Eric Bylenga

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SU HIF-4 Carb Repair

Whilst first inspecting the MGB prior to purchase I noticed that there was something wrong with the right carburetor, while on the left one I could easily move the piston up and down, the right one was decidedly stuck.

In order to drive this thing home, I needed to get things working properly again. We tried running the car as it was but fuel came pouring out of the bottom of the carb bowls.

Here's what things looked like prior to work. To explain, the bottom braided cable is the fuel line passing through the front carb to the rear by the shorter braided cable. The little clear one that goes into the steel line is the vapour line to the charcoal canister. The top beasty pipes I believe are the vacuum lines, one to each carb.

Carbs in engine compartment

The carbs are held to the engine by 4 bolts that are a bit of a trick to remove, but here you have it from the backside below. The upper bar thingy is the throttle shaft and the lower is the choke. You can clearly see here both throttle plates.

Removed Backside

Below is the disassembled dashpot assembly. as you can see the piston is machined to slide up and down inside the dashpot. The motion of the piston is retarded by 20 weight oil inside a little chamber at the top of the cylinder. My problem on the rear carb turned out to be a lack of this oil! Some water seeped in and seized up this assembly. I had to use a whole lot of WD40 to soak in and some gentle vise work to work that baby off. You can also see in this photo the needle that rises and falls with the piston to let more fuel into the engine.

Dashpot assembly

Here's the bottom of the bowl, yep, it's a SU Carb built in Birmingham. So, look here's my second problem and the reason fuel was pouring out of the bottom, broken seals! Thankfully the replacements are only $3.00USD at Moss Motors.

bottom Bad Seal!

While we're in the neighbourhood, it's good to make sure the floats and jet is in good working order. I removed my floats and stuck them in a cup of water... Yep, they still float. The jets were kinda gummed up. Loosened those up with WD40 so they could be adjusted easier as well. University Motors has a great video for adjusting your floats properly. Thanks John!

Float

Here were are ready for re-install, armed with my Moss catalog, which very kindly has some nice diagrams that are handy for putting things back together again.

Ready for Reinstall

Here we are mostly installed again, the other goofy thing I had to fix was a third part fuel pump that the kids at school had installed on the vapour line for some reason... Yea, that wasn't going to work. Incidentally the original pump works just fine!

reinstalled

Here we go! I removed the spark plugs and manually turned over the engine to make sure nothing was seized. After putting things back together I tried to start it up. Amazingly it started up just like that! I was amazed... Witness my excitement below!



The day after this video I was finally able to drive my beast home, a really exciting day!

Links:

Moss Motors
60 MGB HIF Float Bowl Adjustment

Last Updated: June 6, 2020