Running rough, just off idle

[2TFool]

So I've just about finished this RD 350 project which I've spoken about here a couple of times and just put some pictures in the project section.
It starts up first kick and idles nicely but at a small throttle opening as you pull away, the Engine sounds to my non-expert ear, lean, As you open the throttle, there is a definite switch to running properly (?)
This is all happening with the air screws one and a half turns out. If I increase that to 2 1/2 turns out it improves it slightly, but that transition as the throttle is opened, isn't as smooth.
The carburetors which are 34mm Mikunis have,
300 main jets
35 pilot jets
The needle is in the second groove from the top.
I have a large selection of main jets, but only 35 and 32.5 pilots.
I'm tempted as a first measure to put the carburetors in the ultrasonic cleaner as the circuits maybe gummed up but the fact it starts so well and idles makes me think that's not it.
Suggestions?

[m in sc]

what about at 1 turn out? or less? up to 1/8 throttle its basically all pilot jet and airscrew

[2TFool]

I'll try one turn out or even less. It's always been in my head that you started one and a half turns out and anything less would be too lean.
I'll try 93 octane too, if it knocks will I hear it above all the other noise? Is there some other way to check for pre ignition?

[automan]

#1 Pre-ignition events do not last long, 10 or so then a hole in a piston.

Not to be confused with simple detonation.

Needle 2nd from top? why, the needle is wrong?
300 mains? Rich, someday lighten up on that.

cut-away is?
needle jet is?
jet needle is?

[m in sc]

if the timing is conservative, and it is, thats ok. squish and compression make a big difference on detonation issues. they are also 2 different things.

fwiw i ran 330 mains on my rd w vm34s. im thinking the jet tube was a p4 but if have to look. i swapped em out years ago but syill have the 34s set up in a box as they came off.


remember, air screw out = leaner.

[1976RD400C]

As a reference, my 34mm carbs on a 400 runs good with 280-310 mains, 6F9 needles, P-O or P-2 needle jets (adjust needle clip accordingly) 35 pilot (now with UFO inserts 25 pilot)

[teazer]

There is a lot of conflicting information on the internet about pre-ignition and detonation.

Preignition simply means that a hot spot of some sort causes the ignition to happen before the spark is timed to fire.  That results in pressure rising earlier in the cycle than it should and can lead to a rise in operating temperature with potentially damaging results, but it is still a normal combustion event and the flame speed is more or less the same as a normal burn. Flame propagation is around 20-50 M/sec

Normal combustion creates peak pressure around degrees or so ATDC.  With early ignition that could potentially be before TDC as the piston is still rising.  That can create excessive pressure and heat and can lead to detonation, but it's the detonation that cause the damage.

Detonation by comparison, is exactly that.  The mixture detonates (explodes) and the speed of flame propagation is extremely fast and creates a sonic pressure wave which moves at somewhere around 1,500 M/S. It is that sonic boom that is heard and which causes the damage. The combination of excessive heat and the sonic wave combine to turn pistons into dust.

But back to jetting.  "It sounds lean".  Determine whether it is lean by doing what M in SC suggested.  Try a smaller and smaller air screw setting and see if it gets better or worse. The next thing you mention is that is gets better when the throttle is opened and that's usually where the side and then the needle start to have more of an effect. So it sounds like that may be OK.

Try adjusting the air screw and then let's think about the results of those tests.
 

[JKV45]

As noted - close the air screw.  Richer idle mixtures tend to blend better as you start to open the throttle.

To me, it sounds like you could go richer on the needle (moving the clip down) if richening the idle mixture doesn't cure the issue.

Are you using ethanol-free gas?  Gas with ethanol will make your settings leaner.

Higher octane isn't going to help, and may make it run worse, unless you need it to control detonation.

[automan]

pre-ignition is not normal. it is the #1 killa of 2 strokes.

detonation due to timing or c.r. is rectified by adjustment of such.


pre-ign leads to deto as much as 60° btdc. your 50m/s flame speed is half correct.

[2TFool]

Ok, i'll try that today.
And for a point of reference, I'll find out what needles and needle jets I have. Is the needle marked in someway or measured?
Thanks for all your replies, I really appreciate it.

[2TFool]

I tried screwing the idle screws in, it bogged the motor down and no improvement.
Not sure how to measure what needle I have or needle jet or slide cut out?
I included a picture of the carburetor, main jet removed, next piece of brass in is slotted and engages on a pin to prevent rotating, I'm going to guess that is access to the needle jet, but not sure how it removes.   
Also a picture of the spark plugs. That's 10 minutes, running up and down the road at idle throttle setting adjusting the air screw.
There's about 10 miles in total on those plugs. They're very oily.
Running 36 to 1 premix.
The reason for the 100 octane Ave gas is 155 pounds per square inch pressure in the cylinders and lots of porting and squish band. M in sc says use 93 and as I gain courage and confidence, I might well do that but for now, I'm not too confident.

[85RZwade]

The slotted brass dude is your needle jet (emulsion tube) and pushes out the top. Screw the main jet in and push or gently tap on it to push the needle jet out. The needles, slides and needle jets should be marked just like main and pilot jets, with teeny tiny numbers.

[JKV45]

When you open the throttle from idle, does it bog/flatten-out or sputter/stumble?

For starters with the idle mixture screws, setting them to get the highest RPM is a good place to begin.  Usually going slightly richer helps the transition.

[m in sc]

my lc runs 155 ish zero issues premix 32 1.

thats super rich. drop the needle a clip and go to a 32.5 pilot at 2 turns out

[2TFool]

I'll make some adjustments and report back. Thanks guys.