boat just wont run right in water, all out of ideas

[Shea_G]

Did an chevy vortec 350 swap in my 97 regal 2100 lsr (21' fastrac hull in board). Vortec heads, edelbrock performer intake and holley 660 cfm carb(tried a mercruiser 750 cfm carb before and did the exact same thing) with delco est voyager ignition system. Its runs great on land, starts right up and idles nice. Revs up to 5500 rpm no problem. When i get it on water itll run just as good until half throttle or about 3500-4000 rpm. After about 3500 rpm it wont go any faster and itll still rev only up to 4200 but it'll go slower at high rpms. I can do 65 kph alone but with 5 ppl it wont plane and i can do about 20. Im running an alpha leg with 1.84 gear ratio and a 21" pitch 4 blade prop. My fuel pressure is steady around 6 psi. The timing is set to 12 degrees idle and advancing but only up to 26 degrees but that is because the delco voyager has a pretty flat timing curve so i know it is still functioning as it should. And i know the prop isn't spinning, checked multiple times. Im all out of ideas at this point. If anyone thinks they can figure it out id be very grateful. Been to the lake 5x trying new things and never got it to run right and summer is half over.

 

[Chad Flaugher]

Have you tried a different prop pitch?

 

[Shea_G]

I tried a 3 blade 21. No difference. This engine ahould be able to spin that prop no problem with that ratio. The characteristics aren't that of a wrong size prop i dont believe. It should over rev if anything.

 

[BidD]

If it's powering out under load, have you checked your fuel filters, fuel quality and air intake? Plugged filters can give you the problems you are describing. Might also be a vacuum leak somewhere on the intake system. Hopefully its something simple. Not sure how the drive works, have to let the professionals answer that one.

 

[Shea_G]

Haven't "checked" the fuel filter but thats why i put theb pressure gauge in. Maybe its worth pulling and inspecting. What would i be looking for? Im running new 94 octane gas. I've ran it with no air cleaner and it made no difference but usually use a edelbrock foam one. Haven't checked vacuum issues but its a mechanical carb.

 

[wahlejim]

Is your fuel tank venting properly? Anti-siphon or vent tube malfunctioning? It definitely sounds like it is starving for fuel under load.

 

[Bondo]

Maybe its worth pulling and inspecting. What would i be looking for?

Ayuh,.... Water, 'n Crud,.....

Has the carb been tuned to the motor,..??

I don't do Holleys, so I ain't no help,....

Sounds like a carb/ fuel delivery issue though,....

 

[Shea_G]

It does sound like a fuel issue. No it hasn't been but I was on a marine 350 before and before this carb I was using a different one and when I swapped them it made no difference. I'd think if the tank was the venting it'd kill the engine. I had a quad that did that before and would run great for a couple minutes then it'd die after the vacuum became to great. Doubt it would affect it only at a certain definitive place in the throttle

 

[Shea_G]

O and I did a compression test today and all cylinders had 200 psi give or take 10 so that's out the door.

 

[NHGuy]

Are you sure it's a 1.84 drive and not a 1.48? If it's a 1.48 you'd have a normal drive for a 350 engine.
Have you tried trimming the drive up a little at a time when at 3500 rpm? Once you do that the engine should gain some rpms and the boat some speed. Also it wouldn't hurt to confirm speeds, you could do that with a gps app on a smart phone.
What do you think that boat weighs? My 4000 lb 22 footer is right in the center of it's correct wide open throttle rpm's with a 19 pitch propeller. The drive ratio is 1.5. It's engine is a 1997 350 mag with the factory Webber AFB carburetor. I forget what the CFM rating is but it's a 4 barrel with demand style secondaries.
Are your tabs down or up? You want them retracted for the last few hundred rpm, or maybe just a few degrees down, but not much. You have to figure that out by trial and error.

 

[Scott Danforth]

I am going to go with a fuel delivery issue.

 

[skippy2235]

I am thinking something timing?
I am not a pro, but most V8, run about 34-36 deg timing when under load. mine is at 36 deg.
24 i think is low.

 

[Shea_G]

Are you sure it's a 1.84 drive and not a 1.48? If it's a 1.48 you'd have a normal drive for a 350 engine.
Have you tried trimming the drive up a little at a time when at 3500 rpm? Once you do that the engine should gain some rpms and the boat some speed. Also it wouldn't hurt to confirm speeds, you could do that with a gps app on a smart phone.
What do you think that boat weighs? My 4000 lb 22 footer is right in the center of it's correct wide open throttle rpm's with a 19 pitch propeller. The drive ratio is 1.5. It's engine is a 1997 350 mag with the factory Webber AFB carburetor. I forget what the CFM rating is but it's a 4 barrel with demand style secondaries.
Are your tabs down or up? You want them retracted for the last few hundred rpm, or maybe just a few degrees down, but not much. You have to figure that out by trial and error.

I'm sure it's 1.84. It was a v6 boat originally. Just did the swap. I haven't tried trimming it up but it's not really a fine tuning proble. There's something seriously wrong. I can't plane with more then four people and before with the v6 which was a very tired v6 I could plane with 10.

 

[Shea_G]

I am going to go with a fuel delivery issue.

Yea that's what I'm thinking too, where do I start? Carb is working fine I think, fuel pressure is good (haven't checked under load because gauge is near engine obviously ) I've tried two carbs and there was no change

 

[Shea_G]

I am thinking something timing?
I am not a pro, but most V8, run about 34-36 deg timing when under load. mine is at 36 deg.
24 i think is low.

Well it's a vortec to ideally it should be 32, but like I said I'm using the voyager ignition system which has a flat timing curve so I either total time it to 32 and have a 18 degree idle timing or have 12 idle and 26 total which is what I'm doing now.

 

[NHGuy]

Make sure there are no kinks in the fuel line. Suppose you could add 6 degrees to the timing, but then it would be kind of advanced at idle for a normal cam.

 

[skippy2235]

While on water with engine running, move the timing around and test it out, see what happens.

 

[Shea_G]

Make sure there are no kinks in the fuel line. Suppose you could add 6 degrees to the timing, but then it would be kind of advanced at idle for a normal cam.

Yea even 12 is kind of high. It's set to manufacturer specs.

 

[mr 88]

What is the stock carb for your motor without the mods. I was talkiing to a engine rebuilder the other day and he told me there is a big difference between marine carbs and what you might throw on a car. Another guy I know did the Holley/quadra jet switch and lost 10 mph alog with a significant rpm drop. He has called the Holley tech line and got no response from them. I would try to find a 350 magnum mercruiser stock carb and work from there. Your first post you noted you had a 750 cfm and went to a 660 but didn't say why you did that or exactly what the 750 came off of or if it was rebuilt etc. I would be looking at your carb situation before going any further.

 

[Shea_G]

What is the stock carb for your motor without the mods. I was talkiing to a engine rebuilder the other day and he told me there is a big difference between marine carbs and what you might throw on a car. Another guy I know did the Holley/quadra jet switch and lost 10 mph alog with a significant rpm drop. He has called the Holley tech line and got no response from them. I would try to find a 350 magnum mercruiser stock carb and work from there. Your first post you noted you had a 750 cfm and went to a 660 but didn't say why you did that or exactly what the 750 came off of or if it was rebuilt etc. I would be looking at your carb situation before going any further.

It's a auto 350 so stock carb is irrelevant. I swapped carbs thinking maybe that was the problem but also because 750 is too big for a 350. This is a auto carb on an auto engine appropriate for a 350. I'd see some tuning be in order but not this bad I wouldn't think and it's ONLY under load. Also swapping carbs made no difference so it'd be a big coincidence if both carbs had the same problem but if there's any way to rule out a carb problem I'm ok with trying it.