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MG MGB Technical - Slight miss at idle? - OK otherwise

Hello all,
After loads of work, a few threads here and much reading of the archives, I've got my '77 B with SU conversion running well, with one exception.

I have a slight regular miss at idle. The engine moves around a small amount and the exhaust note is not regular. Idle speed is even.

I've gone back through my HIF's and performed a complete rebuild. They seem tuned correctly. I have a new 45D distributor with new cap, wires, points, condensor. Vacuum line for distributor coming off manifold.

I've replaced my spark plugs (gaped .025), have correct dwell (50), have timing set to 15 BTDC.

Manifold vacuum reading is 15 with fairly steady needle.

I've tried closing off vacuum line to brake resivoir (no difference).

The car runs great with good acceleration.

I've completed this type conversion on other rubber bumper B's with never an issue.

Any ideas?

Thanks in adavnce!
David

David Steverson

PS Added (missing) charcoal canister and have carbs vented correctly to this, vents not plugged anywhere.
David Steverson

David,

Have you checked compression on this engine?

Allen
Allen Bachelder

Allen
I meant to mention that, checked it with all plugs removed, 120 psi across all 4 cylinders.

Also, carb. rebuild included new throttle shafts and solid throttle plates from Joe Curto, also new floats, seals, etc.

David
David Steverson

I wonder if I have a vacuum leak? I think the reading should be around 20. Could that cause the miss?
David Steverson

I have this sort of lumpy idle myself but I was always told it was normal so I dont worry about it.
Ross Kelly

David,

I'm really glad it wasn't a compression problem ($$$). Alternatively, a vacuum leak is quite plausible. With the engine idling, spray some carb cleaner around all the intake plumbing. If you find a spot where it affects the idle (either way), you may have found your problem. If this works, thanks go to John Twist; if it doesn't, blame goes to me.

Cheers,
Allen
Allen Bachelder

If you had a vacuum leak you would probably have had to move one of the carbs further from the starting position to get the correct mixture. What process did you use to setup the carbs? "seem tuned correctly" is no good, you have to set them up yourself from scratch for both airflow and mixture balance.

If it's a *regular* miss but an otherwise even idle speed as you say it does sound like one cylinder, and I would be more inclined to think of ignition more than fuel (90% of SU problems being caused by Lucas electrics). Have you disconnected each plug lead in turn and noted how much each changes the idle speed and exhaust note? When you hit on the misfiring cylinder the idle speed will drop less than the others and there will only be a single beat in the exhaust. When you disconnect a good cylinder you will get a double beat in the exhaust instead of a single and a bigger drop in idle.

But if its enough to shake the engine are you sure you have the plug leads in the correct order?
Paul Hunt

I called Joe Curto yesterday, he gave me several pointers. The one that made a difference was to disconnect the vacuum advance on this rebuilt 45D distributor.

I have (had) it connected to manifold vacuum. I've read archives about whether one should use ported or manifold vacuum. Now, I wonder if I need to get a different vacuum advance to work with manifold vacuum. Is that possible?

Paul, I have gone from scratch in tuning these SU's, actually about 4 times now. Always the same end result. I did try removing plug wires one-at-a-time. I found # 3 and 4 had less effect on the engine when removed. *However* the engine was running smoothly enough with the old Weber/electronic ignition setup, so I am hesitant to say I have any bad cylinders.

I have now tested for vacuum leaks and found zero.

Since disconnecting the vacuum advance, I do have a better idle, but still not very smooth (i.e. the engine moves around at idle).

Thank you all for the help thus far!
David
David Steverson

OK, if 3 and 4 had less effect than 1 and 2 then it does imply problems with the rear carb, but see below. Is there any difference in the colour of plugs 3 and 4 compared to 1 and 2?

If you had carb vacuum then I'd say there was a chance you had a peforated vacuum capsule diaphragm, but that should be compensated for by setting-up, albeit having to richen the rear carb more than the front, which would make that carb richer when running but OK when idling.

The difference between manifold and carb vacuum is nothing more than manifold gives full advance at idle, carb vacuum gives none. You don't need to change anything if moving between the two (although almost everyone else says you do) as UK engines kept the same engine and distributor, including vacuum capsule, when changing from carb to manifold in September 76.

With yours being manifold it's possible you have the wrong vacuum capsule or it is faulty and giving too much advance at idle, but that is more likely to give an *irregular* misfire than a regular, and all cylinders should be affected equally.

With a new distributor etc. it's possible that there is a defect on 3 and 4. Swap leads and plugs 1 and 2 with 3 and 4 (or at least one of them if they won't both reach) and see if that moves the weak cylinders. Also I'd be tempted to rotate the distributor drive gear through 180 degrees, move the plug leads round two positions, and see if the fault stays with three and four or moves to 1 and 2. Alternatively you could try a different distributor, rotor, cap, points, condenser, leads and plugs.
Paul Hunt

David have you checked for a slightly tight tappet.
Denis
DENIS4

Or way loose, for that matter, and a good point. Valve clearances must be correct before starting on ignition or carbs. As well as checking the clearance at the strict 'rule of nine point' also check to see if the clearance is bigger either side of that point on any valves.
Paul Hunt

Oh crap, I did not think of that. Still, it was running smoothly enough with the weber and electronic ignition.

I feel that either the SU conversion or the points distributor change is the culprit.

I talked to Bob at Brittek and he says he has a 45D that works with manifold vacuum. So i ordered that. An expensive way to diagnose a problem, but if that fixes it, I'll return the other distributor. If not, I'll flog one on EBay and will then know (amybe) there's something about the SU's.

On a side note, I drove the B to work yesterday. On the way home, while sitting at a light, a fellow rolled down his window and told me he used to work for Barnum and Bailey and used an MG as a clown car....

David
David Steverson

Did you ask him if it drove funny??
J Heisenfeldt

Nice one J, I wish I had thought of that!

It turns out I had a float with a weak tab. I would set the level and it moved on it's own when fuel came in. I have 2 new floats and a smootly running B.

Thanks to all for your help and suggestions!

David
David Steverson

"he has a 45D that works with manifold vacuum"

They *all* work with manifold or carb vacuum, same as 25Ds. There is no difference whatever in the distributors, the only difference is when vacuum is applied by the source and how much. There *were* different distributors for North American carb vacuum engines and manifold vacuum engines, but that was because they were different engines and Abingdon were continually chasing emissions limits. The UK had the same engines and distributors throughout rubber bumper production, even though the vacuum source changed from carb to manifold in 77, again for emissions reasons.
Paul Hunt

This thread was discussed between 20/07/2008 and 28/07/2008

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