73 Eldorado sputtering & surging under load

[Odradek]

Hi,
I've a 1973 Eldorado that is currently driving me crazy.
Had it laid up for the winter, and didn't put any additive in the fuel like I usually do.
That said, on my first ride out it was fine, for about 10 miles, and then it started running a bit rough.

This started a period of about 2 months where I've spent most weekends trying to get it running right, and I'm still failing.
I've installed new fuel filters, new fuel lines, had the carbs off and disassembled twice, and have cleaned the point and set th ignition timing a few times.

I was having a problem with serious backfiring, but either the last carb clean or the last timing adjustment has sorted that.
Now the bike is starting and idling fine. It will rev up like normal, but, as soon as you start to ride, the engine under load starts spluttering and surging, as if it's about to stall.

Can anyone offer a few pointers as to what I should be looking at next ?

I've even resorted to asking Google AI or help.
It says the gas cap vent maybe be blocked or the distributor advance may be seized.

[Antietam Classic Cycle]

Have you removed the petcocks from the tank and cleaned the filter screens or at least verified that fuel flows well? Possibly a failing condenser or even ignition coil. What's the history of the bike though? Has the engine or even just the top-ends been rebuilt recently? It might be best to start from the "beginning" - retorque the head nuts, and adjust the valves. Fresh plugs. How old are the plug wires and caps?

[Odradek]

Thanks.
Yes, I guess the petcocks are next to check.
Stein-Dinse, where I usually buy parts are out of stock for the next 8 weeks they say.

Heads were retorqued late last year and the bike has hardly any miles since then.
I've no info on the bike's service history before I got it, 3 years ago, apart from discovering it was restored by Mark Etheridge at Moto Guzzi Classics sometime in the 90's.
I do have a spare coil sitting on a shelf in the garage. Honestly can't remember if I changed the ping wires, though it did get a new distributor cap about 2 years ago.
It does seem to regularly need new spark plugs though, and I've never figured that one out.


EDIT :  I've ordered a new condenser and some petcock gaskets. Coming from Germany so they could take a week to arrive.

[Antietam Classic Cycle]

What country are you located in? That helps with parts recommendations.

[Odradek]

I'm in England.
There are one or two Guzzi parts specialists here, but I've found for older bikes they often tend not to have things in stock.
Have had best results ordering from Stein-Dinse in Germany, who carry a huge inventory.

[Tkelly]

I had a bike that did something like that until I rode it with the choke on,then it ran fine,I don’t remember what the fix was.

[TN Mark]

Did you empty the tank of the old fuel and fill part way with known, fresh gas? If not, why not?

What's the condition of the inside of the tank? Rusty or fairly clean. If the current petcocks and/or the filters are clogged, you've found the result of a rusty/dirty gas tank.