Throttle jerkiness after carb balancing

[hans3]

I just rebuilt the twin Zeniths on my 2800CS and attempted to perform the carb balancing process. The car now has an annoying “jerkiness” to the throttle when going from off throttle to on throttle. Example: Coasting in 4th gear, I hit the gas to accelerate past someone—the engine hesitates (bogs) then abruptly accelerates. Another case: downshift to 2nd so that I can make a 90 deg. right-hand turn—after making turn in 2nd I hit the gas, engine hesitates and then abruptly starts to accelerate. Throttle is not “linear” like it was before the rebuild.

I performed the synchronization using 2 Mityvac vacuum gauges. Per blue book, carbs seem spot-on balanced at 1700 RPM, with both gauges showing 20 mm HG. Exhaust CO is 2.5 % per my Gunston gas analyzer. Idle balancing was trickier because both vacuum needles fluctuated around 17 mm HG—due perhaps to leaking valves. Timing, dwell, and valve clearance are to spec.

Question to the experts is whether this throttle jerkiness is because of a carb imbalance or rich / lean mixture issue?

Thanks for any inputs -H

 

[oldcoupe]

Jerkiness

Did you replace the two vacuum tubes to the distributor the right way round? I got exactly your symptoms with the tubes reversed.

 

[MMercury]

I just rebuilt the twin Zeniths on my 2800CS and attempted to perform the carb balancing process. The car now has an annoying “jerkiness” to the throttle when going from off throttle to on throttle.

Question to the experts is whether this throttle jerkiness is because of a carb imbalance or rich / lean mixture issue?

I am far from being an expert.
I can't tell from your post if the problems developed only after the your carb balancing act or if its been that way since the rebuilds. In any case, it sounds like you are describing one or more conditions known by many different names: poor throttle response, flat spotting, progression issues, trailer hitching, etc. Go through the archives, as there are probably many other impolite descriptions.

I am inclined to agree that your problems seem related to inconsistent fuel mixture. However, I would not entirely rule out ignition as a factor - especially if you are using a distributor that has a vacuum advance or retard component. They are not always leak free. You are probably aware of the trouble shooting guides in the tech section. (See for example: http://www.cscoupe.org/tech/zeniths/bmw1/bmw1.html **) You will note that almost every guide suggests carb adjustments assuming properly adjusted timing and valves. Since you mentioned it, leaky valves, may contribute to your problems. This begs the question, how leaky and why? One burned valve or worn valve seat is a recipe for misfire. It is easy to overlook on an otherwise decent running engine but I am not aware of a simple tuning fixes.

In very general terms, the flat spotting can occur if you have vacuum leaks or if your accelerator pump/s is/are leaking and not injecting enough fuel to richen the mixture when you abruptly punch the throttle. This is particularly true under a load.

The trailer hitching phenomenon with the foot off the gas tends to be caused by too lean a mixture. Can be caused by over-advanced timing, dirty idle jets, floats that are incorrectly set, leaky accelerator pump diaphragm/s, leaky valves (reducing compression) or even throttle plates that are not completely closing where they - expose intermediate transition/progression ports, in turn permitting un-metered fuel to enter the cylinders. You also getting any backfiring when this happens? Consider that if your idle circuit has some problems (I am not saying yours do) and other adjustments are made to compensate for those issues, obtaining a decent idle, under those circumstances, can affect part throttle operation.

I think Oldcoupe may be on to something when mentioning a vacuum hose or vacuum leak. Those secondaries are vacuum controlled as are so are many things according to the pic below. Besides, Zeniths have a reputation (deserved or undeserved) for internally warping which accounts for part of the proliferation of Webers. BTW, I do not mean to contradict the approved methods of carb syncing - especially for fine tuning - but often times it is easier to compare the sounds coming from each carb by a length of hose to the ear.

**7. Poor transition (throttle response) when engine is warm (misfiring)

CauseExplanationRemedy

7.1Fuel-air mixture too weakCO value adjusted incorrectlyAdjust carburettors correctly

Leak at insulating flange on throttle butterfly unit. - Renew insulating flanges Skim throttle butterfly unit flat

Worn throttle butterfly shafts - Renew throttle butterfly section

Stage 2 throttle butterfly air gaps too large. - Carry out basic setting procedure for stage 2 throttle butterflies (see Repair Manual, 13 11 412)

Too little preload at sprung stop for stage 2 throttle butterflies - See Repair Manual, 13 11 412

Incorrect injected fuel volume and spray direction at accelerator pump. - Adjust injected fuel volume and direction of spray (see Repair Manual, 13 11 054)

Float level too low. - Correct fuel level (see SI 13 01 76 (54))

7.2Fuel-air mixture too rich - Float level too highSee 7.1

Incorrect injected fuel volume and spray direction at accelerator pump - See 7.1

7.3Stage 1 throttle butterflies not opening together - Carburettors not synchronized
Synchronize carburettors; see Repair Manual, 13 11 004

 

[hans3]

Lots of good insights here to consider. Vacuum advance / retard are connected correctly, but I will double check they hold vacuum. No backfiirng at speed, but exhaust does "burble / snap" at idle. Problem existed after rebuild but became more / less pronounced as sync setup was changed.

 

[tochi]

Lots of good insights here to consider. Vacuum advance / retard are connected correctly, but I will double check they hold vacuum. No backfiirng at speed, but exhaust does "burble / snap" at idle. Problem existed after rebuild but became more / less pronounced as sync setup was changed.

In addition to everything else, if you have a dashpot like that pictured it can sometimes be maladjusted so that it keeps the throttle cracked open when you want it closed. That may not be part of your problem but it could certainly add to others.

 

[dang]

Under hard, short-term acceleration there isn't all that much going on in a carberator, that's why you can dump fuel down a carb and keep an engine running by working the throttle. Its when the engine is doing something other than accelerating is when all the air/fuel ratios are important. In other words, under normal operation the vacum in the venturi draws the fuel into the mix, which is when jet size and air/fuel mixture adjustments make a difference, but under hard acceleration fuel is forced into the mix. The first thing to look at when you have a lag in acceleration is the accelerator pump (rubber diaphram) or something that could affect the accelerator pump system (restricting fuel from dumping into the venturi).

The vacuum lines to the distributor sounds interesting too. Keep us updated...

Dan

 

[hans3]

Influenced by these responses, I rechecked everything. Timing had drifted a few degrees to the retarded side, perhaps because of dwell drift. Verifying dwell and resetting timing did help to mitigate the throttle abruptness. However, the carbs still failed to balance at idle.

The front carb continues to waver the vacuum gauge between 17 and 23 mm HG. The rear one is now more stable at 17 mm HG. At 1700 rpms, both are still consistent at 21 mm HG. I guess there’s some other intake manifold leak that’s causing the discrepancy. I don’t believe it’s the carb because this imbalance existed before the rebuild. Intake gaskets perhaps? Will have to experiment next winter. Car pulls strong nonetheless. Thanks again.

 

[blumax]

Jerkiness

when going from off to on throttle--sounds like lack of fuel delivery when first called upon.

The injectors in your Zenith carbs should squirt evenly when throttle is advanced to avoid or prevent bogging down before vacuum overcomes and plenty of fuel flows. If not you have that delay and then non linear acceleration effect

Pull off the airbox and check to see if you are getting a uniform--well directed squirt from each carb injector when the throttle is actuated. That squirt has to be into the center of each carbs primary throat--If not--for example if only a weak dribble it may be gummed up or have tiny dirt particle affecting it--you have identified a possible or contributing cause. It may be that only one is failing.

Injectors can be removed--very carefully--use needle nose pliers and careful twisting motion. Clean orifice with very fine wire maybe after a soak in solvent since the injector orifice is very fine. Check by blowing air through it. Be very careful when re-installing as if dropped--ya got a "fishing" problem--they are brass and if dropped--into the intake manifold they go and a magnet won't help.

The other suggestion Dang made as to the diaphram in the accelerator pump could be right on as well because its proper function is essential to the squirt.

 

[blumax]

Jerkiness

There are a few other factors that will affect correct operation of your rebuilt carbs. I'm sure you may be well aware of them. According to my old time Zenith trained mechanics the following are essential to proper operation beyond rebuild of internal carb parts only.

Were new carb base gaskets installed following rebuild and before re-installing them?
Were all the carb mating surfaces and bases honed to assure they are perfectly flat?
Are the throttle shaft openings in the carb body castings perfectly round--not oval from wear?

How many miles were on the carbs at re-build?