The Hitachi SU some insight required.

[That-hurt]

Hello all.

I stretch my hand out across the pond to shake the hand of my fellow 240Z owners on distant shores.

I seek advice.

Car: 240Z 1973 Standard engine. Rebuilt from the bottom up and no issues with compression etc. 6 branch header and using a 2 inch exhaust with no centre box just a rear box.

State of tune. New Pertronix ignitor using a 3 ohm flamethrower coil and no balast resistor : New plugs, rotor arm, leads and dizzy cap. timing set at 10 degrees BFTDC. done using strobe light.

Carbs: a brand spanking new set of Z therapy 3 screw hitatchi carbs these ae running SM needles. i have watched the Z Therapy video several time and done all the adjustments etc.

This what I have encountered. you can tell me if I am right or wrong.. shout at me whatever.. but I do need to see if it is me making stupid mistakes or another issue I have not or cannot see. I have been at this for about 20 hours on and off and its time to ask the audience..

I checked the float levels when the carbs arrived just in case they had been thrown around on the plane over to the UK. I cut a piece of card 9/16ths or 14mm slid this under the float to make sure the arm was just contacting the valve. both were fine. I cranked over the engine and marked 23mm down on the side of the float bowl {not 23mm from the lid} and checked the levels with a piece of clear tube. both read 23mm or as near as dam in.

I then checked the pistons both dropped with a nice clunk. I topped up the dash pots with ATF as per Z therapy video. both had the same resistance. I set the jets down 2.5 turns. mine have the flat plates on the bottom and not the knurled nut. I made sure all linkages were free and not binding. Its cold here just 6 degree C so I had to use the chokes to start the car.

The car fired up and ran at fast idle I let it warm up and gradually decreased the choke. once warm I pushed the choke in.. the car faltered spat back through the front carb {lean} and back fired with popping from the exhaust from the rear carb {rich}. I turned both idle screws up so I could get it to idle, the idle was rough and un even. I evened it up as best I could with the uni-syn.

This where it gets a little funny. In order to get the fastest smoothest idle I have had to screw the jets down 4.25 turns front and 4.5 turns rear. I have done this several times and as I screw the front carb up the car starts to hunt and stumble. So its was leaning out on the mixture. I now have it running and set at the 4.25 and 4.5 turns. Its balanced precisly with the uni-syn and revs easily from idle up with not stumble off idle . There is no smoke from the exhaust although it does smell slightly rich.

There is no spitting back but when revving it there is a slight popping from the exhaust as I close the throttle.

My gut feeling tells me that its the fuel levels in the bowls.. but they are correct. Am I missing something?

I pulled the plugs.. but to be honest it needs a good run to really check this out, they look a little sooty.. I have ordinary unleaded in the tank at the moment.

All help and advice welcome.. I read somewhere that altitude comes in to the equation with Su type carbs. we are at 1600 feet above sea level its raining damp amd misarable..been like this for two weeks. I wish I lived where it warm and dry. :disappoin

sorry for the long post.. thanks in advance.. I think Bruce Palmer frequents the site so he may be able to shed some light on this. I am also thinking its the SM needles?

Andrew for an cold wet April in Sheffield England.

[Zedyone_kenobi]

Hello from the colonies!

Great post and fantastic description of the problem. So you have the car running well, but are not sure why the settings are off. I have been playing chase with my SU's for well over a year, and we should start with the basics.

First are you positive you have no vacuum leaks? It sounds like you may have a leak with your idle hunting all over the place. Also, if you need to put that much fuel into the system, it may be getting some extra air in there you may not know about.

Second thing I would check since you already ruled out your fuel level, is the needle set at the proper height in the piston.

Third, are you getting the fuel pressure required to the SU's. I think it is around 3.5 to 4.1 psi.

I would highly recommend you take a trip to the auto parts store and buy a combo vacuum/fuel pressure meter (if you do not already have one). It will come in very handy in diagnosing problems in just these areas.

I have to assume you are running a european dizzy, and I thought those were required to run about 17 deg BTDC. You may be able to advance your timing some.

By saying you are running 3 screws Hitachi's I am assuming that to mean round tops, and not flat tops. Great move with Ztherapy by the way. Great product.

Oh another thing, check to see if the fuel line from the bowls to the carb is not bent excessively. Sometimes in shipment they can get kinked or creased.

Keep posting your findings. We will all be very happy to help!

[olzed]

Sorry to here of your crap weather there Andrew.

Down here we are having the best Autumn ever. Three weeks of glorious sunshine. We just came out of a so so summer though.

Sounds as though your carbs are lean. Needles??? Altitude,Yes. Sooty plugs from slow running test is not conclusive.

Brian.

[Gary in NJ]

is the needle set at the proper height in the piston.

This. The nozzle adjustment simply exposes sections of the needle. If the needles are "low" then you have to expose more needle by lowering the nozzles.

The cold air also effects the mixture adjustment. The colder (denser) the air, the more fuel you need.

These two factors could contribute to nozzles that are lower then expected.

Edited April 25, 2012 by Gary in NJ

[That-hurt]

Hi Guys.. you may all be asleep when I post this reply so I bet there are plenty of snoozing zzzzzz's bouncing off the walls sorry could not resist the pun..:laugh:

Many thanks for your replies thus far.

I will certainly look at the timing issue, my haynes book says 10 degrees however i have seen reference to 17 degrees on other sites so i will give that a whirl.

I have almost convinced myself that i do not have any air leaks as everything is buttoned up pretty tight.. I stand to be corrected :stupid: and I will get stuck into every nook and cranny to see if there are any leaks.

An old mechanic told me to use a blow torch!.. turn the gas on low, feed it through a small hose with a metal probe on the end and waggle this around in front of things in the engine bay. If you have a leak then the engine will pick up on the revs a little.. Sounds a bit dangerous but it may work....

Could the valve for the brake servo cause a leak.. sorry just thinking aloud... What if the inlet manifold is a bit wonky and is letting air in.. food for thought.

I am glad its warm and sunny over there.. over here we have just had flood alerts.. no really flood alerts. Its rained that much that we are developing webbed feet .. awful country the UK, lord only knows why we live here at all. I have to waterproof my Z in every conceivable way if I did not then it would just dissolve.. Pictures taken on a rare sunny day, don't ask me when the last time I saw the Sun was weeks ago...

Here is a little picture of the UK Z.. hope you like it.. its a rare beast over here just 200 or so left on the road..

[Gary in NJ]

If you suspect a vacuum leak from the brake booster, you can always disconnect the hose at the carb and temporarily plug/cap the orifice. In fact, that's true for just about any suspected vacuum leak with the exception of a leak at the manifold mount. That type of leak can be traced with a can of carb cleaner.

Don't use US/North American specs for ignition timing. Our distributor is different.

[Zedyone_kenobi]

Just to be sure not to confuse our friend, The US spec is 5 deg BTDC. What is called out in our manuals is 17 deg BDTC for european dizzies. If you look on the side of your dizzy and tell us the number stamped into the housing we could clear that up. But I do not think timing is your issue.

[That-hurt]

Hi guys. Still raining here in the uk.. It's been weeks oh for some dry weather!

I thought I would report back and ask for some advice . I think well I know I have traced some air leaks . I used a can of carb cleaner and squirted it I have found when I squirt some on the intake manifold where it bolts to the head there is a definite sucking in of the fluid . All gaskets are new. I have nipped up the bolts to the head but it has failed to cure it. I am going to remove the carbs and take off the inlet manifold. Have you guys a solution to the problem? I shall order a new gasket and depending on your advice go from there..

[240260280z]

Intake manifold mating surface must be checked for flatness

You may need shimming washers to distribute the clamping force from the common nuts that secure both the intake manifold and the exhaust. Generally if the thickness of the intake or exhaust is significantly thicker than the other, it must be addressed.

[Diseazd]

Andrew......Whenever you engage the chokes, you are lowering the nozzle under the carbs. It is very common for the nozzle mechanism to bind and not snap back up tight. If this happens, you'll never get the carbs to run smooth. Reach your hand under each carb and push up on the nozzle with the chokes in the off setting. If you feel a little snap, you'll know that they weren't completely snapped back. IMO, don't start 2nd guessing Z Therapy settings. You can take it to the bank that they didn't send you carbs that were out of adjustment. Also, regarding vacuum leaks.....just hook up a vacuum gauge at the brake booster outlet on the intake manifold.....reading should be constant at 18 to 20 hg.

[That-hurt]

thanks guys.

I have had the lot off today. checked everything for flatness etc. Put it all back together with new gaskets and so far so good. Used carb cleaner on the manifold. just one sucking a little air. On my car its the front closest to the dizzy. it has one manifold stud that is really tight against the manifold so no socket made by mad will fit over it. I have to be a contortionist in order the get the spanner on it and then try to get enough torque on the nut. I managed to get it nipped up a little tighter and it appears to have done the trick...

I will report back once I gwt to re tune the carbs.. thanks for your help.. Andrew

[Gary in NJ]

Andrew......Whenever you engage the chokes, you are lowering the nozzle under the carbs. It is very common for the nozzle mechanism to bind and not snap back up tight. If this happens, you'll never get the carbs to run smooth.

Andrew was concerned that he had to turn the mixture screw 4-1/2 turns - at both carbs. This suggests that the carbs, for what ever reason, are lean. If the nozzles didn't reset to their full-up position, that would create a rich condition.

I think he's on the correct path with the vacuum leak(s) he has discovered.