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MG MGA - High idle causes

I cannot bring the idle down on my 58. I have backed the throttle screws all the way off, backed off the fast idle screw, checked that the choke is off (jets are not pulled down) and checked for air leaks. I'm pretty sure the front carb is running too rich (black, sooty carbs). But shouldn't backing off those screws cut off the air supply completely and, therefore, stop it from idling no matter what? Yet it continues to idle at about 1500. Weird. I pulled the dashpot and looked at the jet on the front carb. It is roughly flush with the bridge.

The only changes I've made is adjusting valves (they were too tight), new points and static timing. I'm going to double-check my settings for dwell and timing before I go further.

What effect does timing have on idle speed, if any?

Any other advice? Thanks again for any help.

Jim
J Plegue

As the timing is advanced your idle speed will increase. If the front carb is flooding due to a bad needle and seat, float or fuel pressure the gas is running under the throttle plate and mixing with the manifold air from the other car increasing the idle. When completely closed the throttle plates are not a leak free joint against the carb ody, a little air will get thru and fuel can run thru the small gaps. Normally running a little rich will not soot up the carbs more than normal unless you are getting backfires thru the intake. Address what appears to be flooding first, it may solve the rest of your problems. Also check that the needle is in the proper location in the damper, if it is too far in it is the same as lowering the jet and will cause a rich mixture.
John H

Jim

Are the pistons sliding freely and bottoming out with a solid clunk when releasing the lifting pins?

Steve
Steve Gyles

I disagree with John I am afraid the only way for an idle to be high is if the air butterflies are not closing properly. Perhaps your engine was set up "faulty" thus masking the fact that the butterflies were not closing fully.
This can be caused by many things throttle cable too tight, poor adjustment between the 2 carbs or more than likely poor overall of the carbs by somebody.
Bob (robert) yes Y8 is toast again :)

Jim,

Think of an engine as a runaway air pump. It runs as fast as it can pump the air thru. The amount of air is controlled by the butterfly valves. If they are completely shut, there is not enough air going thru to sustain combustion and engine running/idling.

If the idle screw is completely backed out and the engine is still idling, it means that air is still getting in. Either the butterfly valves do not close fully or there is a huge air leak somewhere. In either cases, the carbs will need some type of attention or a rebuild.

The butterflies not closing could be due to some obstruction preventing closure. It could also be that when the throttle shafts were last replaced, the person who did it did not drill the shaft pin hole properly in order for the butterfly to fully close at the same as the shaft lever hits the stop on the carb body. This job requires a bit of attention...

Karl

Karl Leclerc

Hi not as technical as other causes, but have you checked that the pedal isn't being held but carpet or mat stuck under it etc.

gordon
g c pugh

I know that the pistons are moving and hitting with a solid metal sound. I know that the throttle/pedal is free. Does it make sense to remove both carb tops and, either by feel or with a mirror, compare the relative positions of the throttle butterflies? Should there be no gaps around the top and bottom, i.e. completely closing the inlet?

Seems odd that the front carb is flooding since the jet is flush with the bridge. Shouldn't that be a relatively lean position? Maybe I should check the float bowl.

One good thing about this: I'm starting to understand the innards of the SU carb.

What do you guys think? I really appreciate all the input. Thanks again.
JPlegue

If the float is not shutting off the supply of fuel it will run out of the jet no matter the height. It may also be running out the bowl vent line. Use your mirror to see if the butterflies are closing completely. If one of the screws is loose or someone had the carb apart and loosend the screws the butterfly may not be closing completely. If the car is running with both carbs completely closed you have an air leak somewhere combined with fuel leakage into the manafold.
John H

Properly set up the butterflies will close tightly enough to stop air flow and completely kill the engine. Engine still running means at least one of the butterflies is not completely closed. If both throttle valves are completely closed it will kill the engine even if there is a substantial air leak in the manifold area, as there will be no fuel mixed with the leaking air.

A worn shaft can be far enough off center to drag the butterfly on the side of the throat preventing full closing. This is quite common. If you step on the throttle pedal, then slip your foot off the side to let it snap back up briskly, the throttle valve may seat better and allow slower idle. You can do the same by pulling on the throttle cable at the carb and letting it snap back briskly. Fix for problem that is to loosen the screws in the shaft holding the throttle plate, realign the plate while closing the throttle completely, then tighten the screws. If teh throttle shaft is thet badly worn it may be time for replacement.

The accordion clamps on the throttle shafts may be out of sync. Loosen one of the clamps, then close both throttle plates completely, then tighten the shaft clamp.

This last one is not so likely, but if someone rebuilt the carbs and pinned one of the idle control arms on the shaft in the wrong orientation it could prevent full closure of the throttle valve.
Barney Gaylord

It is almost certainly one or both of the butterfly valves not closing. SU's are one of the simplest carbies known to man but if they are not 100% your car will not run/behave properly.
Barry Bahnisch

I just went through this with my 59. I concluded you cannot accurately tell if throttle plates are fully closing with the carb on the car. Looking with a mirror and flashlight just doesn't tell you the story. Take one carb off and look through the body at a bright light and see how much light is passing around the throttle plate. (Light passing = air passing in operation.) Do as Barney says; loosen the throttle plate screws and move the plate around until it is properly centered in the carb body (i.e. least amount of light, if any, showing around the plate.) Put the carb back on the car and see if that helps. You may have to do the same on the other carb.

Steve
Steven B

I just read the responses quickly, but didn't see throttle shaft wear as the probable cause. It doesn't seem to take much play there to leak enough air to make it idle too fast. I have heard that spraying carb cleaner around the shafts will make it speed up even more if there's too much wear. I've been stuffing grease around mine and it seems to help the idle. Too lazy to rebush/install oversize shafts, I guess.
Tom

J,

What was the state of your car and carbs before this problem? Did you just rebuild them? Are you starting up a car that hasn't run in a long time? Did the problem just start out of nowhere one day? I agree that the butterflies probably aren't closing, and I think you should listen to Barney, but before you tear down the carbs maybe go back and re-try the basic setup from scratch. I would check the easy stuff first by disconnecting the throttle and choke cables and slacken the linkages between the carbs. Make sure the shaft springs have been installed correctly and also the return springs that attach to the heat shield. Get yourself a jet centering tool and make sure the jets are centered. Set the jets to two flats below the bridge. With everything slack and the carbs operating independently, see if you can bring the car to a normal idle. SU carbs are easy but not if you're unfamiliar with them. If you don't have a basic setup problem, I'll wear a White Sox jersey next week.

Mark
Mark

I've had the car for five years; driven it lightly and never tuned it up til this year. It has been prone to plug fouling in the front two carbs unless I've been driving it hard. Also, always prone to slightly high idle (maybe 1,200) except when it's good and hot. Then the idle would come down to around 1000, though not consistently. Now it's idling at around 1500. I figured because I just adjusted valves, replaced points and reset timing that maybe I had caused the situation. I'm double-checking dwell and timing right now and then I think I'll start by trying to eyeball the butterflies to see if they're closing completely. The shafts don't seem to have any play in them. I sprayed carb cleaner on em while it was running. Made no difference. I'll also check the float level on the front carb. Anyway, I'm getting one heck of an SU education here.
I'll keep you all posted. Add any advice you think might help. Thanks.

Jim
J Plegue

Hey J.P., - Where in Michigan? I'm just around the other side of the lake.
Barney Gaylord

In the boonies, actually. Extreme northeast Oakland County, north of Rochester. Pretty area but too many dirt roads between me and the paved stuff.

Jim
J Plegue

That's not the boonies, just a coffee break beyond the Detroit suburbs. No big deal with dirt and gravel roads, as long as you're not knee deep in mud. But it would be about 11 hours round trip from Naperville, so it may be quicker to fix your carbs by remote. Otherwise a couple of months from now I may be going by there on my way to eastern PA.
Barney Gaylord

Jim,

You mentioned something in your original post about "black sooty carbs" Did you mean plugs or carbs, I can't envision why carbs would be black and sooty.

BTW the offer still stands, I'm only 30 minutes away in Brighton.
T McCarthy

Just a synaptic malfunction. I meant to write plugs. I may yet take you up on your offer but I want to give it my best shot first. Thanks.
J Plegue

Hello,
I once had the problem that if the carbs are not screwed "evenly" to the engine the linkage between the throttle shafts can cause that the shafts are not able to move freely (carbs monuted slightly "bent" to each other). Thus the shafts were not returning to the rest position only by the force of the springs.
Additionally to that if the shaft are rotated one against the other, only one of them can close fully (but that was alreasdy mentioned). If you have the plates out of the car remember that the flanges have to be oriented the right way.
Simon
Simon Beyertt

Here's an update. I went back and reset the dwell to get it to 60 and reset the timing. Removed the carb tops and used a mirror to look at the butterflies. They sure look closed to me. But as Tom in Solvang wrote earlier, you can't be 100% sure without removing the carbs (Can you tell I'm trying to avoid that?).

Took the car out for a 20-mile drive, and here's what I know for sure:

1) It runs great. Seriously. Starts right up, seems to have a bit more power than before. No missing, no pinging. Runs a little cooler than before -- 180-190.

2) It still idles high. BUT when I blip the throttle it comes down slightly -- from 1200 to 1000+. That would seem to indicate sticking butterflies? Or maybe sticking dampers since it seems like it comes down slowly?

3)Front carb is damp around the bottom. Rear carb is not. Front spark plugs are black and sooty. I raised the jet 3 flats (screwing it in, counterclockwise if looking from above, right?)and rechecked with Colortune. No change. Checked the float level. It's actually a little lower than it should be.

At this point, I think I'll go up a few more flats and see if it makes a difference. Otherwise,I think the next step would be to R&R the front or both carbs.

Or leave it be and enjoy how it runs right now. Sure was fun driving it yesterday.

Jim
J Plegue

Thinking outside the box again, as is my want, you may have the vent holes to the carb pistons blocked by the gaskets or the rear filter box on upside down. Look at the second article on my website page: http://www.mgaroadster.co.uk/hot_press.htm

Steve
Steve Gyles

Not a bad thought but I've got the filters completely removed right now. I'm guessing root of my problem is the damp/leaky front carb. Thanks, though.

Jim
J Plegue

Barney has mentioned that if the throttle plates are properly seated and you turn back your idle adjustment so both carbs are "closed" the engine should stall due to insufficient air flow to draw in fuel. Try that. If the engine does not stall then look at your front carb throttle plate. (It really is not that big a deal to take a carb off, especially since your filters are already off.) Put the front carb back on and see if the engine will stall. (Of course, you have to increase the idle setting to get the engine started and then loosen up the idle adjustment screws.) If it still won't stall, do the same with the back carb.
I thought my throttle plates loooked good through a mirror but there can really be some large gaps and it just takes a little to get the idle up.
Steven B

Yeah, guess I'll have to remove the front carb (at least)because with the screws backed off it still idles at 12-1500.
Is it totally insane to drive the car without the air cleaners? Unfortunately, I have several miles of dirt (and sometimes, dusty) road to travel to get to the main road. It's probably a "once or twice but don't do it often" kind of thing, eh?

Jim
J Plegue

I am having the same problem Jim outline in his first post. Just had my SU's rebushed and new throttle shafts installed by Apple Hydraulics. Looks like I will have to pull the carbs and see if I can adjust the throttle discs.

Will let you know what I find.

John
jjb Backman

This thread was discussed between 11/06/2007 and 24/06/2007

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