I'd like to say no,it isn't likely that the flipped seal would affect your runability. But, these aren't the heaviest valve springs either. The cup would slide over the guide- towards the head. You wouldn't have to pull the head to reverse it anyways. Your model # isn't quite right, so I don't know what the application is. Was this engine originally set up with a manual throttle control?

Yes, model number was listed in error. It is a 121602. I noted that the springs were not real heavy duty. It was originally auto throttle and I put a manual control on it. What else should I be looking at or diagnosing for the engine miss?

Had it ever run well w/ the manual control?

Hi Tom, Thanks for your continued interest. It came here as a junker that didn't run. The tranny on the mower was shot, the gas intake to the carb was corroded, The valves were out of adjustment and it had dirty oil. I did NOT even attempt to start it. I was told it did NOT run when I received it. So with a few new gaskets and adjustments (most recently pull rope), it runs but acts like maybe it is starving somewhat for fuel. Could be I didnt get the carb clean enough.

I would suspect the carb, especially since you noted corrosion. I'm not up on B+S engines though. That is where I would be looking.

You are probably right as this carb has no discernible IDLE circuit and is the kind that runs WOT upon startup. The lack of the idle circuit may be the cause for rough running at lower engine speeds as I have installed a manual throttle control cable to the throttle shaft on the unit. This knowledge came to me courtesy of another user here in this thread. I give credence to Tomplum for enlightening me with this information. This carb has the primer without the choke assembly.

The brass bolt holding the carb bowl on is you MAIN JET.
You may have to clean out the hole(s) some.

These carbs were pretty much set to run almost too lean from the start to keep the EPA happy. Even a slight reduction in orifice size because of varnish etc. puts them "over the edge" for being too lean.

Also look for air leaks because of gaskets, worn throttle shaft etc.

Thanks for the input Bill. I am going to run it and inspect it again. It does seem like it runs really lean too. I have added Chem Tool Berryman B-12 to the fuel at double the dose and feel it has helped in cleaning. Valves and head are clean and clear as I had it apart before and cleaned it well. Thanks again!

For a final follow-up: This engine still idles a little rough. I did not change the orientation of the intake valve seal. The engine appears to run better as RPM's increase. It doesn't stall out (amazingly). At WOT, the miss or stutter is much less apparent. For a junker engine that cost nothing, I feel it is a pretty good deal! Thanks to all for your input; I am still running it with carb/intake valve cleaner in the fuel and intend to cut grass with it in the spring.

Once it is in use ,I recommend adding a little Marvel Mystery Oil to the gas instead of the B12.

Thanks Andy. I will heed your advice and try that! Thanks again everyone.

You might want to simply remove the Main Jet and soak it IN 100% carb cleaner.

"As I recall, the seal was CUP shaped and I may have installed it upside down"
Which "seal"?

Click the link

Here is a link that might be useful: Carb disassembly

Hi Bill. I know the deal about cleaning the main jet orifices. The "seal" I have mentioned is the intake valve seal. The exhaust valve did NOT have one. This is one of those non-adjustable WOT carbs, known not to have an actual idle circuit. I removed and cleaned the head and all head parts, but I didn't lap the valves. They may NOT be seating 100%, but the carb was soaked and cleaned with no parts replaced except maybe the bowl gasket. I realize the brass bowl bolt is the main jet. I will try a couple different things, recheck the valve lash, run some MMO thru it and see.

This is an update on the runnability status of the Briggs 6.5HP OHV lawnmower engine. Yesterday, the mower was fueled with gas containing 4 additives: 1. Ethanol Killer 2. Chem Tool B-12 Fuel System Cleaner 3. Stabil fuel preservative and 4. Marvel Mystery Oil. The engine was run while mowing grass (weeds mostly) and it ran perfect. I am no longer concerned with an unsteady idle, because I do not MOW with the engine idling. It will idle though. I feel the key here is to religiously use additives in our fuel! Thanks to everyone who showed an interest and posted in this thread!