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Reduced Power after Rebuild

S

swhandyman

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"I rebuilt my 1974 OMC 225 307

"I rebuilt my 1974 OMC 225 307ci Chevy engine not long ago. My power has been down ever since, most notably when loaded. I had a few distractions, like a saturated float in my new Edelbrock carb, but these fixes didn't help. I finally did a compression test on all cylinders yesterday and came up with 140 lbs dry/ 145 wet on each with some small variance. My compression before rebuild (when healthy) was 165 lbs wet and this is what is stated in the manual. Could it be that the short block rebuild mechanic put in the wrong pistons? I didn't notice anything unusual when I put on the heads. I believe, but don't know, that he re-used the connecting rods."
 
I have to re-check the timing

I have to re-check the timing since I haven't looked at it in awhile. Plugs are new. I set the valves and will re-check those too.
 
"Steve, when you check your ti

"Steve, when you check your timing, check while under a load, start engine on trailer in water let get to operating temp if boat ramp isn't crowded and put into gear, then check, Also if you have an advance timing light check your total timing at 3000 rpm if possible.You also stated you have a edelbrock carb, what size is it."
 
"Steve, its me again, when you

"Steve, its me again, when you made the valve adjustment, how tight did you set the lash 1/4,1/2,1 turn.once you get zero valve lash go just 1/4 more turn, no more, will work fine"
 
"This makes sense to me, Harry

"This makes sense to me, Harry. Thanks. I believe I did the lash correctly, but will check, as well as the other items. I'll post my results next Monday. The carb is 600cfm with an Edelbrock Performer manifold -- these are common for this size marine engine. I also installed a Mallory electronic distributor with electronic advance. But the key is, will any of this get me up to the rated 165lbs compression? The valve lash seems the best bet for this."
 
"Steve, if the engine was rebu

"Steve, if the engine was rebuilt by an enginebuilder there should be a build sheet with an item list of parts that were used in the shortblock rebuild. Pistons used,rings,bearings,part #s and clearences etc. that way you could verify the right parts were installed."
 
"You are the man, Harry! [b]I

"You are the man, Harry! It was the valve lash. I had followed the manual of 3/4-turn. I also may have tightened too much before the turn because I was looking for the rod not to "turn", as stated in the manual. I was able to recover to 165lbs wet on two cylinders, and >150 lbs wet on the others (in all cases dry reading is within 5 lbs of wet). I'm hoping that a little run time will cause the >150 valves to seat better, otherwise I may have to face the fact that I burned a lot of valves. I'll repost with an update after next weekend, complete with a full set of before and after compression readings."
 
"Good shooting.
I use a compr

"Good shooting.
I use a compression gauge to set valve lash on all marine engines.
I crank the starter, tighten the nut until comp drops, then back the nut off until the rocker rattles, then I retighten the nut halfway to split the difference."
 
"Steve, Great, glad that was t

"Steve, Great, glad that was the problem and not anything more serious. I really don't know why the book calls for such a tight lash adjustment.I always go just 1/4 turn after zero lash,the plunger in the lifter will compensate for the difference and pump up for it.Now what are you running your timing at on that small block chevy, is it set at initial or total advance."
 
Harry: 10 deg at idle. Verif

Harry: 10 deg at idle. Verified with timing light that timing advances as RPM increased. Mallory electronic dist has factory settings -- I don't know what initial vs total advance is. Can you fill me in?
 
"Steve, If the timing light yo

"Steve, If the timing light your using has a dial on it that you can turn then you can set it either way. Total timing for that motor should be 28-32 degrees. With someone operating the throttle while in neut. you aim the timing light at the timing tab,set the dial on 30-32 degress,rev the engine to around 3000 rpm and set the timing to 0 BTDC on tab.3000 is used as a general #. Depending on the springs in the distributor your total may come in earlier than that or later. At a certain rpm the advance will come in completely and that will be your total it will not advance any more. A little later would be better i would think due to your cruise rpm is probably around 2600-3000."
 
"Steve, I believe the Mallory

"Steve, I believe the Mallory Unilite has the medium springs from the manufacture and at one time sent springs in the box so you could taylor your advance to your liking."
 
"steve i'm glad i helped i

"steve i'm glad i helped in a little way, " setting up the valves " Is understood to mean setting lash, wanted me to go into detail on how to do it, is in the book.

So heres how i do: with the motor running. " old school " i will endeavour in the future to be more exact in my opinion and try to express it in a more concise, manner."
 
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