1973 Evindrude 115 tops out at 3k any help greatly appreciated.

1973 Formula 180 thunderbird
Evinrude 115hp outboard

At max throttle the engine does not go past 3,000 rpms.

We just rebuilt the motor and everything is to spec. Except the pistons are 1/5 of an inch shorter. All new seals and bearings. New pistons and wrist pins. Rebuilt carbs, fuel pump, filter, primer bulb, engine starter. All new gas lines, new batteries. It throws a mean spark and the power pack checks out too. Idles like a champ goes reverse and forwards, the damn thing just wont spin above 3,000 rpms at WOT. Also we are running the recommended per the repair manual oil/fuel mix, which off the top of my head is 2x the amount of oil as normal. We have put about 3 hours of the first 10 hours of recommended 1 part of breaking her in. However not being able to get her past 3k is keeping us out of the power band and never really breaking 9mph on flat water. Before the rebuild we could hit 30+ on flat days. Why the rebuild you ask? She ate a piston ring while warming up at the dock right before we were heading out.

Any advice would be welcomed. Hopefully i have given a large enough scope of what has been done. Basically a complete overhaul of the top end. Last year we did the bottom end. Now we are just pulling our hair out in frustration as the fishing season drifts on past us.

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

Is the timing advancing? Compression good? Are the throttle blades opening fully? Have you done a complete link and sink? It sounds like a link and sink type issue

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

Yup we checked all that and have done the link and sync. M buddy thinks it's fuel/air related. I think something is not right withe the link/sync.

Is normal to have to do a link/sync more then once? To me it really feels like a car stuck in gear at wot.

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

Why are the pistons 1/5 of an inch shorter ??

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

The new replacement pistons wrist pin hole is 1/5 of an inch lower. So the piston sits that much lower at top dead center. We actually took 1/5 inch of the piston skirts because they were bottoming out on the block. From our research the 115/135 pistons are the same, but the 140, have the offest pin hole. We could not find a rebuild kit with 115 pistons. All the ones we found have the same pistons listed for 115/135/140. Compression on all 4 cyclenders is 90 exactly.

73 is a odd year and they changed things in 74. From 74-90ish are the same but 73 had some differences.

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

Wow where do I start...... you have a Frankenstein motor now. That 1/5" actually changes the game. Were any modifications made to the ports? Yes 1973 had a smaller crank and bearings to begin with (changed in 1974) and since the pistons now have a lower compression and length I think you are passing the ports.

Frank Acampora had modified some motors with porting and maybe he will pipe in.

Having a compression of 90 lbs is okay but still low.......... the original V-4 motors of 1973 were destined to have high compression and run on premium fuel......... many un-cared for "leaned" out and melted pistons.

My wild guess is based on that 1/5" in relation to the porting. There is a chance the ports can be modified to make up for that.

On a good note.....running is a good thing

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

Passing the ports how? The top of the piston still clears the ports in the cyclinders. Do you mean passing them as in to far past? In comparision to the old piston the ports have the same amount of clearance/opening when the piston is in the correct position.

Honeslty i am a car guy, and this whole 2 stroke fuel/oil mix thing is new to me. Lots of learning as we go type of thing.

Do you think the pistons being the way they are now would be the cause of our 3k max rpm issue?

Other then the low compression she runs great. Just hit a wall at 3k.

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

When ever you change the length of the piston skirt you will screw up the port timing for performance. Bob VT had it right, You now have a "frankenstein motor"

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

So your saying the pistons are the reason we top out at 3k rpms?

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

In 1974, OMC changed the location of the piston pin in their crossflows. The change affected the height of the pin in the piston. In order to make sure the piston still retained the same displacement in the cylinder, the OMC engineers also had to change the length of the rod. This change took place in the 74 model year. Possible that you have somehow mixed older rods with newer pistons, or vice-versa. This would account for the change of the piston height in the cylinder. It should not affect the actual ignition timing, but will affect (slightly decrese) the amount of fuel charge going through the intake side of the crankcase. In addition, the deflector on the piston crown will be lower and will affect the charge flow through the combustion chamber. Probably best to pull the block apart and make sure you match the proper pistons and rods for your 1973 year, then reassemble. Normal compression on that 115 block after overhaul should be in the 125+ range.

Re: 1973 Evindrude 115 tops out at 3k any help greatly appreciated.

Have you got the pistons installed the correct way.