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MG MGA - Very High Idle
| Mystified of Kirkham Having just rebuilt my MC (other thread) and bled the brake and clutch, I decided to do a late night test drive. Fired up the engine and it soared away to 3000RPM without any need for choke. The engine has been running fine all winter and was last used yesterday prior to spotting the MC leak. Choke and throttle linkages all okay and choke works correctly and closes fully. Revved engine - pistons lifted in unison. Took the pitons out and had a look around. Butterfly valves intact and closed ok. Turned idle adjustments down to nothing - throttle bar fully closed. Checked floats, no leaks, fuel levels ok. Dizzy tight - no wandering timing. Totally mystified. Revs still 3000. Must be something simple but I can't see it. Things come in threes, what's next? Steve |
| Steve Gyles |
| Hi Steve have you trapped the throttle cable somewhere Gordon |
| g c pugh |
| Steve, Do you have a brake booster on your MGA? If so is it disconnected? Sounds like a vacuum leak, if the throttles are not against their stops. |
| James Johanski |
| Gordon Checked that; full and free movement. James No booster. An air leak seems an option as throttle is totally closed. Not sure where one would have sprung from as I have not disturbed anything and it was working perfectly on Monday. Can't sleep for thinking about it! Running USA time. Steve |
| Steve Gyles |
| If it runs at higher speed like that with good running and correct mixture without heavy choke, then it is not a huge air leak, but one of the throttle valves is not closing fully. One of the accordion clamps on the intermediate throttle shaft (between the dual carbs) has slipped a bit on the shaft, now holding one of the throttle plates open a little while the other is closed. Most likely the rear throttle is a little bit open while the front one is closed (because the throttle cable pulls the rear one open while the shaft spring tries to close the front one). Loosen one of the clamping screws to free one of the clamps from the shaft, and both throttle plates should snap shut for proper idle speed. After that you get to balance the air flow again before tightening all of the shaft clamps. Another possibility is the two small screws securing one of the throttle plates in the shaft may come loose. This allows the throttle plate to move off center and jam in the throat before fully closing. Cross fingers and hope you did not lose one of the screws down the throat, through an intake valve into the cylinder. |
| Barney Gaylord |
| Thanks Barney, sounds logical. I did check the throttle bar and the clamps were tight. I also put the balancing rods in the pots and they rose in unison when I revved higher than the 3000. But I guess it just takes a slight movement of the bar to throw it all out of balance so that will be my first job tonight to rebalance. Fingers crossed it's not a butterfly valve and a screw gone in the engine. I did feel the valves when I had the pistons out and did not feel anything untoward; there again, I did not feel them specifically for missing screws. Steve |
| Steve Gyles |
| Nobody seems to have mentioned the choke cable. As you mentioned it is probably one of the small things? Neil |
| Neil Purves |
| Neil Choke and cable operating fine. I think Barney's theory has the most merit at the moment. If not, I am truly baffled. When I first fired the engine I had the choke pulled. The engine went up to 3500 RPM very smartly but was running rough, missing frequently on at least one cylinder. I fed the choke in, the engine picked up on all 4 and settled at 3000. I did a number of switch offs and restarts and each time it accelerated straight to 3000RPM running on all 4. I checked all the cables and checked underneath the jet assembly to make sure the choke had gone fully in - it had. This sequence of events would tend to align with Barney's analysis, in that one carb is doing more work than the other, although the balancing manifold tends to mask it a bit. The carb cross shaft did seem tight so, at the time, I discounted out of balance as a cause. I will recheck properly tonight and report back. Steve |
| Steve Gyles |
| Check that at both ends the outer sleeve of the throttle cable has not come out its metal ferrule. This can be difficult to spot since the cable looks almost the same. |
| J H Cole |
| John That is not the problem. The throttle shaft is physically closed on its end stops on both carbs, idle adjusting screws both fully retracted. I could not close the throttle more, even by hitting with a sledge hammer! - still get 3000 RPM idle. Just got in from work. No sign of slippage between the shafts. Looks like carb removal and inspection tonight. Steve |
| Steve Gyles |
| Dead end on Barney's thoughts. Both butterfly valves in sync and fully closed. All screws in place. Just started up on cold engine. Exactly same symptoms as I described to Neil 3 posts above. Monitored operation of throttle cable and choke cable, both okay. Chokes definately going all the way back in to jet assemblies. I am floundering. Carbs off after dinner; let's see what we find. Steve |
| Steve Gyles |
| Steve, maybe one of the throttle plates is loose on the shaft, inside the throttle body? Unlikely, but it would explain it. |
| A Pearse |
| Barney Now I have the carbs on the bench and it is easier to see the throttle valves. You may be right. I have them still connected with the throttle shaft. I closed the front butterfly valve. This left the rear butterfly slightly open (30 thou gap - see image). Is this sufficient do you think to cause the symptoms? Steve  |
| Steve Gyles |
| Another question. I bought the carbs as reconditioned units and I have not tampered with any of the ancilliary fittings. Is the cam plate fitted correctly? It is as supplied. I have my suspicions. It looks 180 degrees out. I cannot see what useful purpose it achieves this way round. It rotates anti clockwise and strikes the underside of the idle stop. Steve  |
| Steve Gyles |
| Steve, This is what it should look like. Image attached. Mick  |
| M F Anderson |
| Steve, Also, a photo attached. Mick  |
| M F Anderson |
| Steve, I don't think the cam is your problem, even as it is, wrong way round. It only has an effect when it is pulled up and connects under the adjusting screw. I reckon more likely it's the 30thou gap. Good luck, Barry PS Only 28C here today!! |
| BM Gannon |
| Steve, It is almost certainly the throttle valve - looks like the shaft coupling slipped and possibly the idle had been set off only one of the idle set screws. (Don't why I might have any first hand knowledge...) First try loosing the coupling and backing off the idle set screw(s) to see if you can close the throttle valve. If the throttle shaft coupling was loose, the throttle shaft return springs may slip and are not able to close one of the throttle valves all the way. Do you have the accelerator return spring fitted (Moss part number 329-015)? This spring is very often missing. The shaft springs alone are not reliable enough to snap the throttle closed when you take your foot off the gas. Since the gap at the top and bottom of the throttle plates appear to be equal, you probably don't need to fiddle with the throttle plate itself. Once both throttle valves are in an equal closed position, you may need to start from scratch on readjusting and balancing the carbs. Or it may be as simple as tighting everything up and resetting the idle. As for the choke cam see Barney's site for details on setting up and adjusting the choke, http://www.mgaguru.com/mgtech/carbs/cb102.htm . Good luck, John |
| jjb Backman |
| Barry, Can you send some of your weather up to Sydney? It was over 43C (110F) today. Steve, You said that you rebuilt your choke. What is a choke? Never used one! Mick |
| M F Anderson |
| Mick. Thanks for the diagram. Confirms my suspicion. My cam has either flipped round 90 anticlockwise during my previous rebuilds or it has always been like that. I will never know. Barry. I may have confused you. I was not suggesting that the cam plate was the problem. It was just that when I was carrying out the visual inspection on the bench I could not see its purpose. That said, with it that way round, I noticed that when the cam struck the underside of the idle stop, the connecting rod had the effect of acting as an additional pivot point for the choke. If the choke cable was then pulled out further, the choke was pulled out more than that on the other carb! Wet and 8 degrees here. Good MGA weather. Pity it's sick in the garage. Maybe tomorrow; should be heavier rain - more to my liking. Steve |
| Steve Gyles |
| Steve Sun has just made an appearance in Brussels, temp at 5deg.C so may even take the roof down for the drive home. Okay, that was rubbing things in a bit too much?!! I really am stumped as to the cause of your problems. I look forward to hearing what the solution is. Neil |
| Neil Purves |
| Steve, I'd say you've located the problem. BTW: We're supposed to get all the way up to -11deg C today, and -12deg C tomorrow. Keeps the snow nice and firm! GTF |
| G T Foster |
| Engine sorted, running like a dream. Looked like it was the butterfly valves because I did not alter anything else. Top marks to Barney again. It just seemed so strange that when I switched the car off on Monday all was fine. Next time I started up and I had this problem. It was not as if the throttle cross bar coupling was loose. It was dead tight. Most strange. Steve |
| Steve Gyles |
This thread was discussed between 13/01/2009 and 15/01/2009
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