1985 470 Mercrusier RPMs question

[Pst76]

Took my 19ft 4Winns cuddy out for first test run after restoration. All went well but have a couple questions.I am running a 14 pitch 3 blade prop.At WOT 2200 rpm, speedo reading 38 mph. Not sure accuracy of speedo but was very pleased with performance.Out of the hole and up on plane very well. This is my first I/O and am wondering what range should be and if I'm close with existing set up.<br /><br />Thanks for any info.

 

[vipzach]

Re: 1985 470 Mercrusier RPMs question

What motor and drive do you have? 2200 rpm at WOT seems pretty low, and a 14p prop sounds pretty small for a 19ft boat.

 

[MrBill]

Re: 1985 470 Mercrusier RPMs question

RPM's should be up around 4400+. The drive probably only accomodates up to 14-1/2" prop, but it seems like you should be up around 19 or 21 pitch. Check RPM with accurate meter, and get WOT up in the proper range for best use of the power curve.

 

[Pst76]

Re: 1985 470 Mercrusier RPMs question

Motor is a 470/170, 3.7 Mercruiser 4cyl.With an Alpha 1 Drive.

 

[vipzach]

Re: 1985 470 Mercrusier RPMs question

I think you need to get that tach fixed, I don't think you could be going 38 mph with a 14p prop, turning only 2200 rpm's. I have an 18ft boat that turns 4500 rpm's at about 36 mph. <br />Maybe you read the prop size wrong, and your tach is just reading wrong.

 

[Pst76]

Re: 1985 470 Mercrusier RPMs question

Tach is new,I have setting switch set on position Marked #4 for 4cyl motor. I'll have to double check. I set idle using a remote tach to 800rpm and verified same reading on Dash tach. I thought 2200 seemed a bit low for speed. Speedo reading seemed ok as I clocked against my brother inlaws boat side by side against his GPS speed set up. I will double check everything.<br />Thanks

 

[jeffkrol]

Ressurecting old thread

Ressurecting old thread

Seem to have the same situation w/ a 1984.. Tach hovers at 2000RPM w/ WOT... and I could almost guarantee that's not right.. Any answers yet?

 

[Don S]

Re: 1985 470 Mercrusier RPMs question

Jeff, if you have a question, just ask it in a new thread. Tacking on to a year old thread is a sure way to NOT get an answer. Most readers only read the first post and answer it. Seldom reading the other replies.

To answer your question, get a good shop tach (all kinds on the market, some in the timing light) then verify the actual rpms against what your helm tach reads. If it's bad, replace it. That's all there is to it.

 

[bruceb58]

Re: 1985 470 Mercrusier RPMs question

I bet your tach select switch is pointing at the 8 cylinders position.

 

[Chopstix99]

Re: 1985 470 Mercrusier RPMs question

I run a pair of 488's which is basically a 470 with a 4bbl carb. Have a 25' hull Mark Twain aft cabin. I run 13.75 x 21 props and I max out around 4000 rpm. GPS ground speed @ 3200 rpm is 30 mph. I can get 38.5mph out of the boat but can't see burning the extra gas. Then engines are rated 4600 - 4800 WOT. I could probably re-prop and get the R's but don't see this as needed or even wise on 20+ year old engines.

 

[jeffkrol]

Re: 1985 470 Mercrusier RPMs question

Jeff, if you have a question, just ask it in a new thread. Tacking on to a year old thread is a sure way to NOT get an answer. Most readers only read the first post and answer it. Seldom reading the other replies.

To answer your question, get a good shop tach (all kinds on the market, some in the timing light) then verify the actual rpms against what your helm tach reads. If it's bad, replace it. That's all there is to it.

Actually was hoping pst76 solved the problem and would post an answer. But you notice I did do a search before posting a repetitive question that has been answered over and over again Problem with a lot of threads(like this one) is that a problem gets solved w/ no "this is what was wrong" followup.
Having a 84 Chris Craft deckboat w/ a 470 and a low reading tach, just seemed logical to post here. And after doing some of the fun electrical troubleshooting (no start this spring) and getting interesting readings at the coil as to voltage I really didn't want to get into the whole description of my findings and my lack of skill. From the number of responses it seemed to work.
Thanks for the advice though.