RPM difference between engines
 

I have a set of slightly modified Merc 420's and at the top end there is a 200 rpm variance between engines. From idle to 4700 rpm's the engines are the same but when I push it to WTO I get 4900 out of one engine and 5100 out of the other. Both engines are timed at 34 degrees but I have a feeling that the slower engine might be slightly off. I've been told that the timing marks are not 100% accurate. So just curious if I should bump the timing up 2 degrees and see what happens or should I look elsewhere for the issue?

i would try it,if you see no change,time it back to 34.

I dont think its props because the rpm would be different at all rpm. Also, I dont believe it to be ignition related, because it should show a difference say at 4500. I would get some liquid filled quality fuel psi gauges, and have a buddy watch the fuel psi when you go wot. Electric gauges suck. I just went thru this on my setup. Engines totally equal to 4000, 4500, then as i went wot, one engine 4500, the other 5000. I was loosing fuel psi on one engine due to a fitting on the inlet side of the pump, sucking air. No fuel leak, but just loose enough to cause a problem. Tightened fittings up, when back for a ride, both engines 5600Rpm.

When it was stuck on 4500, engine ran fine, just wouldnt pull.

Thanks Joe- I'll go over the fitting and make sure everything is tight. How are the new engines running what kind of speed are you seeing?

Be a good idea to put a fuel psi gauge on and make sure you have good fuel psi .

5600rpms, 88mph. Have one other set of props I Wanna try. I think 90 will happen! They are running good though. Oil temps, Oil psi, water psi,etc all good.

Sounds like you hit your goal! The difference between my old engines (63) and now (70) makes the boat feel much better. 90 MPH would be even more fun. I may have to twist your arm for a ride in it when I make it to Chicago!

So you gained 7mph??? You had 454 365HP mills before right? How do you like the cams?

Gimme a shout anytime your in town...

I had 330's. I still need to have the props cut down. Hydromotive said I would gain 2 MPH or so by having them cut to 14.75". Right now my RPM's a are a bit low as well. My boat with 420's, according to Formula, was good for 69-70 MPH. So when all said and don't I should be around 74-75 so that would give me a 12-13 MPH increase.

As to the cams, I do like them. I can get them to idle down to 600-700, acceleration is great, and they sound good as well.

do a compression test. chances are just as good the lower rpm engine is a little more tired than the other...

Good deal. What pitch you running now? I'd imagine with those cams, getting the engines to 5300-5400 will be the ticket. The old 420's liked 5200, and those cams are slightly hotter I believe. Either way glad they worked out for you!