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MG MGB Technical - Winter Tuning Delays Anger!

Hello All,

Winter is here and I have kept my MGB parked long enough that I am no longer quite so frustrated with it. After my engine swap last spring, the damned B ran terrible (worse than the worn out motor) during the summer and fall. The temperatures here in Nevada get over 100F and the B does not like that very much. It starts up just fine when it is freezing cold, yet during a summers day it would be hard pressed to drag race a Model T Ford.

My question is, what causes extreme stumbling when you press down the accelerator? Is it timing or can it also be incorrect carb adjustments. I am running a Weber 38 DGES which is supposed to be correctly jetted for a high perf. MGB, yet I believe it needs a jet kit.
The engine also stalls so much that I must always keep my foot on the accelerator a little to keep it running. The engine idle is around 1,000 rpm.

Any Magic Bullet Cures would be appreciated. The new Volkswagens with 1.8 litre Turbos are looking mighty fine right now also.

Jon Wotherspoon

Jon,
The Magic Bullet might just be a drive in a new Turbo bug. I drove my sister-in-law's last week and was disappointed. It'll probably help get your B back running in a fashion that's acceptable. I'm sorry I couldn't provide the bullet that'll help you with your B though!
SteveO

Check for vacuum leaks.
Andrew in Austin

If it runs great in cold weather, but poorly in hot weather, I would think it is jetted overly rich. The stumbling on acceleration sounds like a mis-adjusted accelerator pump. Is this one of those two-barrel, down-draft Webers that opens both barrels at once when you press on the accelerator? What other mods have you done to your B?

Jerry
Jerry Causey

Jon,
Could also be a mix of carb, electrics and cam. How good are points/coil/alternator output etc? How non-stock is the cam from the rebuild? Fact that it has trouble idling could be lumpiness (a term here for 'hot') in the cam profile.
Regards
Roger
Roger

Thanks for your suggestions everyone,

The car has:

pertronix Ignition and Coil
Crane Cams Sport Cam (not supposed to be too aggressive)
Weber 38 DGES (Both Barrels open simultaneously)

The distributor is still the original Lucas, the alternator charges fine etc.

How does one adjust the accelerator pump? I will order a set of leaner jets and try them out.

Any other suggestions would be appreciated

- Jon
Jon Wotherspoon

Before you order jets take a look at your spark plugs. If they are black, try opening up the air corrector jets by one drill size and see if it helps or hurts. Should help a bit if you are running rich.
Hmmm Is your idle screw all the way in or out? Could also be a sign of which way to go. If it is similar to the DGV they say a large part of the low rpm operation is from the idle circuit.
I'm running a dgev on a 2.5 (nonMG) and found that opening the air corrector jets and the primary and secondary jets made a noticable improvement in drivability. Economy is down a bit but power is noticably improved from idle to interstate.
.

The plugs are very black. I purchased a second set in early fall and tried them and they were black within a short time. My new exhaust got black pretty quick as well.

It could be the idle jets, except that during heavy climbing on the highway, I must push the accelerator to the floor to do 55mph. I am at 5 or 6 thousand feet here, so that must have something to do with it.

Can stumbliing be caused soley by ignition or carb problems, or do they have to both be out of alignment? It happens so often that it is not safe to drive on the public roads.
Jon Wotherspoon

Opening up the air correcter jets should be an improvement and a lower restriction air filter may help lean things out a bit. The K&N style that came with mine was fine but I ended up switching to a long-flo for clearance issues.

Get a pin vise from sears - $9 or so for the small one which includes some smaller bits.I would think one of these would do it as my 2.5 needed sizes just bigger than what came with the vise. Use the pin vise to open the jet to the next bigger drill bit size. My bits were in .003 increments, one was a bit loose and the next was no go, so I opened it up to that bigger size. Much easier to open air jets that to put in smaller fuel jets - and the air jets are not submerged if fuel. While in there write down your current jet sizes so you can keep the car in action while waiting for new jets if you need them.
.

Had the same problem - first, I adjusted the Weber idle screw. Cut down on fuel being dumped in and it ran a lot better. Then, replaced distributor cap and rotor. Now she runs great.
Terry

Agreed, get your ignition in shape before messing with the carbs. Then set up the idle first.
.

http://www.redlineweber.com/HTML/Tech_Docs/DGAS_Outlaw/38_DGAS-Outlaw_Idle_adjustment.htm
.

Jon,
Make sure your timing is spot on. It might pay to have your dizzy recurved as well, or get one that is already curved. Lastly if you have your head ported it will really let that carb breath. Are you still running the hacked up old exhaust manifold? If so try and find a header, even a cheap used one (thats what I did). I would get the distributor in order and tuned up first and then go from there.
Bill
Bill Mertz

Thanks Bill,

The head was converted to lead free fuel requirments and was lightly polished.

I will get a new exhaust manifold for it, although I have a feeling that isn't the problem.

I guess a UK spec 45D might work good at this stage.
Jon Wotherspoon

Jon,
Tell us a little about your engine swap. You say it runs worse than the worn out engine, are you exaggerating or is this true? What parts were swapped from old engine to the new one (carbs, distributor, manifolds, etc)? Was the camshaft "dialed" in? Valve settings? Have you put a vacuum gauge on it? Anything else you can think of?

Wayne
Wayne Pearson

This thread was discussed between 22/01/2002 and 24/01/2002

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