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MG MGA Very Poor Fuel Consumption BBS discussion at MG-Cars.net
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MG MGA - Very Poor Fuel Consumption
| I have just measured my fuel consumption for the last tankful - 15.78MPG! This is down from my normal, but still poor 22.6MPG. I am running an 1800 engine, 5-speed box, 3.9 Diff. All items in good condition. Engine has done 14800 miles. Good compression and oil pressure. I tuned the carbs before the last tank load. Balance spot on. Light brown plugs. I then fitted electronic ignition to a fully reconditioned original distributor, replacing the Metro distributor that gave the light brown plugs. I also fitted brand new vacuum advance - tested and okay. I had great trouble tuning the engine with the electronic unit, but I believe I finally got it sorted to 34 degrees at 4500RPM vacuum disconnected. After discovering the poor MPG yesterday, I checked the plugs. Very black. I checked the chokes. Both functioning OK. Float levels, if anything set a bit low rather than high. No obvious leaks. The only thing I found was a fouling front piston - very sticky using the riser. I have now cured this. Could this alone have caused the problem? I have also needed very little choke from cold and push it in within about 10 seconds, adding to my concerns that I am running rich. I have used the car this morning, so will check the plug colour at lunchtime and report back. All that said, performance seems great and very responsive. Any ideas? A very baffled Steve |
| Steve Gyles |
| Went out for a 10 miles hard drive at lunchtime. Plugs now light brown which is heartening. So it would seem that one sticking piston was causing very rich running. Can someone explain why this is so? The only other possibility is that someone is syphoning my tank in the car park! Still don't know why I need so little choke. It will be another week or 2 before I get another accurate MPG reading. Steve |
| Steve Gyles |
| Steve; I had that problem many years ago with a morris minor. My solution to find the siphoning culprit was to remove the pipe to the fuel tank and substitute a container for gasoline. I then filled the regular tank with severly contanimated gas (old glycol and hydraulic fluid). Two days later we had the culprit. It was a fireman in the next building.He seized his engine with the glycol. Apparently that was all it took as nobody else had any more problems. I was working for a major airline maintenance group and we had all it needed on hand as we were working with Rolls Royce 12 cyl merlins. Sandy |
| Sandy Sanders |
| Hi Steve. You might also want to check to make sure none of your brakes are binding or partially staying on all the time. Brakes that drag a bit can drastically increase fuel consumption. Good Luck, Glenn |
| Glenn |
| I suspect it may have something to do with your non-use of choke. I have something similar on my Magnette. The plugs are a nice colour, and I don't need to use any choke to start the engine - actually too much choke kills the engine. I've checked the carb lifting pin and the engine note rises and falls as it should. The fuel consumption is apalling (less than 20 MPG (Imp. Gal.) ) |
| Dan Smithers |
| Dan Glad I am not alone. I suspect there are a lot of others out there who are not aware how poor their fuel consumption may be. I always thought I was getting about 25mpg on local runs and 30 on long runs until I started accurate measurements. Yes, I am one of those sad people who now keeps tabs on expenditure. I have built a simple but effective spreadsheet for fuel costs. One of the benefits is that by also entering miles at each fuel fill up I get an instant MPG per tankload plus an average MPG since I started the spreadsheet about a year ago. I am still deeply suspicious about the electronic ignition being one of the causes of my problems. Despite getting the dynamic timing to appear spot on I find that the starter almost stalls as it fights to get each cylinder through its firing stroke. I am also aware that the exhaust note is very voluminous (booming) as if a baffle has gone (it hasn't). Any views? Steve |
| Steve Gyles |
| Just had an interesting conversation with James at Bob Wests. He said the advance profile on my original distributor will be the problem. He cited numerous similar experiences with recently refurbished original distributors. Although my timing is spot on at 4000-4500RPM it is far too advanced at static, idle and mid range, resulting in all the classic symptons I have described in my previous post. It certainly tallies with my experiencies of late. If I retard the ignition from its present position, the car starts well and works ok up to about 2500RPM then goes flat. Looks like I will be reinstalling my Metro distributor tonight! Steve |
| Steve Gyles |
| Hi Steve, Yes, I have been nicking your fuel, it smells like TVO, no wonder your car is so slow. Sounds like your timing is to far advanced, your engine is kicking back when you try to start it, my timing is set at 14 degrees BTDC at idle(no vacuum). What carbs are you running? Is the vacuum take off pipe before or after the butterfly? The MGA and early MGB's have the connection after the butterfly and dont produce much vacuum at idle, if any. You need to get it on a rolling road to set it up. My Coupe is on the rich side ( brownish plugs ) and 1 3/4" carbs, I'm getting approx 28-30 mpg on a run along with entusiastic driving ( same transmission as you ) Regards Terry |
| Terry Drinkwater |
| Hi Terry Long time, where have you been? Missed your advice and comments. I run MGA carbs. My short term plot is to go back to what was working well - the Metro Distributor. I will then take stock. James at Bob West is not keen on some of the reconditioning coming out of current suppliers. I will reserve judgement until I can do a comparison with the performance from my 'trusted' distributor. Steve |
| Steve Gyles |
| If I retard the ignition from its present position, the car starts well and works ok up to about 2500 RPM then goes flat. Steve, my car suffers from the above syndroms. I can not get it to PULL after I reach the 2800 RPM range . It just goes flat and lugs down. If I advance my timing ,will this allow the engine to run better in the higher RPM range. Plugs are tan colour....carbs professionally rebuilt and the mixture Colortuned to a blue flame at app 2000 RPM. PS: it also seems to be getting poor gas milage at this presnt time. Thank Gord |
| Gordon Harrison |
| Gordon Sounds identical. What is your distributor history? Steve |
| Steve Gyles |
| Steve I suspect that your distributor is the problem, not the electronic ignition. If it is a pertronix or similar it merely replaces the points. It is still the distributor mechanicals that determine the advance curve. HTH Larry 58 A with Pertronix |
| Larry Hallanger |
| Origional one that came with the car. I rebuilt it (complete cleaning / new points /condensor etc.) and installed it into a fresh rebuilt 1500 engine. When I check the timing (at idle) with a light and the advance mechanism installed I get the pulley notch at about 5 o'clock (when I am looking at the motor).My plans call for a static tune with my dwell meter ....check the vacuum advance mechanism and then a recheck with the timing light. It was interesting to note that Bob West QUESTIONS the refurbishing timing set up. Did he say if advancing the timing will help ? Gord |
| Gordon Harrison |
| Gordon No. He (James - Bob's mechanic) said that my set up is already too far advanced at static. The profile on the distributor (weights and springs I guess) is no good. It will either be okay at idle & low revs and rubbish at high revs or vice versa. He recommended me to discard the distributor and revert to my trusted Metro version. Larry Yes, I agree. Unforunately the baseplate on my Metro disributor is different and will not accept the Ignitor (Pertronix) version I have, so it's back to the points for the time being. Off to the garage to do the switch. Steve |
| Steve Gyles |
| Steve, Why did you fit a Metro distributor in the first place? Is it a good substitute for the original or is it only good for your modified engine? I have got electronic ignition on mine but carry the old points in case it goes wrong (Insurance). I keep thinking it would be easier to change the distributor at the side of the road than the points plate etc and Metro distributors are easier to find than A ones. |
| Malcolm Asquith |
| Malcolm Bob West reckons it's an excellent distributor for the MGA and MGB engine, having the advance curves as near as damn it correct for both engines. I have just come in from the garage after refitting it. Tuned up perfectly with all the right timing references spot on. Exhaust sound also back to normal. Road test in the morning. I feel optimistic. I will put electronic ignition in it when her indoors gives me another £70! Steve |
| Steve Gyles |
| Steve. To answer some of your questions. The reason a sticking piston results in a rich mixture is that is the design of the SU system. When the throttle is opened, there is somewhat more vacuum applied which results in more fuel being drawn from the jet. That is why there is oil in the pistons and the dampener rod works against that oil to slow the rise of the piston, thus, providing the same effect as results when a carb has an accellerator pump. As to timing, about 32 degrees, overall, at about 3,000 rpm is the generally accepted standard. Examining the advance curve of the Lucas 25D4 distributor used in the early MGB provides a good starting place when setting up a distributor. (Specification number 40897.) This might provide a starting point for your research. Without knowing what mechanical advance stop is in your distributor, it is hard to make any comments about what is happening at any other RPMs, nor is it possible to comment about what the curve is. It needs to be tested on a distributor test machine and the actual curve plotted. The curve, as seen on the engine, is a combination of the distributor's mechanical advance curve and the amount of static timing dialed in. Frequently, when old, fatigued springs are involved, as in an original distributor still in use, the mechanical advance comes on too quickly due to the inability of the springs to restrain the weights which move the points cam in relation to the points base plate. Thus, when set dynamically, at a high engine rpm, the low rpm timing can be over advanced. The same thing can happen when setting the ignition timing statically as that is the "dialed in" portion and only works properly when the mechanical advance mechanism is in proper order. Gord. A test light is used to set the static timing, not a dwell meter. The dwell meter is used to read the effective dwell (time the points are closed and charging the coil) of the distributor. This is only governed by the condition of the points and their gap. The use of a dwell meter will only tell you if the points have about the correct gap. The use of a test light will tell you when you have the car statically timed to any specification. Les |
| Les Bengtson |
| Thanks Les. Road test this morning with the Metro distributor was fine. Feels like the MGA I have been used to for the last 9 years. I have now decided to go back to the people who reconditioned my original distributor to find out in detail exactly what they did. Steve |
| Steve Gyles |
| Steve, Thanks for the info. I will go and look for A Metro distributor. The question now is which one. I am assuming they vary across the range of engine sizes etc.? Malcolm |
| Malcolm Asquith |
| Malcolm I suggest you give Bob West a ring. He will tell you what you need. 01977703828 Steve |
| Steve Gyles |
| Steve Just read the comment on the use of a Metro distributor for the MGA - what type did you fit , was it a straight swop or did you just use some of the parts? A few more details would be good. John |
| J H Cole |
| John I bought the unit complete from Bob West 9 years ago. I am just going away for a few days. If you need a quick answer may I suggest you give Bob a ring. Steve |
| Steve Gyles |
This thread was discussed between 07/11/2006 and 11/11/2006
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