Mechanical secondary opening ratio

« **on:** July 28, 2015, 04:37:13 PM »

I have a carb tuning question for the group. I am running a tunnel ram with QFT 750 mechanical secondary carbs. When I bought the carbs the linkage between the primary and seconarys was set up with a 40% ratio (primary's open approximately 40% then secondary's start to open. The carbs also came with a 60% arm and a 1 to 1 arm. Has anyone ever played with this? The boat runs pretty good but like everybody else I'm looking for more. Any help would be much appriceated. Thanks.

« **Reply #1 on:** July 28, 2015, 04:48:07 PM »

You want more cruise economy? How does it run throughout the throttle position range? Do you have any way you can check AFR from cylinder to cylinder while at part throttle? I don't see it influencing WOT horsepower as long as all butterflies are completely opening.

« **Reply #2 on:** July 28, 2015, 04:54:26 PM »

It's a big day cruiser and I have a little issue out of the hole. At about 1/3 throttle it will pop 2 or 3 times when the motor has a good load on it. Once I'm on plane I have no issues anywhere in my throttle range. I have checked the adjustments on the accelerator pumps and they are good. The motor is a 557 ford 10.5 to 1 with aluminum heads. On paper the combination I'm running should make about 750 hp. I would like it to be clean out of the hole.

« **Reply #3 on:** July 28, 2015, 05:12:24 PM »

Quote from: 76biesemeyer on July 28, 2015, 04:54:26 PM

At about 1/3 throttle it will pop 2 or 3 times when the motor has a good load on it.

It's popping out of the carb?

« **Reply #4 on:** July 28, 2015, 05:18:29 PM »

Exhaust.

« **Reply #5 on:** July 28, 2015, 07:58:55 PM »

Quote from: 76biesemeyer on July 28, 2015, 04:54:26 PM

It's a big day cruiser and I have a little issue out of the hole. At about 1/3 throttle it will pop 2 or 3 times when the motor has a good load on it. Once I'm on plane I have no issues anywhere in my throttle range. I have checked the adjustments on the accelerator pumps and they are good. The motor is a 557 ford 10.5 to 1 with aluminum heads. On paper the combination I'm running should make about 750 hp. I would like it to be clean out of the hole.

If you slow the action of the secondaries wroth the 60% package it will likely clean up the transition, it sounds like you have a lean condition happening as the venturi velocity starts to fall.

Does this carburetor have a power valve?

Do you know which one is in it?

GT

« **Reply #6 on:** July 28, 2015, 08:07:26 PM »

They do. I would have to look at the spec sheet to tell what size they are and unfortunately I'm out of town. I was talking to some gear head friends and we were thinking I should try the 1 to 1 setup but we aren't sure. It doesn't take that long to change I guess. It can be done while floating?

« **Reply #7 on:** July 28, 2015, 08:11:46 PM »

Quote from: 76biesemeyer on July 28, 2015, 08:07:26 PM

They do. I would have to look at the spec sheet to tell what size they are and unfortunately I'm out of town. I was talking to some gear head friends and we were thinking I should try the 1 to 1 setup but we aren't sure. It doesn't take that long to change I guess. It can be done while floating?

I think that will turn it into a pig. Could be wrong.

GT

« **Reply #8 on:** July 28, 2015, 08:55:36 PM »

I'm going to the lake on Friday. I will post the results. It should take long to swap out the linkage.

« **Reply #9 on:** July 29, 2015, 02:57:45 PM »

I used the 60% arms on a single 950 quick fuel I had cause I had a little bit of a dead spot between the primaries and the secondaries... Man what a difference!! The little carb woke right up!! Pretty sure their is 3 or 4 holes on their as well you can move the rod to once you find the one it likes then you can really dial the throttle response in even more...

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Mechanical secondary opening ratio

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