I needs Some help. Merc questions

[pstock800]

this is what I have. merc 40ci the pipes are hand built chambered. it has a hustler tower. KA7A carbs mag is locked. Fairbanks Mag. I had it in the water today to test fire it. it fires right first pull every time. My problem is that its only firing on the top to cylinders. at low rpm the bottom 2 fire every once in a while but when I give it gas nothing. I can see the fuel going in when I open the throttle.. Any ideas.

[Merc1]

bad mag

[jeff55v]

Fix the Fairbanks mag, then save it for use only to dry out a wet motor.
The Keikhaefer mag is a far superior design.

[ctpdsr]

Agreed - go with a Kiekhaffer model 4D-3 or higher. New wires & boots.

[Fastjack]

Mark , I have a couple of mags that Ken had done , If you want to try one .

[PRO-MOTIONRACING]

Why is everyone disrespecting the mighty Morse-Fairbanks magneto? It could be one of the carbs is flooding. Give the primer bulb a good squeeze and see if the carb floods over... It's probably the Mag.

[Dave M]

Agreed, Likely an ignition sort of issue. If you know Mags, take it apart and check the points, etc, might be an easy fix. If you plan to race that engine, I suggest going CDI.

[pstock800]

Today I changed the mag to a merc. And wow what a diference. Still not turning enough rpm wide open . Maybe I'm off a tooth or 2 on the timing but it was fun to try the boat out.

[Guest]

Although the FM magnetos were a good mag in their day they really don't have the spark timing precision needed at high RPM. When Mercury looked for an improvement in spark scatter from the FM unit they determined that a larger rotor provided better individual cylinder timing. I have run both mag's on an engine in my test tank using our 40 MOD while watching an oscilloscope and it shows it up with ease. Any time you belt drive a unit you will find that the timing varies on each cylinder due to belt dynamics. This is made much worse due to the small diameter of the FM rotor. The best mag's are the KD-3,4, and 5 which have the self aligning mid section. The older KD's require a tool to get them aligned (body/cap/housing) correctly. Also mag's with the big diameter point pivot are the most desirable. We have a few new alignment tools for sale for both the FM and the KD mag's on our Aeroliner web site.

When setting up a KD mag you need to make sure the spring tension is between 33 and 37 oz. I like it to be at the higher end. Point setting for the KD mag is stated at .007". I set new points at .009" and used ones at .008" which allows them to work in to the optimal gap. After assembly I run the mag on our test stand from 300 to 10,000 RPM. Once I verify stable spark through the range then I spend 30 min. at about 6,000 RPM to see if there are any heat related issues. Another sweep to 10,000 and The mag is ready to use. We set the mag timing at .365" piston displacement on the number 1 cylinder.

To solve the magneto spark scatter I have converted a number of engines to the Electromotive ignition system that we have developed for the 4 cylinder engines. Spark precision is better then .25 degrees and no more belt induced spark scatter. Let me know if you would like info on the ignition system.

Alan