so....I've got a little tuning problem.
12-07-2010 | 07:48 PM
#32
Ok, This is a wild shot in the dark;
Is the idle screw set properly? If the screw isn't set properly, and you've got the throttle servo holding the carb at idle instead of the screw, when you brake, the carb will close too much and depending on your LSN setting, the truck will act as if it's leaning out. You said after a high speed run it leans out, which could be because as you apply the brake, you're also closing the slide/LSN. Then you said it goes back to normal. Maybe as a result of letting off the brake? Does that make any sense? Hit the brake and see if the carb slide moves at all?
Also, are you sure you're not getting bubbles forming in the line when the motor is up at high rpm? High rpm vibration can cause the fuel to foam and cause these exact symptoms. When you stop the truck/lower the rpm, the vibes go away and the fuel settles almost instantly.
As far as the boot being a seal, sure some of them seal just about as much as a bearing seal, some not at all. But while the carb is subject to back and forth pressure pulses, it isn't subjected to exactly the same back and forth pressures of a bearing, and normally, air leaking past the carb slide is compensated for during a tune. For example, looking down into the carb throat, air flows through the usual opening, but a minute amount of air can also leak past the sides of the slide barrel when it's closed (usually close to the opening), but that's also compensated for with your tune. If the barrel is really sloppy in the carb body, or if the slide barrel or the carb body become worn to a taper, the movement/angle of the servo linkage can cause a variation in the amount of air that leaks past, and that can cause problems, but it would have to be fairly sloppy.
In other words +1 on what ambros303 said.
Is the idle screw set properly? If the screw isn't set properly, and you've got the throttle servo holding the carb at idle instead of the screw, when you brake, the carb will close too much and depending on your LSN setting, the truck will act as if it's leaning out. You said after a high speed run it leans out, which could be because as you apply the brake, you're also closing the slide/LSN. Then you said it goes back to normal. Maybe as a result of letting off the brake? Does that make any sense? Hit the brake and see if the carb slide moves at all?
Also, are you sure you're not getting bubbles forming in the line when the motor is up at high rpm? High rpm vibration can cause the fuel to foam and cause these exact symptoms. When you stop the truck/lower the rpm, the vibes go away and the fuel settles almost instantly.
As far as the boot being a seal, sure some of them seal just about as much as a bearing seal, some not at all. But while the carb is subject to back and forth pressure pulses, it isn't subjected to exactly the same back and forth pressures of a bearing, and normally, air leaking past the carb slide is compensated for during a tune. For example, looking down into the carb throat, air flows through the usual opening, but a minute amount of air can also leak past the sides of the slide barrel when it's closed (usually close to the opening), but that's also compensated for with your tune. If the barrel is really sloppy in the carb body, or if the slide barrel or the carb body become worn to a taper, the movement/angle of the servo linkage can cause a variation in the amount of air that leaks past, and that can cause problems, but it would have to be fairly sloppy.
In other words +1 on what ambros303 said.
Out of curiosity, if I am getting foaming of the fuel / bubble in the line, how much is acceptable, and how do I resolve it? External fuel filter perhaps, to act as an accumulator?
No problem on the wild shot in the dark...like I said, this will be a good learning opportunity in problem diagnosis, and hopefully others will benefit from the discussion as well. I've got the throttle linkage set up so that the idle screw is what's holding the slide open, not the servo arm.
12-07-2010 | 09:36 PM
#33
12-08-2010 | 11:00 AM
#34
Tech Fanatic
Joined: Sep 2008
Posts: 801
I haven't messed with it much since last week. I've got a race this coming Saturday, so I've been working on getting my truggy prepped....plus it's been DAMN COLD here in NC for the past week! Jeez, when did winter arrive?!?!
Out of curiosity, if I am getting foaming of the fuel / bubble in the line, how much is acceptable, and how do I resolve it? External fuel filter perhaps, to act as an accumulator?
No problem on the wild shot in the dark...like I said, this will be a good learning opportunity in problem diagnosis, and hopefully others will benefit from the discussion as well. I've got the throttle linkage set up so that the idle screw is what's holding the slide open, not the servo arm.
Out of curiosity, if I am getting foaming of the fuel / bubble in the line, how much is acceptable, and how do I resolve it? External fuel filter perhaps, to act as an accumulator?
No problem on the wild shot in the dark...like I said, this will be a good learning opportunity in problem diagnosis, and hopefully others will benefit from the discussion as well. I've got the throttle linkage set up so that the idle screw is what's holding the slide open, not the servo arm.
If the issue is fuel foaming, you should be able to see it by holding the truck off the ground and spinning it up a bit. Droplets will start to dance on top of the fuel in the tank, kinda like backwards rain. Then it'll start to foam and bubbles will flow through the line. It's pretty obvious when you see it.
A filter/accumulator is a good thought, but won't work because the filter will just fill up with air, and the air will eventually be fed into the line. This type of thing was tried with adding small header tanks to helicopters, but it was just a matter of time before the tanks filled with air, and of course they're subjected to the same vibration. Some people just wound up wrapping their tanks in spongy foam.
The only way to resolve the problem is to reduce the vibration translated to the tank, or to change it's frequency. You can never really balance a single cylinder engine, but a heavier flywheel can help to lower engine vibe frequency. Loose bolts/parts vibrating against each other will also have an effect on harmonics. Worst case scenario, you can try to isolate the tank from the chassis (something like the foam mentioned earlier), put strips of plastic between the motor mount and chassis, or even stick a few strips of dynamat to the tank, but that's never pretty.
You know, after re-reading your post, I'm kinda leaning towards an air leak or carb adjustment like everyone else. If it was foaming fuel, the truck would also lean out during an extended high speed run and I'm not sure if that's your situation. Just thought I'd throw it out there though, just in case.
Good luck this weekend
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