32 MPH with 17' trihull with Chevy 305. Does this sound right?

To begin with, even though the engine sounds and runs great at anything above idle, I do have some carburetor issues that should be resolved shortly.

I have a 2800 lbs trihull IMP Pawnee with a Chevy 305. WOT with a decent 14x16 cruise prop is 3500 RPM and max speed is 32 MPH verified by GPS (No wind or current).

I also tried an old chewed up 14x14 non cruise prop and got 4400 RPM at WOT and the same 32 MPH verified by GPS (No wind or current).

The engine does not sound that much louder at 4400 as it does at 3400 and it sounds much louder than when running at 3000 which give me 26 MPH on the 14X16 and 24 MPH on the 14X14.

The numbers above make me suspect a bad Tach, but I don't know what kind of speed this boat should be making. 32MPH for a big V8 in a little tri-hull seems slow to me.

This boat has plenty of torque and acceleration. I was able to get it and a second boat up on plain while towing it back to the shore. Pulling a person on a tube doesn't even phase it. It's the top end that's I believe is lacking.

Engine info:
1982ish Chevy 305 HO with 4MV Q-Jet and stock intake bolted up to an OMC Stringer 800 with a 14X14 / 14X16 prop

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

Get behind the tack see where the switch is for 4-6-8 cylinders cycle the switch back and forth and set it to 8 cyl.

Then if you want you can get a shop tack and verify the tack.

Now your motor should be running wot at between 4200- 4600 rpm.

Thats where the prop comes in, you need the prop that will let you run between the rpms.

For example 14x15 prop lets you run 4400 then it would be the one. Speed is secondary, i know you want to go faster and and i believe it should.

Now i have a 70 hp trihull in my signature i do with 2 people 35 mph no problem.

This all is assuming you have no water in the bilge, or hull the motor is tuned correctly, and you verify your tack, then 40-45 should be easy for that motor and that boat.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

17' boat weighing 2800 lbs? Man that's heavy! Is that from a brochure? or you weighed it?

My old 17 footer has a V6 and does 52 mph, but it weighs 1740 lbs

Those even pitch props like 14 and 16 were usually the pin on type only used on the old electric shift drives. 1964-1977.
800 series drives made 1978 to 1985 use splined props and usually have odd numbers for pitch like 15 and 17.

Definitely compression test the engine. Then do all the usual tune up stuff - fuel filters, plugs, cap and rotor, point set....

top end rpm issues are usually fuel system related.
Check that the carb butterflies are opening all the way with the linkage.

fuel filter, tank vent, anti-siphon valve

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

I'm sorry, the boat is an 18 footer. With the Chevy small block the boat is supposed to weigh 2600. Add 40 gallons of gasoline and your up to 2840. Add myself and a friend and we're up to 3200 lbs. This boat also has VERY high sides. I'm 6' 2.5" and the freeboard is about as long as my arm length. I have to lean over the sides to touch the water while still and I can't touch it while on the plain. Another interesting characteristic with this boat is how high it raises when it gets on the plain. It goes high higher than any other boat I see at the lake.

This is an early 70s model with the Stringer pin drive and electric shift. The drive works perfectly. Last year after I bought the boat, I cleaned the aluminum saddle fuel tanks and fuel lines out because they were full of varnish. I also cleaned and replaced the gaskets in the carb and replaced the cracked cylinder heads. The previous owner had the water lines reversed after winterizing the boat and in the spring overheated the engine. I forget the actual numbers, but compression was very high on this engine.

I spent many hours cleaning the old carb. It was full of varnish and scale. I think that there is still something wrong with this carb. Maybe some passages are still clogged or something. If the carb has a fuel flow issue wouldn't the engine tend to overheat or ping at full throttle? Maybe the engine is running lean of peak EGT.

I know the carb has idle issues, but I find it hard to believe that replacing the carb will give me a lot more top end when the engine is running pretty everywhere but idle.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

at 3200 lbs, I would say high 30's is about all you would expect for speed.
Weighing the boat might be a good idea or if you have never had any of the floor out, using a hole saw under seat bases and seeing what the floatation foam looks like would also be advised.
Many old boats are carrying an additional 400 pounds or so of waterlogged floatation foam.

A carb that isn't getting full fuel flow, acts like someone simply is backing off on the throttle under load. Very much like you describe.

Running in this "leaned out" condition can overheat the valves in the cylinder heads and eventually melt piston crowns. So, it would be good to get fuel flow verified.
And runn another compression test to make sure it hasn't already begun.

There are quite a few threads on these message boards where people have destroyed their engine by running lean.

There are kits to tee into the fuel line between pump and carb to verify that pressure is holding steady at 3-8 psi or so at WOT.

If the fuel pressure drops at full speed, there's a flow issue from the tank.

Running out of a 5 gallon outboard type tank can be a useful test too, but the pressure test is better.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

Thanks for the info. The safety foam in this boat is unlike my other boat.

First, there is no water in it.
Second, it is very very hard and does not absorb water like the other boat's foam does. The safety foam in my other boat is soft.

I know that I am getting fuel all the way up to the carb. I completely clean everything up to that point. I also replaced the fuel pump and fuel filters. I also have two tanks to switch from. Both act exactly the same way. Before I did so the fuel filter in the carb would plug up after about 30 minutes and I would not be able to make full power. When the filter was clogged I had about 10 seconds of full power and then it would start to die unless I lowered the throttle.

This is not the same problem I have now.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

Next place I would go is the advance mechanism in the distributor. If the flyweights and springs are rusty and gummed up, they wont advance the timing. Common issue on boats due to dampness of the bilge.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

rs2k said:

and replaced the cracked cylinder heads.
.....but compression was very high on this engine.

Click to expand...

hmmm. could you have 305 heads now on your 350 by mistake?

A marine 350 should be a low compression motor.

For the whole 15 minutes it takes. I'd run another compression test.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

Boats and Personal Watercraft
1973 IMP BOATS
IMP MARINE
PAWNEE
Stern Drive Power Boat

Length: 18'Model Name/Description: PAWNEEBoat Type: Stern Drive Power BoatHull Material: FiberglassBeam: 7' 8"Engine: 1
165 HP
GasolineNet Weight: 2600


http://www.nadaguides.com/default.a...&m=0821&y=1973&ml=I&gc=MR&gtc=MR&d=1600092376


Sound's like your timing is a bit off as Howard has suggested...big old boat there and it has enough pop to dig out of the hole. Yet it runs weak at the upper rpm, is that what i am reading? Maybe a trip to the inboard threads, lots on good advice can be had there.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

I'll check and see what the advance is like at higher RPMs. What should it look at at what RPM?

You're right Tail_Gunner. This boat has no problem getting out of the hole. It got itself and a second boat out of the hole last year. You can't even tell when there's a tube or two behind it. It's just seems to be week in the upper RPM range.

I have a small and light 17 foot trihull with an old 100 HP evinrude. That thing planes just about instantly and has a top speed of 31 MPH. That one could really use a higher pitched prop though. This big 18 footer take a few seconds to plane, but it never has any problem doing it.

Maybe I'm just not used to this larger boat, but another issue with this large boat is that it does not want to go any slower than about 18 MPH. It starts to plane at about 7 MPH and gets fully on plan at about 18 MPH. That's not a problem, but it won't do any speed in between. It want's to go one way or the other. Getting it to do a speed in between takes constant throttle management. The small 17 footer I have will do just about any speed I want. It gets a little sensitive around 5 - 10 MPH, but it will do it when I find the right throttle position.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

can you find a old 18-20 pitched prop and give it a run?

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

I thought about that which is why I got the 16 pitch prop. It originally had the 14 pitch prop. The only really difference I saw between the two was the 16 pitch prop cruised faster at a lower RPM. They seems to get on plane the same and had the same top speed.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

rs2k said:

I thought about that which is why I got the 16 pitch prop. It originally had the 14 pitch prop. The only really difference I saw between the two was the 16 pitch prop cruised faster at a lower RPM. They seems to get on plane the same and had the same top speed.

Click to expand...

"It originally had the 14 pitch prop. The only really difference I sawbetween the two was the 16 pitch prop cruised faster at a lower RPM.
---------------------------------------------------------------------
Yup it did exactly what it was supposed to do

Your more than probably way under pitched..just a shot in the dark would be 21p 3blade alum..

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

I think you may be right, but I think I also may have some kind of engine issue which is preventing my engine from making full power. I hope to know more in about a week.

Re: 32 MPH with 17' trihull with Chevy 305. Does this sound right?

rs2k said:

I'll check and see what the advance is like at higher RPMs. What should it look at at what RPM?

Click to expand...

Take the points plate out of your distributor and look in with a flashlight.