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MG MGB Technical - Is overrun harmful ?

If you have read my post below my apollogies because I am probably doubling up here. I just wanted to find out if overrun is harmful to the engine. After I rebuilt my engine, it seemed like I had to choose between pinging, which I am assured is very harmful or overrunning.
I have asked the question once before on this board and the answer from many people was no, it isn't harmful.
Lately it can go on for 30secs to a minute, and is pretty embarrassing ! It usually ends with a light puff of smoke coming from the bonnet and a horrible smell. I had the bonnet ope today and the motor looked like it was shaking violently !
Any advice would be appreciated.
Regards Craig
Craig

One tip is to floor the throttle pedal after you have switched the ignition off. this will draw air through the carbs and cancel the run-on.

This is essentially the effect of an anti run on valve (which never quite work as intended). Also allow the engine to idle briefly before switching off, i.e. do not purposely rev the engine hard and then switch off - apart from this having no purpose except for leaving unburnt fuel in the cylinders, it can also encourage run on.

Otherwise check out paul hunts webpage as he effected an interesting solution to run on....

~PHIL
Phil

Where would I find that web page Phil ?
Thanks for your tips, I'll try them. I have tried flooring the pedal and didn't think it helped much?
I thought I was actually pumping more fuel though (but then again mine has an electric fuel pump that somes on with the ignition.
I'll take your advice and let it idle for a little while before turning off and see what happens.
The way you are talking Phil, sounds like it is a normal occurrence. Would adjusting the timing help or is it just the way fuel i these days. It never used to be this bad before the rebuild?
Regards CRaig
Craig

Craig-
Paul Hunt's website can be found at http://www.mgb-stuff.org.uk/
Steve S.

Providing your electrical fuel pump is not live when the ignition is off you will not draw significant fuel through the carbs, although i suppose some of the fuel in the float chambers may be drawn into the intake... Occasionally mine runs on further if i floor the throttle but most times it will stop the overrun.

Couldn't honestly say whether the ignition timing would alter these characteristics indefinitely - i would suggest it depends on the state of tune on your engine, and whether it is in standard or fast road form.... I get by by running Shell Optimax fuel which is the highest octane fuel we can buy here at 98RON, normal super unleaded fuel is 97RON i use this if i cannot find Optimax, i very very rarely use 95RON as my car throws a fit over it ;-> the extra 5p per litre is something i do not mind paying because the car performs so much better with it.

You can buy octane boosters but i have no experince of these. I am going to attempt them this year when i travel to Le Mans, as last year i noticed a slight decrease in performance due to the high ambient temps, despite running 97RON fuel all the time whilst over there.

You don't say if you use unleaded or leaded fuel but i assume unleaded. There is some arguement for saying that running on is caused by soot deposits at the valve openings and within the cylinder bores. Unleaded fuel burns in a more sooty fashion (my brother has an old carb fed vauxhall which he took for a rolling road tune up last week. the tuner could tell by his exhaust tailpipe that the last tank of fuel my brother put in was unleaded!!) - i do not know the cause of this but assume it has something to do with what they put in it instead of lead - this leads to soot deposits in the engine which conduct heat and form hotspots. These most likely cause the post ignition detonation of the fuel mixture .

Shell claims Optimax to have additives which clean this soot and therefore, combined with the higher octane, may help to reduce running on....

Just some thoughts for you....

~PHIL
Phil

I've tried all sorts of conventional methods to rid my car of run-on but all cures have been either
unpredictable (sometimes it'll run-on...sometimes not), or temporary. De-carboning the head and
piston tops lasts only a month or two, at best.

I think it's the lousy fuel everyone's been passing off as "premium". Remember when premium grade
gasoline used to mean 101 octane, or more? High compression engined MGB's are supposed to
sip on premium 98+ octane. In the US, "premium" now mean 92 octane, and this sometimes dips
lower down to 91 octane during seasonal crude oil shortages. 91 octane was considered "regular"
grade fuel back in the days when the MGB was in production.

Just prior to shut-down, I engage a high gear (3rd or 4th), then gently lift my foot off the clutch while
simultaneously switching off the engine. Using a high gear saves the car from lurching about.
With time and practice, I've gotten the hang of this down to an art, and now I can engage the clutch
during the last, few, dying engine rotations just after the ignition is switched off.

Of couse, this is not practical for parking valets and guest drivers....but then, nobody is supposed
to be touching your baby, anyway ...except you (...right?).
Daniel Wong

But what is worse? Run-on or forcing the engine to stop turning?
Steve Simmons

Other than a little extra clutch wear, I think forcing the engine to stop turning would do no harm at all. I do the same as Daniel, and in fact no longer even think about it. It's just part of shutting it down. If I didn't, it would tend to run on about 1 time in 5....
Rob Edwards

I actually use lead replacement fuel here in Australia, which I must admit doesn't seem to be too bad. Before I had the engine rebuilt, I had been running Optimax and then went back to the LRP and did not notice any difference between the two pinging wise.
Before they brought out the LRP, the super was terrible and you could really tell the difference between it and the premium fuels but the LRP here does seem to be good.I though hey must have boosted the octane up to silence any knockers when they introduced it but to date seems good. I would still use the Optimax if I thought it was better as cost isn't an issue but I was unsure if it was a good idea to use lead additives. What do you guys use with your fuels?
Craig

Craig,
I can't speak for anyone else, but I run straight pump 89 octane (R+M/2) -- usually the cheapest I can find....
Rob Edwards

Here in Ohio I use 94 Octane Sunoco. No problems whatsoever. My timing is probably slightly retarded, as I set it using a vacuum gauge and then backed off everso lightly to eliminate pinging under load.
Andrew Blackley

I discovered Phil's solution some time ago. If the engine is running on, flooring the throttle causes the engine to stop sputting and run "properly" for half a second or so until the unused fuel in the engine is burnt up. And yes, it's very embarrassing to shut off the car and have the engine gasp and sputter - heads in the parking lot turn and I'm sure everyone is thinking "British cars..."
Adam
Adam Birnbaum

When I bought my 73 B, the previous had been using "regular" grade fuel and the car had serious run on problems (10 secs). I switched to 91 or 92 octane, (the highest we can get in Canada) and it made a huge difference, (shuts down immediately). I have also tried some octane boosters but make sure you get a good quality one.

Mike
Mike

To directly answer your question , yes , it is _extremely_ bad for an engine to ' run on ' (diesel) as it is similar to hitting the piston crowns with a hammer , this if course can ruin the connecting rod bearings and also results in cracked pistons from time to time .
I don't see where you mentined what engine / year car / carby setup you have but 90 % of the time I find this to be a simple tuning problem on non- pollution controlled cars , of course late model U.S. spec cars are different but normally setting the timing correctly follwed by balancing ghe carbys will stop this from happening .

Gently stalling the engine in 4th. gear when you kill the ingnition is the way to go untill you sort it out .

-Nate
Nate

I did not see any mention of what spark plugs you are running. If you are running a hot plug it could work similar to a glow plug in a diesel engine. I am running N9YR plugs in my '69 B. It will diesel in hot weather, this generaly only lasts for one or two seconds. I think that dieseling is just a characteristic of high compression engines. As far as ignition timing I am running aobut 7 degrees advance at Idle.
Robert

My 79 MGB with Weber carb runs on occasionally. Burning regular unleaded gas. I use the 4th gear let the clutch out method at shutdown to avoid run on. I do it almost as a habit now. No detriment to the drive train as yet.
Chuck

[Quote][B]High compression engined MGB's are supposed to
sip on premium 98+ octane. In the US, "premium" now mean 92 octane, and this sometimes dips
lower down to 91 octane during seasonal crude oil shortages. 91 octane was considered "regular"
grade fuel back in the days when the MGB was in production.[/B][/Quote]
91 octane and they call it "Premium"? What?
Andy

My roadster has always tended to run-on even on leaded, but with the change to unleaded, even with a so-called octane boosting lead-replacement additive it was very much worse, as was pinking. This is the second year now since I developed the solution (for me, YMMV) mentioned above and it continues to work perfectly. In fact I now run a slightly higher idle speed to counteract the gradual drop that occurs when stuck in traffic for a long time. Before I ran as low an idle as I could get away with as higher idles made the running-on much worse, which meant that I had to pull the choke a little to prevent stalling in traffic.

http://www.mgb-stuff.org.uk/wn_fuelframe.htm then click on 'Running-on'.
Paul Hunt

My car also runs on, but I do use the cheapest unleaded I can find. I was going to install an anti run-on valve, but I see none of these posts has recommended it. Why?

Andrew
Andrew

Andrew

Check out Pauls Website - he has tried it and then engineered a better solution.

~PHIL
Phil

Andy,
The numbers _may_ not be as bad as you think. As I understand, the UK uses RON (Research Octane) whereas in the US gasoline is rated as the average of RON and MON (Motor Octane). MON is usually lower than RON, so this would skew the ratings downward. That's why I said "89 (R+M/2)" in my post above.... Generally we have three grades available -- 87, 89, and 91. Setting my '69 B's timing to around the book specification tends to give me a little pinking on 87 and negligeable pinking on 89. Of course, I consider the factory timing spec simply a baseline from which to tweak...

Have a look here for explanations of the two scales:
www.unionbayracing.com/fuels.html
Rob Edwards

Like Phil says, I tried the MGOC valve as it was intended to be used but it did absolutely nothing for runon, it didn't even stop the engine even though it was allowing copious amounts of air into the inlet manifold. So I thought laterally and used it like the North American valve with total success. I did offer the MGOC details of what I have done but received, perhaps unsurprisingly and not for the first time, no response. Between those two I did consider using the North American valve itself, but it would have required more work.
Paul Hunt

The only way to stop this is the car in gear solution. the reason the runon valve doesn't work is that the engine is in fact running backwards in many instances. thus admitting air to the inlet manifold is now solution - the engine is drawing air from the exhaust side of the system! how it gets the fuel I don't know, except that it is probably " reverse scavenging " it during the overlap period

It is not curable - it is an inherent feature of the design - my 1973 B did it from new- and worsened by the drop in octane value over the years - thus prevention by engaging gear is the only solution

Chris
chris

I watched a Dodge Challenger with a new 360ci 385 hp crate engine run in reverse after being shutoff. This car had alot of overlap in the cam- real choppy idle.
Robert

It only starts running backwards after it has 'dieselled' using fuel and air drawn from the intake, which is why an engineered solution to prevent that initial 'diesel' does work.
Paul Hunt

I often hear people remark about "the good old days" when you could buy 101 octane gasoline. Part of the confusion is that, back in those days, I believe that the gas companies only used the Research Octane method. As Rob Edwards points out, RON will always yield a higher number than MON. Today gasoline is advertised (if you call that tiny printing under the price advertising) as the average of RON and MON. This means that even if Sunoco still sold the same gas that was their 260 grade (101 octane), it would probably only be in the mid-nineties under the current nomenclature.

My car doesn't run-on very often. When it does, it usually only makes one last "vroom" and quickly dies. I usually turn the ignition off and, just as the motor slows almost to zero, I let out the clutch with the tranny in 3rd gear. I use 3rd simply because that puts the shifter forward and is a little less "in the way".
Paul Noble

This thread was discussed between 20/04/2003 and 24/04/2003

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