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Horribly Rich/poor Running Until Warmed Up


mikebontoft
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I admit I should probably start asking my questions in a single thread... my build thread when I post it. But I could use some reassurance and advice.

I lack experience with carbs. I understand the theory and have a little bit of workable experience, but not much. Last carb I worked on was an electronically controlled quadrajet.

Anyway, here's my issue. I have a '75 with the stock M10. when the engine is cold, it runs like absolute crap. Pig rich. Old plugs were carbon fouled. Turns out I have a weber 32/36 DGAV carb, too. Choke operates correctly and I've run through the tuning process with little luck.

Looking down through the carb, I noticed that when you move the throttle and it sprays fuel, the engine almost dies out completely until the added fuel is gone... there's the rich condition.

The longer I let it warm up, the better it runs. Once it's at operating temperature, there's really no issue... though it does still seem to have some hesitation to it.

Smokes on decel too. Hate to say it, but I'm probably adding shitty compression to the list of things to address. BUT... that's why I'm asking, maybe I have a leaky bowl or something.

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If the carb is not new and in unknown condition I would rebuild it and check the jetting, the rebuild kit is relatively cheap and you will have a better idea of the condition of the carb when you are done.  you can use C.D.'s prescription for the jetting to achieve a good baseline.  the smoking on decel is probably your valve seals, just push the clutch in when you decel until you are ready to pull the head for some other reason (unfortunately the head has to come off to replace the seals).

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" Looking down through the carb, I noticed that when you move the throttle and it sprays fuel, the engine almost dies out completely until the added fuel is gone... there's the rich condition."

 

What you're seeing is the accelerator pump functioning properly--presuming of course that the spray is coming from a small nozzle on the forward edge of the carb throat, aimed toward the primary barrel. The squirt shouldn't be very large--a few cc's at most--and shouldn't on its own cause a near-stall over-rich condition.  Unless a PO has enlarged the accelerator pump nozzle (which is specificallly sized like a jet) it should be OK.   Does this near-stall happen with the choke plates closed or nearly closed, and go away once they're open? 

 

And...with the engine cold, do the choke plates close completely when you "set" the automatic choke by pressing down on the accelerator once or twice before starting the engine?  If you don't reset the choke as above before starting a cold engine, it's not gonna function until you press the accelerator.  Then once the engine has started, do you have a faster-than-normal idle while the engine is cold?  Part of the choke function is to increase idle speed (the "fast idle") via a linkage that connects the choke plates to the accelerator linkage to keep the engine from stalling while it's cold.  If these two choke functions aren't both working, then your choke isn't performing properly.   

 

Let us know what you observe so we can pursue this problem...

 

cheers

mike

Edited by mike
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Get the Weber book, some jets and a rebuild kit and do a rebuild as suggested above. It's pretty easy and will give you tons of info about your carb and you'll be starting off with a good working carb. Make sure everything is moving easily and not sticking. While rebuilding mine I soaked everything in carb cleaner to get any crud out of it.

 

From what you said it sounds like a choke issue as Mike suggests.

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Good, that's what I had hoped to hear from you guys. I will rebuild the carb, as I figured I may have to do.

I will have to verify after work, but I suspect that the choke may not be closing all the way after all. I don't remember, but I know it wasn't flat. Thought that's where it was supposed to be. they do set into place when I press the pedal all the way down.

It doesn't matter whether I have the choke open or closed though (with my hand) It still idles high.

The accelerator pump appears to be working then. it sprays a fine stream, not a ton.... but as soon as it hits the slightly open throttle plate the engine bogs down. Without releasing the throttle linkage, engine speed increases to where you would expect it to be with the throttle open a bit.

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so... I have pictures and even a few videos I'll get up when I get home.

Today it wouldn't start with the choke closed (and it does close all the way) I blocked it open a little more and it at least ran, but was notably rich. Idle speed went up almost as far 2k rpm just by playing with the choke plates. About 3/4 open ran best.

I did notice that the top plate around the fuel inlet is soaked with fuel. I wonder if the float level is too high. I can look down that little port and it's almost completely full.

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tried again today. Engine was about to die so I hurried and opened up the choke blades so they were vertical. It smoothed out and increased in RPM. It's too rich.

I will check timing before teardown though. I just ordered a rebuild kit and power valve through worldpac. will be here Friday. only $28 for both together :)

I'll have to check the other jets, but the primary air correction is 110 and secondary is 160.

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Here is a pretty basic base line formula for the jetting of a 32/36 dgav 160 air. 135 fuel primary. 140 fuel 170 air on secondary. The emulation tube should be f7 on both primary and secondary and the idle jet as a 60. And the transition jet a 60.

The transition jet helps on the smoothness when you stand on it. In my opinion a 32/36 should be a smooth linear carb wether it light acceleration or on the gas hard. This is achieved in the transition jet and the secondary air/fuel jets. Like others have stated before your no where with out correct timing

With all this being said I do live at sea level so if your at elevation you will have to adjust accordingly.

Weber makes a great tuning book that costs about 20 bucks. It explains all things weber and then some. But best of all it gives the information on all the strange numbering on the emulation tubes and what they do

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You can adjust the electric choke so that the plates aren't closed quite so far.  If the engine runs well when it's warmed up, you probably don't need a carb rebuild.  There's nothing in the rebuild kit that will solve a 'rich when cold' condition.  Especially if opening up the choke plates solves the issue.  Just loosen the three screws (blue arrows) and turn the canister until the choke plates aren't closed so far.  You can tweak this adjustment over time, and will likely notice a need for a tweak when the weather gets colder.

 

For those who aren't familiar with the choke, those plates restrict the air flow into the carburetor.  This causes the air pressure inside the carb throat to drop (i.e. it creates a vacuum) because the engine is trying to suck air and the choke plates are restricting that.  This increase in vacuum in the carb throat causes additional fuel to literally be sucked into the carb enriching the mixture proportional to the amount of restriction caused by the plates.  The electric canister has a wound spring inside and a heater element.  The spring is what causes the choke plates to close and turning the canister will increase or decrease the spring tension and cause the plates to close more or less (for any given ambient temp).  Once power is applied to the heater element, the spring heats as well and 'relaxes' and that heat-induced drop in spring pressure allows the choke plates to open as the engine warms up.  A choke with a water line going to it also uses engine coolant to control the temperature of the spring.

 

There isn't a magic setting for the choke plates.  Give your engine what it wants, and installing a Wideband O2 sensor setup is the best way to keep an eye on how things are running and will allow you to make more informed changes in the carb.

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Edited by Original_Customs
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Oh points how I love you so...

still need a carb rebuild though. fuel is leaking out every gasket and the idle mixture screw is in all the way with choke open... idles at about 2000rpm. I looked for the idle adjust screw per the guides I've found and it doesn't seem to be present, which can't be true... guess I'd better look again.

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