SU carbs and high revs

[Victor Laury]

Originally posted by 240ZMan

I do have some small vacuum leakage on the throttle shafts (spraying carb cleaner at idle causes a slight decrease in idle speed).[/quote

They will do that when they are new. the real test is to release the linkage and see how much you can wobble the shaft (don't ya love all this high tech discriptions )

Originally posted by 240ZMan

The aircleaner is an early one with the lever to move between winter and summer. The carbs are 3 screw roundtops which I was told by the original owner are supposed to be '72s. Head is an E31. Beyond that all is stock.

That sounds pure 72 ish. Just like mine, with the exception of the E31. that should be a good as gold set up.

[inline6]

"The two main changes I made with the carbs apart were to reset the floats (both were a bit more than the .55" recommended in the video) and to set the needles with the shoulder flush with the piston (They weren't even close). "

Is there a slot shallow slot on the bottom of your SU pistons that goes all the way across? Did you set the needle flush with the bottom of the slot or with the bottom surface of the piston?

There is an even better way to set it than either of these two, but if you did it the first way I described above, please do it the second way. The second way is how the factory manual sets the height. Use a good straight edge, and set the needle shoulder flush with the bottom of the piston (not the groove, or slot). After doing this, you will need to adjust you mixture screws. Have you synched the carbs with a Uni-syn or other sychronizer tool?

Reverberation: I think you can rule that out; not the problem. Set your throttle to open 102%. By that I mean, visually check that while pushing and holding the pedal to the floor, you can't operate the linkage at the motor by hand to get any additional movement. Set the pedal stop at 1 turn lower than where full throttle occurs. This will make sure that the slop in the linkage won't cause a problem.

When you say you ran without the air cleaner assembly and the problem was resolved, what does resolved mean exactly?

Did it weeze its way to 6 grand? or did it pull hard to 6 grand in 1st and 2nd? or did it pull past 6K?

You should be able to pull past 6 grand easily. There is definitely still a problem.

What brand air filter are you running?

Garrett

[240ZMan]

Garret,

I used the second way as you described it to mount the needles in the pistons so I think I'm ok. Before that one of the needles was so low that even with the nozzle turned all the way in (closed) I was too rich.

On your question about running without the aircleaner, the problem was better in that I could pull to 6k (but not past it), but to your point, it was weezing, not at all strong. The engine is also vibrating a lot above 5k, but as I've never been in another Z I don't know if this is normal or not.

I'm encouraged to hear you and others say I should be able to pull to 6k and beyond as I'm excited to unlock this car's potential. But this is becoming a rather frustrating problem as I'm just about out of things to try.

Daniel

[inline6]

Persistance will find the problem Daniel. Keep with it.

What about the synchronization of the carbs? Are you familiar with that process? Your latest note with the new bit of info about lots of vibration makes me think further about that being the possible problem. Couple of additional questions:

Where you have the mixture screws set for each of the carbs: are they set the same amount of turns in relation to each other? In other words are they both set at exactly the same thing, whatever number of turns that is? And where are they: 1.5 turns out each for example...

Incidently, the best way to set the needle heights (which isn't going to fix your problem) is to pull the pistons, set the needles so that they are visibly out a little bit too far, then tighted the set screw just enough to snug the needles to keep them from falling out. Next, screw the mixture screws all the in until they stop (full lean). Finally, put the pistons in the carb bodies and set them down gently. Put pressure to make sure the piston bottoms out on the carb body-- at this point the shoulder of the needle will bottom out on the nozzle, and the nozzle will push the needle into the piston, so that you have an ZERO clearence between the top of the nozzle and the shoulder of the needle. Remove the piston gently and then tighten the screw that holds the needle. Reassemble as usual-- remembering to reset the mixture screws by following the regular procedure.

I had a brand new set of 4 screw SU's (never used) and I set the needle heights with the straight edge (as the factory manual instructs) and when I opened the throttle plates and provide backlighting so that I could examine the needle relationship to the nozzle, I could actually see a gap difference between the shoulder of the needle and the base of the nozzle from the front carb compared to the back carb.

It is this factory machining tolerance that is the reason why the best mixture setting can be different when comparing your front and rear carbs. If you set the needles as I described above, you should be able to use the exact same setting (number of turns) on both carbs. It certainly makes it easier to remember your settings!

Garrett

[240ZMan]

Garret,

I am familiar with synching the carbs and have followed the procedures from both the Haynes book and ZTherapy video. At this time I have both carbs turned out 3 turns, although I can't sense any difference from 2 1/2. But with this issue I figured I'd go a little extra. Much more out and the idle becomes erratic.

I've gone back and reset the throttle to full open when floored. Didn't expect any change, and didn't get any either.

Question to all: Could there be some issue with using the 73 N33 intake manifold and the 72 carbs?

Otherwise I'm thinking I either:

1) replace the needles/nozzles wiht a set from ZTherapy

or

2) buy another set of carbs from eBay.

Don't like either approach as I feel like I'm beginning to throw money at the problem without understanding it which has never worked for me in the past.

Still stumped.

Daniel

[Victor Laury]

I'd say, you can't go wrong with the new needles and nozzles from ZT. Even if it doesn't cure your present puzzle, the cars respond to the ZT "SM" needle nicely.

[Chino 240Z]

Not sure if your last adjustments were still running rich? Might or might not help? But while tuning my new ZT carbs last night I was running to wet and I had to drop the fuel level in the bowl alittle more than what was called out .55 or 9/16, probably another 1/8" on the front and about another 1/16" on the back to get the fuel level down to where I could play with the mixture nut. I think I was in those fuel bowls 8 to 10 times till I found a level I could live with. I had the .101 nozzles installed with SM needles and I am running alittle more fuel pressure than normal so that could be some of the reason why I am too rich. Any how I was amazed at the sesitivity the fuel level was before I could fine tune it with the mixture nuts. I was getting nowhere until I tried alittle different from the exact specs or normal setup. You may need to think this too?

Oh BTW, did you use new gaskets both sides of the insulator if you removed your carbs? I didn't at first cause they looked perfect but still had a vacc. leak only at high rpms. Installed new gaskets like I was suppose to and that problem went away. A problem may be a couple of little misses or problems combined together and might look like only one problem? Does that make sense? oh well, keep digging. I still am not conviced my carbs a putting out their full potential but I know for sure they are on their way after last nights joy ride around the block, got the adreniline flowing! and the shakes once back in the garage before the police showed up. he he he

Craig

[inline6]

I was just reading your original post and noticed this --

"And prior to me tuning the carbs the sudden drop off didn't exist."

This is really confusing as all you did was change float height and needle height.

What are the chances that something else has gotten off track during the tuning process?

"At this time I have both carbs turned out 3 turns, although I can't sense any difference from 2 1/2. But with this issue I figured I'd go a little extra. Much more out and the idle becomes erratic."

Not that this will fix it either, but I think you are running them unnecessarily rich by this description. I run mine leaner and leaner until I can feel the hesitation, (usually during part throttle). When I get that hesitation, I turn them and 1/8th to 1/4 turn richer and the hesitation goes away.

It's too bad you are not near by. I'd love to get my hands on your car for a few minutes and see if I could find out what is wrong.

Any idea what is causing the vibration? Obviously where I was headed was if you have different amounts of air going into the front vs. rear carb, then that will cause engine vibration at higher rpm and could cause the problem of running into wall at 5k rpm.

Is the problem load dependent? In nuetral, sitting in the driveway will it climb to 6k and beyond quickly? What does running without the air cleaner cover and filter, but keeping the backing plate (with air horns) do for it? Do you have a timing light? Can you observe the timing marks by operating the throttle by hand from idle? What do the timing marks do as you rev? Full advance is about what? at about what rpm?

Garrett

[Kyle]

Hi Guys-

Man this sounds familiar. I'll add a few thoughts-

What sort of oil is in the dashpots, and is the level set properly?

Do both pistons rise smoothly when the throttle is quickly hand opened to WOT? Raise the dashpots all the way by hand, then let go of both. Do they slap back down at the same time? Did you maybe bend a needle?

It's puzzling that the air cleaner could make a difference... unless... you put the carb-aircleaner gaskets on upside down?

Needle setting shouldn't really make a lot of difference at WOT.

I agree that this sounds a lot like a fuel delivery issue. Try running it up to 4,000 in gear, letting it sit there at low throttle opening for a few seconds (to let the float chambers refill) then smashing the throttle again. Does that help?

Other things to consider - point bounce or distributor arcing, valve float or adjustment, etc.

The stock needles should work great with a L24/E31, I don't think SM's are necessarily the panacea. SM's are well suited to L28's, in my experience.

Keep us posted!!

[240ZMan]

I had to travel for work for a few days, so no time to work on the Z. Hate it when my work life interferes with my Z life ;-).

You guys asked some questions, and this thread has gone on long enough that I feel I should summarize what I've tried.

<b>Configuration:</b>

73 240 with E31 head, N33 manifold, 72 3 screw carbs, Pertronix electronic ignition and coil. Everything else is stock

<b>Issue: </b>

- Engine won't pull past 5k rpm in any gear at WOT with the air cleaner assy on.

- If I back off the throttle just a little, it will pull higher, but it's not strong at all.

- It will rev past 6k in neutral.

- If I remove the airleaner and cover, it revs a few hundred rpm higher before it stops increasing.

- If I remove the aircleaner backplate, the engine will pull past 6k, albeit more wheezing, than pulling.

- I live at around 6,000 feet elevation.

<b>Things I've checked and tried:</b>

- new distributor cap and rotor

- timing is set to 8 degrees advance at idle which is as far as I can go before pinging.

- full advance comes somewhere around 2-3k rpm (don't remember exactly where) and is around 30 degrees

- all new rubber fuel lines (needed replacing anyway)

- bypassing mechanical fuel pump and running with electric only

- raising and lowering fuel float setting

- running with choke on

- adjusting mixture to excessive rich, and excessive lean

- throttles open fully when gas pedal is floored

- cleaned pistons and needles and ensured that both needles are seated at the proper height.

- pistons move up and down with no sticking

- dampers properly filled with Marvel Mystery oil

- carbs balanced with UniSyn

- One needle is slightly bent at the tip but does not stick at all (spoke with Steve at ZTherapy and he didn't think that had anything to do with this).

- compression ranges from 145 - 160 cold

<b>Next steps:</b>

- I ordered new needle and seats for my floats as one showed lots of wear and I have an intermittent idle problem when it's hot out. I'm going to put those in this weekend, although I dont' expect that to make a difference.

- When I put the E31 head on I made sure that the crank tdc mark was accurate, and that the cam timing was correct as well. I was surprised that even though my timing chain has 200k miles, it didn't show stretch. Hence my cam gear is on the #1 setting. Still, I keep wondering if something is off and I plan to try the #2 setting this weekend as well.

- I haven't adjusted the valves, but the mechanic who sold me the head said he had adjusted them shortly before taking the head off. I'll probably do the adjustment this weekend as well.

- I suppose I could buy a new set of needles and nozzles. I asked Steve at ZT and he really didn't think that was the issue. I want to respect his opinion given that his company sells them, yet he didn't advise I do it.

Other than that I think I'm out of ideas. (Does this qualify as the longest thread yet?)

[Kyle]

I don't see fuel filter in your list... but it's a long list and I could have missed it.

I've had my car to 6,000+ feet and it's been fine. Power is down, but still manageable. I'm pretty sure, but not positive, that it revved fine as high as I wanted to take it.

Have you tried bypassing the electric fuel pump? It's a stretch, but maybe it's restrictive.

To be sure, and to rule out in-tank pickup, maybe you could rig up a gas can as a fuel cell, run straight from it to the mechanical fuel pump (no fuel filter) and see if it makes a difference.

If you still have the stock needle/seat, it might be worth a shot swapping in new ones. FWIW, I have *never*, in 10+ years of Z'ing done more than a cursory inspection/adjustment of the floats. Maybe I'm lucky, or maybe it's not that critical.

Have you checked with other high altitude owners? What do they say about this? And maybe... just maybe... your valve springs are tired, so they're floating. I don't know of an easy way offhand to check it. Maybe a vacuum gauge? Hmmm... I kinda like that, actually. If it's fuel starvation, I *think* you'd see vacuum spike upwards; if it's valve float, you should see it drop and/or fluctuate maniacally? This is just a guess, mind you, but it might be a helpful diagnostic. If your engine has 200K on it, it might be getting tired. Your compression readings support that possibility.

[240ZMan]

Kyle, Good catch on the fuel filter. Actually it was the first thing I changed, but I forgot to list it.

I was going to try your suggestion on bypassing the electric fuel pump, but then decided not to with the following reasoning: at WOT it stops reving at just before 5k. I can hold it there for some time, and even tried it up a long grade. As soon as I let up a little it revs higher. I figured if there was a fuel delivery issue upsteam of the carbs then this should have drained the floats enough to cause a miss of some sort. But it didn't.

Also, power right up to just before 5k is actually pretty good. It just stops reving any higher, literally as if there was a rev limiter. Plus, in the past I have been able to rev this head and motor to 6k, although it wasn't strong above 5k. So I don't think it's floating valves. Also, removing the aircleaner backing plate shouldn't make a difference if it was the valves floating I think.

I've found there are 2 schools on float settings: one who feels they need to be set extremely accurately, and the other who never even deals with them. My own experience so far lines up with yours.

I'll see about getting some fittings to hook up a vacuum gauge into the cockpit to see what's happening. I think that might give me some clues. I'm a little less comfortable with the gas can idea so won't try that one right away.

Thanks for the suggestions!