Engine woes- read on if you dare

[madkaw]

I about forget we have this forum which is most appropriate for what I'm going thru.

 

It hasn't been a very good week and the Z doesn't want to run right.
I have actually been able to find time to work on it, which usually is a problem. But after hours of troubleshooting and working on the car I seem no closer to an answer.It's one of those troubleshooting nightmares that has caused me to walk away before I do something stupid----like give up Z-ing. Symptoms are erratic and only sometimes can I duplicate. I am scratching my head and loosing heart and motivation. I love my Z, but it hasn't been fun lately just working on it while we have had the best weather all year.

I will try and describe my issues-though it will sound vague; I have been around the world and back so my descriptions might seem lucid.

The engine sounds like it has a miss and won't idle as smoothly as it used to. It feels down on power. Laboring might be a good description of the idle.
At first-- cylinders 4-6 seemed to be the culprit. Plugs looked dark and #4 was the worse. But how does half a mikuni run rich? I also had number look dark, but for the most part, everytime I checked 1-3 looked lean or no color.
Now most of this has been idle time in the garage, but those general plug reading were after a drive also. I have seen the plugs 4-6 look both clean and dark during all this troubleshooting, but #4 has been the most consistantly fouled.  
  My AFR's bounce around quite a bit at first . I have dual exhausts so I only have the O2 sensor in one side or another. At first the sensor was in cylinders 1-3 and the reading were not bad, but erratic, not stable at idle. it would run at 14 and start a bouncing routine and then look lean. I switched the o2 sensor to the rear bank and it was pig rich-like 10.5-11 at idle-hence the dark plugs. I had to turn the pilot screws on 4,5,6, in a 1/2 turn just to get the AFR's to 13.5.
Keep in mind that my jetting is at a known good setting that has worked great before. I've ran these pilots jets and screw settings with no issues before. I have checked floats and they are spot on. I have pulled the covers after running and they are all at the same level. All my pilot screws were replaced and all the tips are in excellent shape.
I suspected ignition or my Megasquirt because of the randomness of the issue. But I replaced all components with a 280zx dizzy and wires and no change. Replaced the plugs too. Ignition timing checked with the dizzy and the megasquirt.
I checked all valves and adjusted as necessary-nothing to crazy.
Vacuum guage only show.ed 12HG at idle taken from a port on my vacuum log. I thought that a bit low.
Sprayed everything down with started fluid and could find nothing.
I removed the complete vacuum log assembly and capped off all port openings. So there is nothing hooked to the intake.
Checked torque on intake bolts and noticed one exhaust flange nut very loose at the very end of the header.
I hooked up a coolant pressure tester to see if pressure was building anywhere --- as in a bad HG. I set the tester at 13psi, the same as the cap and pressure rose slightly to 15psi and stayed. No bubbles in radiator and level seems to be the same. Nothing in the oil.
Compression test done
1-205
2-205
3-205
4-215
5-205
6-210
Not sure what to make of those readings, but cylinder 4 jumps out at me a little. I'm pretty sure these are within percentage limits of each other.

So I proceed to switching out carbs. I swap the 3&4 for 1&2. I run the car and bingo, #2 is now fouled-I found it. #2 would have been #4 cylinder so it must be one side of my carb. So I switch out pilots, and e-tube(which should have much to do with idle). Now #2 isn't fouling, but the engine still idle like sh!t. I should say that I checked all the O-rings when I had ALL the carbs off.  
This is what i meant about nothing being conclusive or concrete. I have run and cleaned plugs several times and nothing seems very consistant.

I had to walk away after about 10-12 hours in the last two days. Not sure if a bad intake gasket would cause such issues, or a avccum leak in general. At first my gut said HG, and I'm not totally convinced it's not, but no definite eveidence to warrant pulling the head off.

Valve timing? Well it's not so easy for me to determine this. I have an adjustable sprocket and I degreed this cam in many moons ago. I will have to really ponder on how I will check my valve timing. I have looked down in the hole to see if my chain adjuster popped out or something, but didn't see anything alarming. Could the chain jump a tooth? Unlikely if not impossible, but there is no quick way for me to determine this.

My gut says HG or Valve timing(ignition is safely eliminated), but don't want to go on a gut feeling.
Give me some ideas guys         

 

[JMortensen]

I have to admit I didn't read the whole thing, but a Mikuni can run rich on 1/2 the carb if the throttle shaft gets twisted. Basically one bore is closed and the other is open a bit. I think you can usually find this because the balance tool will show a difference from one barrel to the other. Friend of mine had this problem and fixed it by putting wrenches on the flats at the end of the shaft and tweaking it. Want to say he had to take the shaft out, but it's been 15+ years, can't remember exactly.

[madkaw]

Trying to picture how I twist the shaft-slamming the throttle hard?  I have noticed a difference between two barrels, didn't think it was significant enough and thought I worked it out by synchronizing. Honestly didn't know to check for that-but I will. I need a better syncro tool. I have the ole thumbwheel and bubble.  I have had these running very smooth before so I was doing something right-lol.

[Leon]

I doubt it's valve timing or head gasket. Your compression test and coolant pressure test eliminates the HG. Valve timing would affect all cylinders. Have you tried a power balance test, disabling one cylinder at a time and noting RPM drop? That may give you clues as to the offending cylinder and if it follows the carb. To me, it sounds like you've narrowed it down to the carb being the issue since the plug fouling moved when you moved the carb. Twisted throttle shaft also came to my mind, as well as some other mechanical issue with the carb, e.g. leaky starter circuit, leaking accel pump, etc.

[madkaw]

Well I went out to the garage last night to curse my car and I stared at the linkage and thought of what Jon suggested. I just stared at the linkage as I flicked the throttle like a sad kid. I then noticed the rod movements and how they weren't working in sync-AT ALL. So how did I not see this. I then noticed that the ball was slopping around in one of the sockets-that ain't right. The ball end had gotten oversize through wear  guess.

The more I investigated the more I thought I had something-and I may, but I ran out of steam to finish up. It's obvious now that I had the main linkage/rod in a bind since the secondary adjustment rods were not right.

At least I found something definitive and  will roll with it if I have time today

[Tony D]

"But how does half a mikuni run rich?"

 

The jets come loose and screw themselves out...or fall out the bottom.

 

Twisted Shaft will cause synch issues. They have a penchant for that when the idle stop is backed out too far and overzealous throttle application is done---the carbs  slam shut, then you slam them back open...but the throttle plates are actually cut on an angle to close very tightly on the bore and they can stick. Upon opening, one will stick, the other move freely, and at the edge of the slot for the plate the shaft torsion has a peak and it twists. I can't count how many twisted shafts I've seen! Very common.

 

Those are my two top guesses on quick perusal of the OP. Let's see what everybody else said now...

[Tony D]

Another thing that gave me pause was the thought that you adjust your idle screws by AFR. 

 

I am ready to curse the readily available AFR Gauges as a bane on automobile DIY'ers. 

 

Adjust your idle according to the Mikuni Specs, not the AFR and some magic number. When adjusted properly, see where your AFR resides. It's a secondary check, not a primary. Adjusting carbs based on it... is wrong and I think you are causing more problems than you are solving doing this.

 

Instrumentation can be great if you know what you're looking at, but it can run you down the wrong path if you aren't thinking about what you are seeing.

I watched JeffP redo his entire fueling map several times as for some reason it kept getting lean and lean and lean. As shown by the AFR on the Dyno and his WBO2.

 

Then we would come back and it was pig rich and he was tearing his hair out. Put in the original program and it was fine again. Then why the change? Run on the dyno and it goes lean again.

 

Repeat for better part of a day.

 

Then realize, when he added 5 gallons of C16 it went pig rich. As he made pulls, it got hot and went lean because there was so little fuel in the tank. The Third set of Eyes at the Dyno picked up on that, and life was good afterwards.

 

Don't get blinders (looks like you found some stuff you missed...so it's that intent concentration that leads to overlooking things, or chasing symptoms...)

 

When in doubt, go back to Basics. Make SURE you have followed the intitial setup properly. When I come in to a guys place, and start, that's what I do. It confounds them sometimes...but generally when I'm done with setup, there are just a few really basic issues needing to be addressed, and the car is running considerably better when I arrived. 

Never skip the basics, and when in doubt, throw out your ego and like a noob reading it for the first time take out the book...and go step-by-step, one-by-one.

[madkaw]

I have tried to stay systematic and start with the basics on this endevour. I have learned my lessons from my past. I guess the arbitrary symptoms is what gets me . One time #2 is fouling, then I try to duplicate or verify and #2  looks good. Then I did more driving or running and #2 looked fouled again. 

As far as AFR's, I am kind of using those numbers as a guide line right now. I had found a tune that was pretty consistant with a certain set of jets installed. In this way I know that my engine shouldn't being running in the 12's afr when it used to be in the 14's. There was never that huge of swings even with weather changes.

I guess I need to research a good way to measure the butterflies to make sure I haven't twisted something.

I have a buddy that has some SU's sitting around, I might see if he will let me borrow them so I can throw them on there to see what happens.

[RebekahsZ]

Don't hate your Z. I tried for 5 years to do the triple carb thing against all my friends advice against them-but they look so cool! I could never get all my carbs to run well, and I could never get both throats on each carb to flow exactly the same (I tried EVERYTHING). It took a super cold winter to make me throw in the towel. Don't hate the whole car, but think about joining the LS club. We wil welcome you with open arms.

[madkaw]

I just had to step away for a while. I had these carbs running very well for some time. What complicates things is I have other engine issues.

Not ready for the dark side yet. I love my L6 motor even thru this. I still want to try many other options with my L6 , ie- FI and turbo FI with my Megasquirt.

What hurts is I have tried not to cut corners and I still have had issues, but that's part of the hobby I guess.

[Mr.Twitty]

I totally understand your frustration with your Z. I am tempted to sale mine.I have been fighting with idling issues myself on my turbo z. I have replaced fuel filter(twice),New fuel pump,fuel pressure regulator,every vacuum lines and fuel line replaced,New pcv,air filter,timing. adjustment,New plugs and wire,New oxygen sensor,New MAF and Damn near an entire summer raking my brain. It's just sickening the money and time. Rebekahsz comment got me thinking about crossing over to the dark side(LS club).. I would like to go find out how hard the swap would be

[inline6]

You found that your linkage is messed up - potentially worn ball sockets...  That can definitely cause driveability issues.  Do you have throttle plate return springs on each carb?  If not, then the issue might be exasperated because you may think that the throttle plates across carbs are in sync when they are not.  Out of sync carbs will definitely cause poor running.   

 

If there is slop in the ball joints and especially if you don't have return springs on each carb, then at "off" throttle, each carb's throttle plates may be at different positions.  If twisted shafts, then even the plates within each carb body can be out of sync and at different positions.  If that is the case, setting pilots is going to futile.  And, what you have done with them so far could be contributing to poor running problem.  

 

I think I'd start by removing the throttle linkage rods from each carb and checking them out closely.  Perhaps you need new ball joints...   

 

You need to verify that all the throttle plates on the carbs are closing completely, especially those within each carb body. Back off all idle/throttle screws that open the throttle plates for idle.  With the throttle linkage arms (those with the ball joints) removed... close the throttle shafts and listen for the plates to snap shut in the bores.  If carbs are off the car, you can shine a light on the back of the throttle plates and verify the plates are closing completely.  Address twisted shaft problem if it exists. 

 

After verifying that the shafts are not twisted (or correcting), put the carbs back on the manifold - carefully so that they are level as possible and as close to the same horizontal plane as possible.  The bolts/studs probably have a bit of play in them, and the slight misalignment on the manifold causes minor syncing issues with the linkage - not a big deal, but while you are there... take a minute or two.

 

Install throttle return springs for each carb.

 

Set initial setting of idle/throttle screws.  I cut a strip of paper... and put it between the screw and where the screw hits the arm of the throttle shaft.  I turn the throttle screw in until when I pull on the paper I can just feel the paper drag.  I do this for each carb.  This way I know that the idle screws the same distance from contacting.  When all three are set that way, turn each of the 3 screws the number of initial turns called for in the manual - to set initial idle setting.  For my Mikunis, I think it is one turn or maybe one and a half.  

 

Adjust/set all three throttle arms (ones with the ball joints) to the same length.  I used calipers to set mine to within a few thousandths.  Having them the same length is important.  Loosen linkage for pivot balls on the main throttle linkage rod.  Install adjustable throttle rods.  Be sure that the throttle plates for all three carbs are shut and re-tighten the linkage for the pivot balls being sure to keep carb throttle linkages undisturbed. This will ensure that each linkage works in unison as the main throttle linkage rod operated.

 

Set all of your pilot screws at the manual spec. as well.  For my Mikunis - it is 1.5.  

 

At this point, the car should start without issue. You should then put the sync tool on at idle and adjust the throttle screws (hopefully the same amount) until desired idle speed is achieved.  You can deviate in throttle screw setting across carbs if necessary to get the flow through the carb bores from one body to the next to be the same - however they shouldn't require substantially different settings.

 

After the flow is the same across the three and the idle speed is where you want it, check to see how the flow across the three is with rpm's held at like 2500 or so.  It should be equal across all three.

 

Now you can take a look at AFR between the two banks to see where you are... There really should be no need to run the pilot screws at different amounts out... at least not substantially different.  

 

Hopefully getting everything synced up like this will solve your problem.

[ryant67]

^inline6:  If only the Mikuni manual could have stated the procedure as clearly and completely as you just did, wow!  Thanks for taking the time to share - I'm pretty new to playing around with these carbs, and there is a lot of information out there that presumes a level of competency that a beginner just doesn't have.  You spelled everything out in layman's terms, which is refreshing.  

Edited August 11, 2014 by ryant67

[madkaw]

Inline6---I have actually done many of the things you have mentioned. From the paper feeler gauge to the caliper measurements on the linkage rods. I am confident that the rod linkage is properly set-up. The one worn ball socket is not that bad and  have adjusted to where the throttle plates all open at the same time. I am confident that the carb return springs are intact and work well. As far as throttle plates being the same on the carb, I am not as confident  because i haven't come up with a best way to measure besides eyeball with a light. To the naked eye the bores are the same. I would think it would be noticable to the naked eye if this was actually causing the cylinder to almost foul out. I have taken the carbs apart at least twice and blown them out. As far as pilot adjustments, they have always been adjusted out the same amount of turns with good results. These carbs ran great!!

 

I have since removed the intake and could find nothing with the gasket. What I didn't like was what I saw inside the combustion chamber from my view thru the ports. Looks like oily grunge sprayed against the side of the chamber. Not heavy, but a pattern on certain cylinders that make me wonder what is causing this. Is this oil from the piston side or the valve side? My plugs don't indicate a oil foul, it cleans off too easy-like carbon.

I know I have issues with my rockers being improperly machined and this manifests itself with uneven pressure on the top of my valves(lash pads). Does this have any play in this, I don't know. At this point I am about ready to pull the head and get this issue fixed. I know I have an issue with these rockers, but what other issues will it cause. I need to remedy this because it will only get worse and cause damage that might be fixable. I need to make sure my guides haven't been damaged and my cam also. I know it sounds like desperation, but these tangent issues complicate troubleshooting with the engine. 

This year is slowly becoming a wash with my Z.     

[inline6]

Didn't know your expertise level with the carbs...  Looks like what I said may help someone else which is good.  

 

You said earlier that you have no definite evidence for taking the head off...  And swapping carbs, you fouled number 2 cylinder instead of 4.  And you fixed that issue.  

 

Have you driven it on the road at all after fixing that issue?  Fouled plugs can sometimes be hard to "un-foul".  Just idling in the driveway may very well not do it.  Do you have a glass bead cabinet to clean the plugs with, or some known good plugs laying around to swap with?

[madkaw]

If I had an issue with a plug that looked bad, i would change it with another cylinder that was firing well. I used a propane torch when I did clean up the plugs. Somewhere in there i just bought new ones to help eliminate a plug issue. I did drive the car here and there and it didn't run 'like it used to' . The frustration is the arbitrary nature of the issue. Yes swapping out carbs moved the issue, and then I think I have it fixed, then later on the same cylinder fouls again.

I wouldn't be pulling the head for this but I have this other valve issue that needs to be addressed. Hell-I still have an oil leak from the rear main issue that I have never solved-it's almost embarassing. I will end up pulling the whole motor and putting it on a stand to fix everything.     

[madkaw]

Well the engine is coming out and coming apart. I will update what I find. It is time to fix this cam/rocker geometry issue and hopefully fix everything else too.

[madkaw]

Engine is apart and I do have some issues. The head is at the machinist and looks like the bad rockers took their toll on my valves and guides. I will be replacing guides and valves and touching up the seats. The cam is at Schneider for evaluation. The rockers are on the way back from Delta with a fresh regrind. 

You can see from the pics that there is a lot of junk on the back of the valves(intake). Lots of soot on the top of the pistons.

Right now I am in the process of cleaning everything up to put back together for when the head is done.

If the cam needs to be reground, then I will get more aggressive with the grind-why not-lol

I will probably bump up CR a bit too.

 

The engine builder questioned my PCV system. He worried that I was pulling too much crud into the intake. My PCV was hooked to my vacuum log. 

 

Any input?

 

[NewZed]

The head is at the machinist and looks like the bad rockers took their toll on my valves and guides.

How does a bad rocker affect a valve and guide?  Side loading?  How would that happen?  Just wondering.  And did Delta do the first regrind on the rocker arms?  I thought that they had issues?

 

My impression on carbon buildup was that it comes mostly from combustion gases and fuel mix flowing over the back of the intake valve at certain times during engine operation.  For example, when the throttle is closed the piston can exert a vacuum but there's no air to be pulled in.  Flow stops, gases back-flow, condensation on the cold valve occurs.  Same reason crud gets in the intake runners and all the way up to the throttle body.  Not so much what's coming down the intake runner.  No idea how to stop, except spend more time with the throttle open.

[madkaw]

Side loading it is. The lash pads showed evidence of this with wear marks on just half the bottom of the pad. It increased the tolerance between valve and guide. The rockers were supposedly new from Nissan included in the Schneider cam kits offered years ago. Delta cams rockers will be checked as well as anyone who does my rockers.

 

I'm all for more wide open throttle:)