shredjesse

Hey guys, onto my next question and new carb situation...

Got a 383 build in my 1968 Chevrolet corvette. I have a QFT SS 735 vacuum secondary carb on there. Cam is 218/224duration at 50/1000’s lift, 570/565 lift. Got the whole puppy tuned up, tons of vacuum (16inhg @ idle) and running fine with no PCV hooked up, idle at 800 pretty clean. 4 corner idles turned in to 3/4 turn.

Hooked up PCV system, idle jumped to 1100rpm with exact same configuration. Tons of vacuum still. Timing is 12 initial, 34 total, with vacuum advance adding 18 at any given point (so 30 degrees at idle with no load, dropping down when load is put on the car). I know the PCV setup is good, it's a brand new billet setup from Jegs. The vacuum for lights and what not was coming off the carb PCV connection, I moved that down to a nipple on the intake manifold. All connections are pretty darn snug. Used a very minor amount of synthetic grease to help get the hoses on, as they are 5/16th on 3/8th connectors.

Adjusting down the primary throttle plates idle screw, I was able to get it go down to 915 or so RPM at the lowest, and it takes one hell of a spring setup pulling on the throttle arm to make sure it closes that much. I eventually hit the point though where the screw was not putting any pressure on the arm of the carb, and was able to get it back into adjustment by holding lightly on the arm and tightening up the screw until I felt it move the linkage when it reconnected.

Has anyone else experienced this? What are others doing to work around this sort of setup? I'm thinking that closing the secondary throttle plates a bit would help as well?

The secondary throttle plate does visually appear to be notably more cracked open than the primary, especially at this point. I am imagining that if I close them just a hair more, I'd get the primaries back into a reasonable range of adjustment.

Ryan Menzies

Shred,

Yes, use the secondary idle stop screw to close them up a bit. BUT given that you have a 4 corner idle set up you will have to even out the transfer slots on primary and secondary to a little square or a "D" shape.
So pull the carb off and look at both primary and secondary transfer slots from the bottom and set them even.

My first thought is being a 3/4 turn out on the idle screws and running smooth might be an indication of a tad rich regarding IAB or maybe IFR "jets".

Is the PCV working correctly and free? 300 rpm jump by connecting the PCV seems like a lot but I could be over thinking it.

shredjesse

I'm not 100% certain on how to check PCV valve, but it definitely is moving air freely. Engine idles great at the current 950cfm, quite smooth even. Car also pulls better than ever after doing some adjustment to the vacuum secondaries as suggested by others, along with a larger accelerator pump discharge nozzle.


I'm not quite sure I understand the adjustment aspect that you are stating of the transfer slots. I've read plenty about it but never quite have been able to visualize it on a carb. Any tutorials or videos you think are any good for this? What issues could I face if I just pulled the carb and brought the secondaries in a 1/2 turn?

Ryan Menzies

Pic of transfer slot.. item number 2





And this is a very good read for a total carb tune.

https://docs.google.com/document/d/1...hCcppKdlc/edit

You can adjust 1/2 turn as long as you stay in the transfer slot....even a tiny bit if you have to go that tiny.

Give your PCV a shake....see if it rattles. There are obviously more in depth ways to check it but this will give you an idea if the pintle valve is still in there....

shredjesse

Definitely rattles, I feel pretty confident it's good.

Okay, I see now. So as long as that remains exposed I'm good to go? This is from the bottom of the carb? So just go in a 1/2 turn, and make sure I keep that exposed from the bottom?

I reviewed your article. That info wasn't in there. Even searched for the word transfer, only showed up twice.

Ryan Menzies

Yes, as long as there is some exposure to it you would be good to go. Once this is done then the "other settings" can be discussed.

I think the article is more refined on tuning after the basics like this done. It may not apply now but bookmark it for sure. It will come in handy should you dive into things more with timing etc.

Also Lars (on this forum) is a guru with tuning. Perhaps he will chime in.

gkull

It's usually a total waste of time even answering these kind of questions because of the disparity in beliefs about not using PVC and not using full vacuum to the vacuum advance.

Just think about what you have done wrong so far?

The secondary throttle plate does visually appear to be notably more cracked open than the primary

it takes one hell of a spring setup pulling on the throttle arm to make sure it closes.

Hooked up PCV system, idle jumped to 1100rpm

Timing is 12 initial, 34 total, with vacuum advance adding 18 at any given point, so 30 degrees at idle with no load



The carb has a manual on setup and adjustments. Then there is tons of online guides. All will tell you how the primary and secondary t-blades the exact same. Remove vacuum advance and have the PVC connected and install a vacuum gauge to set the 4 corner idle screws. Why would you even think that a 3/8th inch hole of air would not increase the idle?

You should have dual mild springs for safety on the throttle return. Excessive just wares things out.

PVC is not a good thing because it is a variable vacuum leak.
Using full vacuum to vacuum advance is also a variable. Variables are not go for precise tuning. 18 degrees of vacuum advance is way to big of a number. They make adjustable vacuum advance cans you can change the springs as to at what vacuum the advance starts and with an allen adjust the total amount. 6, 8, 10, 12 are reasonable numbers with modern fast burn chamber designs.

If I setup my ignition to 12 and adjusted my 4 corner idle screws to get the best idle possible and then add full vac 18 degrees of timing. The perfect idle I just set is now down the toilet. If you plugged it into the ported you don't get additional timing and screw up all the adjustments.

The smarter person would limit mechanical advance to say 14 degrees. Set the initial timing at 20 for the total of all in 34 degrees.

With the 20 degrees initial and motor up to temp adjust the 4 corner idles to get the best idle. Then hook the vac advance to ported and nothing changes. It is as you set it. Vacuum advance will be added when you are cruising down the road at lite throttle if you have springs that make it come in at over 16 inches.

shredjesse

I've heard a lot of people reference 20 initial, but with my MSD setup the smallest advance stop MSD makes is 18 degrees, limiting me to 16 degrees initial at most. I believe there's one place making 10 and 14 degree advance stops though? I'd love some more info on tuning with those setups.

Full manifold vacuum advance really helped my configuration. It cured engine run on for me on shut off, as I could close the throttle plates that much more and get a good idle. It also yielded massive fuel efficiency gains, taking me from 10.25 average MPG to 13.5 or so averages... all without giving up any power it feels like. It also cured pops on decel, etc etc.

As for the 18 degrees vacuum advance, that I will admit has not been tuned to by me, nor is perfect. MSD claims the distributor should only do 15, but I am seeing 18. It's just what the vacuum configuration I have delivered when I hooked it up. The distributor is 30 years old and was my dad's, heh. The initial engine tuner didn't even hook it up at all and just went 12 initial and 33 total, which didn't yield great results or drivability. They have mucked off this engine build so much though I can't trust anything they've done.

As for a detailed manual on my carb, actually... no there isn't unfortunately. I wish there were more information on it out there. I can't even find specifics on adjusting the secondaries!


All of the above said, it appears adjusting the secondary is okay to help reduce idle and get rid of this overly strong spring. That's the main thing I was looking for, everything else is working pretty darn good for me.

I'll consider later running more initial timing, a reduced vacuum advance, but same total timing. See what sort of results that yields.

gkull

Even 16 degrees initial could be a good number to use with the 18 degree stop. In the past I have tapped the mech advance slot an put a short bolt. Then a dab of epoxy to make sure it never came out.

Just set the T-blades the same, 18 degree stop, plug the vacuum A. set the total timing to 34 set the 4 corners and idle rpm speed. Of course with the pvc hooked up.

As to run on after the ignition is turned off...... it doesn't have anything to do with throttle blade opening. I can rev my motor to 5000 rpm and shut off the ignition and it is dead.

Run on is only caused by hot spots in the chamber and it usually is a glowing tip spark plug because you chose the wrong heat range. What kind of heads do you have? If it is the common 3/4 deep reach for aftermarket aluminum heads Go buy autolite AP 3924 and you are set.

shredjesse

Interesting. I'll try changing out the plugs. I do infact have aluminum heads, but zero specs because again, the engine installer and builder mucked up the entire process and provided zero specs on an engine that has had two catastrophic problems in under 2k total miles. THAT is another problem in itself :-/

I was able to resolve run-on by running additional advance via vacuum advance, which in turn increased idle, letting my close the primary blades to reduce the idle back down to normal. This in turn prevented run on since the car couldn't pull enough fuel to ignite on it's own.