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Datonedatsun

New carbs 4000 rpm start

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Just recently purchased "rebuilt carbs" and slapped them on got everything hooked up besides the gas pedal, everytime I start my car it rocketed upto 4000+ rpms and I shut it off... i made sure the butterflys were completely closed and it still does it, if I starve the carbs of air I can get it to idle pretty low but cant seem to change the idle otherwise using the mixing screws or idle adjuster screws... any ideas what maybe causing this? 

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Interesting problem. Maybe we should get more information? 
which car do you have? 
Stock?
where did you get the carbs?
Did it run well before? I’ll assume not if you are upgrading the carbs.

Basically what’s the history of the car?

 

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70' 240z had su round tops and I got the rebuilt ones from a seller on Facebook, same su roundtops ran okay before but piston kept sticking and they were super old so I figured I'd just slap a freshened up set on car is completely stock 2.4l 

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58 minutes ago, Datonedatsun said:

 i made sure the butterflys were completely closed and it still does it

, if I starve the carbs of air I can get it to idle pretty low 

How do you "starve the carbs" if the butterflies are already closed?  Doesn't make sense.  Unless a passage was left open that bypasses the butterflies.  Sounding like an error in the rebuild.

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 How about something simple like leaving the carb return springs off. I can verify that the engine will instantly rev up with no springs.  How are you "starving the carbs of air"?

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35 minutes ago, Zed Head said:

How do you "starve the carbs" if the butterflies are already closed?  Doesn't make sense.  Unless a passage was left open that bypasses the butterflies.  Sounding like an error in the rebuild.

In taking a microfiber and covering the entrance into the carb where you can see the piston and only letting it suck in a minimal amount of air, doing this is allowing it to idle pretty low, as soon as a I uncover the piston it skyrockets again if that makes sense I'm pretty new to this and these su carbs confuse the crap outta me... 

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25 minutes ago, Mark Maras said:

 How about something simple like leaving the carb return springs off. I can verify that the engine will instantly rev up with no springs.  How are you "starving the carbs of air"?

I've got the return springs on so not that...

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We hen you say “starve” how are you doing that. 
so tell us what you did. Ie.. just replaced the carb, new spacer seals. What linkage did you unhook.? Big vacuum leaks will cause increased idle but not 4000 rpm

try disconnection the throttle linkage from the carbs and see if will start

 

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15 minutes ago, Datonedatsun said:

In taking a microfiber and covering the entrance into the carb where you can see the piston and only letting it suck in a minimal amount of air, doing this is allowing it to idle pretty low, as soon as a I uncover the piston it skyrockets again if that makes sense I'm pretty new to this and these su carbs confuse the crap outta me... 

That's what the butterflies are for (throttle valves).  Something isn't fitting here.  If the butterflies are "closed" then the microfiber should have no effect.  Got a camera?  Take a picture of a closed butterfly valve.

p.s. the SU concept is unusual.  The pistons only respond to engine vacuum.  There is no control mechanism for them except vacuum.  the throttle valve controls air flow, the pistons just respond to engine vacuum.

image.png

Edited by Zed Head

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Looks kinda ghetto didnt put heat shield back on so I couldnt pin the springs down so I just wrapped one around to pin them back...

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You'd be surprised at how much air can get through that small gap around your throttle valve.  I think you need to adjust those so that they're fully closed.

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15 minutes ago, Zed Head said:

You'd be surprised at how much air can get through that small gap around your throttle valve.  I think you need to adjust those so that they're fully closed.

Is that butterfly gap about what the full choke position opens them to?

doesnt full choke drop the nozzle and open the butterfly?

Edited by 7tooZ
Added thought

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I can't make out both butterflies, but the one I CAN see is not what I would call "closed". Different cars need different amounts, but IMHO, that's open much further than idle.

And some other weirdness, although I'm not sure it's related:

1) It looks like you're using 72 spacer blocks between the carbs and the intake manifolds, and they are upside down. I'm not sure that's causing you a vacuum leak or what, but if you're sucking air in there and then opening the butterflies to "compensate" maybe?

2) The linkage between the two carbs looks a little too short. Looks like the ball on the left side in the pic (front carb) is close to falling out of the receiving hole on the carb. I'm not sure what all the years parts you're working with here, but that's a problem that often occurs when there are 73 or 74 parts involved. I don't think you've got that, so I can't explain the issue there.

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Sorry late reply, gonna go pull them off and adjust the butterfly and flip the spacer blocks I'll post some before pics of the carbs before I took them off 

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Sorry late reply, gonna go pull them off and adjust the butterfly and flip the spacer blocks I'll post some before pics of the carbs before I took them off 

20190824_184719.jpg

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