Leak down test

[arch252]

I recently posted about a Briggs 17.5 hp with a high compression issue. I resolved that by replacing a camshaft that had a busted compression release. The engine starts easy now and runs strong but is surging badly. I have replaced the old carb and no change whatsoever. Adjustment to the air mixture screw caused no changes in the surging or engine operation whatsoever. I've sprayed around the intake gaskets to ensure there were no leaks. I get free flowing fuel to the carb. I've reset the valve lash (.04 in /.06 ex) and inspected both push rods

I finally did a leak down test. Set the engine to TDC on the compression stroke. Locked the crank down, removed the intake rocker to ensure the intake valve was fully closed. I went up to 100 psi and it dropped to 60 psi on the leak down with air blowing up strongly through the carb.

I'm about to pull the head but wondering if this is going to be an intake valve issue or can a head gasket cause this?

The engine does not smoke when it's running. But for the surging it runs great.

 

[Hammermechanicman]

You must vring engine to a point where both valves are closed and block the engine from turning then do the test.

 

[cpurvis]

You're probably setting the piston at the 'wrong' TDC.
There are two--one, at which both valves are completely closed and have clearance between valve stems and rocker;
and two; the other TDC in which the exhaust valve is almost closed and the intake valve is just starting to open.

#1 is the one you want.

 

[Born2Mow]

I finally did a leak down test. Set the engine to TDC on the compression stroke. Locked the crank down, removed the intake rocker to ensure the intake valve was fully closed.

The valve WAS fully closed, if you really were in the compression stroke. So what prompted you to remove just 1 rocker arm ? Why not none or both ? Just curious.

I went up to 100 psi and it dropped to 60 psi on the leak down with air blowing up strongly through the carb.

Of course the pressure dropped, because we're not talking about valves for plumbing. But how long did the drop take ? It's the RATE of pressure loss that's important. If it did that over 24 hours then you got no issue.

I'm about to pull the head but wondering if this is going to be an intake valve issue or can a head gasket cause this?

Well, how would it leak back through the intake if it were a head gasket ?

What were the results of a standard compression test with the carb removed ?

 

[StarTech]

Born2Mow, Do you really understand how a leak down test works? It is a percentage leakage rate. Most Briggs engine can be anywhere between 10% to 40% depending on wear and still be good. When Arch252 said he se at 100 psi and the leak rate was at 60 psi that is 40%.

Also with Briggs engine the PVC system vents back into the carburetor so yes a blown head gasket can be passing into the crankcase and then out the PCV vent into the carburetor. With OP disabling the intake valve it is assume that he has 310000 series Briggs. These engine do tend to blow the gasket between the cylinder and the push rod gallery. What I do is to leave the rocker cover and listen at the push rod galley to see the air escaping is more at the gallery then at the oil fill tube. Now if the air escaping is coming from the intake side of the carburetor (carburetor removed) then it is a valve or valve seat problem.

Also if you use a low pressure leak down tester and disable the valve train it is possible to test not only the up cylinder but the mid and lower parts. My leak down operates at 12 psi so I can easily do this.

 

[arch252]

Follow up, I was on the compression stroke, I use a glove over the compression tester hose to determine when it is pushing air out and then I find TDC. The valves were fully closed. I removed the rocker from the intake after I heard and felt the air blowing out of the carb, removed the rocker just to be sure. There was no air passing by the exhaust so I didn't remove that rocker, I was satisfied that valve was fully closed. It dropped from 100 to 60 immediately. I pulled the head and sprayed carb cleaner in the intake and passed by the valve so I lapped the crap out of that valve until I finally got it to seal. Put on a new head gasket and all is well now.

 

[slomo]

If you adjust the air mixture screw with NO results, that circuit is clogged. Clean the carb.

This time of year, people are dragging out mowers with last years E-10, water, grass and dirt in the tank. Carb is gummed up. Surging = a dirty carb.

slomo

 

[StarTech]

If you adjust the air mixture screw with NO results, that circuit is clogged. Clean the carb.

This time of year, people are dragging out mowers with last years E-10, water, grass and dirt in the tank. Carb is gummed up. Surging = a dirty carb.

slomo

I wish people stop blaming every fuel problem on the E10/E15 fuels. It is not always that as it can be the additives that clog the carburetors. Fuels do evaporate and leave these additives behind clogging up the works. Even the fuel stabilizers can clog up these carburetors if the fuel is let to dry up in the carburetors.

And Slomo I just repaired a new carburetor this Spring that the idle air/fuel mixture was not adjustable and it wasn't an E10 introduced problem. It was a factory flaw. When with an engine first you need to know if it ever was adjustable. Also surging is always a dirty carburetor, it can be one that is simply too lean to start with as OEM is trying to meet EPA regs and think every engine is identical which they are not.

 

[Hammermechanicman]

This time of year, people are dragging out mowers with last years E-10, water, grass and dirt in the tank. Carb is gummed up. Surging = a dirty carb.

slomo

Music to my ears.

Cha-ching $$$$$$$$$

 

[cpurvis]

A surging engine and not responding to mixture screw changes can be caused by an intake manifold leak.