MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Tuning: Rough idle and slow to drop back to idle

Afternoon folks, how are we all?

For a bit of fun this afternoon I thought I would have a go at tuning up the car and trying to get it running smoothly and idling well.

I feel it is in about the right ball park (in terms of carb tuning) but I think there is still something amiss. It idles OK-ish, but not totally smoothly and if I rev the engine it takes a looooong time for the revs to drop back down to the idle speed.

Some setup info:
- 1500 engine, 270 degree "fast road" piper cam.
- Ignition system: electronic module, timing set OK to 10 deg BTDC.
- Single DCOE 40 on a cross over manifold.
- Vacuum advance active via a T piece and a vac line to each inlet tract.

I have a vacuum gauge also hooked up to the inlet tracts, I am only pulling about 15 inHg. The gauge seems to react fine to revving etc and needle is stable at anything about ~1200 RPM, a bit wobbly at idle.

So, I think ignition timing is OK and I think I am in the right area in terms of carb tuning. If I lean off the mixture much it will start to die (and I can compensate with choke and it comes back). Or, doing the opposite, leaving mix where I think is about right and pulling out choke also causes it to flood and bog down.

So, why is it idling slightly off? Why is it so reluctant to settle back down to idle when revved? Why am I only pulling about 15 inHg of vacuum?

I will wrap things up for this post as it is getting rather long, but I am sure there will be questions.

Cheers for your input everyone.
Malcolm
M Le Chevalier

Malcolm,

Could it be a vacuum leak?

Charley
C R Huff

That was what I was thinking was most likely... glad at least one person is thinking the same.

What is the easiest way to test for a leak (assuming I don't have any carb cleaner to squirt everywhere, nor a smoke machine!)

Any one else got any thoughts?

Cheers,
Malcolm
M Le Chevalier

Single DCOE 40 on a cross over manifold.

Is the webber an old orginal? Or is it a fairly new from ebay (drop shipper)

There has been a big problem with the newer webbers ... china is flooding the market esp ebay with reproduction webbers and calling them orginals... and there not doing to well as you can imagine.


But thats were my concern would be, is the webber truly orginal or is it an expensive china knock off someone is telling you that its orginal webber

Prop

Prop and the Blackhole Midget

I take it that you have full vac. advance at idle,
Maybee there is a leak in the vac. advance diaphram on the dizzy
If you don't have any carby cleaner etc to check for manifold leaks ,just lightly trickle some water over the joints and you should get a missfire if leaking or if you're lucky you might hear the slurp noise as it gets sucked in
good luck
willy

William Revit

good morning
I out a 45dcoe on my 1500 and was advised to fit a vacuum-less dissy. I cant remember who suggested this but I think it may have been on here. I did fit the new dist , chinese for about £40 and the whole thing seems to work well, reasonably slow steady tickover and no flat spots on acceleration
rgds tony
a boyle

I agree Tony, you are better off without vac advance with a side d.Weber
willy
William Revit

What sort of vac are you pulling at 2000, 3000 and 4000 and what revs for max vac (testing with just revving engine statically not driving?)

Peter
Peter Burgess Tuning

Hi Peter,

I didn't record exact figures but this is what I remember.... I was aiming for an idle of about 800, at 800 it pulls about 15 inHg, but the needle flickers a little between 14 and 15.

If you increase the revs to about 1500 the needle becomes steady and rises to about 16 or 17.

If you boot it, it flicks down to near zero and back up to 20-ish then very slowly drops back to idle.

A thought this morning, should I expect slightly low readings on a rebuilt engine? i.e. until everything is well bedded in?

cheers
Malcolm
M Le Chevalier

I'd also say it sounds like an air leak, but only because it takes so long to get back to idle revs.

Of course, it could be the throttle return springs. Can you manually close the butterflies on a webber, like you can on an SU, to test that?

Otherwise, a normal engine would be 20ish hg at idle, and flickering between 13-15 at idle with a hot cam. But according to the blurb, the 270 is a pretty mild cam, so not much overlap that would cause vac level to drop at idle.

When you rev it, it should drop to 0 and then come back to 25-30 as you let the revs drop. But it would only get to 25-30, if the throttle shuts properly/quickly, and or, if you don't have an air leak.

As for bed in on a new engine, if you've got good compression, you should also have good vaccum. It's the seating of the valves against that seats, and the sealing of the rings against the bores that provides both.

So what's the compression like?
Lawrence Slater

Hi Malcolm Can you do the test again, slowly increase throttle opening and record vac readings up to say 4000 rpm?

Peter
Peter Burgess Tuning

Will do Peter, might be able to get out tonight and do it. What you thinking?!

Cheers,
Malcolm
M Le Chevalier

And in response to a few of other points:

Prop, I have no reason to suspect the carb is dodgy.

Tony and Will (not being disrespectful) but I don't buy into all this "DCOE shouldn't use vac advance" stuff. I have done a fair bit of research and pondering on the issue and came to the conclusion that vac advance was the way to go. I would be happy to discuss on a new thread if you like?

Lawrence, yeah, I really should buy a compression tester one of these days... :-)

Cheers,
Malcolm
M Le Chevalier

OK-personal choice
William Revit

Looked into a couple of places where I thought vacuum leaks might be happening, but I didn't get chance to warm it up and do any proper tests. I started it up, it was running crappy, so I gave up because it was starting to rain.

Pulled plugs and they look very sooty so we are pretty rich and lumpyness probably due to fouled plugs?

How many times can you clean up a set of plugs before they are just fit for the bin? I have bought a new set, they are in the post, but don't want to fit them if they are just going to get sooted up immediately.

Cheers,
Malcolm


M Le Chevalier

Just a thought, a mate has a Caterham with twin 40 DCOEs and he was having some trouble with poor running and tracked it down eventually to one of the main venturi having come loose and rotated.
David Billington

"How many times can you clean up a set of plugs before they are just fit for the bin? "

You can clean them indefinitely. They're only toast if the electrode/nose is worn(erroded) down or the insulator is cracked.

Just wire brush them. They won't be the cause of you low vac.
Lawrence Slater

Thanks Lawrence, agreed it is not the root cause, but sooty (and wet!) plugs show me I am too rich, right? and surely a duff plug/spark isn't going to help smooth running.

I really should do some more sanding and bodywork (soooo boring!) :-)

Malc.
M Le Chevalier

Malc

Assume nothing

Remove air filter - run again.

Check/clean any pilot jets - run again

Check float height and if needle is seating satisfactorily.

Blow out main jets.

Is correct needle fitted?

and yes, check for any leaks.
Mark O

You're one plug short...
Alex G Matla

Thanks Alex :-p

Stupid 1500 design means getting No.1 plug out is a faff which I couldn't be bothered with the other night!
M Le Chevalier

Alex
I noticed that as well
Of the plugs that are there the Lh one looks dodgey to me--glazed up on the insulator-
David Billington.
Had exactly that problem on a racer MGB engine once
The locking screw for one of the secondry venturies had got itself loose and allowed the venturi to rotate cutting off the fuel supply to that barrel.
It was really strange especially on the weird idling B engine and took a while to find.
Like all hot Bs with a single Weber it idled on two cylinders with Nos 2&3 not doing much until it got a few revs up - in this case about 2000rpm BUT just as it started to pick up the centre cylinders Nos3-4 would start to miss because of the mislocated venturi
Not an easy one to find until you sit back and have a big think about it, as I imagine your mate had to do as well
willy

William Revit

Just one thing to add
Your aim of an 800 rpm idle might be a bit low.
Low idles with wilder cams and the associated stronger valve springs is not benificial for the life expectancy of the cam.
The higher load on the lobe for a longer time (low rpm) causes more wear.
Might be good to look at an 900 to a 1000rpm idle
Onno K

Afternoon folks,

An update and some progress for those still interested.

Peter, I performed your test, at about 1000 RPM I was pulling ~15 inHg but with a bit of needle wobble as you increased revs the needle became steadier and then rose to about 16 inHg at about 2750 RPM, it is a little hard to tell as vac gauge is in the engine bay and I can't see it whilst sat in very well when sat in the car and vica versa.

But anywho, the above is moot as I fiddled again. I found, entirely by trial and error and looking at the vac readings that the the rear barrel (that feeds 1 and 4, due to cross over manifold) needs loads more fuel than the front. I ended up winding the rear mix screw out loads and the front (which feeds 2 and 3) in to almost nothing. And by doing this it started pulling ~17 inHg @ ~1400 RPM. (Note: whilst doing this, it turns out I had the choke half out).

This indicates to me I have a vac leak somewhere on 1 or 4.

Listening a bit harder I thought I could hear a whistling/sucking sound around the back of the engine. I am wondering if there is a leak at 4 between the manifold and head, probably due to the impossible to reach nuts under the manifold.

I then decided to quit whilst I was ahead, as it was lunch time! :-D

Cheers,
Malcolm
M Le Chevalier

Malcolm
If it's leaking bad enough that you can hear it--
Get yourself a piece of hose/tube----one end up to your ear and have a poke around with t'other, You will soon locate your leak area
willy
William Revit

This thread was discussed between 28/03/2015 and 03/04/2015

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.