Time of day to tune SUs?

[Jehannum]

After an unfortunate coolant leak yesterday, I decided to tune the carbs up. I followed the FSM directions, set the mixture to about 2 turns out (5000 feet, 88°F out), synced 'em up at idle and 3000RPM, and gave it a quick rip around the block to see how it felt (great!).

This morning, I set out to drive the car to work, and it ran horribly. Backfiring, misfiring, all manner of bad behavior. The water temp and oil pressure were about the same as yesterday afternoon when I took off, so the only difference I can think of is the temperature (60°F). With the choke all the way on, the misfire issue was taken care of, but not the backfires.

The FSM indicates leaner operation at higher ambient temperatures, so the smoother operation with the choke makes sense. Should I be tuning in the morning, and accepting the (poorer?) performance in the afternoon?

[Gary in NJ]

Assuming that the engine was fully warmed up when you did your carb sync, time of day from one day to another shouldn't make that much difference (from "great" to "bad behavior"). Seasonal changes would make a difference.

Was the backfiring under load or deceleration?

It sounds like you may have the mixture set too lean. Before you started the sync, did you set your float heights (14mm from the top of the cover) or confirm that they were correct? The number of turns out on the nozzle adjustment is meaningless without he float height baseline.

Edited May 21, 2010 by Gary in NJ

[Jehannum]

Assuming that the engine was fully warmed up when you did your carb sync, time of day from one day to another shouldn't make that much difference (from "great" to "bad behavior"). Seasonal changes would make a difference.

Was the backfiring under load or deceleration?

It sounds like you may have the mixture set too lean. Before you started the sync, did you set your float heights (14mm from the top of the cover) or confirm that they were correct? The number of turns out on the nozzle adjustment is meaningless without he float height baseline.

Backfiring under decel, misfire under load.

I assumed that the float heights were fine, because I had Art Singer rehab them after I pulled the motor last year (new floats, new gaskets, new hoses). How do I adjust float height?

[Gary in NJ]

How do I adjust float height?

You may not need to. The point I was making was without a known float height, the number of turns out on the knobs doesn't provide much information.

A little backfiring under deceleration is usually the result a lean mixture. Go back and resync the carbs. Since you don't know your float height, don't bother with the chart in the FSM. Just adjust them for maximum rpm. After you have done that, resync/balance them.

[motorman7]

How do I adjust float height?

Float height is critical, especially at idle. With the height correct, the idle is smooth as silk, when they are off, the idle is rough. That is usually a good indicator.

Got this from Arne's Blog. This is what I do to check float level. Thanks Arne!

"So rather than trying (once again) the traditional methods of checking the float level, I tried something I'd read about a while back. I pulled the fuel hose from the bottom of the float bowl and connected a piece of clear tubing instead. Left the other end of the clear tube open, but ran it up and alongside the float bowl. Then I cranked the engine over (coil wire disconnected) to operate the fuel pump and fill the float bowls. The fuel fills the tube also, and you can physically see how full the bowls get. No guesswork. I've got several manuals that say the actual fuel level should be 23 mm from the top of the bowl, but don't give any clue as to how you might measure this. But with the tube I can see it.

I did the rear carb first, it was close, about 2 mm low. Adjusted and rechecked, got it right first time. Moved on the the front carb, and found that the level there was at least 10-12 mm low! Took two rechecks to get it correct.

Once that was done, the rest of the carb settings were easy. The results were very good. The car runs great under most all conditions now. No signs of being lean, runs clean and smooth now."

[beandip]

First off, when you turned the mixture adjustment all the way to close, counter clockwise. Then you back them out two turns. This is ONLY a starting point in the adjustment process. This setting is so the engine will run. Secondly I don't think you understand what the function of the choke really is. The purpose of the choke is to RICHEN the fuel mixture. This is necessary when the engine is below normal driving temp. especially when first starting in the morning.

Now to properly adjust the Carburetor , SU , the engine must be up to driving temp. Not just by the gage but actually completely warmed up. The whole procedures has been outlined in detail a number of times on threads in the past. Do a search and you will find all the information. It is NOT difficult to do.

It is extremely important to have the float levels set at the correct levels also. You didn't mention that you did anything in this area so I didn't address this . Once the levels have been set properly the level adjustment is rarely ever needed to reset. unless there is a failure of the fuel valve or the float it's self has a leak and fills.

Gary

Edited May 21, 2010 by beandip

[Duffman]

Rich, would love to see pics of Arne's method of checking float level. Probably easier to see than just read.

[hogie]

This is what I used to check the level of mine. Find a bolt that will fit the bowl drain and drill it out to fit a right angle fitting. Mine was from a one man brake bleeder kit. Attach a peice of tube and away you go.

Yould also place a tube on the bottom of the bowls nipple, but you wont be able to run it in that condition.

<a href="http://s190.photobucket.com/albums/z82/wangchung_01/240z/240z%20carbs/?action=view&current=PICT0008.jpg" target="_blank"><img src="http://i190.photobucket.com/albums/z82/wangchung_01/240z/240z%20carbs/th_PICT0008.jpg" border="0" alt="Photobucket" ></a>

[Jehannum]

Ran it out to lunch to get an alignment done, and it ran just great - no backfires, no sputters, didn't even have to choke it. 20° swings really shouldn't make that much difference?

The last time I did a float adjustment, I flipped the bowl covers upside down and adjusted the armature until I could slip a 1/2" drill bit between the float and the top of the bowl. I'll go ahead and check it that way, as I don't have any clear tube that small. What I meant (but didn't convey) was "do I check the float height by forcing it down on the valve, or just let spring pressure hold it?"

Secondly I don't think you understand what the function of the choke really is. The purpose of the choke is to RICHEN the fuel mixture. This is necessary when the engine is below normal driving temp. especially when first starting in the morning.

Yes, I understand what the choke does - it pulls the jet down away from the needle (hell, I don't even understand why it's called a choke - might as well just call it a mixture control). That's why I said that it makes sense that it runs better in the morning when its cold out, with the choke on (if the temperature swing is, indeed, what's causing the issue).

Edited May 21, 2010 by Jehannum

[Gary in NJ]

do I check the float height by forcing it down on the valve, or just let spring pressure hold it?"

With the cover upside down and level (I hold mine in a vice on the inlet boss), measure 9/16" (or 14mm) from the underside of the cover to the top of the float. Make sure that the top of the float is parallel with the cover. Bend it into position if it's not. When taking the measurement, the valve should be closed (pin inserted) otherwise the mixture will be too lean.

You'll need to replace the float gaskets. While you have the floats off, might as well remove the piston housing and give 'em a good cleaning.

Edited May 21, 2010 by Gary in NJ

[Travel'n Man]

If you don't have new float gaskets when you start working on the float bowls - you'll wish you did........they will curl up like a pringle potato chip!!

Nissan still warehouses them!

Edited May 21, 2010 by Travel'n Man

[epsark]

Okay, not to hijack this thread, but I have some questions on what is being said. I just got my car back from the shop and it has been running just fine after sitting for 7 months with occasional starts that the shop did. Matter of fact it strated right up the day I picked it up, with the choke on until it warmed up a little. So, the other day I go to leave work and it was in the upper 40's and had dewed everywhere and needed the choke as usual. A co-worker was looking at it with me while it warmed up and he noticed after a bit that there was something leaking. Shut the engine down and opened the hood. Found that gas was leaking. Turned the engine back on and gas was flowing, ie old faithful, out of the pressure relief nozzle ontop of the float bowl lid.

Now the car has no mechanical pump, just a Holley red pump and it does have a pressure gauge that almost never varies between 4-5 PSI. Yes I know it should be no more than 3 PSI, but it has been working this way for some time now. Last time I checked the filter, glass see through kind, it looked okay and that was one day before this happened. The pump being electric is on it's own switch and can be turned off and on indepentantly of anything else. So, turned the pump off started the engine and ran the level out in bowl, and then turned back on, same thing once it filled. Tried the knocking on the side of the bowl to see if the needle was stuck open. No good, still same problem. While waiting for my fiance to pick me up, I went back into work and got my tools to take the cover off. Took it off and it appeared the needle was moving freely, but really not tto positive. Put it back together and tried again, still same results. Oh, I should mention that when the co-worker was still there he plugged the relief valve and the problem then went to the rear carb. Oh yeah, these are SU's that are the round top 4 screws on my '73 240Z with K & N filters. He held both closed and of course gas came out of the carbs.

So, on the drive home was thinking it over and thought maybe this was imilar to the old bugs I owned and the floats were hollow and with age a small crack formed and finallaly filled with gas holding the float down. Then with the problem following to the rear, the pump may have over come the other float as well. Not perfectly logical, but with a brain used up after a 12 hour shift it allowed me to sleep. When I got up, I decided to call Bruce and see if he might have any ideas. He mentioned that the floats were solid and so not subject to my theory. His idea was "crap" had gotten into the needles and were holding them open. I asked if I should buy the rebuild kit and he said heck no, just try cleaning them and if it didn't work then call back and we would figure what would be needed and get those. A hell of a deal if you ask me. Great service too. So, I got the tops off and took them home and cleaned the needles with carb cleaner and really took a closer look. It seems to me that they are closing just fine and not hanging up anywhere in their travel. I also picked up a new in line filter and an actual pressure regulator to add into the lines. I haven't had a chance to put them back on or install the new parts, but hopefully tomorrow I will. Working 6 12 hour days straight takes a bit of time out of your day. Anyway, I then had a chance to read the forum and here is this thread talking about the float bowls and the floats. Gary, beandip, mentions: "Once the levels have been set properly the level adjustment is rarely ever needed to reset. unless there is a failure of the fuel valve or the float it's self has a leak and fills."

This goes againts what Bruce said about the floats being solid and does match a little more to what mine sound like: hollow. It sounds that it would be prudent for me to check the clearance on the floats again to make sure they are set properly. Any other ideas? BTW the car does run while gushing gas, and no I didn't do so for very long for fear of losing the car so shortly after getting it back again. I'm really hoping that maybe the fuel pressure just finally over came the floats and putting the regulator in will solve the problem.

If the floats are hollow and do occasionally crack and subsequently can fill with gas, where can new ones be obtained? I don't remember seeing Black Dragon having them and MSA can be so hard to navigate at times, but I'll double check those sites again.