I'm expecting my carb to come back from rebuild soon and when it does i'll need to reset my timing since my distributor was out also.

it was suggested to me that rather than using a timing light to set timing it would be better to use a vacuum gauge.
I was told to hook the vacuum gauge up between the carb and the vacuum can and than turn the distributor until I get the highest vacuum and rpm reading than back it down about 1" of vacuum.

does this make sense and is it really a good way to set timing or should I stick with the regular way by just using my timing light?

I've never heard of setting timing using this method but since I'm new to working on my car that doesn't say much. I'm curious to know if this is a useful technique to try and to learn how to do.

 

It would be worth experimenting with, but you may end up with too much total advance. I'd comfirm by marking torsional damper for 36 deg and confirming with vac can disconnected and centrifugal all in at 2k-3k rpm.

 

Never heard of that before for setting timing, but use the vacuum guage when setting the carburetor up to get the highest vacuum reading.

John

 

ok John, now you have me thinking........ this info came from one of the guys that I was choosing to rebuild my carb. I have the notes on the procedure to use (as outlined above) but possible I misunderstood the use or just remembered incorrectly. Would this procedure make more sense as a way to set proper mixture and idle when I get the carb back rather than setting timing?

 

 

Yes, with respect to the carburetor setting. Adjust the carb to get the highest vacuum reading. Since I never heard of using vacuum reading to set timing, I would stick to the tried and true method of a timing light and dwell meter. This way you know the setting so as not to damage your engine.

John

 

ok thanks. that makes more sense now
i use the above procedure to set up the carb, than once the carb is set correctly I go thru setting timing the regular way with my light.

Thanks!!

 

I would set timing first, then adjust carb.

 

Barry, if it were me I would first manually estimate the dwell setting (since the dist was out), set the timing, set the dwell angle, and re-adjust the timing then adjust the carburetor in that order instead.

Sure you already know but just on the side. Once you put the carburetor on, start the car for just a little bit then shut it off to double check for leaks. If all is ok, which most times it is, restart and look around to ensure no leaks while it is running (this is all assuming your alone). Then do the timing/carb stuff.

John

 

Ratpack types faster than me....now you have consensus

John

 

thanks John & Rat
i'll follow your advice and order of steps

yep, with all that I had off the motor - distributor, carb, intake, valve covers, waterpump, etc....... i'll be looking VERY carefully for leaks EVERYWHERE once I get her restarted again.
Looking with my fingers and toes crossed that I don't find any!!

 

Barry, Always do dwell first because the timing changes as the dwell does. Timing a 365 has a few tricks to it which are well documented in the archives. Do a search. Fifty plus years ago, when owning a timing light was a bit of a luxury, there were many methods to approximate timing, including the vacuum gauge trick. The most common was to advance it to get the highest idle speed, road test for ping and retard it until the ping stopped. I'll bet you never heard of using vacuum windshield wipers to determine optimum drag racing shift points either.

 

I'll bet you never heard of using vacuum windshield wipers to determine optimum drag racing shift points either.[/QUOTE]
That is a new to me. I remember the vacuum wipers on some of the older cars - mostly that they were a pain in the butt during rain storms. Glad they were usefull for something.

 

Vacuum setting the ignition timing helps compensate for engine mods that don't necessarily fit in the manufacturers specified timing settings (CR changes, cam, headers, etc), and for altitude change at your location.

You should of course always check what you got with your vacuum guage, with a timing light to make sure you didn't do anything stupid.

I learned that from a German mechanic (turned carb and electrical autoshop teacher) that was on the factory Porsche racing team for many years prior. The vac guage worked fine for me for years, and it got rid of the pinging problem on my modified 11.25 CR L-79, motor still runs fine, after 29 years and countless excursions over 6500 RPM.

Doug

 

Barry,
I agree with you trying the vacuum method, but I also think you should check the setting with a good light with a dial advance setting. With a light you can not only check base timing but also the smooth operation of the advance as well as the full advance setting. If you don't have a good timing light, perhaps you can borrow one or buy one from Sears, etc. (about $60 or so)

By the way, nice web site. Next year is paint and interior. Looking forward to it, but not the wait. Also, I don't want to take over 3-4 months for a good paint job. I'm searching for some 'local' shops.

Good luck

 

As AZDOUG said the vacuum method is helpful when factory timming specs cant be used do to modifications to the engine.But if its stock always use the factory specs and marks.Fine tuning the carb settings with a vacuum gauge is always the best method.

 

The 327/365 has the best tailored OE ignition map of any production Corvette engine, and I recommend it as a starting point for any SB with a high overlap cam. See your service manual or AMA specs for the details of the map.

The OEM initial timing recommendation is 12 deg. with a range of 10-14 deg., which will yield 34-38 deg. total wide open throttle timing, which is all in at 2350 revs.

This initial timing range along with the 16 deg. @ 8" OE vacuum can (plus a couple degrees of centrifugal since the cent. curve starts at 700 and this engine needs at least 900 for acceptable idle quality) will yield in the range of 28-32 deg. total idle timing, which is what is required for a SB with such a high overlap cam. Less total idle timing is needed as overlap is reduced with milder cams, but this timing map is a very good starting point for Duntov, 30-30, and LT-1 cams or aftermarket cams with equal or greater overlap.

If you can run 14 initial without detonation, run 14. If it detonates back it off in two degree increments until the detonation goes away.

You can verify the total idle timing requirement by disconnecting the vacuum advance and advancing the timing until maximum idle speed and vacuum is obtained, but this is tricky because you get ever decreasing idle speed/vacuum with each increment of added timing, and it's easy to end up with too much. It's also an interative process as you may have to back off the idle speed screw to maintain the target idle speed range, since increasing timing from some low starting value will increase engine speed.

Once the optimum total idle timing is determined, you tailor the initial timing, vacuum advance, and centrifugal advance to achieve a timing map that provides the proper range of "ideal" timing for idle, cruise, and WOT, but Chevrolet Engineering has already done this for you.

If you don't understand the effect of mixture density, A/F ratio, and exhaust gas dilution on flame propagation speed, which determines the optimum timing for any operating condition, stick with the OE map and initial timing range recommendation. You can't go wrong with it. If you want to learn more about the effect of these variables on the timing requirement, you will find several essays I wrote on this subject in the archives.

Duke

 

JSB, yep, I'll do dwell first and nope, I've never heard of the vacuum wiper trick.

AZ, I'll just do the vacuum gauge for the carb and my light for the timing

Gilmore, I have a nice dialback light from Sears I got back in Aug.
Thanks for the kind words about my website.
BTW, my bodywork & paint job was almost 11 months!

Kid, yep, the car is stock so that just how I plan on doing it

Duke, Thanks! your answers as always are detailed and very informative. Many times a bit over my head but always herlpful and I learn something new. i can always count on you and Johnz for the most detailed and to-the-point answers!

Thanks all!!

 

Barry, reason I put a three step instead of the two step was because you had the distributor out and thought this safer to keep it running (not to take more of your time up ) "...,set the timing, set the dwell angle, and re-adjust the timing then adjust the carburetor in that order". Just the way I always had done it.

Sounds like you'll be firing it up before New Years!

John

 

cool, I understand John

nope, I doubt before New Years at this point - still waiting for the carb to get back from rebuild. Was hoping it would be back already but it's not and he hasn't called for my credit card info to bill me yet so it's not even on it's way yet.......
hopefully next week I'll hear from him and it will be on it's way home to me.

oh well, at least it's giving me time to get some work on the interior done while i'm waiting on it. I repainted all the molding panels above the beltline and re-dyed the headliner. Just need to re-tap a boogered up threaded screwhole where one of the sunvisor screws go in and than I can start putting the interior back together.
Hopefully my driver seat will get back from Al Knoch soon also.........

than I get to pull the shifter console and replace the ashtray door........

than I get to take the car back to the paint shop for him to re-wetsand it and buff it out again.......

than IF the budget allows I get to install an A/C system.

than I need to get under the car and start cleaning up the frame and underside from 2 seasons of driving and 11 months of paintshop dust.

than I need to get new tires

Darn, not sure if the winter is going to last long enough to get all my projects done!!

we won't mention all the project that I know i want to get to but because of the budget I know I have to put off until at least next winter.
geez, with all this work why did I bother buying a body-off restored car!! Foolish me thinking i could buy a car already freshly restored and just drive it without it needing a lot of work.