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'72 240Z Backfiring Through Carbs at Low RPM.


Connor280ZX

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*Typo in title, its actually a '71.*

If i give the car more than a touch of gas while driving at low RPM (1000-2000) The engine stumbles and the carbs backfire repediately. I have to slip the clutch higher to get going off of a stop in order to prevent this. It seems to run fine at higher RPM's. Does this when its cold, and when it's warmed up. I don't think its running lean because the plugs are nice and brown.
 
Description of engine:
L24/E31 (PO says he installed a Crane Cam years ago, not sure what stage)
Twin 3 screw SU Carbs
 
I've just done a small tune-up:
 
-New NGK plugs (.035) Gap
-New Dist. Cap and rotor
-New fuel filter
 
There is some oil in the carbs, but i'm not sure how to measure how much.
 
There are some strange mickey moused vacuum lines here and there.

 

Let me know what you guys think.

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Edited by Connor280ZX
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Go to Z Therapy and buy the video "just SUs".  It will explain how to maintain SU carbs.

 

http://www.ztherapy.com/

 

Note: backfiring through the carb can be timing or the carb set too lean.  The video will expalin it all.

 

Also there are over a hundred posts here, at ClassicZcars.com and listed in Google searches dealing with SU carbs. Many many how-to posts are on-line.

 

 

All of those vacuum lines have a purpose.  See the FSM.

 

Download a copy of the factory service manual (FSM) from elsewhere on this site or Google a FSM download, and buy a Haines manual.

 

Watch the video until you understand SU carbs. You will then be able to tune a set of SU carbs in about 15 minutes.

Edited by Miles
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Low oil in the carbs can cause a lean condition when opening the throttle that goes away once at a steady speed/ To check the oil in the carb remove the black know at the top of each carb and pull out the damper rod. There are two (somewhat faint) lines. Add oil to get the oil level between the lines. There is a never-ending debate about what type of oil to use. For now use whatever motor oil you have handy. I get good results with 10W30. The damper only holds a tiny amount of oil so add carefully. Use a syringe or eye-dropper if you have one.

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3 in 1 is straight 20wt oil and is what a lot of people use.

 

The line with the bolt in it is the overflow line. It shouldn't be blocked, it should be routed back to the air cleaner if memory serves. You probably don't want it to route to the air cleaner right now with your fire out of the carbs problem, but it shouldn't be blocked like that.

 

Have you checked the timing? What is it at?

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Thanks alot for the tips, guys. I took some oil out of another two sets of SU carbs that came with the car and filled up the carbs that are in the car now. Oh, and timing was at 0 BTDC, now its at 3 BTDC. Problem is completely gone. I can punch it from a crawl no problem now. Thanks again.

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The line with the bolt in it ISN"T the overflow line...it's ported vacuum. Usually used for distributor vacuum advance, but only if the distributor is set up for it! some distributors use it, others do  not.

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The line with the bolt in it ISN"T the overflow line...it's ported vacuum. Usually used for distributor vacuum advance, but only if the distributor is set up for it! some distributors use it, others do not.

Are you sure that's a vac advance line? Because my vac advance is already hooked up to the intake manifold...
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The vacuum advance hose is the one that is plugged. Someone has incorrectly connected the dizzy advance to the intake manifold vacuum. This vacuum source does not give you the correct vacuum signal your dizzy needs, it must be attached to the line at the carb (ported vacuum) for it work as intended.

 

Set your timing with the dizzy hose removed (and plugged, engine side), you have quite high manifold vacuum at idle with your hose connected like it is, and it is advancing your timing too much.

 

Your float vent lines are connected correctly to the air cleaner.

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