78 mercury firing on all 6 but running on 3

Redfeathers kill · Jul 3, 2010

I just bought a boat with an 78 merc out of the water she sounds great but in the water i can only get about 2100 rpms its seems to be running on only 3 cylinders. We checked for spark on all 6. All were getting good spark but when it is running you can pull off 1,3,5 with no change in the sound of the motor. If you pull of 2,4, or 6 the motor just about kills itself Could this be a bad cdi. for what its worth it when we covered the top 2 carbs (1 at a time ) the motor would just about die but when we cover the bottom (3rd) carb no audible or visual signs of the motor being affected by it. My mechanic thinks maybe the stator but he has the same motor with the opposite 3 cylinders not fireing and he tried a stator and then he got no spark at all. The big differance is that my motor shows spark on all 6 plugs but yet 3 of the cylinders are not running. please help me diagnose this problem because i cant afford to just keep dumping parts into this motor to see what works. Thanks

CharlieB · Jul 3, 2010

Re: 78 mercury firing on all 6 but running on 3

When you checked spark did you use an adjustable gap tester set to a 7/16 inch gap? Just checking by watching a spark plug is NOT an acceptable method of determining proper intensity (strength). You need a hot blue arc with at least a 7/16 gap.

We the serial # to be sure of your ignition.

Check out the free Ignition Troubleshooting Guide available from CDIElectronics.com

Their guide clearly spells out testing procedures to determine the problem.

Test twice to be sure, there is NO RETURN on electrical parts.

Redfeathers kill · Jul 3, 2010

Re: 78 mercury firing on all 6 but running on 3

No we didn't use an adjustable gap tester. we just looked for spark. Is it possible that the cdi could allow a visible spark but still be bad enough to cause the 1.3.5. cylinders to not fire. F.Y.I. those plugs did feel cold after running the motor, like they weren't getting combustion. Thanks for the help. I am very new to boat motor opporations and am not the most handiest guy for motors of any type. So i will probably make you think (What an idiot) more than a few times during these posts. Sorry for the frustration in advance. :redface:

quantumman29 · Jul 3, 2010

Re: 78 mercury firing on all 6 but running on 3

this could be a switch box issue,check out the switch boxes....

mike

Redfeathers kill · Jul 3, 2010

Re: 78 mercury firing on all 6 but running on 3

This motor has to switch boxes correct? each one runs 3 cylinders? It was suggested that i switch the 2 parts, basically swapping them to see if the other 3 cylinders go dead. Does this sound right or is there any other suggestions. Thanks for all the help guys and maybe girls.

latin monkey · Jul 3, 2010

Re: 78 mercury firing on all 6 but running on 3

you might not be getting fuel to those cylinders

Redfeathers kill · Jul 3, 2010

Re: 78 mercury firing on all 6 but running on 3

latin monkey said:
you might not be getting fuel to those cylinders

does anybody know what carbs cover what cylinders. I have 3 carbs and 6 cylinders. when i cover the upper 2 carbs it bogs the engine but when u cover the lowest and last carb the engine does not react. I guess my next bet would be to get some carb cleaner. thanks everybody for your suggestions.

CharlieB · Jul 4, 2010

Re: 78 mercury firing on all 6 but running on 3

When you say 3 carbs, 6 cyl's, that tells us you have a stacked 6.

That covering the lowest carb makes no change in the idle shows it is time to clean all carbs.

But the three cold plugs indicate you also have an ignition problem that needs attention. Your model should have two black boxes, the ignition modules, mounted one over the other. Each one receives power from the stator on the red and blue wires, three wires from the triggers tell the module when to fire each cyl coil (the green wires to the coils). The white Bias wire alternates the boxes firing order.

Two tests are needed to determine if you have a weak winding in the stator or if one of the boxes is dying.

First test, switch the blue and red wires from each module to the other, if the cold spark plugs switch to the even # cyl's then the stator is at fault.

If no change then switch the rest of the wires between the boxes, if the even # plug are now cold then the module is at fault.

Redfeathers kill · Jul 4, 2010

Re: 78 mercury firing on all 6 but running on 3

Thankyou so much charlie im on my way out to the yard with my ratchet and carb cleaner. My machanic was explaining a similar test so i went and picked my baot up last night. The guy is nice enough to let me do these simpler tests to keep my labor cost down. (After note) I did some checking this morning on the serial number and found that this motor according to the s.n. is not a 78 but rather a 1984. I dont believe this will change anything everybody has been stating but if it does please let me know. Thanks again guys.

Redfeathers kill · Jul 4, 2010

Re: 78 mercury firing on all 6 but running on 3

another quick question Charlie. the thing that is confusing me is that even though my 1,3,5 cylinders are cold after starting the engine, when you take the plugs out and look for spark, (not the right way either, just looking for any spark at the plug) i do have visual spark at all 6 plugs, is it possible to have spark there but it just is to weak to ignite the fuel. Sorry if this is a dumb question, as i said before i dont work on motors very often, But on the other hand if anybody had a question about wood im your guy

Thanks again guys.

Redfeathers kill · Jul 4, 2010

Re: 78 mercury firing on all 6 but running on 3

(Two tests are needed to determine if you have a weak winding in the stator or if one of the boxes is dying.)

Well I did as instructed and the first test, switching red and blue wires around, did not yield and results. So i continued on with switching the rest of the wires and after starting her up(which she seemed to run much stronger) was able to determine that the opposite 3 cylinders were now hot. I think that my next step would be to order a box and install. Is there any other units that could have been damaged due to the box failure or do i just start with a new box and see what happen? Thanks for any advise!

Redfeathers kill · Jul 4, 2010

Re: 78 mercury firing on all 6 but running on 3

Also is there a chance that i hurt the motor running it before we realized it was running on only half the cylinders?

CharlieB · Jul 4, 2010

Re: 78 mercury firing on all 6 but running on 3

A visable spark on a plug is not a good indicator of that spark once placed under compression in the cyl. The additional air squeezed into the spark plug gap in effect widens the gap almost 10 times the gap that it sparked when you have it out of the motor. It takes substantially more voltage to fire that gap under cyl pressure. We use an adjustable gap tester set to 7/16 inch to force the ignition to produce the energy necessary to fire the plug while out in free air for our observation.

I doubt if you hurt the motor, attempting to run it with a weak ignition. I would be more worried about the lower cyl running lean from the dirty carb, but with three almost dead cyl's You probably didn't get too far for very long.

Test compression on all cyl's to be sure that they are all near the same, the closer to 120 the better, as long as they are all within 10% each other you should not worry.

Search iboats for the ignition module, also check CDIElectonics.com for prices and freight.

Ideally, these modules are changed in pairs. If you do, mark the good one and sell it to someone needing a module for a three cyl, it will work fine.

Redfeathers kill · Jul 4, 2010

Re: 78 mercury firing on all 6 but running on 3

Didnt get far at all mostly around in a couple circles to let my mechanic mess with it a bit and check the basics. It didnt take long to realize we were running on only 3 cyl. Ill get that compression tested also. My dad has a compression tester and leak down tester, but i doubt he has ever used it, what is the proper technique

for testing the compression, can i find a post on that here? Thanks again for all the help.

Redfeathers kill · Jul 7, 2010

Re: 78 mercury firing on all 6 but running on 3

well i put a used cdi box on and it is getting spark on all the cylinders it is able to run on 2,4,6 and on 1,3,5. When u pull off any plug while its running it makes a noticeable drop in rpm and overall smoothness of the running. I thought this meant every thing was good so i took it to the lake and tried her out. To my dismay it did almost the same thing as before it was running as if on only one bank (about 2100 rpms). After running it for a little bid 2-3 min it started to get faster and we were able to plane out and then the motor started to pick up rpms. It seemed like it was running pretty decent then. I only ran it for a little while because i was low on gas. There is defanatly an improvement but it still is not running full bore. Any ideas guys could i have a week charge coming from my stator or could this be a rectifier problem.

CharlieB · Jul 8, 2010

Re: 78 mercury firing on all 6 but running on 3

The stator has Low Speed coils and High speed coils, two sets of each, one for each ignition module. A failure of a high speed set will not allow one ignition to fire hot at RPM and limit power to under 3000 while under way.

The fact that you said it was slow to speed up, but got better after a while causes me to wonder if you cleaned the carbs and set the float levels carefully. Also initial idle mixture should be set @ 1 and 1/2 turns out from very lightly seated for initial start up, then reset ON THE WATER in Forward gear to best idle speed plus @1/8 turn out. The additional part turn out past best idle is needed for the additional fuel required for acceleration, otherwise the motor will hesitate, bog, and be very slow to speed up.

Test acceleration by RAPIDLY opening the throttle control from idle in Forward gear to WOT. If ANY hesitation, lean cough, or bog, open (turn out) the idle mixture screws the width of the screwdriver slot then retest acceleration again. Repeat to perfection.

Redfeathers kill · Jul 9, 2010

Re: 78 mercury firing on all 6 but running on 3

Thanks for the advise. I will get the adjustments made and double check the floats My new cdi box is on the way from iboats and should be delivered here today. Maybe i might have this problem resolved for the weekend. That will make my wife very happy. And you know what they say "Happy wife Happy life". Thanks for all your advise Charlie.

Nickledimed · Jul 9, 2010

Re: 78 mercury firing on all 6 but running on 3

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Nickledimed · Jul 9, 2010

Re: 78 mercury firing on all 6 but running on 3

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Redfeathers kill · Jul 9, 2010

Re: 78 mercury firing on all 6 but running on 3

I just got my new switch box from i boats and it came with a warning paper saying that a common mistake with the ignition is that the switch box is not the problem but the result of another problem. Its got me thinking. according to this paper it says that a shorted rectifier could cause the switch box to fail. How would i check to make sure the rectifier is not at fault. Thanks for the help

78 mercury firing on all 6 but running on 3

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