So I’ve been working at honing in my jetting on my carb and it’s going well for now, but I seem to be having an issue with what I believe is frothing fuel in the carburetor bowl. Once I hit WOT I steadily climb in speed to about 32-33 MPH and then it’ll hit max rpm and bog down to about 29-31 MPH. I feel like my setup has a little more juice to it to maybe get me to 35 but I believe the fuel is vibrating too much. I have really bolted down and secured the engine to eliminate as much vibration as possible and can’t do much more there. I guess I’m asking how I can maybe isolate the carb vibration or keep it from being affected by the vibration of the motor to get my maximum out of the engine? I’m still in the works for using heater hose to extend the carb manifold but struggling with the hose clamps and air leaks at the moment so holding off on that, but hoping there’s something g I can do to keep the stock carb setup from vibrating and losing speed? Thanks!
Frothing fuel and carb vibrations?
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crassius
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on extended intakes, it is often necessary to fasten carb to frame by making some kind of mount for it
typical symptom is called surging where bike gets up to speed, then slows till vibs lessen, goes back up to speed, then slows again
typical symptom is called surging where bike gets up to speed, then slows till vibs lessen, goes back up to speed, then slows again
Yes this is exactly what is happening but without an extended intake. I haven’t been able to keep out the vacuum leaks so I have yet to use the extended intake. It’s surging with the stock carb and intake setup... I’m just trying to figure out how to prevent this, since I’ve cranked the engine mounts as tight as they will go?
FurryOnTheInside
In Memoriam
I agree, and there's a risk of crushing the tubes if mounts are overtightened. I can't remember what your bike looks like at this point, can you remind me? (Pictures!)
I would suggest checking the precise fit of the mounts, the flex that exists in the frame itself, and look for ways to reduce the vibrations being created by the engine in the first place.
I would suggest checking the precise fit of the mounts, the flex that exists in the frame itself, and look for ways to reduce the vibrations being created by the engine in the first place.
Okay I’ll try and get some pictures but I guess I can’t expect a whole lot since the bike I’m enquiring about is an EBay clunker but it’s held up great so far I’ve been babying it lol. I just didn’t know if there was a way to isolate the carb so it would reduce just the vibrations to it? My thought on using the heater hose as an extended manifold maybe it would help with the vibration a little but that’s still a work in progesss lol I can’t find any hose clamps locally that aren’t the standard worn drive ones which have proven to vacuum leak for me considering I’ve cut and tried 4 different times with the heater hose thinking maybe one of the times it won’t rev up so high lol. It isn’t a huge factor for me but it would be nice to hit 33 and climb just a tad bit more since it feels like the power is there but that’s right when the surging happens at almost the top end of my rpms.
FurryOnTheInside
In Memoriam
No bike is without flex, even frames made for this hobby have flex. I know that wider or shorter tubes, or made from aluminium alloy or cro-moly instead of high tensile steel will theoretically flex less; but in reality it is the same because manufacturers use this to make frames with adequate stiffness but lighter.
I don't think the hose does anything to reduce vibration in the carb, since the carb is secured to the seat tube and so is the engine.
Is there no vacuum leak when you use the standard intake?
I really didn't think the Jubilee® style hose clamps were not tight enough to make a seal, I just like to use clamps that are incredibly strong and tight so I can feel confident that nothing can loosen with vibration, and they look tidy. I guess the cause could possibly be the clamps, but plenty of other people's bikes use the same as yours.
I have my way of dealing with frame flex on a large bike with long tubes (my head mount), and I don't have a springer fork or sprung saddle or wide low pressure tyres so that the engine and frame isn't given much freedom to move under me, but the vibes from the engine are not severe in the first place..
I would definitely be looking for ways to reduce at source the vibrations that originate from the engine at high RPM. It could be that your engine just isn't fantastically well balanced for the top of your RPM range.
I was thinking that same thing on the end the crank most likely isn’t balanced which is what it is this is still my first build and it’s holding up great so far and teaching me a lot about how to fix these puppies that’s for sure haha. I have a Zeda80 kit sitting here in the shop waiting to go on a bike but I’m still having a lot of fun messing with this first build lol. And yea as far as the manifold extension it doesn’t leak at all with the stock setup but as soon as I add the hose and clamps it leaks. I am just unable to tell which clamp may be leaking so I’m assuming it’s both. When I hold a propane torch (slowly leaking gas) right up to the hose clamps while the engine is idling it bogs down as if it’s going to die so I figured it’s a vacuum leak at the clamps. It is boggling my mind though because I have seen many people with the setup I am trying to achieve and there’s seem to be working fine. I may just give in and either order or make a rigid extension. I just liked the idea of the hose because I can easily cut and try different lengths finding which spot I like best for my engine.
Bill Carson
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OK. For the engine mounts, I buy an extra Goodyear heavy duty tire tube from Wal-mart, cut strips as wide as the mounts, wrap around the frame twice and crank mounts down. For the intake extension I use clear vinyl water hose from Lowes. Buy it a little smaller than you need, then carefully warm it up with a lighter, propane torch is too hot, and work the warm tube over the carb and intake, also, use the smallest, but widest, stainless steel clamps to hold the joins.
For the carb, first, remove the cup and bend the float valve tab DOWN carefully to eliminate float vibration, then use another piece of the inner tube between carb and frame and secure carb (not tube) to frame any way that works.
The streets where I live are TERRIBLE, and it took much time and patience to work this out, but it was worth it because it totally eliminated this issue for me.
See that’s odd you say that because from most posts I’ve read everyone seems to not recommend putting anything between the frame and engine mount. Most people say it actually increases the vibration but I may look into your suggestion. When you say “float valve tab” are you referring to the Y shaped piece of metal that contacts the float in the float bowl? And you’ve got me really curious about the clear vinyl.. how long has it held up for you? This heater hose had been fighting me from the beginning and I’ve worked with that clear vinyl plenty of times and it does flex really well under hot water. That’s what makes me nervous using it to connect to the intake manifold since the manifold gets relatively warm from the engine... wouldn’t it heat up and get soft possibly leading to a collapse?
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