1994 SNOB - Altitude Change Issues

[Jonny_Quest]

Howdy. New here to this forum. I'd like a little advice on a couple of issues.

 

1994 Ski Nautique Open Bow. 5.8 / 351 PCM EFI Pro Boss. (Don't know if this is what some refer to as the GT-40) 1:1.23 PCM transmission. 625 hours. I've owned the boat for 5 years.

 

Issue One. I recently moved from the Houston, TX area to the Salt Lake City, UT area. In Texas, I ran an OJ Legend 3 14X16 prop. W-O-T was about 4,600 RPMs. Boat really ran well. Fast forward to Utah and altitude. I'm typically running at 4,200 feet - with occasional trips up to 5,500 feet. When I'm running at idle for anything longer than 10 seconds or so, the engine seems to "load up" on heavy acceleration (skier pull-up acceleration). Very poor performance. Feels like a 4 banger vs. a V-8. However, if I go to neutral and give the throttle 3 or 4 "revs" up to 3,000 RPMs or so, and then immediately go into gear and accelerate, the performance seems to be normal. Also, the exhaust is a little more "gray" than I expect and I get a clear smell of un-burned hydrocarbons. Reminds me of the muscle car exhaust smell of the late 1960's.

 

Suggestions? I've heard that these early EFI units can be a bit troublesome and that parts are extremely tough to find.

 

Issue Two. I "propped down" when I came to altitude as I know that the engine performance is reduced by altitude. Texas prop was OJ Legend 3 blade 14 X 16. Just a bit "too tall" for this altitude. 4,400 W-O-T was all I can get. I decided to use an old OJ Legend 3 blade 14 X 14 that works OK, but my RPMs run about 500 to 600 too high at W-O-T. I bought an ACME 1868 4 blade 12.5 X 14.25 (based on a recommendation from ACME). Acceleration and hole-shot is good, but too many RPMs for my liking. 600 to 700 too high. I'm looking at the ACME 422 or ACME 654. Anybody have experience with these slant-back SNOBS with the 5.8 running either prop at altitude?

 

Thanks,

 

JQ

[GregDavis]

Call Skip Dunlap, He has 45 years experience with Props, and 40 years experience with Nautique. 865-605-0340

[E_T]

@Jody_Seal‌ should be along and have some good insight for you.

[DW]

At 4,200 ft you are at close to 85% of the sea level air density & 5,500 it is close to 81%. Assuming by your comments that the EFI system is not compensating for the difference, thus some external adjustment probably needs to be made or a (defective) sensor needs to be changed to get back to the correct air/fuel ratio.

 

You could take either of those props (the used Legend you already have) and add some cup to it to reduce the WOT RPM's, that would be a low cost option.

[6balls]

I think you are running the standard 5.8 pro boss which was not with the GT-40 heads. If memory serves maybe 285 hp on yours, 310 for the GT-40. I've driven the carbed 351 in another older nautique, though, and it seems quite strong, too.

[Siouxcitysmitty]

FWIW - I had a '93 nautique with the 285 hp engine. It was carbed and when I got the Holley dialed in for my 8K elevations here in Colorado, it ran reasonably strong. Certainly more top end and hole-shot than my current EFI '04 Malibu RLXI with high altitude prop.

 

Memory is fuzzy, but, around 44 at WOT. Think my prop was stock, and, sorry, don't remember RPMs at WOT.

-

Best of luck figuring it out, and, once fixed, it should run fairly strong IMO, at those elevations.