Stromberg Idle Surging

[paw140]

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?

[Billmack]

There are kits for the very similar tr6 carbs to make the jets adjustable. This would probably work for you. Also recheck the float heights and check for vacuum leaks

[69S4]

I’ve had that symptom on my S4 when one of the large O rings was not sealing. Might be worth replacing them both with new ones.

[paw140]

Thanks for the hints, guys. I was thinking about a vacuum leak, but I can't understand how a vacuum leak would cause such an *unstable* idle. I would expect a vacuum leak to be more constant. I did replace the large O-rings that seal the carbs to the adapter, but it's possible they aren't seated properly.

I took the temperature compensators apart Friday night and tested those per some videos and blogs that I found. They seemed to open a bit early, so I adjusted those a little bit. I also blocked off the vacuum line that operates the headlights, thinking there might be a sporadic leak in that system, although it seems to hold a moderate vacuum ok. I haven't fired up the car yet to see if those fixes did anything - instead I decided to tackle replacement of the 52 year old Rotoflex couplings... many of the bolts WILL NOT come out of the couplings so I'm going to have to cut some of them off... so much for a simple job. Once I'm done with this, if the idle still surges, I'm going to remove the carbs, inspect the sealing and the float height, look for any issues, etc.

[RogerFrench]

How much, if any, of the original emission control equipment remains, like secondary throttles etc?
I agree with the provisional air leak diagnosis, by the way.
Your ignition seems a bit advanced. Do you still have the vacuum retard distributor?

[USA64]

Just a few things I didn't see mentioned: the piston return spring, the piston damping oil. crossover pipes, the secondary throttle.

[paw140]

Hi Roger,

All of the original emissions equipment is installed (crossover pipes and secondary throttle). I've been thinking about eliminating this, as it just seems to add unnecessary complexity and opportunity for leaks. I have not hooked up the vacuum to the distributor for the vacuum retard feature (I was told by Ray at RD Enterprises that it isn't needed). Regarding timing, my owner's manual says that with Strombergs it should be set at 14 degrees BTC at idle and I think 8 or 9 degrees static. Does this sound right?

I just changed the damping oil to 10W40.

[RogerFrench]

I assumed your timing was static , sorry. If the vacuum isn't connected, did you blank off the connection?
I suppose it should work with all the gubbins there, but there's a lot of leakage possibilities as you say. I got rid of all mine.
I take it the air valves rise and fall easily, landing with a click, not sticking up at all?

[paw140]

Roger,

I plugged off the vacuum port in the intake manifold that would have normally gone to the distributor retard capsule and the throttle bypass actuators, both of which are not hooked up. I was told by someone (maybe it was Ray at RD as well?) that the throttle bypass actuators aren't needed either.

Regarding the air valves, I've read that there is supposed to be a metallic 'click' when they seat, but I wouldn't describe my sound as 'metallic' or 'click'. The valves don't bind, but they descend very slowly and make a *very* soft 'thud'. If I fully open a valve and then let it fall, it probably takes a couple of seconds to fully close. It seems the dashpots add quite a bit of resistance. Does this sound normal?

[RogerFrench]

Hmm, I may be having a senior moment but I don't know what the throttle bypass actuators are!
Your airvalve descent sounds normal, you can try without the dampers to be sure there's no sticking.
Short of a full examination of everything, I'm not sure what to suggest. How's the fuel pump?

[paw140]

Fuel pump is brand new. I'm starting to wonder if one of my floats has a hole in it, or the float adjustment is off, or maybe I have a massive vacuum leak from the large o-ring being unseated...

I'm still in the midst of replacing my rotoflex coupons and waiting on parts, so it will be a a few more days before I can get back to looking at the carbs.

Maybe I have the wrong terminology, but regarding what I am calling a throttle bypass actuator, each carb has a vacuum actuated diaphragm valve towards the front of the carb. My understanding is that these let more / less air in depending on the engine vacuum. I think they are for emissions requirements.

Thanks,

Paul

[baileyman]

Back when I had Strombergs that kind of behavior was either not enough damping oil or not thick enough. I see you have replaced, but perhaps it is worth revisiting.

John

[paw140]

Update...

I finally finished replacing my rotoflex joints and rear brake discs. What a task! There were several times when I thought I wouldn't be able to complete it due to seized bolts, and then getting things lined up to reassemble. I ended up having to cut off the bolt heads that join the brake disk / hub / outboard flex joint and then use spacers and a nut to pull the bolts out through the back. I believe they were the original joints... the car has been sitting since 1980, so some bolts just don't want to come loose. I eventually got it all done.

Now, on to the idle issue. I removed the carbs and the first thing I noticed was that the large o-ring on the front carb was pinched / not seated correctly. It was probably leaking. I took everything apart. Float heights were correct, everything was clean, etc. The only other thing I found was a small piece of rubber from the fuel line (where it was cut) had gotten into the inlet hole on the back of one of the inlet needle assemblies. Today, I got everything put back together and got the car started... same issue, but not as bad. I now have a synchronizer, so I checked carb sync and found that the back one was sucking way more air than the front. Once I got that adjusted, the idle started to get more stable, but is still surging, just not as bad. Then I started to check out the air valves on the carbs and noticed that the rear carb piston will raise on its own, at which point the car will nearly stall, then the piston will go back down, at which point the engine will start to surge. This cycle repeats. If I hold the piston down with my finger, the idle is stable. If I raise either piston manually, the engine will stall. I think I'm starting to figure this out, but still need more investigating. Next, I'm going to remove and inspect the crossover tubes and make sure the gaskets are seated and not leaking. I'm also going to check the timing again. Now that I have some semblance of an idle, I may be able to better adjust the timing.

Any suggestions are welcome.

Thanks!

Paul

[paw140]

I finally figured it out. There was a massive vacuum leak where the crossover tubes connected to the exhaust manifold. I had cut my own gasket for this connection. When I took the tubes off, I noticed that the flanges for the exhaust manifold side were not flat, but were curved such that even with the nuts tight, there would be a gap in the middle. This time, I used high temp RTV silicone and now they are sealed. My idle is no longer surging and seems pretty stable. Thanks for the help, everyone!