How much friction should the Hitachi piston have 74 260Z

[Guest rick458]

on a 74 260Z with flattops how much friction or drag should the carb piston have (with the plunger removed)

Mine are both dragging and want to stick at the bottom

 

the car has sat up for 10 years the oil level is right at upper line

 

the engine will fire up and Idle up to 1100 or so RPM backfire (gently) and drop off to 400 RPM as per tach and stumble speed back up backfire and drop off until it looses enough momentum to catch again

 

Occasionally it will line out at about 800 RPM and idle very smoothly

any clues or theory's to explore

[Guest Al 260Z]

There should be no drag on flat-top piston with oil plunger removed (exactly the same as SU-type). Probably you need to remove air piston covers (the flat-top domes) and clean and degunk 'em. A blast of Berrymans B-12 carb cleaner and a wipe with a clean rag should do wonders. Very easy, except if the cover sticks to carb body - then you need to pursuade it loose ever so gently. Be careful not to break the epoxy (?) tapered locating bosses, do not mix up parts from front/rear carb, and don't bend the needles. Don't tweak the throttle butterfly stop screws which look like typical idle speed adjust - these are factory set. Idle speed is adjusted by air/vacuum screw thingy on the cross-over tube. Idle mixture is the big screw thingy on the front of the front carb.

 

I'm a big fan of flat-top Hitachis, but am one of very few. To be honest, my 260 is running on Weber DGVs which is how I got it. They don't give me much trouble, and I haven't had the spare time to try to re-set-up the stock Hitachis although I have several candidate sets. I'll get around to it one of these years. I am convinced that well adjusted Hitachis should perform a bit better than SUs. They are more sophisticated and are admittedly more difficult to understand and work on however.

 

Don't sweat the carbs too much until you are sure that the rest of the engine is properly dialed in and back in good running form. Recommend regular doses of a couple of teaspoons or so of Marvel Oil to all cylinders via spark plug holes to help free up the rings. Get the engine good and hot and set the valves. Also check oil flow to the cam while cranking with spark plugs out. Check dizzy and make sure vacuum and mechanical advance are working. Lube the dizzy shaft with several drops of motor oil under rubber plug under rotor. Check dizzy timing. Check for vacuum leaks with a mighty-vac. The biggest headache with restoring old sitter car is dealing with the rotten fuel and fuel deposits. After 10 years, chances are good that the gas tank, pumps, lines and carb bowls are full of tar and snot. In your case, with engine running, you can try to purge with new gas and additives like Techron or Marvel Oil. The more you keep 'er running the cleaner the fuel system will get, just keep an eye out for plugging at the fuel filter and screen in bottom of electric pump.

 

Flat-top Owners Unite!

 

Later,

Al

[Guest rick458]

Al260Z

I had a tear in the balance hose(the one on the bottom going to the front mixture screw) I replaced that and I backed off the low speed idle screw and tweaked the high speed idle screw

I still need to replace the hose for the anti backfire valve

but I sure know more about the carbs and intake setup now

[Guest rick458]

I did the marvel in the cylinders thing

and changed the oil after that

 

I am really suprised the cooling system is still intact

I will replace the cooling hoses, and am replacing about a mile of

Fuel and Vacuum hose I am installing a second fuel filter before the mechanical pump

 

finished rebuilding the brakes this morning (still need to bleed em)

need to get and replace clutch master and slave cylinder

and figure why I have no headlights or interior lights

and charge the AC

[Guest Al 260Z]

Rick,

 

You're on the right track, it sounds like. I've been rebuilding a Volvo 145 which sat for 10+ years. First rebuilt the brakes... Gas was really nasty and stunk up my garage for weeks. All the rubber fuel and tank hoses were contaminated with rotten gas stench. Tank was full of horrible tarry snot - it had to get boiled/coated at radiator shop. I had radiator recored at the same time. I've been working out the last vestiges of rotten gas from the carb. When I first starter 'er a couple of weeks ago, gas just bubbled up out of aftermarket DGV carb. Opened it up and float was glued to bottom of carb fuel bowl by old gas tar. For the first little while I couldn't get DGV secondary barrel to work, it would just cough and bog when you stepped on accelerator. Alternating doses of Marvel and Berrymens have cured that. It also took repeated doses of Marvel into the spark plug holes to get the rings freed up. First compression test had #1 at only 50psi. So I'd take 'er for a drive to warm 'er up, bring 'er in, pull plugs, squirt in a tablespoon or so of Marvel, and let 'er sit overnight. Did that several times, and she seems to be running real strong on all four finally. Tomorrow Volvo goes to get R12 removed so I can trash aftermarket air cond, then on to muffler shop to get busted up header replaced. After I can afford four new tires, a voltage regulator, and a tie rod, she'll be marginally road worthy.

 

Keep your eyes out for a 73 or 74 factory service manual. There's lots of info on the carbs in there. Let me know if I can be of any help.

 

Later,

Al[/i]

[Guest NorCalZ]

as a matter of fact yestereday I was having a big problem with my carbs and it took me forever to get my car started. I drove home from my friends house and I have never touched my carbs ever in my life, but I said **** it. So I took the domes off and cleaned everything up and put some 10 weight oil in and really lubed up everything. The car works like a charm. It felt good knowing what I thought was wrong was the culprit. Sorry I brought this up but I think you want the pistons to slide when you turn it upside down. There should be no resistance. At least taht is my case and my carbs work decent.

[Guest rick458]

I took the tops off and cleaned them up real good, I ran some seafoam through the intake, I have replaced every hose on the vacuum side ecept for the brake booster line I blocked it off.

I balanced the carbs by ear and am running stp fuel system cleaner though it.

I got the tank sealed and re installed, and saw that my exhaust manifold leak was only a headpipe to manifold leak with a busted front stud.

I still need to put in the new clutch Master cylinder, and replace the cooling system hoses.

but she sounds real good with no load on her and runs smoother each time I fire her up

things are looking up there isnt much rust, the interior is pretty much gone

but piece by piece little by little she is coming along

[Guest Al 260Z]

Whether SUs or Flat-top Hitachis, the air piston action should be the same and you can check by inserting finger past choke plate and lifting up. If oil dampener is in place there should be substantial resistance to lifting the piston all the way to the top, but it should advance smoothly. On releasing piston, it should drop smoothly and rapidly, and bottom with a dull "THUNK".

 

Usually, any deviation from proper action is due to dirt accumulation between the air piston and the piston dome. Removing air dome for cleaning should be standard annual or bi-annual maintenance practice. Berryman's B-12 cuts gas gunk like nothing else, but it's probably best not to breath too much of the fumes! Dirt in carbs will remind you to change your air filter (or service if K&N type).

 

No continuous resistance on lift indicates lack of oil or missing plunger in air piston oil dashpot. Excess friction may also be due to a bent or off-center needle dragging on the main jet. This is only due to improper disassembly and handling, it is not likely to happen if carb is left on car. Be extra careful not to bend needle when removing pistons. Don't mix up up parts from front/rear carbs. Try not to break delicate tapered epoxy air-dome locating tabs. If these are gone you must carefully adjust air dome position and perform the piston drop test until you get the desired "THUNK" with no needle/jet contact. With carb disassembled, inspect needle for straigtness (a mini machinist's square applied at various angles around the needle works well) and inspect main jet bore for signs of out-of-round wear. You can bend the needle to straighten it, but there's no cure for worn jet - it should be replaced together with needle which will also be worn out-of-round.

 

The main thing that really kills SUs or Flat-tops is wear of the throttle butterfly shaft and bushings. Excess gap here will make a nasty vacuum leak that makes consistent idle impossible and screws up the mixture over all regimes. Ztherapy offers rebuilds with new shafts fit with ball bearings and lip seals. Hopefully their machining specs are up to snuff so that the throttle butterfly ends up perfectly centered and square to the air bore of the carb.

 

Ztherapy's SU video is highly recommended for any SU or Flat-top enthusiast (even if he does refer to Flat-tops as boat anchors!). Very straight-forward and thorough. An investment of $15 (?) will pay for itself in no time. The SU book from MSA has useful info about adjusting a/f ratio by tweeking with needle taper, and also discusses some other performance boosting tricks. The book is better for more advanced carb and performance nerds.

 

Don't forget: don't screw with carbs till everything else is checked out: valves must be set, cooling and compression must be good, and ignition must by spot-on.

 

Later,

Al