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MG MGB Technical - Bad idle on 74 b

Hi folks,
Just done a change of cam followers and push rods, which has reduced top end clatter no end. However a problem remains which I should have tackled before i put carbs back on. Idle speed when warmed up rises and falls 500 up to 1200 rpm. Turn screws down and it will stall. Restart turn idle screws to about 750 rpm then its away on its own again. Eventually it will drop to 500 again but its annoyingly all over the place. I did put a new 45D dizzy in recently but im not suspecting that or should I? No stop screws are touching when they should not be. Choke cable and stop screws are clear.

I checked and plugged vac lines to servo and dizzy, all gaskets seem good.

Help with what next would be great to stop this wayward idling.

Cheers

Jeff.
Jeff

Jeff,

Was it okay before the dizzy change?
However, It does sound like an air leak and therefore I would start by spraying wd40 or carb cleaner around to see if that induces an immediate, semi controllable change.

You don't say what type of carbs but I'm guessing HIF's on a 74?

I would then look at a number of possibilities:-
1, throttle spindle wear
2,O-ring seals on the choke/cold running system (targeted spary with Carb cleaner will help with these two)
3, float adjustment
4, bi-metallic jet locating thingy in the float chambers which causes the mixture to change relative to fuel temperature.

Best of...
MGmike


M McAndrew

Does it similarly vary when the throttle is opened just enough to increase the idle speed slightly? Or does that vary as well?

Did you set up the dwell and timing? Carb air-flow and mixture balance?

Any change in exhaust note when this happens other than from the rev change?
Paul Hunt

Paul/mgmike,
The carbs are HIF's. The idle never was very stable but it's taken a turn for the worse. I did a static timing set up as per Haynes manual for 18v engine, 10 degrees btdc. The old dizzy was incorrect for that engine, ie 25d.

I did attempt to redo the air/fuel but kept running into further trouble. Exhaust note was splashy so I carefully enriched mixture bit by bit on both carbs. The exhaust note thickened but got smoky on the rev up. I still couldn't get the idle to settle, it randomly drops from 12/1500 to 500 almost stalling. Give the throttle a blip and its back up to 12/1500 . I know the links and cables are not sticking and both throttles snap closed on the spindle springs. It does seem like an air leak but Ive done checks with vac take offs etc. It will stall tho' if I drop the idle right down so that leads me to think excess air is not getting in. I even added lots off aluminium foil to the heat shield to divert some heat away from carbs. I figured if spindles were badly worn, the idle would remain up and not suddenly fall back at random.

I need to strip carbs and check innards. By the way, if I remove the oil filler cap there isn't a change in speed or note. I did strip the head a couple of years ago and valve guides etc seemed quite good and the car has only done about 300 miles since.

I could try the old dizzy to see if it makes a difference but I have my doubts. Carbs were on the weak side stalling with slight lifting of pistons so as I say, I tweaked them richer so the revs stayed even when lifting pistons.

I do wonder if butterfly run on valves are playing up or if some sort of auxiliary air/ fuel is leaking from within one or both carbs. (if thats possible).

So to sum up, engine starts ok cold, warms up then runs fine off choke. From revving, idle goes high then falls right off.

Hope I didn't bore you too much but thats as well as I can describle circumstances.

Cheers

Jeff
JJ Egan

Hi Jeff

I had a similar problem on my B and then proceeded to rebuild both the HIFs with the Burlen rebuild kit together with some new butterfly shafts. This improved the running generally but still had problems with fluctuating tickover when hot.
Eventually traced the fault to a tiny hole in the vacuum diaphragm on the distributer!
Bought a cheap new dizzy ( one of the Chinese ones ready assembled for fitting !), fitted this and hey presto its all perfect. I can get a rock steady tickover at about 700 rpm consistently.
CO measurements at the last MOT were 3.6%.

Cheers

Adrian
AdrianS

Use a timing light on the crank pulley and observe if the timing is changing in sympathy with the 'HUNTING'. If it does disconnect the advance pipe, plug it and repeat. If the timing is steady at tick over, that should rule out fluctuating timing, but can't really believe such a big speed difference would be caused by the vacuum unit. If it is Dizzy related it's more likely to be a problem with the bob weights and springs, especially if they are not attached!!! Or a badly worn skew gear. You can try eliminating the poppet valves as a cause by putting on a blob of super glue. I would get rid of them anyway, either solder them up or, better still, replace the butterflies with plain ones. They're not anti-run-on, they are there to scavenge fuel droplets from the inlet manifold when the throttle closes.
Allan Reeling

Thanks Adrian,
I have been looking at a rebuild kit tonight on Burlen site. Carbs have been badly mauled in the past so a kit is due anyway.
I noticed tonight that the carb ID is AUD 616 so it has no valves set in the butterfies. This car is a pain as its a half-car from late 74 and a bit of a mongrel for bits.

The new dizzy I put in is like yours, a Chinese plug and play. I have disconnected the vac and plugged the line but it made no difference. Rubber elbows are a bit tatty tho'. By the way I bet your B is a yellow roadster haha.

Sorry Alan, when I spoke of anti run on poppet valves, I meant the over-run valves to prevent backfiring. My Carbs dont have them so thats one less problem. Next step is to put old 25D dizzy back and see what happens.

Cheers boys,

Jeff
JJ Egan

Having an alternative distributor is a useful diagnostic. These engines will run with almost any distributor, you'll get variations in performance and the amount of timing you can run without pinking, but you shouldn't get any other problems such as not maintaining a steady idle. In fact the 25D4 has come back into its own, with being able to tweak the timing with the vernier adjustment for best advance instead of having to get the spanners and timing light out.

You can test the diaphragm by sucking on the tube that goes to it. If you can draw air through it, it is punctured. It takes a bit of effort, but you should be able to move the points plate a bit, then if you seal the end of the tube with the tip of your tongue it should hold that position until you unseal it.

The over-run valves in the butterflies reduce emissions by effectively slowly closing the throttle rather than snapping it shut, other BL cars had a damper on the throttle spindle. North American cars had a gulp valve to prevent back-firing on the overrun, but that was because they had air injection, exhaust gas recirculation and all the other emissions stuff that UK cars didn't have.
Paul Hunt

Cheers for that Paul. I had another play with it this evening disconnected throttle spindle, sprayed WD 40 around but no perceptable difference. I did try as you suggested with vac adavance tube and unit does suck and hold. No difference in idle woes with it disconnected. Give it a rev and it holds at about 1200 rpm for some time, then it falls off to where the idle screws are set, almost a stall. Dizzy swap has to be next.

Cheers

Jeff
JJ Egan

When it is idling high with the throttle pedal released, have you tried pressing the butterfly cams onto their idle screws?
Paul Hunt

TRY BLANKING OFF THE CRANKCASE BREATHER PIPES AND SEE IF THAT IMPROVES THINGS.
Allan Reeling


Problem solved yeeehaa .

I put the old 25D dizzy back in and the idle problem had gone. Tuned carbs and now I have a rock steady idle.

Problem found in Chinese 45D was base plate screws loose. When I looked closely under a spot light, and rotated the shaft I could just see the base plate move allowing points gap to close up as the tip of the cam was reached. Of course I wouldn't have seen that when resetting points in situe. I think I'll leave the 25D in place for now.

Who'd have thought that problem would have caused such a random and widely fluctuating idle. As John Twist of university motors says....."most carburettor problems are distributor problems"

Thanks for all the input guys, you have been most helpful.

Jeff
JJ Egan

"TRY BLANKING OFF THE CRANKCASE BREATHER PIPES AND SEE IF THAT IMPROVES THINGS."

Trouble-maker :o)
Paul Hunt

Well I thought i'd cracked this misfire but it came back. Just shows how new black rotors and capacitors can be rubbish. It runs sweeter than ever now by using an ancient capacitor and rotor arm which I found in my tat. Stuff I kept from a Morris 1800 I had in the 70's!

Just shows, never chuck old good stuff away, sure to need it someday. The new federal motors 45d lucas copy will be thrown as far as I can make it go. Not a chance of getting a refund now, although it was sold as a Lucas replacement on ebay. Absolute rubbish: weak springs, bob weights that wobble all over the place and nasty capacitors and rotors. The lesson is dont buy one.
JJ Egan

Jeff,
poor quality new parts are not uncommon - especially rotor arms and condensers, you should only change a condenser if it's faulty

as for rotor arms proper (not copies) Red rotors available from Simon BBC I think but their source is the Distributor Doctor, who also sorts dissys - http://www.distributordoctor.com/
Nigel Atkins

"never chuck old good stuff away"

If you haven't found a use for something yet you haven't kept it long enough.

Red rotors have been recommended here for some time now, unfortunately long enough for cheap copies to be manufactured with similar problems. For ignition stuff it pays to buy from specialists and not the usual suspects.
Paul Hunt

This thread was discussed between 10/11/2014 and 30/11/2014

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