Dwell Vs. Altitude

[Shadowbird]

I am trying to tune my 1973 Mercruiser 888. The dwell in the manual says it should be 26-31 degrees with a point gap of .017 and timing 10 degrees BTDC. I run my boat at about 5000 ft. above sea level. Do these settings change due to altitude? I have run at these settings and it runs terrible. If I advance the timing to 30 degrees BTDC it runs pretty good. Any thoughts?

 

[freddyray21]

Re: Dwell Vs. Altitude

dwell should have nothing to do with altitude carb settings will though. You may need to lean the carbs down. at that altitude you need more air or the carbs will load up and it will run poorly

 

[Shadowbird]

Re: Dwell Vs. Altitude

dwell should have nothing to do with altitude carb settings will though. You may need to lean the carbs down. at that altitude you need more air or the carbs will load up and it will run poorly

Thanks. I rebuilt the carb. My understanding though is that I can't truly tune the carb until my ignition system is correctly adjusted.

 

[Almeja]

Re: Dwell Vs. Altitude

>"If I advance the timing to 30 degrees BTDC it runs pretty good. Any thoughts? "

1) Advance mechanism on the distributor not working.

2) Leaving vacuum hose connected while setting timing.

3) Timing mark off or using the wrong mark.

 

[bruceb58]

Re: Dwell Vs. Altitude

Your advance at idle should be 10 degrees and it should go up to 30 or so at high RPM. Is that what you mean?

 

[calwldlif]

Re: Dwell Vs. Altitude

First
Boats at high altitude are geared and
propped for performance.
Next
is this a new to you boat?
Finally
30 deg advance is close to what
total advance S/B
if your setting this at idle I suspect
your not getting any additional from
:%:$:%
dist. This is not normal.
I can't believe it will crank at 30 deg base time
Either way all tune up specs are the same regardless
what altitude.

 

[Boatist]

Re: Dwell Vs. Altitude

I agree the dwell should always be the same.

At high alitude lean the carb to get proper air fuel mixture.

You can advance the timing maybe 2 degrees.

Reduce the pitch of the prop about 2 inches and check your WOT RPMS.

Check your advance curve and make sure distributor is working correctly.

Do not expect the same power and speed at 6000 feet as it sea level. Less air and less compression means less horse power.