Are Zentih carbs the zenith of carbs???

Lotuss7

Lotuss7

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Searched FAQs and threads and haven't quite gotten to the simple answer I was looking for.

I have a stumble / miss / hesitation primarily at low rpm at neutral or low throttle.

An issue when I purchased the car a month ago. Car had seen little use over the last few years and had been sitting for close to 2. I went through the baseline routine: new wires, plugs, cap, rotor, coil, swap to Petronix. Adjusted valves, set timing, adjusted carbs All undertaken with a well respected local BMW mechanic who is senior enough to have tweaked E9s in the day.

All done car runs smoother overall, better tip in, seemingly more power...however stumble / miss is still there. Double checked alternator output and power to the coil. OK. Also looked at intake manifold leaks. None found.

Considerable time spent readjusting the carbs with little improvement. On scope they are seeing a leaning out of the carbs at partial throttle, but can't seem to dial this out.

Although intake leak wasn't detected the symptoms read like this to me and I noted a few threads on planing manifolds and carbs to get proper seating. In fact the previous owner mentioned having done this. To be re-investigated.

Mechanic has noted this stumble / hesitation was a common issue when the cars were new and brought up the Webber conversation option.

OK here is the question. When right are the Zeniths equal to or better than the Webbers. No hesitation, linear power etc. Want to keep them and if that is the case I'll make the investment to sort the issue out.
 
Lotuss7 said:
.... and brought up the Webber conversation option.
Click to expand...

Well, you I suppose could try to reach Mark Webber to have a conversation about this. But, after his win today at Silverstone, I'm going to guess he'll be quite busy.

OK, but seriously - asking about Webers vs. Zenith is sort of like asking whether Buddhism is better than Hinduism. Everyone has an opinion, and all of them are right.

Given that your car had been sitting for an extended period, it is likely that your carbs are sufferring from fuel turning to varnish which can clog jets, passageways, accelerating pump valves, .... Whether it will be easier to fix your old Zeniths, or to sort out a new pair of Webers is an unknown. But with a car that sat unused for 2 years, carb problems are to be expected - I wouldn't look at anything exotic, like planing the intake manifold, until you have worked on carburation some more.

Note that no carburetor is perfect - that's why automakers switched to electronic fuel injection in the 80's and 90's. I don't know how severe your car's stumble is, but you may never get rid of the last bit. It's part of the charm of a vintage car.
 
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Yes.

Zeniths are equal or better to Webers. Some owners have even converted back to zeniths. Some rockstar circles won't touch a new Weber compared to an Italian weber.8)

So you have a stumble. When? What rpm? A coil going bad will show up as a high speed miss or seem hard to get full rpm.

Make sure your dwell is set, timing is set, advance is working, no retard, and rock steady. Let's assume someone took all the emissions, egr, solonoids, etc off the car. There's no vacuum leak from the brake booster, and no air cleaner vacuum hose dangling open. What's your idle speed?

The number one cause of off line stumble on Zeniths are the accelerator pumps also referred to as squirters. Look down the barrel, operate the carb linkage, look at the brass nozzle for fuel squirting out. Not a dribble, not a drop. A real honest to goodness squirt. 2 or 3 cc's.

Advance tip- the pump body has a check valve but it also has a relief hole up higher that can be blocked with a little epoxy.( So there, to those L/K/M jet race fans- full rich at low rpms on a carb.)

If both carbs are squirting- report back.

The number two reason- listen to the carbs at idle- sounds like bacon frying? Gas coming out as an overflow/ spilling down the throat?

Report back.


It's a simple device once you understand what part does what. No, please don't take the carb apart to fix a vacuum leak that doesn't exisit unless you are absolutely sure there's a leak. A cheap stethescope probed around a carb will tell you first. Better to first fix what ails you first than start up something new. Plus, you can do everything from up top and not have to get around that nasty starter on the rear carb.:twisted:
 
Lotuss7 said:
I have a stumble / miss / hesitation primarily at low rpm at neutral or low throttle.

An issue when I purchased the car a month ago. Car had seen little use over the last few years and had been sitting for close to 2. I went through the baseline routine: new wires, plugs, cap, rotor, coil, swap to Petronix. Adjusted valves, set timing, adjusted carbs All undertaken with a well respected local BMW mechanic who is senior enough to have tweaked E9s in the day.

All done car runs smoother overall, better tip in, seemingly more power...however stumble / miss is still there. Double checked alternator output and power to the coil. OK. Also looked at intake manifold leaks. None found.

Considerable time spent readjusting the carbs with little improvement. On scope they are seeing a leaning out of the carbs at partial throttle, but can't seem to dial this out.

Although intake leak wasn't detected the symptoms read like this to me and I noted a few threads on planing manifolds and carbs to get proper seating. In fact the previous owner mentioned having done this. To be re-investigated.

Mechanic has noted this stumble / hesitation was a common issue when the cars were new and brought up the Webber conversation option.

OK here is the question. When right are the Zeniths equal to or better than the Webbers. No hesitation, linear power etc. Want to keep them and if that is the case I'll make the investment to sort the issue out.
Click to expand...

Without considering rubber compounds, tread design, tire pressure and size, how do Bridgestone tires compare to Yokohamas?

You neglect to mention exactly which Webers you are contemplating, e.g., down drafts or side drafts and which size/s. In short, Webers are not necessarily better; however, they have the relative advantage regarding availability - especially jet, choke and emulsion tube selection. This can be particularly helpful when dealing with a modified engine.

At first blush, your having used an oscilloscope to determine an off-throttle “lean” condition seemingly confirms your hesitation problem is carburetion-related. One might expect that the same scope would permit you to narrow this lean condition to a particular cylinder or cylinders and/or manifolds. In turn, this might make hunting for a vacuum leak much easier. Nevertheless, it is unclear that you have exhausted other possible carburetor-related causes, including the most obvious - dirt and wear. Not knowing anything about the current carburetors’ condition (other than the engine sat for quite a while), as others have noted, other things to consider include weak or tired accelerator pump plunger/s; gummed up progression ports, improperly set floats, worn throttle shafts OR carburetors that are poorly or improperly synchronized.

Although a scope might provide a good indication of your engine's mechanical health, stock parts that are worn or parts that have been modified could lead to changes in otherwise “normal” engine operation. Changes in currently available fuel might warrant deviation from recommended ignition timing. Spark plug type and gap can also affect engine operation. Replacing common ignition items and setting the valve lash is aways prudent, but that will not generally overcome a sloppy, worn or maladjusted distributor. Are both the vacuum and centrifugal advance mechanisms operating properly?

Hth
 
Thanks for the feedback and yes Mark Webber had a good day although I was rooting for Alonzo. Sidedrafts / downdrafts really not the specific question (although I was thinking downdraft), nor solutions for potential issues with the Zeniths quite yet ( though very much welcome!) Question was whether there were design problems at the onset and whether this is the carb to stay with.

I could anticipate some debate between Zeniths and Webers but with this I would assume each performed respectably and consistantly. From sidedraft DCOEs on the Seven and Cortina, Strombergs on the TR and SUs on the TD, each of which outdate the Zeniths, the type of issue I am experiencing is nothing any of the above cars exhibit and I would feel is naturally associated with owning a vintage car...even if somewhat less pronounced. Charm this ain't.

Bottom line...I want to keep the Zeniths and am prepared to sort out the problem... just want to know they are worth the investment.
 
Oh...and as with any new car there is a learning curve that I am at the bottom of and although can't say I wouldn't rather have the miss be missing...part of the joy of owning older cars is the problem solving componet. Lots of homework to do and I very much appreciate the input. Thanks.

Each outing in the Lotus is an adventure because your never know whether you'll get home under your own power. The Bimmer was bought to fill another role and I am looking forward to having it do what it was built to do. A refined GT car which will run flawlessly. Now to sort out the flawessly bit.
 
just my two cents

after a very unpleasant clean and rebuilt of the carbs by a local mechanic, i decided to accomplish the "thing" myself with the great help of some of the most experienced forum members here

i am still in the up-phase of the learning curve of the zenith experience, and i would never regret having decided to go this way

i can feel every improvement with a sort of happiness that is difficult to tell, remaining stock is a pleasure that is worth all the difficulties

the engine behaves great, full of power and quick response in all situations, the only flaw is a not 100% perfect iddle, that i am still investigating

here, as to your question, i feel the impposible of exactly knowing what are your particular feelings about it, i mean some people used to modern cars will estimate unaceptable a tiny stumble now and then, while others will cosider it almost perfect

...difficult issue

regards
 
Done. Working with the Zeniths.

I am pretty confident the electrical side is sorted, timing etc. valves are set correctly. Focus will now be given to the carbs after one more check which will be compression.

It is summer and the thought of tearing into the carbs right now and having the the car out of commission isn't something I am likely to do. Live with it for now. Feeling like a Fall project.

The idea of rebuilding myself is attractive, but lots of work travel these days and time is scarce. Having them gone through locally or shipping them off for a rebuild is the way I'll probably need to go. Want this to go right...is there a Zenith guru out there I should be talking to? U

Thanks
 
Stevehose,

Thanks. I recognize these guys from when I was thinking of having some SUs rebuilt. They were in pretty good shape so did myself, however.

Like that they bench test. If I spend the pennies I want it spot on.
 
As alluded to earlier, plug the dist. retard vacuum connection where it exits the rear carb to eliminate that whole vacuum circuit as an error source. Plug the carb-side fitting.
 
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