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Clemmiek

Black Smoke at high rev/ cold start

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Clemmiek

Hi All,

 

Relatively new to this forum, but I am looking for any additional information to resolve the black smoke issue I currently have with my 1982 mk2 Polo Breadvan 1.0 litre (Weber 32 IBF carburetor).

 

Over the past year the car has undergone a major body restoration which was finished approximately November 2018, but with the cold weather now set the car was placed in storage for almost 5 months. Spring 2019 the car was removed from storage but had developed several starting issues which have since been resolved i.e. new fuel, fuel pump, spark plugs, fuel lines, starter motor and distributor. The car then ran well for about 2 weeks, then became really lumpy at idle and pretty much undriveable. The car has recently passed its MOT, which during the test the technician tweaked the carburetor to bring its idle speed back to an acceptable level however this meant the car would run very lumpy at higher revs and stall when they were brought back down.

 

We have since refurbished the carburetor along with various other jobs, but the car now plumes out black smoke when cold and when under load when warm. Ignoring the smoke, the car idles and runs well but there is obviously an underlying issue. Initially over-fueling was my thoughts, but we have now reset the carburetor several times but with the same result. The mixture screw is approximately 1.5 turns out and the car is no longer lumpy when idling.

 

The only remaining jobs I need to complete are an oil/filter change and the rubber mount underneath the carburetor. Holding down the carburetor to seal any voids in the mount does not improve the smoke or affect the idle speed.

 

I have searched the forum and found a few ideas from different posts and believe we have exhausted most ideas; so we are looking for any further ideas before we strip the internals and look for any wear inside the block.

 

Below is a summary of the what we have examined:

 

·         Filters – both air and fuel filters have been replaced despite showing no signs of dirt or degradation.

·         Fuel lines – cleaned and replaced if necessary.

·         Fuel pump – replaced.

·         Spark plugs – replaced and cleaned several times in the past few weeks.

·         Fuel – tank emptied, re-filled with fuel cleaner additive.

·         Distributor cap – replaced with new points.

·         Earth straps – both replaced.

·         Weber 32 IBF carburetor – jets and body cleaned with car cleaner, serviced with genuine Weber kit i.e. new gaskets, washers, o-rings, float needle/seat, solenoid and diaphragm. Float height has been confirmed as correct and all flow tubes are clear.

·         Carburetor rubber mount – looks OK but is being replaced in the next few days.

·         Oil cleaner (Wynnes smoke clear) additive.

·         Vacuum lines have been checked.

·         Thermostat will be replaced in the next few days.

·          Oil will be replaced this weekend.

 

Any further ideas would be greatly appreciated. I can add photographs of the engine if required.

 

Many Thanks

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steveo3002

sounds like carb tuning issues to me , is the choke working as it should

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Clemmiek

Yes, the choke is working fine.

Edited by Clemmiek

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steveo3002

well smoke suggests too much fuel /not enough air ...id concentrate my energy on the carb   

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caretakerplus

I am wondering if the hot/cold air flap in the air intake is restricted, the effect of that may well only be noticeable at large throttle openings.

If running with the top removed from the air cleaner cures the problem, then that is probably where the restriction is.

As it idles OK with the mixture screw one-and-a-half turns out, there is obviously nothing wrong in that department and altering the position of the mixture screw will NOT improve the situation.

The only other point that comes to mind, is that the main jet might be of  incorrect size or (long shot) the choke tube (venturi) is of the wrong size.

 

Regards.

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damunk

does exhaust pop? would have thought over fueling, unburnt fuel pop noise when revving it high n letting go

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Clemmiek

Thanks for the comments.

 

We see no noticeable difference with the air box on or off, the flap appears to be operating correctly.

 

The exhaust only pops slightly when the engine is cold and under high revs. Once warm it tends not to pop.

 

I'm wondering if the float height may be incorrect as the instructions from an older web page stated a height of 38 mm, but the Haynes manual for a weber alternative carburetor states a height of 28 mm +/- 1.0 mm. I'm not sure if this will cure any over-fueling issues, but will investigate.  

 

My thoughts were a vacuum leak from the base mount, but after inspection it appears to be fine, but has been sealed with gasket paste to be sure.

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Clemmiek

Just an update:

 

A combination of small things seems to have cured our smoking issue.

 

1. Cleaning the ends of the HTC leads - no serious build up of dirt and all had resistance readings of around 5k ohms.

2. Quick clean of the spark plugs - done before but were dirtied again.

3. Adjusted the float to approximately 30 mm +/- 1.0 mm. Was initially running quite lean, but an adjustment of the mixture screw evened it out. 

 

Thanks for the input.

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steveo3002

sounds like it was the float

 

you could check the plugs are the correct grade

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