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PostPosted: Tue Jul 02, 2013 9:54 am 
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Joined: Fri Apr 02, 2010 2:56 pm
Posts: 331
Location: Finland
My 260z is giving me hard time now. I have Mallory Unilite dizzy (vac advance model, vacuum tube blocked earlier on), and I modified the advance curve with the appropriate springs and set the max advance. Before doing the mod, she always started instantly and idled well. After the mod, cranks, won't start. I have checked that the dizzy is correctly installed with no1 being met at compression stroke tdc, each and every plug are nearly new, clean and dry, and they have a good spark. Initial timing confirmed with lamp when cranking - currently 10 degrees. Battery fully charged. Fresh fuel in the tank, fuel pressure as should be (gauge installed), and the carbs have fuel in them. The car has been in a dry, warm garage, and prior to the mod she started well. I have rechecked 5 times everything in the dizzy is correctly put back, nothing is visibly broken in it, everything is super clean and I even went back to the original springs, but still she won't start. She makes a slight sound of trying to start at a regular interval, nothing more than that.

Is is possible that the dizzy is internally damaged in a way that is not visible to the eye and that it still gives spark, but not consistently? I really have no ideas anymore.. Only thing I could imagine doing, would be trying another dizzy, but I don't have one..

The carbs worked the previous time I started her, and they are filled with fuel. I cannot be 100% they feed the engine though, but what could have happened to them in a dry, warm garage in 2 weeks (last I started her was 2 weeks ago)?

heeelp.

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PostPosted: Tue Jul 02, 2013 11:31 am 
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Location: Finland
well...................... I begun to be suspicious about the fuel supply as it was lacking the normal sound when you turn the butterflies. it didn't sound like fuel is squirt into the intake even though the carbs were filled with fuel. so I opened the top covers, talked to the dcoes nicely, put the covers back and fired the engine at the first attempt. this really seems strange but that's what happened. 2 days wasted for this, dizzy dis- and assembled 5 times, tiking played with a dozen times, coffee had in huge quantities, french vocabulary further enhanced and then it comes down to a mystery like this. well at least she's runnign now.

I have done the by-the-book weber initial settings in order to begin to play with settings (idle speed 1 full turn in, mixture screws 2 full turns out) and it SHOULD idle very slowly at this point, but instead is revving really high. Any ideas? Is it jetting? (I'm running 140 mains, f9 45 idle, 200 air, (and chokes are 34))

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PostPosted: Tue Jul 02, 2013 1:19 pm 
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Glad you found the problem! Weird that the bowls wouldn't take fuel until you opened them.....still thinking about that.

As I undstnd it, the basic Weber settings are simply to ensure the car runs when you first install them. You have to adjust (slowly) from there.

Try 1/4 turn adjustments to each idle screw first, and see if you can lower the rpm that way. Changing the mechanical opening of the butterflies is easier to reverse than changing the mixture.

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PostPosted: Wed Jul 03, 2013 4:30 pm 
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Frank, in fact the fuel bowls were filled with fuel up to normal levels but for some reason beyond my comprehension, it was not fed into intake.. Remember, these carbs were recently completely disassembled, ultra sound cleaned at a major airline service facility and they came out super clean. i have new fuel filter at the back and front too. Pumps and all gaskets and seals are new, everything tight, double checked, linkage works, nothing is binding it... a complete mystery and I do hope this won't happen when I'm on the road.. Now all I wanna do is finally tweak her until she's driveable but schedules aren't on my side again..

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PostPosted: Wed Jul 03, 2013 8:51 pm 
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Joined: Fri May 23, 2008 9:34 pm
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Location: Hamden, CT
10 btdc at idle is too low for Webers. Unplug the vacuum advance and set it to 16btdc. Then make sure the main linkage is loose/slack and try setting the idle screws again. BTW, 900-1000 is a correct idle for Webers.

Try these jets: 50F9 idle, 40 pump, 55 exhaust.

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1970 240Z - enhanced F54 L28 w/P90, Weber DCOEs, 4:11 R200, Nissan T5 5-speed


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PostPosted: Thu Jul 04, 2013 2:38 am 
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Location: Finland
Ok, I'll try with 16. Some other guys suggested starting with 10degrees. Vac advance has long been unplugged in my case, i also heard with webers you shouldn't use vacuum advance.

I'm also aiming at 900-1000 idle. I currently have 45f9 installed but I do have 50F9 too. not sure what pump and exhaust jets I have, must check. Thanks for the tips!

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PostPosted: Thu Jul 04, 2013 5:18 am 
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I checked and confirmed that I already have 40 pump, 55 exhaust.
Hopefully try and set timing again and adjust mixture tomorrow.

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PostPosted: Thu Jul 04, 2013 6:36 pm 
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Location: Hamden, CT
BTW, last summer my engine started missing suddenly when idling/driving at low speeds. But would run fine above 3k. Turns out one idle jet was clogged...blowing it out restored smooth operation. Considering the small size of the orifice it makes sense these could get easily clogged.

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1970 240Z - enhanced F54 L28 w/P90, Weber DCOEs, 4:11 R200, Nissan T5 5-speed


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PostPosted: Fri Jul 05, 2013 2:22 am 
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Location: Finland
Yeah it would be great to have a small blowing device in the glove box in case they get clogged on the road. My jets are all new, the engine has been running only a few times since the installation of the Webers. But modern fuels are bad in the sense that if you have any in the tank and car stands a few months, the fuel starts getting bad. I have basically new gaskets in the dcoes, but now that I had to remove the top cover, I noticed some resin type matter on the surface of some gaskets. I will have to look for some additive that helps during the longer periods of not driving.

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PostPosted: Fri Jul 05, 2013 10:19 am 
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We use StaBil here. Works pretty well.

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PostPosted: Sat Jul 06, 2013 5:18 am 
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ok now I'm starting to see why I'm having starting problems. It seems as soon as the engine gets hot and I stop it, next time there's no fuel fed to the intake. Vapor Lock. I do have soft mounts but seems I really need a heatshield too. So I'm still not tuning her, just working around different issues (2 days ago I had flooding carb because I had accidentally messed the float adjustment when trying to find the fuel feed problem last time..)

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PostPosted: Sat Jul 06, 2013 7:45 am 
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Are you still running a manual fuel pump off the front of the engine, or have you installed an electric fuel pump at the fuel tank?

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PostPosted: Sat Jul 06, 2013 8:17 am 
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electric at the back with pressure regulator and gauge, set at 2,9psi as per instructions I was given by a weber specialist

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PostPosted: Sat Jul 06, 2013 1:58 pm 
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Vapor lock and percolation were two problems the flattop SUs had. Guys tried various tricks to prevent it, including wrapping their fuel lines with aluminum foil or clipping spring-closed wooden clothespins along the line. Looked stupid but it worked. Some guys even ran their fuel lines thru reservoirs of ice water under the hood, refilling them as needed (which was often, in Hawaii).

Vapor Lock means the fuel gets just hot enough inside the line to change state from liquid to vapor. Since the fuel pump is designed to pump liquid and not vapor, no fuel got to the carbs until the engine cooled down enough for the vapor to change state back to liquid again.

Percolation means that the fuel bowls become so overheated, the fuel inside boils (far more volatile than water, so it happens far more easily). That forces fuel into the carb throats when the engine is off and can either load the engine ('flood' it) or drip fuel out the air horns. If your carb is perched above the hot exhaust header and dripping fuel down onto it, you can destroy your car. I watched a 1953 or '54(?) Corvette melt to the ground one hot Saturday because the sidedraft carbs perked onto the manifold.

Electric pumps at the tank usually overcome vapor lock problems, but not always. Heat shields, air ducts onto the fuel rails, and fuel line insulation all help.

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PostPosted: Sat Jul 06, 2013 2:06 pm 
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I'm still not sure if I had vapor lock or something else. Might be sticking needle valve as well. the car had same fuel in the tank for a long and engine only running a few times. the fuel may have gone bad and cause a sticking valve. I need to change my oil now because flooded carb 2 days ago resulted in ruined oil (float level went wrong accidentally when top cover was off), and after that I'll be able to run the engine and use some carb cleaner. I now have fresh fuel in the tank.

I also need to take off one of the carbs to adjust butterflies as they don't cover the first progression hole completely.. a loads of small things to mend but I learn something every time I have to fix a problem :)

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