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Discussion Starter · #1 ·
Hiya,
I am new to this board but I really need some help. I have a customer with a 1970 240Z that is giving us fits. The car has a rebuilt motor that was bored 30 over an dhas a "mild" Crane Cam. The duration of the cam is 232/242 @.050. The problem we are having is the car cannot be driven without it starting to load up bad and popping thru the carbs. The carbs have been rebuilt and synced but that has not help. It is only pulling about 13-14 inches of vacuum at idle in park and about 10-11 in drive. With it in park it runs and sounds great. no popping or anything it only acts up in gear. My thought was that the cam had too big a duration and not enuff vacuum for the SU's but I wanted a second opinion before I swap in a stcok cam. Also the car is an automatic with everything else stock. It has fresh plugs and wires,Cap and Rotor and points and a new coil. I also have 2 seperate air cleaners on it instaed of the big stock one. Any help is greatly appreciated. I have been sorting it out for almost a month now as I inherited this from the owner after the fact. I also cannot find a timing spec for this car either. And is the vacuum advance supposed to be on full vacuum or ported? Sorry for all the questions. Thanks for any help u give.

Moe
 

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The carbs can work fine with the setup you describe.

Assuming the carbs are in good working order, and all else being similar; the situation you describe is indicitive of the carbs being set too lean. A very lean condition will typically cause backfiring through the carbs as you describe. Your situation sounds like a textbook example of this. You mention the the carbs are sync'ed but you make no mention of the mixture adjustment. Look into this first.
To learn how to tune these carbs I recommend watching the Ztherapy video on them.

Don't worry about not being able to pull enough vacuum with that cam. There are people (including myself) running hotter cams with these carbs.

Timing - 7 degrees BTDC (Haynes manual)
not sure about what you are asking about for the vacuum advance....can you rephrase...
 

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go to zhome.com they have two s.u. write-ups on adjusting them.
they may help.That is a lot of duration for an auto trans. they have a fairly tight convertor.I would make sure the valve adjustment is correct and determine which distributor duel point or single.lookfor vac leaks
you should use ported vac on vac advance.
 

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The carbs are too lean try this , screw in trhe jet adjustment untill close then back them down 2 1/4 turns then drive it if it still pops richen 1/4 turn. all this depending on if the valves are set properly and the carbs are ballenced with a uni-sen . Another thing dump the potnts dist and get a electronic ignition from a '79 or 80 one that has the module on the side of the dist marked 80 12 ,this will make a big difference . Bone yard cost $50. Good luck Gary Portland Ore by the way with an automatic set the thming at , 10 to12 deg advance . Good luck
 

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Also make sure you have the carb dampers properly full of 20wt oil. Too little or too thin will cause the lean pop condition also. If the dampers pop up too quick you would think that it would run rich but this is not the case. With the damper popped up too high there is not enough vac to pull fuel from the jets. Might also contribute to the low manifold vac, but that's likely a stretch. Vac port for dizzy is on the front carb base right by the upper left mounting stud.
 

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Under a load, can you get it to accelerate up to a certain rpm, or does it pop through the carbs at all revs? If it's the former you may have distributor vac and/or mech advance problems.
 

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Discussion Starter · #8 ·
We found the main problem to be that the customer assembled the engine with the oil pump out of alignemt causing us to not have the correct timing. We made a modification to the distributor hold down slot to get the timing where it should be till we can re do the pump. I have it timed now at 8 BTDC and that got rid of 95% of the popping I think it will be about perfect at 10-12 BTDC. It now only pops when the engine lugs at slow speeds in 3rd gear. I still believe a 232/242 Duration @.050 cam is too big for an Automatic Trans 240. I think we may try a looser converter in it too to keep the engine in it's rev range longer. The trans also needs some adjustment so we are gonna address that too. A valve adjustment is on the agenda.
Thanks for all the help. I'll keep you posted.

Moe
 
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