Holley jets

[Jamcat]

Can anyone give me a primer on what jets/powervalve/pump squirter/ pump cam Holley considers to be the "Marine standard calibration" for a 750 DP on a big block chevy? My 750 DP came off a custom marine smallblock and I'm sure it was optimized for that engine.

I have put it on my boat with BBC, large oval port heads, 11:1 compression, performer RPM intake, moderate cam recommended for jetboats (I have the specs but not on me) and bassett thru transom headers.

Is there any certain specs/jet ranges that the jet boat community considers a 'given' on this type of set-up? It has been a long time (8 years) since I was trying to tune my combination and it's like I am starting from scratch as I never did get the carb completely sorted before circumstances required me to store the boat.

[RLP]

I would set it up to what the engine likes.
Every onehas there own idea doing the carbs,i dont run power valves,when ya do this ya have to go way up on jet size.
If ya want to run the power valves ya need to take a vacum reading to see what she needs.
You can always take the list # off choke tower and call Holley and they can tell ya how it came out box.
If it came off a healthy small block ya may just try it with new plugs and check plugs after run in water and see if they are fat(black smut) or little to no color(lean) or a lite chocolate (right on).
Most big blocks like a 850,but ya can set the 750 up to work.

[Just A Jet]

I agree with RLP, I would just go ahead and check to see if the PV are pluged yet if not I would plug them and put 77 in al four jets. then do a plug test.
If the jets are bigger leave them and go with it.

[SS-201]

Plugging the power valves can be a night mare, It's usally at least 10 jet sizes on the power valve. Use a carb with one power valve and the size for marine is 4.6 - 6.5. The best way is to test with a vacuum guage to read the vacuum at the point of desired opening. I also put on a 770 avenger and works great.

[Last Mohican]

This will at least give you what it had stock for jets and power valve.

http://www.mortec.com/carbs.htm

[Last Mohican]

Just last weekend I took a Holley 4780 (800 cfm) off of my boat and replaced it with an out of the box 80443 (850 cfm marine). The difference was night and day. The 800 had 64/74 and plugged power valves. The plug reading were good with this carb but it had a bad flat spot and I could only get it to turn 4200 RPM. I fought with it all last summer.

I bolted on the 850 with no changes I picked up ~800 RPM and the transition is smooth from idle to WOT. I can hold it at any RPM with no issues. It's like a different boat.

I have a buddy with a mild 454 in a 20' Mako jet boat. He has been fighting a similar issue with his boat. We pulled the carb off his boat and bolted on my 850. Same result. His boat ran better than it ever has.

He decided to go buy a new carb for his. Instead of getting the marine 850 he bought a 4781S. (850 auto) out of the box his boat ran like crap. The 4781 had 80s front and rear with 6.5 power valves. My marine carb had 88/96 with 6.5 power valves so we jetted his street carb just like my marine carb. It still ran like crap. He took the auto carb back and ordered the marine carb. We'll see what happens next weekend.

I know we could have tuned either of those carbs to work eventually but the new carb was worth worth every penny for the time saved. According to Holley there are major differences in the marine and street carbs. There is more to it than just coatings to prevent corrosion.

Short story, Every application is different. What works for others may or may not work for you even if you think it is the same.

[Jamcat]

I would not try plug the PV on anything but a 'full race' setup where I was not concerned at all about economy.

Last Mohican- yes- I am tending to agree with you on the fact that may be other differences than just jets/PV. I have fought the good fight trying to make a non-marine carb work good. I mean, yeah, I can aggravate with the jets and read plugs and make it work- but it seems from experience that there is HP to be gained with a marine carb on a boat. My main question is does anyone know what else may be different? Different boosters maybe? Different emulsion tubes? etc...

OR, is it that each carb from Holley is bench flowed and jetted/tweaked at the factory and when you go away from what was in it for a certain application then you're calibration is just gonna be 'off' a bit?

[Last Mohican]

According to Holley, almost everything is different. You might give their tech line a call and see if you can get more info. I'm guessing the main difference is in the metering block. There may also be differences in the main body and boosters as well. All the "tunable" parts (squirters, jets, accelerator pumps, pump cams etc...) seem to be the same. In fact all four carbs we tried had 31 squirters and the same color pump cams. I did not however check cam location.

There are a lot of people who will argue that there is any difference. What I do know is for me the proof was in the pudding. Between two boats and four carbs the "magic" tune was an out of the box marine 850 for both boats.

[RLP]

I would say if ya dont know carbs very well get ya a new carb(marine)so it will also pass coast guard rules.
But i can tell ya the carb ya have can be set up to work.

[Just A Jet]

Just last weekend I took a Holley 4780 (800 cfm) off of my boat and replaced it with an out of the box 80443 (850 cfm marine). The difference was night and day. The 800 had 64/74 and plugged power valves. The plug reading were good with this carb but it had a bad flat spot and I could only get it to turn 4200 RPM. I fought with it all last summer.

I bolted on the 850 with no changes I picked up ~800 RPM and the transition is smooth from idle to WOT. I can hold it at any RPM with no issues. It's like a different boat.

I have a buddy with a mild 454 in a 20' Mako jet boat. He has been fighting a similar issue with his boat. We pulled the carb off his boat and bolted on my 850. Same result. His boat ran better than it ever has.

He decided to go buy a new carb for his. Instead of getting the marine 850 he bought a 4781S. (850 auto) out of the box his boat ran like crap. The 4781 had 80s front and rear with 6.5 power valves. My marine carb had 88/96 with 6.5 power valves so we jetted his street carb just like my marine carb. It still ran like crap. He took the auto carb back and ordered the marine carb. We'll see what happens next weekend.

I know we could have tuned either of those carbs to work eventually but the new carb was worth worth every penny for the time saved. According to Holley there are major differences in the marine and street carbs. There is more to it than just coatings to prevent corrosion.

Short story, Every application is different. What works for others may or may not work for you even if you think it is the same.


I think the jets play a big factor in your testimony, If he runs the PV so be it but no need to trash the carb.... just run the carb at full throttle for a burst and check a plug, White= jet up..Black down.. brown runer till she pops!!

Not NASA< just CHEVY