Playn

I have a 1990 7.4L that just recently could not get over 3400 rpm out of it. The engine has 526 hours on it. A couple years ago the compression ran as follows (1) 144, (2) 141, (3) 142, (4) 142, (5) 144, (6) 142, (7) 147, (8) 144

We have just done a compression test with the following results; (1) 145, (2) 130, (3) 120, (4) 100, (5) 130, (6) 115, (7) 130, (8) 130

I was going to sell it so at minimum I need to get the engine right. If I keep it I would like some more hp and speed. The boat is a 1990 Baja 250ES. It weighs about 4000# dry and usually runs about 56 at 4600 with a 21p. I don't want to put a ton of $ in it either way. Anybody got any suggestions for the winter project?

blown formula

If you are wanting to sell the boat rebuild the engine to stock spec's. Be aware the stock Merc pistons require metric rings that are quite expensive. You could probably replace the pistons & new rings for what the metric rings cost. You do not need forged just cast pistons. I just did my 454 Mag MPI & replaced a forged piston, all new rings, sleeved a cylinder, new rod, crank & cam bearings....(only ten hours on engine when melted a piston!)
We used oversized rings and filed to fit the rings.
Total cost- $550 parts & machine work-3 days incl delivery
$1000.00 labor to pull & rebuild & reinstall. 2 days
total $1550.00 pretty reasonable I thought..... but then I spend a LOT of money overall with this mechanic & I was back in business in 5 days!! I had other issues to work out, but the engine was in and ready to go.
Save the bucks for the next boat & get at least a 502 block in it,,, much stronger & ready to modify!!

FloridianSon

I took a 7 year old salt water block and rebuilt it, Wrong big hole in # 8 cylinder wall from block failer must have be getting thin then we went 30 over.Called 1800 runsnew.New 502, replaced heads with the oval Merlins,cam change and I am up ****creek with around 450HP who need's a paddel

Offshore Addiction

of course you can rebuild it as long as there not an exceptoinal amout of corrosion,you can also replace it with a new long block,for a little more money....I would first recommend tearing the heads off and inspecting them,sounds like you might have bad exh and they've wiped out your exh valves.....very common problem.....even if theres no water in your oil.....

timucin

rebuilt!
american v8's never die ,they just go faster...

blue thunder

I second offshore addictions advice. I would do a leak test on the cylinders with low comp readings to validate the problem as valves. Depending on how you run and care for your engine, the rest of the engine could be in excellent shape and may run like new with a proper top end job.

BT

blown formula

Arizona Speed "freshened" the top end of what was supposed to be a very good 140 hour 454 Mag mpi...... end result was it is low on power now and sound like it has a miss in it...... the increased compression from the valves has now shown the amount of blow-by the rings...... now I have the expense of pulling it again and doing it all!! If your need to tear it down to do a valve job, do yourself a favor - go ahead & do the rest. Unless you need new pistons the honing & new rings (metric Merc) should only run about $500.00 more.... do it right the first time!!

Playn

Thanks for the input guys.