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My 240z is a Gas Guzzler


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Do a regular compression test, then repeat it with a spoonful of oil in each cylinder. If the compression is higher, rings are bad.

 

"regular compression check" == Hot engine, pull all six spark plugs, put the tester in one cylinder at a time. FULL THROTTLE PEDAL, Fully charged battery, hit the starter until the cylinder dead-heads; repeat measurement three times per cylinder (or more if you get wildly erroneous results.) Take average values of each cylinder (discarding any wildly inappropriate numbers.)

 

You are certainly running pig-rich, so your mixtures can be dialed back a hair, but listen to jmortensen on the nozzles.

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ONE nozzle stuck in the down position will drop my 260 from 22-26 to 11-15 mpg. Similar for both nozzles stuck just partially down. Fully down...

 

Geee, I wonder what two fully down would do? Bables slipped of their holders, frozen down by years of sludge and disrepair, pulling the lever only moves the cables now, instead of the nozzles which a stuck, frozen, bending the light metal levers attaching them to the bellcranks of the lever actuating wires...

 

"None are as blind as those who won't see..."

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I pulled the air box off an dit had oil around the filter, also it was really black around the carb intake parts. I pushed the nozzles and they both returned, so they're not stuck. im going to do another compression test, and put some oil in to see if the numbers change. I don't have the tester that screws in i just have a quick one that has a rubber plug around it that you shove in the spark plug hole. Possibly air leak around it that gives such low numbers? ill post the test numbers later

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okay i did the compression test again and i got the same numbers. Adding a spoonful of oil only increased the numbers by 2 or 3 psi. They all were still in the 70-85 range. whould rebuilding the carbs solve my problem or is that just a waste of money. I have a slow drip leak from my gas tankk drain plug but it couldn't be it. i have no egr yalves or any of the emisson control stuff, just a tube from the fuel pump straight into the float bowls, Is this bad? i don't have to worry about emisson tests...its too old. lol

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i second the valve adjustment statement. my stock 240z is having similar issues but i know for a fact that its running way rich and after playing with the carbs off and on for the last 2 months i pulled the plugs to find that cylinders 3 and 4 were black as hell and the rest were fine. my best educated guess is mal adjusted valves so i suggest you check your valves before you do anymore tampering with the carbs. also check your timing. good luck

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try another compression gage. the gage can be wrong. pull out all 6 plugs when doing the test and open the throttle.

 

low cranking pressure is typically caused by a stretched timing chain. Try advancing the timing chain to the 3rd position. read the repair manual carefully how to do it.

 

bad mpg can also be caused by retarded ignition timing. Make sure the mechanical advance to working and make sure the vacuum advance is working. Do you have a timing light?

 

are you using points or electronic ignition?

 

work the choke levers and make sure they return to the non-choke position.

 

adjust the idle mixture for best idle (highest rpm and highest vacuum).

 

Look for gas leaks.

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  • 3 weeks later...
okay i did the compression test again and i got the same numbers. Adding a spoonful of oil only increased the numbers by 2 or 3 psi. They all were still in the 70-85 range. whould rebuilding the carbs solve my problem or is that just a waste of money. I have a slow drip leak from my gas tankk drain plug but it couldn't be it. i have no egr yalves or any of the emisson control stuff, just a tube from the fuel pump straight into the float bowls, Is this bad? i don't have to worry about emisson tests...its too old. lol

 

You realize the EGR significantly improves mileage at mid-throttle, right? It's very possible that without an EGR your combustion chambers will overheat, and can scorch your rings along with deforming the piston/cylinder wall. It takes a fair amount of heat, but if you're running around that rich all the time and still mashing the throttle I'd say it's possible that you're well over 2500 degrees in the combustion chamber.

 

I'll assume you're not going to tear down the engine to find out, so double-check that drain plug. If you're dripping liquid from your gas tank drain plug, it could certainly be evaporating through it. Does it constantly drip fluid, or only after fuel pressure builds up? You may actually want to take it in to a smog shop to have your fuel lines pressure-tested (it's part of a standard smog test, and will tell you a lot about your lines).

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Okay well my 72 z is a major gas guzzler. i put 5 gallons in and it only gets me 35 or so miles, which is ridiculous. My setup is a l28 thats running duals su's the good ones. i don't have the charcoal canisiter or fuel return lines on it. I was wondering if anybody has some insight on how i can get more than 6 miles or so to the gallon?

6 MPG is a little bad...I get 9 MPG city and 28 MPG highway (at 100 MPH).

Of course, the 9 MPH city is my fault because my 260 is heavily modified and I was born with a lead foot.

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okay today i did another test with a different gauge and got 100 even when i pulled all 6 plugs. Also i took both carbs off the clean them out and see if the floats are set right. I don't have an air flow meter so i can't really tune them right so im waiting on that. What the best tool for sensing the cfm on the su's, i hear unisyns are kinda useless. Any cheap but effective tools to use?

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Just a wild guess here, but try reaching under the carbs and push up on the nozzles. I'm thinking maybe they're stuck down.

 

I had to replace one of mine after fighting with trying to adjust them. I found one was stuck and would not close all of the way. Once I replaced it I didn't get that smell anymore.

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i Was to cheap to buy a uni-syn so i tried the old hose to ear trick and i think i got them pretty well balanced. my only problem now is the fuel mixture. i have the front carb pretty well mixed... i push up on the piston and it doesn't sputter or rev high. my problem is the back carb. i've pushed up on the piston and turned it richer each time because it sounds like its going to die. so i guess i'll just wait and check the plugs in a couple days and see if they're fouling

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i Was to cheap to buy a uni-syn so i tried the old hose to ear trick and i think i got them pretty well balanced. my only problem now is the fuel mixture. i have the front carb pretty well mixed... i push up on the piston and it doesn't sputter or rev high. my problem is the back carb. i've pushed up on the piston and turned it richer each time because it sounds like its going to die. so i guess i'll just wait and check the plugs in a couple days and see if they're fouling

 

Hmm, it almost sounds to me like they're pretty far out of balance. since pushing up on the piston should rev higher the engine on those 3 cyl. though it'll run rough.

 

you can build your own vaccum meter with a 2 ltr bottle and a long hose. mark inches on the hose, then put 2 cups water in the bottle, insert hose seal w/ duct tape and hold other end near carb, it'll suck water up vs the pressure in the bottle, just match the pull on each carb. long hose keeps you from sucking water into carb.

 

Phar

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okay well i think i got it somewhat tuned. now when i drove it around i got 45 miles on ten bucks or about 4 gallons. so im not as bad as before but its not the greatest mpg. im still leaning towards efi for it so im keeping my eyes open for one or if anyone in the NW has one for cheap pm me.

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