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78LB620

Throttle sticking Stock 78 Carb

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Hey guys. My throttle started sticking out of the blue on Sunday. I was able to just kick it down and it resumed normal idle, but this morning on the way to work it stuck almost wide open. I did some looking around and searched Google on another site (Z specific) guys were pointing to the bcdd valve.

 

I took off the air cleaner today and the throttle cable was not freyed and it was slack and I moved the throttle through its range and let it snap back. It seated where the cable would have been taught again, so I am assuming that the sticking is in the throttle.

 

Anybody have any suggestions? I don't want to spend like 280 bucks from the dealer (if I can even find one available) Thanks in advance

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happens to me often unfortunately. I used to use just carb cleaner and douch off the in/out of the carb and it would last a few months. I started doing that then following up with some Sea Foam Deep Creep, wich is a peroleum based penetrating fluid and I just spray it all up in and out of the carb and work the linkages back and forth and it really works out good.

 

Bill

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sometimes the fast idle cam will stick and cause a high idle, try cleaning it with some carb cleaner and see if helps.

 

 

on another note: I was driving my 78 620 and was having no signs of problems, when I came up to a red light, I pushed in the clutch and the engine was reving high so I pulled over to the side to see if the idle cam had stuck, nope, it turned out for some unknown reason that the idle speed screw was screwed in to far, readjusted it and have not had a problem since.

 

damn gremlins.. :D

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happens to me often unfortunately. I used to use just carb cleaner and douch off the in/out of the carb and it would last a few months. I started doing that then following up with some Sea Foam Deep Creep, wich is a peroleum based penetrating fluid and I just spray it all up in and out of the carb and work the linkages back and forth and it really works out good.

 

Bill

 

Thanks for the quick response Bill. I was thinking the same thing as a starting point, this thing has been sitting and is filthy under the hood. I will try out the Sea Foam treatment.

 

Maybe you can help me out with this vacuum line that was dangling when I removed the air cleaner:

I hope you can see this little guy, it looks like it is only about 1/16th of an inch ID

f8fb1d35.jpg

 

It is coming out of the valve down below center in this shot:

521aa42e.jpg

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the tiny hose connects to the tiny tube coming off the power brake booster metal line. forget what I wrote here, my mistake.

521aa42e.jpg

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It's the vacuum signal to the anti back fire valve... the round thing with the missing hose on it just below. If you can't connect it up make sure the hose is plugged and not leaking vacuum.

 

 

 

 

 

. 521aa42e.jpg

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It's the vacuum signal to the anti back fire valve... the round thing with the missing hose on it just below. If you can't connect it up make sure the hose is plugged and not leaking vacuum.

 

 

 

521aa42e-1.jpg

 

OK, I've marked this up to help clarify. The hose with arrow 1 connects to a tee below the valve (anti back fire valve?) marked 2. Where does connector 3 hook up? If 420n620 is correct that connector should go on metal tubing marked 4. It doesn't seem to fit there but I guess there could have been another piece of hose joining the two. It looks like the PO stuffed something down in that tube, or just some filth and dirt naturally got down in there... that should be fun times getting clear, eh?

 

sorry for the crappy markings just getting used to photobucket. My Haynes manual should be here tomorrow but I thought that the hose could be the cause of the entire idle problem (guess not if it is for the brakes)

 

I cleaned the carb inside and out with some seafoam deep creep and it didn't seem to help. I found out if I turn off the key and let the vacuum release the sticking seems to stop, and when it sticks it seems to rev on its own, so it isn't really from depressing the accelerator. If that makes any sense.

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I just went out and looked on my 620, there should be a small tube on the driver side coming out of the anti back fire valve that you got marked #2, which I don't see it but it goes to the tiny tube #4.

 

#1 and #3 look to be the same line, where is it going to, I can't tell.

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You in colorado, it hasn't been under 40 degrees where you at? The throttle can ice up and then thaw out while you driveing

 

Otherwise just use "carb cleaner" it will clean and lubricate the linkage

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I just went out and looked on my 620, there should be a small tube on the driver side coming out of the anti back fire valve that you got marked #2, which I don't see it but it goes to the tiny tube #4.

 

#1 and #3 look to be the same line, where is it going to, I can't tell.

 

yeah , you got it. 1 and 3 are the same hose the bottom is hooked to valve 2 and the top should go into 4, which is the power brake assist. Sounds about right.... thanks for the check on that.

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You in colorado, it hasn't been under 40 degrees where you at? The throttle can ice up and then thaw out while you driveing

 

Otherwise just use "carb cleaner" it will clean and lubricate the linkage

 

No freezing yet. I hope it holds off for a couple more months :).

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It does not have to be freezing to get carb icing. If you are getting any temperatures in the 40's, especially with high humidity or light rain, then you can get throttle body icing and that condition will definately cause throttle body sticking and flooding.

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Yes, I've had it happen at about 37 degrees. Throttle sticks due to ice. Turn the engine off for a few minutes, then it runs Ok... This is the problem with the weber since it doesn't have a hot-air feed like the stock carb. Also a problem with headers.

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I have an '80 L20B, and if that intake is the same as mine, the #4 on your pic connects to...nothing, at least on my truck. Perhaps your year Datto is different. My truck is original as far as I know, and was bought in CO with NO air pump stuff. Although it doesn't connect to anything, it DOES have a factory cap on it. It's a piece of hose that doesn't have an open end. The id on that hose is pretty small, as you said, it seems like 1/16. That is a manifold vacuum port, BTW, so if it's unplugged, you have a vacuum leak. I can guarantee you that because I use that exact port to hook my vacuum gauge up to! Works great and sure is convenient, thanks Nissan :D !

 

Get yourself some hose that fits, and plug the end with a screw, or try to find a vacuum cap that will fit. Maybe your secondary throttle is hung up? Give it a check, I've had that happen to my the idle on my truck.

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