Motorcycle Carb Jets?

[the_journeyman]

Currently, my Mikuni PHH carbs are set up very strangely. Each carb as a 105 in one throat, and a 95 in the other. Not sure why they were set up this way, they're very lean when running on the fuel jet (SNOW WHITE plugs, very short on power. Stomp pedal at 4K RPM, makes a lot of noise but doesn't accelerate quickly) so I need to go with a larger main.

 

Mikuni recommended settings are 140 for the fuel jet.

 

So, out of curiosity, I took a pilot & fuel jet into my local motorcycle dealer. Turns out, my mains are the same as a common Mikuni motorcycle jet. Has anybody tried this before? The shop didn't have any 140s in stock, but he ordered in a few for me.

 

I'll post results once I get the 140s installed.

 

JM

[the_journeyman]

Better and worse, HELP!!!

 

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

[beermanpete]

It sounds like the idle is way too lean. A hanging idle is a symptom if a lean idle mixture. You probably need larger idle jets, or perhaps smaller idle air bleeds.

[the_journeyman]

It sounds like the idle is way too lean. A hanging idle is a symptom if a lean idle mixture. You probably need larger idle jets, or perhaps smaller idle air bleeds.

 

I think on the Mikunis the air corrector only affects the main circuit, but I'll check the manual to be sure. It's currently at 57.5 on the pilot, I'll look around and see if I can get my hands on a couple larger size pilots. Idle mix has been VERY touchy (a tiny adjustment makes the plugs go from lean to quite rich based on readings) so a larger jet might make sense and no be so hard to adjust.

 

Thanks!

 

JM

[the_journeyman]

Post-driving plug readings:

 

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

[beermanpete]

If you want to get everything "perfect" you should sync the carbs last. Set the valves, then sync the carbs, then set the idle mix., then sync again.

 

How much the idle mixture setting will have on the plug readings is dependant upon how you drive the car. At large throttle openings the idle circuit does not contribute much, if anything, to the air/fuel charge. Make sure the float level is the same on all 3 carbs. Make sure the air filtes are all clean and do not have differnt flow restrictions. If your engine is tired and pulling oil on a few cykiders it could be interfering with your plug readings.

 

How are you setting the carb sync? Unisyn? Mercury column?

 

Since you found different size main jets have you verified the air correction jets and emulsion tubes are all the same?

[the_journeyman]

How are you setting the carb sync? Unisyn? Mercury column?

 

Since you found different size main jets have you verified the air correction jets and emulsion tubes are all the same?

 

I'm running 200 air on all six. I didn't actually check the stamp on the emulsion tubes, but they looked the same.

 

All I've got is a vacuum gauge. Seems the middle carb was WAY off, so my vacuum gauge may not be very effective.

 

After driving a bit and having trouble idling, here's my latest work.

 

Haven't checked the valves yet, BUT I started checking the plugs 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

[the_journeyman]

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

[beermanpete]

A regular vacuum gauge is ok. It is just slow since you have to keep moving it around. If you hvae not already done so, add a restricter the vacuum hose to dampen the needle bounce.

 

Setting the vavles first is a good idea. Any changes to the valve setting will affect the air flow and therefor the carb sync. Besides, it is an important tune-up proceedure. Tight valves cause a rough idle and might cause them to burn out.

 

It sounds like you might have some issue with the throttle linkage if you cannot get the idle lower than 1300 RPM. When the idle screw is backed out all the way is there any play in the linkages to each carb? Are there throttle stop screws on each carb? If so, how are they set?

 

Also, make sure the ignition timing is not too far advanced at idle, this can cause the idle to be too fast.

[the_journeyman]

It sounds like you might have some issue with the throttle linkage if you cannot get the idle lower than 1300 RPM. When the idle screw is backed out all the way is there any play in the linkages to each carb? Are there throttle stop screws on each carb? If so, how are they set?

 

Also, make sure the ignition timing is not too far advanced at idle, this can cause the idle to be too fast.

 

Timing is ok, IIRC it was 10º at idle. The 1300RPM idle was fixed after I replaced the bushing, I can properly adjust the idle now. I'll check in on the valves before I do a final sync, but I'm going to make sure I don't have any other issues like the two linkage issues I found. It ran better this morning than it has been. Funny thing is, last night I had just slapped the linkage back together and hadn't even tested it. I jumped in it this morning and it started right up. I had to set the idle speed because the screw was still backed out, but once I got it set it ran good.

 

The idle hangs out around 800-900, but continually drifts up just past 1,000 then settles back to 800-900RPM setting. It kind of slowly "hunts" between 800 and 1000RPM.

 

I'm going to check the plugs when I get home and see if I still have the carbs are still Lean, Rich, Lean since I fixed the stripped linkage on the middle (rich) carb.

[the_journeyman]

I'm still struggling with the #2 carb reading rich on the idle mixture. EVERYTHING I do, it still reads rich, identically rich on BOTH plugs. 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. The plugs on the cylinders fed by #1 and #3 are quite lean looking, and again they're reading the same on both cylinders for each carb.

 

I've not checked plugs since going the 1/4 turn on #1 and #3. That'll come when I get home. I was just thinking that the pilot screws should be closer than that.

 

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

[Leon]

I assume you haven't even checked valve lash yet, which is supposed to be done before any carb tuning. Set your valves first, or else you're just chasing your tail.

Edited December 21, 2011 by Leon

[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