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MG TD TF 1500 - Xpag compression readings

I am suffering 'running on' with my 1250 Xpag engine (TF/30582) I have done all the usual checks and even purchased some super recon. carbs from Tony Hebden. My pal thinks that I need to decoke the cylinder head (which is a Peter Edney unleaded one) purchased in 1991? by a previous owner. Today I undertook a compression test on the cylinders which were between 170-180psi within the 10% limit which I always believe to be OK. Are these readings normal for an Xpag please? Any comments on the above would be warmly welcomed. Thanks, Peter.
P J Wilgoss

Has the running on just started...started after doing anything to your engine/carbs? Is your engine running hotter then normal? Is your timing set correctly ...running on I think has more to do with mixture and/or timing?
gblawson(gordon)

Those compression readings suggest to me that you need to use a higher octane fuel. I would try this before you do anything else. This will certainly improve running on and pinging.

Cheers,

Paul.
Paul van Gool

Hello Gordon and Paul, thanks for the prompt replies to my query. The timing has been set up with a timing light to the recommended setting, the engine temperature is perfect at about 80degreesF I run on British 'super unleaded' I have replaced the plugs with NGK equivalent to Champion NA8s carb needles are standard. The car starts on the button and runs well with an even tickover and revs freely. Yours, frustrated.PS Paul, do you think the compression readings are high? Peter
P J Wilgoss

A post script to my previous ramblings, the running on just started one day and all the stated work followed as a result of this, but as you will read has not helped! Peter
P J Wilgoss

Peter-
The compression readings are high, indicating that things have been done, but they haven't changed so it's not the problem.
Depending on how long this has been happening, various likely possibilities:
Very short term (one or a few tanks from the same source) - bad fuel, misidentified or changed from what you think you are using.
Longer - Timing disarray due to distributor advance trouble, or lean idle condition (airleak), or simply too high tickover. What is the tickover speed? 800rpm or so is good, over 1000 gets fuzzy.
Since something has been done to raise the compression, and maybe other changes, it's conceivable that plugs and/or needles might be better changed in some way, but again, if it worked earlier it should still do.

FRM
FR Millmore

Compression readings are pretty high. Previous suggestions are right on & should be checked out. If it all of a sudden appeared out of nowhere and never happened previously, the fuel could be the culprit.
Check your plug color- if porcelain is white, you're running lean and the combustion chamber is hot enough to diesel. Tweaking the mainjet nut down will richen it up and not cost anything. If you have black plugs, maybe it has been running so rich you've built up an extreme amount of carbon.
It might sound old fashioned, but a vacuum gauge might give a clue. For instance, last week, my wife had an intake gasket deteriorate on her TD.
Make sure your tank pickup, fuel filters and pump are clean and rust hasn't built up in the float or needle areas. I had that all happen constantly until I coated the gas tanks.
Retarded ignition could also lend to hotter than normal running.
If it isn't pinging, you are lucky. Over adavanced would promote detonation, but not really have anything to do with run-on, since the ignition is off and out of the picture.
jrn Northrup

tidbit of info, from Glenn's "MG.Morris Magnette Repair and Tune-up Guide"...
"Each SU has a lifting pin (not quite correct)for checking the air-fuel ratio. To check the mixture, lift the piston about 1/32" with the lifting pin, or with a screwdriver inserted into the air intake port. The engine should speed up momentarily slightly if the mixture is correctly adjusted. If the engine speed increases and continues to run faster, the mixture is too rich. If the engine speed decreases, the mixture is too lean."

Note, mixture problems are not always the fault of carbs; they're caused from fuel shortage and airleaks, too.

Plug your diagnostic terminal into the ECM and check engine faults when all else fails! HAH!
jrn Northrup

Peter, I still suspect the fuel. How old is it? ULP has a very limited shelf life unlike the old leaded fuel. If the car has been standing for months it is likely that is the problem. I have an old motorcycle that had really old fuel in the tank. Wouldn't run properly until I replaced it with new fuel.

Cheers,

Paul.
Paul van Gool

yes, thanks for all the good information. The car is run regularly and I have filled up with UK super unleaded on each visit to the petrol station (which is my nearest at 4miles a Waitrose supermarket). I fill up my midget and MGB GT here as well with no problems. Peter
P J Wilgoss

Peter.

I don't have the answers but, as I'm not a million miles away, I wouldn't mind swapping notes if you've a mind to, particularly any experiences you might have on expert local help and parts suppliers. I acquired a 1953 TD in July and I'm on a steep (re)learning curve, having left hands-on, full-blown car maintenance behind with my late '70s Capri.

Cheers,

- Tom.
Tom Bennett

Peter - The following statement caught my eye - "The timing has been set up with a timing light to the recommended setting"

When you say the recommended setting, recommended by who or what? The shop manual recommends a TDC setting, using a static timing method. If you used a timing light with the engine running and set the timing at TDC, the result would very likely be an excessively retarded spark, which would in turn, require a higher setting of the idle screw. An excessively wide of the throttle plate at idle can cause run on because there is nothing to limit the flow of fuel through the carburetor. Cheers - Dave
David DuBois

I'm having a hard time buying this!

I've had run-on for years. I've done EVERYTHING to eliminate it, except to change the camshaft for another profile.

Compression is good (130 X-the board), and timing bang on (pertronix) I must admit that I run a bit of advance, but not to starve the valves or cause any other problems.

When the ignition is shut off, it also shuts off the source of fuel, and supposedly, the source of spark, yet it continues to motor on, eventually running backwards and expiring. I know this can be very hard on the engine.

My cylinder head is a polished work of art ... no sharp points anywhere. If I shut off at speed and examine the plugs, they're absolutely perfect. I've had everybody plus his brother work on it ~ nothing changes.

Highly frustrating, but I've learned to live with it.

Gord Clark
Rockburn, Qué.
Gordon A Clark

Gordon,

Shutting off the ignition doesn't shut off the fuel. You still have plenty of fuel in your float bowls, more than willing to suck into the engine to ignite as the engine staggers through its run-on cycle.

Run-on such as you are experiencing is caused by hot spots in-lieu of spark plugs igniting fuel in the cylinders. Not as efficient as spark plugs but the fuel will ignite just the same.

Dave DuBois is exactly correct when he says that poor timing will cause wider throttle openings, sucking in more air and making the problem worse. In your case you say you are advanced which may counter that slightly, but if you have excessive compression ratio due to a shaved head, low octane fuel, and a lean running condition you can expect the results you are seeing.

Just for fun last week I leaned my carbs just one flat each. My car did not get appreciably better mileage, but it did suffer from run on, wouldn't start easy (was harder to set with the choke), idled 100 - 150 RPM less and ran 4 degree C hotter. I went one flat each down and all those symptoms disappeared.

My engine is .060 over, standard porting and valves, points, Standard cam, the head and block are slightly decked (wear and corrosion dictated this) and I calculated my CR as about 8.5:1. I run about 87 octane fuel which is a combination of RON and ROM figures here in the states. I get 25 mpg per US gal. My carbs are set with the fuel level .160 in. below the bridge and the jets are about .073 below the bridge. My idle is 650 RPM and my timing is 11 degrees before TDC at that idle.

Have you ever thrown a pint of Sea Foam in the tank? While your plugs look great, you could have some hot spots (carbon) built up.

I hope this helps.

warmly,
dave
Dave Braun

Run on is an issue at idle and not at speed. Try the lifting pin with hot engine at idle (as per my previous post above).

A vacuum leak will screw up idle adjustments. Doublecheck manifold hold down nuts. Spray around intake mating surfaces with starting fluid.

If all else fails, pull out enrichener (choke cable) and see if it shuts off cleanly. As a side effect, you might notice quicker starting.

Until you get the problem squared away, step on the brake, switch the key off and ease the clutch out to stall it.
jrn Northrup

peter, an alternative to spraying starting fluid to find and induction leak is an unlit propane touch. regards, tom
tom peterson

I wish folk would quit complaining about run-on without telling us the usual idle RPM.
Too fast idle is the #1 primo cause of run-on, regardless of whether the fast idle is due to timing, mixture, air leaks, or just the idle stop screws or other linkage maladjustment. Note that Dave B states "My idle is 650 RPM"; at that speed any engine should shut down immediately. Set it at book. As I said above, 800RPM is generally fine and up to 1000 usually works. If your engine won't idle at these speeds, fix it.
The second main cause is heat. Assuming it's not really HOT, it should still shut down fine, IF the idle speed is correct.
The third most common cause is trying to shut the engine off the instant you stop. It is (internally) hot AND it hasn't actually settled to idle yet; give it 30 sec. I used to spend almost as much time telling customers how to stop their engine as I did telling them how to start it.

FRM
FR Millmore

I'd expect if the plugs were too hot, you'd experience pinging but try sticking in some really cold plugs and see if that resolves the issue. It can't hurt.
jrn Northrup

The nice thing about run-on in a stick shift car is you can just shut it off in gear and let out the clutch. Unlike the early 70s automatic tranny emission contolled nightmares that you could walk away from while they knocked and pinged away.
George Butz

After reading your (collective) inputs, I realize, I'm guilty of a lot of the things brought to light.

First, my carbs are way overdue for a rebuild and I'm sure I'm sucking air somewhere. The jets are probably off-centre and the floats I'm sure have to be adjusted again (frequently). But most noticably, I do have a high idle - about 1,000.

I'll try a set of N3 tomorrow.

But I still cry when it starts to run backwards, and I do indeed, release the clutch immediatly.

Thanks for all your input.

Gord Clark
Rockburn, Qué.
Gordon A Clark

Gord-
"First, my carbs are way overdue for a rebuild "
>>>Other than maintaining damper oil near the correct level, there is only one thing on SU that needs periodic attention. That's cleaning of the piston/vac chamber, more often on early carbs and or those with poor/no filters. 12,000 mi on an MGB with factory filters, twice a day with no filters on a dirt track. NOTHING else wears or changes for very long periods of time. When I get a "new" car, I go through the carbs, set them up correctly, and NEVER touch them again, 10, 15, 20 years. I have a set of slightly modified HS2 that have been on a half dozen different engines, with NO attention other than initial adjustment when swapped. Slight mixture adjustments get made at change of season, anything else reflects problems elsewhere in the engine.


"and I'm sure I'm sucking air somewhere."
>>>how are you sure?

"The jets are probably off-centre"
>>>They don't move themselves - if they were right to start, they still are. Even if they were slightly off, in the real world it likely doesn't matter unless the needles are worn, which you can see while cleaning the chamber/piston. Severely off means the carb would have had problems from the beginning.

"and the floats I'm sure have to be adjusted again (frequently)."
>>> Again, I adjust 'em when I get "em, and never touch them again, unless I have to replace the float valves.

Following is quote from earlier writings of mine:
""...used SU's you get, it gets expensive when you have to get them rebushed"
The rebushing myth, like most such, has some truth and a lot of undeserved attention and belief. Yes, it's nice to have no slop. But, nearly all the wear normally seen is in the throttle shaft, very easy replacement. The little wear usually found in even the most worn carb bodies can be compensated in the carb setup. I have cared for thousands of SU/ZS cars over my 40+ years of doing this commercially. I have NEVER rebushed a carb, and I have the shop, tools, and knowledge to do so correctly. I have seen a couple that people had rebushed "professionally" that were as sloppy as the average worn one, and worse than what they started with.

Blaming carb body wear for difficulty in performance is generally a way of getting around saying "I don't know how to set this engine up" Blaming the carbs for "going off tune" is a way of not saying "I didn't get it right the last time and now the new plugs I put in are fouled (or cooked)" If someone can't set up an SU or ZS system, they certainly can't set up a Weber, or anything else for that matter. In the rare case where the thing comes out of the box near to correctly set up, then you will no doubt get better running than you did with the screwed up predecessor. Furthermore, setup on a SU/ZS is a matter of adjustments without parts, if you have the right needle. On a Weber or similar, you have to buy arcane and difficult to get $$$ pieces, which you drop on the ground or into the engine."

"But most noticably, I do have a high idle - about 1,000."
>>> A point that few people understand is that setting idle speed too high gets the distributor into the advance curve. Then when the temperature drops and the mixture gets a bit leaner, the engine speed goes up, which gives more advance, which increases engine speed, which gives more advance etc. Same thing in reverse when the temp goes up - richer mixture leads to slower idle, which looses advance, which slows the idle, etc. It should be obvious that sticky or worn advance mechanisms, sloppy bushes, and the like are the prime cause of unstable idle, which is usually why idle speeds are set way high.
Unless you are certain the distributor is perfect, save the shekels on the carbs and put it in the dizzy.

"I'll try a set of N3 tomorrow."
>>>Don't! I take it your are using N5, and they are OK. N3 won't solve your run-on, especially if you give the engine the 30sec cooldown I recommend, but will likely add plug fouling. Better yet, use extended nose plugs, they really are much better and less sensitive to both overheating and fouling. N9Y or equiv ought to do. Check first that the pistons won't hit them, you never know what's been done to these engines today.

FRM
FR Millmore

Hello everybody, this is Peter the chap who started all this very useful and informative debate. I am shutting the engine down through the gearbox most of the time but I have to try it 'on the key' now and again just to see if a 'miracle' has happened - mostly not. I will say that the engine has been timed with a light in accordance with the workshop manual but I am going to take the advice given to try a static timing which I feel (in my water) would be better. The head is a Peter Edney unleaded one so almost certainly would have had a skim as part od that process. This was done before my ownership of the car and would provide the answer for the high compressions I would think. Anyway thanks to you all and 'onwards and upwards' to a good result. Peter
P J Wilgoss

Hi Peter-
Wondered where you'd gone!
Still don't know what your "tickover" is, or was before the problem started.
You haven't answered Dave DuBois' observation on timimg settings - above "Posted 25 September 2009 at 18:58:45 UK time". The Factory manual ONLY has static procedure, so what are you using?
You said, way back: "the running on just started one day and all the stated work followed as a result of this, but as you will read has not helped"
Certainly the compression is the only parameter that didn't change when the problem started, and hasn't been changed by all the stuff you've thrown randomly at the problem.
Following my last comments above, the most likely culprit is in the distributor, specifically the advance mechanism.
All of which comprised my first post above.
Please let us know if or how you solve this.

FRM
FR Millmore

Hello FRM, Many thanks for your continued interest and you are correct that I am not being detailed enough with my counter explanations probably due to domestic pressures and the fact that my old friend (aged 86) has decided to purchase a MORGAN DHC of 1960 vintage for which I am trying to arrange insureance on his behalf. Never mind that, I will add to this thread as soon as possible (ASAP). PS,the Morgan is rather nice!!! Peter
P J Wilgoss

Peter-
Now, that may be the best reason ever given for a diversion from a task at hand!
Good for him, and good for you for helping out. Give him my best wishes & hopes for another 20 years of motoring pleasure. I distinctly recall staring wishfully at a Morgan +4 DH in 1964. My stepmother had a '57 4/4 Series 1, near to the slowest car on earth, maybe! Well a tie with my '54 803cc Morris Minor convertible!

FRM
FR Millmore

Yesterday I checked all possible settings which included the static timing of the engine (contact points just opening at TDC) the car starts and runs very well but still 'running on' when swithing off. I shall be having the cylinder head off over the coming winter for an overhaul and to find out the compression ratio, thats all I can think of. The trouble is we all know that one job always leads to another! Peter
P J Wilgoss

After reading through this thread, I think that your problem is the quality of the fuel. One quick way to test this theory is to add lead substitute to the petrol the next time you fill up. I have done this in the past and found it made a difference.
What will knowing the compression ratio give you? You already have a good idea from the compression test, also you don’t need to pull the cylinder head off to determine it, it can be done by careful measurement with the head in place. Here are some guideline figures, from the XPAG tuning manual.

C.R. Thickness
7.25:1 3.0177” (STD)
8.6 :1 2.928”
9.3 :1 2.8977”

These values assume that you don’t have high compression pistons.


John


52 TD
J Scragg


9.3 :1 2.8977” should be 9.3 :1 2.8966”

Please excuse my typo, also these values assume standard bores, if they are oversize then the figures will be slightly higher.
J Scragg

Hi Peter, what is the octane rating of the fuel you are using? I don't know what British super unleaded is.

If I use fuel of less than 98 octane RON my engine runs on as well. I have a high compression engine too.

Others in our club report similar circumstances. If the compression ratio is increased a higher octane fuel is required. All running on problems I have encountered over many years have been related to fuel.

Hope this helps,

Paul.
Paul van Gool

Hello Paul, thanks for the information. I have assumed that a head converted to use unleaded fuel would do just that without other additives required. However, I will purchase an octain booster and give it a go. Peter.
P J Wilgoss

Hi Peter, I am running a 51 TD which has been bored to +100 thou with a Leystall Lucas head. The CR is over 9 ( I forget the exact figure) and I have tried using standard fuel and the higher octane but it still suffers from running on.

Having tried most things to find a cure nothing works except after a run at normal engine temperature I leave the engine to idle for 30 or so seconds. No running on. It works for me.

Cheers,

David
David Tinker

This thread was discussed between 21/09/2009 and 15/10/2009

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.