsible.


Perhaps on this particular application you may be correct, but I wouldn't generalize like that. I've cured several small engine woes by oversizing the main jet a touch. Sometimes they just aren't calibrated correctly. Now granted, these have been chinese clone engines of various sorts, but the point remains.

Also keep in mind that the carb has been replaced and the o/p has stated he believes the idle mixture setting was wrong (stated that it was bottomed and needed to be 1.25 turns), so the carb could be slightly different or outright wrong.


I too would verify fuel supply first before getting too crazy with anything else. I'm really curious as to why the idle mixture was able to be adjusted from bottomed out to 1.25 turns out with no apparent effect.

 

[chad8]

I too would verify fuel supply first before getting too crazy with anything else. I'm really curious as to why the idle mixture was able to be adjusted from bottomed out to 1.25 turns out with no apparent effect. [/quote]

Because there is an air leak. Very basic small engine diagnosis.