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MG TD TF 1500 - Engine Question
thanks for the help on my previous question (starter issue). The problem was with the distributor, which has since been solved.
New problem today and am hoping for the right insight again.
It's been cold (temps not above 30F) until this weekend, and today it was 55, so I wanted to run the car. I set the choke, turned the key, let the fuel pump click click click its way, and started.
The engine ran very high RPM at idle, so I pushed the choke in, and it really sputtered, and nearly died so I pulled it back to 50%. Again, ran at high RPM, but gradually over the next 3 or 4 minutes, the idle speed got weaker and weaker, requiring me to increase the choke just to keep the engine running. With choke all the way out, at 5 minutes, RPM steadily dropped, even with full throttle, and then finally died.
I opened the bonnett,started the engine again, and exactly the same pattern - 3500 Idle RPM with 100% choke, 2000 RPM at half choke, and the gradual death of the engine, even increasing choke/throttle.
Apologies for the basicness of the question, but am a newbie, so your experienced suggestions are very much appreciated.
|Do you have "tickler" pins in top of the fuel bowls?? If you do have them, did you notice that they were dropping as the engine quit?? ie. running out of gas??|
thanks; I don't have tickler pins on top of the fuel bowls.
I would suggest that your fuel pump is not strong enough to keep gas in the float bowls with the engine running.
|G. L. Raham|
thanks; I had considered that the fuel pump MAY be the culprit. Is it possible that the pump can be OK to fill/start, but cannot maintain sufficient fuel to keep it running? Is there a good test I can do short of getting a new pump and then finding out that that is not the guilty party?
|Is this a new problem - ie, did the engine start and idle OK before and nothing has been touched?|
3500 idle speed means the thing is getting a lot of air. Either the carbs are set up horribly or there is a large vacuum leak. Pulling the choke out more would compensate for a vacuum leak, but as the engine warms up it wants leaner mixture, so it could be wet fouling the plugs. After it sits a few minutes, the plugs dry off and it fires again. But this can only continue a few cycles before the plugs are fouled permanently.
Fast idle should be around 1500 rpm at 1/2" cable pull, and only at this point should the jets start to move to actually enrich the mixture.
If the pump clicks as much on the restart as on the first start, then it is running the float bowls out of gas. Even a very feeble pump should be able to keep up at idle though. Might be a vacuum in the tank from a blocked vent, though it usually takes more running for that to show up. Might be a mostly clogged line to the pump. Book spec was one pint (imperial) per minute; run the fuel line into a jar and see.
Your fuel pump may be strong enough to fill the float bowls without any demand from the carbs. A soon as the motor starts, the pump cannot keep up. There is a test you can do without removing the pump. Disconnect the fuel line to the rear carb and pump gas into a quart size container. According to the Work Shop Manual, the pump should deliver one pint of fuel in one minute. If your pump cannot deliver at this rate, then you have the answer to your problem.
This can be a dangerous test as you have the gas in close proximity to the starter switch. I would recommend that you cover the switch and keep the open end of the fuel line well away from switch.
|G. L. Raham|
|David - See the article, Fuel Delivery Troubleshooting Guide in the SU Fuel Pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ for tips on finding problems in the fuel system. I agree with Fletcher, the problem sounds like something other than fuel delivery. You might want to check the exhaust pipe to make sure that the is no obstruction in it (like a mouse nest). Cheers - Dave|
|George, FRM, thanks.|
I disconnected the hose from the back carb, put it to a glass container, and turned the key to get the pump going. It clicked (as usual) every 4-5 seconds (click...click...click) spitting out fuel. After one minute, the fuel in the (354ML/12OZ) container was about 10% full, so rougly 35 ML. One imperial pint is about 570 ML, so that's less than 10% of the spec.
I wonder if the pump is bad, or there is a line block?
FR- I confess to being a bit confused by your answer; if ther is TOO much air which is the cause of the very high idle speed, then having more choke ought to make it better, and not worse, since less air? With full choke, the idle goes crazy; with no choke, it stumbles and wants not to run.
I know the carbs are badly in need of a tune, and the fast idle adjusted (it was running too high), and indeed, the plugs are wet after running. I need to get this issue set before I can do the tuning, since I need more than four or five minutes, with some of that manually lifting the pistons to let more fuel in so it doesn't stall.
In some manner, blow air into the fuel line after disconnecting from the pump. If the line is open, you should hear air bubbles in the gas tank.
|G. L. Raham|
You need to study basic SU theory. There is no "choke" as such. On SU, cold start enrichment is done by lowering the jets, giving more fuel. "Choke" or "Strangler" as the Brits did say, cuts the air supply - that is the common upper butterfly in American carbs. Both systems have a linkage arrangement that cracks the throttle plate to give fast idle, an entirely different function in addition to "cold start enrichment".
While raising the piston manually does in fact raise the needle, causing an effective increase in jet size, the airflow increases more than the fuel flow, so the delivered mixture actually becomes leaner - but there's more of it.
With the line disconnected, the pump should click about 1-2 per second (heartbeat rate!), so you have a restriction in the line on one side or the other. If there is a filter, it should be on the carb side of the pump, as one on the suction side can damage the pump such that it will work but will burn points up rapidly. Barring add-on filters, remove and clean the pump screen (the brass plug on the pump that is not a union). I do not know if TD has a screen on the tank pickup; if so, that may be blocked or there is a blockage in the line.
If the pump were sucking air it would pump very rapidly, maybe 5+/sec. It is unusual for an SU pump to work, but slowly, from a pump fault.
| There's "usually" a fine filter in the pump- rust can plug this. You can check/blow out/replace it by unscrewing the plug in the bottom... that is IF THE PLUG COMES OUT!. |
I've had rust plug the strainer that is "usually" on the fitting sticking up in the fuel tank; ...and tank fittings, too.
I say "usually" because I've discovered these items removed. I'll blame "previous owners."
I've had to routinely scrape fine rust out of float bowls, too, until finally breaking down and coating the gas tanks.
I realize, caked up float bowls won't restrict delivery with line disconnected, it is just that fine iron oxide can build restrictions in tank fittings, line, pump and carbs, all at the same time. You can ask my wife!
|All- thanks for the helpful suggestions; an update:|
I removed the line from the fuel tank to the fuel pump and switched on; as predicted here, the pump did then run very very quickly. Disconnected the brass plug from the bottom, took out the screen and blew some debris out, reconnected the line, and still slow. Removed an after-market fuel filter, and again, it pulled rapidly. Blew out the filter and hose, and siphoned some fuel manually from the line from the tank, and the fuel came freely. Re-installed the whole thing, and now the pump is behaving as it should - about 1-2 clicks per second, and good, solid clicks. The carbs bowls were quickly filled.
I think my fuel problem is solved, so thanks for the tremendous help.
F- thanks also for the education on the "choke" (sic) mechanism. Most enlightening FYI, the fast idle screws were very, very far in, so that is the root of the very quick idle. Turning them out seems to have fixed that problem as well.
|Probably the best description and tuning methods for SU's is in the New England MGT Regiter's book "The T-series Handbook". Greatly detailed, yet easy to understand. Other chapters deal with ignition, general engine functions, etc. George|
|I'd say your problem is not solved, but the issue at hand may be resolved.|
If you blew out the line/filter and it passes fuel, the problem will return. Eventually the rust will comeback to plug the system again. You may get years, months or days of clear sailing.
I've removed tanks and shaken washers & nuts around for many minutes, then blown/flushed rust out. That in itself will restore new life to the tank interior, but a coating wouldn't hurt.
Long periods of inactivity account for ingress of moisture and corrosion.
Magnets placed down at the bottom may collect some rust, but I've watched that brown stuff zing right past a powerful neodymium rare earth magnet like it wasn't there.
Bottom line is- the source of your problem is all behind you, the gas tank, in my humblest of opinions.
|David - If I read your last post correctly, you have an after market, in line filter between the tank and your fuel pump. If that is correct, I would suggest that you get rid of that filter before it damages your pump. A modern day in line filter will trap the finest of rust particles, ones that would be passed by the filter in the pump, and will clog as you found. When the inlet line to the pump is clogged, it causes the pump to stall in a current on condition, which will eventually burn out the internal swamping resistor around the pump's coil. Once this happens, there is no arc suppression for the points in the pump and they will burn much quicker than normal. Worse, when the burned points are replaced, the new points will burn very quickly. The SU fuel pump and SU carbs are very tolerant of fine rust particles, so a modern day, high efficiency filter is not needed, but you do need to clean out the tank and have is sealed or treated with zinc phosphate to keep any further rust from forming. Cheers - Dave|
This thread was discussed between 16/01/2010 and 18/01/2010
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