the_journeyman

Yes, I searched, and "dieseling" didn't really get me the relevant results.

1978 280Z, Mostly stock L28 & P54 head, only KNOWN upgrade is the Crane XR700 ingnition.

The carbs when I bought the car had a 95 fuel jet in the front throat (odd number cylinders) and a 105 in the rear throat (even numbered cylinders) in the Mikunis. This is NOT normal from research & an e-mail to Wolf Creek.

I recently installed 140 fuel jets, based in the Mikuni recommendations for the L28.

It takes throttle MUCH better without bogging on throttle up. However, it is slow to return to idle, and when I cut the the switch off, it diesels a bit.\

Pilot screws are at 1.75 turns OUT. Idle speed is just under 1000RPM. Going to 1.5 turns results in a momentary idle and dieing. Pilot jet is 57.5.

Is dieseling a rich or lean indicator?

Any help is appreciated!

Thanks!

JM

the_journeyman

After driving it, the 140 fuel jets seem a tad rich. It revs out and pulls well, but seems to have a little lag. The pilot to main hesitation is still there but greatly improved. I'm going to putz with the pilot circuit before i go around swapping fuel jets again.

JM

the_journeyman

Post-driving plug readings, these will be idle readings as I was rolling through the neighorhood before I checked 'em. I'll look into a Colortune. Maybe they're not expensive since I'm trying not to spend a fortune on this car.

1st Carb - Looks pretty good, light brownish.
2nd Carb - Looks rich, pretty black
3rd Carb - Looks pretty lean, ashy white

Do I need to re-sync the carbs? I just did that a couple months back.

Float level is good, I adjusted that a couple months ago when I did the sync.

Out of curiosity, before I left for work I went leaner on Carb 2's pilot screws and richer on Carb 3's pilot screws. My idea is that if it's STILL reading "OK, Rich, Lean" then I'm betting my sync is off (too much air entering 3rd, not enough entering 2nd, 1st ok. I did get a LOT of lean popping from one of the carbs when driving just off idle, even after the car was warmed up.

JM

the_journeyman

Haven't checked the valves yet, BUT I started checking the plus a little closer. The valves must be pretty good and not causing the different draws because each pair of plugs on each carb matched perfectly.

The 57.5 pilots must be lean because it finally idles, but I'm 2.25 turns out, AND the idle WILL NOT adjust below 1300RPM. I turn the idle adjustment until it is no longer touching the carb linkage. So, on to figuring that out. Plugs still read pretty lean is the funny thing.

After adjusting middle carb (reading quite a bit richer than carb 1 & 3) richer with the sync screw things are much better, smoother and the car pulls through the revs a lot cleaner.

Still have work to do, but it's a bit closer.

JM

hoov100

resynch the carbs, and don't forget you can always adjust them in 1/8'th and 16'th increments.

the_journeyman

Yea, I'm going to have to find another method. My vacuum gauge is driving me crazy.

JM

theramz

It will never run right or sync with different jets. The throttle linkage must be equal at closed position for all carbs. Close all mixture screws and then back them out 1 to 1 1/2 turns. Set the idle speed a little fast. With engine idling turn each screw in untill it starts to die, turn the screw out 1/4 turn from there. Do this to all mixture screws and repeat the process. If the carbs are in need of a rebuild you will still have problems. MSA has a good book for multi carbs.
Dieseling is from to lean or to far advanced.

the_journeyman

Advance is good, I can't remember the number now, but it was right where it was supposed to be for a stock motor. I have removed the mis-matched jets at this point and replaced all six fuel jets with 140s.

I was ready to roll the 280 out with a for sale sign Sunday evening. I had spent a good part of Sunday afternoon tuning, driving, checking plugs etc. About the time it seemed to be running right, I couldn't get the idle to drop below about 1300RPM. It was then I noticeed a $&%* bushing had backed out of part of the linkage allowing the main throttle rod to droop on the #1 carb end.

Replace bushing and start all over again. Damn.

Then I find one of the linkages is stripped. The piece of linkage that attaches to the long rod (don't really know the name of it, but it's the rod that all three carbs are connected to by the adjustable linkage) that controls all the carbs is stripped. So, that explains why the two cylinders on the middle carb ALWAYS read rich. The carb wasn't opening the same as the other two. There was about 3/16 of an inch of play on the middle carb linkage. So, when I hit the pedal, it wasn't opening as far as #1 and #2.

The good news: After taking all the adjustable linkages loose, backing out the throttle stop screw for the carbs and re-setting the linkages to the proper length, it started right up and idled smoother & the idle didn't hang when I let off the gas pedal.

I've still got some tuning to to, but now that everything is properly connected it should be easier. My next mission is to eliminate some of the play from the pedal to the main control rod.

JM

the_journeyman

I'm still struggling with the #2 carb reading rich on the idle mixture. EVERYTHING I do, it still reads rich. I try to adjust it out with the throttle sync and it just makes it diesel & idle rough. I'm currently a 1/4 turn richer on the pilot screws on the #1 and #3 carbs. It *almost* diesels when I cut it off.

It runs good on the main circuit, but feels lazy, but that may be because the 140 fuel jets are a bit rich for a stock motor.

JM

hoov100

Check for a vacuum leak.

the_journeyman

Thought I'd drop an update in here, just in case this thread can help some else along the way.

I could hear one carb drawing more at idle than the other two just standing by the engine bay. When driving, I had trouble with a lot of lean popping, especially when cold, but it was still randomly present once warmed up.

I used a dirt cheap & kinda redneck (I do live in the NC mountains ) method. I used a cardboard paper towel tube to help me balance the carbs.

First, I set all the carb sync/balance screws to the same number of turns. Then, I made sure the linkages where set where they matched the position of the butterfly lever and weren't putting any pressure, up or down, on the lever.

Second, I started the engine. It was evident one of the three carbs was drawing more than the other two. I could hear noticeable intake "gulps" from the middle carb (STILL GRRRRRRRRRR.... still dealing with that middle %^$& carb) and nothing from the other two. I could also hear two stronger exhaust pulses in a pattern from the exhaust pipe. Slightly touching the gas pedal showed a hesitation & the occasional lean pop in the intake manifold.

Now for the redneck magic I put the paper towel tube to my ear and leaned down and put the tube on each filter and listened. It was very obvious the center carb was drawing more than the front & rear carb. I used the center carb as a benchmark since it's the hardest to adjust because of the linkage issue. I compared the sounds went back and forth, listening and adjusting the throttle screw on the rear carb until the sounded the same. I repeated the process on with the front carb. Much smoother idle & throttle response was the result. Except, now my idle was too high, about 1300 RPM, and when I attempted to adjust it out, it just lifted the screw from the throttle lever and didn't lower the idle any.

To fix the high idle, I disconnected the linkages from all three carbs. Then, I adjusted the throttle adjustment screws out, equal number of turns until I had the idle down to about 800RPM. Then I went back and adjusted the individual linkages until they were not putting any pressure either direction on the throttle levers. After I put the linkages back on the carbs, I restarted the engine, got my tube back out and listened to each carb again. I had to make one tiny adjustment to the rear carb (barely a 1/16th of a turn) to get the intake vacuum hiss the same.

Butt-dyno results? Hesitation gone, once warmed up the lean popping is gone. Starts easily using the starter circuit, and good low-RPM throttle response!

Still to do - work out mains. It runs smooth on the current fuel jets, BUT is a bit lazy. It's currently got 140s, so I'm going to try 130s and see how it feels. Someone, somewhere, at some point mentioned the 140s were probably a bit large for a stock engine.

JM