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MG MGB Technical - Chaps with Weber carbs...what am I doing wrong?

Thread on Weber 38/38 is below....I just don't get it...or I've been in Fuel Injection world too long with my Saab fixations over the years. No matter what I do it will not idle happily and stumbles to pick up. Once going it runs great. But come to a stop and move off from rest and it's all splutters and hesitations....the car is bloody mint otherwise,(and unrestored). PITA
P J KELLY

PJ,
Have you any books on Weber Tuning? I would suggest at least the one by Pat Braden and there are other, newer ones out there, if you haven't gotten into this too deeply. Some of Weber tuning is finding the right jets and air bleeds to deal with mixture control under the varying conditions of load and one engine's dynamics vs. another's.

I went thru this process with a 32/36 DGV kit on my GT years ago and after much reading, playing with jets and air bleeds, and the advice of a couple of successful weber users, I eventually returned to the OE HS-4 SUs. However, I think your set up could work much better than the 32/36 DGV and possibly at least as well as the SUs IF you can get it sorted out for your engine. I would not trust that whoever sold you this carb has zeroed in on the best jetting, etc for your car - I don't think that is an assumption we can make, regardless of how much their advertising says it works right out of the box. My opinion is that the most they can claim is that it gets you into the ballpark, but not necessarily up to homeplate.
Bob Muenchausen

You might want to Brit-tek a call. I had to jet down my mains because I was running rich. Maybe the idle jets are too rich.

Shareef Hassan

Pete,

Have you tried the procedure to get "lean best idle" that the Weber dealers describe? A good source is at Redline Webers site: http://redlineweber.com/html/Tech/Table_of_contents.htm

At home I have an article that was printed in one of the racing magazines that describes Weber set-up for Nissan engines (the L-18 is similar to the MG engine). The article is in MS Word format and I could e-mail it to you tonight. It provides a step-by-step method for finding the best jet configuration for your desired performance level.

The disclaimer is that I have been following the method for over a year now and still haven't gotten my eninge to run right. If you recall, you responded to one of my threads a couple of months ago about compression and blow-back. (The blow back was the result of a plugged exhaust as I discovered later) It is my opinion that breathing is a major factor in the poor Weber performance on our cars, and the fellow who responded to your earlier post about the manifold allowing the mixture to separate was on the right track.

Doug
D. Cook

I'm running a 32/36 on an 18V which has 98,000 miles on it. Recently I had to adjust the following: (1)float level (had to lower it), (2)spark plug gap (had to OPEN it), (3)new spark plug wires (resistance too great). The car ran well at open throttle but was hesitant at part throttle and at idle. As a first step, remove the air cleaner to determine its effect. If no change, evaluate 1 through 3.

Let us know.
Chuck Edwards
Charles Edwards

Sorry to be monopolizing and I'm sure you know, but the fuel pump used on the late MGB will over-power the Weber. You must use a pressure regulator.
Charles Edwards

No Charles I didn't know about the pressure regulator.....how can the pump overpressure the needle valve??????????????????????/
P J KELLY

Charles and Pete,

I have a low pressure regulator and Holly gage installed in my fuel line. The gage indicates 3 lbs and 3.5 pounds on the pump stroke. That's within Weber recommendations and I'm still getting too much fuel.

Doug
D. Cook

Have just splurged on an inline pressure reg....$20....was told to dial it down to 1lb only and see if it made any difference. Cant hurt...will update tomorrow
P J KELLY

PJ. You have been given some bad information. The stock SU fuel pump, including the one used on "late MGB"s put out a max of 2.6 psi fuel pressure. The specification for the Weber downdraft carb is 3.0-3.5 psi. Hence, if you are running the stock fuel pump, you do not have a problem. David DuBois, after contacting Burlen to get the information on the currently manufactured fuel pumps, has a tech article posted on my website, www.custompistols.com/ in the MG section. You might want to read that. As Bob M suggested, the Pat Braddon book, "Weber Carburettors" is an excellent source of information and includes a section on troubleshooting the downdraft carbs. The Weber 38/38 has been used, with good success, on the smaller BMW sedans. So, we know the carb itself is capable of being set up to perform correctly. The question is, has anyone ever set one up specifically for the MGB engine using the commonly available intake manifolds? Or, have the currently available 38/38 carbs simply been set up to some "general" specification which might not be optimal for the MGB?

I suspect the first place to start in your researches would be with whomever sold the carb to see what testing was done on this particular set up to ensure that there would be no problems when bolting it on. Les
Les Bengtson

Sorry your problem is still on going. After diagnosing and fixing thousands of vehicles and if I didn't have any of my fancy modern equipement, this is what I would do. Not in any particular order.


1. Never make any assumptions, always test.
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2. Compression tests. cranking, running, wet, dry. A large fault here would generally affect entire rpm scale.
3. Vacuum readings. Min, max - especially at idle where the problem lies. Look for fluctuating needle, too low or too high reading. fluctuating could be caused by valvetrain problems. Too low/high by plugged exaust, incorrect vavle timing etc.
4. Valve timing, lift.
5. Check exhaust back pressure, should be under 2 psi- unlikely to be the problem is not fitted with catalytic converter

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6. Check primary ignition system. Check points and condenser with DVOM. Make sure mechanical and vacuum advance working. Check for excess wobble in distributor shaft. Check for excess slop in distributor rotation. Too much free play could indicate problem with dist. drive gear, cam, timing chain.
If you have electronic ignition you MUST use oscilloscope for sure proof diagnosis. You can use voltmeters, tests lights etc, but they cannot really tell you what kind of signal is being produced by electronic igniter.
7. Check secondary ignition system. Firing order, cracked wire insulation, broken rotor,cap etc. . Check plug gaps and condition, resistance of cap, rotor, wires , coil. A scope would greatly help here as you cannot see slightly fouled plug, nor can you see number of coil oscillations (coil integrity) by using voltmeter, test lights.

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8. Check fuel pressure and volume
9. Check fuel delivery at idle, cruise, full throttle. This is where I would greatly concentrate since you say that the engine misfires at idle. You need some way to measure the air fuel ratio. Again, an exhaust analyzer is the best way to test this. You can use the Lean Best Idle to get good ratio. You need to shoot for around a stoichiometric ratio of 14.7:1 at idle and cruise for a piston 4 cycle engine on pump gasoline. While about 12:1 will give most power, I find that richer that 12:1 and leaner that 17:1 will start to cause misfires. I would concentrate on idle circuit and check float level.
10. Vacuum leaks. Carefull attention to internal leaks such as EGR valves, PCV, Brake boosters if so equipped.
11. Power and grounds.


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While the above is not any kind of definitive list, it should help zero in on the problem.
Since you have not made any kind of comment about seeing black smoke upon acceleration I would suspect that you are too lean at idle. Symptoms include hunting idle, misfires, backfire through intake, extremely clean and white spark plugs, stumble upon accelaration, higher than normal engine temperature, low power output. Running an extremely lean mixture for prolonged times will cause valve damage. Some of my worst headaches come not from faulty components, but mis-mached/ wrongly enineered parts. Have you tried swapping carb? Sorry to ramble on or repeat what you may already know, good luck.
Joaquin

Joaquin...thanks so much for your..."article"...much appreciated. I do have a Vac. Guage on the car and it does jump around inconsistently. It is stable and shows good vacuum at highway speed and there is good power above 2500 rpm. So I don't think I have valve or valve timing related issues. Everything is new here anyway and I have reset and checked the valve timing 3 times now...I was right the first time. Ig system is all new and I have swapped out many components with no change. I have to suspect the carb isn't set correctly or jetted right. The tail pipe has carbon but the is no black smoke on acceleration. I do have an exhaust gas analyzer, and can get a fast idle to be pretty clean...under 2%CO, but at idle it stumbles and the CO goes up to 6% or higher. I am now concentrating on fuel pressure, float level and lastly rejetting this bloody carb in an effort to get it right.
Pete
P J KELLY

PJ: try staggering the butterflies so you only idle on one choke. The 38 has an external sync lock screw that makes staggering easy. Loosen the screw and the secondary should snap shut allowing you to make mixture adjustments with only the primary open. You may have to open up the mixture screw a half turn or so to smooth out the idle. Then you can lock down the screw so you only idle on the primary but the secondary opens when you apply throttle. Try this before changing idle jets. Good luck.
Terry

Terry,
does this mean that the "sync" obtained is always slightly off between barrels or are they supposed to be truly in sync in the first place? My not too good understanding was that both barrels were always "on", not progressive - therefore no real primary or secondary throats - both would be "primaries"???? Or is there an accomodation in the linkage for idle adjustment as you describe and then once the throttle pedal is moved, it pulls both throttles up to an "in sync" condition? Just wondering.
Bob Muenchausen

Pete and Terry,

The throttle "butterflies" should both be closed completely in order to establish a good idle with the Webers. If either throttle plate is not seated it will allow the hole for the main jet to be exposed and excess fuel will be pulled into the air stream. That's why the set-up instructions say to make sure there are no obstructions, such as gasket material, blocking the opening to the manifold, and why the throttle shaft nut is only tightened slightly to prevent binding.

Doug
D. Cook

Hi Doug - did you mean progression holes for the idle jets rather than the mains? And that these holes should be covered, rather than the the butterflies be completely closed? Or do these 38/38's progressive carbs work that much differently than the DCOEs?
Mike Polan

The Weber 38 is indeed a synchronus not a progressive carb. My previous post was confusing- substitute choke "A" and choke "B" for primary and secondary. (or barrel "A" and barrel"B"-why are there so many different terms for the same thing???)

And I did mean staggering the butterflies (or throttle valves) so that the progression holes are not exposed simultaneously until just before the mains take over, just like the DCOE. (the idle circuit controls not only idle but transition off idle to when the mains are required) This MAY preclude the necessity of changing idle jets and is quick and easy to try. It has reportedly worked for some similar problems.

Thanks to Bob and D.Cook and Mike for pointing this out.
Terry

Thanks, Terry. I have seen many different sorts of linkage setups on various carbs over the years, and it could have been entirely possible for there to be a staggered response in the initial movement of the throttles, but a change to "in sync" movement slightly afterwards.

A progressive carb runs on only one barrel until the second is needed, so why not a staggering of the barrels at idle of a non-progressive carb?? I am just not that familiar with this carb and wondered if I was reading things right. I agree with you, no sense running through unneeded jets and air bleeds if not needed - it is PITA exercise anyway, if you don't have a dyno.
Bob Muenchausen

This thread was discussed between 06/01/2004 and 11/01/2004

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