the_journeyman

I searched, but either I can't search good or it's not been covered

Ok, so I've got my 1978 280 in the garage. It starts easily & idles smooth, runs pretty good above 3K with a really nice growl from the intake

However. From a stop, a good, hard take off with lots of throttle results in the motor choking for a second, then picking up. It won't even break the rear wheels loose, and I'm certain it should be able to.

Are there any baseline settings for carb jets & the various fuel and air screws when used on a stocker? Only *known* change is the Crane XR700 ignition & Taylor plug wires.

Plugs are pretty black too. 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.

I've got a couple carbed bikes, and I KNOW carbs can run better than what these are.

Where to start?

Thanks!

evandubya

?!? Carbs on a 280?!?

hoov100

You need to lean out the base idle mixture and you probably need to adjust the accelerator pumps as well, also you probably need to go through them if it's been more then a year and a half.

70's EFI is dogshit, I would rather tune tripples by ear then deal with 70's EFI.

the_journeyman

Yup, there's this bag of wiring wadded/wrapped into a plastic bag zip-tied against the fender I'm assuming is the old EFI stuff.

EDITED to add: Also, FI doesn't give you that nasty little growl triples with hi-flow filters gives you either!

I've located a PDF of the PHH series service manual, so I'll refer to that for cleaning. I just picked it up, so I have NO IDEA what was last done or when.

I've got a little experience turing Mikuni motorcycle carbs, but these are a bit different! I know enough to plug read and such and that's about it. Off to the garage.

Thanks for the input!

JM

the_journeyman

AFAIK this is a pretty stock L28. Previous owner had no knowledge of any previous mods.
Current Jetting:

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 some unverified factory settings for a stock L28

Main Air - 175
Main - 150
Idle (pilot) 50
Accel Pump - 45

JM

evandubya

Okay, makes sense.

Six_Shooter

Upgrading to newer EFI is pretty easy though.

the_journeyman

Ok, as I research more and more, I a link to Mikuni's site AND found a recommendation for jetting with a stock engine (which I think mine is) 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

the_journeyman

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 (3-4.5K) 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/spe******ts make any sense of my posting?

Thanks

JM

Six_Shooter

Brown-grey is basically right on the money. A white plug would indicate lean.

the_journeyman

I'm thinking that after the short drive they went from black to brown-gray, they my be white by the time I get them home today. I'm going to try to keep the RPMs & my foot in it to see what color the mains give. I was idling in on the pilot circuit through the neighborhood, so I'm thinking they're set about right, even though I still need to make the adjustment to pilot size to run at 1-1.5 turns out on the idle.

JM

the_journeyman

Revival with more questions!!! While I'm working out some the fuel jets, I'm still tinkering.

Lean, Rich, Just Right (working on pilot circuit) 57.5 Pilots, 2.75 turns out starting, fresh plugs.

Carb #1 - snow white plugs (lean)
Carb #2 - Black, sooty (rich)
Carb #3 - Light brownish (about right)

I adjusted the idle (pilot) screw for each carb.

Carb 1 - 1/4 turn richer
Carb 2 - 1/4 turn leaner
Carb 3 - No change

Do I need to be looking somewhere else instead of monkeying with pilot screws? Should I check on float levels? I did check for intake & vacuum leaks with Ether and found none.

Thanks for the input!

JM