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?
what about at 1 turn out? or less? up to 1/8 throttle its basically all pilot jet and airscrew
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?
#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?
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.
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)
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.
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.
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.
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.
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.
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.
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.
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
I'll make some adjustments and report back. Thanks guys.
Quote from: teazer on August 24, 2026, 01:26:24 PMThere is a lot of conflicting information on the internet about pre-ignition and detonation.
Wait until somebody tries to tell you that you have pre-dotonation going on
Well, maybe I do?
I switched to a 32.5 pilot and dropped the needle one (now all the way down)
and experimented with the air screw.
I also adjusted the throttle slides and idle stops as best as I could. It's best at 1 3/4 out (air screw)
It starts right up and idles fine but put a load on it as you pull away (and (I'm going to try to use words) it sounds like it's four stroking (?) the exhaust tone is sharper, almost a clack sound (way too lean?)in the chamber. That scenario when pulling away on an incline. A lighter load less severe.
Perhaps the idle circuits are slightly gummed up but would it start and idle the way it does if they were)
I did blast brake cleaner through them. Perhaps I should put them in a ultrasonic cleaner?
Main jets 300
Pilot jets 32.5
Needle jet .158 P-8
Slide 2.5
Needle 6F8 (clip in top groove)
Appreciate your consideration!
you def need to start with fresh clean carbs. so def take them all apart and clean them. blow thry every hole.
then start over w clip in the middle. im guessing the pilot is ok. it really is based on throttle position. idle to 1/4 pilot. 1/4 to 3/4 needle, and wot is main. there is of course some overlap but its about throttle position. p8 is a relatively rich jet tube but not ridiculous. seems like your issue is primarily there. but a clogged jet tube or air jet will cause a rich condition
How my bike with 34mm carbs reacts to different needle jets: P4-needle must be in position 1 (full lean) P2-needle in position 2-3 P0-needle in position 4 (rich). I would try smaller than P8.
Quote from: rodneya on August 26, 2026, 02:06:48 AMQuote from: teazer on August 24, 2026, 01:26:24 PMThere is a lot of conflicting information on the internet about pre-ignition and detonation.
Wait until somebody tries to tell you that you have pre-dotonation going on
I heard that at work, many times.
(one of the Big Three)!
had this on the other day. even though its a big block ford, later in the video he goes into a good vut basic ignition timing and definition of detonation, pre ignition, ping etc. worth a watch if you aren't familiar.
https://youtube.com/watch?v=ze2CROQ34sM&is=GtXol_QzxU7rlBo7
Ok, a couple of things,
I asked AI about what's going on with my carburetion and it suggested that my slide at 2.5 was a little big and that 2.0 might be more appropriate. The idea being too much air was flowing initially and a smaller pilot jet couldn't keep up. If you didn't want to buy two new slides go up pilot jet size.
Again that's what AI said, what do you think?
I'm about to make a trip over to a bike shop and see if they have any larger pilots. If not, I have to buy them online. Can I buy Chinese? I can certainly get a large selection for a lot less money.
I see online that correct float height is 24 mm from the body of the carburetor to the top of the brass pivot that the floats engage on. Is that correct?
The afore mentioned brass pivot is held in with a pin that is peened over on one end. I've tried tapping it out to remove the float needle assembly but it doesn't want to move. Is there a trick?
stop relying on ai for this.
a 2.5 is fine. look into the needle issue clean the carb 1st. as far as the pin be careful, i tend to use a small set of needle nose and put one on the pin leg the other on the pin, pushing the pin out
agree with Mark on this- Stop using AI. If you look closely under the information presented by an AI bot it gives a disclaimer "AI can be wrong, please cross reference with other sources" WTF? Useless! If you have to verify with other sources- get a good RD manual that has all the OEM specs in it as your baseline reference.
Also, beware Chinese jet assortments on-line. I once bought a kit for my KDX220 and they came UNMARKED. No way to tell what sizes they were. Again, useless :bang:
Well, I tried a whole variety of pilot jets from 17.5 to 45 and throttle slides 2.5 and 2.0.
I tested it on an incline right outside my house and all at less than quarter throttle, the zone where it does not run well.
I think I could hear it knocking on the 17.5 and the 45 was super smoky and bogged down maybe 27.5 was the best, but none of these settings, cured it of its unhappiness at less than quarter throttle.
I don't think it's pilot jet size. I've thoroughly cleaned the carburetor and there are two orifices that are fed by the jet and they are clear.
Once I'm past quarter throttle (I calibrated the twist grip) it seems to be running fine (there may well be some fine adjustment to be made there but nothing like below quarter)
I have posted Engine and carburetor details elsewhere but as a quick recap, it's an aggressively ported 350 with 34 mm mikunis, 152lbs compression and 100 octane gasoline. It was a race bike that came to me, and I saw fit too resurrect it and put it on the road. Steep learning curve in regards to jetting and tuning and I really appreciate this forum!
sure the carbs are synched off idle really well? also, race bikes don't really perform well under partial throttle, especially old school porting.. there's a cost to all that porting. and that's it.. pretty sure you found it.
remember, to over simplify it. at lower rpms, until you really get the gases flowing, aggressive porting is going to 'bleed out' mixture to a degree. this will cause it to be sort of sloppy. I think you've done all you can.
go w the pilot jet that gives best performance at 1.5 ish turns out on the airscrew. this is about where its optimal for air speed in the pilot circuit.
I installed the UFO inserts in my 34mm carbs and WOW, they transformed them to be ultra smooth and crisp. You really notice the improvement at 1/4 throttle. I got them from Niche and they came with the slide. You can get them without the slide, but you must modify the cutout to match the insert.
I was beginning to wonder if the problem wasn't the pilot jetting but a function of porting, pipes, compression, the way it had been tuned.
The best pilot jet still doesn't make it run very well, it doesn't sound or feel right until the throttle is opened to just past quarter, probably of little concern on the racetrack.
I have ridden it "on or off" no throttle or quarter throttle. Any danger of starving the top end of oil riding it like this?
I'll look into those UFO inserts,