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Launched it today. Question about prop

footbrake

footbrake

Contributing Member
Launched it today and everything was great. I could only get 3000 rpm w.o.t. out of it. Motor was smooth, good oil pressure, good temp , everything fine except the rpm"s. My marina says it should be able to get to 4000-4500 rpm, with the 2.5,120 hp,OMC and it could have the wrong prop on it, which would cause the lower rpm. I need an explanation. Thanks. Here's some pics also IMG_0003.JPGIMG_0007.JPGIMG_0009.jpgIMG_0014_2.jpgIMG_0018.JPGIMG_0032.jpg
 
That IS true. If..the prop is " over-pitched" it could be dragging the engine down. I lost some RPMs when I re-pitched my prop to 18 pitch from 16.
 
it's more likely a lack of power (gummed up advance weights in the distributor a probable culprit), but what pitch prop is on it? should be a 17P I'm thinking, maybe a 15P if that's a bigger boat than it looks - looks 17.5 foot or so

you want to verify the tach accuracy with a shop tach too.
 
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It's an 18.5 footer. We're gonna pull the prop tonight to see what exactly is on there. I have an extra prop that came with the boat and I'm thinking that is the right prop that he may have damaged and had fixed, and used the one on there now as a fast replacement which may be wrong, just to get him going again. Is there a set rule for the boat or the motor and outdrive as to what the correct prop is for the boat? that is size/pitch whatever else.
 
We cannot perform a WOT RPM test until the engine is in near perfect tune.

I agree with hystat...... start by checking the progressive ignition advance and total advance.
See your OEM specs for this.

No gasoline engine will perform near it's potential if the ignition advance is lacking the correct spark lead.

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Had everything checked including to see if the throttle was opening fully . Got to take the prop off it's a 13 3/4 x15. The other prop I'll have to look up it's a michigan prop with just J80 stamped into it.
 
OK... let me hold your feet to the fire here... but as one friend to another.

What is your OEM TA, and what did the mechanic see when he checked this?
All too often mechanics will set BASE advance, and look no further.

Just say'n!


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a 15 is already the shallowest prop you would ever want. You are missing some power, or the hull is waterlogged or something. I would start with a look at the advance weights/springs.
 
Yes, found out today about the 15 prop being the shallowest. tomorrow we're gona check the tach ,advance weights and springs, and set the timing at 8 deg. at idle and and see if it come in at full advance about 2500rpm
 
footbrake said:
tomorrow we're gona check the tach, advance weights and springs, and set the timing at 8 deg at idle and and see if it come in at full advance about 2500rpm
Click to expand...
If you can find your engine's advance curve, it will look similar to these Mercruiser curves..... or, it may not be in graph form at all.
Either way, you'll need to know what your OEM specs are for both Full Advance in degrees, and at what engine RPM this occurs, before doing any testing or adjustments.


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The very first thing is to verify the tach.

Then do what the others have posted.

Say everything is in within specs, i would disconnect the esa module while underway.

To shift out of gear you will need the esa module plugged in or shut the engine down.
 
Ok, everything checked out, tach,timing,dwell,advance all good. What my mechanic did notice was that the front of the engine was adjusted unusually low, which would tilt the outdrive up on the 400 stringer. He manually raised the adjustment about 1". Yesterday we took it out and I had the lever where I had it the first time out and it was at 3000 rpm again. I knew I had more lever to go so I put it down and it went to 4100 rpm and got on plane beautifully. I think the first time out I really didn't have it WOT. Now I have to practice docking and undocking in a current. It's a little nerve wracking doing this being a new boater.. Thanks for all the help everyone.
 
footbrake said:
Ok, everything checked out, tach, timing, dwell, advance all good.
Click to expand...
What was the "full in total advance", and at what RPM?
FYI, as said, checking/setting BASE advance, is checking/setting BASE advance ONLY!
It will do ZERO for high end performance if out of spec.
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Ok, the timing was set at 8 degrees btdc. Full advance came in just about 2200 rpm at 30-32 degrees. It felt pretty good there and finally got over 4000 rpm wot. Any other suggestions to tweak this a little more?
 
footbrake said:
Full advance came in just about 2200 rpm at 30-32 degrees.
 It felt pretty good there and finally got over 4000 rpm wot. Any other suggestions to tweak this a little more?
Click to expand...
If an advance such as 30-32 degrees @ 2,200 rpm continued to advance above 2,200 RPM, we could not pull this off on a SBC V-8 engine.
It would begin to detonate @ 2,200 rpm.
I know that this is not a SBC... it's a GM 120 hp 4 cylinder.

My question would be; What are your OMC OEM TA/RPM specs for this particular GM 120 engine? This is serious stuff.

Too much advance at the the wrong crankshaft angle, can and will cause detonation.
Severe detonation will cause damage.


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