OMC Engine boggin issues...

drexton311

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I am almost positive that it is 100% stock. I bought the boat from the original owners family and they were meticulous about keeping it stock. So assuming it still has the factory gear ratio, do you think I have a prop issue?
 

bruceb58

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I ran a 15x17 on a 20' boat with your exact engine/drive so your prop is the correct size.
 

HT32BSX115

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I am almost positive that it is 100% stock. I bought the boat from the original owners family and they were meticulous about keeping it stock. So assuming it still has the factory gear ratio, do you think I have a prop issue?

If you have the "right" prop on it, you will need to determine if the engine is actually producing rated power. a compression/leakdown check can tell you is there's a cyl 1 or more, that's not producing enough power. Otherwise, timing and/or carburetion/fuel delivery is likely the problem.
 

drexton311

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I am going to do a compression check on it as well as check the timing. In addition, I think I will replace the plugs and see what my fuel pressure is out of the pump. I appreciate the input from you guys.
 

drexton311

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So I changed the raw water pump, serviced the stern drive (gear lube), replaced the plugs (the old plugs looked good, no funny color, etc.). I noticed that my vacuum choke on the carb will not open fully when the motor is warm, when I manually opened it, the engine smoothed out. No vacuum leaks that I could find, so that means possible low compression? Can a partially closed choke could cause a WOT back-fire? Think I?ll put an electric choke on. NOTE: I did not get a chance to check the timing yet.

So, I completed a dry compression check (cold motor).

​Compression as follows (PSI):
1 - 110
3 - 110
5 - 105
7 - 105
2 - 105
4 - 105
6 - 105
8 - 110

So I guess this means the little motor is worn out? So my next question, how can I get a couple more years out of her? Is there a timing adjustment, miracle additive, etc? Nothing leaks and this boat has been well cared for. Also, another question, should I have warmed the motor up before the compression test? I did not complete a wet compression test, I assume compression readings would have been better if I had.
As usual, thanks for all the help!
 

bruceb58

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You need to test it when its warmed up with the throttle fully open.
 

HT32BSX115

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So, I completed a dry compression check (cold motor).

​Compression as follows (PSI):
1 - 110
3 - 110
5 - 105
7 - 105
2 - 105
4 - 105
6 - 105
8 - 110

So I guess this means the little motor is worn out? So my next question, how can I get a couple more years out of her? Is there a timing adjustment, miracle additive, etc? Nothing leaks and this boat has been well cared for. Also, another question, should I have warmed the motor up before the compression test? I did not complete a wet compression test, I assume compression readings would have been better if I had.
As usual, thanks for all the help!
Yes. The engine is a little worn out. Yes, you can probably get more time out of it but don't expect much more performance.

OTOH, you could pick up a used 350 cu-in pickup engine of a similar vintage to replace the 305 and probably get many more years of service!
 

drexton311

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I think that was my mistake, I did not let the motor warm up and it was not at WOT. in your experience would I experience better PSI if competed in those conditions? I did test the compression with all plugs removed.
 

drexton311

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yes, when I go to replace the motor, it will be with a 350. I'd just order a long block crate engine and put the marine bolts ons from the current motor since they are all in good shape.
 

HT32BSX115

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I think that was my mistake, I did not let the motor warm up and it was not at WOT. in your experience would I experience better PSI if competed in those conditions? I did test the compression with all plugs removed.

The standards are with the engine having ran before the test AND with the throttle butterfly full open............But it doesn't always make a HUGE difference. You might get 10 psi or so improvement.

Your numbers indicate worn rings which would be more or less the same for the entire engine......... I wouldn't blame valves because it's unlikley ALL 16 valves would be leaking the same. Running the engine (prior) and/or squirting a little oil in the cyls would probably give you 10psi, but that would be a temporary thing.

Another problem could be a flat camshaft ............that happened to me with my previously installed 1987 OMC 460, although I had GOOD compression numbers (even without using WOT during the test) Some of the valves were only opening slightly. The engine would only turn about 4000 when it would normally do 5000 RPM.

Once you fix (if needed) any fuel (carb/pump/filter/pickup/anti-siphon valve etc) problem, get the timing correct, if the engine is still not producing enough power simply because it's "tired" you could go to a lower pitch prop to get your *some* WOT RPM back and subsequently get better performance.

Putting a truck 350 V-8 would be a very cost effective way to get a good running boat. Not sure I would spend a lot of money on a nearly 30 year old boat putting a "new" marine longblock engine in it (my 87 FourWinns was 19 years old when did the OMG460/Mercruiser 454Bravo swap)

. The difference between a new marine longblock and a wrecking yard truck engine might not be enough to notice (assuming you get a "good one" )

Cheers,


Rick
 

drexton311

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So I have an update...

Today I checked the timing and guess what?! It was sitting at 8-10 degrees AFTER TDC. So I checked the points on the distributor and there was ZERO gap at the contact. I adjusted to .017 per my OMC manual and engine settled down at 0 degrees TDC. It appears that it was purposefully set to 0 degrees as the distributor was tight, no play whatsoever. I reset the timing to 7-8 degrees Before TDC per the manual ( I run 87 octane non-ethanol gas always) and adjusted the idle as specified. The motor appears to be more awake than it has been since I bought it about a month ago! I am going to take it out tomorrow to see if there is any increase in power and hole shot. Needless to say, my dash tach is correct as my timing light has a digital tach built in, and I easily hit 4k (briefly) which is already 1k more RPM than I have ever hit without a backfire. I did not complete another compression check as I was so excited to correct this issue and if there is no performance change, I will and post results. I do have a question, with the timing being 16 degrees out; would this not skew my compression readings when I did my test?

As usual, the feedback here is world class, thanks!
 

drexton311

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Oh and one more thing, the dwell was ready 37 degrees, it is now 29 degrees as per the OMC manual.
 

HT32BSX115

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I do have a question, with the timing being 16 degrees out; would this not skew my compression readings when I did my test?

As usual, the feedback here is world class, thanks!
That would be ignition timing. It wouldn't affect the valve/cam timing. so no difference! Running an engine retarded like that would REALLY kill the HP!

Probably wouldn't hurt to check the total advance of the distributor if you have a way to do it.
 

drexton311

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Lake update:
Success! After the timing and dwell adjustment, the boat acted like a different animal. Hole shot was powerful, planed out almost instantly with no one in the bow and a full tank of gas. Power was much more noticeable, man what a difference! I easily hit 4k RPM and topped out MPH at 38. I think we can pull a skier now.
Now that she runs, I actually think the 2 barrel is hindering me a bit, the motor now feels like it wants to do more, maybe a 4 barrel is in order? There is no way this motor would run as it did with 105 ad 110 PSI in the cylinders, I must have a leaky connection in my compression kit or something. Thoughts?
 

HT32BSX115

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There is no way this motor would run as it did with 105 ad 110 PSI in the cylinders, I must have a leaky connection in my compression kit or something. Thoughts?
having the throttle butterfly closed during a compression check would give you false (low) readings.....
 

drexton311

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I plan on re-checking the compression on a warm motor with the throttle at WOT. I am also, going to use a rental gage in addition to my gauge to make sure the one I purchased is reading correctly.
 

HT32BSX115

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I plan on re-checking the compression on a warm motor with the throttle at WOT. I am also, going to use a rental gage in addition to my gauge to make sure the one I purchased is reading correctly.
You probably already know that your engine should have 150-160 PSI compression. So if you're at ~110 or so, you're already a tad worn out......according to the spec sheet at http://boatinfo.no/lib/omc/manuals/1...anual.html#/86

If that last WOT RPM reading was 4000 RPM, you're still a little low (but it's simply due to teh engine being "tired")

You can get a little more performance by replacing your prop with one of slightly lower pitch. I.E. go from your current prop if it's say, a 17p prop to a 15p prop etc...... and see if it will now turn 4600 (or closer to 4600)

If you can get it to turn max recommended RPM for the current condition of the engine (once it's tuned up as best as you can tune it) propping the drive for max RPM will get you the max available performance which will really help for holeshot (for skiing/wakeboarding etc) and most bang for buck for the rest of the summer.

Then you can consider rebuilding/replacing or replacing with a truck 350 this winter at your leisure!
 
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drexton311

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HT32BSX115, I appreciate the input. I am going to check again this weekend and if 4k is the highest I can hit, I'll look at the prop change as you mentioned.

I am not confident in my original compression test so double checking that will take place first. (You are correct, my OMC manual says compression should be 150-160PSI.) Until then, I have a leaking power steering pump to look into...
 

vasy

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Dec 14, 2014
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4 barrel would burn more gas at wide open.
Mine 302 does 4500rpm on a 2 barrel and does 50mph on the water. Sure 4 barrel might give me 2-3 more but I'm not in that much of a hurry :)

If you setup a 4V right, you can cruise all day long on the primaries, and only get into the secondaries when you push it. Generally the primaries on a 4V will be smaller than the primaries on a 2V, assuming you'll be running a 600cfm on the 400, so you'll be more fuel-efficient too at low speeds.

Just a good new carb setup will run you $500ish plus tuning...

Check and take care of that compression fist before you play with the carb.
 
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