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Carb oil leaks out

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Greg Q

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I have a 1972 MGB roadster. I had the carbs (SU HIF4's) rebuilt in April and have been working to get them adjusted (I'm having a little trouble with the mixture). In between efforts, I have found that the oil in the damper tubes is leaking down into the carb throats and fouling the pistons.

This is happening in both carbs to some extent, but the rear one is much worse. It had been three weeks since I last started it when I tried it yesterday. It ran poorly and coughed and spit so I shut it down. I checked the carbs and found that the piston in the rear carb was the color of motor oil and wouldn't move when I pushed on the lifting pin. While I can take it apart and clean it, I don't understand why this happens.

Is it a commom problem for the oil to leak out in SU carbs, or is this an indication of a more serious problem?

Any suggestions would be appreciated.

Thanks.

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<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Greg Q:
I have a 1972 MGB roadster. I had the carbs (SU HIF4's) rebuilt in April and have been working to get them adjusted (I'm having a little trouble with the mixture). In between efforts, I have found that the oil in the damper tubes is leaking down into the carb throats and fouling the pistons.

This is happening in both carbs to some extent, but the rear one is much worse. It had been three weeks since I last started it when I tried it yesterday. It ran poorly and coughed and spit so I shut it down. I checked the carbs and found that the piston in the rear carb was the color of motor oil and wouldn't move when I pushed on the lifting pin. While I can take it apart and clean it, I don't understand why this happens.

Is it a commom problem for the oil to leak out in SU carbs, or is this an indication of a more serious problem?

Any suggestions would be appreciated.

Thanks.

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<hr></blockquote>

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you might try a slightly heavier oil. (I've heard of people using engine oil in the carbs). It will slow the response of the carbs, which may or may not be acceptable to you, but might stop the loss of oil - - you're not over-filling are you?
 
The books I have, and the people in my local MG club both recommended using engine oil (20W50)so that is what I put in.

I filled the dampers to a level 1" below the top of the neck. I have seen several pictures and read discriptions of the correct level which seemed to be higher than this. When I filled the tubes to these higher levels I had oil sqirting out the vent holes in the top of the damper caps.

I'll look at my books again to see if I'm doing something wrong.

Thank you for your comments.
 
I have seen several references that indicate the dashpot oil level to be 1/2" or 1/4" inch above the top of the piston tube, I forget which. I am not sure why? I tried that but like you found too much oil accumulation entering the carbs. I have always run about 1/4" below the top of the tube.
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Starting with rebuilt carbs, I began using 30w, non-detergent engine oil, and filling to 1" below the top. I find that I still lose a little over the winter when the car sits for long periods, but overall, the level has been pretty stable. I've heard just about everything being recommended, including brake fluid.
 
I want to thank everyone for their replies and the information they provided. I guess it isn't as simple a question as I thought.

I am going to clean my carbs up this weekend and reduce the oil level to see how that works.
 
Aloha,

I agree that the most probably cause of oily carbs is over filling the dash pot damper tube. Since the area within the dash pot is a low pressure area, oil above the top of the tube would be drawn into the dash pot area and then drain to the carb bore. If you keep the level of the oil about 3/4 inch below the top of the tube that rides in the bore of the dash pot it should stay put.

There are many opinions on what type of fluid you should use in the dash pot. Most of them generally differ based on the viscoaity of the fluid, light machine oil, transmission fluid to motor oil or even an STP type product. Most manuals if read recommend motor oil. I use 10w30 in my dash pots, however I'm in a very mild climate. The purpose of the fluid is to keep the dash pot from fluttering in the carb. A primary function is to maintain a rich fuel/air mixture upon acceleration. As the butterfly opens and air flow increases thtough the carb, the dash pot will rise allowing more air into the engine. Since air is less dense than gasoline, air flow volume can get ahead of the fuel flow. The damper retards the dashpot and allows the mixture to remain in proper ratio or on the rich side allowing power on acceleration. A lean mixture will result in lack of power and slow acceleration. Therefore, in my opinion, use the same weight motor in your dash pots as you use in the engine and check the level periodically, topping up as necessary.

Opinions are like noses, every body has one and they are all slightly different.

Dave
 
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