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MG MGB Technical - Update on daul SU conv


I just finished switching my '80 LE over to daul SU HIF4 carbs. The car starts fairly easily, but still runs rough when it's cold. The symptoms are stumbling when I press the accelerator and roughness until it fully warms up. Once it warms up, it is much better. Joe Curto rebuilt the carbs, so I know they were done properly. My question is: Would adjusting the mixture slightly richer help with the stumbling and wanting to stall when the engine is cold? I have to keep the choke on a bit until the car fully warms up. It's also very warm here (usually '80's in the daytime). It just seems like I have to keep the choke on for too long compared to our other MG's.

Thanks in advance,
Dave
David Plantz

Hi Dave,

I don't think I would be very concerned about this. The much more crucial question is how does it run once fully warmed up. The individual differences in heads are almost infinite. Likewise, manifolds. So I wouldn't bother comparing it with other MGs (and I know you've had a bunch!). If you experience a performance problem that can be corrected with the choke, before the engine is fully warmed up, go ahead and use it. If you need the choke AFTER the engine is fully warmed up, yeah, then you're running lean. If it runs really well when fully warmed up, whatever it takes to get it there is immaterial. You and I have both had a bunch of Bs. No two of mine were ever alike.

Conceptually, these engines are ancient. The beauty is that we can get them to run at all - even up to 21st-century standards once well warmed up. The fact that guys your age can do it is just plain amazing. But I just can't get myself to care how long I have to run with the choke pulled out a bit - as long as I do reach the point where it's no longer needed.

FWIW,
Allen
Allen Bachelder


Hi Allen,

Good to hear from you! As you probably know, this car has been sitting for over 3 years, so the fact that it's running is a big step for me. I haven't had much of a chance to drive it, but it really seems fine once it's warm.

Now I just need to order a new cooling fan motor (one is bad) and a set of the K&N air cleaners that will clear the brake booster.

Cheers,
Dave
Dave Plantz

Modern fuel injected computer managed cars have spoilt us here. We forget how much is going on when we turn the key and drive off easily with a cold engine. The B series is a heavy engine with a lot of water in it, it takes about 2 miles for mine to warm up. While it is cold it will run fine on a light throttle, but as soon as you ask it to do any work, increase speed or climb a gradient, it will stumble if you do not give it some choke, enough choke to get past the fast idle cam and onto moving the jets down. This is to be expected and even desired as if it is set for correct mixture when warm it must run lean when cold. After the first 10 years you get quite used to this and it becomes one more little diversion offerd by ownership of one of these cars. The Rover SD1 actuaslly elevated the choke fot it's SUs to minor control status as a little plastic lever in the centre console
Stan Best

This thread was discussed between 02/09/2008 and 03/09/2008

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