Do I need a new prop or something else?

[HeavensFence87]

Hi all I purchased a 1985 Reinell 192 Sunchaser with a 260hp 5.7L gmc motor this year and have done quite of bit of work on it mechanically. I've changed spark plugs, wires, points, distributor cap, rotor, and I rebuilt the carb. My boat gets up to 43 mph which is plenty fast for me but it only gets up to 4100 rpm wot with a light load. The seloc manual recommends 4200-4600 rpm at wot. My prop is a 3 blade stock 15 × 17 pitch aluminum prop, it does have some minor dings but nothing terrible. If anyone has experience with this set up I'm wondering if I need to change props or is something still mechanically wrong that is keeping me from achieving a higher rpm. I'm thinking to drop to a 15p prop or even a 4 blade with 13p if I could find one. Any input would be greatly appreciated I'd also like to dabble with waterskiing and wakeboarding

 

[Leardriver]

Are you at higher altitude? Based on your numbers, a 15 X 15 sounds right. For a boat that does tow sports, I like to be able to hit the top recommended RPM, in your case 4600, with a loaded boat in the configuration that you are likely to be using the boat, such as 5 people and a full tank of gas at 85 degrees.
You may give up a mph or 2 on the top end, and have a much more usable and enjoyable power band in the range that you are likely to use it.

 

[HeavensFence87]

I am at a higher altitude about 5000 feet I do agree about the prop change. I'm even thinking of a 4 blade prop thats 15p but I'm worried that is a little too much prop will drop my wot RPM to 3800. I simply cant find a 4 blade prop with 13p

 

[alldodge]

Being a mile high, I wouldn't go to the 4 blade, just try the 15P 3-blade. Being at 4100 your not far off

Have you chage the fuel settings do to high altitude?

 

[HeavensFence87]

I have rebuilt my quadrajet carb, other than tuning the carb so it runs smooth at 800 rpm while in the water I havent adjusted it. What other settings are there? If this isnt the right spot to discuss that plz pm me

 

[jimmbo]

What is your outdrive's gear ratio? if it is something like 1.5:1 a prop pitch of 20 - 22 would sound about right, if closer to 1.4:1 an 18 - 20 would be closer. A ratio of 1.6:1 would require a prop more along the 22 - 24 range. However, if the engine is not in proper tune or is poor mechanical shape, the will put out less power than normal and lower than normal pitch props to raise the rpm will get chosen.
What is the compression? Is the timing advancing as the engine speeds up?
The Secondaries are opening as you wouldn't get much over 2500 - 2800 if they weren't. Whether or not the air valve above the secondaries is opening enough is unknown. As a 350 turning 5000 would never fully utilize all the airflow a Q-jet can supply, yours at 4100 is certainly not going to need the air valve to be fully open.
One thing often missed. When the throttle is at WOT, are the throttle valves, primary and secondary opening fully? The choke has to be fully open or an interlock will prevent the secondary shaft from opening. When I say Secondary, I don't mean the air valve. I mean the actual throttle shaft. Sometimes the linkage/cable is not adjusted/anchored correctly and the carb doesn't open fully

 

[HeavensFence87]

So I watched the secondaries and bothers the throttle plates and air valves are fully opening a WOT. My gear ratio is 1.41 according to seloc manual. The choke is fully open at WOT as well. I did have an issue where my throttle was actually installed too low on the side of the boat so it interfered with the fiberglass ledge before reaching WOT after moving it up an inch or two I went up from 3700 to 4100 rpms, now the carb is fully open at wot. I'm wondering if it's possible I have some crud on my anti siphon valve or something. My timing seems to be advancing properly I should get s light to make sure bit there are no misfires at WOT

 

[HeavensFence87]

By the way I really appreciate all the help.

 

[Maclin]

If you only boat at that mile high lake then I would get into the carb and lean the jets out. But record what they are now...

 

[Sea Rider]

5K ft of altitude it's not that wow altitude to worry too much, we boat at double or triple from that altitude. If your engine has a "manual time advance distributor" can advance the timing + 3° from current one if was set to work at sea level + a less pitch 3 blade prop to pull wot revs up to at least middle wot revs or max rpm range factory stated. Use the least fuel octane available to work better along + time advance and new prop.

If posible to move distributor, mark it before hand so it's placed back again same when at sea level boating.

Happy Boating

 

[HeavensFence87]

I absolutely never sea level boat I live in Utah and the lake I frequent most is 4500 feet above sea level. I'm not very knowledgeable on how to adjust metering rods either. Is there any video to watch on how to accomplish this? I think I'll stick with sea riders advice for now and prop down and see where that takes me.

 

[Sea Rider]

Yep with less pitch prop from current one combo will perform much better if plan doing no other thing by the moment.

Happy Boating

 

[jimmbo]

On a Q-jet there are lots of adjustments that can be done to tweak it for altitude. Not just jets and metering rods on the primary side and metering rods on the secondary side. Power piston springs were available in several calibrations, the air valve opening rate is adjustable, deeper in the carb are airbleeds to mess with. Plenty of things that can be changed/modified, so that someone who DOES NOT have a thorough understanding of the design and engineering of the carb, can do a lot damage in very little time.

 

[Sea Rider]

The issue dealing with high altitude issues is that will never, ever make any engine to perform same as when at sea level as you can't possibly add more non existing oxygen to the fuel/air mixture for top combustion, unless you build some sort of a device that will inject oxygen in amount accordance of the butterfly/throttle opening.

The higher any engine works the worse it will perform specially if it was adjusted to work top at sea level and then taken to a high altitude level. A matter to test a less pitch prop, say a 17 if you already have one, anyway will rev less that at sea level when at wot as loaded.

Happy Boating