SU carbs and high revs

[Doehring]

You wrote your timing is advanced 7° degrees. I think the manual mentions 17° advance. Can you find out the advance at 5000 rpm? I read it should be 32° (Rebello?). I have the same problem as you and it became better when I put the advance at idle (650rpm) to 20°. Perhaps I have to do even more. Before that with my car the funny thing was that it felt like a rev limiter already at 4500 rpm but when I lifted the gas pedal for a short time and pressed it again, the revs went up to 5500 and more before after some time the shaking started again.

That's why I also believed in a fuel problem. In another thread here your (and my) problem was already discussed and there the solution after a long discussion and some trial and error actions was the timing.

Good luck

Rolf

[240ZMan]

Rolf,

Don't take this wrong way, but I'm glad I'm not the only one with this problem ;-)

How did you advance your ignition so far and not have detonation? I tried 10 degrees advance at idle and found I had significant pinging at lower revs (and couldn't tell at higher revs because the other engine noises are too loud). I can only imagne what 20 would be like!

Thanks,

Daniel

[Doehring]

Daniel,

there was absolutely no problem to advance the timing. Are you shure your timing marker at the engine and the marks on the disk are correct? My first Z had once gotten another disk because of the added smog pump and the correct timing was not at Cylinder 1 but at No.4 !

What I did, I took out the No. 1 plug and put into the hole a slim stick instead. I turned the engine slowly and when the piston came up the visible end of the stick went down, like a lever. At the lowest point I looked at the marker to see if it is in line with the mark for 0° on the disc.

Another possibility that you can't advance your timing enough might be that the dizzy isn't in the right position.

Did you find the other discussions in the engine forum?

Good luck

Rolf

[Victor Laury]

There are several mechanical advance curves. which one your car has is dependant on several factors.

Where was it's market?

Which Transmission?

What year?

Was the dizzy swapped at some time?

The only way to be sure is to lift the breaker plate and look at the stamping on the advance cam (flat rectangular plate with 2 slots the advance weight ride in)

The number stamped on that plate is the distributor advance.

Multiply that by 2 and you get the crank advance.

So if you have a 7.5 distributor, you have 15 degrees crank. Set the your initial timing at 18 and you've got 33 total advance (pretty safe at normal compression and pump gas)

If you have an 11 dizzy, you have 22 degrees at the crank, set that at 18 and it I'll rattle itself to death, quickly.

If you can't run more the 7 degrees initial, you probably have a 13.5 dizzy (U.S. Smog equipped).

Which one is best? That can be argued. My preference is the lower mechanical advance numbers of the "Euro Distributor" as marketed to the U.S. When I had my EI dizzy built, I had Gary set it up with a 7.5 advance. Sot I time 17 for regular and just a hair under 20 for premium. If my compression was enhanced, I'd have to lower those, or run race gas and run a high overlap cam.

Nissan, in it's smogging wisdom, equipped the U.S. Market Roadster ( a sports car!) with the worst possible dizzy, a 17.5! You had to time that at TDC! Absolutely the lousiest idle and low-end performance ever. That's ok, cause we all fixed that with the pre smog 7.5 dizzy!

[240ZMan]

Thanks Rolf and Victor for the feedback. I'm going to focus my attentions next on the timing. The dizzy I have is a used replacement and I know nothing of it's history. I'll open it up tonight and see what there is to see. I'm also suspicious of possible binding on either the mechanical or vacuum advance.

To Rolf's suggestion, I'm sure the timing mark on my pulley is accurate as I just had the head off and had the chance to get everything set right for TDC.

One question on the issue of dizzy advance: I notice at idle that if I pull the vacuum hose off the dizzy the timing doesn't change one bit. This is different from any car I've ever owned where it was important to remove the vacuum line and plug it. Also, if I put the hose up to my tongue I can feel some vacuum, but there is no change in idle speed if I leave the hose open to the atmosphere. I'd expect the idle speed to rise a little as the mixture leaned out.

[Doehring]

Hi Victor and 240ZMan,

to me it's always astonishing how much knowledge the forum members all together have. Up to now I never read or heared what Victor wrote about the different advance curves and the dizzies. Thanks !

Rolf

[240ZMan]

Well, the saga continues, but I've finally tracked down the culprit.

I checked my dizzy and cleaned it up as well. It's an "11". The vacuum advance moves without binding and the mechanical weights can be moved without any binding I can feel.

I also tried setting the floats significantly lower. None of this made any difference.

Then I tried making a run without the aircleaner including the backing, attached. In other words, just the mouths of the carb breathing straight out. PROBLEM SOLVED!. Just for grins I put the old stock '73 aircleaner with square horns back on the problem returned. This is very strange because I know I never had this issue with the old aircleaner.

I also noticed from experimenting that with the aircleaner back on, if I kept it floored in 1st gear until the engine "settled in" at around 5k, and then lifted the throttle just a bit, the engine would rev higher! I could get it to pull to 6k, albeit the power was falling off at that point.

So I went back and checked that the throttles are opening properly with the pedal floored. I'm using a mirror in one hand a lifting the cylinder with the other and from that perspective they look horizontal. But I'm suspecting that this is either 1) an optical illusion perhaps?, or 2) there is some resonance with the aircleaner on AND the throttle in a specific position that doesn't occur when the airhorns aren't there.

I tried backing the linkage off a few turns and took one more pass. I don't think that was enough and by then it was too late to drive at redline ;-) So I have backed it off another 5 turns and will drive it later today, as well as enlist the family one more time to floor the pedal and check the throttle position.

My concern is that in order to get the throttle set to let me pull the full revs, I'm going to limit the opening during the entire rev range below 5k. And I'm surprised that this is so sensitive. I'd think that others would have experienced this before me.

I'm all ears to hear what others think!

[240ZMan]

Just tried one more thing (one of the advantages of working from home on a slow day). I adjusted the gas pedal stop so that when fully floored the throttles open a little bit more than 5 degrees shy of fully opened (when they hit the stops). It pulls a bit past 5k now, but still stops reving completely after that unless I lift slightly on the throttle. Seems to me that I'll need to really further reduce the throttle opening to get the revs to climb which will reduce power - which seems pointless and not a strategy I want to follow any further.

Sor right now I have 2 theories:

Theory#1: For some reason my needles/nozzles combination doesn't flow enough fuel to meet the peak demands of the engine. With the aircleaner on, the horns enhance airflow and the engine runs out of the incremental fuel it needs around 5k rpm and simply stops increasing revs due to an excessive lean condition. When I back off the throttle a little bit I reduce the airflow and the mixture comes back to more normal and the revs climb a little further. When I remove the aircleaner completely, including the back plate, the lack of airhorns means the peak airflow of the engine is reduced and hence the fuel flow just barely meets the needs of the engine (I say barely because it really doesn't even want to pull to 6k as the power is falling off so badly). I'm assuming it's not a carb setting as I tried running with the choke on and it made no difference.

Theory #2: There is a resonance between my airhorns and the carbs right around 5k rpm at full throttle that limits airflow. Reducing the throttle a little changes the resonance point and allows the engine to rev past that point.

If theory #1 is right, then I suppose the solution would be to get new needles/nozzles.

If theory #2 is right, then what is unique about my situation to cause this. And how do I solve it?

So 'm wondering what others think of my theories.

[Victor Laury]

Maybe your cam is advanced 1 tooth?

[240ZMan]

I just recently had the head off, and when I was putting it back on, I thought I might need to move the cam gear to the #2 or #3 postition to account for chain stretch. But when I put it back on in the #1 position, the notch on the gear was to the right of the mark on the tower, just like the book and ZTherapy's video say it should be. So I'm going on the assumption that the cam timing is good.

[Victor Laury]

Vacuum leaks? Hows your throttle shafts?

I can't beleive the air cleaner theory, is it the stock air cleaner for the carbs? (round top carbs and 70~72 cleaner or flattops with the triangle port cleaner) . Nissan did a lot of engineering on those cleaner air horns and they are about as good as they come.

[240ZMan]

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

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.