After installing my rebuilt 1.7 to a 2.4 engine with dual twin Webers (used), and installing it into my 914 some 8 months ago, I have been working on improving the engine operation in the idling and acceleration rpm modes. until recently I could not get a smooth idle, nor remove a flat spot from low revs acceleration up to plus 2000 rpm approx.

I initially focused on the carbs, 1) eliminating vacuum leak, 2) trying different main, idle, and air jet sizes, and 3) adjusting floats so that both compound floats are 12 mm spaced from the carb cover under side. I have a dash mounted air/fuel ratio gauge, but unfortunately the gauge shows no ratios much above 14 but at least indicated richer vs, leaner with each trial and adjustment. I eventually relied on warm engine runs of cruising above 3000 rpm for 3 to 5 minutes, shut engine off, cruise to a holt, and inspect plugs for correct color on the electrodes and porcelain, to find the correct air/ fuel mixture. During these tests I learned that adjusting linkage is critical as small errors make a significant difference. I also learned that a synchrometer (carb throat air flow gauge) and a portable tachometer is essential to finding balance between the carbs and the effect of adjustments, particularly finding the best air/fuel adjustment valve positions for what Bubblehead calls Lean Best Idle (LBI).While all of these efforts improved the engine operation, it remained unsatisfactory.

Then as I searched these pages I found advice on how to eliminate unintended impact from the blocked off and remaining choke parts in these carbs -- two vertical placed brass "pistons" designed to assist cold starting by opening up to let more fuel in. I followed the advice carefully which is to remove the brass retainer "caps" and springs that hold these pistons down in place. Then using a 3/8" x 24 tap, cut threads into the top section of these holes the pistons slide into, and screw in plugs to force these pistons to stay in their closed position. Apparently, with time and deposits/dirt these pistons can get stuck open with a relatively weak spring unable to keep them in the closed position. We do not need a choke in S.Cal to start our cars! This modification immediately resolved the rough idle problem I have been battling.

Further search in these pages brought my attention to the distributor as I continued to overcome the flat spot issue. Recognizing all along that timing is part of the overall equation, I adjust to 30 degrees BTC, not higher as this engine has a slight vibration at higher advance positions. I have a Petronix igniter so no dwell angle issues. I recently found two articles (including a well written description by Bubblehead Tech) advising these type IV engines with Webers require a distributor with vacuum advance to provide more advance at low engine rpm's, so called SVDA distributor. I had a 009 mechanical only advance distributor. Changing to an SVDA distributor with a vacuum hose connected to the carburettor has completely eliminated the flat spots and significantly improved the pereformance and drivability of my 914

In summary: Engine is running perfectly at sea level with: 1) 160 main jets, 220 air jets, 60 idle jets, fully "decommissioned" choke parts (described above), floats adjusted to 12 mm (both floats in each carb), and 2) installing an SVDA distributor with Petronix igniter.

I thank the many contributors to these pages who have taught me the little that I know and having helped me "tune" this engine to what I believe it should be.

A good running 914 is a wonderful thing.

Good write up!

I use an Innovate AFR and watching the display while driving is nearly useless.
It jumps around and varies too wildly.

So I log the data with a homemade logger and look at it with excel after the drive to get average values during hard "pulls".

Another thing that plagued me for a long time: I had mismatched accel pump diaphrams. Or one was just faulty.
I found the problem when the two sides just didnt "look" like equal squirts with a full wide open actuation.
So I devised a way to lower a 5ml measuring device into the carb while actuating.
My carbs were off by 5:1 side to side,

I replaced both accel pumps and it ran like a different car.

How close did you manage to get the flows with the synchronometer?

I'm running EFI throttle bodies and I get 5-6 on the #1 and #2 cylinders and 6-7 on the #3 and #4. Running 12 degrees retarded timing at idle.

After much small adjustments to the linkage and measuring air flow with a warm engine I was able to get them sinked within a 1/2 point -- about as close as I can judge with the meter -- using the idle speed adjusters and resetting the A/F mixtures every time I adjust any other item. Now runs very smoothly, and starts cold instantly now that I have warm idle down to 900 rpm.

nice write-up - it's not often that solved problems are detailed