Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunderbol

My 1973 merc 1500 thunderbolt 150 HP is cutting out on WOT and giving me lazy performance on my Hydrostream Vector. seems to be dropping a cylinder, or 2. when I pulled the sparkplugs to check for spark as soon as I turn the key to ON position they start sparking out of control withought even cranking the engine over. I would think this has something to do with my problem. Any ideas where to start? Thanks, Kris

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

Anytime there is a voltage flux in that type of system the switch box will fire the coil, multiple firings can be caused by a broken wire, bad ground or bad trigger. A single firing of more than 1 cyl, is usually a cap or rotor which can normally be cleaned.

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

I'm going to check and clean distributer cap and rotor now, also check wires. What exactly is the trigger?

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

Also what is switch box?

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

The trigger is the part of the distributor housing that the timing plate rotates thru the holes in the plate cause a magnetic flux which the switch box senses and fires the capacitor in the box which in turn fires the coil. The switch box is the box that is attached to the inside of the front face plate.

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

The switchbox aka electronic ignition module mounts flat on the inside of the front cowl support. It's retangular in shape and has terminal boards on either side. Wires from the distributor hook to the Port side of the box and power wires/coil wires connect to the Stbd side.

The trigger is built into the distributor housing and is non-repairable. If you have a bad trigger you must replace the entire housing.

Here's a wiring diagram which will help you relate the different parts of the ign:

http://www.maxrules.com/oldmercs/Wiring/1966ona/35.jpg

And here's instructions on how to do some tests to your ign system which may help you narrow down the problem:


"MERCURY BATTERY-POWERED CDI TEST FOR DISTRIBUTOR MODELS

This test is for the 332-2986 switchbox used from 1967-1978 on all the inlines.

This test assumes your coil is good (problems with CDI coils are rare).

DISCONNECT BATTERY

1. Turn off ignition;

2. Disconnect all 3 distributor wires on the Port side of the switchbox (and the ?mercury switch? if present);

3. Remove the High Voltage lead from the ign coil to the center of the dist. cap (remember it unplugs from the coil and unscrews from the cap);

4. Reconnect the H.V. lead to the COIL only;

5. Position the free end of the HV lead approx. 3/8" from ground (block, shrouds etc), and find a way to hold it there;

6. Jumper the brown and white terminals on the distributor side of the switchbox to each other.

RECONNECT BATTERY

7. Check that you have +12V at the red terminal (even with the ign off);

8. Turn on ignition and verify +12V at the white terminal (same side as the red terminal);

9. Ground the black terminal on the distributor side of the switchbox - this should cause a spark each time you touch ground.

If you get spark with the distributor bypassed, and it won't fire with the distributor connected, the trigger is bad and the entire distributor housing assy must be replaced.

If you get no spark using the test, the switchbox is probably bad. In that case, be sure to check for correct power on the switchbox, check all connections, and check the coil's resistance to make sure it's OK."

In your case, you're getting those spurious ign firings. If you still get crazy sparkings after you disconnect the distributor, you've got some sort of issue with the switchbox. As per the above test, you should only get spark every time you ground the black terminal on the distributor side of the switchbox.

If you find that the multiple firings do in fact go away with the dist disconnected, most likely the issue is with the trigger. A CDI Marine replacement trigger assembly is the way to go and will likely last the remaining life of the motor and then some. I've never had a CDI replacement part blow out.

Here's hoping it's the switchbox, there's a lot of used ones kicking around and it'll be considerably less work and expense to replace that part, than to tear apart the distributor.

HTH & let us know what you find...........ed

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

Well I took the cap off to find it very dirty with good size chunks stuck to terminals, I cleaned all with carb cleaner and lightly sanded wit 400 grit. Also on the rotor terminals. Put back together, pulled the plugs and grounded them on the engine. Turnd key on and all is good!! no crazy sparking, put everything back together to notice when I'm putting my throttle at wide open my distributer is stuck, not rotating to advance spark, and my carbs only opening up about 1/3 of the way... So I sprayed a bunch of carb cleaner in to free up old hard grease, unhooked throttle linkage and started to free up by hand it was seized badley with grease as hard as JB Weld lol. Finally things started moving again after half hour of unseizing by hand lol, re greased both nipples and hooked everything back up, now I have 2/3 more throttle on my controle and is way easier to push, opening everythin up completly to WOT, hooked on the muffs and started er up, hooly shiist what a difference, this really excites me to have such a easy fix, and to get so much out of it... I just bought this boat completly restored last week for 8000$ and did 53Mph the way it was... Can't wait to try it in the lake now, probably goin to **** myself, 1973 Hydrostream Vector!!!!

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

So I took the boat out today for a test, it now goes wide open for about 15 seconds then bogs out and almost dies, I can feel when all of a sudden it's like it cuts out a couple cylinders, it runs fine at slow speeds up to almost half throttle then just boggs, I checked for vapor lock in tank and even loosend the cap right off still same thing. Any help? Could this still be a timing problem?

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

Sounds like time to rebuild the fuel pump(or pumps) on the motor. Kits are available for short $ and the work is easy. Make sure you get two kits for the triangle-shaped fuel pumps that mount on the transfer covers, or the one kit for the cowling mounted fuel pump.

Also, make sure the fuel lines are 3/8" ID and there are no air leaks, and all the filters are clean.

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

I switched to my #2 gas tank and filled with supreme mixed at 50:1, just to see what happend because I read on here those old motors dont like todays regular fuels, also thought maybe there was a clog in my tank #1.. took to the lake and low and behold she worked great!!!! no bogs and ran top notch!! ran her to 60MPH on GPS with 2 people in the boat with me. I havnt got a chance to try it with just me yet. also I just put in a new tach and a 21P Vengence stainless prop.... not yet tested but my 26P aluminum didnt seem to rev out enough.. just to verify max rpms should be 5000 - 5800? finaly time to dial this thing in.

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

Yes, that 26P prop is likely too much pitch.

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

double check on the way I hooked up my tach...

comming out of the front of merc controle I have black, brown, and white wires.
I hooked black to Neg -, and white to Pos +, then Brown to Sig on the back of my tach, then I went to to motor and moved the brown wire from the non distributer side of the switch box to either one of the yellow wires on the Rectifier (I used the bottom wire), I then set the back of my tach for 12 Poles,

I have not yet tested because the other day the ladies left my radio turned on and just turned it down to no volume at the dock when i went to get the truck..from now on im removing the faceplate every time. I got a bran new marine battery flat DEAD...

again can someone verify max RPM for my engine?

Thanks Again,
Kris

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

It sounds like you have wired it properly. Max RPMs for Merc IL6 motors are all a bit different. Figure 5500RPM-6000RPM

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

1973 1500 MAX RPM is 5800. Make sure your timing is set to no more than 21' as today's gas is lower octane than back in '73.

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

I tried testing tach with no luck. So I completly bypasses merc controle and hooked a 12" wire from tach Sig to the wich I belive is the rectifier, ( I am now second guessing that) then hooked + and - directly to battery, still nothing. With setting on tach at 12 poles.

It is a faria marine tach Instructions for tach say to connect sig to a terminal or wire originating from the unrectified side of the alternator.

Also can anyone verify how many poles my engine has?

I just installed the way I have read on this site with no luck,
unless I'm messed up somewhere, please help

is the rectifier the box mounted on the face plate under the switch box? It has 2 yellow thick wires and a thick red one in the middle, the bottom yellow is where I am trying to get my signal from, is this right??? It's on non distributer side.

thanks, Kris

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

The original rectifier on that motor was a large cigarette pack sized component with three studs for ring terminals. Either of the yellow wires are correct to power the tach. If the rectifier is shorted to ground (it happens), you will not get a tach signal from the yellow wire. You might disconnect the yellow wires from the rectifier and see if you get a tach signal.

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

Is there a way I can test to see if it's even giving off a signal with a volt meter?

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

Sure, The stator will put out a high AC voltage, when the motor is running. Disconnect the stator from the rectifier for this test.

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

Can someone plz post a pic of rectifier for this engine to be sure I'm hookin up to the right thing. Also when I test yellow wires from stater with meter it's getting all kinds of reading... But still nothing out of my tach, is my tach setting at 12 poles correct? What is going on here... As soon as a hook up power to tach needle jumps up to 0 RPM but no higher even when I rev engine. Someone please help this is driving me crazy, tach is bran new

Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde

Trace the two yellow wires from under the flywheel, down the starboard side of the motor, and over to the rectifier. You should have only those two wires from under the flywheel. The tach signal wire is brown, and should be connected to one of those yellow wires.