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Weber running too lean, can’t adjust enough?


Speedymaru

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Hello, I have a 78 620 that I just recently started daily driving. I’m trying to get the air/fuel ratio correct, and right now it’s running too lean. The plugs are whitish, and it hesitates when throttle is applied quickly. I have been trying to richen it up a bit using the fuel mixture screw, but I’m a bit confused. I get the truck running, then turn the screw in until it starts to drop down in RPM, then I back it out and the rpms come back up, and I’m supposed to keep backing it out until it drops down again, but no matter how far I back it out it doesn’t ever drop back down. Does this mean I need to rejet it? I’m not a carb expert by any means. Also, which jets would I be replacing?

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Been awhile but I think u should not turn the mixture screw out more than 1.5 turns, if no affect then jet maybe too small. What is the history of the carb? Is it new? Have you looked at a diagram of a 32.36 weber to make sure u are messing with the right screw? To get more details on adjustment and diagram of carb check out piercemanifolds webpage.

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I don’t remember what the correct sweet spot is but you’re right, it should be somewhere between 1-3 turns out and if it’s more than I think it needs rejetted? I’m not sure about the history of the carb, it was on when I bought the truck. The guy said that he had just picked it up for the truck but I don’t think it was new. And yes, I am turning the adjustment screw. Thanks for the info

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Sounds like a vacuum leak to me, or a plugged idle jet. This is true on dirt bikes I work on. When you have to run with the choke on, it usually is a plugged idle circuit. Definitely running lean if plugs are white. It will be running hot too in a lean situation. Also check for vacuum leaks. Take the carb apart if you haven't already and have a look see inside. They are very easy carbs to work on and there is lots of info on the 'net.

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Sounds like a vacuum leak to me, or a plugged idle jet. This is true on dirt bikes I work on. When you have to run with the choke on, it usually is a plugged idle circuit. Definitely running lean if plugs are white. It will be running hot too in a lean situation. Also check for vacuum leaks. Take the carb apart if you haven't already and have a look see inside. They are very easy carbs to work on and there is lots of info on the 'net.

 

Dirty jet. Maybe not fully plugged yet but some blockage.

 

Can you get it revved up into the secondary? This will usually allow it to run because the other jet is OK.

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Also check the tightness of the screws holding the top on to the main body.

I've seen the Weber body gasket dry out, letting the screws loose tension, and become a major air leak around the center where there are several fuel passages.

Have even see the gasket rot, and parts of it are missing.

 

By the way, the mixture screw is only for idle, and has very little effect on anything above idle.

So go buy a rebuild kit, and go into it.

I'm thinking most likely crud, like Mike says.

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 then turn the screw in until it starts to drop down in RPM, then I back it out and the rpms come back up, and I’m supposed to keep backing it out until it drops down again,

That's not exactly right. Back it out until it starts to rise, then go about another 1/2 turn and call it good.

 

You probably need some jetting done though. The idle jets are on the sides of the exterior of the carb, while the fuel and air jets are inside the float bowl. If the carb has been sitting for a long time, the jets can get hard buildup causing them to clog. This is especially true for the idle jets since the orifice is so small to begin with. You can use a single strand of wire brush to clean them out.

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I like to use small propane bottle with a 5/16 vacuum hose about 2' long to pinpoint vacuum leaks it's much more precise than a can of carb cleaner. We used to find about 50% of the Webbor adapters not sealed properly at our shop.

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Okay cool, thank you all for the great info. I’ll tear into it when I get a chance and check it out. Does the carb have to be rebuilt if I’m just cleaning the jets? I know it’s better to, but I’d rather not wait for the rebuilt kit to come in. If it is necessary then I can wait

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Rebuild on these is only a gasket and needle/seat. Some other stuff too. But if u dont damage top cover gasket,ur good to go. Have had top off mine half dozen times,cleaning and jet changes. Also ck inlet filter screen(where fuel line connects) just blast it clean. 

Idle jets do effect engine above idle speed. To like 1/3 throttle,idle and main jets overlap each other. Remember,BECAUSE DATSUN. so anything is possible

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And I’ve tried to use propane before, but I’ve never been able to have good results with it? I’ll give it another try though

The secret is the regulator, I have 3, 1 has a push button that give a puff when pushed. If yours is just a nob, use a balloon on the vacuum line to barely inflate it to get your adjustment as low as possible. Maybe just buy a smoke machine, it works to find all kind of things

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To keep the gasket in tact, remove the lid screws and knock the top cover with the heel of a screwdriver. That usually pops the cover off without damaging the gasket.

 

Do you know what carb adapter you have? One piece or two piece?

 

Yes, remove the carb..... so you can inspect the adapter. The bolts often vibrate loose so check them and the gaskets. Loctite???

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