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Kabstract

1972 240Z SU Carb Full throttle upon start up

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Posted (edited)

Hi guys, I've never posted here before but I think this is where I should ask this. Anyway, I have a 1972 240Z (early) which was disassembled and all the parts have been or are going to be replaced, repaired or serviced. I have a reman E88 head from California Datsun and the block is good and within spec. The car has the 3 screw SU carb on it. I did some gaskets and replaced a couple things on it that came with a refresh kit. 

 

Time came yesterday to try and fire it up for the first time and after some playing around it finally did. However, it came on as if the throttle was stuck at full open. It went like this:

 

1. Turn over and fiddle with fuel and choke

2. Car roars to life

3. Doesn't go down to idle, instead keeps reving up

4. I shut the car down

 

It does this every time the car starts. It does take a few seconds of turning over to get it to start.

 

 

I checked the throttle linkage, the choke, float levels, butterfly valve, dashpot oil, and mixture screw etc. The high speed idle screw is backed off a fair bit at the moment too. Everything seems to be okay or at least in working order and nothing indicates to me why the car would start and relentlessly rev instead of going down to idle. 

 

With the air filter off, when looking into the carburetor, the piston is down while the car is off, but when the car starts it flies all the way up to the top, which is as I understand is how the car would behave under full throttle application. If anyone has any ideas, let me know. Thank you.

 

 

 

I am just learning about carburetors so forgive me for being dumb in advance. I have some manuals and have looked online and can't find anything that has worked. I don't have any pics at the moment. I have removed a lot of the emissions stuff and my carb looks something like this one, albeit not as clean.

 

Image source: https://www.classiczcars.com/forums/topic/57927-removing-emissions-clutter-for-cleaner-look/

 

Thanks!

1497475086375.jpg.a52cc333c47b08bde7cb88f43c3aafa4.jpg

 

SOLVED:

Okay, so I did some thinking and I took the dashpots off the carbs and check the butterfly valves again. They are functioning smoothly, but they weren't quite 100% closed. I checked all my adjustment screws again and determined it was in fact the fast idle screw that was the issue. I guess from the previous owner's setup, it worked where it was, but with everything cleaned up and refreshed, the screw was set too tight which was opening the butterfly up just enough for the engine to rev up like crazy on start with no load. I back the fast idle screw almost all the way out and now the car starts (reluctantly) and idles now, albeit roughly.

 

It's just a matter of getting my mixtures right and balancing etc but at least it's not reving to the moon on start up now. Hopefully this helps some other dummy like me one day!

Edited by Kabstract
posted solution to OP

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Posted (edited)
3 hours ago, bunkhouse said:

Have you checked that the throttle return springs are hooked up? There should be one on each carb that hooks onto the linkage and anchors on the heat shield below.

Yes these are hooked up and functional. The throttle linkage snaps back into 0% throttle when the linkage is turned and released.

 

 

 

EDIT: Okay, so I did some thinking and I took the dashpots off the carbs and check the butterfly valves again. They are functioning smoothly, but they weren't quite 100% closed. I checked all my adjustment screws again and determined it was in fact the fast idle screw that was the issue. I guess from the previous owner's setup, it worked where it was, but with everything cleaned up and refreshed, the screw was set too tight which was opening the butterfly up just enough for the engine to rev up like crazy on start with no load. I back the fast idle screw almost all the way out and now the car starts (reluctantly) and idles now, albeit roughly.

 

It's just a matter of getting my mixtures right and balancing etc but at least it's not reving to the moon on start up now. Hopefully this helps some other dummy like me one day!

Edited by Kabstract

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3 hours ago, bunkhouse said:

 Good logic in checking the butterflies. They have to open for the engine to rev. Be sure to check the float levels. I wouldn't assume they're correct. The previous owner's "set-up" wasn't even close.

Yeah for sure. I will do those tonight.

 

The previous setup was odd to say the least. Weird wiring and shoddy fuses in weird places, you know, the usual backyard mechanic stuff. I'm just glad the car starts and will idle now. I started it again a few hours ago and it only took a couple of seconds to get it to fire up and it idled at about 1500rpm for 30 seconds until my temporary electric fuel pump wiring rattled loose and cut the fuel off. At least I can finalize some more of these things now. Thanks for the responses, and hopefully this will help someone else with SU carbs. It's always the simple things that get overlooked and yet somehow mess things up completely, like floor mats under clutch pedals, etc.

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If you are new to SU carbs you need to purchase the video "Just SUs" from Ztherapy.

 

Link: http://www.ztherapy.com/

 

On the left side of the web page under products click on Videos and then on "Just SUs". 

The video explains theory, repair and tuning of SU carbs.

 

You will not regret buying this video.  Watch the video several times.

 

Ztherapy also sells parts for the SU carb.

 

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