1991 Resto - I Have A Feeling It's Going To Be A Long Winter...

[thatsmrmastercraft]

Originally Posted by MikeyOrange88

My carb has the same 'added plate' (black with round stud on it) that the throttle cable attaches to, and pretty sure it is stock. The secondaries open via vacuum, not mechanically on that year also, correct? Vacuum on my '88 anyway. I've had issues with getting them to open previously, so was curious. Is your picture before or after you moved it? Thanks.

You are correct on the vacuum operated secondaries. If you can't get your secondaries to open, you could have a ruptured secondary diaphragm. You need an intact secondary diaphragm in order for the secondaries to perform.

[mxhideout]

Pic was before it was moved. Ok, because when we moved it to WOT they would open 2/3 of the way. So you're saying that vacuum opens them the last 1/3?

[MikeyOrange88]

mxhideout: Not a carb. expert, but my understanding is that they are 100% vacuum operated, on our vintage engines at least. Just so were communicating correctly, the rear two barrels are the secondaries, the front two are the primaries. The primaries open by the throttle lever via throttle cable connected to the carb. The secondaries open by vacuum when the engines develops enough low pressure at speed/load, somewhere above 3000 rpm or so (seen several opinions on when).

thatsmrmastercraft: Understood, mine are working now. I was just looking for some of that 'free horsepower' he mentioned!

[mxhideout]

Yes, we were looking at the secondaries (rear). I have more experience with bike stuff, but my dad is the full-time mechanic. If you want more free HP, just add more stickers!

[MikeyOrange88]

Originally Posted by mxhideout

Pic was before it was moved. Ok, because when we moved it to WOT they would open 2/3 of the way. So you're saying that vacuum opens them the last 1/3?

Secondaries are only operated by the vacuum, no mechanical action on this setup. However, if you are able to develop more throttle/rpm by moving the throttle cable connection point, and a consequently a higher vacuum, then they would open more/wider. There are also several different springs that can be used within the secondary vacuum assembly to alter when they start opening.

[thatsmrmastercraft]

OK, lets start from scratch on carb operation in our boats. The throttle lever causes the primary throttle blades to open and go fast. When approaching 3/4 throttle, the secondaries will open due to the secondary spring overcoming decreasing manifold vacuum. Location of the throttle linkage will not affect secondary operation.......only primary operation. Secondaries open strictly by vacuum.

And stickers do indeed ad horsepower. It has been proven on the dyno.


Good Holley carb operation info http://www.holley.com/data/TechServi...ech%20Info.pdf

[mikeg205]

More stickers... my ears perked up... -

[thatsmrmastercraft]

Originally Posted by mikeg205

More stickers... my ears perked up... -

[mxhideout]

Gotcha, thanks.

[MikeyOrange88]

Originally Posted by thatsmrmastercraft

OK, lets start from scratch on carb operation in our boats. The throttle lever causes the primary throttle blades to open and go fast. When approaching 3/4 throttle, the secondaries will open due to the secondary spring overcoming decreasing manifold vacuum. Location of the throttle linkage will not affect secondary operation.......only primary operation. Secondaries open strictly by vacuum.
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Understood. I was trying to carefully craft the wording to indicate that a higher vacuum (lower pressure) would increase the degree that the secondaries would open (wider opening, allowing more air in. Correct? My wording was probably poor, but I stated twice that they are operated only by vacuum.
Certainly not a carb expert, but I do like to drive the boat fast! I've learned a tremendous amount on TT, and continue to, as I read it almost daily. Thanks for keeping us straight.

Sorry, can't put stickers on the boat. Heck, I've not been able to drill any holes in it yet.