1970 S: suddendly a hole to 2500 rpm

The bike (new Pazon Sure Fire) rode great, idle to 1000 rpm, no holes of carburation.
Today a hole to 2500/2700 but rode greta under and up this hole.
What i must to check?
One information: the bike was first time today with the petrol tank with tankerite into (but used 10 days after the work).
Thank you.
Ciao
Piero

This would usually indicate blocked up idle passages in the carbs, Piero.

So it runs on the needle jets, with nothing from the idle circuit.
Can be quite tricky to get those little passages clean, may take several attempts.

You should probably keep an eye out for particles of rust or dirt (or tankerite) in the fuel too,
in case any missed the tank flushing and cleanout.

Most engines will not start or run at all if the idle circuit is plugged so I have to disagree with the previous comments.

The RPM that you are describing a flat spot is the transition between the idle circuit and mains.

I would ask what size carbs, needle and jet size and needle position. Could just be needle position that is not allowing fuel soon enough or main is too small.

dennisgb said:

Most engines will not start or run at all if the idle circuit is plugged so I have to disagree with the previous comments.

The RPM that you are describing a flat spot is the transition between the idle circuit and mains.

I would ask what size carbs, needle and jet size and needle position. Could just be needle position that is not allowing fuel soon enough or main is too small.

Click to expand...

Hi.
930 Amal, 220, 06, central needle position.
But the bike rode great before, never ad the hole.
Ciao
Piero

try new spark plugs.

pierodn said:

dennisgb said:

Most engines will not start or run at all if the idle circuit is plugged so I have to disagree with the previous comments.

The RPM that you are describing a flat spot is the transition between the idle circuit and mains.

I would ask what size carbs, needle and jet size and needle position. Could just be needle position that is not allowing fuel soon enough or main is too small.

Click to expand...

Hi.
930 Amal, 220, 06, central needle position.
But the bike rode great before, never ad the hole.
Ciao
Piero

Click to expand...

Then Rohan could be right. You got some dirt in there.

You need to determine if the hole is too rich or too lean. If its too lean it may cough, spit, bog, or die. If too rich it stumbles and misfires like 8 stroking.
Jaydee

jaydee75 said:

You need to determine if the hole is too rich or too lean. If its too lean it may cough, spit, bog, or die. If too rich it stumbles and misfires like 8 stroking.
Jaydee

Click to expand...

Yes,
I mean.
But remember that the bike (before the tank tretment) rode great without holes.
Ciao
Piero

But knowing if the hole is a lean or rich gives a clue as to why it has a hole.

Currently all we know is you have a hole.

kommando said:

But knowing if the hole is a lean or rich gives a clue as to why it has a hole.

Currently all we know is you have a hole.

Click to expand...

Hi.
Lean.
Thanks.
Piero

Blocked idle jet(s).
You are starting and running it on the needle jets.

Check for dirt or debris in the carb, and make sure all the little fuel and air passages are clean.
Just having fuel evaporate from no use can leave gummy deposits, in places hard to reach....

dennisgb said:

Most engines will not start or run at all if the idle circuit is plugged so I have to disagree with the previous comments.

The RPM that you are describing a flat spot is the transition between the idle circuit and mains.

I would ask what size carbs, needle and jet size and needle position. Could just be needle position that is not allowing fuel soon enough or main is too small.

Click to expand...

The transition between the idle circuit and the mains, is not sequential. The idle circuit transitions to the needle which gradually transitions to the needle jet which transitions to the main jet. All stages of carburation are, essentially, additive. A plugged idle jet will have repercussions up the line to a varying degree and at WOT may not be felt, but may show up, in a plug chop, as a bit lean.

What I think most of the responders have missed is that the problem didn't occur until after the tank was treated. This is an important time to install fuel filters, few people do this for reasons that escape me, it is very common with the BMW crowd and I sell plenty of ide jets (and fuel filters) to such owners.

Your carbs are dirty. Given how the idle jets are, essentially, cast into the body of the carb, you can relatively easily run a .079 drill through the air side, but you'll have to work a solvent and compressed air through the fuel side if you hope to achieve that wonderful cadence you had before the tank service, but do the compressed air in reverse, that is from the business end back to the float bowel or you may make the situation worse. Add fuel filters, enjoy your ride, and move on to worry about something else... Or purchase a Mikuni conversion and have total access to all your jets and fuel passages.

RPC