Chris Runnels' 1977 Triumph Spitfire
Uneven spark plug readings on a Weber 32/36 carbureted car
When I bought my 1977 triumph spitfire, I had a fuel delivery problem with my Weber 32/36 carburetor. Cylinders 1 and 4 would run rich or lean according to the "mixture" setting as expected, but no adjustment setting or re-jetting would allow cylinders 2 and 3 plugs to change, as they would always run lean. In other words, the plugs from cylinders 1 and 4 would be white at lean settings or black from rich settings, and plugs 2 and 3 would always be white, or lean, no matter the mixture setting. First lets look at the typical Weber setup for the spitfire.
As you can see fuel/air appears to flow into the manifold between the plenums for cylinders 1 and 4 from the primary venturi, and seems to flow between 2 and 3 from the secondary. Although the fuel/air mixture SHOULD atomize and flow to whichever plenum is sucking the mixture, that was simply not happening in my case. Before going further I tried the usual suspects for carb problems, as follows.
1. Check for air leaks at intake or exhaust manifold by spraying carb cleaner or starting fluid( with engine cold only! ) around the block to manifold connections. If the engine revs, you have a leak.
2. Check carb to manifold connections with spray,
3. Look for any other vacuum leaks in the system.
4. Check for loose spark plugs or faulty ignition system components.
5. As my carb was new, I believed it to be functioning properly, but you should check that your carb is not gummed up.
6. Check fuel pump pressure. ( should be between 1.5 and 3.5 psi .) any more and you could flood the carb.
7. I also pulled the head and did a leak down test on the valves using gasoline poured into the bottom of the cylinder head while it is upside down, ( with old plugs in) to check for air/fuel leaks in the valves while they were at rest.
8. Check and set timing per Haynes manual
9. Make sure no "DPO" has changed the cam or any other "mechanicals" that would cause a fluctuation for the norm.
10. Check the float level of your carb. Set it according to factory specs, as they don't always come set coreectly.
In my case, none of these yielded any results. I spoke with Weber and pierce manifold, and neither had a solution for me.
Through some further research I found a kit called a "Sync-Link" for the Weber and purchased it for around $30. This kit changes the 32/36 to a "synchronous" carb rather than a "progressive" setup. A progressive carb uses 1 barrel ( primary) until the pedal is pressed just about to the floor, then it opens the secondary. A synchronous carb uses both barrels evenly all the way through as the pedal is depressed.
Upon installing the kit, I noticed a big difference in "off the line" performance, and drove it for about 20 miles then returned to check the plugs. To my surprise, all 4 plugs now read slightly brownish grey, which is exactly what you want. I am finding that the car uses a bit more fuel now, but to get the engine to run evenly and correctly, I am just fine with that.
I also believe that the fact that my engine is a "low compression" model with 7.5 to 1 dished pistons may be part of the problem. Maybe the engine does not have enough vacuum with it's lower compression setup to pull the fuel/ air mixture through the series of enormous angular plenums in the Weber type manifold, all the way to the cylinders, as the motor was designed to run with very small Su/ Stromberg carburetor plenums.
Truthfully, I really don't know. Either way the engine is much happier now, as is it's owner.
DISCLAIMER :) I am only a weekend mechanic, with very little knowledge or skill, and these are only my guesses as to why it worked. I am simply in hopes this will help someone else who may be having troubles with the Weber setup, as I know how frustrating it is to not get answers from our usual, knowledgeable sources. Please don't take any of my guesses as to why it worked as fact, only that I wanted to offer an option for anyone else who is having the same problem.