Motor has run GREAT twice now..

[MassillonBuckeye]

1978 Merc 70HP Thunderbolt ser#4827***. Have had it on the water twice. Have had it idling in the driveway a few more times than that. Starts and idles like a dream. On the water, I get to about 1500 RPM I'd estimate and can't get any more OOMPH out of her. Just small "bursts". RPMs and the boat jumps a bit. When its dogging, you can hear the carbs opening, as the exhaust gets a little throatier, but not more power. Classic case of fuel starvation?

The only problem I have with that is it has run GREAT twice now. Once each trip. No rhyme or reason other than the second time I had increased the throttle fairly slowly and it just keep gaining power. COOL! Popped up on a plane and sounded real nice. We ran for about 45 sec or so, then I brought it to a stop. To have it be a dog foer the rest of the day. First trip on the water, at a seemingnly random attempt, it throttled up with power, but I was in a no wake zone so I couldn't do much about it. That time it seemed to last a bit longer. When I brought it home the first night, I opened it up to find this:

I thought EUREKA! We found er! And proceeded to remove the bolts and washers holding and crimping that fuel line... Someone had ran the fuel line between those bolts with washers and that piece of rubber holding it down.. Genius!! And also thought to myself YOU BIG DUMMY for even taking it out like that. But anyway, fast forward 2 days to seatest #2 and I have the same results. So it wasn't that crimped 1/4 fuel hose. Like I said earlier, I'm in the process of rebuilding the carbs(looking for the rebuild kits), cutting in mercury quick disconnect fittings and replacing the leaky primer bulb then we'll go from there. My question is, if the carbs were gummed up, how could it run great those two times out of the blue? Anyone else experience this?

 

[CharlieB]

Re: Motor has run GREAT twice now..

Two things will limit RPM and power under load, limited fuel flow is one, low high speed stator voltage is the other.

You need a DVA meter or DVA adaptor to test the high speed output of the stator.

NEVER ASSUME anything and start hanging expensive ignition parts on as there is NO RETURN on electrical parts, you need to PROVE the fault.

download the CDIElectronics free Ignition Troubleshooting Guide from their Support pages. Follow their directions to PROVE whether you have an ignition problem.

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

Two things will limit RPM and power under load, limited fuel flow is one, low high speed stator voltage is the other.

You need a DVA meter or DVA adaptor to test the high speed output of the stator.

NEVER ASSUME anything and start hanging expensive ignition parts on as there is NO RETURN on electrical parts, you need to PROVE the fault.

download the CDIElectronics free Ignition Troubleshooting Guide from their Support pages. Follow their directions to PROVE whether you have an ignition problem.

Well, I've already made a DVA adapter according to a schematic I came across on here. Haven't tried it yet. Was thinking about verifying I have it wired right before I blew something up lol.. I'm pretty sure it's right.

Wouldn't it be backfiring, or flooding if the ignition was fouled up? Definitely seems like its running lean. No smoke at all. It still revs higher with the cold start lever. I guess it could still be ignition?

Thanks for the input.

 

[BonairII]

Re: Motor has run GREAT twice now..

Not familiar with your motor, but is your timing advancing correctly when you throttle up?

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

Not familiar with your motor, but is your timing advancing correctly when you throttle up?

I had read about something along these lines about maybe sticking because it was tensioned by a spring. How do I verify that it IS advancing? What am I looking for?

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

I can see that there is a linkage that is moving a plastic piece under the flywheel. It starts moving at around 75% throttle I'd estimate.

 

[CharlieB]

Re: Motor has run GREAT twice now..

The plastic piece under the flywheel is the trigger assy. Movement of the trigger is timing advance which should begin to move just a bit before the carbs begin to open, continue advancing until full advance stop screw contacts and stay there while the throttle continues opening to WOT.

Prop synch and adjustment of timing and throttle is absolutely CRITICAL to both performance and longevity of the motor.

A FACTORY service manual will become your Bible.

Check these links

http://1manual.com/Mercury manuals/Service_Manual_Index.html

http://forums.iboats.com/showthread.php?t=309079&p=2072211#post2072211

 

[carholme]

Re: Motor has run GREAT twice now..

The engine service manual you require is P/N: 90-86135--5 for s/ns 4576237 and up.

You can try ebay but you won't find it at either of those links.

Gerry

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

I believe I have the proper manual.
http://www.ebay.com/itm/23075490528...X:IT&_trksid=p3984.m1497.l2649#ht_1277wt_1396
Is what I bought.
I cleaned the carbs. I blew a chunk of something out of the fuel hose that was connecting the fuel filter to the carbs. Not sure what it was because it went flying. Had some mass to it because I felt it bounce off my shoulder lol. I'm gonna slap em back on there and see what happens. Then breakout the DVA adapter, although I'll have to put it on the water to test. I wouldn't be surprised if the trigger was shot due to the rotting wires at the rectifier end.

I've been researching the link and sync procedure. I'm still a little cloudy. The grid on the flywheel is garbage as well. Something tore into it while running apparently and it made it unreadable.

 

[CharlieB]

Re: Motor has run GREAT twice now..

You still need to DVA test the stator high speed windings as an UN-loaded motor can rev to the moon with a dead high speed winding but once load is applied will not even pull up to 3K

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

You still need to DVA test the stator high speed windings as an UN-loaded motor can rev to the moon with a dead high speed winding but once load is applied will not even pull up to 3K

Gotcha. Well yeah, I definitely want to test it all before I start replacing. I went ahead and ordered a DVA adapter from maxrules. I'm afraid to hook my homeade jobby up in fear of ruining something. I'll probably get it in a few days. Can you detail the proper test procedure for the stator and trigger? I have found the values, but am having trouble pinpointing exact testing procedure. One probe goes to engine ground? Could that be where the switchbox and everything grounds? That screw? One probe to ground, the other on the appropriately colored lead from the trigger or stator? Thanks for the input guy. We'll get 'er.

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

Brown wire at switchbox AND a brown wire connected to a yellow on the Rectifier.. I thought it was either or. If you had factory tach, it runs off the brown wire at switchbox. And if you had aftermarket, you connect it to rectifier. Is this correct?

And the tach harness.. Has black white and green. I assume I connect the green and black to tach? Pretty sure the gree in the harness lines up with brown in control. White is +12 and goes to my battery gauge. Which is the only gauge I'm running at the moment.

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

Put it back on the water today after replacing all the fuel lines and installing a fuel filter and still doing the no power thing. Had a new set of plugs with me so swapped them and same deal. Fires right up, but no power past about 20% throttle. Took the new plugs out after running for a few minutes and heres what they looked like. Top to Bottom, Cyl 1 plug is top in picture.

I thought for sure I was going to bring it home and have no spark on cyl 1 but it does. There was a bit of unburnt gas when I first took them out. Other two have that black soot you see there. Going to DVA it tomorrow. How do I get the plug wires off of the coil packs? Service manual says to remove, but I can't find where it says how to do it. I pulled straight out a bit and they weren't budging so I figured there was another way.. Or they just don't come off.

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

Now I know these motors have a tendency to run lean on that top cylinder, but does that mean seeing unburnt gasoline on the plug after 5-10 minutes running? Where do I go from here? DVA stator test this afternoon. Can I unplug the plug wires from the coil? Do I really have to unplug them to test the coils? I'm suspecting weak spark on cyl 1 which is the top plug in that picture.

 

[CharlieB]

Re: Motor has run GREAT twice now..

Many coils have a threaded post that the wire screws on to.

It appears #1 is not firing. Yet you claim it has spark? Have you tested spark using an adjustable gap tester set to 7/16 inch? The neon 'flash tube' spark testers are pretty much worthless as far as troubleshooting marine/industrial ignitions, there engines operate at maximum load, unlike our cars, so ignitions are required to operate at peak capacity all the time.

Spark test with a 7/16 inch gap, if #1 fails, switch two coils and retest, if the problem moves with the coil switch then your problem can be the coil or plug wire. Make sure the coils are well grounded, a poor ground can cause a weak or no spark condition.

If the weak spark stays on #1 then you will need to DVA test the triggers and the switchbox to determine which output is low.

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

Many coils have a threaded post that the wire screws on to.

It appears #1 is not firing. Yet you claim it has spark? Have you tested spark using an adjustable gap tester set to 7/16 inch? The neon 'flash tube' spark testers are pretty much worthless as far as troubleshooting marine/industrial ignitions, there engines operate at maximum load, unlike our cars, so ignitions are required to operate at peak capacity all the time.

Spark test with a 7/16 inch gap, if #1 fails, switch two coils and retest, if the problem moves with the coil switch then your problem can be the coil or plug wire. Make sure the coils are well grounded, a poor ground can cause a weak or no spark condition.

If the weak spark stays on #1 then you will need to DVA test the triggers and the switchbox to determine which output is low.

So I switched the switch box leads between 1 and 2 and that same coil still didn't fire. Coil is bad right? Since the #2 switchbox lead made #2 coil coil fire before the switch. I think we're getting somewhere now . I was confusing myself because I wasn't changing the location of the plug wires with the coils so my brain had to look at it upside down and sideways. The other thing that keeps messing with me is the order of the coils. #1 coil is bottom, #1 cylinder = Top.. but yeah.. Drrrr.

And yes, using one of the adjustable spark gap tester set to 7/16".. Seemed so far at first! That spark is really jumping on the working coils!! I threw away that lighted inline tester. Junk!!!

 

[CharlieB]

Re: Motor has run GREAT twice now..

With the #2 switchbox lead connected to the #1 coil AND still a weak or No spark there is a problem with the #1 coil, ground, or spark plug wire.

At the same time you should be sure that the #1 switchbox lead once connected to the #2 coil provides a hot spark, proving the switchbox is Good, supporting your determination that your problem is the #1 coil, ground or plug wire.

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

With the #2 switchbox lead connected to the #1 coil AND still a weak or No spark there is a problem with the #1 coil, ground, or spark plug wire.

At the same time you should be sure that the #1 switchbox lead once connected to the #2 coil provides a hot spark, proving the switchbox is Good, supporting your determination that your problem is the #1 coil, ground or plug wire.

Thank you. Yeah it wasn't making sense at the time because it was late, and the plug wires weren't moving location with the coils so I was still thinking it was cyl #1 not firing. But yes, coil#2 Definitely fired nice blue spark running off #1 switchbox lead.

Now about my rectifier/tach.. Rectifier is benching good, tach only worked for a short period of time when I first installed this new rectifier. Then stopped. And the needle now seats around 2-300 RPM instead of 0.

 

[MassillonBuckeye]

Re: Motor has run GREAT twice now..

Shes running great now boys!! Swapped Cylinder 1 coil and she really goes! Spark GAP tester was instrumental in helping diagnose the spark issue. Those inline spark testers with the neon light bulbs ARE JUNK!! It showed spark, but was obviously not enough.

Tach working now as well. Although its no seating on Zero, it seats at about 200 RPM for some reason. So I'm just subtracting 200 from my calculations. I think I need to fine tune my idle mixture and idle speed now. It's idling a bit high. That is maybe for another thread

Thanks for all the tips. Hopefully this helps someone down the road.

recap: 1978 Mercury 700 70HP, no power, seemed to run fine on muffs. Had spark on inline neon spark tester, but weak. Used spark GAP tester set to 7/16" gap and had NO spark. DVOM tests on coil = BAD. Replaced coil, motor runs great. Threw away inline neon spark tester

 

[mercuk]

Re: Motor has run GREAT twice now..

Shes running great now boys!! Swapped Cylinder 1 coil and she really goes! Spark GAP tester was instrumental in helping diagnose the spark issue. Those inline spark testers with the neon light bulbs ARE JUNK!! It showed spark, but was obviously not enough.

Used spark GAP tester set to 7/16" gap and had NO spark. DVOM tests on coil = BAD. Replaced coil, motor runs great. Threw away inline neon spark tester

Well done Dude! Its so satisfying when you trouble shoot your own engine and finally, all the hard work pays off!!!..

I'm glad its all come together nicely because I know how frustrating it is when you know somethings going wrong, but you just can't put your finger on it then, BLAM, you got it. Good stuff.

Just a quick Q tho, can you show a link to this "spark gap" tester please because I'm from England, and I don't think I have ever come across one yet, apart from them crappy neon things lol.
Cheers, and one again, well done fella...