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 Post subject: Leaky throttle shafts
PostPosted: Sun Sep 17, 2017 1:11 pm 
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Location: CT
I know someone who has air leaks around his SU throttle shafts. Who wants to talk about that?

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PostPosted: Sun Sep 17, 2017 7:12 pm 
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Hey I know that guy too! :lol: Great minds think alike Frank!

So I've been trying to pinpoint the cause of a strange performance issue in my 260z. What is happening is she runs completely out of power around 4.5k rpm, and will backfire multiple times when shifting at that rpm. However if I shift at 4k the power is smooth and there is no noticeable backfiring happening. She runs absolutely great at any rpm lower than 4.5k, starts well, and idles perfectly.

I have been thinking about this issue and the only thing I could come up with was a possible problem with the advance mechanism in the distributor preventing full advance from being achieved, causing retarded timing hence the backfiring and lack of power.

After putting a timing light on it and confirming the advance works perfectly, a mechanic friend of mine (who's light I was borrowing to check the timing) noticed he could hear a vacuum leak somewhere. Incidentally I still don't hear it but that's neither here nor there.

Anyway he checked the usual places like intake manifold gasket and carb bases. Then he gave the throttle shaft on the rear carb a shot of carb cleaner and she shut right off. I started her back up and he gave the other carb throttle shaft a shot too, same result.

Leaky throttle shafts!

Interestingly enough these carbs are Z Therapy units with only 3kish miles on them. I bought them back in 1998 and they have been installed since then, but my Z basically has lived in storage the whole time. The last 1,000 miles have been since 2016. And like I said, she runs beautifully up to that rpm ....... weird.

Jay


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PostPosted: Mon Sep 18, 2017 6:16 pm 
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I spoke with Vinni Bedini about my z issue today and he wasn't convinced that it was resulting from the throttle shafts. He has instructed me to try changing my fuel pump back to my old OEM pump (currently running a cheap aftermarket unit). He also suspects I may have the timing set too advanced.

Next chance I get I'm going to do that, I'll post the results.


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PostPosted: Mon Sep 18, 2017 11:42 pm 
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We're all standing by for latest developments. If Vinny says something, I tend to believe it.

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PostPosted: Tue Sep 19, 2017 5:11 am 
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I agree. He explained that the amount of vacuum in the intake at higher rpms or wide open throttle is so low it shouldn't matter if the shafts are leaking, they wouldn't cause the problem I'm having. Makes sense to me. I'm glad I called him.


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PostPosted: Wed Sep 20, 2017 6:29 pm 
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Well Vinny called it. He was spot on about the fuel pump. I swapped out my two year old aftermarket pump for my 20 year old OEM one and it runs much stronger. I ran it up to 6k rpm with no effort now. I'm sure she would have pulled further. 4.5k is no longer a problem!

She still pops when I shift at higher rpms though. So I'm going to try backing off on the timing like Vinny suggested and see what that does for me.

Jay


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PostPosted: Sun Sep 24, 2017 8:03 pm 
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New update. I had every intention of starting with the timing but the leaky exhaust manifold got to me. I had a blown gasket due to a nut that had loosened up where the manifold and exhaust downpipe come together right under the carburetors. So I swapped in a fresh gasket and replaced all the nuts with new ones, then fired her up. I didn't realize how loud it was leaking until then..... So much quieter. But in a good way, still plenty of exhaust note left just no more harsh tapping sounds.

Anyway I had to take her for a spin to Zee how she sounded at speed. I ended up pushing her to 6k multiple times with not one single pop through the exhaust.

She is now running perfectly..... Possibly better than she ever has!

I am strangely curious how she's running so well with leaky throttle shafts though.

Guess I shouldnt worry about it.

Jay


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PostPosted: Sun Sep 24, 2017 10:10 pm 
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This is great news Jay, but just remember, it didn't happen until you met us! :lol:

We're 8)

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PostPosted: Mon Sep 25, 2017 6:39 am 
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Ha! You certainly are! :mrgreen:


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PostPosted: Thu Sep 28, 2017 10:27 pm 
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So your car was running slightly rich and between shifts the fresh air from the leaky exhaust gasket would ignite the unburnt fuel in the exhuast. This isn't really a back fire. The question is if you put back in the non oem fuel pump does you fuel pressure really go up. Not that means you will run richer, if your floats and needles are working they should hold back a far amount of pressure, maybe 8 psi reliably.

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Bruce - Early 260Z - 305 Light Blue Metallic - All Stock


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PostPosted: Fri Sep 29, 2017 5:30 pm 
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I actually think the old (new) fuel pump didn't produce enough fuel.... Either gph or psi, not sure which. Maybe both? She was definitely running out of fuel at 4.5k rpm and completely losing power with that pump, and now is a whole different animal.

As for the popping noises through the exhaust (not technically backfiring), the cause seemed to be unrelated to the fuel pump. I was assuming they were related because I had to push it to the point of losing power to get the popping....they seemed linked to the same culprit. Not so. I think the leak by the manifold was allowing the fresh air into the hot pipe and burning whatever unspent fuel/fumes were in there? Not from being rich, more from not being 100% efficient. I can't really think of any other explanation for it.

The carbs are actually not rich enough. In fact I'm hoping to play with it this weekend to try and fatten up the mixture more, the vacuum leak from the throttle shafts has her running very lean. My plugs have 1,000 miles on them and they are spotless. I'm glad they are not fouling, and she does idle well enough, which is good. But she "hunts" a bit at idle though....up and down. And runs hot at low rpm or idling, despite having a new fan clutch, shroud, belt, fresh 50/50 coolant, and a cooling system that pressure tested perfect. I think if I can richen things up that may be alleviated. I hope anyway. :roll:

Jay


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PostPosted: Wed Oct 04, 2017 7:34 pm 
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I thought I'd take my Z for a ride tonight but instead she took me for one. :roll:

I planned to make my timing and carburetor adjustments today so I began by letting her warm up while I looked around under the hood. I noticed the fan wasn't spinning but the shaft in the center of it was. I figured the clutch must be bad. After removing that and the shroud I saw that one of my fan blades was actually stuck into my radiator core! I couldn't think of why that would have happened as there were no impact marks of any kind on the inside of the shroud. I called Vinny to get his opinion and he said that was a first for him also.

Fortunately I keep old parts lying around. The fan clutch was new last spring and when compared with the old one it is 1/8" or so taller to the fan mounting surface. Why it didn't happen immediately when I installed it is a mystery. None of the hardware was loose.

Jay


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PostPosted: Thu Oct 05, 2017 9:29 am 
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WHAT??? :shock:

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PostPosted: Thu Oct 05, 2017 5:38 pm 
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I think the fan blades must flex at higher rpm, which combined with the taller clutch became an issue now that she'll rev happily to 6k.

Wasn't a problem when I was limited to 4k.

New one on me.

On the bright side my cooling system should be tip top in a week or so.

And I can finally finish my mixture adjustment. :mrgreen:


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PostPosted: Thu Oct 05, 2017 7:02 pm 
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You had a shroud? That's what saved the blade from flying thru your bonnet.

When we were teens we spent the weekends cruising our main strip looking for girls, drags or fights. We all stuffed into one car. One night we were in Ray's '56 Chevy 265 with triple Strombergs and caught a race against a brand-new (as in brand-new) Mercury Cyclone Caliente 289 4Bbl. He had more kids in his car than we had in ours, so we thought we had a fair chance. We did a pretty fair job but as we were approaching the conveniently-marked 1/4 mile point on the highway, he got up into the high revs and began to edge ahead. When he got even with my door his hood eksploded and his fan belt cut a slice up his driver's side fender, across 1/2 of the hood, then disappeared back inside his engine compartment. There were multiple bad noises and two fan blades flew out of the hood and far fender. His fan belt had broken (or come off) and gotten tangled in the fan blades.

I can't picture what might have caused yours to just break off like that. Good thing you had a shroud. Radiators are cheaper than bodywork.

For goodness sake, take advantage of this opportunity to change your water pump while you're in there with your fan and radiator off anyway. :thumbs_up:

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