Not no spark, but LOW spark on front cyls?

I have an '84 VF700F I'm trying to get running. My situation is that the front cylinders (2 + 4) have crappy spark compared to the rear (1 + 3). Signs are pointing to a bad spark box, but I have spares and get the same result! I suppose it's possible that all 3 I have on hand are bad, but I was wondering if there's something else I can test to see if it's maybe a wiring issue / bad ground / etc?

To be clear, what I'm describing is: the 1+3 coil gives good spark (holding the plug up to the cylinder head) while cranking, but the same plug connected to the 2+4 coil gives a much weaker spark. But I swapped the coils around and saw the same thing, so I know it's not the coils. The plug wires are new and I double checked them at 5k ohms. The pulse generator is also healthy at 487 / 493 ohms.

Can I swap the red / white spark boxes just as a test? Or will it mess something up? It looks like I should be able to, but wanted to make sure first. I saw other threads ask this question but not get an answer.

Also, can someone explain how exactly do these coils work? It seems like: the black/white wire is 12v all the time, and the yellow or blue wire becomes an open circuit at the command of the spark box, right? If that's true, then why can't I make the plug fire manually just by unplugging that wire? (akin to opening the points manually in a distributor...) I was thinking if I could make the plug fire manually and see a healthy white spark, that would prove a faulty box.

Other thing is: I've deleted the fuel pump and gone with gravity feed. (I already read all the threads about this and I'm going to at least try it for a while.) I know the blue wire that goes to the 2+4 coil also goes to the fuel cut relay, so I'm wondering if my problem might be related to that. I've just unplugged the relay, and saw no change. Do I need to jump two of the leads? Or ground something there?


Thanks!

 

Alright, I went ahead and swapped boxes, and the issue is unchanged! All the components seem healthy so it points to a wiring issue, right? Any ideas?

 

Make sure all your connections are clean. Like, scrubbed to shiny metal clean.

Coil connectors, coil grounds, ignition key connector to harness, battery terminals.

Tried switching plug wires to see if the low spark follows the wire?

Sent from my SM-S911U using Tapatalk

 

OK, the issue here was a flaw in my testing. There's 2 leads and plugs per coil, and I was comparing one plug from one coil to one on the other. But I had all 4 leads disconnected from their plugs during this test. So basically, lightning was trying to shoot down both leads, but one of the leads wasn't grounded on the end, and for some science reason, this makes the one plug fire poorly. As soon as I tested BOTH 2+4 plugs simultaneously, I saw great spark. I guess the discrepancy between the two coils was a result of this partial firing issue being less bad when the leads are super short, like they are on the 1+3 coil.

So now it's back to the drawing board trying to figure out why cylinder 2 is totally dead. Compression test looks good. Must be a carb issue, but I JUST redid them, and the other three are fine... I think it's running rich, because I can fully close the idle mix screw and nothing happens. But I guess if it's completely not firing at all, then all adjustments are moot......

 

For anyone following along, going on the too-rich hunch, I decided to check float level on the #2 carb, and the cheap amazon float had sunk! and on top of that, the main jet was just hanging out in the bottom of the bowl. Not sure if it backed itself out or if I didn't tighten it enough... Too bad I tossed the OEM floats when doing over the carbs before! Oh well, time to order some more parts.

 

makes no sence to delete japanese-made floats for chinese garbage, does it ? mainjets dont just fall out, u diddnt tighten propperly.

in over 35 years of carb work i never saw one bad plastic float, not one. bad floats were in the old brass and solder days.

 

Sure, in hindsight. But at the time I was thinking these were wear items, good today but who knows for how long, might as well freshen up along with everything else.