Rough when driven at constant speed

[Vorticity]

My car is a 1973 240Z with 3 screw round top carbs, and a Crane Fireball XR 700 ignition (optical pickup).  All emissions control stuff was removed/plugged by the PO.  I haven't driven it much over the past 1.5 years, for various reasons.  One reason was it randomly died, then just as randomly would restart and run fine.  I traced that to a float valve that randomly stuck closed.  Solved that problem, then decided I should give it a tune-up.  Replaced cap, rotor and plugs, checked valve adjustment, and set the timing.  Then balanced the carbs using a unisyn (badly out of balance) and set the mixture as best that I could using a Colortune.   The distributor's vacuum and mechanical advances all work, and the timing is set at around 7 when idling at 800 RPM, with it going to around 35 at about 3000 rpm.  It's still not running right.  It's:

1)  Difficult first start of day.  After a lot of cranking, it will idle rough, die and backfire through the carbs, even with the choke on and the ambient air temp being 80 degrees. Once the temp gauge starts to move, it settles down to a nice steady idle.  Will start very easily when it's been driven in the past few hours, though.  

2) Will sometimes run-on (diesel) upon shut-down when the engine is very warmed up.  Setting the timing so low at idle did help this, as did setting the idle lower than the 1000 rpm it was originally at.

3) Will feel like it is missing when driven at a constant speed, especially if going slightly uphill.  Seems to happen most around 2500 rpm when going 45-50 mph.  If you put your foot into it, it'll accelerate just fine.  In fact, it accelerates better than ever now. 

Some thoughts:

1) I really couldn't get the mixture setting to where the Colortune looks blue.  It's more orange than it probably should be, but turning the jets up will lean it out too much, causing rough idle and popping through the carb before the Colortune will turns blue.  In the end, I just left the jets turned down 2.5 turns, which makes the color tune look the most blue without making it idle rough.  

2) I checked that the optical pickup on the Crane is right near the number one wire on the distributor cap, and have played with moving it a bit.  It'll run horrible if I move it forwards or backwards, so I think it's correctly placed.  

3) Carbs have 10w20-50 oil in them.  Blipping the throttle makes the Colortune go blue for a bit then it'll settle back to slightly orange.  However, if I had to guess, the roughness at constant speed at moderate RPM is due to flutter in the carb piston(s).  

4) I have checked for vacuum leaks by spraying carb cleaner around possible leaking spots, like the throttle shaft bushings.  No leaks.  

5)  The coil gets quite hot to touch after running for a while.  The ballast resistor is wired correctly.  I'd suspect the coil or the Crane Fireball, but why would it accelerate smoothly after roughness/missing at constant speed?  

6) To me, it the exhaust has always smelled like it runs rich, and the plugs look slightly sooty, especially the plugs not directly across from each carb.  In other words, plug 2 and 5 look fine, but the others look slightly sooty.  My perception of running rich was why I bought the Colortune, and that thing confirms that it is slightly rich, but I can't lean it out or it idles badly.  Maybe stinky exhaust and slightly sooty plugs are normal on these things?  In any case, I don't think the richness causes the main symptoms.   

To recap, main symptoms are difficulty during the first start and warmup of the day, run-on when shutdown after long drives, missing/stuttering a bit at constant speed.  Any thoughts/ideas are appreciated.  Maybe it'll trigger something.  

Matt

 

 

 

[Mark Maras]

 1) "Difficult first start of the day" is not enough fuel. Misadjusted choke cables are often the reason. Have you checked the float levels. They are the primary mixture adjustments.

[heyitsrama]

+1 on the float levels

[Patcon]

The oil in the carbs seems really heavy

[7tooZ]

I checked that the optical pickup on the Crane is right near the number one wire on the distributor cap, and have played with moving it a bit.  It'll run horrible if I move it forwards or backwards, so I think it's correctly placed.  
 

Matt when you move the Crane optical pickup did you reset your timing? 
I think the oil is on the thick side. I use Mystery Oil for a setup almost identical to yours.

I always start a tune up with a valve adjustment, and new plugs.

when you says the coil is hot, is it like to hot to hold your hand on?

 

[Vorticity]

On 8/19/2020 at 5:22 PM, Mark Maras said:

 1) "Difficult first start of the day" is not enough fuel. Misadjusted choke cables are often the reason. Have you checked the float levels. They are the primary mixture adjustments.

The choke cables were the problem with the difficult first start.  The PO had installed the round tops, and also put a mounting plate under the center console to strengthen the choke lever attachment.  However, the mounting plate was actually catching on the screw holding the choke lever, keeping the lever from moving more than half way.  I don't think it would ever go all the way forward, either.  I was told that this was just the way it was when switching to the round tops while using the flat top choke cables and lever.  A little playing around with the nozzles showed me that they never went all the way down when the choke was on.  In fact, they barely moved.  Basically all the choke was doing was moving the throttle plate a bit.  I had to take apart the center console and trim the mounting plate for clearance.  I've now got full range of motion on the chokes and the cables are set right.

As for float levels, I set them using some clear tubes attached to the drain holes.  Set them so that the fuel is 23 mm below the top of the float bowl with the car running.  Is that right?  Maybe it's too low, because now that I can actually use the chokes, I discovered that pulling the chokes on just a little will cure the the feeling of missing or stuttering at constant throttle between say 1800-2500 RPM.  Noticing this, I turned down the nozzles a half turn (now at 3 full turns down), and it runs perfect.  Maybe I'll get the colortune out and see what it looks like now, but I hate to mess with it any further.  

As for the oil in the carbs, I found several recommendations for 20w-50 oil.  On a whim, I switched it out for 3 in 1 oil, which also gets lots of recommendations and is definitely lighter weight.  Not sure it makes any difference.  I think the real difference was getting the chokes working right and getting the mixture set better.  

Any other thoughts are appreciated.  Always helps to have another brain or two!  Thanks everyone.  

 

[Vorticity]

On 8/21/2020 at 8:55 AM, 7tooZ said:

Matt when you move the Crane optical pickup did you reset your timing? 
I think the oil is on the thick side. I use Mystery Oil for a setup almost identical to yours.

I always start a tune up with a valve adjustment, and new plugs.

when you says the coil is hot, is it like to hot to hold your hand on?

 

Thanks for helping me brainstorm.  

I did play with setting the timing after moving the pickup.  To tell the truth, I've messed with that pickup so much and retimed it so many times that I am unsure what all I've tried.  I'm taking notes from now on.  Anyways, I just think it's set right now, based on how it runs after finally getting the carbs all set.  It seems that the run-onn/dieseling is cured, the cold start is cured and the rough running between 1800-2500 RPM is cured.  I'll have to see how it runs on first start tomorrow morning to be sure.  

I did a hot valve adjustment and new plugs to start, too.  Only the #1 exhaust was out of spec.  

The coil gets too hot to hold your hand on for more than a few seconds, but not like a hot stove or something.  

Did switch out the oil to 3 in 1 per other recommendations.  No difference noted.  

 

 

[Mark Maras]

3 hours ago, Vorticity said:

The choke cables were the problem with the difficult first start.  The PO had installed the round tops, and also put a mounting plate under the center console to strengthen the choke lever attachment.  However, the mounting plate was actually catching on the screw holding the choke lever, keeping the lever from moving more than half way.  I don't think it would ever go all the way forward, either.  I was told that this was just the way it was when switching to the round tops while using the flat top choke cables and lever.  A little playing around with the nozzles showed me that they never went all the way down when the choke was on.  In fact, they barely moved.  Basically all the choke was doing was moving the throttle plate a bit.  I had to take apart the center console and trim the mounting plate for clearance.  I've now got full range of motion on the chokes and the cables are set right.

As for float levels, I set them using some clear tubes attached to the drain holes.  Set them so that the fuel is 23 mm below the top of the float bowl with the car running.  Is that right?  Maybe it's too low, because now that I can actually use the chokes, I discovered that pulling the chokes on just a little will cure the the feeling of missing or stuttering at constant throttle between say 1800-2500 RPM.  Noticing this, I turned down the nozzles a half turn (now at 3 full turns down), and it runs perfect.  Maybe I'll get the colortune out and see what it looks like now, but I hate to mess with it any further.  

As for the oil in the carbs, I found several recommendations for 20w-50 oil.  On a whim, I switched it out for 3 in 1 oil, which also gets lots of recommendations and is definitely lighter weight.  Not sure it makes any difference.  I think the real difference was getting the chokes working right and getting the mixture set better.  

Any other thoughts are appreciated.  Always helps to have another brain or two!  Thanks everyone.  

 

  Congrats on the success. 23mm down is good. I'd try turning the nozzles up 1/4 turn just to see the effect and then probably reset them to 3 turns. 20 wt. oil is recommended. The thicker the oil the slower the piston and needle rise during acceleration but doesn't prevent the piston from going all the way to the top. That slow rise (restricted venturi) and the open butterfly, speeds up the air coming thru the venturi (piston bottom), drawing more fuel from the nozzle. That slower rise replaces the accelerator pump that is found on most carbs. I ran ATF in mine with no complaints.