Quick Carb. Question!
 

The guy that is working on my Carb. (750 Holly) says it has a blown power valve and also said it has #78 Jets in the front and Back? He thought that might be why I was fouling plugs, and thinks that I should try a smaller set in the front.

What do you guy's think? What size should they be? What should we try?

It's on the motor's on my 1987 33 ft Fountain, I think they are old 420's that have been worked over by the last owner. Maybe 365's?

If you have a blown P.V. then it was running too rich at idle, that might be why your plugs are fouling.

The 78 jets sound a little big also. I would go down two sizes and change your plugs so you can get a good reading.

Good luck
Bob

Pull carb apart & go to autoparts & get new power valve. If power valve is blown it will be giving motor lots of extra fuel. Do not rejet untill you put new power value in. I think something is wrong with the jetting, either you will need smaller ones in front or bigger ones in back Front ones + 6 or so steps should be equal or still smaller that the rears.

The blown power valve would definetely cause the plugs to foul at idle. Does it have power valves on the primary and secondary? If so, I use 74's on the primary and 81's on the secondarys on a 780. But you really need to check the plug color after a WOT run and shutoff to determine the correct jets. Cam, intake and exhaust all play a part. Good Luck. You can have him put poerevalve savers in the carbs. They are little check valves that go in the port that signals the power valve. This saves them if there is a back fire.

78 s sound about right if its a double pumper with power valve front and rear

He did say it had two Powervalve's!?

I would put new power valves in and leave the jetting alone for now. Put in new spark plugs and check them to see how they are coloring. In my holley carb book there are a few 750's shown with 2 PV's and jets set at 80/80. You don't want to lean it out and burn it down. What is the carb list #"?

Some (that know alot more than I do) feel that you should just plug the secondary powerb valve, and just run off the jets.

If the power valve is plugged the jetting on that side of the carb will need to be increased 6 to 8 jet sizes. Then you still need to watch your spark plugs close to assure your mixture is correct. Note it is better to be a little fat. You won't burn pistons down because you are rich.

I've got an 830 4150 series on my 502. The engine has an HP 500 cam and stock 502 heads. It made 490 HP @ 5300 recently on a dyno. The carb air temp was 77 degrees on the dyno and we had 78 primary, 78 secondary, with 6.5 PV front and back. My engine builder thought the engine was rich and rejetted the carb when we were done with the following - 73 primary, 75 secondary, 6.5 PV front and back(we didn't have those jets with us at the dyno). Looking at the above posts it seems I am quite a bit leaner. My engine builder also said he doesn't like a PV in the secondaries. Should I plug this and jet up? If so does Holley make a plug for doing this?