90 HP won't turn over with plugs in.

[Kenny323]

I have a 90 hp Chrysler. Model 908h3a. When all the plugs are out the motor will turn over fine and the flywheel spins freely. If you put one plug in it will barely turn over. Same if all four are in. I've had the starter checked and tested under a load, checks fine. I have tested with several batteries even using jumper cables directly to the starter , same results. Plugs out fine , plugs in no go. The result is the same for a single plug in each individual cylinder. Even took the lower unit off , same results. Any help would be greatly appreciated.

 

[pnwboat]

Testing with jumper cables directly to the starter can be sometimes mis-leading. Unless the jumper cables are good quality heavy gauge, they will not pass enough current to the starter. Same thing with the battery cables. Make sure both the positive and negative cables are making good connections and the negative cable is bolted directly to the engine block. Might also have what ever battery your using tested to make sure it's in good shape.

 

[Jiggz]

Disconnect the negative terminal from the engine block. Wire brush it clean and shiny and also the mating surface on the engine block. Reconnect and tightened. Squeeze some fuel oil mix into each cylinder and re-install all spark plugs. Using a large size screw driver, short the starter solenoid main terminals (two large post).
-If the engine turns over try cranking it through ignition switch. If it will not turn over, the solenoid needs to be replaced.
-If the engine does not turn over, parallel a second battery to the existing battery using jumper wires. If it still does not turn over after shorting the starter solenoid, it is time to open and inspect the cylinders.

 

[tommarvin]

We had this same problem,when we forgot to put the negative battery cable on the battery after new battery boxes installed..
Load test your battery(s) the battery(s) batterys can still be bad with good volts.
Make sure all battery cables and terminals are clean, and bolted down with nylon lock nuts, never wing nuts.
You can test a solenoid it's fast and easy, You-tube explains it perfectly.

 

[Kenny323]

I got it to spin enough to crank by hooking it to my truck. I am using 4 gauge jumper cables but they seem to build a lot of heat. Should I get heavier?

 

[Jiggz]

So did you clean the negative cable connection on the engine block? If not I still recommend doing this. If you were able to crank it over by jumpering it with a larger battery, maybe it is high time to get the biggest CCA battery you can afford. I always recommend dual purpose starting/deep cycle batteries for this motor and make sure you get those regular non-maintenance free type. Did you also try shorting the solenoid to get it out of the equation?

 

[fhhuber]

Heating 4 gauge cables... is expected if jump starting a large farm tractor. You should not be pulling that much current.

Do you hear clanking as it turns with the plugs in? (like pistons hitting the plugs)

Something's not right.

 

[Kenny323]

I shorted the solenoid and it is fine. There was no clanking sound at all . I am going to clean every possible connection again just to be on the safe side. The motor turns freely with the plugs out.

 

[jerryjerry05]

First rebuild/ clean the starter.
Then do a compression test. Low comp can cause this.
Comp OK?? Then drop the lower and see if that makes a difference?

4or 2 gauge cables is what most outboards use(my 225Merc had 4).
Some smaller use 6ga.

 

[Kenny323]

Alright, cleaned the contacts, was able to start one cylinder at a time. Plugs out of the other 3.Runs fine on one cylinder at idle.Everything sounds fine internally not clanking , or anything. Good compression on each cylinder. I guess it's time to get the wiring diagram out and search for anything that could be the problem.

 

[Jiggz]

I shorted the solenoid and it is fine. There was no clanking sound at all . I am going to clean every possible connection again just to be on the safe side. The motor turns freely with the plugs out.

What do you mean, "the solenoid is fine"? Does that mean the motor cranked over with all plugs in when it was shorted with a screw driver with just the original battery connected and not jumpered from your truck? Troubleshooting is mostly about elimination process. Unless you know for sure what the real cause is (e.g. will not start because the battery is missing), process of elimination is the only way to go.

 

[tommarvin]

Get a battery load tester at HF tools 22 bucks,load test the battery is the battery ok?
If the battery's ok, with all the plugs in, when you turn the key does the starter turn the flywheel hard enough to start the motor?
Don't do any more jumping, so we can eliminate that.
Are you saying you have a ignition problem,

 

[Jiggz]

Just to be clear, the word "crank" when referring to starting a gas engine means turning it over to get it "started". Before the electric starter, a manual crank starter is used to turn over the engine to get it started. This is where the term "crank" was derived from. As soon as the motor fires and is running, then the proper term is "started" or "running". Now, for the final question, "Does the motor crank over or turn over and will not start or run or it will not crank over or turn over as indicated by your title"?

 

[jerryjerry05]

Tom, read the post. OP said it runs so obviously not a starter problem.

What are the compression readings? "good compression" is not an answer??
Some do a comp test with their finger??

 

[Kenny323]

The compression is around 120 in each cylinder. I am going to take the starter apart and clean and inspect everything. If I am putting 12 volts directly to the starter with a good ground, I am thinking something may be wrong with the starter. Even though it checked fine when tested, it may have enough voltage drop to keep it from spinning the flywheel when under a full load.At least that's my hope. I will post back with results. Thanks for all the replies.

 

[tommarvin]

Check all your battery ground cables , are they all clean, tight, not broken .
We had this same problem and a ground was off.

 

[Kenny323]

Took the starter apart and cleaned everything. The brushes looked good , as well as everything else. Had a slight burnt smell though. Put it back together , hooked the cables directly to the starter with nothing else connected. Same results as before, it will spin the flywheel fine without plugs , put them in it slows down. I'm guessing there is a short inside somewhere that shows up under a load.

 

[Kenny323]

I also tried grounding several different ways, directly to the starter , different places on the block, same results.

 

[Jiggz]

it's most likely it is time to get a new starter. That fact you have burnt smell, you should see some burnt wires. That in itself is a bad sign the starter is no good anymore under load.

 

[tommarvin]

I think your right, a wire is grounded out somewhere, and could be hard to find..
Check every inch of wire, pull every wire hard that goes into a connector.
Do you have a battery that you load tested that you know is good, so you can test everything, on the boat, from the battery, check the battery cables,especially the battery ground cables, and everything else.
The flywheel will slow down with the plugs in under load.
Will the flywheel turn fast enough with the plugs in to start the motor, or is the flywheel not turning anywhere close to fast enough to start the motor.
Can you post a video of the slow flywheel.