Runtong jets and upgrade

[PsychDoc]

I’m breaking in my engine and getting ready to change the jet for a proper tune- the engine is the standard 66/80 cc China girl- (specifically ‘flying horse’ epa approved with “upgraded high performance profile carburetor” ) so my questions/thoughts...
It starts and runs easily- 4strokes quite a bit but that has been improving somewhat. I’m told the ‘stock’ jet is likely #70 and will be too rich. Ok- so I need a jet. The bigger issue I am considering is a modification of the carb. The choke plate obstructs the ¿throat (is that the throat) blue arrow. That should be easy enough to trim down. Red arrow- this is a vent to the fuel bowl yes? So don’t mess with or obstruct that, right? And the big opening -yellow arrow, what’s the point of that?
So what I’m thinking is the edge of the throat is a sharp (like cut your fingers sharp) edge. Air can’t possibly flow around that smoothly. I’m thinking either round the edge as much as possible with file, sand paper, grinder, chainsaw, whatever. Or build a small “velocity stack” at the opening to smooth the flow. The carb is a runtong 14mm Dellorto copy I believe. Thoughts?
Also included air filter pic if that matters.
And if any of you have a spare jet laying around feel free to pm me and I’ll give you my address to mail it out

 

[Mcdonns]

I’m breaking in my engine and getting ready to change the jet for a proper tune- the engine is the standard 66/80 cc China girl- (specifically ‘flying horse’ epa approved with “upgraded high performance profile carburetor” ) so my questions/thoughts...
It starts and runs easily- 4strokes quite a bit but that has been improving somewhat. I’m told the ‘stock’ jet is likely #70 and will be too rich. Ok- so I need a jet. The bigger issue I am considering is a modification of the carb. The choke plate obstructs the ¿throat (is that the throat) blue arrow. That should be easy enough to trim down. Red arrow- this is a vent to the fuel bowl yes? So don’t mess with or obstruct that, right? And the big opening -yellow arrow, what’s the point of that?
So what I’m thinking is the edge of the throat is a sharp (like cut your fingers sharp) edge. Air can’t possibly flow around that smoothly. I’m thinking either round the edge as much as possible with file, sand paper, grinder, chainsaw, whatever. Or build a small “velocity stack” at the opening to smooth the flow. The carb is a runtong 14mm Dellorto copy I believe. Thoughts?
Also included air filter pic if that matters.
And if any of you have a spare jet laying around feel free to pm me and I’ll give you my address to mail it out
View attachment 89064
View attachment 89063

Well, Doc..:

That big open hole (the yellow arrow) really is only a casting cavity, and does nothing. It has no impact on airflow, and actually provides visible access to the throttle gate and spring. There’s nothing to do, there.

The small hole below the throat (the red arrow) is a vent hole, so you’re absolutely correct in leaving that be.

The throat area is actually not requiring any mods, unless absolutely everything else from filter to exhaust tip is matched/ported/timed perfectly and you intent to gain a second or two in a five-minute flat-out run. The intake environment of a two-stroke is violent and bi-directionally pulsing. Velocity stacks do absolutely nothing, [again] unless everything from end-to-end is perfectly addressed. The choke-plate isn’t really an occlusion or creating any issue, unless you just feel the compulsion to put the file to it.

You’re absolutely correct, in that the greatest thing you can do is jet it. Even then, anytime you get into modifying anything in the end-to-end airflow of the complete system, there’s still a good chance you’ll be readdressing the jet.

I’m pro’ly gonna get corrected on some of that, but I personally believe you’re good with what you have, until you have the jetting sorted out, then you can spend a couple hundred hours and several more hundred dollars on treating the asthma these li’l buggars are prone to.

 

[gary55]

Not much to add to that, but maybe.
2 Stroke Engine Products - Sick Bike Parts

 

[gary55]

If you want to throw some money at the carb system without changing your intake you can go to Dellorto Direct and buy a real Dellorto SHA 16/16 for close to a 100 bill. Anything larger than a 16/16 is not going to help as the intakes are around 16mm id.

 

[PsychDoc]

Well that settles it- that carb is looking better by the minute.

Mcdonns did you settle on a jet for your bike?

 

[Mcdonns]

Well that settles it- that carb is looking better by the minute.

Mcdonns did you settle on a jet for your bike?

Yessir, I have. Jus’ t’day, I put in a #65 (and, yes, the factory jet was larger, but unmarked — did look like at least a #70). The throttle pop is VERY responsive, quick return to idle, and (as yet) still a little heavy in the higher RPMs. That may clear up a bit more, with run hours. Your elevation and atmosphere are pretty similar, so you may be in that same range. My jets came in #05 increments, so I didn’t go to #60, but I suspect a #64-#63 would be the sweet spot. With Texas August comin’ up, though, I’m gonna leave it rather than riskin’ bein’ too lean. Gary threw ye a great link t’ domestic jets, and you may be in a better position to test the #64/#63. Curious what your result is.

 

[PsychDoc]

Sweet! I just had a brief ride and it 4 strokes consistently- the only smooth running is under heavy load- like a steep hill, or mid throttle if I’m opening it up but after it gets close to top speed it’s baxk to stroking.
The idea of ordering 5 - 10 jets and having 4-9 “spares” feels wrong to me. Not that I don’t have a shed full of that kind of thing. I know there are dozens of members here with several mid #60’s jets gathering dust.
And for no particular reason- I tested how long it takes to run my float bowl empty- 1/4 mile.

 

[Freddy Snottgrass]

You can get a speed NT carb for around 15 bucks on Amazon. I have two deloroto clone carbs and have yet jet them properly. Went back to the stock NT and love it.

 

[GreasyChris]

With these clone carbs I find that adding an extra hole in the emulsion tube makes them perform better. I've put a yellow arrow where I added my extra hole. This helps the fuel charge atomize more.

 

[Deleted member 27811]

If you want to throw some money at the carb system without changing your intake you can go to Dellorto Direct and buy a real Dellorto SHA 16/16 for close to a 100 bill. Anything larger than a 16/16 is not going to help as the intakes are around 16mm id.

Zakly watt I did. Went from a Runrong to real Dellorto from Dennycycle for 60$. BIG difference. 16-16 w/ 68 jet.