1978 911 SC sputtering / bucking in 3rd and 4th gear

[gszeto]

I have a 78 911 SC, all stock with a recent top end rebuild,· when driving at freeway speeds (50+ MPH), car will sputter / buck between 2800 to 3300 RPM in 3rd and 4th gear, the shifter will shake and rattle, this happens when I'm light on the throttle, if I apply a little more gas, car doesn't sputter, car doesn't sputter out of this RPM range or when going faster

My mechanic thought this was due to CIS mixture and has adjusted it several times w/o success, would really appreciate any insight / guidance on how to resolve this issue.

Here's a video of the shifter bucking
https://photos.app.goo.gl/9nobEwGz6xMbhDFW8

Thanks!

[pmax]

When did this start happening ?

The rpm is solid. Why would your mechanic muck with the CIS ? I would check the engine mounts and ... change the channel.

[pete3799]

Agree with above.....check engine and trans. mounts. Bolts all tight?

[gszeto]

Thank you! I just talked to him today and will let him know to check the mounts, he was speculating it might be a short in the CD unit.

JFYI, he adjusted the mixture and the frequency of the issue def. went down (from 75% to 20% of the time) so he thought there was a correlation.

But if it was the mounts, wouldn't this occur at all RPM range and not just between 2.8 and 3.3?

[Reiver]

The best mod I've done to my '83 SC is putting a wide band AFR gauge in place of the clock.
I can monitor my CO in real time and at all conditions...idle/coast/cruise/wot etc.
It makes tuning the CIS a breeze and will show any malfunction issue immediately.

[Bphawk1]

I had a similar experience with my stock 83 SC I turn out that the negative ground by the transmission had become detached due to age and rust not making a good contact. The negative ground on the battery was not tight as well.

[VFR750]

I’d look for vacuum leaks and air leaks around the seals around the fuel injectors.

Air leaking past the fuel injector sleeves will cause leaning out, especially at light throttle when the system is trying to lean it out for emissions.

Had similar problems and instead of pulling the injector and replacing the sleeves and the two sets of rubber O-rings, I applied black rtv over the injectors and sealed them to the aluminum manifolds.

I saw a pca club racer do this on his car and tried it myself. It worked.

[Bill Douglas]

Also check the distributor cap is correctly fitted. I had this sort of thing and the cap was off by about a millimeter or two. And check the leads are fitted into the cap properly too while your there.

[stlrj]

Something similar happened to me in my 87 Carrera after the recent rains here in the bay area. Seems the last time I filled there must have been some water contamination at the pump so I tried a bottle of "Heet" (yellow bottle) from O'reillys and all my sputtering disappeared.

Cheers,

Joe

[gszeto]

Thanks to everyone for all the advice, the root cause does appear to be the motor / trans mounts which have now been replaced, I wanted to drive the car for a bit to verify as the problem was intermittent before, I've now driven on the free way ~50 miles in 3rd and 4th in the same RPM range where the shiftier used to buck and it no longer does.