78 280z, slight stumble, low vacuum

[Zed Head]

The other way to richen the mixture at idle but with little effect anywhere else is by the idle air bypass screw (it might be called the lean adjustment screw or something similar) on the AFM. Turn it in to richen, then adjust the idle speed with the throttle body screw.

It's behind the rubber plug on the bottom, side of the AFM. I have a 76 and I had to adjust mine to keep the flapper off of the fuel pump contacts.

Haven't been emission tested since then so I don't know where it will come in.

[cozye]

The other way to richen the mixture at idle but with little effect anywhere else is by the idle air bypass screw (it might be called the lean adjustment screw or something similar) on the AFM. Turn it in to richen, then adjust the idle speed with the throttle body screw.

It's behind the rubber plug on the bottom, side of the AFM. I have a 76 and I had to adjust mine to keep the flapper off of the fuel pump contacts.

Haven't been emission tested since then so I don't know where it will come in.

Yeah, I thought about that too, but the lean condition actually exist at idle and at slight throttle openings such as cruising or light acceleration. Thats when you get the intake pops, etc..

The best way to richen it up I found was to jumper the "full enrichment" position on the throttle position switch, but you use a lot of gas doing that ( i got like 13 mpg), and it's a hack. I'd rather figure out the real problem then just put in a hack.

Where I've got it now makes it plenty driveable and is a work around for now, I've got the AFM advanced 5 teeth, and the timing on 15 degrees advance. That almost completely gets rid of the stumble, appears to get decent mileage, and doesn't loose power so much. It's a compromise for now.

it seems to me that there are a few 78 owners that have this issue. I know of 3 so far.

[FastWoman]

Behind the rubber plug? There's a rubber plug?

Hmmmm... Not on mine. Just the screw, which is roughly flush with the housing (i.e. so there would be no room for a plug). Should it be screwed in quite a bit more than that? Perhaps that was an adjustment the PO made? (I was hoping that adjustment went unmolested.)

Grrrrrr....

Well, I'll do the test drive tomorrow. I'm guessing I'll be too rich at half/full throttle and will indeed need to wind up the AFM again -- and advance the idle mix screw quite a lot to richen the mix at idle. Anyway, one thing is for certain: Given the right fuel/air mix, the engine runs fine. (Same in your case, Cozye.)

[Zed Head]

Behind the rubber plug? There's a rubber plug?

The rubber plug is actually pretty deep, about 3/8 to 1/2" I would guess. And mine fits flush so the screw must be in pretty far. I can't see the head of the screw. But my AFM is a rebuilt one from MSA (or wherever they source theirs from) so I don't know if it was ever properly adjusted. Apparently the stock AFMs are factory-adjusted. But I did end up screwing mine in even farther and the engine idles fine, and accelerates smoothly.

I have rethought my original comment about no effect anywhere but idle and I think now that there must be some effect just off idle until enough air is flowing to offset it.

One more thing to play with...

Edited November 2, 2010 by Zed Head

[Pomorza]

Bit off topic here. I've had this same issue earlier when I bought my Z. The previous owner decided that rich was just not rich enough, and had it set so rich that it barely idled at anything lower than a 1k. When I went to pass emissions I failed with CO levels in the 2k range.

Secondly, that ticking noise is the injectors. I have the same noise (it drives me nuts) when the car gets warm after driving on the freeway. I've cleaned and cleaned them with everything under the sun (other than take them off) and nothing seems to help.

Jan

[cozye]

Secondly, that ticking noise is the injectors. I have the same noise (it drives me nuts) when the car gets warm after driving on the freeway. I've cleaned and cleaned them with everything under the sun (other than take them off) and nothing seems to help.

Jan

You might consider having them cleaned off the car professionally. Mine still tick a little, but I cannot hear it in the car now and it did seem to help.

[cozye]

Well, I'll do the test drive tomorrow. I'm guessing I'll be too rich at half/full throttle and will indeed need to wind up the AFM again -- and advance the idle mix screw quite a lot to richen the mix at idle. Anyway, one thing is for certain: Given the right fuel/air mix, the engine runs fine. (Same in your case, Cozye.)

Definitely let me know what you think. I think we are fighting the same problem, and two heads (with two cars!) are better than one.

[75zhead]

I know this is a old thread but I have a idea.

Your fuel pressure is a hair low, volume maybe too?

Easy test is unhook fuel line after filter and run pump for 15 seconds in to a plastic QUART milk jug. Should fill it half way or more. I did this test and only got 25% fill. Half of what it should of been.

I was having same problem on my 75 z. Just got the car and it had sat for a long time. Well it had a new airtex fuel pump on it and the guy I bought it from said the tank had been cleaned 3 months earlier. Read bad reviews about airtex.

I knew it was running lean because of the intake pop. So I bought a MSD fuel pump to install.

Well when I took off the airtex guess what I found ?? a chunk of rust caught in the fuel pump nozzle. Obviously they changed the fuel pump BEFORE cleaning the tank!! New pump and rust dug out helped a lot. But as soon as it cools off enough I intend to remove and clean and reline the fuel tank. At the same time I will remove and clean fuel dampener and blow the lines while dampener is off (it acts as a flow stopper to tank).

I too cleaned every thing from fuel filter to injectors and thought I was safe, but on 35 year old cars, got to think fuel tank to injectors. Rust catches in the darnest places.

just a thought ...hope it helps

Great tip about cranking idle screw down to see if it dies, checking for vacuum leak.

love my Z

Rich

[cronoz78]

I guess Im a tad bit late to this party.

Cozye, I believe I have the same issue as you. I have pulling out my hair trying to figure it out. Im about to replace the injectors then try the fuel pump (which has been replaced but does look real old). Did you ever figure out the issue?

Thanks!

[Wade Nelson]

Would I be out of line to suggest it's time to hire a professional mechanic and get it fixed instead of jerking around on the Internet?