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Turbo_the_world

76 280z, low idle, low vaccum

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I have a 76 280z l28. A little about the issue, car does have a slightly larger cam .450 lift, 270 duration. Pallnet fuel rail, 210cc injectors, deleted most of the emissions items and made sure to cap off all vaccum lines. Timing is right athere 15 degrees, any lower and it will die out. I'm only getting 5in of vaccum at idle, and wot I'm getting about 10-12. Idle stays are 500rpm cold or hot. Running a little better when warmed up but not much. Fuel pressure is at 35psi and I have the stock rising rate regulator. Engine stumbles at first when the intake is opened. Could use some help if anyone has some ideas.

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Throttle position sensor: dirty contacts, poor ground?

Vacuum leak: on or around intake manifold, afm, at head-intake gasket surface?

AFM contacts dirty, loose, poor ground?

Too lean at idle? What are your AFRs like?

Distributor: vacuum leak at vacuum advance or damaged advance diaphragm, distributor cap/ rotor worn or corroded?

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Looks like he blocked off the Idle air adjustment with a bolt. That isn't going to help. I have my 280Z pulling 12 in with a 480 lift 280 degree cam.

 

WOT should be  zero inches of vacuum. Not 10 or 12. You should be measuring the manifold vacuum reading at the manifold port fitting beside the brake booster fitting. Not the " Ported Vacuum " fitting on the Throttle body.

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Regardless, without ECU controlled idle speed, he needs a throttle body or blade bypass.  The 280Z's had the bypass in the TB, the ZX's went to a hose bypass with an inline valve.  

 

Hard to get a good vacuum reading if the engine is about to die.  He needs idle speed control.  Set the idle speed then tune the engine.

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If the intake manifold gasket is brand new, did it exhibit the problem before installing the new gasket?

 

A quick way to check for intake manifold leaks is to use some vacuum tube. Put one end to your ear and the other along the sealing surfaces (at bolt holes, gaskets, capped vacuum lines, etc). You'll hear a distinct "whistling" when the tip of the tube is near the leak.

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Do make sure that your PCV hose is hooked up and operating properly. The hoses often split on the underside. Do NOT remove the PCV system and run Vent to Atmosphere from the block breather. That will adversely affect drive-ability and cause acids and other contaminants to build up rapidly in the oil. PCV " deletes" are one of the worst thing you can do to a street driven engine.

 

The AFM also has an idle  mixture adjustment hidden down low. It is sometimes blocked off with a removable plug. Even with 76 models, which the FSM says does not have an adjustable idle mixture. ( The FSM plugs the adjustment screw to meet EPA standards. And then states that the idle mixture is non adjustable. Not true. )  ) You can fine tune the idle AFR mixture with this air bypass screw. Really helps with big cam shafts.

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This is an MSA 60mm throttle body.

That's what I run. Where's your large Idle air adjustment screw? You have it blocked off with a bolt.That's no good and is part of your problem.

 

Edit:

 

I also have the idle air adjustment screw back.

 Please post only current pictures. Other wise you confuse the issue.

Edited by Chickenman

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larger cam shafts also require a LOT more static ignition timing. They will tolerate extra timing because the mixture density, and thus effective " Dynamic Compression Ratio "is very low at low throttle openings.

 

Set your static timing to 15 degrees BTDC. Vacuum advance disconnected. Then connect the vacuum advance pot to a manifold vacuum port. The small fitting beside the brake booster fitting. This will add an additional 15 degrees of timing at  or so at idle. So a total of about 30 degrees at idle. Don't worry, because of the low dynamic compression the engine will not be adversely affected. You should see your idle vacuum increase by at least 5 HG Idle should be noticeably smoother. Aim for an idle speed of about 1,000 rpm with your cam.

 

Do NOT use the vacuum port on the TB . This is Ported vacuum and will be zero vacuum at idle. With a larger camshaft you need MANIFOLD  or Full vacuum at idle for the vacuum pot.

 

Note you may have to decrease the spring tension on the vacuum diaphragm to make sure the vacuum pot stays " pulled in " at idle. See Jason Grays distributor page to see a How to Do. All Nissan 280Z dizzy's have an adjustable vacuum pot. Both for spring tension and stroke length. While you are at it decrease the stroke length to 15 degrees instead of 20 ( Add 5 degrees of static timing and you should decrease vacuum timing by 5 degrees to bring cruise timing into spec )

 

Link to Jason Gray's distributor page:

 

http://newprotest.org/projects/510/jasonGrayDistributor.pl

Edited by Chickenman

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You were on here before, right, a short while ago?  A whole discussion about PCV systems and vacuum leaks?  Hard to keep track of who does these things.  Did you block the PCV valve on the bottom of the manifold?

 

If not, the big hole in the valve cover will be a problem.  Stick your thumb over it while the engine's running and see if anything changes.

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A little more help - the air space under the valve cover is the same air space that that breather tube connects to.  The rocker area and the crankcase are the same space.  If you're going to seal the bottom you need to seal the top also.  Or just seal the intake manifold (where the PCV valve is and where that big tube ends up).

 

You have a bunch of different problems to work on.  Your 12% over-rate injectors are probably making up for the extra air.  When you seal up the vacuum leak that you're working on, you'll have cut off that extra air.  With no idle speed control the engine probably won't idle anymore.   And you'll be running rich becuase the injector rates are higher.

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Well, I have the car running at 800rpm at idle now, was a combination of timing and the fact I found a throttle set screw. Still having problems with it on the road, in my garage the throttle is fast and runs fine through all rpms. Under load, the car gives out at 2k rpm, no power and seems to stumble a little.

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