force 40hp wont turnover due to very high compression

[sx3]

My Force 40hp 1992 wont turn over due to very high compression. Motor has been stripped and I can find no fault with it(No carboning up etc) When motor was rebuilt it turned over and started fine but after a couple of months layed up the fault has returned. It is extremely hard to turn over by hand but with plugs out it spins freely including using the starter. Stater motor is good. Any ideas? Thanks Stafford

 

[Frank Acampora]

Re: force 40hp wont turnover due to very high compression

Try removing the plugs and putting about one teasponnful of premix in each cylinder. Then crank it fast with the starter. Now try starting it. Try this a couple of times. Old oil does get thicker and may be causing too much compression and drag. I am assuming that you have already checked all connections to the starter and they are tight and corrosion free. You may also need to remove the starter and check the brushes and commutatior. A poor starter with worn brushes and dirty commutator will crank over an engine with no plugs at a good clip yet not be able to turn it against compression.

 

[fucawi]

Re: force 40hp wont turnover due to very high compression

if you have a voltmeter it is worth checking the battery voltage while you try to crank ...should not drop below 11.5 at the very worst and it is also worth checking the volts at the starter ie the case of the motor to the actual bolt that feeds the brushes ...should not be more than 0.5v below the battery . This should reveil bad battery /solenoid or connections....

 

[JB]

Re: force 40hp wont turnover due to very high compression

Howdy, SX3.

Welcome to iboats.

Each of the above point out likely causes of your trouble. . . IMO it is not high compression, it is a weak starting system.

The more common fault is hidden corrosion in the battery cables or connections. Fucawi's tests would isolate this, but I think taking the cables off, scrubbing the connectors shiny and reconnecting should be as routine as changing impellers in doing preventive maintenance.

Good luck, you are in good hands.

 

[Bondo]

Re: force 40hp wont turnover due to very high compression

Ayuh,.... I once had a similar problem,...

It finally came down to the battery to starter wire....
It had corroded internally...
The motor would start, if it was jumpered directly to the starter, but not at the battery...

I cut the wire apart to find the corrosion,... only a couple of strands were left, of the many....
It would pass voltage, 'n tested Good....
But, it couldn't pass the amperage to spin the starter...

 

[sx3]

Re: force 40hp wont turnover due to very high compression

Thanks for the info. Starter motor has been stripped and checked and have used another battery but still the same problem. Jumper leads straight to the starter by-passing the solenoids still has no effect. Using a starter rope the engine will turn one revolution and then the compression gets tight and becomes hard to turn over. It almost feels like the exhaust port is blocked and the pistons are building up compression on each stroke. This is not the case as the ports are clear and the pistons free of carbon. All earth points have been cleaned and the cable checked. Any other ideas? Has anybody had reed valves stuck shut, I thought this may be the problem.

 

[JB]

Re: force 40hp wont turnover due to very high compression

Reeds could not cause your trouble, sx3. Hydro-lock might, but the fluid has to come from somewhere and should pump out very quickly. It is a long shot, though. Could one of your carbs be dumping so much fuel into the crankcase that it is hydrolocking the piston?

 

[tater76]

Re: force 40hp wont turnover due to very high compression

Hows your lower unit?

 

[Frank Acampora]

Re: force 40hp wont turnover due to very high compression

Tater makes an excellent point. Since all the gears are in mesh constantly, a lower unit problem can stall the engine. If the upper end checks out, drop the lower unit and try to crank the engine. If it now cranks well and starts, then the lower unit or water pump is causing the problem. At a minimum, before dropping the lower unit try to spin the prop shaft in neutral. If it does not spin freely or makes noises, you know there is a lower unit problem. If it shows no problem, then try dropping the lower unit.

 

[sx3]

Re: force 40hp wont turnover due to very high compression

Thanks guys for your suggestions. The lower unit has been serviced and spins freely no noises. I can rule out hydrolock as the fuel is not connected and I have spun the engine over without plugs. I will try putting premix in cylinders and leaving overnight to see if it frees up rings. Other than that I may try connecting two batts in sequence to give 24v and BRIEFLY try to start her. Thanks

 

[GA_Boater]

Re: force 40hp wont turnover due to very high compression

Other than that I may try connecting two batts in sequence to give 24v and BRIEFLY try to start her. Thanks

Not a good idea, sx3. Better to hook the batteries up in parallel, neg to neg and pos to pos. 24v very well may fry something. With a parallel hook up more current is available for the starter. Have you tried putting in one plug at a time and see if you can ID which cylinder could be causing the prob.?

 

[sx3]

Re: force 40hp wont turnover due to very high compression

Hi. Yes have tried taking one plug out at a time but there is no difference. Two strokes are basic engines and I am at a loss as to what is occurring. I will try premix (may include carb clean type product) and leave overnight and as you say put 2 batts in series and try to crank her over. Get feeling that once it starts and runs for a while it may clear the problem. Its summer here down under just want to go fishing... Thanks for info

 

[fucawi]

Re: force 40hp wont turnover due to very high compression

Its still a 95% chance its the starter motor or associated wiring/battery...did you test it as suggested ....looking and cleaning is not testing ...test dont guess

 

[GA_Boater]

Re: force 40hp wont turnover due to very high compression

SX3 - DO NOT RUN 24 VOLTS. You will burn up something and I don't care how careful you are. Hook the batteries in parallel, NOT SERIES. You want more current, not voltage.


In the first post you said "When motor was rebuilt it turned over and started fine but after a couple of months layed up the fault has returned." If I interpret what you are saying correctly, you had this problem earlier, rebuilt the motor, it ran fine for a while and after sitting for a few months the original locking up on start returned. Is this right?

You also said "It is extremely hard to turn over by hand but with plugs out it spins freely including using the starter." You should be able to turn it by hand and it will be hard as the pistons come up to TDC. This is normal and you might need a wrench or socket on the flywheel nut to turn it through TDC. If the plugs are in, can you spin the motor by hand?

In a later post you said "Using a starter rope the engine will turn one revolution and then the compression gets tight and becomes hard to turn over." That kind of answers my second question, but not completely. Have you changed spark plugs lately?

Get back on these three questions. This is turning into long distance fault elimination. You don't have a high compression problem and it is one of two things. Or maybe both things - Electrical (bad cables, battery or starter) or simple mechanical interference.

 

[ratherbeboating]

Re: force 40hp wont turnover due to very high compression

everybody is giving you great things to check. one small thing to check is the flywheel key. if the flywheel key is sheared timing can be advanced and the spark will fire prior to tdc and will cause what seems to be high compression. hope this helps.

 

[sx3]

Re: force 40hp wont turnover due to very high compression

Hi. YesHad this problem before after it was laid up for winter. The motor was stripped to short block stage and starter brushes checked and comutater cleaned. Exhaust ports checked and cleaned although not necessary and bottomend serviced. Pos batt cable was found to have corrosion at batt connection so was cut back to clean wire. Unit reassembled and she turned over beautifully and ran. This was beginning of winter so I winterised it and laid her up. Now we have the same problem but as the boat is at our bach(Beach-house) I am unable to do all the recommended things until xmas. Will keep you posted. Not sure on batt cable as I used jumper leads straight to starter but no change. Flywheel key,no have not even got it to turn over so pre ignition is not a facter, Thanks guys Merry xmas.

 

[GA_Boater]

Re: force 40hp wont turnover due to very high compression

Thanks for the update, sx3. One more question. When you use the jumper cables are you jumping the battery NEG post to the motor also?

 

[Jiggz]

Re: force 40hp wont turnover due to very high compression

Very interesting troubleshooting. Reading from all the posts so far it seems all possible causes have been mentioned but has to yet to eliminate the initial suspect of too high of a compression. How about removing all plugs and then insert the compression tester one cylinder at a time and do a reading on each cylinder. That ought to eliminate any too high compression symptoms. If all compressions are within normal range, the source of the problem is obviously either a blockage or mechanical interference. If one of the cylinder has abnormally high compression (which I sincerely doubt since the piston's stroke is constant), you might want to investigate this one further. While compression testing try cranking the engine for about 10 secs each and listen to the sound of the cranking between each cylinder. This will give you an indication which cylinder has or creating the problem. Can you post pics of the spark plugs? Make sure you spray oil in the cylinders if the fuel is not connected while doing the test.

 

[sx3]

Re: force 40hp wont turnover due to very high compression

Hi. Thanks yes will take comppression tester with me when I go back to where the boat is. I have purchased new batt cable and will strip starter again to check. Just read a post about a Johnson outboard wit identical problem, CRC into bores and left to soak fixed the problem so will try that too. Will post again after xmas if I can remedy. Thanks

 

[woodieturner]

Re: force 40hp wont turnover due to very high compression

Ayuh,.... I once had a similar problem,...

It finally came down to the battery to starter wire....
It had corroded internally...
The motor would start, if it was jumpered directly to the starter, but not at the battery...

I cut the wire apart to find the corrosion,... only a couple of strands were left, of the many....
It would pass voltage, 'n tested Good....
But, it couldn't pass the amperage to spin the starter...

I'm having the same problem with my Johnson 40hp. To compound the issue, I had my starting battery tested and it has a dead cell, so I am having a tough time pinpointing the problem. The battery connections were so hot it burned my fingers, ie seared off my finger prints burned, sizzled when it touched my skin burned. I tried to clean off the connections, but it could still be corrosion in the cables.
I hope it's as simple as that and not an engine/starter issue.