9.8 Only Runs at WOT?

[db21]

Have a 1971 9.8 Merc, was running fine and then decides it won't start. It used to start on 1st pull and then nothing, thinking it might have been flooded I put it in gear at WOT and pulled it (was tied up to dock) and it finally started. However as I reduced throttle it would cut out abruptly before I could get revs low enough to put in neutral.

It continued to do this so I thought perhaps idle jets were plugged (even though I have gas filter on line). So I cleaned carb as best I could at the cottage and put it back together and still the same.

Could this just be a carb that is still plugged, or what else could it be? I was getting spark and gas - I was wondering if it could have anything to do with the timing advance and spark not timed well at lower revs. Wires looked good as far as I could tell. It just seems odd how quickly it shuts down when I reduce throttle. Cuts out hard at about 3/4 throttle, but was running good just an hour before!

Appreciate any thoughts.

 

[db21]

Re: 9.8 Only Runs at WOT?

Okay I have cleaned the carb yet again and still the same problem. I've seen some threads of people posting with similar symptoms, but with no clear cause. Some suggested it could be a fuel problem, but no one confirmed this is what their issue was for sure. Anyone have any insights? It runs at wide open in gear but then it dies as soon as the the throttle is turned down a bit.

 

[CharlieB]

Re: 9.8 Only Runs at WOT?

Will the primer pump up solid?

If the fuel pump diaphram developed a hole leaking fuel it may be flooding and not starting til the throttle is wide open.

Pump up the bulb, whait a couple of minutes and give it another squeeze to see if it has bled down, if so, disconnect the fuel pump and look at the backside, is it wet with fuel? Disassemble and inspect the diaphragm.

 

[db21]

Re: 9.8 Only Runs at WOT?

Ok thanks - I'll take a look at the fuel pump tonight.

 

[db21]

Re: 9.8 Only Runs at WOT?

Confirmed that fuel pump works like a charm. Good spark, going to clean the carb, again...!

 

[db21]

Re: 9.8 Only Runs at WOT?

OMG, someone please help me before I throw this stupid motor in the lake! I meticulously cleaned the carb for the 3rd time, and still the same problem.
I can pull it over a 100 times and nothing, then if I put it in gear and open up I can normally get it to run for a bit. So that tells me I've got spark and gas and compression. But yet nothing when in neutral and throttle is down. Figured it was idle jet, but it's as clean as a whistle.
Now some of my wires look like they need replacing, but could that be causing the problem? Why would it run at WOT and not at idle???
Anyhow have any suggestions?

 

[oldman570]

Re: 9.8 Only Runs at WOT?

you need to link and sync the carb as they have to be intime for everything to happen as it should. Nothing more than adjusting the linkage for everything to work.
Oldman570

 

[db21]

Re: 9.8 Only Runs at WOT?

What linkage are you referring to? There is a direct linkage from the throttle to the carb and timing, and nothing there has changed since the motor was running fine.

 

[CharlieB]

Re: 9.8 Only Runs at WOT?

There are some very small fuel passages drilled into the carb throat just past the throttle blade, it can be a PITA to get these perfectly clear, but they are critical to passing fuel at low or no throttle opening.

I keep a very fine sewing needle and a couple of wires pulled from a stainless steel toothbrush just for clearing these small holes. You must be sure not to use a wire larger than the hole as reaming any of them out oversize will ruin the carb.

Wally world super tech carb cleaner is about the cheapest, yet one of the best chemicals I've found, use the 'lil red spray tube to verify that each passage flows well. WARNING; DO NOT GET ANY SPRAY IN YOUR EYES!!!! It burns like FIRE.

 

[db21]

Re: 9.8 Only Runs at WOT?

Thanks, pretty sure I've got all the passages clear. Have stripped it right down three times. Used a small wire on all the holes and flushed it out with carb cleaner and compressed air.

Last night it looked like I might have an intermittent spark problem. I have an ignition coil (energy transfer) iginition, can anyone tell me how I can tell if it is working properly? I'm good with mechanical stuff, not so good with the electrical!

 

[db21]

Re: 9.8 Only Runs at WOT?

Okay I've been taking a closer look at the coils and wondering if they are working as they should. I can't seem to get any spark now (although I did have it running at WOT the other day, so obviously had spark then?). I've taken coils off and see low resistance between low voltage coil (can't remember exactly what). How do I confirm high voltage coil is good. Do I measure across the output to the spark plug and the pig tail that comes out? What is the purpose of that thing anyhow? On the one it is taped on top of the coil and under the insulation cap - I've heard that is a ground, but I don't see how it can ground out to anything!? What should resistance be on these coils pn 336-3955.

TKs.

 

[oldman570]

Re: 9.8 Only Runs at WOT?

The small wire on the coil has to go to ground on the block where it mounts. Sounds as tho it is put in the mount wrong if it that wire is facing up when you take the mounting cap off of it. Adding a peice of rubber tape between the wire and the rubber on the coil will help the wire ground to the block if it is pushed down into the rubber of the coil. The points of the older 9.8 motors are maker points and not like others as one would think. They put ground to the coil so the plug will fire. They can be tested by using a ohm meter from the wire from the pints on the coil to ground on the block and turnig the motor over by hand and watching for each set of points to make to ground. If both points are the same reading to ground, the points are good and should NOT be messed with. If not you will have to clean them with lacquer thiner and test them again till you get a reading on the meter about the same for both sets of points. Then remove the plugs and ground them to the block and check each for good spark. If one dose not spark as good as the other then that coil could be breaking down and causing the trouble. If the wires from the points to the coils is hard , brittle or falling off they will need to be replaced.
JM.O2
Oldman570

 

[CharlieB]

Re: 9.8 Only Runs at WOT?

The old points motors also had a habit of arcing across the plastic insulator of the connecter to the points, all can look well until the high voltage running, then they would arc. But these would seem to spark fine at cranking speeds, just would not run.

If your motor starts at WOT but will not idle, your problem could still be in the carb OR leaky reed valves.

 

[db21]

Re: 9.8 Only Runs at WOT?

Okay I did a bunch of test yesterday to try to determine why I have not spark. One thing I did figure out, the hard way, was that I am getting lots of voltage off the wire that goes to stop switch. Got a nice shock while trying to test that. Had by voltmeter on that and the ground, further tests showed upwards of 300V. So that at least told me my magneto was pumping out some voltage...however when I measure the voltage across the coil I get milli volts!? There are two what I assume to be +ve wires coming out of coil, one goes to the stop button, the other to the first coils +ve terminal - a jumper takes this to the other coil. Each negative wire comes from the points (I think that is what they are called) that open and close turning the spark on and off at the right time. When I measure voltage across the coil terminals I get only millivolts - which surprised me as I got loads on the wire going to the stop switch. When I put stop wire to the coil I don't seem to get any voltage either!? Which makes we wonder why!?

Oh when I test voltage from output of coil to the little pig tail ground I get zilch...but then again voltage going in is so small I'm really not expecting much!

Anyhow, my head hurts just thinking about all this. Does this make sense to anyone else? Running out of tests I think I can do. I did take flywheel off, looking in I can't tell too much. Wires look fine and I can test continutity across points and poke around a bit, but not really sure what to do next.

Really appreciate your help!

 

[CharlieB]

Re: 9.8 Only Runs at WOT?

You will NOT get a volt reading on the points wire if that wire, or the points, are grounded. Take another look at the white plastic insulator that wire connects thru to the points, these have a nasty habit of arcing/cracking, laying a carbon trail that shorts out voltage, killing spark.

Your style mag creates spark when the points MAKE a connection, totally opposite most points ignitions that make spark when the points break the connection.

 

[db21]

Re: 9.8 Only Runs at WOT?

Ok did a bunch of other tests on this last night. A found a few things:
- voltage going to stop ground is anywhere from 150-300V
- voltage going to coils is about 12V

What should the voltage be going to the coils?
What should voltage be coming out of the coil?
What is best way to test the coils. I have continuity on the side from the mag with little resistance, but not quite full continuity on the plug side when I measure from the inside of the coil (removed plug wire) to the pig tail on the back.

I've followed all the wires and now have a good sense on how it should work. So question for you CharlieB - since when the pts close the circuit is made allowing +ve voltage to flow through coil and to the ground how would arching on these kill spark? If both pts were closed voltage would go to both coils and both plugs would fire at once - no?

Starting to think that both coils are bad as no voltage on plug end, but have a hard time believing that both would go dead at once! Plus does not address the original symptom of it only running at WOT!? Now I can't get any spark so focusing on getting that fixed, as I can't get it running now regardless of throttle position!

 

[CharlieB]

Re: 9.8 Only Runs at WOT?

The electron charge is built up while the points are open, raising the pressure/voltage, as the points close the flow begins, creating the current and thus the spark.

If there is any short on the ground side, between the coil and the points, voltage cannot rise.

If there is a weak spot like a carbon trail/crack in the bulkhead insulator going to the points, it will begin to arc here as soon as there is sufficient voltage to begin the arc, preventing sufficient voltage rise for the coil to begin to make sufficient voltage to arc a plug gap under compression.

This style ignition can drive a guy nuts as you may test for spark in 'free air' and think you have a good enough spark that the motor should run, yet checking the plug wire with an inductive timing light you won't find a hint of a spark. Cause the system shorted out as the voltage begins to rise.

Check those insulators CAREFULLY, Merc NLA'd them but CDIElectronics makes them, they are just a chunk of plastic.

 

[CharlieB]

Re: 9.8 Only Runs at WOT?

...............................
- voltage going to stop ground is anywhere from 150-300V
- voltage going to coils is about 12V.....................

Voltage to the coils should be almost identical to that to the ground/kill

You won't see that until that side is un-grounded, that is, you find the short and insulate it.

 

[db21]

Re: 9.8 Only Runs at WOT?

Gold star for you CharlieB! I found the short last night on one of the ground wires I couldn't see until I took the whole ignition apart. What's more I figured out why engine only ran at WOT - turns out where the wire was frayed did not ground when it was wide open, as you slowed throttle the wires moved to retard the timing and the frayed wire then came in contact with a bolt where it grounded out. So that explained why the engine cut out so suddenly. What's best is I fixed this all for the cost of a foot of wire!

Thanks for everyones help!

 

[CharlieB]

Re: 9.8 Only Runs at WOT?

Feels great to know that you fixed it, and no shop charged you a couple hundred bucks!