RPM / MPH / Prop on a 1986 268 w/ 5.7 omc

[stevea661]

Finally splashed the boat this weekend and the only problem I had was with the top speed to rpm ratio. The top speed was 30 mph on the gps, rpm was about 3400, the tac is a new replacement telleflex, the prop is 15.8" x 15 pitch, the water was pretty flat with a slight wind chop. The engine is a fresh rebuild with fresh water cooling through the manifolds. water temp was about 72, engine temp never got over 180. I'm aware that if I drop the pitch an inch I should gain 200 rpm. I am using the recomended prop for the boat according to the omc manual. I'm like 1000 rpm off. Here is what I am thinking. The tac is off, There is a differance running at altitude, I was running at about 2500' and the boat is set up for sealevel, timing could be off a little, but would that drop 1000 rpm, carb is wrong for the boat, I got this boat in non running condition, the engine is new but I reused the carb. something with the ignition system. Can anyone point me in a direction to start looking. Thanks Steve

 

[Lazy Daze]

Tach's usually have a switch on the back to be used for either 4cyl, 6cyl, 8cyl. I would guess that if it was off, the tach would actually read higher actual - but just a guess.

Did you "trim up"?

 

[stevea661]

Tach's usually have a switch on the back to be used for either 4cyl, 6cyl, 8cyl. I would guess that if it was off, the tach would actually read higher actual - but just a guess.

Did you "trim up"?

I'm pretty sure I set it right but I will recheck that first because it would be an easy fix. Thanks.

 

[BillyT0020]

The first thing that would strike me is the top RPM of 3400. If that's accurate, then you probably have a fuel ignition, or carb problem. You should have no problem getting to 4400 with that prop. When was the last time your boat had a tune up (plugs, cap, wires, rotor)? If it was recent I would look to the coil first because they are cheap and should probably be replaced if its original. Does the boat rev up past 3400 in nuetrel? You need the correct carb for the engine, if its jetted for a diffent size block then that could be your problem otherwise check your carb linkage and the throttle cable to make sure its opening your carb up all the way at WOT. Check all your fuel lines and replace if cracked, change all fuel filters and take a peak inside your tank to see if there is junk or if the sending unit rusted out. 30 mph at 3500 RPM is about what I get with a 17 pitch SS prop but you should be able to take up past 4200 RPM''s at a minimum and get to around 36 or 37 RPM's.

My bet is its the carb.

Good Luck!

 

[BillyT0020]

Oh and the temperature is WAY high. What temp thermostat do you have? I think you should have a 142* which means your engine should stay around 150* ish. Check the water line coming in from the transom through the oil cooler and into the thermostat. Take it off the aft end of the oil cooler and check for blockage. Whats the story on your outdrive? Was the impeller housing changed with your rebuild? If not do it. Running your boat hot is not good. Im pulling mine on Friday morning to do the same thing.

 

[stevea661]

The first thing that would strike me is the top RPM of 3400. If that's accurate, then you probably have a fuel ignition, or carb problem. You should have no problem getting to 4400 with that prop. When was the last time your boat had a tune up (plugs, cap, wires, rotor)? If it was recent I would look to the coil first because they are cheap and should probably be replaced if its original. Does the boat rev up past 3400 in nuetrel? You need the correct carb for the engine, if its jetted for a diffent size block then that could be your problem otherwise check your carb linkage and the throttle cable to make sure its opening your carb up all the way at WOT. Check all your fuel lines and replace if cracked, change all fuel filters and take a peak inside your tank to see if there is junk or if the sending unit rusted out. 30 mph at 3500 RPM is about what I get with a 17 pitch SS prop but you should be able to take up past 4200 RPM''s at a minimum and get to around 36 or 37 RPM's.

My bet is its the carb.

Good Luck!

Plugs are new, cap / rotor / wires / points / condenser /coil are existing. Fuel water seperator is new. Engine revs past 4000 or more, no problem in neutral. Carb seems to go to WOT. Fuel is fresh. I will check the plugs to see what they look like.

 

[BillyT0020]

Okay I would start by changing the cap rotor wires and coil. Ditch the points and condenser and get a ignitor electronic ignition conversion kit. Best 65 bucks Ive ever spent (ebay it). Never screw with points again. Upgrade to the Petronix Flamethrower I coil when you do the conversion. Set your timing after you do the kit and then it should stay where it is since you have EI. If your running past 4000 in N then it sounds like it could be ignition, the electnic conversion and coil upgrade along with new cap rotor and wires could do the trick and really should be done anyway.

Billy

 

[stevea661]

Okay I would start by changing the cap rotor wires and coil. Ditch the points and condenser and get a ignitor electronic ignition conversion kit. Best 65 bucks Ive ever spent (ebay it). Never screw with points again. Upgrade to the Petronix Flamethrower I coil when you do the conversion. Set your timing after you do the kit and then it should stay where it is since you have EI. If your running past 4000 in N then it sounds like it could be ignition, the electnic conversion and coil upgrade along with new cap rotor and wires could do the trick and really should be done anyway.

Billy

There is a co on ebay that sells a coil / ei kit , I think It's aaa marine. They make it specificaly for omc and it works with the shift assist moduel. If I go with the petronix I have to make up a deal with some resistors and diodes. I've been looking into going with an ei. I guess I'll stop looking and start doing. Thanks

 

[BillyT0020]

There is a co on ebay that sells a coil / ei kit , I think It's aaa marine. They make it specificaly for omc and it works with the shift assist moduel. If I go with the petronix I have to make up a deal with some resistors and diodes. I've been looking into going with an ei. I guess I'll stop looking and start doing. Thanks

That will be an upgrade you won't regret! Let us know if it solves your problem with the RPM's as well.

 

[Larry]

Oh and the temperature is WAY high. What temp thermostat do you have? I think you should have a 142* which means your engine should stay around 150* ish. Check the water line coming in from the transom through the oil cooler and into the thermostat. Take it off the aft end of the oil cooler and check for blockage. Whats the story on your outdrive? Was the impeller housing changed with your rebuild? If not do it. Running your boat hot is not good. Im pulling mine on Friday morning to do the same thing.

Under 180 is fine, he has a fresh water cooled engine (see first post). You would be correct if it was raw water cooled though....

 

[stevea661]

Oh and the temperature is WAY high. What temp thermostat do you have? I think you should have a 142* which means your engine should stay around 150* ish. Check the water line coming in from the transom through the oil cooler and into the thermostat. Take it off the aft end of the oil cooler and check for blockage. Whats the story on your outdrive? Was the impeller housing changed with your rebuild? If not do it. Running your boat hot is not good. Im pulling mine on Friday morning to do the same thing.

The thermostat is a 160. I changed the raw water pump when I had the drive off. I also cleaned out the power steering oil cooler. The system I put in not only runs coolant through the engine but also through the exhaust manifolds. raw water goes through the heat exchanger and out the risers. the manifolds are not cooled by raw water.

 

[BillyT0020]

Ahh, my mistake I thought he was raw water cooled.

 

[stevea661]

Oh and the temperature is WAY high. What temp thermostat do you have? I think you should have a 142* which means your engine should stay around 150* ish. Check the water line coming in from the transom through the oil cooler and into the thermostat. Take it off the aft end of the oil cooler and check for blockage. Whats the story on your outdrive? Was the impeller housing changed with your rebuild? If not do it. Running your boat hot is not good. Im pulling mine on Friday morning to do the same thing.

The thermostat is 160. The system I have is an omc 210 mfg by sanjuan engineering. This is a full system. coolant runs through the exhaust manifolds as well as the engine. raw water runs through the heat exchanger and out through the risers. The power steering cooler was cleaned out when I did the engine. the raw water pump was rebuilt also and the coolant circ pump was replaced

 

[stevea661]

IE from AAA marine (ebay) is comming:smt038. Or at least breathing hard:grin:

 

[Tail Chaser 661]

Simple stuff first. Tach switch like lazy daze said
Make sure the carb butterfly opens all the way at WOT with engine off
Maybe compare movement with throttle cable disconnected if needed.
Make sure secondaries open up also.

If you decide on carb and ignition upgrades.
Edelbrock marine carbs and MSD marine ignitions can be purchased at speed shops like PAW. Much cheaper then Merc and Volvo stuff through a marine dealer.

 

[stevea661]

Simple stuff first. Tach switch like lazy daze said
Make sure the carb butterfly opens all the way at WOT with engine off
Maybe compare movement with throttle cable disconnected if needed.
Make sure secondaries open up also.

If you decide on carb and ignition upgrades.
Edelbrock marine carbs and MSD marine ignitions can be purchased at speed shops like PAW. Much cheaper then Merc and Volvo stuff through a marine dealer.

TC
I went with the AAA marine upgrade because of the esa / shift assist circut on the omc. I'll check the secondarys. That sounds like a possibility. The engine is real smooth. It just won't come up to rpm. The engine is not stumbling at all. I have an old dwell / tac meter that I can use to check the rpm against the tac. A little more trial and error and we will be killing fish. The two bait tanks go on this week. Good luck with yours:thumbsup:

 

[stevea661]

Under 180 is fine, he has a fresh water cooled engine (see first post). You would be correct if it was raw water cooled though....

Larry

You have fresh water cooling on your boat . I also see you have big blocks. I have a small block. If your manifolds are fwc and if so what temp they read on the inferred thermometer. Thanks Steve

 

[Larry]

Larry

You have fresh water cooling on your boat . I also see you have big blocks. I have a small block. If your manifolds are fwc and if so what temp they read on the inferred thermometer. Thanks Steve

Steve,

I cannot recall what the manifolds run at, I have put my heat gun on them but do not recall. I have checked resently my elbows and they are running around 120 on top @1200 RPM's. I can check this weekend and let you know if that helps.

 

[Sundancer]

I think most of the 268's with the 350 ran a top speed between 30 and 34. It's a heavy, deep V hull that is a little under powered. I had the same vintage 250 with a 454 Magnum and the top speed was BARELY 40 mph. With a heavier boat and smaller motor, I'd say anything over 30 mph is gravy. Unfortunately to get on a clean plane you need to be doing around 25 or so. There just isn't a lot of extra oomph in that combination. Someone else might chime in here, but it does appear that you have some issues in the carb/jets/prop combo.

 

[stevea661]

Steve,

I cannot recall what the manifolds run at, I have put my heat gun on them but do not recall. I have checked resently my elbows and they are running around 120 on top @1200 RPM's. I can check this weekend and let you know if that helps.

Larry
My elbows run about the same temp. Exhaust manifolds vary 180-230 normaly around 200. I'm not going to trip on the variations, It will make you nuts, as long as the engine temp stays around 170-180 I'm good.