heymow.com - Lawn Mower Racing Forum

Engine Help => Briggs Flathead => Topic started by: boomboom on April 21, 2014, 08:30:09 PM

Title: Oppy Fuel Problem
Post by: boomboom on April 21, 2014, 08:30:09 PM
Ok guys I need your help. New build, 16hp oppy, I have the motor set with the carb sticking out of the left side of the hood. I have 2 carbs Im playing with - 1 older with 3 bolt fuel pump and 1 with 4 bolt fuel pump. The 3 bolt is bigger but for some reason it will not pull the fuel.  Both carbs and pumps are fresh rebuilds. The 4 bolt pulls the fuel but I run out coming off the corner (this being because of the direction of carb....float raising the needle etc). Now....I ran 5/16 fuel line from tank to carb with inline filter. Tank is mounted under seat BUT is lower than the carb. Should I go back to 1/4 fuel line? Is it too much for the 3 bolt to pull? My vacuum hose to carb is pulling from the #2 boss on the intake (NOT the block) that I drilled and tapped. I recently read a post about a reserve tube off the side of the carb which I will try....but my concern is I know these things like to be on the rich side....NOT the lean side so Id rather run the 3 bolt. Any ideas? Any help is GREATLY appreciated. Thanks in advance!!
Title: Re: Oppy Fuel Problem
Post by: PJG56 on April 22, 2014, 05:18:18 AM
Do you have carbs externaly vented?
Title: Re: Oppy Fuel Problem
Post by: boomboom on April 22, 2014, 06:04:42 AM
Running the stock air filter so the stock vents on top are not blocked.
Title: Re: Oppy Fuel Problem
Post by: Jeff McKelroy on April 22, 2014, 07:13:31 AM
I always had better success not using the carburetor-mounted fuel pump and running an external fuel pump.  Just strip off the original pump, flip the steel cover to block off the feed hole. Next, you see that boss to the left as you are looking at the fuel pump? Carefully drill straight through with a small bit to the fuel feed passage, then tap the boss with a 1/8" pipe tap.  The stock pumps are garbage for what we do.
Title: Re: Oppy Fuel Problem
Post by: Jim Knutson on April 22, 2014, 07:33:50 AM
I have seen some older oppy carbs that had a thicker plastic block in the pump, between membranes, that will pump as much as needed for any oppy build.
Title: Re: Oppy Fuel Problem
Post by: boomboom on April 22, 2014, 10:09:54 AM
External fuel pump sounds like a good idea. I have a few briggs pump off my 17.5 single. Any reccommendations on what pump will be enough to feed this thing? You guys have helped alot so far....thank you!!
Title: Re: Oppy Fuel Problem
Post by: Jeff McKelroy on April 22, 2014, 11:56:53 AM
That pump you mentioned should do you fine.  Just don't forget a filter.  With a pressure pump you can add a filter with a lower micron number between the pump and the carb.  It has been my experience however, to have some kind of filter or screen between the tank and the pump.  Might save you some driveability problems in the future.
Title: Re: Oppy Fuel Problem
Post by: boomboom on April 22, 2014, 02:45:52 PM
Ok Ill give it a shot. Sounds good thank you!
Title: Re: Oppy Fuel Problem
Post by: PJG56 on April 22, 2014, 04:11:53 PM
If you rebuilt the 3 bolt carb and put the gasket between float bowl and top of carb it blocks off centholes and acts as you say.
Title: Re: Oppy Fuel Problem
Post by: boomboom on April 22, 2014, 04:38:11 PM
Yes I did.....do I eliminate the gasket??
Title: Re: Oppy Fuel Problem
Post by: PJG56 on April 22, 2014, 04:45:11 PM
I meant to say if gasket is upside down, gasket only goes on one way. Sorry for the unfinished reply
Title: Re: Oppy Fuel Problem
Post by: boomboom on April 22, 2014, 05:17:36 PM
Oh ok....lol. Yes i made sure the gasket lined up the right way.
Title: Re: Oppy Fuel Problem
Post by: Jeff McKelroy on April 23, 2014, 07:27:29 AM
You are correct Paul, I was thinking of the 4-screw carburetor.