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IDLE and Spark Plug Fowling Problems


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After restoration of my 71 Z ,I had a lot of problems with my SU's.Thanks to the help and parts I got from ZTHERAPY,most of my problems are solved.Still I have two problems as follows:

1-Idle speed should be at 750-800.When it is adjusted to this optimum speed,it sometimes was going down lower and the engine was stalling.Then I found out that touching the linkage between the two carbs was changing the idle speed.After that I discovered that the bushings are worn and air is sucked in in uneven ratios hence changing the mixture.What can I do to eliminate this worn bushing problem ???.I adjusted the idle that it runs between 800-1000 so that its lowest value is 800 in order to avoid stalling.

2-Two of my spark plugs ,mainly 3 and 4 are getting fouled due to rich mixture or poor detonation.The compression measurement showed that these two have lower ratios.My mechanic suggested that this may be due to valves.

Any ideas or suggestions ???

Your comments will be appreciated since my Z is the only one in Turkey that I know of.

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Only way I know to eliminate worn throttle shaft bushings is to replace the carburetor bodies. Since you have a relationship with ZTherapy, you might want to contact them to discuss this.

In the USA a bird such as a turkey is sometimes refered to as a "fowl". Michael was making a joke about the country of Turkey, and the bird "turkey".

Also there is a spelling joke in there. When talking about spark plugs, the term is "foul", when talking about a bird the term is "fowl". Both words are pronounced the same way in the USA.

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Thanks Carl.I'll get in touch with ZTHERAPY and see what I can do about the carb.Also thanks for pointing out the correct term for spark plug fouling.

Do you have any idea about the problem in 3&4 cylinders ?.Definitely it is not due to carb,since the others are OK.

Thanks again

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Hi there Carl offered sound advice but I don't think replacing the carb body is necessary they should be able to re-hone the are that the throttle linkage sits in and either find a replacement linkage or machine it to fit with no leaks.

Were you referring to the balance tube when you say bushings?

In terms of compression you might be able to adjust the valve clearances and that could correct the problem. I haven't heard of fouling caused by badly adjusted valves I'd say that's more to do with poorly adjusted carbs.

What color are the plugs? Are they black and oily or just black?

Let us know.

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The linkage bushings in the balance tube portion of the carb linkage wouldn't cause air leaks since they aren't part of the "induction" system.

I believe that he's referring to the rod that the butterflys are mounted to. These rods are (as I understand it from Scott Bruning the original ZTherapy owner) made of brass which has been plated with a harder material, which eventually wears off and causes air leaks. If you hone the bushings in the carb body (that the rod rides in), you would need to add metal (diameter) to the rods to fill the larger diameter that you've created by honing. IIRC, ZTherapy fixed this problem by installing actual bearings of the proper size. They probably do more than just that, but I'm speaking from memory and it's been a few years since I spoke to Scott.

Sorry, I don't have any idea about the #3 and #4 cylinder issue. Odd that each cylinder is served by different carb and the other cylinders aren't affected.

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Back in the day, before Z Therapy, we used to hone the openings and drive bushings that had the correct diameter for the butterfly shafts into the hole. Then if you ever needed to do the job again, all you had to do was replace the bushings. But, like I said, that was years ago and the shop where we would buy those correctly sized bushings from is long gone.

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Thank you all.I'm refering to the rod that connects to the butterfly.I think honing and bushings will do the job.But where do you suggest that the bushing should be installed,on the rod or in the carb body??.


the spark plugs are black,indicating rich mixture or pure detonation and they are not oily.They are fed from two different carbs so the adjustment of the carbs is not causing this fouling.

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Okay well how old are the spark plugs in general? If those plugs have been dropped in the past or just old they would not work the same and could misfire etc.. How about the distributor cap does it have small cracks in it look underneath and check each point aswell are they worn down? Spark leads are not likely the culprit but that could be another cause get a multimeter and check their resistance.

If they are the two lower compression cylinders then do a valve adjustment first if not improvement it's more than likely the rings are shot. Fill those cylinders with some oil and redo a compression test if you have an improved reading you know the problem.

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I finally solved #3 and 4 cylinder spark plug fouling problem !!!!.

By using a gasoline additive that cleans the engine and the valves the problem is solved.

If you have a car that has not been used for a long time,2-5 years like mine,you might have the same problem..

Thanks to everyone that advised me what to do....

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