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Motorcycle Repair/Which Coil For which plug.


QUESTION: I have a 1985 vt700c Shadow Bobber There is 2 coils on it. and 4 plugs . Which coil do I use for the front plugs. Or do I use both coils for front plugs. I have 1986 v45 Magna 650 coils on it because mine did not work. Bike runs with one coil going to front 2 plugs and other coil going to back plugs. But it still backfires once and awhile. Could it be a Carb problem.  is my email

Hi Richard,

It is correct to run each coils wires to only one of the cylinders.
If you are unsure of the connections just use a timing
light to check if the "F" marks align in the timing hole.

Backfires can be caused by open pipes, leaking exhaust pipes
and lean fuel mixtures due to air leaks around carbs or intake.
Also plugged carb jets or passageways can lean the fuel mixtures.

Sometimes carb spray can find an air leak around the carb mounts,
the engine speed will change.

I would check the timing and exhaust gaskets, if you have no pipe baffles
the lack of backpressure will cause some backfiring. Using one size
larger idle jets may help this.

Your plug color will tell you what the engine is doing so use that as a guide
to tuning it:

If your coils were wrong I don't think it would run so as long as one coil
goes to each cylinder it should fire okay.


---------- FOLLOW-UP ----------

QUESTION: I run open pipes with Shorty mufflers on it with the baffles out. I put them on new ran about 5-10 mins and it run real bad. Removed the baffles and it runs much better. It is a bobber hard tail. Seams the more I run it with the seafoam in it the better it runs :-)
When I got the bike 4 weeks ago it has set over the winter with the tank off but he didn't drain the reserve tank :-( so I am hoping the seafoam will take care of it. The weather is still not the beast here so I am hoping for some riding weather soon. Rain next week :-(.
TY for your help.

Hi Richard,

Hopefully the seafoam clears things out if not you may have to flush the tank
and drain the carbs to get the old fuel out.

The spark plug color can tell you how the jetting is for the open pipes.
If the plugs are mostly tan or brown color things are close.
If the plugs tend to run whitish or hot you may need larger main jets.

The Yamaha brand carb cleaner is good and safe to soak the
carbs in or just to fill the carb bowls and let it sit awhile, then drain
the carbs to clean them out. Honda may have some carb cleaner
too but the other gets good reviews.

If you need info later on about tank cleaning or other let me know,
otherwise have fun!


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Wayne S.


Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.


Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

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