Hi all, I recently acquired a '73 with a '71 su carbs. It has been garaged for the last 25 years. I took out the gas tank and give it a good clean. Changed all the fuel, coolant hoses, and new spark plugs. I took out both domes to give them a good cleaning. The front piston of the carb travel less than the back. Aren't they suppose to travel the same distance when you rev the engine? The pistons travel freely up and down so no binding. I put sae 5w20 for damper oil. It can rev 6000+ rpm, but when I put it in 1st gear, the engine is very sluggish and dies eventually, the compression is good. Any suggestion would be appreciated, and if you know a good mechanic in the los angeles/orange county area, please let me know. tks in advance.

It may be timing.

5w20 might be a bit thick for the dashpot oil, but I don't think it would cause the car to stall. Have you checked the fuel filter and the banjo screens at the carbs? Also make sure the vacuum advance on the dizzy is operating.

Have you performed a complete tune-up? If the valves are not adjusted properly and the carbs synchronized the 2 carbs won't have even air flow which would result in different piston heights. Also, a vacuum leak might cause similar problems as well.

ok, I suspect the air flow is uneven so I'm getting a flow meter. How do you increase the airflow? tks

A friend of mine was having some issues like this one.

We fought with dizzy to setup timing accordingly with no good results.

At the end, the car died on the road; the alternator wires were corroded. they did not provide enough energy to the coil to create a spark under load. My friend had to destroy the plastic cover of the alternator to access the corroded area. Without destroying the alternator, it might be worth a check

Edited August 21, 201212 yr by Lazeum

The front suction piston opens 1/2 and back piston opens fully when revving. Now I swapped the front piston to the back one.

The front one now opens all while the back one opens 1/2. The piston that opens 1/2 now is doing the same thing after swap. The piston looks clean and the needle is good and straight. The gas floats are the same on both carbs. Is this a bad piston? tks

There are many variables that could cause this symptom. If you push the pistons up by hand (engine off) do they move freely and drop by themselves to the bottom? Do they fall at the same rate? Do both carbs have the spring installed? Are the springs that same (free length, wire size, and coil diameter)?

I found out they are not falling at the same rate when I taped the two holes on the piston. The one that is real loose has less grooves on the piston making it slide faster.

Success. The problem for running sluggish was not getting the mixture right and synchronizing both carbs correctly. Now it runs good. tks.

Good to hear you got it running better.