MGB-GT 72 B GT Running On

[MarkS]

Hi All,

Fairly new to the wonderful world of MG, but just wondered whether anyone can offer any advice on a problem I am having.

I'm running a '72 B GT as my everyday car, and the car is fitted with a 'fast road engine'. When hot the car runs on when the ignition is turned off. I've heard that this is a farily common problem, but wonderd if there is anything I can do to resolove it. I've checked the normal things like timing and mixture.

Secondly, can anyone tell me exactly what difference a 'fast road engine' has to a normal engine, and do I need to be setting the timing differently to what the owners manual says.

Thanks in anticipation.

Mark S

 

[Wingtip]

Hi Mark! Welcome to the forum! I am not an MG expert by any stretch, so I am sure others will offer more sound advice, but I know one thing that can sometimes cause the engine to run-on when you shut it off and that is excess carbon build up on the piston tops and in the combustion chambers. I had this happen on a family car once and cured it by running some top end" cleaner through the motor. That stuff makes the car run horrible while it is going throught the system, and lots of black smoke will come out, but it will clean of the chambers. As I understand it, when there is too much carbon in the cylinders, it will glow like cola embers after you shut off the engine and this, in turn, will continue to ignite and fuel/air mixture that may still find its way into the engine after you have turned off the key. No guaruntee that is your problem, but something to consider.

George

 

[aerog]

Check the timing -- 70-72 MGBs didn't have an anti run-on valve, later cars did, but I understand there are add-on kits available. They effectively starve the carbs of fuel ceasing the run-on.

VW "solved" that problem by adding a solonoid & needle-valve assembly to the carb that opens with the ignition on but slams shut with it off, killing the fuel to the carb.

[ 06-23-2002: Message edited by: aerog ]</p>

 

[aeronca65t]

Welcome!

As mentioned, timing (usually "late" or "retarded") will cause running on.

Also, "coking" of the head (also mentioned) could be the cause...this is especially true in a country where leaded gas is sold...lead can help to form these deposits.

Quite often, I've found that an idle speed that is too high can cause "running-on" (over 1000 RPM, for example). If your car is idling at 600-900 RPM, you're probably OK).

I've never heard of a "fast road engine"....I doubt there's any *exact* explaination of what this means...but I'd assume it means that the engine has been mildly modified but is still capable of road use. This could involve any or all of the following: semi-racing camshaft, modified ignition advance curve, higher compression pistons, bigger carb(s), ported head, header, lightened flywheel, etc. Some of these modifications might profit from some changes in timing....others might not. If you could find out what *exactly* was done, you could contact the vendor (of the modified part) and see what they advise. Barring that, you might try setting the timing "standard" and then test-driving it. Then advance the timing in about 2 degree increments, and test again (be sure you're doing this with premium-grade fuel). Judge or time your acceleration performance (ie: 0-60 MPH) and listen for "fuel-knocking" or "pinging" ("pinking", I think, in the UK). Advancing the ignition timing should help perormance, but may cause "knocking". If the you hear this knocking, you've gone a bit too far.Keep trying, but don't go beyond 6 degrees past the standard setting (in my opinion).
On racing or semi-racing engine, many folks set the "total advance" rather than the ignition timing at idle. A good figure to start with in your car would be about 35 degrees (total advance) at about 3500 RPM. You'de need a "degreed timing wheel" to do this.

By the way, if your car has oversized carbs or a very high lift camshaft, it may *require* a high idle speed to keep from stalling.....meaning that "run-on" is inevitable. This is one of the many reasons why an unmodified car may be more suitable for street use (one of my cars is quite modified.....I have a choice between "running-on" or unreliable idle).

[ 06-23-2002: Message edited by: aeronca65t ]</p>

 

[vette]

Hello Mark, Aerons last post covers alot of things I might have said. But just to add my 2-cents, And engine runs on for a variety of reasons some of which are:
-Just plain too Hot. Make sure the cooling system is doing its job. Make the cooling system as efficient as when it was new. I tend to stay away from after market stuff. Just make things as new.
-a lean carb mixture
-timing too far advanced, if the carbs aren't adjusted right but the idle seems to be where it belongs, the idle may be supported by little too much advance timing. this might make things seem to work ok but with the time advanced it doesn't want to stop when you turn off the key. Use The small micro adjustment knob on the housing of your distributor and just turn it in the retard direction about 2 clicks. see if this slows the engine idle speed. (idle should be happy around 600 or 700 rpms.) then does this solve the run-on situation. If engine becomes very under powered with this time adjustment then this isn't the answer.
- just a poor state of tune with the points and plugs can cause this problem as well because if the carbs where adjusted with the poor points and plugs chances are the carbs could be alittle rich causing alittle rich fuel feed after shut off.
- just too fast of idle setting will cause the problem, if the engine is stock it should idle at 700 rpms anyway, then it should shut off.
- and i have seen engines that just want to do this more times than not, in which case i have seen drivers just live with it and compensate in shut down proceedure such as, when you stop: put the shifter into nuetral, wait a second, depress the clutch and turn the key off at the same time, this simutaneous depression of the clutch puts a load on the engine just as the ignition is turned off and the engine quits abruptly. but I still believe that if an engine is tuned and adjusted properly it will stop on it own accord with the key the majority of times.
- as to the term "Fast Road Engine" it is not common to me, but it could refer to an engine which has been modified with the minimum of a different cam and different or modified (rejetted) carbs. If this is the case then run-on she may anytime she feels like it. All of the things I said above still apply but to more or lessor degrees. things like timing can still slow it down, and carb adjustment should be as effecient as you can make it. I happen to have a Healey with a modified engine in the area of cam and carbs. It took me alone time to get it to like to idle around 1000 rpms. I finally kept it cool, did meticulous carb adjustments and ultimately did not have to sacrafice timing and was able to keep it at a consistent 900 rpms. I was happy with that. Then the low restriction muffler I had gave up the ghost and in a pinch I put on a stock muffler and that dropped the idle rpms to 800. I use to have some run-on, now I rarely do even on the hottest days. Keeping the engine cool is very important.
Well there's my two cents.
Good luck,
Dave