Help with prop selection

[jwoodall912]

Not sure if this is prop question or an engine question, so I am posting it here.

Boat - 1990 21' Baja 216 DSR - 4.3LX Mercruiser w/ 1.81 ratio Alpha One​

I have this boat for ten years and it came with a 19P Quicksilver S/S Silverado prop. I do not know how the boat ever ran prior to me owning it. Recently, I rebuilt the carb and replaced everything on the ignition system except the Thunderbolt Ignition Module. I also had a brand new outdrive replaced with a 1.81 gear ratio.​

At WOT, I am only reaching 3200 Rpms and 32 mph. I have read several other posts with this issue, but could not find any one of them with my same boat. I have taken it to two different mechanic shops. The first adjusted the carb a little and said everything was fine. They replaced the 19P S/S prop with an off brand aluminum 17P. I have better planing with the 17P, but still only reaching 3400 Rpm and 32 mph. The second shop completed compression test (150 psi) and checked the timing. They said that everything appears fine with the engine. I was told that the boat is under powered with the 4.3LX engine and that with everything running fine, he feels that the prop needs to be changed. This would require me to drop down to a 13P or 14P. With all the other posts that I have read, I have seen that most people with my same engine and boat size are rocking 19P S/S props and are getting the 4400-4800 RPMs w/ between 40 and 50 mph. ​

Should I be dropping down to the 13P or 14P prop to get the correct RPMS or is it thought that there is still another issue with the engine that is stopping the RPM from hitting correct range? I am trying to find someone with this similar boat and engine to check what needs to be done.​

 

[QBhoy]

This doesn’t sound like a prop issue. If I was asked blind what prop should be on this set up, I’d naturally say a 19 or 21”.
At first read, I was thinking your tach must be our but if you are only getting 30 something mph GPs then something isn’t right.
I think you maybe need to start looking at the fuel system.

 

[QBhoy]

And ignition system too.

 

[jwoodall912]

The fuel system has a brand new fuel pump and does not appear to have any flow issues. The ignition system has new cap and rotor, coil, plugs, and wires. Timing was confirmed to be correct also. I can go back through the system myself and double check things. I was more of trying to find out if it sounded like an engine issue or prop selection issue. My inclination is that it is still an engine issue, even though the engine is running good.

 

[QBhoy]

The fuel system has a brand new fuel pump and does not appear to have any flow issues. The ignition system has new cap and rotor, coil, plugs, and wires. Timing was confirmed to be correct also. I can go back through the system myself and double check things. I was more of trying to find out if it sounded like an engine issue or prop selection issue. My inclination is that it is still an engine issue, even though the engine is running good.

Ok. Really puzzling.
So. Sorry to ask, but are you trimming up when at wot ?
maybe not the issue because I’d naturally think that even if you weren’t...it wouldn’t have such a detrimental effect. Perhaps it might. Having though about it. Am I right in thinking your boat will be 21ft or so ?
Baja are usually quick hulls and even with the 4.3 and 1.8 ratio. I’d still naturally think a 19” wouldn’t be far off. To imagine this is just a prop issue seems crazy to me. If it was, as you’ve mentioned you would be away down to almost unheard of pitch numbers for such a thing.
Is it defo a 1.8 ratio ? Just trying to think of an answer for you and maybe thinking that it would maybe be more likely for a 21 ft baja to perhaps came from the factory with a bigger engine. Perhaps a 5.7 and a 1.47 or 1.5 ratio. This would explain things if that ratio was mated to a later change to a 4.3 ??

 

[jwoodall912]

Yes, I am trimming up. It is a 21ft Baja.

I just had the entire lower unit replaced, so I am sure that it is a 1.81, and that is what came off of the boat also. With the original lower unit and the new lower unit, it was the same thing. From what I know, my uncle was the original purchaser of the boat. I am not aware of any engine replacement on the engine through its' life.

When I first got the boat (ten years ago) it had sat in storage for 3 years or so. I got it back up and working at that time, but it was still only running 3400 at 32mph. I was unaware that this was actually a problem up until recently when I started having other engine problems. Then I saw in the book what the WOT range was supposed to be, and I started down this path of trying to figure out the problem. The engine started pulsing badly about a year ago and I rebuilt the carb (stopped the pulsing). I also did all the ignition work. After the carb was tuned recently, it runs great. Better than it ever has during the 10 years that I have owned it. It idles great and even runs great at the 3400 RPM. I just don't get anything else out of it after it reaches the 3400 rpm. I have seen the big list of items to check (12 things or so) that people always post for the loss of engine power at WOT. I am working my way down that list now.

 

[QBhoy]

Does the throttle remote allow the actual throttle on the engine to open fully ?

 

[jimmbo]

You said the outdrive was replaced, and you say it had a 1.81:1 and that since there is no change in performance, you think the replacement is the same ratio?
Have you checked to see if you have a plugged up exhaust on one or both sides of the engine? Sometimes the backflow preventers break loose and wreck Havoc with the exhaust.

Being an LX, I assume it has a 4bbl carb? If so, Is it a Q-jet or a Weber/Carter? Are the Secondaries opening? A Q-jet can supply enough air on the Primaries as small as they are, to permit a 4.3 to turn 3200 rpm. Both the Carter and the Q-jet have Mechanical Secondaries and an Air Valve above/in front of the Throttle Valves
On the Q-jet, the Secondaries are locked out by an interlock with the choke, so even though the linkage between the Throttle Lever on the carb appears to be moving the Secondary Shaft, it is spring loaded and the Secondaries are not opening if the choke is on, or if varnish and dirt are preventing it from unlocking. Of course if the choke isn't opening completely, and I mean completely, it will lock out the Secondaries. Then of course there is the Mighty Secondary Air Valve. It too can get stuck shut from Varnish and Dirt.
I'm not as familiar with the Carter/Weber 4 bbl but it too has systems to prevent airflow thru the Secondaries until similar parameters are met

Take it to a shop that has a Dyno. Have them verify that the timing is Advancing as engine speed increases. I guess you could do it yourself with a timing light running down the lake... Also the operation of the carb can be observed

 

[jwoodall912]

I will check the throttle remote. That is actually my next thing to check on my list to check.

It has a 4bbl Q-Jet on there. Before I rebuilt the carb, the secondaries were opening up. I ran it several times with the engine compartment open to visually see this. I have not personally checked this again since I rebuilt the carb, but both mechanic shops informed me that the carb is working great, just a little tuning was needed. The last shop was supposed to check the timing advance up to the 3000 rpms, but they are only checking it in a tank, not under a load on the water. It has been hard for me to find someone that will do a water test that works on a 1990 boat. I have never checked timing before, but it may be time that I learn and check it on the water.

 

[QBhoy]

You said the outdrive was replaced, and you say it had a 1.81:1 and that since there is no change in performance, you think the replacement is the same ratio?
Have you checked to see if you have a plugged up exhaust on one or both sides of the engine? Sometimes the backflow preventers break loose and wreck Havoc with the exhaust.

Being an LX, I assume it has a 4bbl carb? If so, Is it a Q-jet or a Weber/Carter? Are the Secondaries opening? A Q-jet can supply enough air on the Primaries as small as they are, to permit a 4.3 to turn 3200 rpm. Both the Carter and the Q-jet have Mechanical Secondaries and an Air Valve above/in front of the Throttle Valves
On the Q-jet, the Secondaries are locked out by an interlock with the choke, so even though the linkage between the Throttle Lever on the carb appears to be moving the Secondary Shaft, it is spring loaded and the Secondaries are not opening if the choke is on, or if varnish and dirt are preventing it from unlocking. Of course if the choke isn't opening completely, and I mean completely, it will lock out the Secondaries. Then of course there is the Mighty Secondary Air Valve. It too can get stuck shut from Varnish and Dirt.
I'm not as familiar with the Carter/Weber 4 bbl but it too has systems to prevent airflow thru the Secondaries until similar parameters are met

Take it to a shop that has a Dyno. Have them verify that the timing is Advancing as engine speed increases. I guess you could do it yourself with a timing light running down the lake... Also the operation of the carb can be observed

Put simply...just about the best explanation of how the secondary system works, I’ve ever read.
Excellent.