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As I wait for my license plate to come in the mail since the dept of motor vehicles offices are all closed at the moment, I have been limiting my driving to going around the block a few times just to start working the bugs out.... I have a very strange drivability issue that is confusing to me. At part throttle openings I can get the car to go into an "oscillation" in low gears. As I accelerate lightly in first (and to a lesser extent second) gear I can get the car into a situation where as the car moves forward it seems like there is such sensitivity to my throttle position/foot position that the car will gradually start to surge back and forth and it seems largely due to my foot sitting on the throttle pedal. If I very carefully slide my foot over to the right and allow it to partially rest on the carpet it seems to effectively dampen-out this motion and the car is once again stable.

I am assuming that my description is so poor that this sounds like crazy talk but I am unsure how else to describe it. It is almost like the powertrain is unsupported by the engine steady rod or remote shifter however both are firmly attached with all new rubber parts and the engine is locked in place quite firmly. engine mounts are also brand new and nothing is lose.

I did notice that the engine throttle response is astoundingly quick to pick up revs (like race-car quick!). I have a lightened "street" flyweel and I guess I was very surprised at how responsive the engine is compared to my old car with a 1300 powertrain!

Has anyone else ever experienced issues like this? It is almost like I want to have a throttle damper to help calm things down.

This is a very odd and surprising effect!

A few things to check out
Carbs have oil in dash pots
2 or 3 resafety return springs fitted on throttle linkage to heat shield bottom bracket
There should also be a little bit of slack/ travel on accelerator cable at carbs so response is not instantaneous. Cable does not need to be dead tight between manifold bracket and throttle linkage
Good luck

Thanks for the suggestions. I did confirm that the dashpots are full to the correct level and I reset the throttle linkage just to double check things. I have the linkage set so neither carb has any throttle action when first applying the pedal however they are set to engage after around 0.060" of initial movement (just like the manual says). In other words, idle is set independently on each carb for balanced airflow so at idle the throttle cable (throttle shaft) is floating with 0.060" before the linkage engages with the finger on each of the carbs. There are also 3 springs on the carb linkage.

I spent quite a bit of time making sure things were set right on all of that so that is why I am even more puzzled...

It actually makes the car a bit unpleasant to drive at the moment!

Most likely due to too short a throttle cable/outer casing.........

The later single carbs, with the short cable are particularly prone to this......

Try a bit heavier oil in the dampers - it will slow down the piston lift a bit - easy to do without messing settings up. I run ATF oil in one car for this reason. Went through a few different grades until one worked right

Ultimatley it sounds like you will need a rolling road session

Gordon

I had exactly this...one of the springs was weak...

Appreciate the observations. I have a few things I can try then to see if I can isolate what is going on here. It definitely seems that it has something to do with the cable or linkage/springs but I have everything set as near to original as is possible so it is still surprising. I guess I can see a scenario where if the engine rocks even a little bit that causes the throttle position to change which could start an oscillation of sorts.

This is the sort of issue that would be solved in software on a modern car!

Yep I had it all set up and running great on the road but real a surge when you just touched the pedal and if you were going slow and the road was bumpy then your foot bounced and made it all worse...!! Took off the springs in pairs and did the simple test and found a weak one!!

Exactly right! If I encounter bumps and my foot moves I can get a nasty oscillation as a result. Sounds like a likely cause then...

Do you have all 3 springs facing down?

Another thought is the PCV might be malfunctioning if you have one fitted

Which damper springs do you have ? I have had similar problems with blue springs in twin HS2. I switched to red springs and the problem disappeared.

What engine are your running out of interest? One random suggestion - how are your engine mounts and the bushes in the top engine stay? If they are flogged out the engine could be rocking back and forth and this can give an oscillation like you describe that will be worst in first, less so in second etc.

timmy201 wrote: ↑Mon Jun 01, 2020 5:36 am Do you have all 3 springs facing down?

Not sure if I know what you mean by facing down. The springs are in their original locations; one on each carb and one on the shaft.

Another thought is the PCV might be malfunctioning if you have one fitted

No PCV. This is an early S with open breathers.

nick rogers wrote: ↑Mon Jun 01, 2020 10:19 am Which damper springs do you have ? I have had similar problems with blue springs in twin HS2. I switched to red springs and the problem disappeared.

No idea on the springs. I believe I am using the original damper springs that were on the car when I bought it but I don't see any color on them. Maybe worth switching to known "red" springs?

Maddog wrote: ↑Mon Jun 01, 2020 12:22 pm What engine are your running out of interest? One random suggestion - how are your engine mounts and the bushes in the top engine stay? If they are flogged out the engine could be rocking back and forth and this can give an oscillation like you describe that will be worst in first, less so in second etc.

Engine is a 1275S. Everything is 100% brand new including all mounts and bushings everywhere. It does act like there are missing motor mounts but the engine is very tightly constrained with basically no movement (have been driving without the bonnet so it is easy to see). I can also rock the car in the garage while in gear and everything is nice and tight.