Evinrude 100XP V4 Hitting wall at 4700 RPM (Long read/ head scratcher)

[fmjshooter]

Hello all been digging through this site in search of knowledge but defeated I come for help. Little background about myself, been working on drag racing cars since I was 14 as a hobby then turned pro then back to hobbyist. Worked on/built engines from pro mod level down to street so I'm not a complete dolt behind a wrench.

Long story short I parked the drag car and decided to buy a starter boat to combine my love of fishing with my love of racing. Found a clean 17' foot Javelin bass boat with a Suzuki 115 on it that had a high idle issue for cheap. Figured what a great way to learn these two strokes. I did learn a ton but eventually killed that motor from what I believe was a vacuum leak at the lower crankshaft seal. So in my searches I found a retired marine tech that rebuilds power heads out of a small shop and he was selling a complete 1990 Evinrude 100xp v4 crossflow with a rebuilt power head. So I dumped that Suzuki and mounted her up. Compression read 119 cyl 2 and 120 on the rest.

After running a few tanks of high oil gas through her (VRO is removed) as per the builders instruction I tried running up to WOT and she would hit a wall at 4000 rpm. She comes out of the hole pretty darn good so I figured this was a simple carb issue and started my diagnostic nightmare...

Here's what I've done so far with no resolve.

Carbs
Completely cleaned with new carb kits with new floats and pins. Blew out all passages with compressed air and visually inspected all visible passages. Only thing I didn't do was pop the core plugs because the builder said he went through them.

Link and Sync done as per the OMC service manual throttle blades are laying flat at WOT timing is advancing and reads 4 degrees at idle.

Ignition
Upon inspection all four coils had chips or cracks in the insulation so I replaced all four with new CDI packs.

Power pack was showing a low output on the orange leads to the coils (120 DVA) Stator was also showing low voltage on one of the two wires from the charge coils (120 DVA). Swapped power pack and stator (stator did indeed have a swollen and cracked coil) with brand new CDI parts.
Stator voltage now reads 180 on one wire and 160 on the other at cranking speed. Power pack is now putting out 180-190 DVA at the orange leads when connected to the coils.

Timer base is putting out 1.5-2 volts dva on all leads and checks good on the OHMS test.

Rectifier is brand new as well

Timing advance reads 24 degrees static at WOT which as I understand should jump to 28 when the engine is running.

I indexed tdc for each cylinder on the flywheel and they are firing at the right time, #1 TDC mark lines up with mark on the flywheel as well.

Spark gap tester shows a healthy blue spark at 7/16" gap and all cylinders seem to be firing at the problem RPM after doing the tach test on each coil lead as described below.

Haven't done a high speed shutdown to read the plugs yet but I did pull them and they are all loaded up with carbon on the base, ground strap seems to be showing correct timing and the porcelain is a dark mocha color with #1 cylinder being a little wetter than the other three.

Fuel

Bumping the primer solenoid at the problem RPM immediately drops the rpm about 300-400, also disconnected the lines from the solenoid at idle and they don't seem to be leaking at all. Maybe I should test them under vacuum?

Ran on a 2 gallon test tank direct to the motor with no changes, squeezing primer bulb also had no effect at problem RPM.


Tachometer

Tested accuracy with an inductive tachometer set for a 4 cylinder four stroke and read 2360 rpm on each cylinder which when multiplied by 2 shows the dash tach to be off by about 15% at high rpm but it does read closer at lower rpm.

So now I'm stumped. If I trust the inductive tachometer I'm actually running around 4700 which makes more sense if I run the numbers in a prop slip calculator.

Only baseline I have for the boat is with Suzuki 115 which was mounted as low as possible on the transom with a 19P 3 blade stainless prop that pushed the boat to 50 mph(GPS) at 5300 rpm(may have been much higher RPM) Current set up on the Evinrude , engine mounted in top transom holes (no jackplate) with a 17p 3 blade stainless omc brand prop with unknown history but visually seems to be undamaged other than a few very minor nicks. Now I'm begining to wonder if the engine is okay and that this prop is has been modified and the engine is too low? Trimming up seems to have almost no effect on RPM at any speed and the cavitation plate does look to be about an 1" or 2" below the water line when running, seems like I could also use more bow lift, perhaps the bow is plowing?

I also decided to pull a main jet from one of the carbs and it's a 57C which shows up on a parts look up as jets for a 90 horse carb for 1990 models but are correct for a 1988 100. The carbs are also 1 3/16" venturi size which I can't find if are correct for this engine. So I'm thinking these carbs are either off a 90 horse or an 1988 100 horse. Would incorrect carbs cause this? Should I pull them back off and check all the jets and pop the core plugs out? What the hecks hiding under there anyways? I also noticed when I look down the carb throats one of the emulsion tubes seems to be slightly lower than the other, meaning on the left tube it has maybe three holes fully exposed and the right tube has about two and a half exposed (don't remember actual # of exposed holes)

Only other Ideas I have is to do another dva output test on the orange coil leads while the boat is running at the problem RPM maybe bad flywheel magnets can't put out enough voltage? Is the logical next step to bring it to a shop and have the engine run on a test prop to eliminate my setup from the equation and verify I'm getting 28 degrees max advance. Am I shooting myself in the dangle by not doing this first to establish a baseline?

This is about how I been looking the past week I'm completely obsessed with getting this thing right it's all I can think about and it's pizzing off the wife and kid! Any help would be extremely appreciated!

Oops forgot to mention when I fired he up this morning on muffs she was surging down 100 rpm at idle now, so hooray either a new problem or a new symptom of the existing one!

[Lockjaw]

I'll take a shot at it. For starters, a Javelin isn't the lightest boat out there, I have a 17'9'' model, and it weighs 1350 pounds, hull only. That said, they do run good, when you have the right amount of HP on the transom.

I'd like to know what your prop to pad height is.

Second can you verify that at WOT the carbs are open all the way?

Third, on my javelin, I measured the trim relative to the gauge and the cavitation plate. At 3/4 up one the gauge, the cav plate is level with the pad. Boat runs good there, but the last 1/4 up seems to really get it up and running.

I've usually found with these motors that its usually something simple causing it to not behave properly. If you have good spark, and adding fuel at the top end doesn't help, then I'd say fuel and spark is good. That pretty much leaves timing, and if the throttle blades are open, or the motor is to low.

[fmjshooter]

Thanks for reading all of that yes the throttle blades are flat at wide open. I originally went through all this diagnostic crap when I thought it was running at 4k but now that I know the tach is off some and might be pushing 4700 I can maybe see the set up killing my top end. Best I was able to get out of her was 37 mph(GPS). On paper this engine is supposed to put out around 110 hp and the 115 pushed it to 50 mph with a 19 pitch prop this thing should be screaming with a 17 pitch on it but then again maybe someone changed the pitch on it. Without a solid base to gauge this thing I just don't know which way to go.
For all I know the guy dicked up the build and I'll have to bring it back

I've little experience setting up boats is it within reason to expect a buried cavitation plate and or dicked up prop kill 700+ rpm?

[perfmarine1]

57C is correct jet for 100Xp. Core plug allows you to clean idle holes, do not remove. If you squirt carb cleaner through and all holes shoot fuel you are ok. I believe you are to low with motor, cav plate 2 inches below bottom is too low. Raise motor to cav plate 1-2 above if cavitates get a raker prop. Are you running full trim? If you are and still hitting a wall at 4700rpm get a smaller prop.

[fmjshooter]

Thanks for that hard to find info on these carbs I pumped alcohol from a syringe through all the passages and they seemed to flow fine. Trim all the way out gave me an extra mph or two but doesn't seem to effect rpm much at all. I'll try raising the motor up two holes and see what happens.

[fmjshooter]

Another question this hull has the step between transom and pad where the aerator inlets/drains mount up not sure how this effects my prop height measurement and what not I'm guessing your measure from the step correct?

[perfmarine1]

You measure from the bottom of boat/ pad. Notch is for factory setback. The more setback the higher you can raise motor. The higher the better, as long as you don't loose water pressure or prop bite. Stock props need to be under water, Raker type props can run higher, and overhub props( choppers and Hoss style) can run on the surface with 1 blade at a time slapping the water. If boat is heavy and HP low you normally can not run a surface prop because of too much slip.

[Lockjaw]

The easiest way to measure it is adjust the trailer with the jack until you can put a level on the bottom of the boat and its level. Then put the level on the cavitation plate and adjust until its level. Take the distance from the center of the propshaft to the floor from the distance from the bottom of the hull to the floor, and that is your prop to pad height. You need to be around 3.5 to 3 inches.

My boat runs good with a raker 2 prop. I have the motor back 8.5 inches off the transom.

[racervboat]

Compression check it better yet leak down,just because its a new rebuilt powerhead dont mean chit.You say you have motor knowledge so put a wrench on the flywheel and check for a soft cylinder by turnig it by hand you may just be surprised? if all good raise the damn motor.

[fmjshooter]

Already rotated by hand and pressure tested, all okay. I went ahead and raised it up and measured it at just around 3.5 inches now was 5+" Will test her in the water tonight or tomorrow and see what happens

If this was my problem that tach is getting loved tenderly with some hot .44 mags even though I'm the idiot that didn't test it first.

[Lockjaw]

I bet you will see a difference. On my Javelin when I got it I couldn't turn it 5K with any prop. I only took it out once or twice before I slapped a jack plate on it, and I didn't properly measure the prop to pad height, and had it way low, like 5.5 inches, and it wouldn't turn up still. You talk about pissed off. Once I got the motor height up, I started gaining RPMS.

A jack might help you too, depending on the what the weight diff is between the Zuki and the OMC motor. If the zuk was heavier, then swapping to a lighter motor would change the balance of the hull and it might not lift properly or as well as it should to get better top end.

My boat runs better with 8.5 inches of set back than it does with just 6.

[fmjshooter]

Well no change at all speed/RPm wise actually was down 100 rpm but that can be attributed to the mid day heat. Boat jumps on plane super fast and responds way better to trim adjustment now and the vent plate is just barely skimming water but no changes on top end. Think I may pull the carbs again and really look them over and check all the jet sizes. Seems to be running fat.

[flabum1017]

Have you had anyone physically look to see where the water is hitting the engine at speed? Cav plate should be above the water at speed..... Can you trim far enough where the engine breaks loose?

[fmjshooter]

Yep at neutral trim it rides just above the water. I spoke to a prop guy today and he thinks something is wrong too, says I should probably be running a 21p not a 17p and getting aroumd 45-48 mph. Said he would let me try an aluminum he's got on the shelf but from what little I know about props wouldn't that just lug the motor worse? Or maybe the 17 on there is so off that I would get more bow lift thus more rpm with the 21p? I'm just lost here

[keefallan]

I have to think that the carbs may not be the right size. I think they should be at least 1-1/4 or 1-5/16. My 112 spl has 1-5/16 carbs. If it is a true 100xp then it should make closer to 110. The gt150's make closer to around 165. The only way to make more hp on these motors is the carbs(unless you raise comp. or porting). The 1-3/16 carbs come on a 90. I don't think there is any way that you can make a 100 plus hp with 90hp carbs. A stock 150 comes with 1-1/4. A gt150 comes with 1-5/16. I wonder if you got a 90 hp p-head? What does the core plug say on it? I could be wrong on this, but I have opened up a lot of 150's and gt motors. I have had a few v4's open too. The mains on a stock 150 are like a 58(if I remember offhand). The v4 has the same internals as its v6 brother....just less cylinders. I have a 112spl with 135 heads and it pushes a tank of a crab boat to 40mph with 52-5300 and a 17 prop.