2 stroke puttering below certain revs?

[AP3]

Hi Again Everyone!

I have another quick question for you guys. Is it normal for a 2 stroke to putter at lower revs, and then smooth out at higher revs?

An example of what I mean here: 

 

Between 2-3 seconds you can hear it smooth out, then go back to puttering.

Cheers,

Andy

[frankS]

Andy. This one could be a little more difficult to sort out. Could be a number of things and you would need to go through a process of elimenation anything from too high  amount of oil in fuel ( 50/1 usual ), wrong or bad spark plug, faulty high tension lead, weak coil, contaminated fuel, blocked fuel line, air getting into fuel system, fuel pump gasket, sticking needle in carbs, wrong float level, rubbish in jets in carbs. Those are the easy fixes, then there's the harder fixes like cdi ignition or loose earth somewhere ( but usually a more consistent miss ), stater ( under flywheel ) loose or cracked side plate, or cracked head or gasket. And others will no doubt come up with even more things it could be. Let's hope it's one of the easy fixes.

Frank 

[jeffb5.8]

My old 2 stroke used to do this, light splutter down low but once throttle applied it would clear and run beautifully.

I put it down to being a 2 stroke it loved to rev and it never let me down, I just cleaned the plugs regularly and I dialed back the oil mix just a fraction (55:1 rather than 50:1 )and used a higher octane petrol, Still had the stutter but not as bad.

What the plugs like after a run

[zmk1962]

Andy,

Just adding to @frankS and @jeffb5.8 posts,   here's a guide to help assess what the plugs can tell you:

Just for clarity, in your video were you opening the throttle (to demonstrate the smoother operation) or was the motor surging (revving up and dropping down) on its own when puttering along?

2-strokes tend to putter  and cough more at low revs (especially around idle speed) than 4 strokes,  they are just not optimised to run at low revs. Having said that, I had a "surge" situation on my carbie 4stroke 15hp. Ran fine at high revs, but would rev up and drop revs when running below half throttle.  I initially suspected fuel delivery issue.  After replacing all fuel lines and fuel bulb to eliminate air bleed, collapsed bulb etc and testing the low pressure pump delivery, I had a look at my plugs. They looked ok but had not been changed for a while. I replaced them with new plugs, and set the gap to the factory recommendation - bingo motor purrs like new.  In hindsight I should have started there !

Cheers Zoran

[Nolongeramember]

1 hour ago, zmk1962 said:

 

 

That guide in not right. The lean one is on the right and optimal is on the left. Also, plugs are just a guide IMO. There are many instances where plug colour doesn’t tell the full story. 

Andy, the engine in the video sounds to me exactly the same the whole time.

It looks like it’s running at low revs and roughish, which is normal for an old school 2 stroke. It should run smooth and clear once you increase revs. If it doesn’t there is an issue. 

[AP3]

Thanks for the advice all!

@zmk1962 This was at a constant throttle. I tried to capture it at the transition point, when it goes from puttering to smooth. At this point it alternates between the two. It doesn't surge in revs, just putters at low revs, and runs smooth at higher revs.

@ Hateanchors It might well be that this is normal for a 2 stroke. My last motor was a 4 stroke so it's new to me. At higher revs it is indeed very smooth!

So far on this engine I've changed and gapped the plugs, pulled the carb off for a good clean, and replaced the idle jet for good measure, changed the impeller, cleaned up all the earth connection I could find (inc. the battery, which is new), changed the gear oil, set the air mix and idle speed, although I might raise the idle a little.

I haven't checked compression, as I haven't got a compression tester yet.

I'll pull the plugs and see what they look like anyway

Cheers,

Andy