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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.

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

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.

arch252 said:

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.

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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.

arch252 said:

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

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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.

arch252 said:

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?

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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 ?

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.

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.

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

slomo said:

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

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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.

slomo said:

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

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Music to my ears.

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

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

It was originally surging, I cleaned that carb and no change. I put on a brand new carb, fresh ethanol free gas, no change at all. I cleaned and checked that new carb, no change. I found that I had a leaking intake valve, I corrected that and it is now running smooth with no surging. I have a video to prove it. I didn't touch the new carb after I corrected the valve issue so the surging was not the carb. Hard to say that a surging issue is ALWAYS a carb issue, that's apparently not true.

StarTech said:

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 set at 100 psi and the leak rate was at 60 psi that is 40%.

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Sorry to disagree. 100 to 60 is NOT a RATE. By its very definition a RATE always involves Time. "Miles" is not a Rate; "Miles per Hour" is a Rate. "Gallons" is not a Rate; "Gallons per Second" is a Rate.

100 to 60 is a percentage of leakage, but it does NOT describe a rate because NO Time was stated. Like I said if it occurred over a TIME of 24 hours there's no problem.

NOW, the OP has finally told us it occurred over a time of "Immediately", indicating in the vernacular approximately 1 to 2 seconds. 1 to 2 seconds is TIME. Finally we have true Rate of Leakage.... 40% per 2 seconds.

Ok I will bite. You guys read on here just like I do that filthy carbs = surging a lot more than gas additives drying up and intake leaks x 50.

Sure the Leaning Tower of Pisa could fall down. I get it. Gas prices might go up, check. And "A" carb might be flawed from the factory, noted.

slomo

My lord Born2Mow are you saying you can read leakage in less than the measureable time it takes most people to do it? Most humans takes a second or two at the best realize a reading. As close to instant as it is there is still time lag involved. Even when a light is turned on it take time for it to be realize by the observer.

But the common leak down test we use on most four cycle engines is where we input air at fix amount with it continuously being replaced as it is lost so we can a fix amount of leakage. Now with two cycles we do a leak down test from a fixed amount over a period time. Any way we are not testing for the exact same thing or using the same testing method. Otherwords with the leak down use on four cycles it doesn't matter if you test for 1 second or 1 hour the difference of 40% is 40% over both time periods.

Most leak down tester for four cycles uses an orifice between the two gauges. Something like .140". With zero leakage the readings on both gauges the read same. But when a leakage is occurring the orifice delays the replacement air from instantly replacing the lost amount so there is naturally a time delay. The two cycle leak down test uses fix amount of air and the same gauge so you are looking a difference pressure reading over a fix time period. No air is replaced during the two cycle test so a 2 psi lost in 2 seconds is different than 2 psi lost in two minutes.

And Slomo there is and will always be an exception to everything. I was once told that electronic calculators could not add wrong but they can if someone screw ups the programming. I had to replace 100 cpu chips in Huntsville because someone did in the 80's in the Victor 600 series.

Surging is either a fuel blockage or air leak, not valve adjustment. Could also be a governor issue. When surging, if you slowly apply the choke, does the surge tend to want to smooth out? If not, then safely grab the throttle shaft, linkage or governor arm. Can you control the surge? If so, governor issue. Was the replacement carb an OE carb? Or, some China knock off?
When doing a leakdown, dropping both pushrods will insure both valves are closed, also for TDC, it doesn't matter what stroke you are on. Fuel does matter! Most engines are rated for E10 or less. Using over 10% ethanol not only damaged seals and gaskets, attracts water, it is also illegal to use. Lastly, I will post this everytime I see a post about replacing a carb. The only reason to ever replace a carb is if it is missing, broken, parts are not available, or you don't know how or are too lazy to clean it. Now days, you can purchase a decent ultra-sonic cleaner for less than the cost of some carburetors. Good luck.

slomo said:

Ok I will bite. You guys read on here just like I do that filthy carbs = surging a lot more than gas additives drying up and intake leaks x 50.

Sure the Leaning Tower of Pisa could fall down. I get it. Gas prices might go up, check. And "A" carb might be flawed from the factory, noted.

slomo

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A carb defect from the factory is extremely rare. And if there was a defect, it would be evident very soon in the engines early life. Carburetors DO NOT develop internal defects. Dirt, old fuel, or missing/ damaged parts are about all you'll have. Please prove me wrong. 40 years of working with carbs, hasn't happened yet.

txmowman said:

A carb defect from the factory is extremely rare. And if there was a defect, it would be evident very soon in the engines early life. Carburetors DO NOT develop internal defects. Dirt, old fuel, or missing/ damaged parts are about all you'll have. Please prove me wrong. 40 years of working with carbs, hasn't happened yet.

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I agree completely with the above statement, although i've only been 30 years working with carbs.

txmowman said:

A carb defect from the factory is extremely rare. And if there was a defect, it would be evident very soon in the engines early life. Carburetors DO NOT develop internal defects. Dirt, old fuel, or missing/ damaged parts are about all you'll have. Please prove me wrong. 40 years of working with carbs, hasn't happened yet.

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Carb throttle shafts and the opening in the carb body do wear over time. This creates a vacuum leak. Yup needles and seats and other wear items....

slomo

slomo said:

Carb throttle shafts and the opening in the carb body do wear over time. This creates a vacuum leak. Yup needles and seats and other wear items....

slomo

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Yes they all do wear. I have seen both the shaft and carburetor bodies worn. Believe it or not I had at least two Nikki two barrel carburetors where the choke shafts completely worn through and broken along the carburetor bodies heavily worn. So much wear on the carburetor bodies that bushings had to be installed. Both of the ones I thinking about were on horizontal Kawasaki v-twins on the Z925A and Z950M mowers. Not something I would thought of until I actual seen it myself as chokes just don't get used that much.

txmowman said:

A carb defect from the factory is extremely rare. And if there was a defect, it would be evident very soon in the engines early life. Carburetors DO NOT develop internal defects. Dirt, old fuel, or missing/ damaged parts are about all you'll have. Please prove me wrong. 40 years of working with carbs, hasn't happened yet.

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Yes, normal defects usually show up very early on. I was only referring to the new carburetor. Damaged parts AKA worn parts do occur. Fuel itself is abrasive over time but is nil wear most times as worn engines actually needs it.

As far trying to prove you wrong I subscribe to the theory of Murphy's Law that anything is possible given the right conditions. Just because you haven't seen it doesn't mean it not possible as corrosion has a nasty habit of doing strange things.

StarTech said:

Yes they all do wear. I have seen both the shaft and carburetor bodies worn. Believe it or not I had at least two Nikki two barrel carburetors where the choke shafts completely worn through and broken along the carburetor bodies heavily worn. So much wear on the carburetor bodies that bushings had to be installed. Both of the ones I thinking about were on horizontal Kawasaki v-twins on the Z925A and Z950M mowers. Not something I would thought of until I actual seen it myself as chokes just don't get used that much.

Yes, normal defects usually show up very early on. I was only referring to the new carburetor. Damaged parts AKA worn parts do occur. Fuel itself is abrasive over time but is nil wear most times as worn engines actually needs it.

As far trying to prove you wrong I subscribe to the theory of Murphy's Law that anything is possible given the right conditions. Just because you haven't seen it doesn't mean it not possible as corrosion has a nasty habit of doing strange things.

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I also am a firm believer that anything is possible. But, corrosion is not a manufacturing defect. Neither is dirt that wears the shafts. Good posts!