Welcome to our resource for MG Car Information.
MG Midget and Sprite Technical - Cannot slow down idle speed!
I am at my wits end on this one so would appreciate any suggestions
My sons 1275 midget (71) had a fast idle (1500 - 2000 rpm). Easy job to fix - or so I thought.
Obvious checks first: choke returning OK, throttle screws right out, carb pistons moving in dashpot OK. No improvement thus carbs stripped down and fitted with new jets, needles(AN), new butterfly flaps and spindles, new float needle and seat.
All this resulted in "idle" speed increasing to 2500 rpm!. So onto search for air leaks, stripped back to manifold gasket and all gaskets replaced. No difference. Stripped again and gaskets smeared with Hylomar and black spacers checked for cracks- no difference. Disconnecting and blocking vacuum advance tube and emissions pipes makes no difference.
The engine starts instantly with no throttle or choke and roars off to 2500 rpm where it seems to run really smoothly despite jets being screwed up flush with bridge and butterfly flap closed.
Only thing left to change is the inlet manifold but cannot see any obvious places where air can get in. I would have thought if there was a large vacuum leak it would run roughly or hunt but it runs at a steady 2500 rpm.
Apologies for long saga. No doubt I am missing something obvious so any thoughts would be welcome
Thanks for reading this to the end!
|Definately sounds like you are getting extra air from somewhere. I know youve mentioned stripping the carbs down,Are you sure you put the butterfly plates the right way so they totally close off the throats of the carbs wen shut? Im just throwing some ideas out there to look at! You should be able to get down from 2500 to something more reasonable. Do you have the needle jets set to the top? If you look down the throats of the carbs are the butterflys set so they are both opening the same? Normally after you get the idle down a bit I would sparingly use either and spray a little around the intake gaskets to see if the idle goes up a little to determine if I had an air intake leak.|
I say sparingly because that stuff is highly flamable.
Just letting you know as Im not sure of your level of expertise . If you already know I apologize. Well that should get you started. Im sure some others here
will help. Hope you figuire it out soon!
|OK - what have you got the timing set at?|
|Chris at Octarine Services|
Are you sure you dont want to discuss non midget related topics instead....
What about the black spacer blocks...these things can devolop cracks that cause an air leak
But like you based of what your saying its probably not a vacume leak because it runs so smooth....still maybe add a cheap vacume gauge to the intake manifold and google how to read a vacume gauge would be a 1st step just to rule out a vacume leak...a vacume gauge read and interppeted correctly is an awsome and powerful diagnostic tool
2 other thoughts that come to mind...
Is the timing set correctly...if its to far advanced can do this ...if you retard the ignition does it drop the rpm at idel....if this is the cause the carbs have to be set at there lowest idel then the timing set then rest the carbs
2nd... now this is tricky...and ive done this
You said you completely rebuilt the carbs...did you also replace the springs that go over the little adjustment screws like on the fast idle ...if so get a magnifiy glass and watch the those springs as you manipulate the carb linkage, ive found that tho those springs are common, ones is not like the others... even though they look like the orginals the aftermarkets spring can barely catch on the linkage and prevent the high speed linkage for fully returning... it catches on the side of the aftermart spring ...I.wish I could explain it better... but once you see it after a full day of fiddleing and head scratching....its a slap to the face and you will say....bi could have had a V-8
Anyway... I hope some of that helps...
Please let us know what happens, becausewe are severly suffering from social media cabin fever... not much spridget matrial to talk about lately....we love Issues like this
Thanks for stopping by
|Prop and the Blackhole Midget|
|Thanks for replies. I have now in desperation put the carbs on from my midget which I know are fine. If anything that made it worse. Checked the spacer blocks which were OK.|
The static timing was 7 degrees BTDC last time I checked but must admit I haven't checked recently, so maybe the dizzy or points have slipped out of timing. That will be my first job in the morning - I will let you know how I get on. If that is not the answer I will find a vacuum gauge before I strip it all down again
|"we are severly suffering from social media cabin fever" I was wondering what he hell it was! Lol!|
|When you say stripped back to manifold gasket did you mean the manifold to the block gasket? You can end up with gaps on the underside where they are not visible from above. |
Another couple of suggestions - though both rather obvious:
- most manifolds have at least one blanked off opening which should be closed with a screw - just check it hasn't gone missing!
- and is the ferrule on the throttle cable at the pedal end seating properly into the "cup" on top of the driver's footwell?
Thanks for your input. Yes I have changed manifold to block gasket, it looks to be sealing OK from top but as you say difficult to see from underneath. Manifold screws/plugs are all OK (I have even removed them and resealed). Throttle is closing OK and visually both butterfly flaps are closed tight when not running so in theory the engine should not even start! (it starts first turn without throttle or choke even when cold).I have noticed though that there is more suction at the inlet on the front carb than the back one which suggests that any air leak is at the back. The flaps must be somehow opening without throttle as their is a strong inlet air flow. So jobs for the day:
Recheck (again) all carb linkages and return springs
If no success change inlet manifold for sound one from my midget (in case welch plugs are leaking or a small crack that I haven't noticed)
And if none of that works I have discovered a car that defies the laws of physics!
thanks all again for input - will keep you posted on progress
remember the order for timing -
tappets (if required), CB points, plugs, timing, mixture
if you can't set up then possibly its something in or with the dissy(?)
it all seems a bit odd
|Could you have a weak bob weight spring, which, when the engine is revved, is putting the ignition to fully advanced position and not returning until the engine is stopped. Just another thing to look at.|
|What happens if you block of the front carb? Will it start? |
Sounds like your butterflys are not sychronized.
Just guessing since Im not there! I had a welch plug pop out of the manifold last year on a back fire and the car wouldnt start because there was no manifold pressure!
Keep going you will get it bud!
|Like everybody else, I am intrigued by Mike Starnes' son's fast idle problem. I must ask - did the problem suddenly happen? Or did the fast idle occur after work/maintenance was done on the car? Depending on your answer, it might throw some light on the problem.|
Irrespective of accuracy of engine timing, air/fuel must be getting into the engine for it to start at all. Have you closed off the butterfly apertures in both carbs by adjusting the idle screws? Even allowing for a possible manifold air leak, fuel must be entering the engine - and that can only happen through the butterfly valves - unless there is a serious fuel 'plumbing' problem.
You need to double check that the butterflies are fully closed and then try and start the engine. (Take the carburetor pistons out and have a look at the butterfly positions.)
Another thought is the fast idle function of the choke control. When the choke cable is pushed home (ie zero choke position), are the jets returning to their 'end stop' and are the choke 'fast idle cams' fully disengaged on the carb linkages?
Thanks for latest input.
The car gradually developed a problem with tickover rising to 1500 rpm. With an MOT due I thought I would fix it with a quick bit of tinkering with idle screws and mixture but could not slow it down. Thus the start of the saga which so far has made things worse (now 2500 rpm with a cold engine).
Sitting down and thinking logically it has to be something to do with the butterflys/throttle linkage because as you say air and fuel must be getting in OK. Throttle stops are screwed right out and jets lifted as high as I can get them to weaken mixture. Also fast idle cams are screwed right out. I can see that both flaps are well and truly closed before starting. The bizarre thing is that it starts instantly and runs really smoothly at high revs!
Blocking the front carb inlet does stall it while blocking the back one just slows it so there must be a clue there. I have taken the dampers out and pushed the pistons down when running. This actually allows be to control the speed. I wonder if I have been sold mismatching jets and needles (the needle is an AN which is standard strength).
Having said that I even swapped the carbs from another car and it still had the same problem (although it ran much rougher).
I agree that if there was a big vacuum leak it would not run so well.
Off to try again and will add checking jets and needles to list of jobs, might see if I can get a pair of weaker needles to try.
thanks all for your interest
Have you had the butterflys apart? The brass disk has a bevelled edge so that it closes against the throttle body. Its easy to put them in the wrong way around and they will then appear shut, but actually allow air to leak past.
Just to eliminate worn throttle spindles, try smearing a good dollop of vasaline around the ends of the spindle where it rotates in the throttle body.
|I trying to think of how to word that, you did it nicely Guy! Does that model have the crank case ventilation hole on the carb bodies or is that just for the cars that came here? Do you know what I mean?|
|2 different sets of carbs do the same thing? We are missing some information here, Im thinking....|
I still think it is a manifold leak. I think the 1275 manifold has two short stub sleeves that locate into the block. If one or both are catching on the edge of the hole it tilts it upwards, stopping the thing from seating properly, even though the studs may be tightened up quite firmly.
|I went threw this last 4th of july on the MGA, I folded one of the carb gaskets over on itself but I couldnt see it... I wasted the whole day. I think you are right guy! It makes sense! You never know till you get it though Ha ha ha!|
|The fact it starts from cold with no choke suggests there's too much fuel at idle so I'd look for fuel from the jet spilling over the bridge - cut the engine, pull the pistons and needles out, and see where the fuel height is. Should be 2 - 3mm below the bridge. If it's too high either the float height needs adjusting or the pump is overcoming the needle valves.|
|Even if he had the seat for the needles to low it wouldnt pull the gas in! Unless the butterfly were open, there was an air leak from the intake to the head or before the butterfly on the head side theres a bad gasket or the manifold is not seated right!|
He will find his issue there! Im guessing of course, but by deduction also what else could it be?
Guy beat me to it!
|Even if the float chambers/jets are passing fuel (ie flooding), assumuing the butterfies are closed, the carbs' venturi/necks would have to be flooded with fuel for it to get past the closed butterfies' 'lips' and there'd be a strong smell of petrol.|
I wish I lived nearer to Cambridge, I'd pop over to witness it for myself!
I know how you feel! I just want to find out what the answer is now! Kinda exiting huh!
If all the gaskets are back proper and still a problem:
Are you sure the return springs are retuning everything to their stops? and I mean check the choke is as well. Its one common denominator that gets overlooked. Especially if the tick over seems OK when you have adjusted with your head under the bonnet.
1) adjust and rev under bonnet
2) rev on acc pedal
3) any difference in tick over?
If there is will pushing / tweaking the linkages back make it settle? Also did the choke and acc cables sleeves come out of their seats?
I have wear in my linkage that was driving me up the wall for weeks but I have made the carbs consistent by part dis assemble, cleaning, making all return springs work properly, making sure there is free play in the linkage so the grease can work and so the whole thing return properly. After that balancing the carbs needs to be done from scratch. (only took half a day .... mutter mutter)
Do all when engine hot.
Sorry if teaching to suck eggs. I now know its 90% simples and 10% advanced.
Method before madness, make tick lists, work through methodically, stick to the plan, come back tomorrow, the answer is in there, (somewhere), etc.
|Dave Squire (1500)|
|I had a problem like this after fiddling around with a screw on the footwell tunnel (engine side). It was the stop for the accelerator pedal and that was preving the pedal going fully home!|
|G Williams (Graeme)|
This was all caused by a "modification" (ie bodge) that the previous owner had made to the heat shield to accommodate a tubular exhaust manifold. The heat shield had been cut in half and the middle couple of inches removed to provide clearance for the exhaust.
This has resulted in two problems over time. The first was a small crack in the heat shield that led from the crude hack saw cut through to the fuel/air inlet on the rear carb. I guess this slowly developed over time causing a vacuum leak thus the increasing idle speed.
The second problem was due to the way that the throttle linkage had been set up. Because the middle section of the heat shield was missing there was no way to feed the throttle cable through to the cam on the linkage rods. So instead it had been attached directly through to the throttle lever arm on the front carb via a homemade bracket fixed to the inlet manifold. The throttle return springs had been connected between the centre cam on the linkage and the home made plate on the inlet manifold - presumably because the heat shield had been cut up to such an extent there was nowhere else to put them.
When I refitted the carbs I set the linkage up with clearance in the lever arm forks as normal (at least on my midget its normal!).
I checked and double checked that the butterfly valves were closed tight before starting the engine. However because of the clearance in the forks the return springs did not keep the butterfly fully closed so the butterfly opened by the clearance in the forks with the vibration (are you with me so far?!). Because of the fact that the throttle was acting directly on the front butterfly the clearance in the forks also caused a lag between the front and rear carbs.
By getting rid of this clearance I now have it running at a steady 800 rpm!!
Not happy that this is a permanent solution so will need to look for a new heat shield and adapt it for the lcb exhaust in a different way and/or look for a manifold that needs less clearance. I'm surprised that it has not caused fuel evaporation problems but it seems to be OK on this.
With the last few days practice at least I get the carbs on or off in less than 10 minutes!
The lesson learnt from this saga is to immediately suspect something that is not standard. The guys who engineered the car in the first place knew what they were doing unlike the previous owner.
Many thanks for everyone's helpful suggestions and input, it is amazing the wealth of knowledge and interest there is out there.
Now off to put the carbs back on my midget!
|Further to what Guy said about the locating rings between the head and manifold, I have seen one where the face of the head and the face of the manifold had been surfaced enough times that the manifold bottomed on the rings before it met the head.|
And, does the A-series have the same issue as the B-series where there are different flang thicknesses of the manifolds through the years, which means that stepped washers are needed to mix them?
With respect to the butterflies, when you installed them did you rattle them around with the screws loose to let them find their "home" place before tightening the screws?
Also, I'm not seeing a mention as to whether we are talking about HS or HIF carbs. Did I miss finding that?
Edit: I see that my post is both too late and not relevent.
|C R Huff|
|well done Mike and thanks for reporting back|
(with hindsight of course) I think sometimes putting a photo on the thread can help as I'm sure the unusual heatsheild and spring fixings would have bought comment and suggestions
as would a tubular exhaust that needs the heatsheild modifying
even though many cars are modified or non-original in some way I think most image a pretty standard set up unless otherwise stated
Agreed, now I understand the significance of the mods I realise I should have highlighted this earlier. However at the beginning of this I did not appreciate that the way it was hooked up would have such a significant impact. Essentially the play in the lever arm forks (about 2mm?) allowed the butterfly to open enough to increase speed to 2500 rpm. Presumably the previous owner had gone through all this and adjusted to suit the mods. Also the crack on the heat shield on the back carb made it worse.
Now I realise the depth of expertise out there next time I need help I will give all details whether I think they are significant or not.
Thanks for the feedback
I was being wise after the event, I did *think* about suggesting you put a photo up but that's all I did do, think about it, until it was too late
I often have to put up photos as I can't explain what I mean or have a clue what I mean
there's me and I guess 4 others that would have instantly spotted what was up with a photo (even perhaps seeing inside your carbs with our special vision) I'd have still guessed wrong and the others would have been on the right track
I thought an exhaust manifold was glowing hot on one side in a photo until Dave O'Neill, I think, pointed out that the manifold was painted red - my eyes are used by the NSA !
|The original Special Tuning LCB manifold requires the heatshield to be cut. The factory issued a drawing of of where to cut the heatshield to accommodate the LCB. I had to do this when I fitted one to my car. I'll see if I can find the original details and post this up.|
This thread was discussed between 17/06/2013 and 19/06/2013
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.