Throttle stumbling

[w3wilkes]

I'm guessing that for about the price of the FAST system you could go with rebuilt SU's from Ztherapy which would work with the big cam  for sure.

[Lenbo211]

Thanks,  Great information.  I've talked to Bruce at ZTherapy a couple times.  $1,400 for a carb conversion so about the same as the Fast EFI.  I'd probably rather do Carb, but Bruce is backed up to September with orders.

Does anyone have troubles getting the carbs to work well without waiting to warm up?  That's the only drawback I've seen on the forums around doing a conversion.

[siteunseen]

I just got back from a post hibernation drive. Cranked right up on the 3rd turn of the key. The float chambers were empty I'm sure from evaporation.

After 30 seconds or less the oil gauge was up and I left out driving slow through the neighborhood but no stumble at all. Ran great!

Never had a warm up issue and my heat/coolant line through the carbs is bypassed. Mine run best cooler. 3 screw round tops adjusted right are solid as a rock. 

[Jeff G 78]

3 hours ago, Lenbo211 said:

Does anyone have troubles getting the carbs to work well without waiting to warm up?  That's the only drawback I've seen on the forums around doing a conversion.

My 260Z race car carbs are amazing.  Granted, I don't drive it around much in the cold weather, but it fires up immediately with the choke lever pulled and within 30 seconds at most, I can back down the chokes.  If I need to move it right away, it still runs well with the chokes on.  I'd say other than the manual chokes giving it away, it runs better overall than my EFI car and you'd never know it's carbed.  My SU's were on a 240 I bought out of a field in 1984 and they sat in my garage until 2009 when I took them apart to clean them and install new gaskets and get everything working smoothly so I could put them on the race car.  They have never been "rebuilt" other than new gaskets, hoses and cleaning.

[Mark Maras]

 If the mixture is correct there's no problem with cold starts. I suspect those with cold start and or cold running problems may have the float levels set too low or the chokes aren't set up properly.

[siteunseen]

That'd be the cheapest way to go carbed, like Jeff said above, buy a 240 motor with carbs and all the bits. Rebuild them for $200, if they need be, and slap them on.

I bought one for $300 complete from a guy that put a V8 in his 240. I have a whole setup for my 280 to switch over when I get super bored. Hell, maybe I'll start on it tomorrow.

Covid-19 paid vacation but now it's getting old. Saving money on belts though. 

Edited May 7, 2020 by siteunseen

[Lenbo211]

OK, I did a full test on the AFM and it all came back correct.

Here is a video of the car running. Hopefully the video gives some of you Z Heads an Aha! moment.

https://youtu.be/nfUWozhWMzA

There are a few things to see so you may want to watch it a few times.

Fuel Pressure gauge. Looks like it is working as expected.

I have vacuum gauge. It is low at idle, but consistent. It is plugged into the line between the intake and the Distributor/Throttle body. When we add throttle, it drops to zero. Is that normal?

idle is around 800 to 900. that is as high as I can get it to go with the idle set screw.

The timing light is shown blinking in the background as well. I have it connected to the negative terminal on the coil. Something I saw when I was watching the video is that the light blinking isn't consistent on throttle input and release. might not even be that consistent on idle, but it's hard to tell if that is just the shutter speed difference on the camera. But light goes away when we blip the throttle and let off. that doesn't seem right. It's like the coil stops receiving or sending a signal.

Could that be the TPS?

I wonder if the TPS isn't telling the computer what to do correctly so the signal from the disti gets lost in transit to the computer or coil?

I tested the TPS last week per the FSM. The Idle switch check came back fine and the full switch check came back fine. When I did the Throttle Valve switch insulation check, I didn't fully understand what it meant by "infinite" on the ohmmeter. I'll check that again tomorrow if I can figure out what an "infinite" result would be.

One thing I know is this car was parked 10 years ago for a reason that died with the guy who parked it.  The cam could be an issue, but it wasn't the reason he parked it so I'm probably chasing what caused him to park it.  

 

[Dave WM]

infinite resistance, an open circuit. Your vacuum should be connected to brake booster port or some other manifold vacuum, you have it connected to ported vacuum, not correct for the reading of manifold vacuum. I suspect its all working as it should, how does it drive?

[Lenbo211]

2 hours ago, Dave WM said:

infinite resistance, an open circuit. Your vacuum should be connected to brake booster port or some other manifold vacuum, you have it connected to ported vacuum, not correct for the reading of manifold vacuum. I suspect its all working as it should, how does it drive?

Thanks for the answer on resistance. I did a little more looking and realized that infinite resistance = 1 on my meter. Yeah, I did have it on the brake booster earlier and it bumped up to 15 when I hit the throttle.  but the point is the thing stumbles when I add throttle so I moved the vacuum gauge so I could get a reading on what the ported vacuum was doing on the throttle body to see if that was normal. Did you watch the video?  Maybe it's not clear in the video, but it idles OK, but runs horrible when I add throttle.

 

[Zed Head]

Never mind.  

Edited May 12, 2020 by Zed Head

[Zed Head]

11 hours ago, Lenbo211 said:

Could that be the TPS?

I wonder if the TPS isn't telling the computer what to do correctly so the signal from the disti gets lost in transit to the computer or coil?

 

The TPS only has three positions.  Idle, "full" throttle, and neither of those.  Full throttle adds 27% more fuel than the base fuel enrichment.  Idle adds a little bit of idle enrichment.  Off idle the AFM is in control.

Edited May 12, 2020 by Zed Head

[Zed Head]

Do this coolant temperature sensor "tweak" and you'll probably get rid of the stumble.  It might run okay, or it might be gassy in other areas.  The low RPM, low throttle, stumble is a typical lean mixture problem, on many old Z's even with stock cams.

http://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

p.s. and just for conversation, if I remember right, one reason big cams run poorly at low RPM is because the cylinders don't get cleared out well, and they tend to push air back through the intake valve before they start pulling.  So you can get lean cylinders and rich cylinders and dirty mixture cylinders all at the same time.  That's where the rumpity rump sound comes from.  It's actually just the sound of an inefficient system.  But, adding fuel can fix the lean cylinders so that at least they'll be firing.

Anyway, the "tweaker" is the simplest way to get richer.  No FPR adjusting or AFM fudging necessary.  Too bad Radio Shack isn't still popular, those little potentiometers are hard to find now.

Edited May 12, 2020 by Zed Head