I'm checking the Timing on my L48 and with vacuum disconnected I have it set at 8 degrees BTDC as specified. When reconnecting the vacuum to the distributor what sort of jump should I have in the advance at idle? Mine is going to something about 16 and that seems high for idle? None of my manuals say what it should go to.

Also, when idling how much can the timing fluctuate and still be okay? mine is going probably around 7.25 to 8.75 as it idles.

 

I don't see anything wrong with the timing accept that there isn't enough of it. Factory timing specs are a compromise between drive ability, fuel mileage and emissions. If you want to set it at factory specs that's fine but you're missing out on some power and throttle response. If you have to pass smog go with the factory specs.

Otherwise, map the advance curve up to 3000 RPM with the vacuum advance unplugged. If it is anything less that 36 degrees at 3000 RPM you need to get a recurve kit. One light and one medium spring will generally get your "all in" timing at about 2600 to 2800 RPM. Set the initial timing at whatever it takes to get 36 degrees at this engine speed. Now hook up the vacuum advance to manifold vacuum and live happily ever after.

BigBlockk

Later.....

 

follow BigBlockk's advice on your timing numbers,

as for the timing flucuations at idle your distributor is most likely out of tolerances and could use a rebuild including shimming up the mainshaft. When the endplay on the mainshaft is too great the timing varies and flacuates as you are describing.
Unless you have done it yourself in the past, there is a very good chance that the distributor in your car may never have been touched or serviced or maintained by previous owners so it would be very common to find it needed a rebuild after all these years

 

great advice guys. it's antique here in va so no emissions.

"map the advance curve up to 3000 RPM with the vacuum advance unplugged."

is this basically advancing the timing by turning the distributor while i'm up at 2600 to 3000 rpm and setting my digital timing light to 36 degrees?

my vacuum advance is connected to ported vacuum. are you suggesting to go to manifold vacuum?

 

You got it.

If your hooked up to a ported source (Usually front passenger side on a Qjet) your timing should not jump when you hook the advance back up because there shouldn't be enough vacuum at idle to move the advance can.

Where is your idle set at?

 

As has been posted already, you'll need to fix the jumping advance before continuing. The ideal shaft end play is .007" but if you can get it to under .010" it will help a lot.

Mapping the advance curve is done to get a good picture of how well the centrifugal (mechanical) advance is operating and what, exactly, it's doing. A dial back timing light comes in real handy and it don't hurt to have an extra body to help you.

With the vacuum advance unplugged start your readings at 1000 RPM. Have your helper (who is in the drivers seat WITH HIS/HER LEFT FOOT HOLDING THE BREAK) raise the engine RPM by 200 RPM. Now you take a reading. Again, raise 200 RPM and take a reading. All the way up to 3000 RPM. At this point, if the distributer is all stock, you will probably be somewhat short of 36 degrees. We'll say that it is somewhere around 22 or 26 degrees. This indicates a lazy advance curve. (Some stock distributers didn't max out the mechanical advance until 4500 RPM)

Now, in the recurve kit you will find three (3) pairs of springs, two (2) weights and maybe a bushing or two. The first thing you'll want to do is take the weights out of the kit and get into your car. Now, drive 100 miles in any direction, get out, spin around 5 times and, with your eyes shut, throw the weights as hard as you can. Preferably into a large, deep body of water. Get back in your car and drive home. DO NOT USE THE WEIGHTS THAT COME IN ANY RECURVE KIT! All you need are the springs and bushings.

Now if you want to experiment with the different weights you can but a short cut is to install the lightest springs. Now rerun your test. The advance will stop advancing somewhere around 2200 to 2600 RPM. That is, the advance will no longer increase with an increase in engine RPM. You have found the limits of the mechanical advance mechanism in your distributer. However many degrees short of 36 you are is the number of degrees you need to add to your initial advance. For instance, if your distributer will give 24 degrees of mechanical advance you're going to need more than 8 degrees initial to get to 36 total.

NOTE: To get the mechanical advance correct at 2800 RPM you will probably need one light and one medium spring. Yes you can mix and match them.

NOTE: If you end up with numbers like 8 degrees initial + 28 degrees mechanical you will need to limit the mechanical advance with a bushing on the advance pin. What you really want is to get the initial advance up around 14 BTDC degrees for good throttle response.

Now, hook up the vacuum advance to manifold vacuum (that's high on the front of the carburetor body on the passenger side next to the choke pulloff) and drive.

BigBlockk

Later.....

 

I set the timing on my 72 base 350 today.

12 degrees at 700 idle (park) with vacuum disconnected and plugged.

With centrifugal advance "all in" .... gives you

35 degrees at 2800 rpm (park) with vacuum disconnected and plugged.

Hook up the vacuum / re-adjust idle to 700 idle in park / good to go!

 

I'm hooked to the front passenger side of the q-jet just forward of he choke linkage and just behind the vacuum motor that hold the secondary flaps closed. I thought this was ported and just north of the throttle plates and yes, low vacuum on idle.

It definately jumps from 8 to roughly 16 when I reconnect it. Looks like maybe the diaphram in my vac advance on the distributer is weak and advancing on weak vacuum.

My idle is set at 750 right now. I have noticed that my driver main jet is dripping at idle, so I'm getting gas in from the main jet on idle and probalbly have the idle speed screw set for too much throttle to bad aid this. Adjustment of my driver side idle mixture screw does nothing since the main is dripping and apparenlty feeling vacuum.

Im a newbie to carbs, but learning a lot every day.

 

Haha! I just busted out laughing.... good stuff.

Okay, I think I understand all the steps now.

Should I tackle fixing the distributer myself? Are there kits or directions I can follow for that?

 

I have a vacuum tester and will check how much I have on the passenger side q-jet connection. This would let me know if it's either too much of if my vacuum can on distibuter is faulty. Any idea what the ported vacuum at idle should be?

 

Ported vacuum should be close to zero at idle. Manifold vacuum on a stock 350 should be somewhere in the teens... Ill guess 16" or so for your engine if it's stock.

Ported vacuum will increase from about zero as you raise the RPM. Manifold vacuum will will be near it's highest at idle and will drop off as the engine reaches WOT. You want the manifold vacuum to go to the distributor advance.

Keep in mind that some engines simply wont run best on manifold vacuum advance, but most will.

If you haven't already read Lars' papers on vacuum and timing you can find them on Barry's web site.
Do yourself a big favor and read them then follow the instructions on setting your advance.

You have a vacuum gauge and a digital timing light so you should be good to go.

Pull out that idle mix screw that has no effect when you adjust it and spray some carb cleaner in the port with the little tube that comes with the can to make sure theres no blockage. If it still doesn't change the mix then you need to dig deeper into the carb to correct it but timing should ALWAYS be correct before you even bother fine tuning the carb.

Heres a link to Barrys site which has some of Lars' papers on it.
http://www.lbfun.com/Corvette/Tech/vettetech.html

 

great advise guys, thanks.

One thing that I am not sure of is if I am on ported or mainfold vacuum currently.

"Bigblockk" says it's "... to manifold vacuum (that's high on the front of the carburetor body on the passenger side next to the choke pulloff)..."

whereas,
"Bob Onit" says "if your hooked up to a ported source (Usually front passenger side on a Qjet) your timing should not jump when you hook the advance back up because there shouldn't be enough vacuum at idle to move the advance can."

I am on the passenger side right near the choke pulloff, this looks like the same location mentioned by each poster.

I need to get out my vacuum tester and see, but was wondering if someone new for sure and could confirm

 

You can see where Bigblockk refers to the drivers side as ported vacuum in this thread.http://forums.corvetteforum.com/show...post1562505300
so he may be referring to the port coming out of the passenger side (not front pass side) below the pulloff.
Heres a pic of the port I'm speaking of.



Use your vacuum gauge to test them.
Manifold port will read high (somewhere in the teens) at idle and ported will read zero or close to zero at idle and will increase as you slowly bring the RPM up.

 

okay, the port you have in the red box in your post just above is where my vacuum advance is plugged into and my advance is advanced when plugged into that port at idle, so I would say it's manifold vac at that port. I will check with the vacuum gauge tomorrow night when I am back at it in the shop

I just read the articles by John Hinckley on timing 101 and advance mapping, and they are spectacular! I feel like I know so much more now.

 

Sounds like you have it right.
All Qjets don't have the same plumbing so it can be confusing.
Sorry if I made things hard to understand... almost confused myself on this one