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G5 bucking


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Hoping for some advice on next steps for a '79 G5 that's bucking.

Background: This was a project bike and got a full (not museum, but solid) restoration. I did most of the work, but couldn't get it running and so took it to an experienced classic Guzzi mechanic. He got it running (with an entirely new Bender wiring harness and other sensibly replaced electrical bits, etc.). However, his brief rides up and down the neighborhood didn't reveal a problem that has been with the bike ever since.

Started out running well, but after 15 minutes or a half hour, it would begin to feel bottled up and decel for a second, followed by a surge, repeated ad naseam, resulting in a bucking sensation. If I parked it and rode the next day, it would start fine and ride fine for a while, but the bucking would eventually return after a bit of riding. I thought this was limited to when the engine was quite warm. At least, until today.

A little research and chat with another Guzzi mechanic revealed that this decel/surge issue can be a symptom of ignition coils faltering when hot. I installed new ignition coils and took a spin yesterday -- same issue exactly. Warmed up in the driveway, did ~7 miles and (on interstate doing ~65mph indicated), the familiar bucking returned (though not so bad). After a few more miles, I filled up the tank and continued, doing ~4 miles before going up what passes for a mountain in these parts (somewhat less that 1K') and then another 25 miles or so of country riding, 30-60 mph, with no problem whatsoever. After stopping at a friend's place for an hour, I rode 5 miles or so, with a good bit of bucking in the last mile, to a farmers' market. Bike got a 10-minute rest, then bucked the last 5 miles home.

So, the new ignition coils didn't resolve the problem. Today, I replaced the NGK spark plugs with some E3 plugs. That was terrible (worse engine firing and more bucking than before). I removed the plugs and the points were black and wet. I looked at the old plugs -- they were gapped at about 0.5mm. I widened the gap to 0.6mm, per the owner's manual and took it for a ride. Still crap -- probably a bit better than the E3s, but -- until today -- I hadn't experienced the bucking when the engine wasn't particularly warm. Today, both with the new plugs and the re-gapped and cleaned up old plugs, the bucking was present right out of the driveway.

Since those rides around the block, I've confirmed that the condensers were replaced in the restoration. I also checked the gaps of the points and they are right at spec of 0.4mm.

This certainly seems like an electrical ignition system failure, but I don't have any sense of what may be going on. Nothing has changed since restoration. Bike has been inside. No corrosion. Haven't removed any ground wires.

Any ideas would be greatly appreciated.



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Thanks, Pete.  I'll give it a try.  Not sure if there's a best practice to make a vent hole in the cap.  It's the old plastic fuel flap with the spring-loaded stopper.  The stopper doesn't appear to have ever had a hole in it, excepting the little square one that accepts the post from the flap.  If I'm to drill a vent(s) hole in it, what size would be appropriate?

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Just try riding it for as long as it usually takes to start playing up with the filler cracked open. Not with a full tank obviously! If it doesn't start cutting out? Then that's the problem.

I'll bet it is though.

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Okay.  Tried a few things this morning. 

First, no fuel cap, with the old spark plugs gapped to 0.6mm (spec per owner's manual, but wider than the 0.5mm I previously had them at), I still had bucking.

Then, I looked up the factory spec on NGK BP6ES plugs (0.8mm gap), and set them accordingly.  Engine wouldn't start at all.

Then, I reset the gap to the 0.5mm I previously used (still with no fuel cap).  Started right up and warmed it up a bit.  Once choke was released, I started getting popping and chuffing.

So, maybe I have a fuel mixture issue.  Maybe an air leak?

Other notables based on comments elsewhere:

  • High test wires and spark plug boots are new
  • Carburetors were stripped, cleaned, and rebuilt
  • Fuel petcocks and filters were replaced (and I did away with the electrovalve in favor of simplicity)
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I definitely don't have KiwiRoy's knowledge of electrickery; but should plugs be that sensitive to gap and only fire if it's really narrow ?

Sounds to me like you have something weak or an issue further up the line.

With all those new parts and much talk recently, of the crappy quality of new condensers, could that be an issue?

What about the grounds and the ignition switch?

When I replaced just about everything electrical on my CX100, Roy pointed out to me, that my 35 yr old ignition switch would cause me grief; in the end he was right, it would only run/run decently once I replaced the ignition switch as well.

fwiw ymmv


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Now it sounds electrical. Weak ignition will only fire a closely gapped spark plug or a very rich mixture.


I've confirmed that the condensers were replaced in the restoration

I would look there next. If you still have the old condensers, they are most likely better than new ones.

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You can test the condensors with your DVOM ( as long as it has a capacitive function )  or an  analog meter .  The DVOM will give you a specific # . Usually .24mFd is a good reading . The analog meter , you will put the r reading on the highest scale and touch the leads to the case and tail of the cap . Then swap the meter leads . You will see a "kick" of the meter before it falls back to O . Do this a couple of Xs to see what you are looking for . When using this test method , ALWAYS short the lead to the case to discharge the cap. If you don't do this , sometimes you can damage the meter .

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