Boat Runs Slower after Rebuild
05-21-2018 | 08:51 AM
#1
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Boat Runs Slower after Rebuild
Spent some time with my boat this winter and ended up making it slower...
The engine is a 454 Gen VI Mag / roller cam / carb in a 236 eliminator eagle. (200hrs)
In January I put my compression tester on each cylinder. Cylinders 3-8 were cranking 150ish psi while 1 & 2 gave me 90psi and 40psi respectively. I pulled the heads off to find a crack on the cylinder two side and a vacuum test showed a bad valve on cylinder 1. Pistons and cylinders appeared to be in good condition. I tracked down some heads locally for a fair price. The replacement heads were 990 rectangle heads with new intake valves and new hi rev springs. the heads had been decked with some minor port cleanup. I was told they would be a direct replacement for my 088 heads, but once I got them on I found out that I need stud type rocker arms so I bought those also and reused my pushrods. I went with the same ratio of rocker arm that was spec'd in the manual cant recall of the top of my head. Well once the boat was all together it was cranking 190ish psi in all cylinders. I set the timing to 34* total advance @ 3500rpm.
so before the rebuild I was turning a 23p Merc Rev4 prop 5000-5050rpm to get 62mph
after rebuild I am turning the same prop 4850-4900 to get 59mph
it leaves the hole a little bit better but I was expecting a big improvement considering I was only running on 6 1/2 cylinders
not sure where to go from here any suggestions?
Thanks,
Todd
The engine is a 454 Gen VI Mag / roller cam / carb in a 236 eliminator eagle. (200hrs)
In January I put my compression tester on each cylinder. Cylinders 3-8 were cranking 150ish psi while 1 & 2 gave me 90psi and 40psi respectively. I pulled the heads off to find a crack on the cylinder two side and a vacuum test showed a bad valve on cylinder 1. Pistons and cylinders appeared to be in good condition. I tracked down some heads locally for a fair price. The replacement heads were 990 rectangle heads with new intake valves and new hi rev springs. the heads had been decked with some minor port cleanup. I was told they would be a direct replacement for my 088 heads, but once I got them on I found out that I need stud type rocker arms so I bought those also and reused my pushrods. I went with the same ratio of rocker arm that was spec'd in the manual cant recall of the top of my head. Well once the boat was all together it was cranking 190ish psi in all cylinders. I set the timing to 34* total advance @ 3500rpm.
so before the rebuild I was turning a 23p Merc Rev4 prop 5000-5050rpm to get 62mph
after rebuild I am turning the same prop 4850-4900 to get 59mph
it leaves the hole a little bit better but I was expecting a big improvement considering I was only running on 6 1/2 cylinders
not sure where to go from here any suggestions?
Thanks,
Todd
05-25-2018 | 09:19 PM
#3
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From: Lake Winnipesaukee, NH
Also clean every electrical contact that can affect ignition...I built myself a new engine 2 years ago and reused the 1997 ignition. First year it was good. 2nd year it deteriorated as time went on.
Finally this spring I got around to cleaning the electrical contacts, and poof no more underwhelming 383.
Maybe you could be that lucky.
Finally this spring I got around to cleaning the electrical contacts, and poof no more underwhelming 383.
Maybe you could be that lucky.
05-25-2018 | 09:40 PM
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From: Grand Haven, MI
Also clean every electrical contact that can affect ignition...I built myself a new engine 2 years ago and reused the 1997 ignition. First year it was good. 2nd year it deteriorated as time went on.
Finally this spring I got around to cleaning the electrical contacts, and poof no more underwhelming 383.
Maybe you could be that lucky.
Finally this spring I got around to cleaning the electrical contacts, and poof no more underwhelming 383.
Maybe you could be that lucky.
05-26-2018 | 08:57 AM
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The only changes made were, heads and rocker studs / arms. I definitely will put the timing light on it again. I was told by a old jet boat racer to put it at 36* total advance but I think I'm sitting around 32 right now.
I have not done any prop changes. I wanted to see it run at least as good as it did before because if I change the prop I'll lose that baseline
I have not done any prop changes. I wanted to see it run at least as good as it did before because if I change the prop I'll lose that baseline
05-26-2018 | 09:45 AM
#7
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From: Pompano Beach, Florida
[QUOTE=pondracer95;4628503]
I was told by a old jet boat racer to put it at 36* total advance but I think I'm sitting around 32 right now.
Well i'm a old offshore racer and I believe that could make a difference.
Well i'm a old offshore racer and I believe that could make a difference.
05-26-2018 | 04:36 PM
#8
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Posts: 2,888
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From: SF Bay Area
Engine Masters did a show where they actually lost some power going to roller tip (heavier) rockers over the stamped steel because the engine in this case was too mild to for them to be of value. Your case seems more extreme than that though.
I was told that 2* advance can make a difference of 50hp. Opinions and situations will vary, but the bottom line is you need everything dialed in before making any comparisons. Fresh gas makes a difference too.
I was told that 2* advance can make a difference of 50hp. Opinions and situations will vary, but the bottom line is you need everything dialed in before making any comparisons. Fresh gas makes a difference too.
05-29-2018 | 07:32 PM
#9
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From: On A Dirt Floor
This other member found what NHGuy found .
Maybe something to be learned when you stop laughing.
.
Last edited by SB; 05-29-2018 at 07:36 PM.
