We're bringin' back the Flat Tops!

[grannyknot]

4 hours ago, motorman7 said:

The paint job is pretty amazing.

So is everything else, fine work.  I predict jayhawk will be coming home from Branson with some awards.

[Captain Obvious]

What he said!

So how's it running? Cold, hot? Everything stable?

[motorman7]

2 hours ago, Captain Obvious said:

What he said!

So how's it running? Cold, hot? Everything stable?

 

5 hours ago, grannyknot said:

So is everything else, fine work.

So here is the Status and some of the issues that I ran into:

ENGINE BAY: the engine is running nicely now and all hoses are connected.  I still have a few small clamps that need to be added to the vacuum hoses, but for the most part, the motor is done.  I have some right angle battery terminals coming in today and will see what I can do to replicate the correct style battery cables (240zrubberparts cables are sold out, have not been able to contact via phone or e-mail).   Not sure what I can do to re-crate terminal covers, but do have some on order that will be arriving too late. I am missing the original smog pump arm, but picked up one on ebay that should be in on Tuesday.  That is about it for the engine items

The only issues I had with the motor operation were a high idle and some RPM cycling oscillations.  Even with everything backed off, the idle was sitting at around 1200 RPM.  Not sure why that is.  I remedied the situation by extending the needles in the two flat top pistons.  Idle is now at around 700 RPMs.  The RPM cycling was odd and I knew it was a result of one of the vacuum systems.   The engine would oscillate from 600  RPM up to 1200 RPM and then back down in a continuous cycle (That would drive anyone crazy).  Plugging the hose going the the anti-backfire valve fixed the issue.  Looks like I need a new valve and those are pretty pricey.  For now, I will leave it plugged. 

I have run the motor for over 30 minutes and temperature remains steady at a bit below 180F.

Clutch and brakes have been bled and peddles feel good.  

 

INTERIOR:  I have a few parts that were in pretty bad shape (the passenger side pull handle and one roll-up handle), so ordered new ones from Motorsport.   All gauges work as well as the clock.  The LED lights look nice in the dash but do not appear to be adjustable.  I also put LED lights in the overhead interior light and the engine light.  I am finishing up the luggage straps and should have them installed in the next day or two.

 

EXTERIOR:  Just need to install the center bumper finisher which I painted last night and connect front turn lights.  Need new lug nuts for the mags and license plate frame.  Will probably pick up from Pep Boys tonight.  Also, Miguel wants to do one final polish on Thursday night before we load.  Can't imagine the paint looking any better, but he wants to do the final polish on the car.

That about completes the list.   I will try and take the car out for outdoor pics in the next day or two.  We load on Friday.

 

 

[Patcon]

5 hours ago, motorman7 said:

ENGINE BAY: the engine is running nicely now and all hoses are connected.  I still have a few small clamps that need to be added to the vacuum hoses, but for the most part, the motor is done.  I have some right angle battery terminals coming in today and will see what I can do to replicate the correct style battery cables (240zrubberparts cables are sold out, have not been able to contact via phone or e-mail).   Not sure what I can do to re-crate terminal covers, but do have some on order that will be arriving too late. I am missing the original smog pump arm, but picked up one on ebay that should be in on Tuesday.  That is about it for the engine items

@nix240z

[motorman7]

Put the show tires on and took her for a short spin in the cul-de-sac.   Got the center front bumper filler installed along with the alternator and smog support that came in from eBay.  Will do the luggage straps tomorrow night.  Motorsport parts are coming in on Wednesday.  It's coming down to the wire.

Edited July 9, 2019 by motorman7

[240260280]

Some flat top tuning tips  I hoovered a long time ago. Just noticed on my HD. Enjoy!

 

Flat top tid-bits

 

 

On the 260 Flat-Tops, jet adjustment is done through float bowl cover removal, and adjustment of the jet height, OR easier, the adjustment of the needle in the suction piston. Along with the set jetting for set jetting for the idle circuit. So there are drillings that are required for idle mixture adjustments. Basically you just make one "drill adjustment" to richen that circuit, and then go with the screws on the external parts for fine tuning from there.
The 74 does have a VERY small float bowl internal volume, so if you are planning on running hard, or adding something like a cam MAKE SURE BOTH YOUR FUEL PUMPS ARE FUNCTIONING PROPERLY!
The volume in the late carbs are decreased by a measure of probably 10 over early round tops, and by probably 2 or 3 over the 73 carbs. This was to keep fuel flowing through them quickly, so as not to have any chance to heat up and cause problems. So if you need more fuel, make sure you have good flow to keep you from sucking the floats dry.

See those little pin spanner holes in the brass round "thingy" in the center of the float chamber?

Turn it counter clockwise and you richen the mixture throughout the main circuit operation....

Funny, if I don't get to them quick enough seems all the ROUND TOPPERS pull the suction domes to REMOVE THE SMOG NEEDLES... Wonder why?
Could it be that the SMOG NEEDLES give a BETTER enrichment curve? Ironic isn't it, that people will tear a carb all apart for ONE component, rather than drill a few small orifices and just install the complete carb and have the BEST of both worlds: better emissions, better performance, a REAL choke system for cold start, AND visual emissions compliance?

Hi Al,
Well,  the car is setup for SCCA ITS class racing.  So basically we have to keep the engine fairly close to stock.  We are allowed to modify the carbs as far as jetting and removal of chokes, & smog equip.  My carbs have been COMPLETELY de-smogged.  Pretty much in the same way as described at another site I saw referenced in a post here.  All of the choke stuff has also been removed.  All the remaining holes and smog passage ways have been epoxied(sp?) shut.  
The idle adjustment stuff has been scraped.  The fuel line goes directly into the carb body (from an adjustable press. regulator).  The inlet valve has been replaced with a Gross-Jet inlet needle valve. Everything else is the same as stock. Oh...I use SM needles also.

The power jet is tuned by changing the metering assembly that bolts onto the side of the carb (it is next to the float bowl site glass).  It is a round thing with six screws in it.  I don't know of any way to test them other than trial and error.

I am building a new engine for my car this summer.  Just got the block back from the machine shop yesterday.  It was bored 1mm over to match the pistons (Nissan 1mm oversize).
If I can remember too, I will "borrow" the digital cam from work and take some pictures of the carbs so that I can post/send them if you are interested.

 

 

What I'm thinking is that the oil you have in the carbs is too thick and creates a flat spot in the response of the carbs because the slider won’t move quickly enough. 
One thing that was mentioned to me from Tony is the fact that you need to have the butterfly plate closed off all the way at idle, if you are trying to get a faster idle out of the engine, the set screw in the center of the shaft is not the place to increase the idle.
Also, you may want to try adjusting the needles in the slider up a little and that will provide more fuel to the engine quicker. Those carbs have  a lot of adjustment between the needle and the jet that is screwed into the bottom of the carb housing. 
Anyway, those are some observations I have made on the carbs, and I think I can get them running well on my car.

 

I use 10WT in mine, or ATF. Some people use marvel mystery oil. Oil will affect the response rate of the carbs, as will the spring tension inside the suction dome. Take a look at some of the Jaguar Tuning Sites, and you get reams of data on tuning SU's. There is more to doing it than just turning the screw!
The flat tops used in the US differ from the ones used elsewhere. The unitized float bowl, and idle circuitry is the key points. You can get just as much fuel out of them for top end performance. It's the tuning of the IDLE and TRANSITION circuits that takes a little tinkering. Once you unlock the idle-to-main transition (tip: make it richer, just like everybody does with the OLD ROUND TOPS) you will find they work just as well as round tops!

 

 

Move the needle up EQUALLY in both carbs MAYBE 1mm.
What you are doing is running at idle on the needle.
Like he stated, these carbs were designed to idle with the plates CLOSED, and run off the idle circuit in the front carb. Unless you want to open it up and drill passages, your easiest fix is to fatten up the main circuit, and keep it running on the main system at idle.

Not the correct solution, but functionally operable.
BTW, the Marvel Mystery Oil should be about 5wt. This allows easier lifting of the suction dome, allowing a fatter mixture across the power band. By running a higher viscosity (10wt or ATF) you LEAN out them mixture under load. 

The SU is a basic carburettor, but respect is due to those who tune it, because simple things are amazingly complex to get RIGHT. You are finding this out right now.
Good Luck!

 

Thin oil, with a needle that's very blunt. So when you slam open the throttle plates, the suction dome rises, and you go to full jet available gas dumping.
Zolorin is correct about the cross sectional area, while a slower opening carb will have a higher vacuum, and suck some gas out of the jet, the needle is covering the jet, restricting the flow. If you expose more of the jet, you get disproportionately more fuel than you do when you simply increase vacuum.
The oil DAMPENS the action, and allows you to trim tip-in response. To actually CHANGE the rate at which the piston rises, you must install another spring, and then work out the oil you need for tip-in.
For throttle response, at WOT, you either run another taper, or run the needle higher in the piston(also running a higher fuel level in the float bowl helps), and run lighter oil, to allow the piston to be less dampened, rise quicker and fall quicker, letting FULL POWER settings of the carb to be reached quicker.
Now, this all assumes that the car is being DRIVEN correctly, and you aren't expecting something akin to a V-8 sort of launch from 1500rpms. That just isn’t going to happen.
For proper tuning of the SU's, you should be concentrating for throttle response above 2500 at least, as this is where the engine actually starts making useable power.

 

 

Not vibrating, but oscillating. What would happen if you hit a hard bump and no damping in the carbs? The piston could slam down and kill the engine from the pistons' inertia! It also slows the ascent and descent somewhat for a gradual and smooth tip in transition...

 

 

Generally the power valve is the culprit. Have you broken off the needle stops on the idle mixture screws to dial it down? The throttle plates are supposed to be CLOSED, ALL the air comes through the starter idle circuit on the front carb in essence. If the plates are cracked, the main jet contributes to the mixture (from the taper you have noticed never plugging---this means very quick main tip-in in driving to keep from stumbling LIKE THE ROUND-TOPS ALL DO!)

If those plates are open AT ALL---close them. This gives all vacuum to the idle circuit, and draws in proper emulsion air to lean the mix... if the air bleeds don't have the proper vacuum; they will 'go rich' as well.

The idle issue is the IDLE circuit. The two SHOULD NOT be on at idle. ONE only. As SOON as you touch the throttle the mains start to work. This is the problem most people can't get through their heads working on these carbs. They are designed for TOTAL closure of the throttle plates at idle. TOTAL CLOSURE just like EFI! They have a separate air fuel bypass circuit to fuel the engine at idle.

This allows it to be tuned totally and separately from any other portion of the engine. Station indicators and running around with the hood off or on a rolling road anybody?

 

 

Seems like stiffer springs would have the effect of "delaying" the maximum rise of the piston/needle. This might possibly be good for smooth driveability- has sort of same effect as using a smaller carburetor. I found that the piston/needle was fully raised on my L20B around 4000 RPM (w/ stock Hitachi springs). I don’t think it is necessary to delay the piston max rise all the way to peak HP PRM or redline. Maybe match the full piston rise closer to the torque peak RPM? The SUs just go into "non constant-velocity mode" and function more like a traditional e-tube carb once the piston fully rises. Using stiffer springs will have the effect of raising the venture velocity (and vacuum) through the rest of the low/mid RPM range before the piston reaches full lift. Stronger vacuum is produced to counter the stiffer spring down force and increased vacuum will also work to richen the low/midrange RPM air/fuel mixture (just like thicker dampener oil richen the mixture under acceleration). So- sounds like your choice of the stiffer red springs may in part be contributing to your rich-at-idle/midrange problem. Before customizing you needle profile to work with the stiffer springs, try using the soft Hitachi springs again and see if your mixture balance at low/high RPM is improved.

 

 

A 260Z will run smooth and clean to 7000rpms with a power peak around 6500 with EITHER the SU's or the Flat Tops. Frankly, the FLAT TOPS will run stronger on the top end because they are possessing the correct taper on the needle for the larger engine compared to the L24 Round Top Transplants.

[motorman7]

Thanks @240260280 . That is useful reference material.

 

Still finishing up details.  Added decals.  Also below is a pic of my fabricated right angle battery cables....no cover though.

[bartsscooterservice]

Very nice job you have done there. Lot's of details... Love the work.

The hardest part is keeping it clean and looking like that, in here I live in a 4 season's climate, that is having a toll on everything. But I am fine with that, just keep it looking nice, and give it the usual attention and cleaning every year. It doesn't have to be a trailer queen for me.

[motorman7]

We will be loading the car tomorrow AM and heading for Branson.  Here are some final pics of the car.  Yes, and a few photos of @jayhawk and @motorman7 in the background.  Looking forward to an awesome event.  Thanks to everyone for the help, input and support.

[jayhawk]

Motorman7 is more than a Datsun expert, he is the Fountain of Youth, who made me 24 years old again! (with key work by Datsun paint expert Miguel).

I am incredibly grateful to him for enabling me to recreate that wonderful feeling  getting a brand new 240Z - the car I knew even in 1973 that I would keep forever.  This is not a car to be sold, but to be enjoyed and driven. Although we will try to show it off for a while, so everyone can see how a heavily used Z can be made new again by a knowledgeable and dedicated lover of Datsuns.

Thanks Motorman7!

Now for a road  trip!

 

Edited July 12, 2019 by jayhawk

[270AD]

Has been great watching this progress. Seems like a great outcome for all involved.  Congrats and well done. 

[rcb280z]

That is a beautiful car!

Have a safe and fun trip.