I know not another backfire through intake 1976 280z I think I tested it all

[cozye]

You can try putting some seafoam in the cylinders and letting it sit overnight, the drive it a lot.

Considering your numbers are 30psi low on all 6, I doubt its stuck rings. I'd have to question. Your 41k miles. It's probably 141k, maybe even 241k. Mine has 113k on it and compression is around 160. I believe 175-180 is what you are looking for iirc. It's in the fsm.

[racebird1]

I have a pretty good history on the car and who owned it since new. I am pretty confident on the milage which was confirmed with the condition of orther components of the car. The cam shaft shows no wear at all and the oil pressure is very good. I am wondering if something may have happened to the rings or cylinders from sitting so long. A coworker has a camera I can stick down the cylinders through the spark holes to see if anything is going on with the cylinders. I will even be able to tell if there is any cross hatch left on the walls. I put some mystery oil in the cylinders and when I go to start it up later today I am going to get a quick vacuum reading and see if that comes up from the higher compression from the oil being in there. Either way if it stays this low is there anything I can do to get it through the year with out the intake backfire and tackle the engine next year? Does it even pay to do the engine myself I see them on ebay for maybe under a grand over what it would cost me to do it myself?

[FastWoman]

Eric, I hate to throw cold water on your low compression/vacuum theory, but when the vacuum runs weaker, the fuel pressure runs higher. In the end, the pressure differential along the path of the fuel -- from rail to manifold -- is kept constant, so that the amount of fuel injected is a function of injection pulse width and NOTHING ELSE.

I do believe our slightly low engine vacuum does relate to our somewhat low compression, though. It's just engine wear. When our engines run less efficiently, the vacuum is going to be lower -- probably from more blow-by. Predictably, the vacuum issue is mostly at idle, not at elevated RPMs.

I have a theory to compete with your low compression theory, and I have no hard data upon which to substantiate it. I wonder whether most or all of our late-model ECUs tend to have this problem, and most people just don't realize it. (In my experience, linear circuits tend to drift and fail in the same ways from unit to unit, usually at substantially the same rate.)

When I bought my car, the PO represented it as running great. Indeed, it ran well enough to drive it hundreds of miles home. The car had been professionally maintained by a Nissan mechanic with whom I had a long chat on the phone. To him, the car was running fine. For many people, a car that starts and runs until the key is turned off is a car that runs great -- even if it's not running efficiently, and even if it's lacking power. I suspect the unaffected cars are simply not running quite lean enough to start popping, and they're running well enough to satisfy the professional mechanics, but I bet they're not running all that great either -- at least well enough to satisfy fussy people like you and me. The thing is, I don't think most people expect an antique engine with 150k+ miles to get up and go.

Racebird, try the potentiometer fix. I think you'll be pleasantly surprised. :-)

Edited February 6, 2012 by FastWoman

[cozye]

Sarah, I forgot about the FPR being vacuum operated. I reckon you have a good point there.

[racebird1]

I had my friend over with the scope and we put it in through the spark plug holes and checked out the condition of all the cylinders. All the cylinders have avery good cross hatch pattern on the walls and very little carbon on the pistons, now this isn't going to tell me what condition the rings are in but I would think that an engine with such a low compression would no longer have cylinders that look so good. I should probably take the valve cover off and make sure the valves arn't a little sticky. Never the less untill I can find a fix I have been looking for someone local to possibly rebuild my engine. I just don't have the time right now to do it myself. I will in the mean time try to find some simple instructions to vacuum time and put the potentiometer in to see if they can help get it through the summer. Anybody have simple quick instructions on the vacuum timing method. Do I simply turn the distributor untill I get the highest vacuum?

[FastWoman]

Racebird, you can simply judge by ear. With the throttle in the same position, and adjusting the timing, the faster the engine runs, the higher the vacuum. I would suggest putting a timing light on the system to at least see what 10 deg BTDC does for you. However, some of us run a bit more advance. I think I'm running at about 13 deg BTDC. I wanted to advance enough that the engine felt like it was running happily (i.e. wasn't completely happy at 10 deg), but I didn't want to keep advancing after that. I stopped advancing when I felt I was starting to round the corner of diminishing return, if that makes sense.

[racebird1]

When I pulled the plugs I checked them all out and found that it is indeed running very lean. All the plugs have a white coating on the electrodes. I have ordered a 5k pot to try on the temp sensor but I also looked at the three extra AFM's and they all have a differant resistance of tension on the flap and I don't really know which one was on the car. I searched the site and can't find out how to tell if it is at spec or how to set it there. Does anybody know how to do this? I am sure they must set them by how much tension it takes to open the flap. I can turn the wheel but to what? I still think I need to pull the injectors again to make sure the screens are clean. A local injection company says they can rebuild them in two days for $20 each. It bothers me a little that all I get from them is a steady stream not any kind of spray. They al trigger fine with no sticking or leaking. The $20 each does not seem too bad.

Edited February 8, 2012 by racebird1

[FastWoman]

Here you go:

http://atlanticz.ca/zclub/techtips/afm/calibration/index.html

For $155, you can buy all new injectors:

http://www.ebay.com/itm/75-76-77-78-79-80-81-82-83-Datsun-280Z-Fuel-Injectors-/160661473384?pt=Motors_Car_Truck_Parts_Accessories&fits=Model%3A280Z&vxp=mtr&hash=item25682b8868

The hoses might be a bit short on these. (They were on mine.) It's simple to cut them off and install longer ones, though. Cut the hose reinforcements with shallow, grazing angled cuts, and then pull off to the side. Don't nick the aluminum barbs!

[racebird1]

Do you use fuel hose rated for fuel injection from a local parts store or get special hose somewhere? I used some from a local store a while back and it didn't hold up all that well.

[racebird1]

I was looking at those injectors and I thought I saw somewhere in a post where they did not think they worked too well but maybe they had other issues. How are they working for you? I did not see any negative feedback. The AFM link was great just what I was looking for thanks

[FastWoman]

Yes, fuel injection rated hose. It should cost in the neighborhood of $4/ft. I've used both the type with the blue rubber inner lining and the type that's just black. The black is sturdier, as the blue rubber lining can become separated.

I'm running those injectors in my car and have had no problems with them. They've been in service for about 1.5 years now. I once read someone complaining that some aftermarket injectors they bought were not balanced with each other, spraying different amounts of fuel. However, judjing from plug color, all of mine are spraying the same -- just right.

BTW, I was very pleased with this style of clamp, which comes in various sizes:

http://www.summitracing.com/parts/CCA-G3912/

It won't bite into the rubber, and it's flexible enough to make a good seal on small hoses.

Edited February 9, 2012 by FastWoman

[racebird1]

I have been going through all the afm tests on the four afm's I have. Two are older much heavier with the back flow valve which are probably from the 76 and two are much lighter and might be a newer version. I did the weight tests with the measured water and the two with the glue missing were way off and one of those was completely sprung. It took about 13 teeth to get it back from fully sprung. The one that was on the car was still glued but was comomg up a hair too tight which is towards a lean fuel mixture which is also part of my problem. I will attempt to go with a setting of just below normal to try for more fuel. I then did the reccomended cleaning of the contacts and such with testing before and after the cleaning and some of the tests it did make a difference from cleaning. Here is the thing, I tested the ohm readings as per instructions and they all passed after the cleaning but I noticed that if I did a more detailed test of the ohm test of the carbon trace the two newer ones came out almost exactly the same and the two older ones came out almost exactly the same but the new compared to the old are a little differant. I measured the resistor strips right on the board so I was getting an accurate reading. I have concluded that there is some difference between newer and older 76 version but I don't really know if it is enough to change anything? The other one with the glue still on the screw was just about right on with the weight test. The fully open weight test is a little tricky because of the friction of the string dragging and the angle really doesn't allow the flap to fully open it is just shy of open. I am going to get a new set of the injectors and see if I can set the afm and timing to get me through the summer.