Zenith balance

[JelmerE12-E28]

Last week i've taken both Zenith carbs of my E12 because they where very dirty and one of them had the acceleration nozzle plugged. After I disassembled them I degrased all the parts and cleaned them in my ultra sonic cleaner using special cleaner for carburetors. While in the ultra sonic you could see filth coming out of all jets! After fresh seals, plungers and gaskets (also 2 thin gaskets for the bakelite spacer) I've put them back on the engine. The cars fired up pretty fast and ran pretty good. When I drove it it pulled smooth and strong thru the revs. While cruising it starts to stumble to an unpleasand level! Might this be the trottle balance between the 2 carbs? I've also cleaned the idle air screws (big ones) and idle mixture screws and when I've placed them back in the carbs I turned them al the way in and than backed them out by 2 full turns.

Any advice?

 

[Stevehose]

Possibly this can help:

http://www.e9coupe.com/forum/showthread.php?t=10942

Also try searching for other threads. let us know progress

 

[JelmerE12-E28]

Thanks Steve!

The idle mixture screw, is that the big one with the oval end (the one on the left which is not complete on the picture) or the small one with the pointed end? (the right, small one on the picture)

 

[Stevehose]

These are later emmissions regulating carbs, the small screw is the air bypass adjustment to meet a cetain CO level at idle. The larger screw is the mixture screw. Screwing the air bypass all the way in will essentially turn them into non-emissions carbs. You would need a CO meter or somesuch to set those up to factory spec so just close them unless someone else here has a reason to use them. If you have an AFR gauge these could be used to lean up your idle AFR reading if it's too rich.

For the mixture screw, do a lean best idle adjustment on each carb with the throttle linkage disconnected and the butterflys (equally) barely open and the barrels drawing equal air. Also put your finger over the idle jet vent hole on the carb cover and check that both carbs go down equally in rpm when you do so. Do you have a synchronmeter?

 

[JelmerE12-E28]

Thanks Steve.

I have 2 kinds of vacuum meters. One has 6 gauges which I used for balancing my S38:



And 2 of these:

I want to use the second one and make a lid for over the carbs out of a plastic bowl.

 

[Stevehose]

For the STE gauge I used a rubber pipe reducer from Home Depot with a smaller PVC reducer inside it to get the gauge to fit tightly. The larger side of the adapter fits snuggly over the carb top and no air leaks by so you get consistent readings.

I had a vintage setup with the plastic tops but I wasn't confident it wasn't leaking.

For vacuum I really like the Carb Mate, it eliminates having to read fluctuating gauges (which may or may not have equal calibration) and has variable sensitivity.

 

[JelmerE12-E28]

That looks good! I was thinking of using a pair of these:

 

[David]

My favorite topic.

Steve, on the Carbmate are you hooking it up to ports above or below the throttle butterfly ?

 

[Stevehose]

You can use either depending on what you are tuning for. Hook it to manifold (below butterfly) for synching at idle. Hook it to ported (above butterfly) to check the balance through the rev range. I went to the dark side and replaced the zeniths with triple webers and i use the Carb Mate exclusively as i believe it to be more versatile than the Synchronmeter, although that is an excellent device also.

Admittedly this is one of my favorite topics also - chasing the holy grail of optimal tune.

My favorite topic.

Steve, on the Carbmate are you hooking it up to ports above or below the throttle butterfly ?

 

[JelmerE12-E28]

I wasn't into carburetors but these made me wanting to get them as good as possible. Ill keep you guys updated when I start balancing!

 

[deQuincey]

check at least the final setup fixing, post 11:

http://www.e9coupe.com/forum/showthread.php?t=8569&page=2

i would recommend to fix:

1- points or dwell angle
2- distributor for ignition, using a strobo light

 

[61porsche]

Stumble

at cruising would indicate that DQ was closing in a possible solution. It is true that this can be ignition related by timing, A simple twist of the distributer would tell. The tach might act up also. Perhaps a check of the coil is in order also if weak.

Next would be fuel. Sluggish pump, etc allowing one carb to get fuel and the other to catch momentarily if the stumble was severe to moderate and delivery was less than needed. Zenith floats can hang up on ill fitting gaskets and not open but partially would be another item to look for.

Spark plugs tell a lot some times.

Balance is a fine art of tuning as Steve attests. The symtom as described points elsewhere.

 

[Stevehose]

clogged fuel filter?

 

[JelmerE12-E28]

Tanks everyone! I really appriciate it very much.

Let me tell some more about the issue:

The car is an E12, build january '76, 2.8l with a 4 speed. The cars was driving pretty good but had a hesitation after you accelerated again after a curve or speedbump. Also the choke didn't work with a cold engine. Under normal acceleration the engine pulled not really smooth so I checked the ignition timing and it was perfect. It already has a pertronix electronic ignition. Next I replaced all vacuum lines and hooked them up like they should because they where messed up big time. This was a big improvemend!! The car accelerated very smooth now but the hesitation was still present. After some research someone told me to check the acceleration nozzles so I pulled them from the carbs and one of them had a kink in it and wasn't squirting enough. Because I have a ultrasonic cleaner I thought it was a good idea to clean the carbs so I pulled them, cleaned them and put them togheter again with new gaskets (the right ones!). This gasket set also has a new bakelite spacer with 2 very thin gaskets to prevent leaks from here.

So this is where I am. The problem has to be carburetor related because I didn't touch anything else and the carbs where completly parted out but offcourse I should check everything else too.

 

[deQuincey]

Tanks everyone! I really appriciate it very much.
So this is where I am. The problem has to be carburetor related because I didn't touch anything else and the carbs where completly parted out but offcourse I should check everything else too.

not necessarily agree on your last comment, as the carbs had deteriorated during their life igintion and other settings would have been accomodated to that evolution, so now you should check and fix

my last experience shows, the following, my engine works perfect, and i am proud of its perfection, so i regularly make checks, and was time for the points gap, checked, fixed at 0,4mm and slightly move the diff, i ended up with a hesitation when up from 4500 revs, what the heck ?, checked again , this time strobo lamp used to meassure dwell, resulting too small, so gap too big, and ended up with a new set of the points, this time extra care was taken, dismantle, use a tungsten file to flatten the surface, ...e voila ! no hesitation !

not saying this will be your case, but,...:?:

do not hesitate to ask for any not functioning parts on your setting, i might have spares

regards

 

[JelmerE12-E28]

Thanks,

I'm certainly going to check the timing again and make adjustments. The current hesitation gets more worse so than I let of the gas or go full throttle but when cruising it starts to hesitate again. It feels a bit like it going faster and than slower and gain a bit faster and slower again etc.

My E28 with S38B38 (with 6 induvidial throttles) did this a little bit to, you could only feel this when cruising at a given speed. I synchronized the 6 throttles and now it drives much smoother.

 

[JelmerE12-E28]

Today I worked on the E12 again. I printed the synchronisation procedure from the BMW workshop manual and follwed the instructions. First I did idle and than at 1700 rpm. Only problem is that the airflow meters only could read 30 kg/h max airflow so I coudn't read it at this engine speed.
I bought 2 special cups that sit over the carbs and have a hole in the top where you put in the airflow meters.

The hesitation is beter when driving but still a little bit present when in lower rpm's. When going in 4th gear at a higher speed it drives pretty smooth now.

Any advice on what to do now?

 

[deQuincey]

your dwell angle is ?

 

[JelmerE12-E28]

It has an electronic ignition. Dutch E9 specialist Brové put it in there in 2008 or 2009.

 

[JelmerE12-E28]

Yesterday I took the E12 out for a longer trip and after a while it idled at around 1500 rpm, it was fluctuating at 1400-1500 rpm. I didn't have time yet to check out what was wrong but sounds like the choke is working. It also pops in the exhaust.