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Poor Performance at idle and mid range

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pcg

New member
I have a 1989 merc 135 V6 that has been giving me fits. Owned it since new and no problems EVER other then maintenance. I'm guessing it is running on five cylinders. It will not idle and will barely put a 19 ft sea ray on plane. If you put it to WOT it will skip a bit less until it starts running good. Seems like a fouled plug. If you run it full throttle for several minutes it continues to run fine but as soon as you pull it back off full it seems to miss again. The boat ran great the first trip out and the second trip this started to happen. New gas this year ad no change from first and second trip. I brought it home and replace the normal stuff, plugs, water separator, fuel filter but everything looked good. No change in performance. I figured it must be crap in the carbs so I pulled all 3 down to nothing. They were perfect, not so much as a spec of crap anywhere and the fuel showed no signs of water. I measured all the coil packs and not sure what they should be but all pretty similar. Checked all the lines and all looks good. Compression all between 114 and 116. I am simply at a loss. I should also add that this was converted last year to pre-mix and is running merc oil at 50-1. Simply not sure what else to try. Only thing left is CDI box but guessing this is not cheap to simply try. Any elp would sure be appreciated. Thanks in advance!!
 
Try pulling one plug at a time while running with plastic pliers,cylinder drop test and place timing light on each plug wire to test each cylinder for spark. Is your timing advancing with throttle? Try a open air gap spark test also, You should get a big blue lighting snap!
 
Loosen the two (only 2) bolts that hold the fuel pump on the engine block and see if fuel leaks out of the pump when you squeeze the primer.

Remember there are 4 bolts on the pump, just loosen the two that hold it to the block.

If fuel is found, rebuild your pump and retest.
 
johnnygjr said:
Try pulling one plug at a time while running with plastic pliers,cylinder drop test and place timing light on each plug wire to test each cylinder for spark. Is your timing advancing with throttle? Try a open air gap spark test also, You should get a big blue lighting snap!
Click to expand...

I would like to try this but have concern with arcing the coils and burning them out. My experience with cdi ignition is never leave the plug disconnected or not grounded or plan on spending money. Am I simply overly concerned? Appreciate the help
 
Docksidemarineservices said:
Loosen the two (only 2) bolts that hold the fuel pump on the engine block and see if fuel leaks out of the pump when you squeeze the primer.

Remember there are 4 bolts on the pump, just loosen the two that hold it to the block.

If fuel is found, rebuild your pump and retest.
Click to expand...

Quite interesting, I would have never even considered such a thing. I will definitely give this a try today. I do smell excess fuel when I pull off the cowl but no signs of leaking. This solution seems to fit perfectly. I have watched the carbs intake while running and no abnormal fuel and all six carbs look perfect. The bulb does pump up hard and seems to stop but no signs of fuel anywhere. Probably would not notice a pin hole leak. Thanks so much for sharing some experience. Experience is priceless!!!!

One other question, when I reinstalled the connection that links all the carbs together (sync) I simply made sure all butterfly plates moved at he same time. Is this correct thinking?
 
pcg said:
One other question, when I reinstalled the connection that links all the carbs together (sync) I simply made sure all butterfly plates moved at he same time. Is this correct thinking?
Click to expand...


There is more involved than that in a sync. If you just removed the carbs and reinstalled with out moving all the screw associated with the linkage, you should be close enough. Most important is that all butterflies close at exactly the same time.


As for the fuel pump leak, it will pump gas straight into the crank case and not into the cowl, it can make the engine run crappy, stall around idle speed and run fine at wot. You will smell all this raw gas as it passes unburned at idle into the exhaust.
 
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Docksidemarineservices said:
There is more involved than that in a sync. If you just removed the carbs and reinstalled with out moving all the screw associated with the linkage, you should be close enough. Most important is that all butterflies close at exactly the same time.


As for the fuel pump leak, it will pump gas straight into the crank case and not into the cowl, it can make the engine run crappy, stall around idle speed and run fine at wot. You will smell all this raw gas as it passes unburned at idle into the exhaust.
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Sad to say I just pulled the pump and all is dry. I put excessive pressure on the bulb and nothing came out the vacuum port. Darn, that seemed like the perfect solution. One thing I did notice was the primer bulb was dry. Seemed like all the fuel ran out of the lines and water separator. Took many pumps to get it primed back hard. I ran it yesterday. I simply do not remember this ever happening before. Usually half a pump the next day if left on the water if I recall correctly. Is there a check valve somewhere in the system?
 
Rebuild that pump while you have it off. Especially if it has not been done in many years. Simple job. Still may be the cause of all your issues.

Probably no check valve in the system. Likely no anti siphon valve on the fuel tank either(Outboard). May have a leak on the fuel system letting air in.
 
Docksidemarineservices said:
Rebuild that pump while you have it off. Especially if it has not been done in many years. Simple job. Still may be the cause of all your issues.

Probably no check valve in the system. Likely no anti siphon valve on the fuel tank either(Outboard). May have a leak on the fuel system letting air in.
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Sounds good, it has never been done so certainly can't hurt. Have you used the Sierra parts or should I spend the extra for OEM?
 
One thing about doing drop tests, you can always stick a spare plug into the plug wire of the cylinder you're testing, and ground the plug body against the block. Then you're not firing that cylinder without a load on the ign.

BTW the ADI coils are notorious for cracking and that'll cause intermittent problems. You might want to pull the coils for an inspection, including peeling-back the rubber bootie to thoroughly check for cracks and arc marks.

When the motor is misfiring underway, you can use an inductive timing lite to check spark strength. A bright timing lite will pick up any weakness. Not sure if you'd be able to safely move the pickup from one wire to the next when running, but you could do one, then stop and move the pickup to the next plug wire, since the problem is repeatable.
 
Ed-Mc said:
One thing about doing drop tests, you can always stick a spare plug into the plug wire of the cylinder you're testing, and ground the plug body against the block. Then you're not firing that cylinder without a load on the ign.

BTW the ADI coils are notorious for cracking and that'll cause intermittent problems. You might want to pull the coils for an inspection, including peeling-back the rubber bootie to thoroughly check for cracks and arc marks.

When the motor is misfiring underway, you can use an inductive timing lite to check spark strength. A bright timing lite will pick up any weakness. Not sure if you'd be able to safely move the pickup from one wire to the next when running, but you could do one, then stop and move the pickup to the next plug wire, since the problem is repeatable.
Click to expand...

I picked up an in line spark tester today and will give this a try. I also ordered a rebuild kit for my fuel pump. I would expect a cracked coil would be worse at WOT?? I'll check anyway. Thanks to all for your help, I am really stuck. Hate to put it away broken.
 
Docksidemarineservices said:
OEM on that rebuild kit.
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Finally got back on my poor performance issue. Unfortunately I had already ordered the Sierra kit prior to hearing back from Dockside who recommended OEM kit. Well, the Sierra kit seemed Ok with the exception of the diaphragm lock pins which were junk. I now ordered the OEM kit.

FYI there was definitely a bad diaphragm in the pump so hopefully this solves my problem. Thanks again, Ill update once the new part arrives.
 
Spoke too soon, seemed to run fine on the trailer but once I dropped it in the water I was back to where I started. Did fix the drain back issue so not a waste of time anyway. Does anyone know if there is a low speed and hi speed ignition col on these? My aftermarket book is pretty vague. Guess I need to buy the factory book.
 
Well, finally back at it. Put a spark tester on all six plugs and two were very weak at low speed. Checked coils, all fine. Ended up replacing the CDI box for one side and all runs great now. Thank to everyone who tried to help?
 
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