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MG MGB Technical - engine will not fire on #1

Here's the story,,1975 MG-B engine Complete rebuild,has 150 lbs compression across when not running,130 lbs when running,Starts and runs but will not hit on #1 cyl. new plugs,plug wires dist cap new dist.rings incerts timing gears and chain,new weber downdraft carb.#There is fire going thru the #1 plug wire and to the plug but it will not hit on this cyl. Idle s rough @ 750 rpm's now for the fun part.Pull the plug out of the #1 cyl and not only will the engine run smother but the rpm's will increase by 300 rpm.Any ideas out there? I'm at whits end. To nice of a car to send to the mill.

John
John

It could be a tight inlet tappet on no one. Or an air leak in the inlet manfold.
DENIS H

Dennis.<P>Tappets are properly set,And I have gone as far as to remove the intake and replace the gasket and replace the complete intake from one of my other cars..Still does the same thing...Thanks
John

If the engine runs smoother with 1 disconnected then 1 is firing but at the wrong time - double check that you have the plugs connected correctly to the dizzy - firing order is 1, 3, 4, 2 - anticlockwise on the dizzy cap.
Chris at Octarine Services

John; Please tell me how you check compression @130 when running.
Sandy
Sandy Sanders

Sandy. Not sure whether you are asking how to perform the test or how the reading could be so high.

The running compression test is performed by using a compression gauge having a flex line attached to the gauge with a terminal on one end that screws into the hole for the spark plug. the engine is started with the compression gauge attached in place of the spark plug, the pressure allowed to build up, the reading taken, pressure released, allowed to build up, then taken again to verify the first reading. The book I have says that 50-100 psi running compression is standard on most engines. It also notes that all of the cylinders should be tested and that all of the cylinders should be within 5 psi of each other on a running engine.

I have performed such running compression checks on my engines as part of the trouble shooting process. With a cranking compression in the 150 psi range, running compression has been 80-90 psi.

Thus, when we are told that the cranking compression is in the 150 psi range and the running compression is 130, I am not sure what to make of the figure. As Chris has noted, on a fully dead cylinder, shorting out the spark plug does not make a difference in how the engine runs. That is the basic method that you use when trouble shooting a dead cylinder--pull the plugs, one at a time, replacing each lead after removal and noting what effect the removal of each lead has. If nothing happens, you have your dead cylinder.

Thus, the fact that the number one spark plug being disconnected results in a smoother running engine (rather than no change at all) and the very high running compression test are two pieces of an interesting puzzle. But there should be a number of pieces between them to link the cylinder firing at the wrong time (especially since it is number one cylinder which is used for setting the ignition timing--one can see number one being correct and all others wrong due to a distributor points cam problem, but not the situation as described happening) and the high running compression reading which is a test of volumetric efficiency and shows that the cylinder is operating in a more efficient manner than most engines do--amazingly more efficient.

It will be interesting to see what finally develops. I would think, however, that I might do a dynamic timing test on each cylinder and note what the timing was. To do this would require making a temporary secondary timing mark 180 degrees opposite of the original timing mark. The secondary mark would be used to read the timing advance on cylinders two and three. I have seen a number of distributors which had variation in cylinder to cylinder ignition timing due to either a bent dizzy shaft or an inconsistently ground points cam. I have seen one combination which had a combination of bent shaft and poorly ground cam combining to make a major problem in cylinder balance. With a dial indicator set at zero on the lowest lobe, the lobes read .018", .047", and .019". With the points set on the high lobe, they would not open on any of the other lobes. With the points set on the low lobe, ignition timing was wildly variable from cylinder to cylinder with timing for the high lobe being significantly over advanced and causing the engine to run better when the spark plug was disconnected as the engine was not fighting the over advanced cylinder then.

But, all of that falls by the wayside when we consider the problem is supposed to be on cylinder number one which is used to set the base timing for all of the other cylinders.

Interesting problem.

Les
Les Bengtson

What type of Ignition is being used. I had an eletronic ign. that would not fire 2 and 4. When I switched back to points, the car ran fine. I used a different Electronic unit and it worked fine.
I the ign. is electronic, try a points unit or a different dist.

Good luck

Steve
Steve

Les;; It is the high reading that is the mystery. I would consider changing the dist-cap as it seems to be cross firing. Hence the RPM increase when pulling the lead.
Sandy Sanders

I do not know what John is referring to here: "...dist.rings incerts... "

The high cylinder compression readings can easily result from incorrect cam timing, and I see no other reason with all common parts. The set of threads on Tony Basham's cars should cover that issue. In fact the whole thing sounds like Tony's problem from a different perspective.

Les's remarks on bent distributor shafts could account for the variance in one cylinder. And it would show up in his suggested multiple timing checks. You can also check it by measuring the points gap in all four positions.

DO NOT disconnect plug wires on a running engine!! I know it is an old time standard method for checking, but it is wrong, and an excellent way to kill coils , caps, rotors, and wires. It can also kill you, especially if you have a weak heart or a pacemaker and electronic ignition or HO coils. Removal of plug leads forces the coil to produce its maximum output, and that spark WILL go to ground somewhere, through insulating materials or your body. Spark voltages can go in excess of 60KV with HO coils, and 40KV with a Sports coil or stock MGB coil. Doing this can easily reduce brand new parts to junk in a few seconds.
To do this check safely, you need to short the plugs to ground. On accessible plug connectors you can do it with a screwdriver held in contact with the head, sliding it up against each plug connector in turn. For shrouded connectors you need extensions, like 2" long springs, that go inside the plug connector and over the plug - short to those. BE CERTAIN the tool is in contact with grounded metal BEFORE you contact the plug lead!
FRM
FR Millmore

Removing a non-firing plug will make the engine run faster as that cylinder is no longer operating as a vacuum pump. It's the same end result as a vacuum leak in the manifold, by removing the plug you are creating an 'air leak', just via another route. Normally all the cylinders are acting as a vacuum pump at idle because the throttle is almost closed - pulling 15 to 20 in. Hg - but the bang and expansion from ignition is more than enough to cover it as well as all the rotational losses.

What's No.1 plug like after running like this for a while? If it was getting fuel but no spark or a spark at the wrong time I'd expect to see it wet or fouled. If it is relatively dry and clean it isn't getting fuel - for whatever reason.
Paul Hunt 2

Problem solved!!!!!!
It seems that in my haste to get the engine put back togather,( it may have had something to due with the 30 degree weather)Some how the Cam sprocket was installed about 2 teeth out of line.Thusly it was firing,just not when it should have..

Thanks to all who gave their input on this subject.

John
John B.

This thread was discussed between 11/04/2007 and 15/04/2007

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