Surging Briggs Twin II

My 1999 model Briggs Twin II 42E707 model on Murray lawn tractor has had an issue where it surges at idle for at least 3 yrs. I took the carb apart 3 times and rebuilt the fuel pump twice in an effort to get it to stop but it persists. Engine runs pretty well under load but will issue a single backfire at shutdown after running it an hour or two.

I recently found a new factory carb at a reasonable price so replaced it this Spring. First time out of the box the engine ran great, smooth idle. Second time, right back to surging again.

I was using the round white gas filters on it as they were listed as the ones to use with the fuel pump engines. Looking through owners book on the engine, it specifies the 5018 filter even though package says non fuel pump engine. So installed a new one of those.

Compression is 100 and 110 on the 2 cylinders. Air filter is new, ignition is new, spark plugs changed in DEC. Fresh 90 octane ethanol free fuel, changed fuel lines sometime in last 3 yrs trying to sort out the problem. Fuel tank looks clean.

Any ideas?

 

When it's surging, progressively pull the choke on and see it hits a point the surging stops. If it does fuel supply/carb adjustment or air leak is likely the problem.

 

Agreed. Near the bottom of the carb at the front of the mower is a screw or a jet, depending on how old or new the carb is. If it's an adjustment screw, back it out a half turn or so. If it's a jet, take it out and clean it with a torch tip cleaner. That is the idle circuit.

 

OK, took my first Welch plug out, that didn't go too bad. There is a cap looking thing(another plug?) in the bore with a slot in it. Tried a jewelers screwdriver to remove it thinking it might be threaded but could not budge it. Didn't want to put too much force on it without knowing exactly how to deal with it? What next?

 

This is on the new carburetor you just put on it?

 

Is this the same carb you have?



If so is it a 3 screw or 4 screw fuel pump...?

 

Yes, its a new carb and yes its the 4 screw FP model.

 

When you get it put back together and if it's still surging try the choke thing I suggested or spray some WD40 down the carb throat and see if the surging levels out. If it does, shield the intake of the carb and spray the WD40 around mating surfaces with the engine running.

If it stops surging you have an air leak. If not you probably have a carb adjustment problem or a fuel flow issue leading into the carb from the FP, The pulse port may be plugged or a gasket out of place.

In a new carb , you may have dirt or some other obstruction, but it's likely something else.

 

Hmm, it might turn out to be something as simple as the carb body not mounted securely enough to the manifold? Gasket looks good but I could not torque it to spec because of where the bolts are so "guessed" it tight and I tend to try not to over tighten things from bad experiences in the past.

Since I have the Welch plug out and the carb off, I might as well check the idle jet. I bought a new screwdriver I think I can grind down just a bit to fit into the bore and into the slot on the head of the cover or screw or whatever it is. New carb, I guess its a fixed jet and will just unscrew counter clockwise with a proper fitting screwdriver and some force? Should I use some PB Blaster or something?

Do people replace the Welch plugs when finished before reinstall?

 

Could be a mounting problem - check for leaks with the WD40. If you can't get the jet out without the risk of damaging it, clean it in place, then spray some carb cleaner through it.

Replace the welch plugs - they often seal fuel/vacuum. Either affects the performance of the carb. Pt Blaster is good for penetrating corrosion. You shouldn't have that in a new carb.

Anytime you tighten mounting bolts use either the recommended torque pattern or a cross bolt pattern if the recommended isn't available. Always use at least two torque stages.

 

Decided to put the carb back on the mower today and make sure everything was torqued properly. Pulled the intake manifold off, this allowed me to properly torque the carb mounting bolts(100 in lbs). Resecured the intake to engine(90 in lbs). All wires, cables, linkages look good as did all the gaskets.

Started the engine and it was surging still. But when I cover the bore with the idle jet, it smooths right out. So need to get a new Welch plug. I don't see the plug on the parts diagrams anywhere? Anyone know what size it is or better yet the Briggs part no? Its the 4 screw FP model no. 693480.

Bottom line is proper torque must be critical on the fuel system on these engines.

 

Hi all, back again, I put the new carburetor on and the mower ran great, idled good....for one day of mowing anyway. Now the engine will not run for more than a few seconds on a cold start. It has good spark to both cylinders, its defintely getting gas.

I notice the idle lever is not resting against the idle adjustment screw. I can push it to where it does contact it, but it does not stay there on its own. Any ideas?

 

It's the carburetor still. The idle lever not resting against the screw is not related. If it's not getting enough fuel, it won't rest against the screw. You may need to put a better pleated paper type fuel filter on it, sounds like you're picking up trash or water.

 

I've got a new red 5018 fuel filter on it now that the manual calls for. The gas is fresh 90 octane ethanol free. When it hits the "no start" situation, pulling the air cleaner assembly off, the housing is soaked in gas. That's what led me to say it was getting plenty of fuel.

Was just out cleaning the governor assembly, I don't know anything about these but it seems to be binding when I move the throttle lever back to idle. All the springs look ok and seem to be attached. At least all that I can see above the big exhaust shroud. What's good to use to lubricate these levers and linkages?

Should I drain the tank, blow out the lines and use fresh gas from a different source? Recs on filter to use?

 

The governor is not the problem, it is doing just what it is supposed to do. With the engine off the throttle is held at WOT by spring pressure. With the engine running the governor counters the springs to close the throttle to what is set by the control.
Since you recently changed plugs, pull them and note the condition, color, build-up, wet, dry etc.
Make sure your choke is adjusted and operating properly.
You can remove the carb and leave the fuel supply connected. Since you have a fuel pump, lower the carb so it can get gas via gravity flow. Observe and see if it begins filling or leaking gas. Check your oil level.
When running is it still surging or is it spitting and sputtering? Any smoke? Black? Blue?

 

Soaked with fuel?

That's definitely a carb problem. Sounds like the needle not seating. A new red filter does just about nothing. Look inside it, it has a screen in it with big holes. It is not a pleated paper filter.

 

Started the mower from cold, it had sat overnight, with the air cleaner assembly off. Choke works fine. Engine started right up and while it was at idle and I was easing off the choke, gas started bubbling out of the top of the carb BIG time. Shut it off immediately as the gas was starting to pool on the muffler shroud. Engine did not run more than about 10 seconds. It was not real smooth, did not notice any smoke of any color.

 

Blew out the fuel lines, emptied the tank, put on a fresh 60 micron paper fuel filter, fresh from the the pump 87 octane ethanol free gas. Swapped the top end(inc float, inlet valve, etc) from my new "parts" carb to the one on the mower, Fired it up and it runs perfect.

Haven't taken the other top end apart yet but did not notice the float full of gas or an inlet valve seating problem. Will take it apart later and investigate.

New carb, new fuel filter, new air filter, new ignition, new plugs, new battery........hopefully I'm set for a while! Thanks everyone for your help!