[IROC_385z]

I know that this has been gone over a million times, but I don't think my question is quite the same.

At idle, in gear, I have about 10".
while crusing in D at about 35-40mph I will have from 14-20", depending on how much pedal I give it.
If I slow down to 20-25 (still in 3rd) and then drop the gas about 1/4 down the vacuum will drop from 8-10". If i gas it 1/2 it will drop to 5".

Which point should I go by to pick my PV? I have a newer 4160 and I think factory it has a 6.5pv. I am thinking that I should have an 8-8.5 in there, what do you guys think?


Engine info: http://iroc385z.gotdns.com

[Rick WI]

The real question is how does it "feel" Is the response crisp with the 6.5 on accel when loading up the throttle? If it "feel" good and is responsive then it is probably not an issue. If it seems like there is a hole in the fuel curve and throttle response is lacking (meaning lean) than up the PV.

[spinn]

dont jump 2 steps . just a half or maybe 1 .

take your lowest vacuum , then half it. if your a hard on add 1.

flat spots try upping the shooter size


cruising will work too.

[bracketeer]

They recommend power valve = 1/2 the vacuum at idle in drive with automatic or in neutral with a standard.

[Rick WI]

IROC, I just took a gander at your specs on the motor. Everything you posted makes sense on the PV question with the exception of the idle vac reading, it seems REAL low. I would expect you to be well above 10 and I am wondering why it's not. Although that is a healthy cam it's by no means huge. I would expect to see 14 to 15 inches based on your specs, at idle. I have done up 383's with duration in the 248 range and had vac at 12".

What makes sense is your cruise vac readings and with only 10 at idle that's a pretty big differential; 20 I suspect under light load hwy cruising versus 10 at idle.

I wonder if there is a slight tuning issue on your idle circuit or if the timing is retarded. Are you running ported or manifold vac advance and what's your base setting at? The only other thing is you may be short on compression a tad. Just curious is all.

[IROC_385z]

WELL, I thought that 10 was low too. I do get around 20 when cruising. As of now I only have the 15* initial and do not have my vac hooked up. I have been chasing down this noise *posted in another thread* that I am now assuming is just piston slap. I do not KNOW what detonation sounds like so I disconnected my vac advanced to help rule it out. I did/will be running it on full vacuum.

The float is set to just seep out when running and the sight plug is off, and the idle circuit is set about 1/8 turn lean from highest vacuum at idle, in gear. The screws do not seem as responsive as they first were; think I may have a vacuum leak somehere? I had been having a problem with backfiring out the carb, which went away after a real big backfire that caught the carb on fire , and now my idle seems lean and I cannot get it rich again. Now when I turn it off sometimes it will run-on for a few revs and then just stop, very violently. I also noticed that 2 of my plugs were brand-new looking (white), 3 where tan on half/white-ish on the other half, 1 was tan/black and the other 2 looked pig-rich black. I already tried tightening the carb and manifold bolts but have not yet sprayed carb cleaner around the base.

The other thing That I have been pondering is my PCV setup. I do not have a baffle, just a grommit, for the PCV valve. Could it maybe be sucking up oil and contaminating the combustion process?

*random thought* Or maybe should I open up the rear idle circuit up a bit and lower my idle-screw some?

This has all got me so confused that I don't know where to start. I want to get the carb running right before I bring the vacuum advance back in, and I REALLY want to get that vac back on cause It ran oh-so-well for a good day. Look up my other posts; maybe the other things I have been having problems with will help tie this all together.

Thanks SO MUCH for stickin with me here, I appreciated it much.

[Rick WI]

I'd replace the PV no matter what after a back fire like that, even if you are supposed to have blow out protection for it. I'd replace it with what's in there.

If you are sucking oil through the PCV you will see it out the exhaust. Even so, worht disconnecting if the rockers covers are vented just to eliminate and variables.

I'll look through your other posts.

[IROC_385z]

I was under the assumption that a blow PV would cause an overly-rich condition?

I checked my secondary transfer slot today...it was closed ALL the way. I opened it up so that only a square-box was exposed. Now the Idle screws have NO effect and the car still runs lean at idle/part throttle. It did NOT do this when I first put the carb on. What else should I be looking to? COULD a blown PV actually make it run lean like this?

I plan to pull the carb and give it a quick go-over with some carb-cleaner this weekend, is there anything special I should pay attention to or look for?

Thanks guys

[Rick WI]

The only reason I suggested you replace the PV is due to your comment about backfires. It eliminates a variable and it's cheap. Your correct that a blown PV will make things rich but you also mentioned two black plugs as well. I know you also mentioned the condition of the other plugs, which seem fine. It's just always good to make sure unknown variables are taken out of the picture.

I also would go through the distributor after you make sure the carb is clean and working. I would make sure that the advance (mechanical) operates smoothly. If this sticks it will make the car run like crap, similar to your shut down problems.

[Tech @ BG]

I was under the assumption that a blow PV would cause an overly-rich condition?

Yes, a blown Power Valve would cause an overly rich condition.


It did NOT do this when I first put the carb on. What else should I be looking to?

Your ignition timing may have moved, or there could be some dirt or debris caught up in the carburetor.



COULD a blown PV actually make it run lean like this?

No.... If the PV was bad it would be open all of the time. going richer.


I plan to pull the carb and give it a quick go-over with some carb-cleaner this weekend, is there anything special I should pay attention to or look for?

Just make sure all of the orifices are cleaned out, and that you put fresh gaskets into it.

[IROC_385z]

So the ignition timing could cause my A/F to read lean?

I still don't understand why my idle screws have no effect on the carb anymore, THAT is what really confuses me. With the transfer slot for the secondaries set correctly I can close the idle screws and the engine doesn't care. I can open them 3 full turns and it won't bat an eye. If I cover the rear transfer completely then the drivers side screw will stall the engine if closed, but not the passanger side. Both will make a differance on the engine at idle up to about one full turn out, then nothing.

[Tech @ BG]

IF the ignition timing is off it can cause some of the problems you are having, but not all of them.

There has to be some type of trash in the carburetor for the mixture screws to be as unresponsive as they are. I'd take it apart clean it out real good, and install fresh gaskets in the carburetor.

[IROC_385z]

Aye capyton....this sunday im ganna do that, i'll let yas know what happens.

[IROC_385z]

Well guys, this officially has pissed me off. I rebuilt the carb, all new gaskets, used carb-cleaner and compressed air in/on EVERY oriface/part and it still hates me. The PV was a stock 6.5, which I replaced with a new 6.5. It looks like it is not a clogged carb.

I fired it up and, no matter how far I unscrew the idle adjustment, it idles sky high. I turned in both idle mixture screws ALL THE WAY and the car still runs! The rear transfer is set correctly.

*sigh* any other ideas?

[benwantland]

Is the throttle linkage arm resting against the idle speed setscrew? Sounds like your primaries are hung up on something. Do you have a choke on this carb? Are you certain that it is all the way off, and its fast idle linkage is not stuck on?