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Help troubleshooting miss on '89 120hp looper

J

john_day

Member
I have a miss that comes and goes and can't seem to find, fix it.

Motor seems to idle and troll fairly smooth.

At cruise, 3800+, the motor will run smooth then it misses. This goes on the whole time running. The rpm's don't drop but I can feel the boat slow just a bit and motor vibrate. Also the sound the motor makes is something similar to the sound from a semi when the driver takes his foot off the accelerator. Naturally, no where near as loud.

WOT is about 5300 and has the same miss.

I have a flo scan and notice when the miss is happening the gauge will vary between 1/2 to 3/4 gph.


I bought this motor just over a year ago and have been working on it since. I'll give a quick list to what I have done and where I am now.

Motor is currently raised to the third hole, QL82yc plugs at 35ths, they look in between brown and black and not dry. Timing is advanced 19 btdc. Manual recommendation is 18.

I have cleaned the tank, replace water separator, replaced all fuel lines up to the pump including fill hose, checked the fuel vent is not plugged, removed the vro, rebuilt the carbs and reset the float up and down several times, decarbed the motor a couple of times, replaced the plug wires, used ql77jc4, ql78yc,ql82yc plugs at 30 and 35th. Raised the motor, dropped the prop pitch 2"s to 15, link and sync numerous times, run the motor on plane with a timing light hooked up to see if I could see an intermittent spark, spark does jump 7/16". Compression is 125 to 126 on all cylinders. Motor was thoroughly rebuilt in '03, I have the receipts. Compression at that time was 140 on all.

I can't determine if one or all of the cylinders are misfiring.

Any ideas?

Thanks in advance,
John
 
john, a couple thoughts. 1st, the noise you here when a truck lets off the brake is a compression release system. They don't have to do that. its a feature for quick rpm drop or emergency braking using the engine.

Question, when the carbs were rebuilt, did you remove the orifice covers and clean? many call spraying the bowel and shooting cleaner through the jets a rebuild. I believe you have a high speed orifice in those carbs . Ran into similar problem with a 90 hp mid 80's evenrude, Also early 70"s 150 merc. dirt in them orifices which seems almost everyone is a afraid to remove and clean. that's all it took.

Why cant you put in a test tank to determine cylinder or bank, a lot safer than working on a moving boat. lol could be a coil even though they are supplying good spark.
What do our plugs look like? any unusual looking one or two?

there are numerous unusual possibility's, But most often its the obvious and overlooked. Just a thought...
 
Have you checked flywheel magnets ?--------Have you checked the drain plug on the lower unit ?----Any metal chunks on it ?------------Have you looked at issues with crankshaft sealing rings?---------Have you looked at all the pins / sockets in the wiring plug in connecters?
 
Last edited:
john_day said:
I have a miss that comes and goes and can't seem to find, fix it.

Motor seems to idle and troll fairly smooth.

At cruise, 3800+, the motor will run smooth then it misses. This goes on the whole time running. The rpm's don't drop but I can feel the boat slow just a bit and motor vibrate. Also the sound the motor makes is something similar to the sound from a semi when the driver takes his foot off the accelerator. Naturally, no where near as loud.

WOT is about 5300 and has the same miss.

I have a flo scan and notice when the miss is happening the gauge will vary between 1/2 to 3/4 gph.


I bought this motor just over a year ago and have been working on it since. I'll give a quick list to what I have done and where I am now.

Motor is currently raised to the third hole, QL82yc plugs at 35ths, they look in between brown and black and not dry. Timing is advanced 19 btdc. Manual recommendation is 18.

I have cleaned the tank, replace water separator, replaced all fuel lines up to the pump including fill hose, checked the fuel vent is not plugged, removed the vro, rebuilt the carbs and reset the float up and down several times, decarbed the motor a couple of times, replaced the plug wires, used ql77jc4, ql78yc,ql82yc plugs at 30 and 35th. Raised the motor, dropped the prop pitch 2"s to 15, link and sync numerous times, run the motor on plane with a timing light hooked up to see if I could see an intermittent spark, spark does jump 7/16". Compression is 125 to 126 on all cylinders. Motor was thoroughly rebuilt in '03, I have the receipts. Compression at that time was 140 on all.

I can't determine if one or all of the cylinders are misfiring.

Any ideas?

Thanks in advance,
John
Click to expand...

john_day said:
I have a miss that comes and goes and can't seem to find, fix it.

Motor seems to idle and troll fairly smooth.

At cruise, 3800+, the motor will run smooth then it misses. This goes on the whole time running. The rpm's don't drop but I can feel the boat slow just a bit and motor vibrate. Also the sound the motor makes is something similar to the sound from a semi when the driver takes his foot off the accelerator. Naturally, no where near as loud.

WOT is about 5300 and has the same miss.

I have a flo scan and notice when the miss is happening the gauge will vary between 1/2 to 3/4 gph.


I bought this motor just over a year ago and have been working on it since. I'll give a quick list to what I have done and where I am now.

Motor is currently raised to the third hole, QL82yc plugs at 35ths, they look in between brown and black and not dry. Timing is advanced 19 btdc. Manual recommendation is 18.

I have cleaned the tank, replace water separator, replaced all fuel lines up to the pump including fill hose, checked the fuel vent is not plugged, removed the vro, rebuilt the carbs and reset the float up and down several times, decarbed the motor a couple of times, replaced the plug wires, used ql77jc4, ql78yc,ql82yc plugs at 30 and 35th. Raised the motor, dropped the prop pitch 2"s to 15, link and sync numerous times, run the motor on plane with a timing light hooked up to see if I could see an intermittent spark, spark does jump 7/16". Compression is 125 to 126 on all cylinders. Motor was thoroughly rebuilt in '03, I have the receipts. Compression at that time was 140 on all.

I can't determine if one or all of the cylinders are misfiring.

Any ideas?

Thanks in advance,
John
Click to expand...

When an engine starts to cut out under speed it does funny things - it can appear to bounce/buck and make funny noises, as instead of the engine pushing on the boat the engine is dragging behind.

To me it sounds like a simpler fuel supply problem; I've got a 50 HP Merc that does this intermittently and even one of 150 Optimax's was doing this when I re-routed and accidentally crimped a hose (run fine low speed then miss above 3000).

Given it's easy enough to test, try this if you can. When the issue is happening, have someone try pumping the primer bulb (to manually pump the fuel). If that prevents the problem then you know it's just fuel supply. We can figure out what to do next in that case.

Given the headaches, if you are planning on running this engine for awhile and can't uncover the root cause, I'd recommend finding a parts engine. Anything that vintage in non-running order can usually be found for $500 (and much less for the thrifty/patient). Then you can swap some parts and check results; I've heard of ignition coils "overheating" and doing something similar but I think that issue should stay until temperature changes (would not be instantly gone upon RPM drop).

Jon
 
Another simple " no money spent " test is as follows.---------Mark the flywheel for TDC for each cylinder ( compensate for the timing advance ) with a paint marker or tape.------Run the engine on the lake at full throttle and see if the powerpack is firing each cylinder at the proper time and only the one cylinder at the proper time.
 
Semolivin and Leaky, Thanks for you response.

Semolivin, I don't think I have an orifice cover in my carb. I have 3 air bleeds, low-med-high, that I did run a fine wire through. Now there are 3,I think, tubes with holes on the side that I did not run wire through. I'm not sure what they are called or do. Is that what you are referring to?

The boat is currently in the water. I'll be pulling her this week sometime.

Plugs are a brownish black and not dry. All look almost identical

Leaky, I have tried pumping the bulb and no difference. Also tried chocking the motor but that causes the motor to bog. The spare motor is a good idea

Thanks, John
 
Racerone,I have pulled the flywheel to change the timer base and don't recall any loose magnets or anything looking out of place or damaged. Drain plug is clean and clean oil. All wiriing was pulled apart cleaned and reconnected with dielectric grease.

racerone said:
Another simple " no money spent " test is as follows.---------Mark the flywheel for TDC for each cylinder ( compensate for the timing advance ) with a paint marker or tape.------Run the engine on the lake at full throttle and see if the powerpack is firing each cylinder at the proper time and only the one cylinder at the proper time.
Click to expand...

I'll try this in the next day or two. How can I determine if only the one cylinder is firing and not others?

I know nothing about crankshaft sealing rings. What should I look for there?

Thanks,
John
 
john_day said:
Semolivin and Leaky, Thanks for you response.

Semolivin, I don't think I have an orifice cover in my carb. I have 3 air bleeds, low-med-high, that I did run a fine wire through. Now there are 3,I think, tubes with holes on the side that I did not run wire through. I'm not sure what they are called or do. Is that what you are referring to?

The boat is currently in the water. I'll be pulling her this week sometime.

n
Click to expand...

The Orifice that I refer to is what you may be calling the low med hi air bleeds. You do have an idle and intermead orfice. looking at the top of split carb are two brass orifices these are removable.
On the face of the carb will be a large brass screw head, cap. this is the access to your high speed orifice/ air bleed. which is also removable. If you get any thing floating in there such as grain of sand, mineral deposit etc will cause headaches. if you cleaned them eliminate that. if you didn't, something to consider.

Before you haul your boat see if this condition exist with boat on trailer, I also was considering counter weights/magnets so don't overlook that. At this point you are not sure if its and engine miss fire, fuel or balance issue. at least if you can do it on trailer its not a boat motor combo issue.
.
 
Well, you guys got me thinking and looking at things so I was running the motor with the cowling off in the dark and on close inspection saw arcing from the spark plug boots to the spark plugs. I know I said I replaced the plug wires but I used the old boots. They look in great shape, no cracking and very pliable, but they don't work. I'll see how available they are and once I replace them I post back with results. John
 
I went to buy the plug wires and was knocked out at the price brp wants, so I bought four spark plug boots. Installed them on the wires and hooked up to motor. Motor idled better and ran a little better but not what I was hoping for.

Once it was dark I removed the cowling and started the motor and the #1 cyl was arcing at the boot. Twisted it, removed it and put back on and still arcing. I then removed the plug and replace with a spare plug and no arcing. I didn't take it for a ride, I'll do that tomorrow.

Any ideas on why the one plug was arc and the replacement didn't?

John
 
john_day said:
Any ideas on why the one plug was arc and the replacement didn't?
Click to expand...

Two possibilities:
1. A tiny crack in the porcelain filled with moisture
2. The permanent arcing has created a so-called creepage path on the porcelain. That means the arc's high temperature has burnt the porcelain and converted it into something conductive.
 
Thanks WernerF,
It's been cold and wintry here lately and the boat is out of the water. I haven't done anything to it since I replaced the plug.

Motoring to the boat ramp I still have a slight miss, much less than before, and the WOT is 5200rpms. Once I hit 5200 it's like the motor hits a wall. It actually will lose 1-200 rpms.

So I'm waiting for better weather and when I get some I'll report back.

Thanks, JOhn
 
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