OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines 850 series hunts

Hi all, I'm new to the forum.

Have just installed a new 850 series engine 6hp on my granberg push mower. Seems like a solid unit. At fully throttle it does hunt very noticeably under load. I installed a inline fuel filter whilst fitting the engine is it possible I have slowed the fuel flow to much or is it more probable I have a tune up issue. Don't have an uncut fuel hose with me right now to try it with out the fuel filter.

thank you for any thoughts

Briggs have two in-line fuel filter types: a type for gravity feed, and a type for fuel pump feed. If you fit a pump-type filter in a gravity feed system, fuel starvation does occur at full load.

I think you need to remove the filter temporarily (substitute a short piece of tubing of the right diameter) and see if the symptoms go away. If they do, either fit a new fuel line or see if you can get hold of a lower-resistance filter. If this does not solve the problem, please tell us, and we can dig deeper.

Bingo, i bet you have diagnosed it correctly. I was completely unaware of the two different filter types I can bet my bottom dollar I have fitted the wrong one.

thank you for your time

You're welcome, greenfield. I'll leave this thread open for now: please let us know when you have tried it without a filter, or with a low resistance filter. If there are still issues, we can discuss tune-up processes.

You could substitute the filter for an inline tap.


Seeing as how there would be a filter built into the bottom of the fuel tank (gause type) i would think the tap would be of more benifit than an extra filter. Well thats supposing that you are not trying to recycle fuel you scooped up out of a puddle at a local car wreck.

Have ordered a couple of the 150 micron filters, the red disc type. Are these the right ones?

The correct filter is indeed red. See picture and information here:
http://www.lowes.com/pd_46671-85794-5018H_0__?productId=1205339

Yes that looks like the one. I'll report back once it arrives

installed the new filter today. 95% of the hunting is gone. Only hunts under severe load now when it is on the governor constant. It hunts noticeably then.

Sounds like more work will be needed, then. If you had a classic engine it would be as easy as making a small adjustment of the mixture screw, but that will be an emissions engine that does not have an accessible adjustment. I suggest the first thing to try is removing the fuel filter altogether: emissions engines tend to operate on the ragged edge of leanness, so even a small restriction on fuel flow might make a difference. Also check whether the level of fuel in the tank makes a difference. If neither of those things helps, and you have checked that the governor linkage to the throttle butterfly is completely smooth and free, it will be necessary to strip and clean the carburetor.

Edit: I need to correct the last bit, this is a brand new engine so the carburetor does not need cleaning. However there are still a couple of things to check. Is your air cleaner sealing properly, not being bypassed by leakage? Reduced air cleaner restriction will make the engine run leaner, as well as letting in dust and ruining the engine. There have been problems at times with ill-fitting ex-factory air cleaners. Also it would be worthwhile checking the external linkage of the governor to see if anything is bent or binding. If possible check the maximum engine speed. I used to do this the hard way, with a stroboscope, but with horizontal-pull recoil starters it was a pain to remove the starter so I could get access to a rotating bit to point the stroboscope at. I've recently started using a cheap plug-lead sensing digital tachometer. It agrees with the stroboscope very accurately, so there's a good chance they are both right.

I put a new 850 on my Rover SP mower, it did not have a fuel filter inline before so in my wisdom I decided to fit a inline filter. It gave me nothing but grief, engine hunted at full throttle, would not run up to full throttle actually. In the end I removed filter and the mower has been fine ever since.