Stromberg Idle Surging
Posted: Fri Mar 19, 2021 10:14 pm
Hi All,
I've been lurking on this forum for a while (what a great wealth of knowledge!) but this is my first post. I am very new to the Lotus Elan world. Last fall, I bought a 1969 S4 (Federal spec) from a friend who was moving to Florida. He bought it in the mid-70's and broke the timing change around 1980, at it had not been running ever since. It sat in his garage for 40 years, and about 10 years ago he started to take everything apart with aspirations of a complete restoration. But this never happened, and he offered to sell me the car. When I bought the car, he had started putting the engine back together and he had the head, oil pan, and valve cover on. Pretty much everything else was in boxes and crates, including the entire interior of the car (seats, dashboard, etc was all disassembled). So this has been quite a project for me, with a nice learning curve.
Right now, I am at a point where I have the drive train in the car, the interior and dash is mostly assembled, and I am able to start the engine and pull it in and out of my driveway. I am getting very close to a real test drive. However, I am having a very hard time figuring out what is going on with my idle. My car has twin Strombergs, the emissions models that do not have adjustable needles. When I start the car, I need to pull the choke all the way out and it fires up fairly easily. It stumbles for a little bit for maybe 30 seconds, and then the idle starts to rise to ~2000 rpm. As I start to push the choke in, the car will die. Once I get the engine good and warm, I am able to completely release the choke, but the engine will surge to ~2000 rpm, then nearly stall out (~500 rpm), and then surge again to 2000 rpm, and continue to surge / die. I have tried adjusting the idle trim screws (they seem to do nothing). The points, condenser, and plug wires have all been replaced and the timing is around 15 degrees BTC, but it is really had to set timing due to the constant surging of engine speed. A few days ago I pulled out the air pistons to inspect the needles and discovered that they were set at slightly different heights, so I fixed that. I thought that would solve the problem, but today I tried to run the engine again and it still has the same problem. I just cannot get anything resembling a stable idle. If I set the throttle stop so that it doesn't stall, it is constantly surging from a near-stall to ~2000 rpm. I also tried lifting the air piston slightly with a screw driver and the engine dies if I do this, which seems to indicate a lean condition, but I'm not confident in this procedure.
At this point, I am considering removing the carburetors and taking them apart again to see if missed something when I rebuilt them.
Does anyone have any thoughts on this?
I've been lurking on this forum for a while (what a great wealth of knowledge!) but this is my first post. I am very new to the Lotus Elan world. Last fall, I bought a 1969 S4 (Federal spec) from a friend who was moving to Florida. He bought it in the mid-70's and broke the timing change around 1980, at it had not been running ever since. It sat in his garage for 40 years, and about 10 years ago he started to take everything apart with aspirations of a complete restoration. But this never happened, and he offered to sell me the car. When I bought the car, he had started putting the engine back together and he had the head, oil pan, and valve cover on. Pretty much everything else was in boxes and crates, including the entire interior of the car (seats, dashboard, etc was all disassembled). So this has been quite a project for me, with a nice learning curve.
Right now, I am at a point where I have the drive train in the car, the interior and dash is mostly assembled, and I am able to start the engine and pull it in and out of my driveway. I am getting very close to a real test drive. However, I am having a very hard time figuring out what is going on with my idle. My car has twin Strombergs, the emissions models that do not have adjustable needles. When I start the car, I need to pull the choke all the way out and it fires up fairly easily. It stumbles for a little bit for maybe 30 seconds, and then the idle starts to rise to ~2000 rpm. As I start to push the choke in, the car will die. Once I get the engine good and warm, I am able to completely release the choke, but the engine will surge to ~2000 rpm, then nearly stall out (~500 rpm), and then surge again to 2000 rpm, and continue to surge / die. I have tried adjusting the idle trim screws (they seem to do nothing). The points, condenser, and plug wires have all been replaced and the timing is around 15 degrees BTC, but it is really had to set timing due to the constant surging of engine speed. A few days ago I pulled out the air pistons to inspect the needles and discovered that they were set at slightly different heights, so I fixed that. I thought that would solve the problem, but today I tried to run the engine again and it still has the same problem. I just cannot get anything resembling a stable idle. If I set the throttle stop so that it doesn't stall, it is constantly surging from a near-stall to ~2000 rpm. I also tried lifting the air piston slightly with a screw driver and the engine dies if I do this, which seems to indicate a lean condition, but I'm not confident in this procedure.
At this point, I am considering removing the carburetors and taking them apart again to see if missed something when I rebuilt them.
Does anyone have any thoughts on this?