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Discussion Starter · #1 ·
My bike is an 83 VT750c

(I think i saw a post about this last august? and i didn't follow it to the end because I ASSumed it was bad plugs.)

now that it is a nice balmy 75 degrees here in sunny San Diego and it appears the rain has stopped for the season i decided to get the bike up and running and take care of yet another pesky oil leak. I pulled all my plugs and the rear cylinder must be getting some blow by (oh joy) by the looks of the fouled plugs.

i found a great website with some photos to help decipher plug condition and cause: Spark Plugs Overview

I digress, to the problem. After changing out all four plugs the bike wouldn't fire. I decided to check to see if i had spark so i pulled the right front wire, attached an old plug, rest it on the exhaust pipe near the head and cranked it. In about 2 seconds the bike started right up and ran for a moment and sputtered out after a back fire from the rear cylinder. Tried it again and it started and continued to run. Turned off the bike attached the plug wire to the plug in the front cylinder and it wouldn't start. I also tried the same thing will all four plug wires and it would only start when one of the front plug wires pulled and a spare plug attached resting on the block.

what does this mean? where do i start to trouble shoot? of course the battery died soon after so now i think the battery is going bad also. But thats a whole new thread..... but should i just go with the lithium-ion?

thanks
 

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I think it makes sense if the spark plug cable has both VDC supply and ground wires inside: the ground of the tip on plug is isolated from ground when installed except thru the body of the plug itself so if ground wire in cable is broken then you'll get no spark- corollary is that when touching the block ground is restored via the chassis and so current can flow allowing spark.
I think anyways, I know nil about spark plug cables but I know VDC very well- electronics guy. ;)
 

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Discussion Starter · #5 ·
however when the spark plug is installed in the block the plug would complete the circuit the same as when a spare is used touching the same block, correct?
 

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I get your point, but only if the ground lead is tied back to the metal portion of the plug body, if it's isolated from that but tied to ground in the cable at the back end... you see that design a lot in DC electronics wiring, maintains floating ground at the main bus.
However all of this is a lot of guess work, I've no idea how a spark plug is wired. :twisted:

p.s.- an easy way to test the theory is to take a GOOD plug and do a continuity test with a meter between ground terminal and the threads, see if they're connected. If I can get my plugs out tomorrow I'll do that cuz now I'm CURIOUS! lol.

however when the spark plug is installed in the block the plug would complete the circuit the same as when a spare is used touching the same block, correct?
 

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Discussion Starter · #7 ·
thanks. these bikes have little differences that the old cars i used to work on and it throws me off. i'll just pull what seems to be a good wire tomorrow and see if it sparks with out being near the block.
 

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The plug wire has a single conductor. And no, you won't find continuity between the electrode and the threaded portion....the terminal tip and the electrode, yes. Clean the ground cable ends from the motor and see if that helps.
 

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Ok so cool to know, but then doesn't that go back to the question of why would it spark and run against the block but not when installed? Now I'm confused too... lol.

The plug wire has a single conductor. And no, you won't find continuity between the electrode and the threaded portion....the terminal tip and the electrode, yes. Clean the ground cable ends from the motor and see if that helps.
 

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If you don't want to try new wires as was recommended, I'd at least try to trim the end about a quarter inch and reattach the ends. They been tugged off the plugs for a lot of years and you may just have bad connections.
 

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If you don't want to try new wires as was recommended, I'd at least try to trim the end about a quarter inch and reattach the ends. They been tugged off the plugs for a lot of years and you may just have bad connections.
Well I can't speak for the pirate but I'm just trying to learn and contribute to the board, that's all. :D
 

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It is possible that you have a weak spark condition in general. Maybe because your battery is low or voltage drops to the coils, etc.
A good coil can fire about 1/2 inch or more in the air but in the cylinder under compression it is much harder to fire a spark.
So that is why the spark gaps are only .035 inch. And when you ground one plug in the two plug system you are giving a good ground on one end so the other plug can fire better.
If you you could apply battery power directly to the coil Positive terminals it may show up the low voltage problem. That is the Black/White wires as per the manual.
But I would make sure the battery is very strong first or jump it with another battery also.
I checked the terminals on mine and found them to be a little loose from vibration, that can do it too.
 

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Discussion Starter · #14 ·
right but the question still stands. why would it start, using the same, when disconnected from how it should be and connected to an extra plug resting against the block? especially if the wires are constructed as you state they are.
 

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Just an after thought=
You said it now is a problem after changing to new plugs?
Did it run OK before?
Are they the correct plugs, and are they gapped correctly?
 

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Discussion Starter · #16 ·
right but the question still stands. why would it start, using the same, when disconnected from how it should be and connected to an extra plug resting against the block? especially if the wires are constructed as you state they are.
sorry i didn't see that there was a second page of ideas. I certainly going to get a new battery as the CCA on this doesn't seem to be what it should and it lost it's charge rather fast.
 

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Discussion Starter · #17 ·
Just an after thought=
You said it now is a problem after changing to new plugs?
Did it run OK before?
Are they the correct plugs, and are they gapped correctly?

yes the plugs are are correct. it stalled out on me last time i road it to work (about 25 miles) it because of the rear plugs being fouled. so i replaced all the plugs. was going to run some restore oil additive to see how bad the actually blow by is. But i can't do that until the motor runs.
 

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Discussion Starter · #18 ·
i've been out of town for work and the weather hasn't been great so the bike has been sitting since late October. tomorrow i'll go through and check all the connections. and get a replacement battery and report my findings. but for now, time to drink.
 

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I bought a Honda last summer that was twelve years old when I got it. I read on a similar board that they indeed had grounding troubles. I took the time one day to remove all grounds, four of them in all. I cleaned, removed paint and corrosion and reinstalled them all. It made a noticeable difference. It eliminated 90% of exhaust popping and a couple of nagging electrical issues as well. A lesson learned. Two pieces of metal, especially dissimilar metals are bound to build up a little corrosion and thus resistance in twelve years. Wouldn't hurt to spend an hour doing that small job on any vehicle.
 

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Looks like you guys have made progress, but I did continuity test the plugs and the body is tied to the ground flange, apparently through engine block, only the supply is in the plug wire. So it certainly stands to reason you have a bad ground somewhere else.
 
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