Badassbaja

First off I would like to say Hello Offshore Only community. I'm hoping you guys can give me some advice on where to start. First some background info. I purchased a 1994 21' Islander. It originally came with a 502 Merc. Prior to me purchasing it the 502 was removed and a 7.4 L carburated merc was installed. Over the last few years I had been keeping my eyes open for a 502 replacement. Recently I found a 2005 Indmar 502 long block for sale which I purchased, rebuilt and installed. The Indmar advertises around 420HP i believe. It was rebuilt to stock specs. It did not come with a carb so after discussing the best options with Indmar they suggested a Holley 4150 marine version (9200 I believe) I think it is a 800 or 850 CFM carb. After a break in period I opened her up and was only able to get 4400 RPM at 55 MPH. This surprised me a bit because I am pretty sure this is the same RPM I was able to get with my previous 7.4L 310HP engine. I fully expected to be capable of over revving as I have not re-propped yet. I figured I would keep the 21P Rev 4 prop on so as to make it easier on the engine during break-in. Now I know an easy way to tell would be go to a steeper pitch prop and see if I am still limited to 4400 RPM. What I am wondering is are my revs being limited by having used some parts from my 7.4 such as the fuel pump, coil or flame arrestor or is it the overspeed rev limiter that may be set at 4400 RPM?? It does seem to accelerate fast to 4400 and then stay steady there and doesn't pulsate. I can't tell for sure if I have a Thunderbolt IV or V ignition system but from the pictures if the distributor I think I have the V. On another not I wasn't sure what my timing should be set at. The Merc spec says 8 degrees BTDC and the Indmar says 10 degrees BTDC. I went with 10 as I am assuming the camshaft specs would take prioity over the ignition system??

Mr Maine

What intake?

28Eliminator

Assuming the motor is put together right, I would be checking a few things out to start out with. What is the total timing at 4400 rpm. I would pull all the plugs to make sure they are all firing. If you are down 1 cylinder, it would cause the low rpm's, and you wouldn't necessarily notice a miss. If those check fine I would be looking at the distributor - rev limiter. Did it come with the 502, if so you don't know if it was working right when you got it. After that I would be checking out compression, or a leak down test to see if there are any problems. The prop will limit it some and you won't see your power increase until you change it, but it sounds like it's running ok and should rev upto 5000rpm without any problem, although I have no knowledge of Indmar setup/rev limiter.
Check a few things out and post back what you find.

US1 Fountain

iTrader: (1)

Put a timing light on it and hold the RPMs up to 3500 and see if the timing stays up at full advance after a few seconds and doesn't drop back down to your initial advance of 10

blue thunder

If you do have a tbolt V you need to put it in base timing mode to set the timing. There is a purple/wh wire to be grounded to get it into base timing mode. How did you set your timing?

Badassbaja

Edelbrock Performer I think.

Badassbaja

I am not sure if it has a rev limiter. All the wiring is from a Merc T-Bolt. I was just speculating that is what may be causing an inability to over rev. If I put a lower pitch prop I can rule that out.

Badassbaja

I will try that thanks!

Badassbaja

It was built by a reputable engine builder. So if I pull the plugs the one not firing would look like...new??

28Eliminator

It might be like new or it could have color but be wet with gas. If they all look about the same color, and are dry then that is what you want. That's probably not the problem, but is an easy thing to check. Also double check the firing order, and that the wires are connected good. a friend of mine took his boat in to get tuned up this spring and it would not even get on plane. I found two spark plug wires backwards, I swap the wires around and it runs like a top.