imq707s

I thought everything was going pretty good in the power department until I tried to pull a wakeboarder this weekend. My boat is a 1985 21' Scarab with Jasper 350, Alpha I drive, and 22P turbo 3-blade prop. My boat runs great at WOT, about 59 on the GPS at 5000rpms. I always thought that it took a little longer to plane off that it should, but pulling a wakeboarder proved it. I had three people in the boat including myself. It took about 30 seconds for the boat to plane off, but once the boat started to plane off and the rpms got over around 3500 the motor pulled really hard and then I'd have to slow down for the guy I was pulling. I know that my boat really isn't designed to pull a skier, but why is the motor down on the midrange power? I pulled the carb off yesterday to rebuild it, and noticed that the previous owner has pinned the secondary throttle blades on the Holley 750cfm 3310 vacume secondary carb. So when the primary blades start to open , so do the secondary blades. I've always noticed a bog when I hit the gas, but I just learned to ease into it and it was fine. I guess I found the cause of the bog, but would that also cause the low midrange power? The carb has 75 jets up front, and the stardard metering plate in the back ( around 78 jet size). I did a WOT runna few weeks ago and noticed that after I stoped and pulled a few spark plugs, they were bone white. Could the lean WOT mixture cause the low power while planing off? What about ignitoin timing, I think right now total advance is around 32 degress, should I bump it up to 34 or 36 and see if that gives the motor any more torque? My buddy has a boat similar in size and power to mine and it will jerk a skier right out of the water and tops off about the same speed as my boat. Thanks for the help, any tips would be great.

John B

You have discovered why 4 bbl. carbs have vacuum secondarys. Take out the screw or bolt and hook up the vacuum linkage if you can. When you open the secondarys too soon all you get is air until there is enough velocity to allow the venturis to pull fuel out of the bowl! Lean out and bog or backfire result.

Ex .carburetor engineer

Intolerant1

Once you get those secondarys working properly you also need to look up the list number for the original jetting. The list number is stamped into the air horn at the front on 4150 and 4160 style carbs. Then go to Holleys website and look up the number which will give you all the specs for jets and powervalves.

imq707s

Thanks for the help, I guess the mixture is turning really lean when all of the throttle blades open and there is not enought airflow through the motor to pull fuel out of the secondarys. I'll hook up the vacume canister and see what happens. HAs anyone had a problem with the secondarys not opening all the way at WOT? How can I tell if they are opening all the way at WOT? Thanks again for the help.

MKast

There is a vacuum diaphragm located aft of the choke linkage.
If this rubber diaphragm is not holding vacuum (rip, tear or hole)the secondaries won't open, possible reason for turning secondaries into mechanical with the screw. The spring under the rubber diaphragm regulates the amount of vacuum needed to open the secondaries, different spring pressures, different opening rpm, As I remember it, fuel is not sucked from the float bowl, positive (higher) ambient pressure on the fuel head in the float bowl pushes fuel into lower pressure area (Bernoulli"s principle) in the venturi. That's why float bowls have to be vented to outside air (ambient).
As I remember it.

imq707s

Thanks again guy for all the great help. I guess I'll hook up the vacume canister and take the screw out of the secondary linkage and see how that works. Hopfully it will just jump out of the water. What about ignition timing? Will bumping up the total timing from 32 to 34 help at all? How much is to much with 87 octane gas? How can I tell if I'm getting any pinging or detonation? Thanks.

MKast

I think you're going to have to address the "Why was the screw placed in the secondary linkage?" After you remove the screw, grab a friend, take the boat out, throttle up, see if the secondaries are in fact opening. Look into the carb with the boat under power. Are the secondaries open, if not fix this first. as far as timing, start with stock. I'm wary of increase in timing, without someone who knows what to look for.

boot

MKast , I am sure you didn't mean to actualy look in the carb while at full throttle Things tend to go BOOM in your face when there are carb problems . What I'm sure you ment was look at the secondary linkage on the side of carb and see if and WHEN it open at what RPM . DO it with the flame arester on as it will have different vaccume . never tune a carb with air cleaner or flame arester off as it will change when you reinstall it . You can buy a hole bag of different tension springs to try from any local speed shop . Just make sure the vaccume canister is working . It has been a long time since I used a vac. carb but I think you can hook a hand operated vacume pump to the canister while engine is off and see how much vaume it takes to open . YOu can rent or borrow or buy (cheap ), a vacume pump from most local auto parts stores . I beleive you want the vacume to open early but only from Vacume ,not because it is bolted together !

Intolerant1

By the way, if that carb is beyond repair, I have a 750cfm Holley with vac. secondarys for sale. Marine version for $200 just rebuilt set up any way you want.

imq707s

Actually the carb is in great shape, and it doesn't look that old. I don't know what the guy was thinking when he pinned the secondary linkage. Maybe the secondarys were not opening all the way so he pinned the linkage instead of trying to put a lighter spring in the vacume canister. I'm going to put everything back the way it should be and try is this weekend. I've got some of the lighter springs I'll have to try. Hopfully it will work great, I'll let you guys know what happens. Thanks for the help, this board is great.