Today I thought I’d investigate the vacuum/timing characteristics of my car in an attempt to understand the optimum method for ignition timing and use of vacuum advance with a standard distributor using Weber 40idf carbs.

It would be nice to keep the original dizzy (if I can get it to work well) with the carb conversion. It’s a bit like one of those Top Gear challenge specials where if the main muscle car fails they get the VW Beetle. I have a fully centrifugal 009 with 050 advance curve lurking in a box in my shed. If I can’t get the original dizzy to work well, then the box is opened.

The first thing I did today was take out the Weber manifold vacuum plugs and fit my new tubes I made on my lathe from 316 stainless to the intake manifolds on cylinders 1 & 2.

Next I connected vacuum test hoses to these points (leaving the “ported” vac pipe still plugged and my blue “bespoke” vac pipe disconnected)



The hoses were run to my Carb Synchroniser panel gauges 1 & 2.
I then connect gauge 4 to the carb on banks 3 & 4; to its single ported vac pipe and ran up the engine with no connection to the vac diaphragm on the dizzy (as you do to time the car).
After warming through and sitting a tad below 900 rpm I could measure the vacuum in the intake manifolds of cylinders 1 & 2 and the ported vacuum of the carb on bank 3 & 4.



To my surprise I had a reading on 1 & 2 and nothing from the ported vac pipe on the carb over 3 & 4 ??

Hmmm. The balance on 1 & 2 was fine, but zero on the carb ported pipe.

I gently rev’d the engine to around 1200rpm and from about 1000 upwards I started to get a reading on gauge 4 from the ported carb vac.



Not a great vacuum but watching and comparing “ported” with “direct” (manifold) vacuum, the results are that my 2.0L engine has manifold vac from idle to around 1500 RPM which peaks around 1000 and drops off to zero as the carb reaches about 15% open. The ported vac however, starts at zero when at idle, then slowly increases up to 1100RPM where it balances out with the direct vac (as that decreases) so overall the maximum ported vac is much less than the manifold vac at idle. It’s not a very good vacuum and only present for a short period of time.
Interestingly with the timing set a 27 degrees BTDC at 3500 with vac pipes disconnected as per instructions; the timing at idle is barely 2 degrees BTDC (with vac pipe disconnected). With the vac pipe connected using manifold vacuum to the pipe on the dizzy diaphragm closest to the body it pulls the diaphragm at idle to just past TDC, retarded by a degree or so (hard to tell without markings)
Since this is the “factory standard” diaphragm pipe to use for the 2.0L with the other pipe (furthest from the dizzy body) left unused, I’m quite surprised by the findings?

Is this timing at idle correct?

I’d have expected it to be a few degrees BTDC?

Can anyone shed any light on the timing at idle with the vac pipes connected as above on a standard 914 2.0L, for comparison please?

Also (as I’m sure to open the flood gates of opposing and conflicting views) what timing are any other guy’s using on a 2.0L with stock cam running Weber 40idf carbs?

I kinda want to keep the 27 BTDC at 3500 RPM, but think I need a few degrees BTDC at idle than I currently have.

Thanks in advance (no pun intended)

The manifold and ported vacuum readings you are seeing are exactly what you expect from the ports on the throttle bodies (or the carbs, in this case). Ported vacuum is only present when the throttle is open a very little amount.

All I really remember about idle timing is that it should be "around" TDC.

If you are truly trying to find the best vacuum setting for your carbs on your engine, you will want to get to a dyno. You can vary the timing and check the power and torque curves and figure out what makes the most power. Without changing the advance curve in the distributor, you'll probably have to compromise some RPM ranges to get a good overall result.

The vacuum additions are probably in the noise, but you can play with them to help you get transition behavior that you like.

Of course, the carb jetting will affect the above, and will be affected by it. So you may wind up having to fiddle with the jetting, then fiddle with the timing, and on and on.

...And of course, you need to make sure that the basic engine is in good shape to start with...

--DD

Just a question about your vacuum readings. Is your linkage disconnected and the idle adjustment backed out evenly on both sets of carbs? If not it may explain your dissimilar vacuum readings. Vacuum leaks at the carb to manifold and head may be an issue too. Are your air bleeds closed?

Kind of hard to say for sure until you verify readings on the same bank. I would think vacuum would be consistent no matter where you measured it from below the butterfly to above the intake valve.

Thanks for your reply.

The throttle linkage is disconnected with both idle screws backed off to the point where a idle speed is just possible (hence a shade off 900 rpm)
Air bleeds are shut
Carbs are balanced side to side using snail flowmeter. (the tiny discrepancy on 1 & 2 vac on the gauges is nothing to worry about as they are equal by snail flow meter)
I’ve spent a good while playing with main jet, corrector and idle sizes and the cars running much better now with no spit back, popping or other out of sync issues.

I agree with you about the idle screws backed off as if the progression holes are exposed or the butterflies too far open it’ll start to collapse manifold vac.

The plan is to try and get “as best as possible” with what I have. I am in a little catch 22 situation as with a timing at idle “around TDC” the idle screws need to be wound in more than just off the throttle stops (which I want to avoid) to get a steady idle. The timing at idle seems to need a few more degrees advancement to rectify this. But in doing so I’ll loose the 27 Deg BTDC at 3500 setting.
I was kinda hoping some one would say you need 3 or 5 (or similar) degrees BTDC at idle? With vac tube connected to the port nearest dizzy body it’s pulling the timing a fraction after TDC at idle. It’s interesting that you say the timing is “around TDC” which would mean I actually have it set about right already.
My gut feeling having tinkered with the car yesterday is that it needs a few degrees more advancement at idle.
The small issue I have is that the car isn’t UK registered yet and this is part of my build process so it’s not like I can even drive the car to see how its performing or get it down to a shop with a Dyno (which I agree entirely is what’s really needed here).
I think I was more interested in gleaning information from those who’d been here before and to confirm what I maybe already suspect.
For now the plan is to make the best of what I can using the best information available. To get my car registered here in the UK it has to have a Ministry of Transport Test. To pass this test so that I’m allowed to drive it on UK roads the running condition and emissions of the engine have to be tested. So really I need to get it right not only for the enjoyment of a nice drive, but to also be able to drive it on the roads in the first place.
I haven’t tried it yet but I’m wondering if later today after work I swap the vac hose from the pipe on the dizzy diaphragm from the one closest to the body to the unused pipe on the outside of the diaphragm (which is normally a ported vac connection) to see how that effects timing at idle ?

Subscribing...mind if I camp onto this thread instead of starting another one?

Anyone have a good "how to" guide on general carb and ignition tuning for our beasts? I've got a '74 2L race car with a Foley engine that was recently converted to dual Dellorto 40DRLAs, never tuned, and I'm just about at the point where I need to do some dyno tuning. I'm a modern-car guy (90s and later) and the only dinking around with carbs I've done in the last 30 years was on motorcycles, cleaning out the carbs and using my Morgan Carbtune to sync them. Haven't yet found the OBD port to hook my laptop into...

The car is an ex-ITA racer in baseline build condition, it has been sitting in an environmentally-controlled garage for over 12 years (popped an engine at Watkins Glen October 2003). Chris and team built and installed the engine and got it "basically" running, then I took it home to throw some sweat equity into it for all the little stuff and to prep for racing. I want to get the car to the point where I can bring it to someone that has all the jets and tubes in hand and can do it up right (and I can watch and learn).

Distributor is currently the L-Jet disty with Pertronix inside, Bosch blue coil; not clear what I want to (or should) do long term for ignition. I'll be installing a wideband O2 system into it (probably AEM UEGO) to monitor AFRs. It has Foley's carb linkages kit and I've adjusted it for proper alignment and tension per his instructions. Verified spark at all four, and quick engine runups (runs and smells like hell) verify all four stubs are seeing heat. So now I'm at the point of leaning against my Mom's service cover (love that thing) and looking down into the engine compartment going "aye-up, it's an engine..."

Two quickie questions: there's a pair of large vacuum ports at the base of the intake manifolds; they are currently taped up with painter's tape. What are these for, and what should they go to? I see in the photos above that you have a hose on that port. And, are the Dellortos supposed to have those vac ports at the base of the carb like in the photo (where you attach the carb sync hoses)? I don't see those but I'll look again.

Lots to learn, pre-thanks for the info.

GA

G.A, thanks for de-railing this thread, I’ll come back to you in a moment.

Dave, Jmill,

Just been tinkering/experimenting in the garage this afternoon after getting home from work.
So here’s what I’ve found today.
If the manifold “direct” vacuum is applied to the outer pipe on the distributor diaphragm (against the standard Factory installation) I get my “in my mind”, ideal advancement at idle, running about 4 degrees BTDC at 900 rpm.
Trouble is that as the engine revs the 27 degrees BTDC mark is reached way too early (2000 rpm) and overshoots to give excessive advancement above 2000 rpm.
So I cant place the manifold vac pipe on the outer pipe of the distributor diaphragm.

On a whim I rigged up a second vac pipe from some spare hose with a Tee and connected it up to the Weber ported vac tubes (the brass one) that in the initial photo had the blue hose attached. I paired up each Weber ported vac tube and connected that to the outer diaphragm pipe and left the manifold vac hose connected to the inner diaphragm pipe. (As per an early 1.7L carb car)
Sceptical if this would work with a 2.0L distributor, I ran up the engine.
Bingo!
I kept a nice 5 degrees or so (hard to quantify without a scale) advancement BTDC at idle and as I gently rev’d the engine up to 3500 rpm it made the 27 degree mark right on target!
I may have answered my own question.
It certainly looks like the best ignition timing installation for Weber twin carbs on a stock cam 2.0L using the stock distributor is to run both a ported and direct vacuum pipe (like Porsche did on the earlier cars) after all.



GA, From what you describe (without photos) is that the two ports at the base of your manifold covered with painters tape are “direct” vac. The two ports with black rubber hoses in my 1st photo are the same thing.
I’m not certain again without a photo, but suspect your Delorto’s will have a ported vac tube.
Unless mistaken, from my findings today, if you plan on running the stock Ljet distributor, you may want to follow my lead above for optimum basic ignition timing.

Hey, sorry, didn't mean to piss in your personal swimming pool. I'll unsubscribe and ignore.

GA

GA, I'm British, our sense of humour isn't always understood. I was being sarcastic.
No offence intended; if you read to the bottom of my post you'll see I'm trying to help you.