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MG MGA - HS4 question

I have fitted a set of HS4's to my 1800. The problem I am having is that it takes about 15-20 seconds after reving the engine for it to return to idle. It seems to sit at about 1500rpm them drop slowly to idle. Everything seems to move freely and the linkage appears to return to it's proper place.
Any help in sorting this out would be appreciated.
Thanks
Kris
Kris Sorensen

Clean inside the dashpots - but only use a very clean soft cloth with no abrasives - once heard of someone that decided to polish up the inside with T cut and couldn't get them to run after that - and don't mix them up - do one at a time and mark them so that they go back in the same place - cheers Cam
Cam Cunningham

Kris. Cam's comments are a good place to start--make sure the carbs are clean and not binding. In addition, inspect the throttle plates (butterfies) on the throttle shafts. If they are a little loose, they can cause the problem you describe. Then, check the throttle plates for the presence of the poppet valve. This was one of the pollution control devices whose function is to delay the engines return to idle, allowing the fuel droplets which collect in the intake manifold to be burned efficiently before the engine is allowed to return to idle. If you shut the throttle quickly, the small amount of gas which has dropped out of suspension from within the fuel/air mixture, are quickly drawn into the engine and burned in an inefficient manner (excessively rich mixture, leading to high levels of unburned hydrocarbons, or HC).

So, check and see what you actually have going on with your carbs. The throttle plates, if of the style with the poppet valve, can be replaced rather easily. If the throttle plates are loose, they may need to be tightened or the may need to be replaced, only an inspection will tell you that. If they are only somewhat dirty, carb cleaner has been working well for many years and should probably continue to do so. If you do have a problem removing residue, switch brands. Some of the "environmentally safe" products do not clean as well as the older products and it is worth checking a couple of brands and seeing what works best.

If it is not a carb problem, it will be linkage, cable, accellerator, etc--some associated system which might have only a marginal impact on the looks of things (such as one spacer block being slightly thicker than the other and binding the linkage), but will have a significant impact on the system. If you can get an extra set of eyes, even untrained ones, to look at the system and tell you "what is different", it would be of help.

Les
Les Bengtson

Les
I do have the poppet valves on the throttle plates. I also have another set of carbs without the poppet valves. So I will switch the throttle plates and try again as well as checking the other items you suggested.
Thanks
Kris
Kris Sorensen

Kris. No problem. As has been suggested on the MGB Tech BBS, make sure the throttle plates close fully. A drop of blue Loctite on each screw is a bit of extra insurance. I have had the screws loosen on one set of new Burlen made SU carbs which caused the exact problem you mention. Now, I use some Loctite when assembling the carbs after rebuild. I would also suggest checking the tightness of the screws on new carbs and adding a drop of the "wicking" Loctite to them to ensure they do not loosen in use.

As to the use of ATF vice motor oil, that should not be a factor in your problem. Some minor tuning can be achieved by the use of different dampening mediums, but, not a great deal. ATF is, from memory, SAE 10 oil with a lot of detergent and anti-foam modifiers added. The use of a heavier weight of oil will cause the pistons to rise and decend slightly more slowly. Not the problem you are describing as the rise and fall of the pistons have less to do with your situation than the shutting of the throttle plates.

Les
Les Bengtson

Les and Cam
As you suggested I replaced the throtle plates and It runs real nice now. Still need to make a few minor adjustments as I go but it seems to be OK.
Thanks so much for your help
Kris
Kris Sorensen

Kris. Glad that seemed to solve the problem. Hope you did not forget the Loctite on the screws. My experience, with a set of two year old Burlen carbs, might save you some head aches later. Loctite is your friend, even with new carbs.

Les
Les Bengtson

This thread was discussed between 12/11/2006 and 13/11/2006

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