1991 90HP Force problem updated motor is shot & rebuild started

ezez · Jun 3, 2009

I have a 906x91c that is having issues at high RPM cruising on the water. At high RPM the motor misses intermittently and it appears to get worse as the motor gets warmer (this might be my imagination). This miss doesn't impact the ability to maintain overall speeds and the boat is still serviceable but something isn't right.

I really don't know if it is an ignition or fuel delivery issue. I have slapped new plugs and a new fuel diaphram in it and the symptom persists. Also, a compression check is a little less than promising as it reads 100/120/120 moving down the motor.

My next steps in order:
1) Decarb motor
2) Rebuild carbs
3) Have coils checked as I don't know of a way to check for missing intermittent voltage/spark

Any help is much appreciated!

chitownborn · Jun 3, 2009

Re: 1991 90HP Force problem

Re: 1991 90HP Force problem

this is what ive done and really works well, to check for spark. get a timing light and tape the trigger on the light closed so it will stay on, go to the water take cowling off, hook up timing light to the first spark plug, lay the timing light somewhere so you can see it, bring it up to wot keep a eye on the light, if it misses the timing light will react, locating the problem coil if not go to the next one and try again till you find it. i had a coil go open at wot and couldnt find it , checked with meter all checked okay this is how i found the no 2 cylinder would fire intermintantly at wot good luck don

Matthew A. · Jun 3, 2009

Re: 1991 90HP Force problem

Re: 1991 90HP Force problem

If your coils are good you may want to check the fuel system for loose hose clamps, bad hose connections or faulty hose. Air leaking into the fuel system will cause the motor to run lean. At WOT this can destroy a motor in seconds. Whilr checking the fuel lines also check for clogged fuel filters and/or fuel pickups inside the tank/s. Cracked hoses as well as bad fuel bulb either with cracks or holes or a sticking check ball inside the bulb. The motor running hot is usually a clear indication the motor is either not receiving sufficient cooling or is running in a lean condition. Although a blown coil doesn't really pose any serious harm to your motor. A fuel related issue that maybe causing a lean condition is one that should be looked into and fixed without further ado. Until the issue has been fixed it is strongly advised that you avoid running the motor anywhere near WOT. Unless you don't mind having to possibly spend a good deal of time and money rebuilding the head. Something this particular forum can be of great help should you decide to take such coarse.

ezez · Jun 3, 2009

Re: 1991 90HP Force problem

Re: 1991 90HP Force problem

Thanks for the help fellas but things have changed.

I decarbed the motor today and took it on the lake tonight to see if there was any improvement. Well, it is now a much more substantial problem and the motor needs to be rebuilt the top cylinder is gone ( I haven't checked the others yet).

Prior to working on it today, I spoke with two marinas and after communicating what was going on and mentioned the spread in compression they both said I was one borrowed time with the motor.

Now do I rebuild or repower? It is going to be one or the other, as I won't be dumping the boat because it is in excellent shape. I am mechanically inclined but haven't tackled a rebuild before. To have the cylinders rebored and the costs of pistons rings gaskets etc what is the approximate total cost to rebuild? How long would it take me to rebuild it?

Frank Acampora · Jun 4, 2009

Re: 1991 90HP Force problem ***updated motor is shot

Re: 1991 90HP Force problem ***updated motor is shot

Rebuild requires only good mechanical aptitude and for most procedures, only hand tools. It does require a good manual. Factory is best, but Clymers is adequate. It requires a flywheel puller and an INCH pound torque wrench. A press is helpful for removal of connecting rods from the pistons.

You can probably buy another used engine for what it will cost to rebuild, but if you rebuild, you know the condition of the engine. A used engine will be an unknown.

Pistons will cost (depending upon what you buy) about 90-125 each and reboring will cost about 50-65 per cylinder. head gasket, fuel pump diaphragm, water pump impeller and various other gaskets will cost around 100.

Depending upon condition of the other two cylinders, they may or may not need reboring. Let the machine shop tell you. It is not necessary to replace all three pistons if only one is bad. However, if two cylinders are out of tolerance, you might as well do all three and have, in essence, a new engine.

ezez · Jun 4, 2009

Re: 1991 90HP Force problem ***updated motor is shot

Re: 1991 90HP Force problem ***updated motor is shot

I have everything needed to do the job. I already have the factory manual as well.

I'll need to research parts but it sounds like it is a $500-$750 job for a complete rebuild.

Working at a moderate speed... does a weekend to tear it down and a weekend to put it back together sound reasonable?

Frank Acampora · Jun 4, 2009

Re: 1991 90HP Force problem ***updated motor is shot

Re: 1991 90HP Force problem ***updated motor is shot

Yep! That is about the time it should take---However---be aware that the studs usually corrode into the midleg so removing the block will usually be difficult and take more time than you thought. The exhaust cover bolts usually also give problems. try not to twist off any of them.

While you have the engine apart, if you have a Dremel and a couple of carbide bits, consider porting the bypass and exhaust. You will spend about two hours on each side of each cylinder for a total of about 6 hours, and you should gain a significant amount of power. Due to increased pumping efficiency, you will notice a small amount of better fuel economy at partial throttle, but at full throttle, be ready to spend more money for gas. It takes extra fuel to make more horsepower.

Send me your email address in a private message and I will send you a couple of photos of port work done on an older model 3 cylinder engine with good results.---if I can find them.

That's a '79 Chrysler 140 in the avatar. It is ported and moves a 21 foot cuddy at 38 MPH GPS speed. My ported 80 something Force 85/90 moves a 15 foot Glastron at 43 MPH.

ezez · Jun 5, 2009

Re: 1991 90HP Force problem ***updated motor is shot

Re: 1991 90HP Force problem ***updated motor is shot

I checked compression again and the top cylinder is shot but the lower two were still 120ish. I than took a look at the damage. Of course the top piston is shot and the walls of the cylinder have some lite scoring, middle looks OK but the bottom appears to have some metal shrapnel in the top of the piston.

Any thoughts?

Here she is in all her glory:

Top Blown Cylinder/ Piston:

Middle Cylinder/ Piston:

Bottom Cylinder/ Piston:

Frank Acampora · Jun 5, 2009

Re: 1991 90HP Force problem ***updated motor is shot

Re: 1991 90HP Force problem ***updated motor is shot

Top is classic detonation damage due to lean running. Top carb was either set too lean ion the low speed adjusting needle or the inlet needle clogged allowing thecarb to run lean.

Bottom piston shows some ring land damage and needs to be replaced. It looks like it may have eaten something; possibly a reed or a reed retaining screw. It is unlikely but possible that debris from the top piston may have entered the cylinder.

It is really your call to make, but if it were my engine, In this case, since two pistons need replacing, and since the good two cylinders are a tad low in compression, I would overbore all three cylinders and install oversized pistons. I suspect you would see a compression ratio of around 145-165 with the repair.

Then, when you reassemble the engine Be certain all three carbs are set no leaner than 3/4 turn out from lightly seated. Install a fuel filter BETWEEN the fuel pump and the carbs.

Set the timing advance to 29-30 degrees at WOT.

ezez · Jun 5, 2009

Re: 1991 90HP Force problem ***updated motor is shot

Re: 1991 90HP Force problem ***updated motor is shot

That is what I figured and honestly, if I am going to rebuild I would prefer to do the whole shebang.

Well, Frank I appreciate the help and hope you'll be around for some hand holding as I tackle this rebuild!

I'm going to start tearing it down this weekend.

ezez · Jun 7, 2009

Re: 1991 90HP Force problem ***updated motor is shot

Re: 1991 90HP Force problem ***updated motor is shot

Well, at a real leisurely rate of work I spent about 8 hours working on the motor this weekend. All the fuel and electrical components were removed and the powerhead is pulled. The only difficult part of that process was pulling the flywheel. I had to heat, bang and pry but it finally came free.

Tonight I'll begin cracking the head open.

ezez · Jun 7, 2009

Re: 1991 90HP Force problem ***updated motor is shot

Re: 1991 90HP Force problem ***updated motor is shot

Well that didn't take long another hour in the shop and we are good to go. I'll need to call around to machine shops and get prices etc.

Also, I'll need a list of everything I need to order. What all gaskets are needed? I know there are good and not so good pistons, any opinions on name brands. How about places to order?

Thanks for all the help to this point. It has been pretty painless thus far.

ezez · Jun 8, 2009

Re: 1991 90HP Force problem ***rebuild progression

Re: 1991 90HP Force problem ***rebuild progression

Are Wiseco piston and rings the way to go?

Should I change out any of the crankshaft bearings? Or just leave them be?

Shopping List:

Pistons (3)
Rings (3)
Wrist Pins and Bearings? (3)
Head Gasket
Lower Plate/PH Gasket
Exhaust Port Gasket
Transfer Port Gaskets (3)
Recirc. Gasket
Carb gaskets???
Crankcase doesn't have gasket...???
Impeller

What else?

ezez · Jun 9, 2009

Re: 1991 90HP Force problem ***rebuild progression

Re: 1991 90HP Force problem ***rebuild progression

Alright, it appears that I am talking to myself now. LOL

I took it into the machine shop and according to the shop the top cylinder needs boring but the lower two are good to go. So, I guess I'll order the top .010 over and the other two standard sized. I think I am going with Wiseco pistons from outboardparts.com. Has anyone else used them?

Should I just buy a gasket kit? Is that the easy way to go? Anything else I might need? Should I purchase carb kits and go through them as well?

GrindKore · Jun 9, 2009

Re: 1991 90HP Force problem ***rebuild progression

Re: 1991 90HP Force problem ***rebuild progression

ezez said:
Alright, it appears that I am talking to myself now. LOL

I took it into the machine shop and according to the shop the top cylinder needs boring but the lower two are good to go. So, I guess I'll order the top .010 over and the other two standard sized. I think I am going with Wiseco pistons from outboardparts.com. Has anyone else used them?

Should I just buy a gasket kit? Is that the easy way to go? Anything else I might need? Should I purchase carb kits and go through them as well?

Yes I defiantly would use WISECO again, much easier to work with than OEM pistons because you do not need a press to install wrist pins.

By the way, I have been in your shoes last year. I bought a boat and inherited whole slew of problems, have done piston rebuild on my own (with iboats.com forum and Frank's advice of course) I feel a lot more confident out on the water knowing my motor inside out as the result. Here is my rebuilding saga thread http://forums.iboats.com/showthread.php?t=266436

By the way, when you are assembling pistons, use special Quicksilver bearing assembly grease to hold down rollers. Regular grease is not designed to burn up in a 2 stroke and may gunk up your piston bearing causing various failures. Also be very careful with the roller cages when you torquing down pistons to the crankshaft. I had one roller misaligned and it popped the cage. It cost me $120 to replace a tiny piece of metal.

GrindKore · Jun 9, 2009

Re: 1991 90HP Force problem ***rebuild progression

Re: 1991 90HP Force problem ***rebuild progression

On a final thought, take this as an opportunity to examine and repair other parts to prevent future failures. For example examine your reed valves. While you are at it, take off screws holding each valve and apply red Locktite compound and tighten the **** out of them. This will prevent the reed screws from falling in to a cylinder while you are under way. Take your carbs apart and soak them in carb cleaner for a night just for prophylactic reasons. Make sure jet needles are not damaged or clogged, replace all gaskets that you have removed even if they are intact. Make sure all the surfaces where you have removed old gaskets are clean and smooth to ensure proper seal. Take your time and learn about your motor, your life may literally depend on it one day.

ezez · Jun 9, 2009

Re: 1991 90HP Force problem updated motor is shot & rebuild started

Who did you order your Wiseco pistons from? When you did the rebuild did you order a gasket kit or buy individually and again who did you order from?

Like you suggested I am going to go through stuff and make sure things are as they should be.

GrindKore · Jun 9, 2009

Re: 1991 90HP Force problem updated motor is shot & rebuild started

ezez said:
Who did you order your Wiseco pistons from? When you did the rebuild did you order a gasket kit or buy individually and again who did you order from?

Like you suggested I am going to go through stuff and make sure things are as they should be.

Read my thread cited above. I ordered everything from outboardparts.com, I bought the entire kit since it has everything you need to rebuild entire motor. Well worth the money in my opinion.

pnwboat · Jun 9, 2009

Re: 1991 90HP Force problem updated motor is shot & rebuild started

You also might want to replace the crankcase seals that are on the crankshaft. They're very easy to break when removing or installing the crankshaft. Number 11 in the diagram below. They look like similar to piston rings, but they actually go around the crankshaft. I broke one when I was re-doing my motor. I ended up ordering a couple of extra, just in case.

http://www.crowleymarine.com/mercury_parts/1364/110.cfm

ezez · Jun 10, 2009

Re: 1991 90HP Force problem updated motor is shot & rebuild started

Placed my order w/ outboardparts.com...now I wait.

1991 90HP Force problem updated motor is shot & rebuild started

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