Unusual Problem

[bikerphil]

My 79SF has developed a strange problem in the past month. The bike starts and runs fine, but after taking off from my house, about 2 miles away, the bike will just die. I'll be going 40mph and it'll just shut off. After pulling to the side of the road, it is very difficult to start. There is a smell of unburnt exhaust gas I notice and it'll take about 3 minutes of cranking to get her to catch, and then everything is fine after that. Every electrical connection has been gone thru within the last year.

Things I've tried so far....

Swap out the TCI with a known good unit
Unhook the tip over switch
Check the connection at the ign. switch plug
Tightened all wires going to the coil

This one's got me baffled....

[cywelchjr]

Check the gas cap vent. Sounds like it might be plugged. It's at least worth checking.

[Ivan]

Sounds exactly like mine when I leave the petcocks closed. If you haven't yet, check for kinked lines, and make sure the vacuum fuel valve is working correctly, if you havent tossed it out yet.

Two miles is about how much road is in the fuel bowls, so that's why we are going with fuel blockages. I also agree with the gas cap vent.

[bikerphil]

I replaced the fuel filler gasket and cleaned the vent about a year ago, but that's easy enough to check again. No octy on the bike. The way I look at it, if it was a fuel starvation problem, it would give me some kind of warning, like sputtering or popping. Today I let the bike warm up for at least 5 minutes before taking off, so I'd imagine it would be out of fuel sooner than a couple of miles. Going to check the fuel lines for kinks now. Kinda leaning towards an electrical issue tho.

[dbeardslee]

Mine did the same thing when I first got her. I'd be riding along and for no apparent reason everything would go dark and the engine would quit. If I let her sit for a while, or put jumper cables on her she would crank back up. I finally traced the problem down to a short in the large white wire behind the fuse block.

[bikerphil]

Ok, I went thru the fuel system first. The gas cap vent is clear, no kinks in the lines. Put in new fuel filters also. Then I went poking around the coil wiring. Cleaned and dielectric greased all the leads. The only conclusive thing I found was a loose coil mounting bolt. The Accel coils I'm running are mounted in a way where the left coil is in it's normal position, but the right one in mounted vertically using only one mounting bolt. This was the loose bolt. I don't know if these coils need a good frame ground or not, but if they do, this one had a poor ground. It kinda makes sense because after the bike would stall, when I go to crank it, it felt like it was firing only on 2 cylinders. I took her for a 20 mile test ride and she was fine. I'll have to ride her again when she's stone cold, that'll be the real test. I think she was just jealous for attention because I've been spending so much time with the F lately, lol.

[red bandit]

Main fuse connection??

[Ivan]

In theory a waste spark system doesn't need a ground. Think of a coil like two links of a chain. The first link is a circuit, the primary, which includes the battery and current breaking control such as the TCI in this case. The other link which passes through the firts is a fill circuit which includes the iron core with spark leads on either end, a plug on those and the engine block completing the circuit between the two plugs.

The only reason I could see needing a ground would be if the coils have an internal condensor, but that would be highly unlikely, since that is usually near the points or in the electronic spark controller.

[Rasputin]

I once had a similar problem with a truck. The battery proved to have been at fault. The plates in the battery were flopping around causing it to short out at the most inconveient times. I would hit the brakes or a bump and everything would shut down. JAT for you. The only way I found it was to do a specific gravity test on each cell. Hope you have solved already but it can happen.

[Larrym]

The Ungroundable...

Usually there is some way that the Iron core of a coil is connected to the engine 's ground either through a separate wire or being mechanically mounted to a frame. (Seen this in lawnmowers, outboards, generators, etc...)

Can't see how our coils would be any different. Secondary does go to two plugs instead of just one. But generating the high voltage/spark at the plugs requires that that both the engine block and the electrode of the spark plug be at the two opposite ends of the Pos/Neg 20K volts. (Complete circuit....)

If you do a resistance check from the spark plug wire end to the metal parts which mount an OEM coil to the frame, what does it read?

(It's times like this that I wish that my bike wasn't 2 miles away in storage...)

I Hope that loose mounting bolt was the problem.

[bikerphil]

If you do a resistance check from the spark plug wire end to the metal parts which mount an OEM coil to the frame, what does it read?

Larrym, on a good stock coil I had laying around, the resistance is nil from the frame mounts to the plug wires. From the mount to the TCI wires, also nothing.

Ras, When my bike stalls, the electrical is still there, just the engine dies. The motor will crank and try to fire but after a minute or two, it will eventually start. Then it runs fine like nothing ever happened.

[Larrym]

Core Issues....

Cool!! (Thanks..)

Those readings would indicate that the iron core is trully isolated from the secondary/primary windings: No tap present somewhere or connected at the "bottom" of those windings. I've seen that used before on some stuff...

Still, that ferrous core is part of the secondary system and is inductively linked to that high voltage. Might/might not have to be held at ground level for the coil to make that necessary "arcing and a-sparking".

I guess we'll find out if you come back from a ride with a big grin, eh?

[Rasputin]

I was thinking more of a momentary short, just enough to kill the engine. It was just a thought though and pretty much all I could think of right now

[bikerphil]

Appreciate your help guys, I'll test her out tomorrow. It's cold here, it'll be in the 50's tomorrow (thin blood).

[DGXSER]

Appreciate your help guys, I'll test her out tomorrow. It's cold here, it'll be in the 50's tomorrow (thin blood).

Pansie!! Its 5 here tonight, and snowing so I ain't riding either, but if it breaks 30 and no snow, I'll take Thunderstruck out for a cruise. 50s would be like a tropical heat wave!!