Solving A Misfire

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This challange has it's own page due to the lengthy content. I hope you find some useful information here.

On the "rollers."

The Challenge:

My big project for 2001 was to resolve several drivability problems that I had been experiencing since rebuilding the engine in the fall of 2000. This turned out to be quite a challenge and quite an education in the subtle art of tuning an A-series engine.

I was quite frustrated with the rebuild, because the engine didn't run as nice as it had before. The previous engine was basically stock, except for a very mild cam and some minor re-shaping of the combustion chambers, intake and exhaust ports, and exhaust manifold. It had always idled smooth, got nearly 37mpg on the highway and was capable of passing California's tough emission standards even without the aid of the smog(air) pump.

For this recent rebuild, I had decided to install a slightly hotter camshaft, but didn't want anything that would make the car miserable to drive around town. On advice from several knowledgeable sources, I chose the Kent 256 cam with lightened lifters. I replaced the standard rockers with sintered rockers of the same ratio. Due to the wear, I had the cylinders bored .020" oversized and installed short skirt standard compression pistons. I kept the stock 1-1/2" HS4 S.U. carburetor and the stock intake/exhaust manifold.

At idle, the new engine was unstable and it never returned to the same idle speed. At a cruise, on the highway, there was a very noticeable misfire. While I did notice a difference in power from what it previously had, it wasn't that impressive considering the different camshaft. The misfire at a cruise went away by pulling the choke out, so I knew the carb was running too lean.

I suspected that my lean running carburetor was probably responsible for the majority of my running problems and focused on it first. I ran the engine with an exhaust analyzer attached, and this confirmed that the carb was lean at a cruise. But, I had to solve why it was lean? I eventually discovered a large number of problems that were all contributing factors:

1) Leaking "Pop Valve" in throttle plate:

The "Pop Valve" that is installed in the carburetor throttle plate for emmisions reasons was leaking. This was causing the unstable idle because is was worn and would not seal correctly against the throttle plate. Replacing the throttle plate with a solid one that didn't use this valve made a huge difference, but did not completely cure the problem.

2) Worn throttle shafts:

The carb throttle shaft bushings were worn and leaking. I swapped in a good late style carb body. This didn't make a noticeable difference, but it did get me thinking about my crankcase vent system, because this carb had the side vent tube.

Since my car is a 1970 California car, it has a charcoal canister mounted on the firewall. This canister is designed to do 2 things: 1) Store fuel tank vapors and engine crankcase vapors when the engine is shut off. 2) When the engine is running, provide fresh air to the crankcase breathing system. I had not used the canister, nor even had it hooked up, because I hadn't spent any time figuring out how it worked. Now that I had some motivation I started reading about the system. I discovered that the running engine actually sucks fresh air into and through the canister. The amount of air the engine gets is controlled by a hole drilled in the end of the pipe coming out of the top of the valve cover. The hole should be 1/16" in diameter. The valve cover is supposed to have a sealed oil filler cap, so the engine can only get air through the canister. The crankcase breather on the clutch housing is then plumbed to the side vent tube on the carburetor.

My engine was set up all wrong. Someone had drilled out the hole in the valve cover breather pipe toa much larger size, and I had always had a breathing oil filler cap. I had been running a Smiths Valve that is part of the crankcase ventilation system for the 1969 car. It was leaking and causing a huge vacuum leak into the intake manifold. I felt like I was on the right path, so I switched the car back to its correct crankcase vent system. This made quite a big difference in the idle characteristics.

3) Carb needle lean:

The carb needle was too lean for the camshaft I'd installed. The Americas originally came with a DZ needle (at least at sea level here in California). The factory recommended rich needle was the BQ. I consulted world famous A-series tuner and author, David Vizard, and he thought the BQ would be a good needle to try. So, I ordered a BQ from Joe Curto in NY. What a huge difference that made. The engine performed much better. However, I still noticed a slight misfire at cruises speeds, but it was very slight. I also noticed the misfire was somewhat more pronounced, while climbing long grades under a bit more load. I suspected the needle was still a bit too lean, but still wasn't sure, although the symptoms did go away if I pulled the choke out.

4) Carburetor Ice:

Yes, it's possible in California, in the summer, to get carburetor ice. I noticed the misfire got worse in the morning if it was cold and foggy as it sometimes is here on the coast in the summer. On one particular instance, we were caravanning with our British Car Club about 140 miles to a car show. It was foggy, cold, and the group was traveling slow, so my engine wasn't working very hard. When we pulled off the freeway at our intended exit, the engine died and wouldn't restart. It had spark and fuel, but when I touched the carburetor, it was ice cold and covered in water droplets. Apparently the water was from the icy condensation that had formed on the outside of the carb during the drive. In the time it took to check for spark and fuel, the carb thawed out and the engine could be started. When we got home, I wrote to the MG1100 email list, asking if anybody had any advice. Long time member and fellow America owner Scott Williams told me that I should have a sheet metal shroud around the #4 exhaust runner, and that the carb air intake horn was supposed to fit down into this to get heated air off the exhaust. I had always had my intake air hornturned down over the exhaust, but had never had the sheet metal shroud. I didnt even know there was supposed to be one. Scott was kind enough to send some pictures of his setup and then gave me a shroud for free. (Great guy eh?!) I installed the shroud and it cured the carb ice problem completely, but I still had that very slight misfire.

5) Valve adjustment incorrect:

My valves were adjusted wrong. I had originally set my valve adjustment to the factory spec., which is .012" for both intakes and exhausts. The spec given for the Kent cam was actually .016", but I thought that was just too large. Oops. I should have followed the instructions. After talking to another well known and talented A-series engine builder/tuner and author, Keith Calver, I learned the error of my ways. While .016" is a bit noisier, the engine began to run a lot better and had a smoother idle.

6) Carb needle selection:

I started searching for S.U. information on the internet and found some great sites that had needle profile search programs. One site had a Windows based Excel spreadsheet program that was available to download. I used it to find a new carb needle that was just a bit richer than the BQ that I was using. I ordered a CN and an EJ needle from Joe Curto. One great site had a story similar to mine about how the author had resolved the same misfire problem. He had gone through many of the things I had. What he ultimately found was a problem in his distributor advance mechanisms. Fixing those cured his problem.

7) Distributor problems:

My distributor had sticking mechanical advance weights and a torn vacuum advance diaphragm. So, there was the source of my unstable idle as well as the source of my lack of great performance with the new camshaft. Not only was the torn vacuum advance diaphragm causing timing and performance problems, it was a big vacuum leak at idle. The mechanical advance weights were all dry and would not open or close properly. While repairing these faults I also replaced the points backing plate, points and condenser. The results were amazing. The idle was stable, the power was strong and smooth, and the misfire was almost completely gone.

8) Spark plug wire problems:

Spark plug wire resistance was too high. While trying to keep the engine looking original, I'd located a set of vintage looking spark plug wire ends. For some reason, these had extremely high resistance at about 14,000 ohms. Although I'd been using them for about 6 months, Vizard's book recommends 6,000-9,000 ohms. I found a different set of ends that were in the 9,000 ohm range. Not really any improvement felt, but the less resistance (to a point), the better.

9) Carburetor float adjustment wrong:

My carburetor float was adjusted too low and my carb jet was worn out. Several of the internet sites highly recommended check the condition of the jet and making sure that the float height was set correctly. My float was set so it kept the fuel level too low and the jet was so worn, that the engine could still get fuel even with the mixture nut turned all the way up. I didn't notice much of a difference after repairing both of the these, but that's probably because I'd already found and repaired so many other significant problems.

10) Carburetor piston return spring wrong:

I'd also ordered a new red piston return spring at the advice of several internet sites. I installed it before putting the car on the dyno or putting in the new CN needle. Someone had apparently cut my spring shorter thinking it would give better performance. The new spring was about 2" longer. I noticed "crisper" throttle response immediately with the new spring. It really made quite a difference.

11) Dyno (Rolling-Road) Testing:

I put the car on a dyno once the new needles arrived. This was the first time the car had been on a dyno and my first experience in using one. I ran the car first with the stock BQ needle. The dyno read-out showed that the CO mixture (fuel mixture) was lean at a cruise and showed it at about .75%. The CO readings with the CN needle were about 1.9% at a cruise, and with the EJ needle, about 6-9%. I stayed with the CN needle. This completely cured the misfire problem, and the change in overall performance was fantastic. It was like I'd installed a new engine.

12) Ignition points bouncing:

One thing many people recommended, for more stable performance and improved acceleration, was to install electronic ignition. Although I'd already solved my driveability problems, I decided to install electronic ignition. The Pertronix Ignitor came highly recommended and fit entirely under the distributor cap. I purchased both the ignitor and their matching 3 Ohm ballast resistance coil. It was a very simple installation. I noticed a difference in how smooth the engine ran and how smooth and strong it responded to acceleration.


It took several months to sort out all these problems and I'm really happy with the results. I also enjoyed learning more about tuning related issues and theory. The car runs better than it ever has, and with the added power is even more fun to drive.

I really have to thank the many people who took time to help me by either talking to me on the phone, or exchanging e-mails with me. Without the advise of David Vizard, Keith Calver, Don Racine, Dean Snepenger, Joe Curto and Scott Williams, this would have taken me forever and probably wouldn't have turned out half as good.

Thanks gentlemen!