Logo

does the trigger fire 2 cylinders simultanously

peter1965

Member
I have good compression and strong spark on all 4 cylinders yet i have rough idle and very poor top speed. I tested the coils, trigger wires and stator wires with an ohms meter and all are in spec. I marked TDC of each cylinder on the flywheel 1,2,3 and 4 respectively and hooked up a timing light the results have puzzled me
when timing light is set on clylinder 1 i see that both 1 and 2 are firing. on cylinder 2, 1&2 are firing again, on cylinder 3, 3&4 are firing and on cylinder 4, 3&4 are firing again. is this normal ?
 
Should have mentioned earlier that when the motor is started and i pull spark plug boot 1 off the spark plug no difference in rpm is observed. The same can be said with spark boot 4. However the motor dies when either 2 or 3 are removed. I have changed the spark plugs around but still the same result. The motor is a 4 cylinder 50 hp blue band. I am wondering can it be possible to have a faulty power pack and still have good spark ?
 
It's possible, but you also might have bad wiring that's shorting together, or the wires are on the wrong terminals.

That said, my money's on a bad switch box. The trigger coil's a pretty simple device that either works or not.

Jeff
 
Do a compression test.Post the results.
Reset the air screws on the carbs.
Start at 1 and 1/8th turn out for inital settings.
 
Compression is 125 to 130 on all cylinders, its a 50 hp with 4 ignition coils. i have had this motor for decades. I initially had a problem with the stator as it would not run at high rpm changed the stator and it ran well for a while. Then not all that long ago i had a problem with a high rpm miss that eventually got worse. Noticed that 3 ignition coils were arcing inspected them and found that the magnets were cracked badly, replaced them and that stopped the arcing. I have checked the timing pointer position and its correct also the primary throttle pick up is set where it should be at @ 8 degrees BTDC although it runs rough at that point. however the motor will not start let alone idle at anything ATDC. The air screws on carbs are set to about 1.5 turns from lightly seating.
Given that cylinder 1 and 4 appear to not be firing well, can I re route the wiring from the ignition coil to the powerpack to check if the issue moves to cylinder 2 and 3 ? or any other way to confirm that the power pack is at fault.
 
all wires are correct and got some dva readings as well.
coils @ 210 to 220 volts
stator blue @ 300 volts, red well over 25 volts voltage increases rapidly as i increase throttle
Power pack @ 210 to 220 volts at each green wire, however as i increase rpm the voltage does not increase significantly maybe 2 to 5 volts and its a slow response not like I saw with the stator.
although this is in spec shouldnt it increase more rapidly as throttle picks ups ?
 
checked resistance and dva of trigger and meets all specs, readings range between 5 and 7 volts spec says greater than 4 also
i will take off the flywheel and have a look at the magnets. Does the power pack have a dva specification ?
 
Motor year of manufacture is 1978, 50 hp , 4 cylinder
also measured kill switch voltage fluctuates between 0.01 to 0 mv, which i guess its what it should be as the spec says 0 volts. I changed all the spark plugs around and still the same 2 cylinders are playing up.
 
4580036 is the serial number took off the flywheel , magnets havent moved at all but i can see that the crankshaft upper oil seal has moved out of its position almost completely off the crankshaft its not sealing properly at all I will get another one tomorrow and replace it. I am hopeing that this is the cause of my issue. But how can this cause 2 cylinders to not fire ?
The trigger looks okay but is covered in some oil.
 
I banged in the old seal (it didnt look all that bad) put the stator trigger and flywheel back on started it and guess what a hell of a lot of smoke and when i took the spark boot of cylinder 1 the engine dropped rpm and wanted to die. I am kicking myself for not checking cylinder 4 as well, its just that it started smoking and the neighbors are not happy with me over the last 2 days. Iam hoping that the bottom seal hasn't popped off but i dont like my chances. I dont mind pulling the head off and doing it properly its just that its snapper season here at port phillip bay. Thanks for all your support it certainly has been an eye opener for me.
 
lower leg was loaded with oil, pulled the powerhead off and yes one of the end seals popped out as well bearings look and spin okay will replace them since i am there, does the exhaust gasket need to be replaced if it hasent been removed? and there is some end play in the crankshaft (up and down) I suspect that's normal but don't really know?
 
No, it shouldn't work like that. This is still a T4 ignition.

There is only two trigger coils because on the first magnet pass each coil only produces half of a sinewave (the zero to 180 degree portion), the second magnet pass finishes the sinewave (180 to 360/0).

That "power pulse) is what switches the SCR in the switchbox to the appropriate cylinder - for example

positive pulse from trigger coil 1 = SCR switches to cylinder 1 capacitor for release to the spark plug coil
positive pulse from trigger coil 2 = SCR switches to cylinder 3,
negative pulse from trigger coil 1 = SCR switches to cylinder 2,
negative pulse from trigger coil 2 = SCR switches to cylinder 4 (or whatever the fire order is).

IF he is getting fire at two plugs simultaneously then EITHER the SCR's are screwed up or the blocking diodes are fried - in either case the switch box would be "leaking" and would need to be replaced OR the wires or coils are cross feeding each other.

Otherwise the DVA readings sound fine. The switch box only converts the AC to DC and stores it in capacitors for release to the coils, it doesn't "increase" the power from the stator - the power released from the switch box will generally never exceed about 320 volts DC (and it will climb from somewhat over 200 volts upwards to 320 according to rpms and dependent on the the exact motor model)...
 
So does this mean that I should not be seeing 2 cylinders firing simultaneously and that its very very likely that i have a switchbox issue if 2 cylinders are firing at the same time ?
I am pretty sure that the seals popped out due to the motor backfiring which happened not all that long ago when i noticed that the ignition coils were arcing.
Does it mean that if 2 cylinders are firing simultaneously one of them is always pre detonating ? and the pre detonation caused the backfiring in the first place which in turn caused the seals to pop. I am thinking that the switchbox possibly destroyed the coils as well.
What i dont want to happen is that i put the motor back together and start it only to repeat it all again. Once again thank you all for the support I am sure it wont be long before i wake out from this nightmare.
 
Nothing to it--for electronics--but consider this: At 6,000 rpms, the pistons make a complete cycle 100 times a second. THAT is fast!

The ONLY way two cylinders could fire at once is a bad switch box.

Jeff
 
Last edited:
I know its close to $500 i will get one because without it i have no motor, it would have been good if i could open it up and have a look inside is there any way of removing the epoxy material without damaging any of the components.
 
You'd be wasting your time--honestly. Go to eBay and see what's available. I see them on there all the time, for less than $150.

Jeff
 
not anything will trounce this extremely ennobling and extremely plenty of consolatory mail Thanks for sharing this commendable content wake up now
 
I replaced both the upper and lower crankshaft seals and put in new bearings since i was there, started motor (had the old power pack still on) all four cylinders were firing. Although motor sounds smooth, I reconfirmed that 2 cylinders were firing simultaneously. I then installed the new power pack and once again 2 cylinders are firing simultaneously. Checked wiring against the corresponding wiring diagram and it is correct. I guess it just may be normal. primary pick up is set to 8 degrees BTDC, motor sounds smooth and doesn't miss a beat. I will check max spark when in the water. Idle is best at between 1 ATDC and 1 BTDC ) 700 RPM. The spec states that idle should be at @ 4 degrees ATDC. Can i cause damage if its set at 1 to -1 ?
 
Not sure this will help, but...

Merc 7.5 hp lack switch box motor: No spark on one cylinder. Changed coil (twice), switch box, then stator--no difference. Changed trigger coil--voila!

Jeff
 
took it to the water today with new power pack installed, motor is still missing at high rpm and still 2 cylinders firing simultaneously could the trigger be faulty even though all tests are within spec ? other than changing the trigger i am at a loss I dont want to even think about the possibility of a new power pack being faulty.
 
After my experience with a bad trigger coil, I'd say it MIGHT be the problem. I always thought these things were either good or bad, based on conductivity, but...

Jeff
 
I have ordered a new CDI trigger should arrive in the next 10 days, at $ 120 delivered its not to bad. My concern is that all the conductivity and DVA tests that i have done so far suggest that the trigger is fine. But as Jeff and Graham explained earlier on, that 2 cylinders should not be firing simultaneously and that either the switch box or trigger wires / coils cross feeding could create such a situation warrants a trigger replacement.
I have read a CDI technical note stating that the trigger is composed of 2 coils, 1 coil fires cylinder 1 & 2 and the other coil fires cylinder 3 & 4. I am somewhat concerned that this implies that cylinders 1 & 2 and cylinder 3 & 4 firing at the same time may be normal (even although it doesn't sound correct). I am wondering Peter Devil Ears or anyone else on the forum, if they have ever checked their motor to see if it does or doesn't fire 2 cylinders simultaneously ? If proven that it doesn't I think it would be an easy and worthwhile diagnosis test for all during confusing times like these ones.
 
I can check if 2 fire at the same time and will let you know, but I don't know if it will assist much, since my engine is a V6. It has two switch boxes and each then fires 3 plugs. However, I do believe 2 will fire together! I assume this since each switch box only has one set of cables routed from the trigger coils. It would surely have more cables in order to fire each plug individually?
 
Back
Top