My friend recently had a motor rebuilt. He got it back and it was running bad. Its a NA 500 HP I think it has a 750 Holley (stock)

When he took the boat back they ended up removing the power valve and blocking it off. The boat runs better, but he is concerned about why they blocked off the power valve. What effect will this have on the engine and this acceptable practice?

Thanks,

Kirk

Usually, you can block the secondary powervalve on a marine app. By the time the secondaries start to open, the vacuum is low enough that the power valve would already be open.

http://www.bob2000.com/carb.htm

Read this

The power valve helps adjust the air/fuel ratio based on engine load. On a race can you can block off since you are either in or out of the throttle. I think it is a bad idea to block it off. The question is what was done on rebuild? Was carb also rebuilt?

With power valves you can usually attain a good midrange plug reading and not be lean on top end. Without it you are giving away a carburetor circut and the midrange will usually run too rich

With power valves you can usually attain a good midrange plug reading and not be lean on top end. Without it you are giving away a carburetor circut and the midrange will usually run too rich, when it is right on the top end. That's been my experience almost always w/o the secondary power valve. If you don't mind a black transom, you can get by w/o the power valve.

Power valves are sometimes refered to as a "economizer valve".... but,,, if you block off the valve... then you have to rejet the carb because if you run the same jetting as before.... the midrange and part throttle will be good... but the top end will be lean... The fuel that flows through the power valve is seperate of what goes through the main jets. It's possible that the main jetting was too rich to begin with... or there was a hole in the diaphram of the valve.... or the diaphram was stiff from age and wouldn't close at the proper vacuum level.... in either way.... blocking off the power valve on a naturally aspirated engine and not knowing what the jetting is... is asking for a lean-out at the top end...

Great info. As I thought, Holley puts them in for a reason. These idiots around here that actually get paid money to wrnech on boats don't know anything. Wish my friends would just do some research and learn themselves!!

Thank you very much for the education...where do I send the check for tuition?

Well, there is one thing yet. I run power valves in the primary and the secondary. Most people plug the secondary and then you up the secondary jets 6-8 numbers. If they plugged the secondary and upped the jet size-fine. If they plugged the primary, they should be shot.

I say get rid of it and get an EFI. However blocking your powervalve would require you to jet up 5 to 7 numbers to avoid leaning out the engine under load. Then when you decided to come to the dock you would notice black smoke coming from your exhaust.

Don't do it. Always use Holley brand power-valves too as others have limited capacity. (talking about the ones from the HELP company)

Roby