G5 decel and surge (bucking?)

[Frey]

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.

Cheers,

Frey

 

[Moto-Uno]

This may be a bit out there , but have you tried loosening the fuel cap ? Plugged vents
in gas caps can lead to a slight vacuum in the gas tank and partially restrict fuel flow .
Cheap and easy and worth a try . Peter

 

[Frey]

Thanks for that, Peter.

Cheers

 

[john zibell]

Also check the float levels are correct and that fuel flows freely into the carbs. There are screens on the petcocks and at the inlet of the carbs.

 

[scottmastrocinque]

In an otherwise mechanically competent engine, only 2 things prevent proper running, fuel and ignition.

As you have double checked ignition, I am thinking fueling.

It seems to behave as if your fueling line gradually gets choked, then intermittently clears then again chokes and this cycle repeats. From your description, this always appears in what would be termed as the “main” fuel circuit and never the “idle” or “slow” circuit.

We’re the carburetors fully disassembled and cleaned and rebuilt?

John has a very good point. Many carbs have very tiny filters either covering the fuel needle / inlet and many have tiny filters that fit inside the fuel inlet stem where the fuel line connects.

If you still have the original Delloeto VHB30, there is a screen behind the fuel spigot in the carburetor.







Also, the petcock in the tank has a filter on it as well. If there is debris in the tank, it could be getting drawn against this and restricting fuel flow.




It’s behaving as if debris is building up and blocking fuel flow but upon rest, the sediment settles and the cycle repeats.

Another cause of your symptoms is the fuel float pivot pin. If it has become corroded and is no longer perfectly smooth, the floats can hang up and not move smoothly any longer.

Peter’s vacuum idea is also very valid and worth checking.

I’m fairly sure you are experiencing a fueling issue.

Good luck.

 

[Vagrant]

All the above is where I would send you also. But, seeing as it got worse, but the same kind of worse with new plugs did you replace the ignition wires and plug boots? A bad resistor plug or cap could be it too.

 

[Amboman]

Don't rule out the condensers - I've had a few that were faulty right out of the box.

 

[Frey]

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:

• 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)
  • I drained a few gallons before my attempt at a ride this morning; no issues with flow through petcocks

 

[Frey]

Don't rule out the condensers - I've had a few that were faulty right out of the box

HMB Guzzi's web site decries the quality of condensers:

"As we got a bit sick about capacitors not working, we changed the supplier. But we soon had to find out, that this was the same bad quality as we had before. So we switched over to ordering original Moto Guzzi capacitors. But even there, quality was miserable, capacitors not working or packing up after a short period. "

They have an in-house designed capacitor module that replaces the capacitors Anybody have experience with them?

 

[john zibell]

HMB Guzzi's web site decries the quality of condensers:

"As we got a bit sick about capacitors not working, we changed the supplier. But we soon had to find out, that this was the same bad quality as we had before. So we switched over to ordering original Moto Guzzi capacitors. But even there, quality was miserable, capacitors not working or packing up after a short period. "

They have an in-house designed capacitor module that replaces the capacitors ). Anybody have experience with them?

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[Tonerjockey]

Hey Frey,

Where is Happy Valley?

~Alex

 

[scottmastrocinque]

Maybe Happy Valley, CA just outside of Redding?

 

[Frey]

Happy Valley contains Penn State and surrounds.

 

[Tonerjockey]

oopps... there's a happy valley area around Santa Cruz Ca... about 30 minutes from me. I was thinkin I could drop by and see if yer G5 had a similar set up as mine.

Sorry about the noise.

Alex