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MG MGB Technical - Rough/rich idle/weber carb on my 78 B

Help me I'm at the end of my tether.
Car; 78 B with Weber 38/38 Eurospec dizzy/new valve job/38K on th engine/new everything to do with ignition and fuel. Car runs very strong but hates to idle...lumpy/smells rich.

Carb settings; idle screws (both) are 2 1/2 turns out from seated...throttle butterflies are closed. Car gets about 23mpg which I think could be better.
I've tried everything...another new cap/rotor/plugs/different leads/ changed coils...still no change...engine compression is good and even...had recent valve job and gasket. Does 80mph easily (at 4750rpm!!!!)

Sooooo.....ideas chaps?....could it be ignition switch or some other gremlin affecting slow running. I do notice the idle gets even worse under electrical load or when the clutch is depressed. Alternator output is 13.5-14.0 volts...battery is new.

What am I missing????????

Frustrated as hell!
P J KELLY

PJ. You may wish to give Bob Ford a call at Brit-Tek. I do not have any experience with the 38/38. I do have some experience with the 32/36. With the 32/36, the basic idle mixture is two turns out, then lean to proper mixture. On both of my cars, the final mixture setting ends up about 1 1/2 turns out. Thus, I suspect your idle mixture is too rich and needs to be leaned out. You might also want to check for air leaks around the carb base. This can be another problem area. Spray some carb cleaner around the base of the carb and see if the idle speed picks up. Les
Les Bengtson



Try this link.


http://www.redlineweber.com/html/Tech/38_dgas_tunning.htm
William Bourne

Thanks guys....but been there and done both...no carb base leaks. I have a vac guage on the car and it pulls real good vacuum once running...occasionally it idles OK for a moment...pulls good vacuum then too. Bob Ford is stumped and short of driving 300 miles to NH....you see why I'm getting nutty!!!!
P J KELLY

Sounds like you need leaner jets.
It has been fairly common to have to up the idle speed with these carbs.
I have one myself on a mildly massaged engine. I'll try to dig up my settings. Of course I'm at 4800 feet elevation so mine will be pretty lean.

Mike!
mike!

Yer know..you might be onto something Mike...I find the car runs much better set up with this carb progressively....with both barrels working simultaneously it runs awful rich no matter what I do...I'll try going leaner...it has 45s in it now...do you know what youn have Mike?
P J KELLY

I just went through a similar thing with my 76B w/newer 45D4 dizzy, Weber 38/38, Peco header and exhaust and worked with my local gurus on it after I had similar problems. When I had it set up I was at 18BTDC and 2 turns out. It seemed to do really well at 13BTDC and 1 1/4 turns out. Made a HUGE difference.
Saul

Pete,

The Webers are infinitly adjustable and will work well on any car, but it will take a lot of trial and error to get the carb set for your specific application. I could tell you what jet combination works best on my car and it would be meaningless for yours.

The Dennis Hale article I sent you earlier is a good method to use, but I personnally disagree on the point of jetting the idle circuit last. I think you should get the idle jets right first then move on to the primary. Two and a half turns on the mixture screw does sound too rich to me. The car should idle well on the idle circuit alone, with no help from the speed screws. With the carb at its base settings adjust the speed screws, by ear, to the lowest possible position to keep the car running. You should be able to hear the point where the throttle butterfly opens the first progression holes. You don't want the speed screws opened beyond that point. Then adjust the idle with appropriate jets and the mixture screw. You can check the plugs to verify a lean condition, white is good.

Once you get the idle circuit lean take the car for a test drive. Note when the richness becomes apparent. If it appears at first acceleration, the pump jet may be too rich. Just follow the Hale method of going lean on each jet in the progression until too lean then back-up one jet size.

If you decide to go this route, you will be opening the jet cover quite often. The cover gasket wears out quickly and is available only in a rebuild kit, which is expensive. Before you start, I would use your original gasket as a pattern and make several replacement gaskets out of stock material.

Doug
D. Cook

I had a similar issue, and it took two years of failed smog tests to figure out that my stock cam replacement was not infact stock at all. Replacement with a new cam smoothed the idle and cleaned up the emissions.
Tony Calleja

If a carb is runnung rich, it may the float level. ?Even if it is correct, you can lower it some and lean the mixture slightly.
Good luck.
steve

P.J. Work in 5 size increments when rejetting your carb. You'll find the sweetspot then be able to hone in on the best preformance. Don't forget to take notes and write them in a tune-up for easy referance. Do all your idles, mains, $ air correctors (which should be the same in both barrels). P.S. That carb. is designed to run progressively.
victor gardino

Hi PJ -
Here's a long story for you..

A few years ago I experimented with a borowed 38/38 on a freshly rebuilt 80 motor in my 74B - I had the exact same symptoms you have - lumpy idle, rpms dropping under electical load(lights & heater blower), also, rpms really dropping out after engine braking.., plus a really great & revvy top end!

I tried everything you've tried -
upgraded Eurospec ignition & Coil
bosch alternator conversion
various plug types
various fuel pumps & pressure regulators
adjusting carb linkage for progressive operation
various valve lash settings
went over everthing with carb cleaner, propane, even ether looking for manifold or carb base leaks
more

Now, eventually the owner of the 38/38 had a use for it, so I ordered a new 32/36 from Brit-Tek & installed it (on the same Cannon manifold) & guess what...IDENTICAL SYMPTOMS!

Next, I pulled the motor and rebuilt it again, changing from deep dish to shallow dish pistons (calc 10:1 compression), and had the Piper 270 reground by Elgin to mild road specs..I reworked everything except the late head, (which I had DIY flowed and rebuilt during first moter rebuild).

Now, the car was even revvier and stronger, BUT, SAME IDLE PROBLEMS!

So now I begin to suspect the late head (It's the only thing I havent changed other than the left rear taillight lense ;-} ). I searched out and finally found a good "L" head -(large intake valves, same chamber volume as the late head). I modified the head just a bit, cutting down the exhaust guide bosses, then had the head rebuilt with hardened exhaust seats & new valves.

When I installed the head (be sure to use the correct rear rocker post for your head), I was fairly liberal with high temp silicone on both sides of the intake gasket around the intake ports, & also coated the carb base gasket with Hi-Tack aircraft sealant.

THE CAR NOW PULLS STRONGLY & IDLES LIKE A DREAM!

Now, here's the problem & the moral - It's quite possible that the head was not really the problem - all along it may have been intake leaks at the carb base or manifold gasket, & extra attention to sealing these gaskets may be what actually solved the problem.

Do you get a fairly audible hissing sound thru the carb? (suggests manifold leak) Mine is alot quieter after this work.

My suggestion, no matter what your tests with carb cleaner show, before you go through any more major expense, pull the carb & manifold and reseal all gasket surfaces, & temporarily cap all vaccuum taps on the manifold for testing.

Also, set your idle with a dwell/tach meter - my dash tach reads about 200 rpm higher than actual engine speed in the low range...

....Pete

Pete Dickerson

Thanks for your comments...I had a 32/36 on this motor first also....SAME problems....so I upgraded to the 38/38...same problems...I sealed and resealed all the gaskets...marginal iomprovement but still not right...I've ordered a jet kit for it and will be playing with that next...will keep you posted.
P J KELLY

PJ. I would be greatful if you could post your follow up experiences. My two cars with the 32/36 run well (79) and great (daughter's 77). But, within the last two years, I have had to replace both the carbs due to performance problems (which may be related to our blended fuel--both carbs were full of white/red crud on the bottoms of the float bowls). The crud required manual removal--it did not want to come out with carb cleaner. I will probably have to replace the jets to get the carbs to function properly again. So, any insight you can provide into how to set up and maintain the Weber would be appreciated. Your long term experience with the 38/38 would also be appreciated. The Weber, being only a single carb, is an allowable change according to our Arizona emissions people. If the 38/38 would perform as well as the HS-4, it would be a great carb for our use. Les
Les Bengtson

PJ -

Also, take Les' advice and install the Weber "backwards" (with the float bowl at the rear) - sure makes adjustment easier....Pete
Pete Dickerson

Les, Try running a bottle of HEET thru your cars every ten to twevle fill-ups. We've got blended fuels here in the Denver area and lots of stations are using Ethanol in their pumps. A bottle of HEET just seems to keep the system cleaner a little longer, although the enviroment probably hates like hades. Vic
victor gardino

This thread was discussed between 29/06/2004 and 06/07/2004

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