Re-Jetting for higher elevation?????

[bigD1031]

So I am in the process of buying a boat that is located at about 1000ft elevation and bringing it up to Albuquerque where the local lakes range in elevation from 4000 to 4700. The engine is a 5.0 cobra I/O, carbed.

Do you guys think I will need to rejet the carb?

Would putting a k & n air filter on it help to alleviate running it rich?

How about hotter spark plugs?

Would it be worth it just to take it to a shop and have them rejet it? I'm pretty sure they have a pretty good idea as to how to do it as they do it for this elevation all the time.

Thanks

D

 

[achris]

Re: Re-Jetting for higher elevation?????

I don't know what OMC recommend but Mercruiser suggest re-jetting is required only above 5000 feet. Just check that you are still in the WOT range. If you are below, get the next pitch size smaller prop.

Chris...............

 

[QC]

Re: Re-Jetting for higher elevation?????

K & N would make it worse, boats only have a spark arrestor, no dust = no aircleaner . . .

 

[bigD1031]

Re: Re-Jetting for higher elevation?????

K & N would make it worse, boats only have a spark arrestor, no dust = no aircleaner . . .

Oh yeah.......duh.......

 

[gdombroski]

Re: Re-Jetting for higher elevation?????

The only change required that I've ever seen is that the timing needs to be different about one degree for every 1000'.

 

[rad1026]

Re: Re-Jetting for higher elevation?????

I'm interested in this question too because I brought my boat from about 1,000 feet up to Albuquerque also. I have never gotten a firm answer on advancing the timing. I know on my old carbureted Jeep I have advanced the timing and that definitely helped with any hesitation in acceleration. Should I look at advancing the timing on the boat?

 

[dcg9381]

Re: Re-Jetting for higher elevation?????

I've never changed timing for elevation, but do recognize that jets sometimes need to be changed for elevation changes 4-5000 feet is probably enough to justify the change.

Advice is to run the boat at that elevation - if it hesitates or drops more than 1-2 mph, then seek professional help. If anything, it'll be rich... Expect some mph drop just due to less oxygen.

 

[rad1026]

Re: Re-Jetting for higher elevation?????

If I go to WOT with a nice consistent push on the throttle it does great. But if I just push the throttle all the way down hard I do get a slight hesitation. That's why I was thinking of timing advance.

 

[Haut Medoc]

Re: Re-Jetting for higher elevation?????

If I go to WOT with a nice consistent push on the throttle it does great. But if I just push the throttle all the way down hard I do get a slight hesitation. That's why I was thinking of timing advance.

I'd be careful of advancing too much, ie detonation....
The hesitation might be the accelerator pump.....

 

[bruceb58]

Re: Re-Jetting for higher elevation?????

Typically, fuel sold at higher elevations have lower octane by a few points to account for the slower burn rate so there really is no need to change the timing.

 

[fishrdan]

Re: Re-Jetting for higher elevation?????

From what I have read, the rule of thumb for rejetting is .001 smaller for every 1000' in elevation. I run a .065 jet at sea level and .057 at 9000', it does seem to help quite a bit.

I changed from a 15x17 pitch to a 15 3/4x15 pitch prop and it helped a bit, but not much as I thought it would.

I think timing probably needs to be bumped up 1 degree for every 3-4000' of elevation. I haven't tried this yet, but going to do it on my next trip to 9000'. A previous post said 1 degree for every 1000', but that would have me advancing the timing 9 degrees at 9000', doesn't sound right? I'm going to play with 2-4 degrees of timing advance and see what how the engine responds.

Of course this is all speculation and you need to know what you are doing while adjusting the carb and timing. Ugly things can happen if the timing is too far advanced or the engine is running lean.