Merc I/O - Chris Craft 260 - Top Speed

[Martin Leaney]

We have just bought a 1985 26' CC with a Merc (GM 305). We use it in southern B.C. at an elevation of about 2500'.<br /><br />We are having some power issues (or lack of power issues maybe). <br /> The boat will do about 20 mph flat out at a max RPM of about 3500 - most of the time only about 32-3300 rpm.<br /><br /> The top butterfly valves on the 4 brl carb open fully but the lower ones open only a little ways.<br /><br /> It has a 3 blade prop, which was on the boat when we got it. The prop is a little dinged up but not bad.<br /><br />I think the hull is a semi planing type. The boat has trim tabs.<br /><br />The boat weighs about 7000 lbs locked and loaded.<br /><br />We were expecting about 28 -30 mph top speed with a 20 mph cruise. Any ideas on problems or is this all we can expect out of this boat/motor combo.<br /><br />over...

 

[tjkprop]

Re: Merc I/O - Chris Craft 260 - Top Speed

Bearing in mind I am no expert here, just another I/O owner in training.<br />I would think the WOT should be closer to 4400 on that motor. Prop could be cause. That's what I would start with anyway. I would think 30MPH would be a reasonable top speed for that boat.<br />I'm sure I will be corrected if wrong..and learn another thing or two...

 

[Martin Leaney]

Re: Merc I/O - Chris Craft 260 - Top Speed

I just took a look at the prop again (I just towed it back to my shop yesterday - new GM Duramax Diesel works great!). It is a 16 x 16P and does look a little ragged on the edges and has a minor bit of a bend also. I think it is aluminum.<br /><br />What prop would work best or is it just a case of having the existing one fixed?

 

[Bondo]

Re: Merc I/O - Chris Craft 260 - Top Speed

You Should be getting Atleast 1000rpms More than you are....<br />You should 1st give the engine a Full Tune-up, along with a compression test... I suspect you may also have Carb issues, with the butterflies not opening all the way.... I think your motor is Way Down on power, Rather than a prop issue, at this point..... If your already turning a 16' prop, you Should be over-reving if anything... They don't make props much smaller than what you have now.... And being beat-up, it Should blow-out even Easier....<br />Have the Engine looked over Real Closely to see Why your not making the power you should have, Then move on to your prop issues.....

 

[Martin Leaney]

Re: Merc I/O - Chris Craft 260 - Top Speed

Thanks for the info! <br /> I am thinking the engine has issues too. Although the engine starts easily, runs smooth and seems responsive there must be something keeping the revs down.<br /><br />I'll let you know what I find.

 

[Martin Leaney]

Re: Merc I/O - Chris Craft 260 - Top Speed

We checked out several items on the engine last night... <br />We found the timing to be good (8^) and the advance timing was good (25^).<br /><br />The plugs were dark black. We will probably change the plugs and leads. We did not do a compression test but will do when we change plugs.<br /><br />The secondaries on the 4 bbl were opening ok.<br /><br />We were getting 5000 rpm with no load (sitting out front of my shop). <br /><br />Does anybody have any other suggestions or input?

 

[oldboat1]

Re: Merc I/O - Chris Craft 260 - Top Speed

think you may be overpropped if the engine is checking out OK. I have the '77 25 foot model with a 305 inboard, but believe the stock prop is either 12 or 14 inches. I get about 4300 to 4400 rpms at WOT -- I/O could be different, I suppose.

 

[Mikedg]

Re: Merc I/O - Chris Craft 260 - Top Speed

It is a 16 x 16P and does look a little ragged on the edges and has a minor bit of a bend also. I think it is aluminum.<br />

I think the prop may still be the issue. If it is bent at all it can really change the pitch and your performance. To see if you have the right prop for your boat before fixing it you can have Michigan Wheel analyze your set-up and recommend the correct prop. The link to that section on their Web site is http://www.michiganwheel.com/MIWheel/PropAnalysis/OutboardAnalysis/OutboOne.asp

 

[Martin Leaney]

Re: Merc I/O - Chris Craft 260 - Top Speed

Thanks all - and thanks mikedg for the link! I filled in all the info and now just have to wait for an answer.

 

[Martin Leaney]

Re: Merc I/O - Chris Craft 260 - Top Speed

I am going to replace the plugs and leads now. I have bought Bosch Premium 8mm leads but I'm wondering if it is worth while to go with a better (than OEM) plugs. <br />Anybody have experience with upgrading spark plugs?

 

[Martin Leaney]

Re: Merc I/O - Chris Craft 260 - Top Speed

We have now changed the plugs and did compression tests on all cylinders. Compression was 120 to 125 on all. Not great but at least they were consistent.<br />We ran the engine a bit and pulled the plugs to check them - they were black. I am thinking the engine is running rich now. <br />The boat came from the Great Lakes area and is now being used in the Rockies & Kootenays in BC. Elevation 2500 to 3000'. <br /><br />How do I adjust for the mixture for the higher elevation? (It is a Rochester 4 Barrel)

 

[Martin Leaney]

Re: Merc I/O - Chris Craft 260 - Top Speed

We also just tried another prop - smaller - and we still are only getting revs in the lower 3k range. <br /><br />I am thinking the carb needs a rejet for the altitude that we are running. ( I have a KTM 520 EXC offroad bike and I know the problems that altitude can cause with dirt bike performance.)<br /><br />Any Rochester experts lurking around here?

 

[flashback]

Re: Merc I/O - Chris Craft 260 - Top Speed

hey muddy, forgive me if i am repeating someone but if all the throttle plates are not opening then you may have a lingage problem. disconnnect the lingage and manually work the throttle to see if it will open completly,...............

 

[Martin Leaney]

Re: Merc I/O - Chris Craft 260 - Top Speed

Thanks Flash - I think we tried that previously but it would be worth another check. Seems to me that the primaries were opening fully and the secondaries were 'mostly' open - 80 degrees or so.<br /><br />The plugs are showing that we are running rich so this seems to be an indication of too much fuel.<br /><br />I have the info back from Michigan Wheel and they say a 16x16 Alum 3 blade is right for this app (which is what we had on originally).