OMC COBRA HARD TO PLANE

[wheeler87]

I have the omc cobra with a 2.3 liter 120hp . 17' sunbird corsair (open) . Here is the problem when I try to take off the boat struggles to get on plane , but once on plane it runs too high of rpm around 5,500 wot sometimes more (depending on the trim ) . I have a 13 1/4 by 17 prop (michigan wheel) What is the rpm supposed to be at WOT ? if the prop were pitched to steep wouldnt the rpm be too low? any info on this would be greatly appreciated...

 

[bruceb58]

Re: OMC COBRA HARD TO PLANE

Have you done all the standard tune up items...plugs, points, timing, etc?

Checked compression?
Verified distributor advance?
Old fuel?

You are also underpropped.

 

[QC]

Re: OMC COBRA HARD TO PLANE

What year is that Cobra? How long have you had this boat? Always behaved like this? Actually, I think your tach is off, but that other info is helpful too. There is a little switch behind most tachs to set it for 4, 6 or 8 cyl. Make sure that is right. 17 inch pitch sounds about right depending on the gear ratio, but it is hard for me to believe that a setup that struggles to get on plane would actually make 5500 RPM unless it was an OB.

 

[QC]

Re: OMC COBRA HARD TO PLANE

You are also underpropped.

Why do you think that bruce? I am not familiar with the Cobra and 2.3, but I had a 84 OMC Stringer 140 that used a 17 and it was perfect for that setup.

 

[cobra 3.0]

Re: OMC COBRA HARD TO PLANE

I use a 17" on my cobra 3.0 too. Just fine for me.

 

[bruceb58]

Re: OMC COBRA HARD TO PLANE

At 5500 RPM, what speed are you going?

QC, you may be right about the tach being wrong. My statement about the prop was based on his RPM.

 

[Don S]

Re: OMC COBRA HARD TO PLANE

You may also be trimmed to high, did you bring the drive all the way down to start and then trim up to best speed and rpm? Have you ever used trim on a drive before?

 

[wheeler87]

Re: OMC COBRA HARD TO PLANE

I will try to answer all youre questions in one post

1 fresh tune up this year...
2 drained gas tank last year due to water so yes fresh gas...
3 compression is ok...
4 it is a 1988..
5 have had it 2 years now...
6 yes , always acted like this
7 factory tach VDO it is set to 4 cyl
8 WOT 5500 rpm 26 to 28 mph
9 Yes have started trim all the way down and go up little bit at a time till rpm and speed are at max , left trim there stopped and tried to start again still hard to plane
10 yes had power trim on last boat although that one was out board,should be the same principal right?

 

[Don S]

Re: OMC COBRA HARD TO PLANE

Get an accurate shop tach and verify the rpm, it may be set of 4 cyl, but that doesn't mean it's readings are right.

Time to stop guessing and start testing and checking things.

 

[wheeler87]

Re: OMC COBRA HARD TO PLANE

will do Don.

 

[blouderback]

Re: OMC COBRA HARD TO PLANE

#9 on your list: Don't leave the trim up. You should always put the trim down just before starting, then, after you get on plane, trim the engine up to achieve the best performance. Trim up until you hear/feel the prop starting to loose bite, then tap the trim down button once or twice. This will be your optimal cruising or WOT trim position.

 

[jfcar]

Re: OMC COBRA HARD TO PLANE

Somethings not right, at 5500 RPMS with a 17" pitch prop with 10% slip you should be going 43 MPH. Check tach, if correct check prop, mite be slipping on hub or dirty bottom. WOT RPM for 3.0 motor 4200-4600 RPM

 

[wheeler87]

Re: OMC COBRA HARD TO PLANE

ok as far as the prop goes it is brand new this year , I am going to check the tach today , bottom is clean as a whistle and waxed.

 

[marunr]

Re: OMC COBRA HARD TO PLANE

I have the same motor on a 17 foot boat and have always run a 15"...that's what came on it when I bought it new in '88. One time I damaged a prop and only had a 17" to get back home with, and I had the same problem getting on plane you describe. It sure wouldn't hit 5500 rpms though.

 

[QC]

Re: OMC COBRA HARD TO PLANE

Just a caution: He has a 2.3 not a 3.0. This is a Ford engine and Overhead Cam . . .

Take a look at this thread from 2001, notes from our very own DWJ (djohns19), now a moderator. Maybe he'll join in . . .

2.3 OMC Thread

 

[wheeler87]

Re: OMC COBRA HARD TO PLANE

the tach checks out ok within 50 rpm (at idle ) could it be further off at WOT?

 

[QC]

Re: OMC COBRA HARD TO PLANE

the tach checks out ok within 50 rpm (at idle ) could it be further off at WOT?

Yes, 50 RPM at Idle is around 6 - 8% which could be almost 500 RPM at WOT . . . How did you get your accurate tach reading?

Also, don't assume that your new prop is good. New just means new, not good. If new was always good, no one would need warranty . . . I personally don't think it is the prop hub slipping, but it is important to remember that when it comes to troubleshooting stuff, if you assume that anything is good, you have just made a bad assumption.

 

[wheeler87]

Re: OMC COBRA HARD TO PLANE

I got the tach reading from my diagnostic machine (basically it is a big multi meter ) measures dwell , advance , rpm , volts and ohms . even with the tach being off we are still talking 5,000 rpm wot which is to high right?or is it not I did look at the post from 2001 as you suggested . In that post it says my engine (1988 2.3L) i capable of 5000 rpm.

 

[jfcar]

Re: OMC COBRA HARD TO PLANE

I read this on another forum about the Ford 2.3 liter OHC. (WOT RPM's are 5200-5600 but don't run there long or often)

 

[QC]

Re: OMC COBRA HARD TO PLANE

Yeah, it may be right . . . Do you know the gear ratio? That will help determine if the speed and RPM numbers are within reason.