RT Carb

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Tyler6357

Tyler6357

Well-Known Member
Mar 15, 2012
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Santa Barbara, CA
Hello
I just got a brand new RT 15mm high
performance Carb from Dax. I plan on putting it on my 2 stroke 66/80cc, center mount, direct chain drive MB. My carb came with 5 different jets, a 60, a 65, a 70, a 75 (already installed), and an 80. My questions are: (1) which jet should I use? My current NT carb has been bogging down a bit at the higher RPMs with a very unstable idle that seems impossible to set, when idle it up it tends to race and when idled down it won't stay going, even after I installed a washer at the intake manifold. I'm hoping that this new carb will solve these problems and provide a little bit more power for the hills I ride. (2) Can I really expect much more power or torque from the RT? (3) Should I remove the washer from the NT carb and put it in the RT carb or will that not be necessary? I plan on installing it this Saturday and any tips or advice is much appreciated.
 
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2door

2door

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Sep 15, 2008
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You have an air (vacuum) leak, probably at the carburetor to intake manifold, or the manifiold to the cylinder. The symptoms you describe are consistent with excess air entering the engine downstream of the carb. If you don't seal the carb to the manifold or if the intake gasket is leaking you'll have the same problem with the RT.

The 'O' ring (washer) needs to be compressed tightly against the lip inside the carb before it will do any good. Simply sliding the carbretor against it will not assure a good air tight seal. Apply some pressure then tighten the carb clamp. The plastic bushing used in the RT does not offer a good sealing surface. Many prefer to augment with a gasoline compatible sealer such as Seal-All to get a good seal.

Is your engine broken in? You need to burn a couple of gallons or more before you start making too many adjustments or trying to tune the carburetor. Put some miles on the engine before you can expect it to perform decently. As for jet size you'll need to experiment and see which works best for you.


Tom
 
Tyler6357

Tyler6357

Well-Known Member
Mar 15, 2012
1,293
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Santa Barbara, CA
Tom:
Thanks for responding, I suspect that the leak is at the carb/intake connection. It was bogging much worse until I put the "O" ring washer on. The washer really helped reduce the bogging but didn't get rid of the problem completely and the idle problem persists. The RT carb seems to have a better connection screw which I'm hoping will seal against the intake better than the NT. The engine should be broken in now, I have run 1 gallon of dyno oil/gasoline at 20 to 1 mix, and almost another gallon of castor 927 oil/gasoline at 32 to 1 on it before this problem started happening. I guess I will just experiment with the different jets until I find the right one. I'm pretty close to sea level here. I'm rather excited to see how much better the RT performs but I'm trying not to get my hopes up too high.
 
Venice Motor Bikes

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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I concur with Tom about both the air leak & breaking in the engine more before trying to jet it.

The engine will keep running better & better the more you ride it; I would wait until you have about 4 gallons run through it before jetting it. That way you can be more sure about what jet size is best.

That 'bogging down' that you were feeling at high RPMs is most likely what we call "4-stroking" (a little too much gas). I consider that a good thing during the engine break in period because it will both keep the RPMs down a little & also give the engine a little extra lubrication during the break in. ;)

Also... The RT carb is a really good carb, but they usually have too big of a jet installed in them for our engines. Just keep changing to a smaller jet (& testing each jet for a mile or two) until it's running good but you still feel a slight '4-stroking' at those higher RPMs... This way you know the jet is on the larger size & the engine's getting plenty of gas.
 
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Tyler6357

Tyler6357

Well-Known Member
Mar 15, 2012
1,293
294
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Santa Barbara, CA
Venice Motor Bikes said:
I concur with Tom about both the air leak & breaking in the engine more before trying to jet it.

The engine will keep running better & better the more you ride it; I would wait until you have about 4 gallons run through it before jetting it. That way you can be more sure about what jet size is best.

That 'bogging down' that you were feeling at high RPMs is most likely what we call "4-stroking" (a little too much gas). I consider that a good thing during the engine break in period because it will both keep the RPMs down a little & also give the engine a little extra lubrication during the break in. ;)

Also... The RT carb is a really good carb, but they usually have too big of a jet installed in them for our engines. Just keep changing to a smaller jet (& testing each jet for a mile or two) until it's running good but you still feel a slight '4-stroking' at those higher RPMs... This way you know the jet is on the larger size & the engine's getting plenty of gas.
Click to expand...
Thanks so much for your expert advice! I had no idea I had to run 4 gallons through before the engine is broken in. After reading what you said, I'm wondering if I should go back to 20x1 dino oil. Are you saying that I should continue to run the NT carb for 2 more gallons? Also, I thought that the bogging was being caused by the air leak and not 4 stroking. I mean, it was really bogging down at all RPM levels before I put the washer in my NT. Now it just bogs a little bit at the high RPM levels. I just figured that the air leak had gotten slightly smaller. I was hoping that the bogging would stop when I installed the RT but perhaps I am wrong about that. I didn't put it on Saturday because time got short and it was just too cold and rainy.

Thanks for telling me about the jets, I didn't know which one to use, I will follow your advice and keep going smaller until it runs well. I guess I will just try the one that it came installed with (75) first and go down from there if needed.
 
Venice Motor Bikes

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
7,271
1,810
113
Los Angeles, CA.
Just install the RT carb & make sure there aren't any air leaks... (If it idles well?, that's a great sign).
Don't keep switching oils... choose one type of oil, & use only that for the rest of the engines life.
Just remember to use the largest jet that will allow it to run well until it's fully broken in; then go a little smaller for the final tuning. Always remember, if you install too small of a jet, it might seem to be running well, but it can destroy your engine from too much heat! (even after break in).
 
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Tyler6357

Tyler6357

Well-Known Member
Mar 15, 2012
1,293
294
83
Santa Barbara, CA
Venice Motor Bikes said:
Just install the RT carb & make sure there aren't any air leaks... (If it idles well?, that's a great sign).
Don't keep switching oils... choose one type of oil, & use only that for the rest of the engines life.
Just remember to use the largest jet that will allow it to run well until it's fully broken in; then go a little smaller for the final tuning. Always remember, if you install too small of a jet, it might seem to be running well, but it can destory your engine from too much heat! (even after break in).
Click to expand...
Very well, I will do that. If it's running a little rough with the 75 jet I will just leave it on for 2 more gallons and then adjust to a smaller jet. My plan is to get a SBP expansion chamber to replace stock after this. I imagine that I might have to adjust the jet at that time. Thanks again for helping a rookie like like me, :)
 
Tyler6357

Tyler6357

Well-Known Member
Mar 15, 2012
1,293
294
83
Santa Barbara, CA
Just a few more questions on this RT carb. I put it on my bike last night, I had to adjust the engine mount forward a little bit to make it fit but I got it on nice and tight now. When I released the gas valve it immediately started leaking out of the float. I finally got it sealed by adjusting the rubber gasket which appears to be a bit large on the carb I got. This morning I choked it up and it started up right away but when I push the choke back down it immediately dies. When I run it at full choke it seemed to idle pretty well (not racing like it was with the NT but maybe slightly high) but the power is definitely diminished (bogging at all RPMs). When I try to start it with the choke down it won't start at all. I don't think I have any air leaks. Do I need to jet up or down? I have the 75 in right now.

Also, I have a bottle boost which I haven't connected yet. Will I have re-jet after I connect it? And, when I put the expansion chamber on the exhaust, which I plan to do in the near future, will I have to re-jet again at that time?? When I re-jet should I just screw the jet in as tight as possible?

Thanks.
 

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Tyler6357

Tyler6357

Well-Known Member
Mar 15, 2012
1,293
294
83
Santa Barbara, CA
Venice Motor Bikes said:
The choke lever is opposite on these carbs!! 'UP' is choke 'off' while the bike is running! :)
Click to expand...
Oh, ok, sweet!! I noticed that it locks in the down position (choke on). The thing is still 4 stroking a bit. I guess I will run a few gallons like you suggested before I change the jet but when I do should I jet up or down? I have the 75 in as it came, the carb came with a 60, 65,70,75, and 80. I have a feeling 70 might be right. What do you think?
 
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