I think I've got a vacuum leak!
- Thread starter Motorbike Wanabe
- Start date
arceeguy
Active Member
If it idles with the choke off, then I doubt you have an air leak.
An air leak is constant whether you are idle or full throttle. If you have a leak large enough to upset the mixture at full throttle, it will certainly be large enough to wreak havok with the idle mixture.
Here are some random thoughts:
If you have another carb that works well, you might want to switch them and see what happens.
A quick check of float level would be to push the "primer" button while riding. The button doesn't actually pump fuel - it pushes the float down so that the fuel bowl can fill up more than normal. Raising the fuel level in the bowl will enrich the mixture. So if you push the primer for a few seconds, the engine may perk up for a little while until the fuel level drops to "normal". I've seen a few carbs that had misadjusted floats, causing flooding or starving.
An air leak is constant whether you are idle or full throttle. If you have a leak large enough to upset the mixture at full throttle, it will certainly be large enough to wreak havok with the idle mixture.
Here are some random thoughts:
If you have another carb that works well, you might want to switch them and see what happens.
A quick check of float level would be to push the "primer" button while riding. The button doesn't actually pump fuel - it pushes the float down so that the fuel bowl can fill up more than normal. Raising the fuel level in the bowl will enrich the mixture. So if you push the primer for a few seconds, the engine may perk up for a little while until the fuel level drops to "normal". I've seen a few carbs that had misadjusted floats, causing flooding or starving.
Motorbike Wanabe
Member
I'll have to try that with the primer. I hadn't thought of that. So, in this case, if it improves, I should assume that the float is set too low?
Catfishjohn
New Member
Are the air leaks at the cap and betreen the carb and intake pipe sealed now that you installed the o-rings?
Motorbike Wanabe
Member
I think that the cap on the top is sealed much better because of the difference that it made. I want to take the carb back off of the intake and check to see if the end of the intake tube is cut on too much of an angle. There's just a little bit of area inside the carb that doesn't have a slot cut into the edge of the carb bore and I want to be sure that the intake tube goes all the way in, past those slots, for the entire diameter of the bore. If it doesn't, putting an o-ring in there is just going to make it worse, IMO. I'll be playing around with it a little this AM, before I "go to work".
AussieSteve
Active Member
Hello Motorbike Wanabe, I had the same problems. I used a fibre washer intended for an auto sump plug inside the carb top cap and then bought an alloy inlet tube and cut a groove for an 'O' ring.
See:- NT carby/inlet leaks - a reliable solution
Also check the inlet gasket and double-check tightness of the inlet flange nuts.
N.B. Before fitting the 'O' ring to the inlet tube, I used to use Permatex non-hardening 'Make A Gasket', smeared lightly on the sealing area of the carb.
... Steve
Motorbike Wanabe
Member
I've been doing some playing around with this engine and I've about run out of ideas. I put some permatex on the inside of the carb and slid it on. I'm not sure if it helped or not but, it did fill in all the slots on the carb flange. I figured that should help some, if not all of the problems. I also put some teflon tape on the cap of the carb, along with the o-ring. I would think that would cure any leaks for sure.
I've changed the needle setting numerous times, trying to get it to run better. I've got it on the 3rd from the top right now and if I go down one more, it starts to really dog out when accelerating. But, as it is right now, when it warms up, I hear, what I believe to be pre-ignition, or detonation when climbing a hill or accellerating. That's not good either. If I run with the clutch on just a little bit, the pinging goes away. That leads me to believe that it's still running lean at that setting. Idles fine without the choke on once it warms up. I even changed tanks to see if maybe the other one was creating a vacuum. Nope. I'm thinking of contacting Gas Bike, where we bought the kits and seeing what they have to say about it. Any other suggestions? I'm open.
I've changed the needle setting numerous times, trying to get it to run better. I've got it on the 3rd from the top right now and if I go down one more, it starts to really dog out when accelerating. But, as it is right now, when it warms up, I hear, what I believe to be pre-ignition, or detonation when climbing a hill or accellerating. That's not good either. If I run with the clutch on just a little bit, the pinging goes away. That leads me to believe that it's still running lean at that setting. Idles fine without the choke on once it warms up. I even changed tanks to see if maybe the other one was creating a vacuum. Nope. I'm thinking of contacting Gas Bike, where we bought the kits and seeing what they have to say about it. Any other suggestions? I'm open.
AussieSteve
Active Member
This sounds like a rich mix. Possibly you've gone from one extreme to the other. You say it dogs out when accelerating. Is that with the throttle wide open, or 1/2 to 3/4 open?
Does the (four-stroking?) stop if you fully open the throttle, (WOT)?
Have you tried going the other way with the slide needle? Mine used to bog down badly after I sealed all of the carb leaks and now runs best with the needle at it's lowest, (leanest) setting. (Clip in top notch)
I also went down in main jet size, to 0.72mm. (My stock jet was 0.79mm but this varies from vendor to vendor.)
This sounds like you might be describing 'four-stroking' rather than pre-ignition. Another sign of a rich air/fuel mix.
There's more info and a link to a YouTube video of four-stroking in this thread:-
NT Carb Tuning Basics
... Steve
Last edited:
Motorbike Wanabe
Member
If I lower the needle (raise the clip) it starts to ping pretty bad. That would lead me to believe that leaning it out makes the condition worse. When I lowered the clip, it didn't ping as much and when I closed the choke a little bit, it went away. I thought that I could put the clip on the lowest setting and that should help but, it didn't. It just bogged down when I tried to go up a hill. Now, I'm thinking that I might be able to lower the clip and change the jet sizing to a smaller jet. That might help with the bogging when going up hill at full throttle. I would assume that the needle, while it mostly works from idle to 70% throttle, would still have some bearing on the overall A/F ratio, especially if the A/F was rich to begin with on the jet sizing. I just put some teflon tape on the carb/intake connection. I haven't had a chance to see how that does but I might tonight or tomorrow.
AussieSteve
Active Member
Are you sure it's pinging? That's not likely.
This indicates that it's too rich. If it bogs more when you lower the clip, (raise the needle), then it's probably not lean in the mid-throttle openings.
Have you tried a spray of carb cleaner near suspected leaks yet?
Have you done plug chops? Both at ½ and full throttle would be good.
And did you check out the 4-stroking video, to be sure that what you think is pinging isn't 4-stroking?
... Steve
This indicates that it's too rich. If it bogs more when you lower the clip, (raise the needle), then it's probably not lean in the mid-throttle openings.
Have you tried a spray of carb cleaner near suspected leaks yet?
Have you done plug chops? Both at ½ and full throttle would be good.
And did you check out the 4-stroking video, to be sure that what you think is pinging isn't 4-stroking?
... Steve
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