New 1978 280Z, Triple Mikunis - Update after changes!!!

[the_journeyman]

Picked this car up, no knowledge of any motor mods other than Crane XR700 ignition module & Taylor plug wires.

 

Plugs are black and a bit wet, which indicates running pretty rich. Heavy throttle application results in a really bad bog before it starts to accelerate.

 

Of note - fuel tank is really pressurized after running for a bit, it goes PSHSHSHHH when you crack the cap open, and it's pressure coming out. According to the gauge mounted under the hood, it shows about 5lbs of pressure at idle. There is a on/off valve in the return line. In off position, it reads 5-5.5psi, as you open the valve in the return line, the pressure drops. Researching on here, I found that 3psi was preferred for the Mikuni setup. I partially opened the valve, and got the gauge to read about 3psi. It ran a little better, and didn't pressurize the tank as bad. Didn't solve the problem though

 

I've got a couple carbed bikes, and I KNOW carbs can run better than what these are. I've even tuned Mikunis on bikes before, but this is new territory.

 

Now, the current carb settings:

 

Carb 1

 

Main Air1 - 200, Main Air2 - 200

Main1 - 95, Main2 - 105

Pilot1 - 57.5, Pilot2 - 57.5

 

Carb 2

 

Main Air1 - 200, Main Air2 - 200

Main1 - 95, Main2 - 105

Pilot1 - 57.5, Pilot2 - 57.5

 

Carb 3

 

Main Air1 - 200, Main Air2 - 200

Main1 - 95, Main2 - 105

Pilot1 - 57.5, Pilot2 - 57.5

 

I have located something that says 'factory' settings for the PHH carbs.

 

Main Air - 175

Main - 150

Idle (pilot) 50

Accel Pump - 45

 

Where to start?

 

Thanks for any help!

 

JM

Edited September 15, 2011 by the_journeyman

[Leon]

Do you have mismatched main jets in each carb? How did you determine that your tank pressure is above atmospheric?

[the_journeyman]

Do you have mismatched main jets in each carb?

 

Yes, each carb has a 95 main on one side and a 105 on the other. From the front of the motor, the odd numbered cylinders are fed with 95s and the even numbered ones are fed with a 105. Plugs are black, pilot screws were mis-matched as well with no pattern do the number of turns. There were as few as 2 turns out, and as many as 4. All are currently at 2.75 turns out.

 

 

How did you determine that your tank pressure is above atmospheric?

 

When I open the gas cap it blows out pressurized air. The fuel pump clicks rather loudly too, don't know if that is normal, but it makes plenty of pressure.

 

JM

[the_journeyman]

Ok, as I research more and more, I a link to Mikuni's site AND found a recommendation for stock settings with the chart.

 

Chart

 

So, it looks I've got to figure out if I've got 40s or 44s.

 

My question NOW since Mikuni recommends a 140 main for a stock L28, what would lead someone to:

 

1) Run a much smaller jet (95 or 105)

 

and

 

2) Mis-match jets in each carb with a 95 in side and a 105 in the other.

 

I'm totally new to Zs, so I need a little help understanding them.

 

Thanks for any help folks!

 

JM

[Leon]

Ok, as I research more and more, I a link to Mikuni's site AND found a recommendation for stock settings with the chart.

 

Chart

 

So, it looks I've got to figure out if I've got 40s or 44s.

 

My question NOW since Mikuni recommends a 140 main for a stock L28, what would lead someone to:

 

1) Run a much smaller jet (95 or 105)

 

and

 

2) Mis-match jets in each carb with a 95 in side and a 105 in the other.

 

I'm totally new to Zs, so I need a little help understanding them.

 

Thanks for any help folks!

 

JM

 

Who knows, maybe someone thought they were progressive, and not synchronous carbs... Seems like whoever set them up had absolutely no idea what they were doing.

 

You need to start over with these carbs, and a wideband O2 sensor goes a long way in telling you what's really going on. I run Webers so I can't give you numbers, but 130-140 mains will definitely work better than mismatched 95/105 mains.

 

As far as tank pressure, I just don't see how it can be pressurized with the car running unless you somehow have more fuel coming back than leaving (if that is the case, tell me your secret). It's likely to drop in pressure without proper tank venting. I would put a string over the lip of the filler as I remove the cap to see which way the air is going. I bet it's drawing air in, sound can be deceiving.

[the_journeyman]

Seems like whoever set them up had absolutely no idea what they were doing.

 

That's what I thought too, I just needed someone more knowledgeable to verify that was unusual. I'm going to try to locate an assortment of jets around the suggested factory settings and see what happens. Mikuni recommends just one turn out on the idle/pilot screw, but the car won't idle. That may be a result of the small mains, IIRC they have a trickle-down effect even if you're not on the main circuit. That may explain why the pilot jet is also so big (57.5 instead of 50) Too bad my jets from the bike carbs won't fit!

 

As far as tank pressure, I just don't see how it can be pressurized with the car running

 

Neither do I, but it is definitely blowing out, you feel it blast outward past your hand when you open the filler cap. It's opposite anything I've ever experienced. It does it both with the return line valve open or closed, a bit less when closed.

[the_journeyman]

Update after some fuel pressure changes:

 

So, after dropping the fuel pressure to 2.5-3.0psi and driving the Z to work for ONE day (7.5 miles round trip) and keeping the RPMs up to help pull more fuel & cleaner through, my plugs are a brown-gray color. I had made a 35 mile trip with the fuel pressure running a 5.0-5.5psi to bring it home. The plugs were black, even after that trip. I'm sure the fuel system & carb cleaner could contribute to a lean mix, BUT this is a pretty drastic change considering 35 miles at freeway speeds didn't make ANY change in the appearance of the plugs, but a 7.5 mile trip of mixed driving post-FP adjustment made the plugs go from black to brown-gray.

 

This tells me I am pretty lean, which is expected if I am running 95 & 105 mains. Another day or two of driving is in order to see what happens. I'll keep watch on the plugs & listen for ping/knock from the lean mixture. I think I am starting to have a good idea of why this car didn't run quite right.

 

This possibly confims my theory that someone didn't understand the carbs were getting too much fuel pressure. They likely kept going leaner and leaner on the mains thinking that was the problem.

 

Can any carb pros/specialists make any sense of my posting?

 

Thanks!

 

JM