Classic/Antique Car Repair: Triumph TR7 surging under acceleration, stromberg carbs, vacuum gauge

Question
Hello,

I've just bought a 1977 TR7 and have noticed that it tends to surge under acceleration.  The problem seems to occur when the throttle is more than halfway to the floor; the more it's depressed, the harder the surges.  The surges seem to go in two second cycles (surges for two seconds, normal for two seconds).  It occurs in all the gears.  As soon as I back off to half throttle or less, the engine smooths right out.

I hope you can provide some insights into possible problems; the Chilton manual doesn't give any probable causes for the surging and I'm reluctant to start fiddling with things at random and risk making things worse.

Thank you for your time and any ideas for a solution.

Answer
Hi Doug,
Surginng is normally an indication of a lean mixture on any engine. Other causes can be a problem with the vacuum advance unit. This can easily be checked by disconnecting the vacuum line to the distributor and plugging it. Then drive the car to see if that is the cause. If that is the cause don't just drive it that way, connect a vacuum gauge and run the hose out from under the hood (bonnet) and put it under a wiper blade and drive the car noting the vacuum under idle, driving and acceleration and e-mail me the results.One more thing to check is to remove the float chamber vent lines from both Stromberg carbs and make a test run.
If vacuum is not the problem, look toward a lean / fuel problem. You can get a rough picture of a Lean or Rich mixture without CO equipment. It is called "Taking a plug reading" This was easy when leaded fuel was used but now that all fuel is unleaded the art was lost. However, it still works, but not as easy. Here is the proceedure.
You need to find a straight area of road that has a wide shoulder and not much traffic.
With the engine temp up, run the car at the condition or speed at which it runs the worst. In your case hard acceleration for about 10 or 15 seconds. Here is where you have to be careful as you have to turn off the ignition at the exact time you release the throttle. The reason you have to be careful is because if you turn the key back too far you will lock the stearing wheel. Practice it going slowly first to be able to do it without locking the wheel. When you can do it correctly, do it for the test. Pull off the road and remove the front spark plug and the rear plug and examine them closely. A lean condition will show up as a very clean center electrode. A rich mixture will be a flat black powder deposits all over the end of the plug.
If it shows clean, look at the most common fault of the TR-7s by checking the fuel pressure with a pressure gauge placed between fuel pump and the carbs. You should get or borrow a gauge and add a two foot hose so it can be placed under the wiper blade so you can see it while under acceleration. This will put the engine in the most fuel useage where a slightly low pressure will show up. The TR-7 fuel pumps are noted for the diophrams getting hard and not flexable so the return spring (the only part that supplies fuel pressure) can't move the stiff diophram to keep up with the fuel usage under acceleration. Just reving up the engine with no load is not usually enough to see the pump pressure fail. The first signs of this is when you have very little fuel in the tank. When the tank is full it don't do it as much. The TR-7 fuel tank is verticle so a full tank applies a little pressure to the pump helping it along.
I hope this helps, If you need further help you can e-mail me at longez8@bellsouth.net and look at my tech tips on my web site. http://www.mg-tri-jag.net
HMF