Prop size or engine problem ?

[Firefighter3011]

I have a 1992 chris craft 268 concept kimg cobra cone clutch originally equipped with a 7.4 liter 454 engine but had been repowered by previous owner with a 8.1 liter 502. Enfine slightly built large cam. 11:1 compression merlin heads , intake ported polished etc.

Now my question is. I purchased the boat last year and it was equipped with a turbo fusion 4 series 14.5x24 stainless four blade propellor. It was brand new and I thought nothing of it other then it was shiny and ding free. Running this boat this season I was only able to reach 3900 rpm at WOT. I did my research and checked the secondaries opening fully. I have awesome fuel pressure at WOT verified by a gauge on the supply line to the carb. Timing is set at 22* base and around 42* at WOT It doesn't want to idle at anything less then 22 advance at idle . I am not very savy when it comes to timing and I will be playing with curves and bushings in the distributor before purchasing a new propellor and installing new cap , rotor , plugs etc in the spring

Boat bottom was clean , just me in the boat with a full tank of fuel. Here's my weight guesses. Boat weighs 5000 dry. I have a 85 gallon tank at 680 pounds of fuel I'm up to 5680 pounds. And myself included at 200 pounds is 5880 add my gear and cabin supplies and I would say I'm around 6500 pounds.

Outdrive is a 1.43:1 gear ratio. Is there anything I am missing here ? I am curious if anybody has the same style of boat that are running single prop outdrives and what they're prop pitch is that they are running or if anyone thinks this is a prop issue or a engine issue. If I'm missing any information please let me know. Like I said I am going to be playing with advance curves and timing in the spring with my current prop to see if I can achieve 4600 rpm which I will be plenty happy with since I hardly ever run at WOT. Any help would be appreciated.

 

[roffey]

You would think with a 24 pitch prop might be to much for that size of a boat?

 

[Firefighter3011]

I pretty much am 90% sure that is the problem. But it still boggles my mind why he would repower with a fresh 502 (72 hrs on it currently) and purchase a brand new prop of the wrong pitch. Only thing I can think of is the previous owner did not put thought into it or maybe the engine didn't make the horsepower he had anticipated. What pitch are most boats this size running ? Everyone I know with a boat the same size and style all have duo props which I cannot relate to

 

[roffey]

Props make me scratch my head. If you could only try before you buy. I currently have a 23 inch prop on my boat and can do about 56 mph. I need to get a spare prop, do I go with a 19 inch for water sport or a 24 or 25 inch for speed. If I go with a 25 inch and can't get out of the water or can't hit my max RPM it's wasted money. At $500 per prop you don't really want to guess, and that what makes this site so valuable. You will get a honest response from people who are in the know.

as I said you will get your answer shortly.

 

[steelespike]

What is your gps speed at 3900?If your 1100 rpm short and try to make it up with a prop rule of thumb puts you at about 18" pitch to make 5,000 rpm.
Seems pretty light to me.Down the road if all else fails check the valve timing.

 

[Firefighter3011]

Gps and speedometer match. Running about 55-57 with just me in the boat. I'm not looking to reach 5000. Im fine with the lower WOT at 4600 rpm. Im thinking if my tune up and timing adjustment doesn't cure the problem I'm going to try a 21 pitch. 200 rpm for every inch decrease that will put me at 4500. Which I'm happy with. Like i said I don't run wide open very often And I'm not sure what you mean by valve timing this engine is brand new barely broken in. Is 200 rpm per pitch pretty accurate ?

 

[Firefighter3011]

I'm also trying to see what size props people are running on the same size. Style and weight similar to mine

 

[dingbat]

Your going about this all long.

The prop is sized correctly when you can hit the motor' maximum rpm.

Lugging a motor by using an incorrectly sized prop is a good way to destroy a motor.

 

[steelespike]

When they assemble the motor there are marks used to put the cam in time with the crank.
Not likely out of time but could be worth a look if all else fails.
We had a head gasket replaced on a 83 honda. Ran fine but when it turned cold it was near impossible to start.
The dealer actually admitted they didn't get the cam timed right.no charge.
​ 150 to 200 rpm per inch is pretty typical.
It is possible the tach is off a little low.
A quick calculation using 57 mph gets 8% slip, possible but at 3900 slip would tend to increase.
I usually use 13% to calculate for your results. Using 13% seems to indicate the tach may be a little low.
At 13% calculates to about 4100. to make 57
Maybe you can borrow a shop tach to see if I'm in the ball park.

 

[Firefighter3011]

Dingbat. That's the whole reason I am doing this is to get the rpm up. How am I going about this all wrong ? I realize the current prop is lugging the motor , hence why I am trying to downsize in pitch to bring my rpms up into range. Range is 4600 - 5000 for my engine ,But engine problems could be my issue before i start messing with pitch. And I will do that. Boat is out for the winter right now I'm just trying to get my ducks in a row for spring. And that could be spike I changed the stock analog tAch this summer to a autometer digital. And the rpm seems the same. I will try using a shop tach if my tune up doesn't bring the rpm up. That'll make it certain that it's not the engine itself and it is indeed the incorrect pitch. Do shop tachs read off the negative side of the coil or do they read the belt , or balancer ? And I didn't really think of a head gasket because I had 180 psi compression give or take 2 psi on all 8 cylinders . Can a bad head gasket still show good equal compression ?

 

[Sea Rider]

Don't know about same equal compression, but a bad gassket depending on its condition will make the engine miss throughout the throttle range, will miss worst at wot due to the greater amount of water droplets going inside combustion chamber..

Happy Boating

 

[steelespike]

If water is getting in the combustion chamber the plug may appear super clean and there may be evidence of rust on the plug.

 

[Firefighter3011]

Well that's a good thing then because my plugs were all black due to the previous owner using the stock csrb with stock power valve. New engine with large can only is pulling 6 inches of vacuum at idle. Power valve was open all the time. Just discovered this at the end of the season. I'm rebuilding the carb and installing the appropriate sized power valve in the spring.

 

[Scott Danforth]

something is not right if you have to have base timing set at 22 degrees to get it to keep running and are running 42 degrees total advance. with that stated, I assume that you have issues with either cam timing or the cam itself. the 11:1 compression is more for drag racing and less for boating. I am going to assume that based on your large cam comment the cam is not a marine/RV cam as well. My first suggestion is to identify the cam if you can. If you can not, I suggest investing in a decent cam for the boat. I can not see you swapping out the motor for a boat friendly 9:1. Mercruiser's compression ratio is 8.75:1, so 11:1 is a bit high.

you also have a 6500# slug you are trying to push thru the water with a 24 pitch prop. I would have assumed a 21 pitch prop myself based on prior experience with pocket cruisers and a single outdrive

 

[Firefighter3011]

Very good information. It may not be 11:1. That's what a buddy of this guy I bought it off of told me. Couldn't contact the builder. I did some research in the marina storage lot last evening and have seen most boats my size with a stock engine and a single prop are using anywhere from 17 pitch to 21 pitch. So I'm thinking im gonna buy a 21 in spring and try my guesses.

 

[Scott Danforth]

If you cant pull the timing cover to get access to the cam number on the end, I would at a minimum pull the valve cover and measure total lift on both intake and exhaust with a dial indicator. then with a degree wheel, check duration. you will be able to get a good idea of a cam spec

also, check with your marina if they have loaner props to determine proper fit. many do to get the customer dialed in.. also look on craigslist and ebay. I bought two stainless props off of craigslist for under $50 ea for my old OMC.