Please help! 85 Interceptor ignition troubles

Hello everyone! I am new to this forum, and after reading through quite a few old posts, i feel like someone here might be able to help me.

I recently purchased a 1985 interceptor vf500 from craigslist. I was told the bike ran before sitting for a while (not sure exactly how long...). that being said, its craigslist so i might as well say the bike has no history lol.

I have a bit of experience working on smaller, less complicated bikes like mopeds, dirt bikes and scooters, and I figured i was equipped enough to tackle this project. now im kind of stuck, but if anyone has some ideas on what i could try to resolve my issue, I am more than willing to attempt repair.

the bike has no battery, but i have a car and some jumper cables, as well as one of those new lipo jump boxes for motorcycles.

Heres what happens: I hook up my power source, put the key in, and press the starter. the bike turns over but will not start, aside from an occasional back fire. the choke doesn't help, nor does playing with the throttle.

I pulled the plugs, and they smell like fuel, shes definitely getting gas. I grounded the plugs on the head and checked my spark, and something caught my attention. to my eye the spark looks very weak! no fat blue spark, just a faint hairline arc in the spark gap, not much brighter than the spark from an electric bic lighter. I believe the spark is too weak to fire the bike up. its like this on all four cylinders. i checked the wires to the ignition coils and they look ok. ive read about people having one coil go bad and having no spark, but never found anyone with all four having a weak spark!

considering that it IS getting spark, just a weak one, i assume its not a cdi failure. Is that correct?
could it be the ignition coils?
could they both go bad at the same time?

what else could cause this - a weak spark on all four coils... the spark plugs look very clean.

i need advise, suggestions, anything at this point! id like to spend as little money as i can, and i would like to diagnose this problem in order of what is most likely the issue. if i have to rig somthing to get my bike running, thats quite alright. i can be quite the mcgiver and this bike is not going to be ridden on public roads, so it does not have to me mva inspection worthy.

thanks for reading

:edit: i forgot to mention that i cleaned the carbs, and put them back exactly how they were. this is a problem of ignition, not the carbs. the bike is getting fuel!

 

It is possible the ignition coils need changing. Remember this is a 30 year old bike. Unfortunately sometimes there is no cheap fix for these bikes. When they run well they are awesome but can be complex bikes to work on and not cheap to fix.

I recommend replacing the plugs anyway, check the coils thoroughly

 

You really need to look at the carbs. These bikes are notorious for having blocked circuits in the carbs. If you are getting fuel and any spark you should get some ignition.

 

Could be a weak engine ground , and you already checked the power connections to the coils.

Might still have carb problems ,but if you don't get some decent spark then it isn't likely to run well enough to tell much about anything else.

db

 

1. Use a voltmeter to watch for voltage drop on the bike wiring.
2. Get a new battery that can deliver full current and voltage.
3. Check trigger wire connections to coils for corrosion and possible fatigued wires. (Happened to me)

 

Update: I removed the ignition coils and noticed a lot of corrosion where the wires connect to the coils. i cleaned the contacts up with steel wool, and got a much better spark. i hooked her up to a jump and she fired right up!

now Im not entirely sure about this, but should this bike be able to run without a battery or does it need the complete circuit. i took the jumper cables off and it immediately died. Is that normal with this specific bike? Ive got a battery coming in the mail.

 

thank you all so much for the help! this is a great community! there really isn't much else out there for these old v4's

 

Alternators won't run a bike (or car) without an exciter voltage of at least 5v.

Corrosion is our biggest enemy on middle age and older bikes, check all connections. Corrosion can pass voltage but not current required, clean and burnish all the connectors. Tedious but well worth it, requires physical abrasive, chemicals and cleaners won't do it. Corrosion creates resistance, resistance creates heat. Wiring is normally adequate, rarely see wires melt under normal conditions, only under dead shorts or excessive loads beyond their rated value.

 

When you say you took the jumper cables off they were connected to the car battery, and there was nothing else connected to the battery wires on the bike?

Probably best to wait till you get your battery and test it then. You will want to be very careful hooking up jumper cables to loose battery leads, real easy to bump and short the red wire.

IIRC a fully charged battery is about 12.8 -12.9 volts.
Running at 3500 rpm you should get around 13.8volts from your charging system.

Your service manual will have specific information as to test rpm and acceptable voltage range.

Not sure whether the bike shutting down once the battery is unhooked means your not charging , like it does in a car.

 

It's not like an older car, and disconnecting battery wires while the bike is running can damage the r/r, or so i've read in several service manuals.

 

Definitely a good piece of info there. Speaking of I still need to put an upgrade r/r on and beef up my charging systems wiring.

db