what prop for a 1980 Mercruiser 488

Randy stoner

Petty Officer 3rd Class
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Aug 20, 2002
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I Just put my boat in the water for the first time since aquireing it. It has a 488 merc four cylinder with a quadrajet four barrel. carb. its starts, and idles nice. Does not miss at all and runs at about 140 Deg. F when warmed up. It is in a 1980 Tempest boat Approx. 19 foot. First time out, it pulls out slow and will not rev past 3000 rpm. The carb was rebuilt and electronic ignition installed. All four barrels are wide open so that is correct. It runs all day long at that rpm but is a dog out of the hole. Prop is a Michgan prop, 14.5 inch Diameter with a 17 Ptich. Everything I have read about this motor this is the prop I need but the rpms should be between 4200 and 4600 WOT. What prop should I run or should I look at something else? When giving full throttle it is really really sluggish so im thinking the pitch is two high. What if I drop to a 15 pitch and can someone recommend one? Please respond here and or to my email at the_randy_man@Hotmail.com.

thanks

randy
 

alldodge

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I Just put my boat in the water for the first time since aquireing it. It has a 488 merc four cylinder with a quadrajet four barrel. carb. its starts, and idles nice. Does not miss at all and runs at about 140 Deg. F when warmed up. It is in a 1980 Tempest boat Approx. 19 foot. First time out, it pulls out slow and will not rev past 3000 rpm. The carb was rebuilt and electronic ignition installed. All four barrels are wide open so that is correct. It runs all day long at that rpm but is a dog out of the hole. Prop is a Michgan prop, 14.5 inch Diameter with a 17 Ptich. Everything I have read about this motor this is the prop I need but the rpms should be between 4200 and 4600 WOT. What prop should I run or should I look at something else? When giving full throttle it is really really sluggish so im thinking the pitch is two high. What if I drop to a 15 pitch and can someone recommend one? Please respond here and or to my email at the_randy_man@Hotmail.com.

thanks

randy

:welcome: to iboats Randy

I would suggest starting with a compression test to test the status of the motor
 

mr 88

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I doubt its the prop, I am surprised it doesn't have a 19" on it. Does this 35 year old boat have flotation in it between the hull and deck ? If so I would take it to a scale and have it weighed,you could be waterlogged which translates to a lot of dead weight underneath your feet. When is the last time it was timed ? I run a 21" or 19", depending on what I am on doing for the day, with the same motor on a 22' center console
 

Randy stoner

Petty Officer 3rd Class
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Aug 20, 2002
Messages
78
I will do a compression test tomorrow. I didn't install the electronic mod and I did notice a bit of spark knocking so im thinking you could be right on the timing too. Ill find out the compression first. What should it be? I know all four holes should be within 10% of each other. I rebuilt the carb and it starts and idles fine with no missing at all even on wot. No floatation stuff in the hull as I had to repair the floor.

randy
 
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steelespike

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Apr 26, 2002
Messages
19,069
minimum compression 100 psi note the condition of the plugs and the location.
Be sure to check the timing setting and advance.
 

Maclin

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It just seems like distributor timing setting to me from here, lol. On some electronic kits the triggering "event" inside the distributor sometimes moves considerably and the timing needs to be reset. Also with the closed cooling I am thinking the engine temp should be closer to 180-190 range, although this would not affect the performance to the "degree" you are experiencing.
 

Randy stoner

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Aug 20, 2002
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78
Ok, I did the compression test. What a pain it is to get the tester in there. First time going from bow to aft, 129, 90, 106 and 87 psi. This was done with the motor idleing. Didn't make sence at all to me and I suspected my tester was not threaded all the way in on the last three holes. Did it again and this time broght the revs up to 1000 rpm and all four holes where 130+ psi. So I don't know if I was loosing compression at idle and the gauge was not threaded all the way in or not. I know the last two holes I was lucky to get to turns on it and the front one six or seven. Plugs all looked good, a light tan with not deposits. Wires are new.

randy
 

Maclin

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While it is best to do the test on a warm engine I have never done one with engine running :eek:

Usual procedure is make sure battery has good charge, all spark plugs removed, throttle wide open then put the test fitting in a hole and crank. Repeat for each cylinder. If you get any low numbers then squirt some 30wt oil in that cylinder and try again.
 

cajun1957a

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Sep 4, 2005
Messages
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Ok did the compression test, all was good but decided to take the boat in and get the ignition timing checked. The timeing was at 22 degrees and should have been at 8 degrees. He checked everything else out as well and said he thinks the gearing in the out drive is wrong. I think he said its a 184 and needs to be 155 or vs vrs. give or take. Anyway to make a long story short, I put the boat in today. Before the test all I could get was about 3000 rpm with a 17 three blade prop. This past weekend I was given a brand new 18 pitch four blade prop. I thought what the heck. Throttled up and the boat roared far more than it had before the timing. The rpms maxed out about 3200 but I could hear and feel the acceleration. I got almost 30 miles per hour in heavy chop with two of us. Still not the 4200-4600 but its not spark knocking and its is pulling strong.

I have heard four blade props are good for pulling tubes or skiers but you loose a few miles per hour and rpms due to drag. I definitely know the boat is running far better and im going to put the 17 back on this weekend and see how that goes as well. I do suspect perhaps the mechanic is right on the gearing but ill also try a 15 pitch prop just to see what that does. If I can get 30-35 mph our of this boat, im happy. its big and old but definitely fun to drive once I figure out those trim plates on each side at the back and the power trim on the lower unit. I know this is still not perfect, but much much better than I had. After all its to take the grand kids out for a boat ride.

randy
 

alldodge

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You sure you didn't get the numbers mixed up. You went from a 3 blade 17 to a 4 blade 18 pitch, think that should be the other way around
 

cajun1957a

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Alldodge, No I purposely put on the 18 just to see what it would do. I loaned out the 17 and didn't get it back in time so I put on the 18, just to see what happens. How much more drag, and loss of speed and or rpms will a four blade do to a boat going from a 17-3blade to an 18-fourblade? I will know big time when the 17 comes back and I get it on but just for reference, what can I expect?

randy
 

cajun1957a

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yes cajun1957a is me....Randy Stoner I messed up signing in a few times and reset my password and user name and then tried setting it back
 
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cajun1957a

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Sep 4, 2005
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Ok here is an update and I sure hope someone can give me some sound advice. As I stated earlier, prior to a tuneup the boat would not rev past 3000 rpm. I have 14.5 inch 17 Pitch prop. I tried at 13 and it would rev 5100 but really wouldn't get up on plane even. I did a compression test and all was good there so I took it into a mechanic and he said the timeing was off by 12 degrees. Was set at 20 when it should be 8 btdc. That was fixed and I don't have any spark knocking now. Then I tried an 14.5 inch 18 pitch four blade and I got 3200 rpm but only 28 mph. I put the 17 back on and would only read 3100 rpm but would touch 30 miles per hour. I do like the four blade cuz it pulls better even if I loose one or two miles per hour. Today I put on a 13.75 inch 15 pitch prop and back up to 5100 rpms but wont get up on plane. Just revs like the 13 did and cavitates. So what the hell is going on here. This motor is suppose to run at 4200-4600 rpms. Its not missing. It start right up and idles perfect. It has a Quadrajet four barrel carb. Today I had someone throttle up hard while I held the secondary plates open and I didn't see any fuel but when I close them and let the vacuum open them up on full throttle it pulled gas. Just doesn't make any sence. It sounds like a old cow mooing but it just has not balls. The lower end im told might have the wrong gearing but would that really do all this. This motor is supposed to be a bear. The boat is a 19.5 foot tempest v-hull with a mercruiser 488/188 Hp. Everyone I have talked to said this thing should run 38-43 mph at WOT and 4400 rpms. The only way it gets there is with a 13 or 15 pitch prop and it just sits and spins. HELP Please

randy
 

alldodge

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Take a pic of the outdrive and bottom of the boat. I would like to know what the setup looks like.

Another question is how much does this boat weight? Wondering if the hull is water logged.

Also you said the compression is good, what psi did you get?
 

cajun1957a

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Sep 4, 2005
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Compression is approx. 130 each cylinder and I replaced part of the floor and had a chance to look into the hull. seemed pretty good and not wet.. But I will take a picture of the outdrive and post it tomorrow.

randy
 

alldodge

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The stern looks clean so I'll assume the hull is also. The pic I was wondering about was to see if the cavitation plate is even with the bottom of the hull. Looking at your pic I'll guess it probably is. Have a way to weight the boat? Maybe take it to a truck stop and get it on the scales?
 

cajun1957a

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Sep 4, 2005
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Update, The guy I bought the boat from had a 17 pitch, 13 inch diameter High Five staneless steel five blade prop he said I could try. Wants 300 for it and its in perfect shape. I went out in the bay this morning and there was quite a lot of wind and heavy chop. Thottled down, the bow come up and planed out quick. Rpms up to 4200 and gps at 33 mph with two of us in. I'm happy. may not be ideal but I'm betting I can hit high thirties on flat water if not more.

randy
 
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