Mercruiser 420 HP specs.
 

I just bought a 1978 23ft Seacraft Seavette. It has the Mercruiser 420HP 454. I need to do a tune up on it and I would like to know the entire tune up specs. Such as timing, spark plug type, plug gap, idle RPM, etc. Some one has apparently replaced the carb. It looks like a Holley 850 double pumper with an electric choke. The carb # is 9022. I understand that the original carb was an 800 CFM. What was the jetting and power valve size on the 800 carb? Would anyone happen to know the gear ratio of the TRS outdrive, and how much would the engine and outdrive weigh?
Thanks! Steve Burnsed

Re: Mercruiser 420 HP specs.
 

9022 is a 800cfm ., the drive is a 1:5: 1 hence 1:1 trans should be a b/w 72c ,tranny weighs 110 , drive is 150 engine is just a LS6 with a crane cam , gold rockers etc.if it has a holley someone changed the intake , if not mistaken the 420 came with a alum dual plane intake and quadrajet carb.


same question you have-
http://www.offshoreonly.com/forums/s...420+mercruiser



some came with the 800 a also
http://www.offshoreonly.com/forums/s...420+mercruiser

use the "seach" option on the right top of screen . find anything you want....

Re: Mercruiser 420 HP specs.
 

My 420's came with Holley 800s and Wiend Xcellerator intakes. The old Merc Cyclone 400 was the same engine except it had a Quadrajet and a Chevy high rise intake.

Re: Mercruiser 420 HP specs.
 

Thanks guys. It does have the Weiand single plane intake. I'm thinking about trying a GM duel plane high-rise intake just to get a little more bottom end out of it. What kind of RPM should this engine be turning? With it not running right I was only getting 4600 RPM. Right now there is no GPS on the boat so I don't know how fast that it is running, however it has a 23-pitch wheel

Re: Mercruiser 420 HP specs.
 

Merc 420 motors came with no longer availiable huge iron manifolds with cast in individual runners. Also had a bigger elbow. Basicaly , twice the opening size of a stock manifold (330 ) I can't see the motor making more than 385 Hp with stock manifolds, Unless they are the High perf ones. Maybe you will be using headers. I have some of the big manifolds,
and related parts pipes etc. , Hard to find. Check out Mercruiserparts.com , and look up exhaust system under 420 . BBB

Re: Mercruiser 420 HP specs.
 

I have the merc 420's.

I have the GM high rise alum intakes and holley 800 dbl pumpers. My jets are 71 primary, 84 secondary with power valves in the primarys only. I also have the original 3" log manifolds which I feel are robbing me of HP. Im going with 4" manifolds next season

Re: Mercruiser 420 HP specs.
 

Oh Forgot to mention the intake manifold's should end in a 218 # And bear the Winters foundry snowflake !
They should have a bronze crossover as well as bronze inserts in the rear. BBB

Re: Mercruiser 420 HP specs.
 

The manual suggests turning the 420 to 5200 rpm.

Re: Mercruiser 420 HP specs.
 

I've got 2 of these if anyone need them.

Re: Mercruiser 420 HP specs.
 

i ALSO HAVE 2 OF THESE ARE THESE GOOD INTAKES? THEY WERE REPLACED WITH THE WIEND XCELERATOR SINGLE PLANE INTAKES

Re: Mercruiser 420 HP specs.
 

Just Joining the band wagon. I also have two of these Intakes just laying around. I like to run the Holley cast iron hi rise with squarebore. Were on cars 65-68 . They run great if you bore out the throttle bores. They are an excellent manifold. Mercruiser would have used them but didn't because they were no longer a production item when Mercruiser got into the hi- performance realm. The manifolds I'm talking about are large oval port. On a naturally aspirated engine, these and a set of 781 or 049 heads with big valves and some porting will outperform a 454 mag with 990 rectangle heads and rectangle intake up to 6000 - 6500 depending on compression , cam, carb, etc... BTW I have the cast iron Holleys. BBB

Re: Mercruiser 420 HP specs.
 

I just scored a GM aluminum high-rise dual plane intake part# 6269318 with the brass in the water jackets. I'm going to see how it works compared to my Weiand Xcellerator. The engine has the Stainless Marine manifolds. Something isn't quite right. The boat is only running 54-55 MPH @ 4900 RPM. I would think that with 420HP it should be at least in the 63-65 MPH range. The problem might be in the boat, I can't seem to be able to trim the drive up much at all until it starts to porpoise.