Dellorto Runtong knock-Off Problem

Built a gas bike on a Skyhawk frame, GasBike.net supplied 66/80 slant with hi-comp head. Trial run was promising but not quite as good (power wise) as a bikemotorkit.com 66/80 straight head engine I put in an old Specialized mountain bike frame.

OK, so I recognize there is variation, significant variation, between motors from China. I can live with that.

Enter the Runtong Delorto knock-off carb that I thought (and read) would be an improvement/upgrade from the original NT carb supplied. Not so in my case. The bike now exhibits about half the power compared to the originally supplied NT carb. The Runtong appears to be a nicely made copy of the Delorto SHA carb but that's about it. Where is the claimed increase using such a carb? It darn sure isn't there from my experience. I checked to ensure fuel flow was satisfactory and there are no plugged fuel or air jets.

I was expecting it to run at least as good as the NT carb, maybe a bit better in terms of overall power. Sadly that isn't so.

Any thoughts before I junk the Runtong Delorto knock-off and go back to the original NT carb?

I suspect the Runtong may be running the engine a bit lean. Sadly when I disassembled the Runtong to check jet size there are no size indications on the jet I.e., who knows what actual size it is. Then there seems to be the problem of, even if I could identify the jet size, where can I find additional various sized jets?

Any help is appreciated.

Don

Slogger,

Thanks for the response. It gets curiouser and curiouser. I bought the Runtong carb from an Amazon supplier for $25 free shipping. It arrived yesterday and I put it on and took it for a test run, the results of which I posted when I started this topic. At the time I ordered the carb I decided to also place an order for another Delorto SHA knockoff with JNM Motors for their version they call their performance carb. Their claim was one could buy their carb, slap it on and ride. Even though it was twice the price of the First Runtong I bought I thought I'd give it a try too.

The JNM carb arrived today. It is not a Delorto (not that they make any claim that it is) but it is a wee bit different than the Runtong carb. The float bowl is metal rather than the plastic of the Runtong carb. It also has a screw at the very bottom of the float bowl, which I presume is for draining the bowl without having to remove the carb from the bike. I attempted to take the float bowl off to see if the jet was numbered for size but after removing the float bowl screws the bowl did not want to come off, it was almost as if the bowl gasket was glued in place. Not wanting to risk breaking something I chose to put the bowl screws back in and tighten them down.

So I install the JNM carb and go for a test run. The difference between the JNM and the previous Runtong is like night and day. The JNM carb fires right up and sends me on my way. I am back to the same top speed I had with the original NT carb but with a bit better low end power, not a lot but enough so that it was noticible to me.

Is the JNM carb change out of the original NT carb worth $50? Not really sure yet but if my NT carb turned out to be like many I have heard of, a leaker with other down the road problems, then perhaps so.

Now I need to figure out why the first Runtong carb I received doesn't perform as well as the JNM version?

Slogger said:

Dellorto 5mm jets from SBP will work.
I have the same carb, on my stock engine I had to lean it out at low speeds to keep from going too rich at full throttle. I never did get it all tuned out. It had a better idle and was easier to start (the first time in the morning) than the NT, but that's about it.
I put the NT back on.
Not giving up on the sha clone, just taking a break.

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Slogger said:

I would guess that the runtong carb is jetted pretty rich. Mine was. From comparing the jets with a bunch I have collected, I'd guess it was about a 72.
The JN carb was probably pre-jetted leaner. The runtong might respond well to some tuning.
I'd keep it for a spare.

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Do you remember where you got your jets for the Runtong? They appear to be 5mm thread but I am not sure of the thread pitch. I have some jets coming from JNM. Hopefully they will work in the Runtong carb

I may end up as the king of Runtong carbs... for what that is worth.

I now have four of them with an original Delorto SHA on the way to me also. I plan to dig deep into them and find what makes them tick. I have three additional gas bikes builds in the works with various China 2 stroke engines. All will get Runtong carbs, except the one that gets the genuine Delorto.

If the Runtong carbs run too rich I will solder them and redrill the jets. I like the carb but need to get to the bottom of their tuning.

Slogger said:

I may be mistaken, but I think the jets (5mm dellorto) should work in most of the sha clones.
The thread pitch isn't perfect, they have to be grunted in.

You can also solder up the original and drill it smaller to adjust your jetting.
There are all sorts of threads on re-jetting that cover how to do it.

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I ordered a jet kit from JNM and am hoping it is of the correct jets for what they call their, "Performace Carburetor." I also ordered a numbered drill bit set and a Starret twist drill. I will, as a last resort, solder the jet and redrill it if I can't find something that works with the Runtong carb.

Digging knee deep into the Big Muddy I.e., I am going to do this and figure out this carb thing for these goofy little motors. I also ordered a Walbro carb from JNM. Not that I plan to try all the various carbs out on one motor bike, I have three other motor bikes in various stages of build (for a total of four). They are quite different even though they share one thing in common. All have some variation of the Chinese 66cc two stroke motor.

Next up, I have some spare parts coming, specifically cylinders and heads that I plan to experiment porting without their consent. It will be back to square one and carb tuning.

Checked the jet size of the Runtong SHA and it is #67 by my twist drills. I also received the five jet set from JNM and their #74 is not #74 per my twist drill set. It is bigger. Now I don't know who to trust. That sucks.

I am starting on build #4 on a Skyhawk frame with a GT5 66cc engine. I think I will start with the leaned out Runtong on that build and see where it goes.