hi

there is a problem that sickenings me for a long time: rough idle on my CARBURETTOR engine Felicia.

no matter what I tried, it seems there is no solution.

and before getting bald from pulling my hair out, I ask for your help. WHAT IS THE BEST FAULT FINDING PROGRAM (Haynes like - decision tree) when dealing with a rough idle ?

Present situation

- checked compression: OK and equal on all 4 cyl.

- new timing chain

- checked valves clearance OK

- checked timing with a strobe gun OK

- cleaned carburettor and checked according to Pierburg 2E3 OK

- new inlet manifold gasket; tighten according to manual using a torque wrench

- new carburettor support gasket; tighten according to manual using a torque wrench

- checked for vacuum leaks (both with carb cleaner and propane) OK

- new spark plugs, central pin color is OK on all 4

- the car has good power for 66000 miles onboard in 14 years. I get 90 mph easily with no passengers

- decent fuel consumption: 40 MPG on average (in city and outside)

You've done all sorts of relatively expensive stuff without first doing one of the most obvious and relatively cheap things - replace the High Tension leads! I had a misfire on a carb car, and that's all it was.

Other possibilities:-

Bad spark plug. Don't assume that a new set is necessarily a good set. Take one of the old ones and fit it to each cylinder in turn. If you clear the misfire that way it was a bad plug on that cylinder.

This is unlikely, but a bad ignition condensor (Kettering arc distributors only). This usually gets worse until the engine stops running over about 1600km from you first feel a misfire.

Bad coil (single coil or double-ended distributorless ignition) breaking down under high load.

thank you for answering, Ken. I will look into high tension leads. I wish I had an automotive oscilloscope to fully diagnose the ignition system. Maybe you can point me a model cheap enough to not sell my left kidney.

on the other hand, if I upload a video showing how the engine looks and sounds when idling, would that help narrowing the problem ? What else should I show or do in this video ?

P.S. Nothing too expensive I did. Timing chain and coolant pump were 60,000 miles old so they had to go, plugs are cheap, and all the labor was DIY.

Check for vacuum leaks at the brake servo and hose too...spray WD40 or similar while the engine is idling...if the engine idles better when the fluid temporarily seals the leak you've found the leak.

I did. no change.

in fact, to be 100% sure there's not a brake servo or hose leak involved, I pulled out the 1way valve hose from intake manifold and sealed the hole.

OK. I would say that if you had an ignition system fault it would show up when driving under load so in my opinion that's ruled out. You have ruled out an air leak or muck in the carb. I think you just need to richen the idle mixture up a touch by turning the adjuster screw out very slowly while the engine is idling until it's idling smoothly. All you need is a small flat bladed screwdriver.

here's what I found peculiar about the fuel mixture screw in my carb;

you would normally have a 'sweet spot' when mixture is ok (smooth idle, highest rpm then lower it from idle screw) and any other mixture screw adjustment would lower engine speed from either having a too lean or a too rich mixture.

but in my case, no matter how much I enrich the mixture by unscrewing the mixture screw... the speed goes higher and higher.

then I can't lower it enough by unscrewing the idle screw, because the screw looses contact with throttle lever. isn't that a b...h ?

Ok, I've read through the rest of the thread, and have a number of extra thoughts:-

1) I had the impression that the new timing chain was an attempt to solve the rough idle, rather than due anyway. You see why it seemed like an expensive fix?

2) Despite what Dave said, I've driven a car with duff HT leads, and it would still pull through to 6_000rpm in second gear. (for second read 3rd, with 15mph/1000 rpm [on private road obviously ]). HT leads are a service item; buying test gear costs more than a new set of leads does!

3) Is the throttle closing properly? From what you're saying, either the throttle is sticking part-open, or you have an air leak into the carb. An engine doesn't run faster without more air and fuel!

hey Ken

I have to give you credit for solving the whole issue

yes, the faulty parts were the HT leads, I dampened the wires with a spray bottle and clear water, and looked for arcs with a grounded, insulated screwdriver.

I saw little arcs jumping from one lead to the other right above the distributor cap, in the rubbery part of the leads. so now I have new HT leads and a sweet purring engine idle.

even the high idle was, like you said, due to a slightly open throttle from an old, sticky throttle cable. :beer: :beer: :beer:

the only other issue I have left with (idle related) is with the car stationary, when quickly pumping the brake pedal at idle, the engine stalls. I know it happens because the vacuum is destroyed by the 1-way booster valve or by the brake booster itself. I checked the 1-way valve and it is OK, so now I know the booster is bad.

my question is: is there a quick fix ? like a gasket or so, or do I have to replace it ? I am not too happy knowing I will have to bleed the BMC too.

According to the manuals I have (Not Skoda, but the vacumn booster works and is made the same way on most cars), it's not possible to repair a faulty vacumn brake booster. I'd redo the one-way valve check before buying a new booster, because it's less work and money to replace the valve.