MG MGB and MGB GT Tech Talk
Hello all,
I've searched and searched and I can't find anything to answer my questions. My 74 Chrome bumper MGB has a manual choke on the dual SU carb setup.
The million dollar question is, are there any non-manual choke options for these carbs?
If there's nothing that can be purchased ready to go, has anyone fabricated something of their own?
Thank You,
Jeff
Actually, there are no chokes on any stock MGB carburetors. They all use what is called a fuel enrichment system, in your case, on the SU HIF4 carburetors, a valve on the side that allows extra fuel to enter the engine when cold. There's no way to convert them to an automatic choke that I've ever come across. It's one of their endearing qualities. RAY
I don't see why it would be worth the trouble to do even if there were a system available. Personally I rather like the control and simplicity of the manual system.
That's kinda what I figured based on my research.
Thank you very much for the reply.
Jeff
Pull the needles out and file the top 3/8" of each needle flat. That will enrich the system for starting without any choke but the needle will be its proper shape from idle to wide-open-throttle. If you remopve the choke mechanism, make sure to safety wire the jets in place because they will get pushed right out the bottom :-) Basil 707.762.0974 basiladams@yahoo.com
Hi Basil,
That's a new one one me, although it doesn't take much, is this a streetable mod or is it mainly for getting the race car started in the pits without having to worry about the choke hanging up on the track?
Joe
Basil - I've been messing with these cars for 40 years, and I learn something new every day! Once you think about it, it makes perfect sense!
I'll stick to my choke knob, however. Until I get that "T" handle knob - my fingers could use the help.
Jeff.
When SU carburetors were used on cars that did not have a manual choke the normal method was to fit an auxiliary starting carburetor to the manifold.
The Austin-Healey BJ8 and Jaguars ran an electrical solenoid operated mini-carb that was controlled by a thermostatic switch. A solenoid pulled up a needle and air/fuel mixture was directed by pipework to the underside of the manifold.
Complicated and prone to problems.
The early Rover V8 cars also had a starter carburetor which was prone to problems. A friend of mine replaced the one on his car with a lawn mower carburetor with cable activation. It worked surprisingly well, but was then manual operation.
Ok, rattling now. There is not a simple solution to running an automatic choke on an SU equipped car.
Sad to say, but if you really need to have a manual choke I'd recomend converting to a downdraught Weber carburetor and manifold. They are available with pretty dependable water or electric choke systems.
Kelvin.
Why? I went out of my way to get a 4 barrel with a manual choke. Suits the car, not to mention using a dash hole.
I would agree with Basil in theory, but I would strongly recommend that you not go at the needles with a file.
What he is suggesting would be to allow a larger effective opening at the jet face at idle or at starting when the carb piston is way down or not up at all. That would in fact allow more gasoline to be drawn from the top of the jet but the problem comes in when you try to figure just how much to file. And then you would really have to file both needles EXACTLY the same.
Basil's suggestion might work well with a race engine in which there would be precious little time for any idling or low engine speed, with most of the engine time being with a well open throttle. It would not be appropriate for street use though.
My bet would be that you would ruin several sets of $15 needles before realizing that the effort was futile and just buy a final stock set and go back to where you started.
The real fix would be to make sure that your carb enrichment system was in good order and that your choke cable and linkage is working smoothly and set correctly.
Jack
Ok. I think I get the point... Manual choke is where it's at and it's going nowhere else. I have no desire to get into and most likely ruin my carbs by attempting to make a mod I really shouldn't be doing. I was just thinking it might be easier for my son (Just about to turn 16 in May) to deal with. But from what I read, it sounds like he will just have to deal with the manual choke.
Thanks again for all the great input.
Next question: (Let me know if I should be starting a new topic... I didn't want to get thread happy...)
anyone have some good pictures of the wiring for the starter relay, anti-run on, starter solenoid, coil etc? Basically the passenger side of the engine compartment for an early 74 MGB.
Our car had a case of the rodents after sitting for quite some time. It's not too bad, but there are some wires that I can't really determine where they should be going.
Before you send me links to wiring diagrams, I've downloaded and printed out several different ones. Unfortunately I don't see the specific wire colors in the diagrams. I'm not at home so I can't take and post any pictures.
I'll try to do that in the next few days. I know a picture's worth a thousand words.
Any assistance is appreciated.
Jeff
http://www.amazon.com/MGB-Electricals-Systems-Essential-Manual/dp/1845842294/ref=sr_1_fkmr0_2?s=books&ie=UTF8&qid=1328759726&sr=1-2-fkmr0
The best money you will ever spend on your MG electrical system.
Jack
What's "to deal with it"? Pull choke,start car.car started,push choke in. Educating is better then coddling
Sorry Basil. I thought that you were suggesting "turning" the circumference of the needle with a file. Just carving out a flat spot might not be too chancy, but I still feel like doing that would be better left to a racer for his needs.
If I saw a set of carbs come in with flats filed on the needles, I might be shooting from the hip, but the first thing that would happen would be that they would go to the trash bin.:)
Jack
these jet needles are very precisely tapered. i couldn't say i would recommend anyone filing on them. except maybe someone that has been trained to do so.
I think you are just playing with me now, but if our serious,and this might be just me,but I want the choke off as soon as possible, it is sucking more gas then normal...wh, en I start mine,the choke is out, as soon Asa's I can give it gas without "feathering" I push the choke in. I haven't timed it but I would say the choke is out for i less then a minute. If. I stall it out,the choke comes back out,but this time as soon as it starts, its pushed back in and off I go
done all the time. Look in Des Hammill's book on power tuning SUs.
Cold engines don't like lean mixtures, so enrichening the mixture on startup is necessary. On the SU HF4 carburetor, a small passage is opened up when you pull out the "choke" knob. This adds extra fuel to the mixture and allows the engine to start much more quickly. Most U.S. carbs used a choke "strangler" plate, that does what the name implies. It chokes off most of the air entering the engine, leaving a rich mixture to help start a cold engine. Many race carbs use no choke mechanism at all. The Weber DCOE provides for easy cold starting by providing a metered pair of jets that allow more fuel to enter the airstream on startup. These are available in different sizes, as are the rest of the components on these carbs. RAY
Someone on the board might have a high-quality original Zenith-Stromburg carb that would auto-choke nicely :)
Flating one side of the needle above where it rides at idle is a neat idea.
Sorry, there, Jeff!
I didn't mean to insult you - yeah, I missed what you were replying to.
Just making sure.
The Kid will love it! Some day he'll be an old guy, with great memories of driving an MG.
Ray, what are you smoking?
The SU carbs' choke just pushes down the jet on the needle to allow more fuel in. Granted, it's not a "Choke" per se, but there's no valve on the side.
Are you referring to the Strombergs?