E3 2500 Zenith Cold Start Rough Running

[E3_UK]

This is frustrating. Last year the car always started perfectly from cold with a smooth fast idle settling down to 800 rpm once warmed. Since the usual winter layup however it has been running like a bag of nails on cold start, as if only one carb is functioning correctly. Nothing can mysteriously just gone out of adjustment sitting in the garage and the fuel tank was filled with new fuel and Stabil stabiliser added so it's not bad fuel (that would affect both carb's anyway). Once warmed up it settles down to the correct smooth idle. What are the steps I should take to check the cause of the poor cold running ?. I really don't think it's a major strip down problem as all was perfect prior to the layup, and I have been running it up to temperature periodically during the winter despite the cold running issues so it hasn't been lying dormant for months on end. I'm just looking for a methodical approach to identifying and rectifying the problem as opposed to blindly stripping everything without needing to.
I should add this is a standard UK car.

 

[Stevehose]

Take the air cleaner off and verify all linkages are operating as intended and not stuck open or shut on the misbehaving carb. Especially the cold start linkage. Ensure all vacuum hoses are in good nick and attached properly.

 

[E3_UK]

Thanks Steve. Vacuum hoses are all ok. Before I start it next time I'll remove the air cleaner then depress the throttle pedal and check both carb's are set the same. I have never had to play around with the auto choke so am a bit wary of removing the covers. If both choke flaps aren't fully shut, what is the likely cause and where should I look ?. Like I said it isn't an adjustment issue as cold start was perfect before so some part of the mechanism is either sticking or at least not operating to its full extent to cause the misbalance.

 

[Stevehose]

One of the choke flaps may be stuck open so it may just need to be freed and lubricated.

 

[E3_UK]

Can I get to the mechanism without removing the cover ?. Sorry but I've never really looked at this side of the carburettors until now so am very unfamiliar with the setup.

 

[Stevehose]

Ho you will want to remove the air cleaner top, filters, and housing (just two nuts on each on the s will give yiu access to everything. You can see if the choke is working without removing the housing.

 

[E3_UK]

Thanks Styeve, just wasn't sure if I had to remove the housing covers or not. I'll check the mechanism as suggested to see if one of the carb's isn't getting full choke.

 

[E3_UK]

What is the operating principle of the choke mechanism ?. It has coolant and electrical connections, what is the interaction between these and the mechanical arrangement that firstly sets then gradually brings the choke off as it warms ?.

 

[E3_UK]

Mechanically everything is free. Both flaps are fully shut when the pedal is depressed.

 

[Sean Haas]

The tech section of this site has manuals on the carburetors. There is a section on choke adjustment that shows you how it works. There is also info here http://www.jaimekop.com/CarbManual/Page01/toc.html with pictures that might be helpful.

 

[bavbob]

Why not go to the Technical Section here, there is info on the Zenith Carbs. Read it, then read it again and again. I finally am getting it and it is quite helpful.

 

[E3_UK]

Why not go to the Technical Section here, there is info on the Zenith Carbs. Read it, then read it again and again. I finally am getting it and it is quite helpful.

Yes I know, I have read most of what I can find, however sometimes someone has a "ureka" piece of advice that isn't always covered in the technical manuals etc. I will dig deeper though, I was just throwing the question out there initially too see if there was something glaringly obvious I should try first but I don't mind starting from first principles to ensure everything is set up correctly.
My bread and butter carb's are simple SU's with manual choke, can't really get a simpler carb', therefore this is like a piece of NASA design by comparison !.

 

[bavbob]

There is a thermo switch on these, that square box with the aluminum cover that has a single black wire to it. This thing is essentially a cold start valve that enriches the fuel at the onset. You can pull it and watch to see if it moves as the car warms up. It may be stuck closed. Test both of them.

 

[Stevehose]

When you pulled the air cleaner off, were the chokes closed on a cold engine?

 

[E3_UK]

Yes I know, I have read most of what I can find, however sometimes someone has a "ureka" piece of advice that isn't always covered in the technical manuals etc. I will dig deeper though, I was just throwing the question out there initially too see if there was something glaringly obvious I should try first but I don't mind starting from first principles to ensure everything is set up correctly.

When you pulled the air cleaner off, were the chokes closed on a cold engine?

No both fully open. Initially it fires up fine but then seconds later the revs drop and it runs very roughly. On cold start one flap was fully shut, the other a very slight gap. Manually moving the flap on each carb in turn didn't resolve the rough idle. Both flaps move freely through full range. Once warm it runs fine but not perfect, a bit of a cough every now and then. I adjusted mixture and idle last year following your procedure. I also reduced the startup revs slightly. At this time it always started perfectly. Problem has occurred during the winter.

 

[E3_UK]

Still struggling with this unfortunately. I now don't think it's a choke issue. The engine fires up ok from cold, rev's rise to between 1k/2k immediately but then a second or two later drop down and the engine runs very rough, definitely not even on all cylinders. I'm ruling out the ignition side, everything was spot on last year and it has done very few miles since. It's running an electronic points replacement setup inside the distributor and again this has been perfect since fitting it three years ago. The issue must be related to the winter layup period. I ruled out bad fuel as I had used Stabil stabiliser as I always have. However, I know things can still gum up despite using this, so what are the most likely problem areas to investigate ?. When it was running, I pulled the wire off each of the shut off valves in turn. On the rear carb' the engine did slow a bit however on the front carb' it died altogether, suggesting that the rear carb' isn't doing very much. Could the float chamber needle valve be gummed up and not letting sufficient fuel into the chamber ?. Is there anything else I can check ?. Bear in mind this is a '72 built UK spec' car so as far as I know it doesn't have emission control as per US cars. Really struggling with this and don't really want to start pulling it apart unnecessarily if there is something obvious I'm missing.

 

[E3_UK]

20180511_161835 by Robin Derrick, on Flickr

Looking at the wiring diagram, the choke units (24 & 25) are connected to a relay (101) and a temperature switch (26) that is mounted on the inlet manifold directly underneath the front carb. With the ignition on, cold engine, there is current at both choke units however only the flap on the front carb gradually opens over about 5 minutes. The rear carb flap doesn't move at all. I really don't think this is the cause of the rough running however as when the flap is operated by hand with the engine running it still runs very roughly. I thought the flaps just opened over time as the electrical elements heated up. What is the purpose of the relay and temperature switch, and how do I check if it's all operating correctly ?.

 

[E3_UK]

Temperature switch

20180511_163953 by Robin Derrick, on Flickr

 

[Belgiumbarry]

"running it up to temperature periodically during the winter..."

perhaps fouled plugs ? idling doesn't burn the plugs clean...

 

[E3_UK]

Plugs are fine, first thing I checked when the problem started. Perhaps I'll eliminate them by fitting a new set though just to be sure.