Help Needed - Sputtering and Backfiring(2016)

Bought a 75 early this year with Amal Premiers. Bike has been running flawlessly. This morning is started up as usual on the second kick and I took off only to learn that 3200 rpm was my new limit - then it just sputters and backfires instead of accelerating. Bike still idles fine. Fuel is flowing from both carbs, and the tank is completely full, just filled it the other day. If this was a carb problem I would have expected to see some symptoms before this, but haven't had a hiccup in over a 1000 miles. Bad load of gas I suppose could be a possibility - filled up at the same station I always do. Before I start tearing apart the carbs - any thoughts on what else could be the cause?

Things to look at:
1. Remove carburetor float bowls and determine if the jet holders have unscrewed from the body.
2. If it has electronic ignition, examine the wires inside the points cavity looking for a wire broken inside the insulation.
Inspect to see if the ignition rotor has slipped due to being inadequately tightened.

Since pulling the bowels is not much fun you can get a skinny socket or an amal socket tool
up in through the drain hole and check the jet for tight. If the holder is loose you can tighten
the jet and holder together. Do be careful though, dont strip it.
Ive had two loose jets on Premiers in the past two years. Hard to believe but there you are.

Also check that the choke lever can fully retract (raise) the choke air slide. If the choke is on, you won't get very far.

spartanman said:

Bought a 75 early this year with Amal Premiers. Bike has been running flawlessly. This morning is started up as usual on the second kick and I took off only to learn that 3200 rpm was my new limit - then it just sputters and backfires instead of accelerating. Bike still idles fine. Fuel is flowing from both carbs, and the tank is completely full, just filled it the other day. If this was a carb problem I would have expected to see some symptoms before this, but haven't had a hiccup in over a 1000 miles. Bad load of gas I suppose could be a possibility - filled up at the same station I always do. Before I start tearing apart the carbs - any thoughts on what else could be the cause?

Click to expand...

3200 in neutral and in all all gears? 3200 in neutral and in all all gears sounds like a resonant frequency related electrical problem. 3200 in first gear would be a much lower throttle opening than 4th gear and would make carburetion unlikely.

I had a similar problem many years ago. One of the main jets had partially unscrewed.

There's a good chance it's a coil breaking down.

Dave

I thought I would attach my query to this thread instead of create new one...

Quick question to the experts: Would the perished rubber between air filter and carb
intake be a cause for stuttering whenever I accelerate, i.e. too much air to fuel?

Many thanks in advance for your replies...

SwissCommando said:

I thought I would attach my query to this thread instead of create new one...

Quick question to the experts: Would the perished rubber between air filter and carb
intake be a cause for stuttering whenever I accelerate, i.e. too much air to fuel?

Many thanks in advance for your replies...


View attachment 98422

Click to expand...

That would make it lean/weak mixture.
And yes.

spartanman said:

Bought a 75 early this year with Amal Premiers. Bike has been running flawlessly. This morning is started up as usual on the second kick and I took off only to learn that 3200 rpm was my new limit - then it just sputters and backfires instead of accelerating. Bike still idles fine. Fuel is flowing from both carbs, and the tank is completely full, just filled it the other day. If this was a carb problem I would have expected to see some symptoms before this, but haven't had a hiccup in over a 1000 miles. Bad load of gas I suppose could be a possibility - filled up at the same station I always do. Before I start tearing apart the carbs - any thoughts on what else could be the cause?

Click to expand...

OP, any news? What is your skillset? Meaning, at what level do we begin this virtual help?

concours said:

OP, any news? What is your skillset? Meaning, at what level do we begin this virtual help?

Click to expand...

This is a thread from 2016.

L.A.B. said:

This is a thread from 2016.

Click to expand...

Lol, oops. The latest one got me.

SwissCommando said:

Would the perished rubber between air filter and carb
intake be a cause for stuttering whenever I accelerate, i.e. too much air to fuel?

Click to expand...

Yes, and if that is perished also check the rubber balance tube that runs between the 2 manifolds over the top as that also perishes and gives the same symptoms.

Part 37

concours said:

That would make it lean/weak mixture.
And yes.

Click to expand...

Even being on the inlet side to the carbs? I could see that on the inlet to the manifolds. Wouldn't that also mean it would not run right without the air cleaner connected?

Chances are if it's old enough for the rubber to be perished it's old enough for the carbs to be knackered too (or at least warrant some sort of maintenance)......
Possibly!!

I don't think air leaks at the filter rubber will be significant enough to affect engine operation much. I've run quick road tests without filter on bike and things were ok.

Btw i've found those rubbers to be surprisingly resistant to damage while manhandling them into place. I have two sets, suspect originals, which look like new.

Cuppy said:

Even being on the inlet side to the carbs? I could see that on the inlet to the manifolds. Wouldn't that also mean it would not run right without the air cleaner connected?

Click to expand...

Affirmative. Upstream restriction affects mixture, as every carburetor relies on pressure differential. "Could" being the key word. Is it THE problem? Tell us more..

As most road bikes are probably jetted a bit too rich, letting in a bit more air could make it run better. I would be looking for something which might be obstructing the main jets - such as water.

Thanks for all of your interesting replies.

The Amals are not the originals, which were knackered, as @EstuaryBoy suggests. These ones were installed new in 2013 and the bike has not been ridden much since until this year, I have no reason to believe that the carbs are kaput but have committed to dismantling them to check, including as @acotrel suggests, for water ingress, which may come from the cracked rubber, although I avoid driving in rain and don't leave the bike standing in it, unless it were condensation.

The balance hose, part 37 in @kommando's post, downwind of the carbs is sound.

It occurs to me that I simply must go through a process of elimination: inspect the guts of the carbs, replace the rubbers, and see how it goes.

There is another potential reason why the motor stutters on acceleration but, to me, this is way out on a limb because before, under the same conditions, it didn't stutter:

I live at 900 metres (2952 ft) ASL and often drive above that, some mountain passes being up to 2500 metres (8202 ft) ASL. However, I drive equally as much below my home altitude of 900 metres. For info, the main jet needle is set to 'normal'.

Amal writes the following:


Perhaps that says it all and, perhaps, I should just try out different jet sizes. Does this have anything to do with the main jet needle, or is this about changing the jet? I am getting out of my league here.

SwissCommando said:

Perhaps that says it all and, perhaps, I should just try out different jet sizes. Does this have anything to do with the main jet needle, or is this about changing the jet? I am getting out of my league here.

Click to expand...

Before you do anything like that I suggest that you replace the connector hoses first.