Force 75 ELPT missing and rough idle

[Dieseldaddy]

I have a 98 75 hp force trying to get it running smooth. Just purchased the boat a month ago and it's my first one. At idle it only fires on top cylinder. Once the rpms climb tho middle cyl starts to fire. But the third cyl never seems to kick in. Also just replaced the head gasket and still have a lil bit of moisture on 3rd plug. Comp is 125 on all 3. I would appreciate any and all tips please.

 

[TwoFish]

Re: Force 75 ELPT missing and rough idle

When you say it doesn?t fire on #2 or #3 do you mean there is no spark to these cylinders or no fuel? Just trying to work out what is happening.

I have the same motor and have a slight miss or an uneven idle when cold. I take it that the motor has a single carb?

Good to see the compression is OK.

As I understand it moisture can enter through the exhaust side. Exhaust plates on the port side of the motor.

Did you have to remove the power head to replace the head gasket? It would have been a bit squeezy getting to the bolts.

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

At an idle if you pull the 4 pin plug off of #2 and #3 cyl coils it does not change the idle. Yes this is a single carb model as for the head gasket I did not remove the power head the bottom bolts were a little tight to get to but not bad

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

As for the exhaust gaskets. I've read some post on here and it does not seem hard to change so I think that's my next choice. I was just wondering about the single cyl idle. It idles fine but when you put it in gear it wants to kill the engine but if you adjust it up then it grinds going into gear.
I know the wires need ghanged because I tried turning them while running and got zapped by both 2 and 3.
Also I am compleatly out of adjustment on the timing. The screw is all the way in and I'm only reading 18btdc. I checked the #1 piston in reference to the timing marks and it's spot on. Is there any adjustment under the flywheel. ??? All help is appreciated.

 

[TwoFish]

Re: Force 75 ELPT missing and rough idle

My motor will idle happily on 2 cylinders but will only rev to above 2000 rpm under load.

As far as I know there is no adjustment underneath the flywheel for timing. There is an adjustment arm that is in front of the flywheel.

Are you pushing the forward hand control (throttle) all the way forward when you are reading the timing. This mechanically advances the timing mechanism to full advanced. You need to do this with the sparkplugs out of the head and crank the motor using the starter motor. The engine will over rev without a load if you try to do this out of the water. The plug leads need to be grounded to prevent damage to the CDMs. If you are checking the timing with the motor idling you are only checking the idle timing which from memory is 8 degrees before TDC. If the idle timing is 18 degrees you are 10 degrees too advanced which would account for the rough idling.

Sorry if you already know this. It takes a while to work out where people are up to with the knowledge of their motor.

In the frequently asked question section at the top of this forum there is a great guide by Frank Acompora for tuning the motor. http://forums.iboats.com/showthread.php?t=431021

If you haven’t done it already retune the motor as this may be the only problem.

As far as I know the high tension plug wires are part of the CDM and are not removable. I’d like to change mine but have not been game enough to remove one. Does anyone know whether the later model plug wires are removable?

Good Luck

 

[pnwboat]

Re: Force 75 ELPT missing and rough idle

Sounds like you've got several problems. First tackle the ignition timing. Easy to correct. The late model Mercury style CDM ignition system should be set to 32? BTDC at WOT static timing method. This is different than the older Prestolite ignition systems which are set to 28? BTDC. You typically don't set the timing at idle, it should be checked at WOT.
Timing is done by adjusting the screw on tower shaft linkage. See the yellow arrow in the picture. The red arrow is the idle speed adjustment.




Next, the lack of spark to # 2 and 3 cylinder at idle. A little tougher but again relatively easy to check. Look up under the flywheel. There should be six magnets epoxied to the inner circumference of the flywheel. Make sure that they are all intact and not broken or missing. See the picture below of the flywheel.



If that checks out, disconnect the Green/White and White/Green wires at the bullet connectors that come out from under the flywheel from the ignition stator. Measure the resistance with a meter between the two White/Green and Green/White wires that go to the stator. It should read between 500 - 700 Ohms.

This is what the stator looks like. There are two versions. The red one as pictured is the good one. There is another version that is black and has a higher failure rate. Note the two Green/White and White/Green wires in the picture. The two yellow wires go to your rectifier/regulator and are used to charge the battery.



As far as the moisture on the bottom plug, it could be the exhaust gasket if you've already changed the head gasket. Exhaust gasket leaks most commonly, but not always affect the bottom cylinders. Also check the stainless steel baffle plate under the exhaust cover for cracks. It'll probably look pretty warped, but this is normal.

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

Two fish I miss understood the static timing I was trying to set it 8 degrees less than the 32. Excellent info

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

Pwnboat I will check the magnets and wires from stator. If memory serves correct it is the red stator. Again excellent info and I am berry appreciative for the help. Like I said first boat so I'm in new territory

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

Well the magnets checked out and the stator ohm reading was 695 I reset the timing at WOT an ordered the gaskets for exhaust

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

Also I added some insulation to the plug wires and back of the cowl to eliminate the possibilitty of the plug wires arcing I'm going to take it out for a while tonight and see how she runs

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

I've read on here that the bolts on the exhaust plate can be tricky any suggestions ??

 

[pnwboat]

Re: Force 75 ELPT missing and rough idle

Just be careful when loosening. If you have one that feels like it's seized and won't turn, stop and gently work it back and forth. You can try tapping on the head of the bolt, or spray PB blaster or similar product to try and break down any corrosion, or apply heat with a propane torch.

Also wanted to mention that the spark plug wires on the 1996 and later Mercury style CDM ignition where the cd module and coil are one unit (see 1st photo in post #6) do have spark plug wires that are replaceable. The wires are solid core, not the spiral wound RFI suppression type commonly used for automotive applications today. The spark plug wire socket on the CDM has a male screw like pin. Just take the wire and unscrew it and it will come out.

 

[TwoFish]

Re: Force 75 ELPT missing and rough idle

Tested my stator as well 685.

Pnwboat, is it common for the plug wires to need replacing?

Sorry Dieseldaddy don't mean to hijack your thread. Thought the answer would be useful to both of us.

I'll be interested to see how you get on removing the exhaust plate. It's something I've been putting off for a long time.

 

[pnwboat]

Re: Force 75 ELPT missing and rough idle

It's not too common to have to replace the wires. The older Prestolite ignition systems with the blue coils and CD packs have non-replaceable spark plug wires and I've seen some that were probably at least 15 - 20 years old, if not older. If you're getting shocked while the engine is running, then they probably need to be changed. Sometimes the connector and or boot on the spark plug end is all that needs to be replaced and not the wire itself.

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

Just as a temp trial fix I wraped the plug wires with electrical tape That stopped the shocking so I will need to replace them. Hopefully I will get the gaskets and change them this weekend. For now other than the moisture on plugs all seems to be ok. Thanks to the knowledge I gained from you guys. Thank you

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

I'll let ya know how the gasket swap goes

 

[TwoFish]

Re: Force 75 ELPT missing and rough idle

Dieseldaddy. Are you running the motor in fresh or salt water?

I’m interested to know as I run mine in salt water. I’m concerned about how this may effect the ease of removing head or exhaust cover bolts.

Also interested to see if you can get to the exhaust cover bolts without removing the powerhead. I’ve put the job of because of the thought of having to remove the powerhead.

Quite enjoying this thread because we have the same motor. Learning a lot from it as well.

Cheers

TwoFish

 

[Dieseldaddy]

Re: Force 75 ELPT missing and rough idle

two fish i run mine in fresh water but due to the look of the pontoons im going to say it was a salt water boat at one time.. it looks like i can take the two nuts from the lower cowl and tilt it enough to get a ratchet in there.. i had the boat out today and it ran better i can actually idle in gear and it dosent stall going from forward to reverse or vise versa.. hopefully i will get the gaskets soon and i will let you know how hard it was on mine... should i use a sealant on the gaskets or is it not needed?? also from another thread on here i found the torque specs at 70 inch pounds is that correct?? again thanks for the info and have fun on the water

 

[pnwboat]

Re: Force 75 ELPT missing and rough idle

My Clymer manual shows the torque for the exhaust cover bolts is 115 inch pounds. Most other motors are between 70 and 90 inch pounds. Standard torque for 1/4 20 bolts is 70 inch pounds. There is really no need for sealant. If you feel you must use some, I have used the Permatex Ultra Gray and never had a problem. Use it sparingly. Don't want any excess to ooze out and block a water passage. There are a few spots in the exhaust cover that has pretty small water passages.

If you're having some idle issues, might check out the fuel recirculation hoses and check valves. The check valves can get clogged with gooey fuel/oil residue and hoses cracked and leak. Check valves are #3 and #5 in the diagram below. A shot of carburetor cleaner usually takes care of it. You can test the check valve by blowing through it. Air should only flow in one direction.

http://www.crowleymarine.com/mercury-outboard/parts/2534_5.cfm

 

[TwoFish]

Re: Force 75 ELPT missing and rough idle

My manual has 2 different torque setting for the exhaust cover. Manual is PDF format off the net so have no Idea which manual it is.

70, 75, 90, 120HP -- 70 in.lbs

75hp ----115 in. lbs

It doesn’t give the year range for either group.


pnwboat, you mentioned cleaning the check valves. Do you need to remove the valves from the engine or can you spray cleaner in them and blow through them whilst they are on the engine?
I’m concerned about snapping then off trying to remove them.

Thanks again Deiseldaddy and pnwboat! This thread has been very helpful.

I have a question about my stator but I’ll start another thread for that next week.