• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Setting mixture on HS4 carbs

Share this page

A dial caliper cost less than 20 bucks. There's no time like the present to learn. :smile: Quite honestly I often amazed at the trouble that, " the world's most simple carburator" gives folks.
 
Thanks to everyone for all of the help. I did what Mickey suggested and seem to have her running well again. I DO feel pretty stupid for messing with the carbs to start with though...I should have started with "something simple" first. I'm pretty sure that one spark plug wire that was a little loose in the dist. cap was the initial culprit !! :frown: I really am trying to learn guys...
 
Gary -

We've all been there... and back... and there again! The key to getting SU's right is fiddling with them, marking your settings, and once you have them set, they rarely give any trouble.

The way you're learning and gaining experience is the best way, and there's actually little you can do to mess these carbs up. As Hap said, they're quite simple once you understand how they operate.

This article is well-written, and tells all you need to know about the critters.

:cheers:
Mickey
 
JP, You're right, driving is the ultimate goal. If a old british car doesn't backfire a little now and then it just wouldn't seem - well British.
 
DGill said:
JP, You're right, driving is the ultimate goal. If a old british car doesn't backfire a little now and then it just wouldn't seem - well British.
Click to expand...

Amen to that - part of the charm
 
Gary, that's how I learned... messing with the SU's on a 1962 MK I (948 cc) MG Midget. Everybody's got to start somewhere.

Your local (also on-line) Harbor Freight, the infamous China tool outlet, is a good place to get cheap tools. They have a digital caliper set (just like Hap's dial calipers but with a digital readout, and they may not be quite as accurate at plus or minus .001 inch) for around $20. They're good enough for general use. I use a pair for my wood turning. There is a thin rod that sticks out the back end for measuring depth and this rod can be used to measure the depth to which the jet is recessed.

As far as the HS4's go, remember the jets must be correctly centered so the needle does not rub on the jet opening. The piston should fall freely and land with a metallic thunk. Use the lifting pin to check. The piston MUST NOT stick. The only part of the piston/chamber assembly that should be lubricated is the outside of the machined surface of the damper tube. Of course, oil should be in the damper, too. All else should be cleaned but not lubricated. Seems to me the piston and chamber are a matched set, so don't get them mixed up. And you're right, the spark plugs are the best way (without a Colortune or some fancy analyzer) to check the way the SU's mixture is adjusted.

As to your spark plugs, and if I remember correctly (and this was with "real" leaded gas)...

You should be looking for a light brown color on the center insulator. This indicates normal running. If you have dry, fluffy, black deposits your setting is too rich. If your center insulator is whitish, your setting is too lean. Before you reinstall the plugs, always clean and re-gap them.

The dry, fluffy, black deposits could be caused by a too rich jet setting, but could also be caused something common to all cylinders; check for restricted air filters, possible float needle leakage, or improper use of the choke if the deposits are on all of the plugs.

If these deposits are on only one or two plugs, look for something specific to that/those cylinders; check for faulty high tension lead(s) or check the valves as you might have a sticking or burnt valve.

If the deposit is a wet looking black, that's oil fouling. If the engine has been rebuilt recently it probably isn't broken in yet. Otherwise, you may have worn engine bits. Check your compression.

I don't know how the ethanol polluted "gasoline" they now sell would affect these plug readings. (Fortunately, we can get unleaded non-ethanol gasoline locally, but it's still low on octane.) Anyone care to comment on these spark plug indications?
 
mgbsteve137 said:
As far as the HS4's go, remember the jets must be correctly centered so the needle does not rub on the jet opening. The piston should fall freely and land with a metallic thunk. Use the lifting pin to check.
Click to expand...

Gary has a '71, which came stock with spring-loaded needles, so I don't think centering applies to his needles (unless somebody has changed them out for the old style needles.

Gary, have you already checked your throttle shafts for leaks? The main weak point of the HS4s is the throttle shaft and bush wearing so that the hole and shaft become egg shaped, creating a variable vacuum leak and making it impossible to adjust the idle mixture. If you haven't already, you should check them by spraying carb cleaner at the points where the shafts exit the carb body (both ends) while the car in running. A change in rpm when you do this indicates that you need to fix this.
 
Yeah looking a studying how a twin SU assembly works will teach you alot, if you look hard enough, every working of it will make perfectly good sense, I never owned any sort of calibrating tool like a colortune, you don't need it. Yeah centering the sold needle jet and re-doing the shaft are the hardest two things to do to these carbs, but the rest is simple. When I get together for big LBC ride and we all fire up our cars, you can almsot choke from the raw gas in the air :smile: It's like 90% of these cars are way too rich, it's simple turn of a bolt or screw to correct this and some people still look at it as if they working on the space shuttle. There's nothing better with these cars than fiquiring out what makes something tick, and sure you can ask questions, but first sometimes you just need to look and study how something works, once you know what makes something works, you not only pocess the knowledge to fix it, but to also make it better.

So in short, don't look at a set of SU as some hi tech marvel, after all these same people used to make meat grinders before they made carburators :smile:, just look at it, study it, and own it !
 
Davester...I did spray carb cleaner on the shafts and they didn't change rpm.
Mr. Waldrop...you are absolutely correct and I'm trying to actually learn everything I can. Carla and I study the Bentley manual, Moss tech tips and University Motors videos and then just tear things apart and look at them. We've lost a lot of our "fear of mechanics" working on this car. She sat in a garage for 18 years before we bought her and needed a lot of work. It actually IS pretty simple once you just DO it. Thanks everyone !!

ps. Carla drove her 25 miles to school this morning and says she's running very well...no problems.
 
Yeah reading manual is good, but it kinda goes in one ear and out the other until you done it enough to get it. What I'm talking about is per-say sitting there with your carbs on a bench or on the car and looking a studying the workings of it, for example, "Ok when the linkage rotates is turns the butterfly to the open postion, ok this tube on the float bowl is vent , and this one is a fuel feed", and so on. This is a common thing with even professional mechanics, they fix things by habits, without studying the operation of a given component. In short a book or manual, or video didn't teach me how to build carbs or engines, studying and understanding the workings of the component did, the answers are right there in front of you if you look for them, plus it exercises the mind :smile:
 
Back
Top