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vespa 1970 90cc. i just installed a new dellorto 16/16 carb. i couldnt get it started at first then i unscrewd the low speed throttle screw and it started right up. now the problem as soon as it starts it revs as high as it can go and doesnt settle back down. turned it off a couple times and started it back up but it goes right back to high speed rev. the throttle cable is slack so the throttle arm is not up. any ideas would be apreciated thanx
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It sounds like you're sucking air somewhere. Carb connection?
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Make sure the carb is on tight, it's easy to think it's all the way on but give it an extra push to be sure.

Also make sure the outer throttle cable isn't getting hung up.
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Thanks for the advice. I did notice the air bellow on the manifold I guess its the thru frame gaskit. Was cracked a little bit could this b the problem and don't I half to take off the engine to replace it.
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stephenlamb wrote:
Thanks for the advice. I did notice the air bellow on the manifold I guess its the thru frame gaskit. Was cracked a little bit could this b the problem and don't I half to take off the engine to replace it.
That bellow is just to keep dirt out of your engine. It won't have anything to do with an air leak (if you have one).
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small frame air leak
Ok so its not the thru frame bellow. So if the carb is attached and tight where should I b looming for the air leak. And do I half to remove the engine to really check the engine side of the manifold. Thanx
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Re: small frame air leak
stephenlamb wrote:
Ok so its not the thru frame bellow. So if the carb is attached and tight where should I b looming for the air leak. And do I half to remove the engine to really check the engine side of the manifold. Thanx
To check the engine side of the manifold properly you will need to lower the engine a bit by disconnecting the rear shock (you won't need to remove the carburetor unless you plan on lowering it more or removing the intake manifold).However... You can check the security of the manifold without doing this. You'll need two 10mm box wrenches, ideally, of different manufacture. The reason I say this is because different brands will have different off-sets; you'll be able to tighten the nut on the top of the manifold possibly 1 to 2 mm at a time, and using the same wrench won't allow you to get the wrench back onto the nut with enough room to turn it, so you'll have to alternate between wrenches as you run out of room (the wrench will hit the cylinder/case). Sorry for making this seem so esoteric.

Getting to the nut holding the bottom of the manifold is simple enough because there is plenty of room. Don't get any ideas about removing the tire to get more access, because you won't find it to be true, unless you lower the engine enough to also remove the cylinder shroud. Of course, remember that if the intake manifold base at the case has a failed gasket simply tightening the nuts won't improve the air leak much.

The second place you'll need to check for a leak is where the carburetor slides onto the manifold. There is a felt gasket that should be greased and stuffed onto the manifold before installing the carburetor. I have been using 2 instead of the usual 1. Grease both of them up and slide them on. The carburetor will then stuff them back up the manifold to close off any spaces that could let air in.
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Awsome thanks for the.advice can't wait to get off work and try these things. I'll let u no what I find to be the problem Or ill b back on asking questions
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1974 smallie 90
ok got home and went to work. i lubed up the manifold gasket it was dry made sure the carb was fully seated against it. i checked the engine side of the manifold and it was tight. so now i couldnt get it to start after about 50 kicks so i took it for the run and jump start after a few times it fired up. geuss what it didnt go into super high rev mode it idled but when i gave it gas it would bog. so i fixed the air leak and now im onto tuning the carb i guess. its a dellorto 16/16. i/ve check the spark good plug was wet and smelled of gas but i havent been able to get it to run. with the air leak it would fire up first kick now nothing. my problem is there is not much to adjust on this carb. to much gas not enough im not sure help with ideas. thanx so much guys
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There is a screw on the driver's left side of the carb. It has a spring wrapped around it. Screw it all the way in and then screw it out 2 full turns.

Then, the idle speed screw is on top of the carburetor. Screw it in till the engine starts and holds. If the engine idle is then really high, just slowly screw the idle screw in until the idle is reasonable.
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