Low speed at 4k rpm

[slimmons1998]

I have a 1983 Glastron, 3.8L V6 changed from a 2bbl to 4bbl carb. I took it out on the lake to test it out and only get about 25mph at 4k rpms. What could be causing this? I know it should be going faster at that rpm. The prop is new so that cant be it. Any ideas???

 

[Don S]

Re: Low speed at 4k rpm

Wrong size of prop. Even new, it could be the wrong size.

 

[achris]

Re: Low speed at 4k rpm

I have a 1983 Glastron, 3.8L V6 changed from a 2bbl to 4bbl carb. I took it out on the lake to test it out and only get about 25mph at 4k rpms. What could be causing this? I know it should be going faster at that rpm. The prop is new so that cant be it. Any ideas???

What speed was it doing before the changes? Also more details on the props (previous and new) would be quite useful.

 

[slimmons1998]

Re: Low speed at 4k rpm

the prop says 14 1/2x19. I bought the boat and this is my first time taking it out since then. The guy I got it from said that it ran rough so I converted it over to a 4bbl and this was from the test run. take off and wot is fine at 4k rmps. He changed the prop before I purchased the boat. How do I find out what prop I need if this is the wrong one?

 

[achris]

Re: Low speed at 4k rpm

4,000 at WOT?

You need to get a much SMALLER prop... You need the WOT revs to be between 4400-4800....

Chris.........

 

[Don S]

Re: Low speed at 4k rpm

The first thing you have to do is find out what your WOT rpms are. That means open the throttle wide open, as fast as it will go, and see what the rpm are. From that information, you can adjust the prop size to get the proper WOT rpm. Thats how it done.
I will move this over to the prop forum. For more details.

 

[slimmons1998]

Re: Low speed at 4k rpm

So I need to adjust the carb more but shouldnt the boat be faster that 25mph at 4k rpms?

 

[Bondo]

Re: Low speed at 4k rpm

So I need to adjust the carb more but shouldnt the boat be faster that 25mph at 4k rpms?
1983 Glastron, 3.8L V6

Ayuh,... Not near enough Info,.... length, drive,..

 

[CharlieB]

Re: Low speed at 4k rpm

Before you make the assumption that the prop is wrong, start making a few checks and simple tests to be sure that you have everything you need to make a decision.

First, don't assume anything.

Now, with the engine OFF, shift into Forward gear and open the throttle fully. Go pull the spark arrester off and LOOK at the throttle plates, are they wide open? While you are there, what size spark arrestor do you have, the four barrel model is taller than the two barrel or it may not pass enough air for the motor to reach full speed under load.

Pull the spark plugs and look at them, are they new, crusty, fouled? How much gap?

What is the compression on each cyl?

Is this a 'points' style ignition? How 'hot' is the spark?

How 'old' is the fuel filter?

Any of these will limit power and can make an engine under perform, falsely causing you to assume the prop to be wrong.

Verify that the engine is capable of performing THEN retest on-the-water to verify WOT RPM before making any prop decisions.

 

[Don S]

Re: Low speed at 4k rpm

The rpms are there, possibly too many of them, the speed is what's missing.

 

[steelespike]

Re: Low speed at 4k rpm

Unless that Glastron is a real brute I would think a 3.6 should be close to over reving with a 19" pitch. If the speed is by a speedo it could be 5 mph off.
If your only reaching 4,000 I would suspect maybe not running on all cylinders.
or perhaps the carb setup is wrong.Don't assume any all is well just because its a new setup.