I built my engines 4 years ago, have minimal time on em and now that all my other issues have been taken care of I wanna see if there is any more here for me...been running great, time to play with it because I think there is a couple more hp there somewhere...

What is a merc 420 stock rpm range??

boat is 86 Baja 320/TRS

engines were 420 and I changed a couple things... stock shortblocks with Merlin heads, 420 cams, Holley single plane intakes and HP500 800 cfm carbs timing at 34

Boat runs a best of 66.9 on gps with a standard quicksilver 3 blad repitched to 24 @ 4500-4600rpm nothing special but the boat loves these props..

runs 63-64 gps with 23P mirage plus, same with standard mirage 4500 rpm

tried a pair of 24P cleavers yesterday, got rpm to 5000 easily, boat went 63, may had a tad more but wt=ater was forcing me to not stay in it and had to work the sticks

I would have to think the boat would run 23P around 4800-5000 with these engines??? put me at 68-70 in right water maybe?

When I built engine s I verified balancers were right, I gotta look to see if I marked em to see if they spun at all, I usually do, I am not sure of jetting right now, but plan to look this weekend, looking for ideas of things to check...engine temps run cool and wonder if I get some more if that will help? usually water temps at 130, oil temps worse case is 17-180 running it hard.. never get warmer then that..

maybe try 36 degrees timing? what is stock jetting supposed to be on the 800CFM HP500 carb?

guys with 330 horsepower run 62-63 on gps with this hull and trs, I would think I have a tad more to find? maybe not...

If anyone has input that would be great...thx

Yikes, Lots of questions lol

With the 420 Cam, single plane intakes, I'd be looking for 5200-5300 RPM.

Jetting wise, I believe the stock 800 holley was like 79F 89R or something really close to that. You can actually look that up on mercruiserparts.com

I don't recall the total advance spec for a 420, but I think it was 36 or 38 total. I dont see why you couldnt run a bit more timing, esp if you keep the engine a little cooler on the water temps, and use good gas. As long as your Merlin heads havent been milled down or anything like that for more compression.

I wouldn't put more heat in the engine.

What exhaust do you have? Gil I am assuming?

Also, on your boat, have you tried spinning your props "IN". On a older straight bottom hull like yours, it might help it get some bow lift and pick up some speed. Since it likes the mirage "bow lifters", and not the cleavers (stern lifters), i'd try and give it more bow lift (turning in).

Doesnt sound too far off from where you probably should be speed wise. Bolting on some small blowers will wake it up. a pair of 250 blowers, 4 psi of boost, and I think you'll be in the mid-upper 70s.

what he said!!

Find yourself a pair of mirage + 21p or send your props out to have labbed if their mirage + props..
another fast bow lifting prop is a turbo txp and a os2, this prop is a vary fast prop..

Lester kept that boat in the water for a lot of years. That boat has some weight to it. 67 sounds good. I had 420's in that 38 Scarab I had and 68MPH was it but I think I ran 5200 at 34 WOT.

I tried spinning in once, I gained maybe 1mph, it was not worth the handling loss, this boat handles LAke Erie amazing (can be real nasty) and I like handling way better when spinning out...

I am mainly just wanting to see if people agree and have ideas, the heads have not been milled, I was gonna try 36 total but was afraid, in a different boat 2 degrees timing actually made a big difference up top, that was a single engine boat though..

I wanna get ideas so I have things to try next weekend...

keep it coming guys!!

you know the boat?

I redid whole thing besides paint since I got it, hope I can just see a couple more mph if I can..

Oh yea, exhaust is take off 496 4" I was told will easily support 500-550 HP minimal..

what props??

I have a lot of experiance with 420's
Just a few things you need to think about
Stock comp ratio is 8.8 with the stock 990 head which is 119cc
Most Merlin heads come 124cc or more
At 124cc comp ratio is now 8.4, that is assuming you used the same thickness head gasket
Next is the intake runner of the Merlin heads WAY toooo big for a 420
Single plane intake manifold will make less power under 5500 rpm than the stock dual plane high rise, more important the torque is MUCH better with the dual plane.
Last is the Jetting of the HP500 carb. It is really 830cfm but merc changes the jetting to stagger left and right in both the primary and secondary's call Holley and jet the stock jetting for that carb and leave it
Timing with that low compression 36

Known Lester for many moons. Good man. Cuts the five hairs that I have left on my head. He loved that boat. You would have a lot of fun with boost!