ignition Misfire and troubleshooting the engine

The MGA With An Attitude
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It is often said that for an engine to run you need fuel, air, and spark. But you also need compression and the correct spark timing. Fuel and air comes from the fuel system and carburetors. Spark comes from the ignition system. Compression and correct spark timing come from the mechanical parts of the engine. While each of these systems in turn is fairly simple, certain interactions between them may be more involved. The best approach to troubleshooting would be to first determine which system is giving a problem, and then go after the simpler problem or problems individually.

This discussion involves various systems around the car. Much of this is covered in articles in other tech sections, so this discussion will often be referencing these other articles to avoid duplication, while tying it all together for a logical approach to troubleshooting, and hopefully not to miss anything..

At 08:06 AM 10/31/05 -0700, Steve Demko wrote:
>"Often times on a cold start up, after a few days of no driving, the car is hard to start and seems to be missing badly, barely running at idle for a minute or two (and then it gets better)."

Start with the easy checks first. If a little more manual choke cures it, then it just needs choke for startup, which is normal. Review the article on Choke Adjustment. Otherwise it may be bad spark, low compression, or a sticking valve. Be sure there is oil in the dashpots on the carburetors for a smooth idle and to prevent hesitation on acceleration.

If it doesn't fire al all, check for fuel and spark. Disconnect the fuel feed hose at the rear carburetor, place it in a vessel, and switch on the ignition switch to check for fuel flow. You should get a veritable gusher here, more than one pint per minute. If not, then review the article on Lack Of Fuel Flow.

If you can start it up but it misfires, then pull plug wires one at a time to see which cylinder is misfiring. Don't even bother with anything else unless you have good spark on all four wires. For a run down on diagnosing spark problems, see Ignition Troubleshooting. If it doesn't start, you can still pull a plug wire and check for spark while cranking it on the starter.

>"Once it gets going subsequent starts during the day are fine with no problems."

When you have good fuel feed and good spark, but it still refuses to start or runs bad, then do a compression test. Review the articles on Worn Out Engine and Bad Valve Blues. If you have very low cylinder compression it will be hard to start and will likely run bad. You need to fix the compression problem before you can expect it to run well.

If compression is good, and fuel and spark are good, you're back to considering fuel mixture or spark timing.

If one of the carburetor float chambers is overflowing, you need to fix the float valve so it will shut off This may only require disassembly for cleaning the float valve. If that doesn't fix it, check fuel pressure. Pressure of 5 PSI or more is a no-no, in which case you need to install the correct fuel pump, or a pressure regulator before the carbs. SU carburetors like 1-1/2 to 3-1/2 PSI fuel pressure. If pressure is okay, check to be sure the float actually floats. If a brass float will slosh when you shake it, you need to repair or replace the leaky float. If a plastic float will not float at least half above the surface, you need to replace the float. If the float and pressure are okay, and it still overflows, you need to replace the faulty float valve.

If you pull plug wires and find it is running on 1 & 2 but misfiring on 3 & 4 (or vice versa), then it is likely that one carburetor is not feeding fuel properly. The SU H-type carburetors have the float chamber attached to the carb body with a banjo bolt and a pair of rubber grommets. The rubber bits tend to deteriorate with time and may leak or may clog up the passages in the banjo bolt. If you find it not running on one carburetor, check the banjo bolt and/or replace those rubber grommets.

>"The engine usually runs fine at idle with an occasional misfire (rarely)."

This may only need a minor adjustment for timing or mixture.

>"but upon acceleration at high rpm it misfires."

This would be mixture leaning out, or the spark becoming intermittent.

Adjust both carbs two full turns toward rich, expecting it to run rich at idle, then take another acceleration run. If it then runs better at high speed, you may have the wrong needles in the carbs. Pull the needles and check the numbers on the shank. If the needles are the wrong number, install the correct needles. Reset the correct idle mixture.

>"Also a lot of misfire is noticed at cruising speed on the highway while running at 60- 65 mph. Nothing bad but you can feel the missing while cruising."

If it still misfires under acceleration or at higher speed with richer mixture, then it has to be a spark problem. This is not what causes it to run bad when cold, but a separate problem. If the spark plugs are old you might try new ones. If all of the electrical tune up parts appear to be in good condition, you might look for a broken ground wire between the contact breaker plate and the body of the dizzy. Also check for proper function of the vacuum advance unit on the carburetor (assuming it still had the original type distributor). These items are covered under Ignition Troubleshooting.

Near zero valve clearance will also cause misfire, as a valve may not be closing completely when hot. This condition gets worse under high load, higher speed, or climbing a long hill with higher throttle setting (hotter running conditions). Check valve clearance.

>"No miss fire is felt at low speeds under 40 mph. Recently I had to replace the head do you think that this may be a valve problem "

If you have all new valve guides, it might be a tight guide. This is a special problem with bronze guides which have higher thermal expansion than iron guides. That will more commonly cause a valve to hang up at high speed under high load when the valves are the hottest. But a tight guide could cause a valve problem when it's cold too. Review the article on Sticky Valves.

There is a fuel feed problem that can cause this symptom. The SU H4 carburetors have the float chamber connected to the main body of the carburetor with a banjo bolt and two rubber grommets (you can wiggle the float chamber a little). If the rubber grommets deteriorate they can leak fuel or possibly obstruct fuel flow through the banjo bolt. If fuel flow is obstructed here it may run okay at idle but go lean in one carburetor at higher speed and run on two cylinders. When it gets worse it will run on two cylinders all the time. These rubber grommets may only last a few years. I carry replacements in my tool kit. If replacement parts happen to be made from the wrong material they may only last a few weeks (or even less).

>"or a possible leaking intake manifold?"

Leaky intake gasket usually causes bad running at slow speed, but not so much effect at higher throttle setting. A badly leaking intake gasket may whistle fairly loudly so you can hear it. Tighten the manifold mounting nuts. Spray carb cleaner around all the gaskets when it's running. If this affects idle speed in any way, it's a leaky intake gasket.

You might also spray the throttle shaft bearing while you're at it, and see how much they can wiggle. A worn throttle shaft can leak, but can usually be tuned in for proper mixture at idle. That may then cause it to run too rich at higher speed. Worn throttle shaft can cause the butterfly to be misaligned and drag on the throat, causing it to stick at closing and give variable idle speed. If it idles fast, step on the throttle and slip your toe off the pedal to let it snap up quickly. If that brings the idle down, then the throttle valve is sticking near the closed position, and you may need to replace the throttle shafts.

A worn camshaft can also cause bad running. This causes different symptoms depending on how badly worn. With mild wear the cam lobe will have slightly reduced height, or a flat spot on top, sometimes refereed to as a "square cam". With correct valve lash adjustment the valve will open and close at the right time but will not open as far as prescribed. This will allow it to run quite well at idle speed up to mid range, but at higher speeds it will lose torque because the valve doesn't open far enough and restricts high speed flow. At higher speed it may lose so much torque as to seem to be misfiring. It may also cause a tapping noise at any speed, but most noticeable at low engine speed when you have less engine noise.

When a cam lobe is worn down a lot it may be refereed to as a "round cam". In this case the cam lobe is worn away so far that it comes close to being nearly a plain circle close to the base circle of the original cam. Here the valve may open late and close early, and may also have very little lift. With just a little lift it may run almost normally with just a slight shake at idle but will seem to be completely misfiring on that cylinder at any speed higher than slow idle.

To inspect for a worn camshaft, measure the valve travel at the top of the valve spring as the engine is turned slowly by hand. With the camshaft removed from the engine you can measure the cam lobes directly for base circle and cam apex height. Subtract base circle diameter from total lobe height to get cam lift. Multiply cam lift by the rocker arm ratio to get valve lift. Standard rocker ratio is 1.42:1

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