Z therapy SU carbs not running right.

[azriel_strife]

hi guys, i have a set of brand new SU's carbs on my car, they ran good for a while, but now they have started acting up.

The problem i have is that the car will run great until operating temperature or a bit above. I notice it the most when I get on the highway, or sustain rpm above 2000 for any length of time. as long as my revs are up, this problem persists, if I stop and let the car idle rough for a couple minutes it smartens up and runs nice again.

at first i thought it was the stock mechanical pump not being able to keep up with the 2.9 stroker. I saw a lot of air inside the lines, it was boiling the fuel in the lines so I decided to install a carter pusher pump at the tank. this resolved the fuel boiling issue but my car still isn't running right. I've thrown a good bit of money at it now and nothing has changed.

I have checked and rechecked the carbs for dirt, engine bay fuel filter was just replaced, and last chance filters are spotless, fuel pressure is right, bowls are staying full, and float levels are correct (I checked them right after it started acting strangely the last time)

2 things that i have not had the ability to check are carb synch and timing. But I can't see those being intermittent problems. It runs so happily when it decides to run right. I did back off the timing a bit to see if that would help with the backfires (lean) but it didn't do much.

I'm contemplating selling these things and just getting a holley manifold and 390 carb. I can't stand being so frustrated with this beautiful machine.

Edited June 5, 2014 by azriel_strife

[240260280z]

back fire from the tail pipe is a sign of being rich

What do the plugs look like. If you can post a picture that would help.

It could also be ignition. What ignition system are you running? What is your timing set to?

[zKars]

Check for vacuum leaks as well. Tighten the carbs down, check the manifold bolts too.

Are both enrichment knobs turned out the same, and approx 2 1/2 turns?

Don't blame the carbs until all other aspects of the tune are corrected first. Timing, valve lash, vacuum leaks, bad plugs, low compression, bad wires, cap etc are all out of ztherapy's control.

[azriel_strife]

back fire from the tail pipe is a sign of being rich

What do the plugs look like. If you can post a picture that would help.

It could also be ignition. What ignition system are you running? What is your timing set to?

It's backfiring through one the carbs. I believe afterfire is when the exhaust has popping (i love the sound as as far as i can tell it's perfectly normal)

I'm not entirely sure of the plugs, I suppose i should take a look, they are new, gapped to .35 so I didn't check them. I believe I am running the stock 240z ignition, but given the problem is intermittent I was looking at things that could work and then not work.

Check for vacuum leaks as well. Tighten the carbs down, check the manifold bolts too.

Are both enrichment knobs turned out the same, and approx 2 1/2 turns?

Don't blame the carbs until all other aspects of the tune are corrected first. Timing, valve lash, vacuum leaks, bad plugs, low compression, bad wires, cap etc are all out of ztherapy's control.

I have sprayed a bit of ether around the manifolds and carb isolators where i suspected leaks but no change in rpm that i could discern.

Both knobs are 4.5 turns out, since this engine is a good bit larger than the l24. I achieve highest idle at 4.5 turns. it runs like a top at idle and low rpm around town, then acts like a pile of junk when i actually try and drive it. it's upsetting.

[zKars]

There is your problem right there. 4.5 turns is a million too many. Rich rich rich. Your plugs are loading up and this is causing your poor performance during actual use.

Please, just humour us, put them at 2.5 and spend a day with it. I will gladly be wrong if this is the not the issue.

From there, please get access to a wide band O2 and a vacuum gauge, and stop guessing about what the problem is.

[azriel_strife]

It won't idle well at 2.5 turns out, I tried what the z therapy video said and it wouldn't run right at all after setting them up how it instructed me to. highest idle is at 4.5 turns.

if i try to blip the throttle at lower mixture settings it stumbles or hesitates. Carb backfire is a lean condition, it does this at higher RPM when i try to give it moderate gas to accelerate. seems much better if i go WOT. certainly isn't acting rich.

I'll give it a shot at 2.5 turns out and try to bring up my idle with the set screws. maybe it will surprise me, I hope you are right.

Wish i could locate someone in my area to help me figure this thing out, i dont have much experience with these carbs.

[azriel_strife]

While the engine is cold i'm going to set my valves (.010 intake and .012 exhaust?), then i'll set up the carbs for idling at 2.5 turns out, then i'll try and set my timing, then reset my idle.

i need to get my hands on a carb sync tool, but local auto parts stores want 100 bucks for one. but i imagine if i do all of the above, it should reduce my issue if not resolve it. lets hope!

Edited June 5, 2014 by azriel_strife

[CanTechZ]

Wish i could locate someone in my area to help me figure this thing out, i dont have much experience with these carbs.

I'm also located in Chilliwack, I'm not much of a mechanic and luckily I've never had a problem with the SU's on my '70, but I will send a message to a friend and see if he can help.

Nice to here about another local 240, if you want PM me and maybe we can compare rides sometime.

Mike

I seem to have lost the contact info for the guy I was thinking of, but you could try Dan at the ZShop in Surrey, I've bought parts from him in the past and I have heard good things about his service as well.

ZShop.ca

Edited June 5, 2014 by CanTechZ

[porkbun]

Sounds exactly like my 260Z with round tops when the valve lash was out of whack. You want to have your car completely tuned up before you try and tune your carbs. If your valve lash isnt set properly your car will run differently when cold and when fully warmed up. If youve got decent ears you can use a length of hose to sync the carbs up. Put one end to your year and the other end at the mouth(?) of the carb and adjust the vacuum screw until they sound identical. Personally I believe the 390cfm is a step up from stock hitachis but id definitely get the SU's running right first before you make that choice

[240260280z]

You can put your knuckles in the throat to "feel" the air flow between your fingers. Place your hand (just use the same one) and knuckles in the same location with the same finger spacing on each carb, you can balance fairly close.

btw you need to balance at two points:

1. At idle using the throttle valve stops

2. At ~ 3000 rpm using the linkage balance adjuster (if this is not balanced one carb will do all the work. )

[azriel_strife]

ok. I spent the afternoon on her today and if anything it's running worse than before.

I set the valves cold to .008 intake and .010 exhaust, set mixture screws to 2.5 turns out, set idle speed, topped up the carbs with atf to make sure i was getting proper accelerator pump action.

The car is running worse now than before, backfiring every time I touch the throttle, it runs progressively better each time I turn up the enrichment screws. It is most definitely leaning out somewhere, I may just be masking the real problem by adding fuel though.

I ordered a carb sync tool, and I will try that tomorrow, but I don't know if the carbs being a little out of synch would cause performance loss of this magnitude.

Thank you all for the input, it's nice to have a community like this, so tight and helpful.

Edited June 6, 2014 by azriel_strife

[rossiz]

sure sounds like timing...