70 hp Ignition troubles?

[h2oboy83]

I just bought a skiff with a 99 70 HP Johnson. Model J70PLBEE
I took it out for the first time and at speed it misfires. After about 20 min it slowed down and then stopped and wouldn't start. After sitting for 2 hours it start but sputtered out. Finally got it fired and made it back to the ramp. Ran it around by the boat ramp and same thing happened. Misfired then sputtered out.
Did some reasearch and thought it was the temp sensor. Ran it with ear muffs and it started right up. When it would start to misfire I unplugged the temp sensor and it ran perfect, plugged it in and it would misfire. Unplug it and it cleared up. Thought I figured it out so I took it down to the ramp by my house and she wouldn't start.
Im thinking it might be the power pack??? The carbs were rebuilt before I bought it and I just replaced the fuel line cause it was cracking so I don't think its a fuel problem. Im mechanicly inclined but suck at diagnosing ignition problems. Any help or ideas would be great! Thanks

 

[jtexas]

Re: 70 hp Ignition troubles?

You sure it's not overheating? Compression test will reveal any damage from overtemp.

If it's not in slow mode with the temp switch disconnected, the powerpack is functioning normally; if it's in slow mode and the motor's not overheating, either the switch is faulty or the wire is accidently shorted to ground.

If the temp switch is throwing the motor into SLOW mode, the alarm should be sounding...if you disconnect the wire and ground it to the cylinder head with the key "on", you should hear a steady tone.

Otherwise troubleshoot the tan wire to the warning horn, and test the horn to make sure it sounds when connected to 12V.

Spark test will rule out ignition: get an inline spark tester (couple bucks at autozone), set the gap for 7/16", clip it to the engine block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it can't jump a half-inch gap in the open air without a spark plug, it's not firing in the hole under heat & compression.

If it has good compression and spark, then you must have a fuel problem.

 

[h2oboy83]

Re: 70 hp Ignition troubles?

Im confident its not overheating. I replaced the water pump and thermostate when rebuilt the lower unit. (did that before first trip) Plus it shoots water out the tube ok. The horn doesn't blow but it does chirp when I turn the ign on. What should the compression be? I will test that now. I also test all the coils and they checked out OK. I'll pick up a spark tester today also.

 

[jtexas]

Re: 70 hp Ignition troubles?

Compression should be about equal, with all three being within 5 or 7% of the average. I'd expect upwards of 130 on that motor, but difference between cylinders is what counts.

Be careful about jumping to conclusions:

"I replaced the water pump and thermostat..." and "shoots water out the tube ok..." doesn't tell you whether it's overheating. The telltale stream tells you the water pump is pumping, it doesn't tell you whether water is circulating the powerhead, and it sure doesn't give you the cylinder head temp. Motors with new water pumps & thermostats, and strong pee streams overheat all the time.

The chirp tells you the horn is capable of making a sound, but unless you hear it blow when tan wire is grounded, you have no idea whether it's functioning properly in that circuit.

"The carbs were rebuilt before I bought it and I just replaced the fuel line..." just means you have new fuel line, but you don't know the condition of the carbs right now, and there are plenty other ways for the fuel delivery system to fail.

You said it went into slow mode after a few minutes of hi-speed operation, eventually shut down and wouldn't restart for 2 hours. And then it ran in slow mode unless the temp switch was disconnected. That's pretty strong evidence for overtemp to the point of seizing.

You should be able put your fingers on the cylinder head, leave 'em there for several seconds, and pull 'em away without leaving any skin behind. Try it after it goes into slow mode.

The temp switch can be tested in a pot of warm oil with a continuity tester, but the cylinder head has to come off first.

 

[h2oboy83]

Re: 70 hp Ignition troubles?

Well I figured I would try to fire it up before checking the compression and it fired right up. I think the fuel line wasn't fully primed after I changed it. Ran like a champ and started to mis fire. Unplugged the temp sensor and she purred like a kitten. Also it starts acting up within a min so I don't think its over heating. Started it up and it ran great on 3 different times without the temp sensor. I still wanna check to see if the horn will go off when I ground the sensor though. I went ahead and checked the compression and got 110-120psi on all 3 cylinders. I guess my compression gauge has a leak but they all bounced between 110 and 120 Im leaning towards a bad temp sensor again. Im going to try and take her out for a test run on thursday and see if it misfires underload at a higher RPM. I'll let you know what I find out. Thanks a ton for the help!!! I was ready to sink this boat!

 

[h2oboy83]

Re: 70 hp Ignition troubles?

Ok boat ran great with the temp sensor unhooked, didn't misfire once! Never overheated, I was able to comfortably leave my hand on the engine. Run great. Shut it off to fish for 15 min then it wouldn't start. Got it home by trolling motor. I tried to start at the house without success. I then noticed I had lost prime on the fuel line. Reprimed and nothing. Tested the spark and had good spark on all 3. I think the misfire was from a bad temp sensor but im guessing the no start is a different problem! Is there a way to test the fuel pump. I don't see a wiring diagrahm for it in my manual.

 

[jtexas]

Re: 70 hp Ignition troubles?

Good news it's not overheating. No doubt you've already ordered your replacement temp switch -- no sense risking a powerhead over a $100 part.

Fuel pump is vacuum-operated, the wiring there is for the alarm. Original equipment on that motor was a VRO gas/oil pump, but by now many of 'em have been converted to premix by disabling the oil side, or replaced with a gas-only pump.

Faulty fuel pump generally isn't going to cause a no-start because once you prime the line, there's enough gas in the carbs to start and run the motor for several minutes. Unless of course the diaprham gets perforated and leaks down the pulse line, flooding the motor. I don't know how likely that is with your model.

I guess you inspected all the fittings for air leaks? Or could be a faulty primer bulb. Did the bulb get firm when you reprimed? Is it holding?

I assume the motor was cold by the time you got home? Hard cold start is usually caused by faulty choke/enrichener, test it by pulling off the discharge hose and engaging the solenoid by pushing the key in. It can also flood the motor if it develops a leak.

Compression, you'd like to see a little less difference between low & high, but your numbers are fine. At some point you might want to see if you can recover some of the lost compression with a decarb, read about that here:
http://forums.iboats.com/showthread.php?t=158076

 

[h2oboy83]

Re: 70 hp Ignition troubles?

OK. I got home from work this morning and went to work on the boat. And of course it starts right up, first try. So it looks like its not a cold start problem but a hot start problem. The plugs don't seem fouled out. I will check the choke solenoid and see if it is maybe leaking and flooding the motor I guess. Are you able to adjust the air/fuel ratio on these carbs or is it factory set? Any other ideas? I'll plan on taking her out on monday and seeing if it won"t start after running around.

 

[jtexas]

Re: 70 hp Ignition troubles?

slow speed adjustment needle on the starboard side of the carb.
how-to adjust: http://forums.iboats.com/showthread.php?t=167352

 

[h2oboy83]

Re: 70 hp Ignition troubles?

After todays testing, I am sure the problem is in the carbs. Same senario. Ran great. stopped to fish, then wouldn't start. I read up about the fuel primer and thought it might be leaking or not working but It tested ok. What was funny is that when it wouldn't start, if I manualy opened the primer and then closed it, it would start right up but run rough. I'd high idle it and the RPMs would climb slowly and then smooth out and jump to 4000. Take the high idle off and put it into gear and it runs fine. I was able to fish all day starting it this way. So I take it the slow idle is out of adjustment. Came home and went through my manual. I synched the carbs and adjsuted the throttle cam and it already idles better when warm. It would hardly idle and would often die when I was on the water. The manual says I need to adjust the slow speed screw while in gear so Im gunna adjust that next time im out and check the timing also.
Does it sound like im heading in the right direction? Also my manual doesn't specify what the idle speed should be or how I can adjust it.

Thanks again for all your help!!! I am finally starting to enjoying owning my boat!!!

 

[jtexas]

Re: 70 hp Ignition troubles?

just a little more tweaking. Manually opening the primer, should give you the same effect as pushing the key in (keyswitch in the "on" position). I belielve adjusting the slow speed needles is going to help that.

Each carb has a slow speed mixture adjustment, those you can do one at a time with the motor idling on the muffs, following the joe reeves procedure. I didn't identify them in the photo, but you can see the middle one clearly, it's the only piece of brass on the carb visible in that pic.

Then you set the idle speed using the idle set screw (identified below). Idle speed is controlled entirely with spark advance -- should be around 750-800 in gear in the water. Set it to 1000-1100 on the muffs, then fine tune it on the water. You can set it with the boat in gear tied to the dock, or even on the trailer. Might need a little fine-tuning.

 

[h2oboy83]

Re: 70 hp Ignition troubles?

The timing was off 4 degress. Had it running like a top. Would start right away. Idled like a champ and would start even when hot, cold or in between. Fished for like an hour and then it started but died. Had to prime it by hand and it started right up. When I got home I found that the primer wasn"t working everytime with the key. Im gunna pull it off and clean it and see what I can find.

 

[h2oboy83]

Re: 70 hp Ignition troubles?

I have found another problem which may fix the hot start issue. The fuel primer jet on the bottom carb leaks fuel whenever the system has pressure. The middle and top do not however. Looking at the design of the primer im not sure how this is possible, but when I pulled all 3 primer jets out and pumped up the bulb the bottom jet sprayed and then dripped till the pressure let out. The middle and top did not. Im sure all I need to do is replace a seal or o-ring but im not sure which will solve the leak. When I opened the manual prime lever, they all squirted fuel.
As far as removing the primer everything looked ok. It activated each time with 12V and the ressistance checked out OK.

 

[h2oboy83]

Re: 70 hp Ignition troubles?

Alot to report.
Misfire: With a brand new temp sensor the motor still misfires almost like its only firing 1 cylinder at idle. Unplug it and it runs perfect. I put 12V to the tan wire and no horn! So Im guessing maybe a cicuit is bad in the power pack??? or should I trace a wire in the harness? The wiring diagram shows a resistor in the harness. Maybe its bad?
Hot start: I started to notice fuel in the case, and in the carbs when I took the air cleaner off. I pulled the carbs and it looks like the float valves are worn. Im not even sure if they had even been rebuilt. The seals look good, cleaned them out good. Also the fuel primer was leaking. Rebuilt it and its good now.