Terrible bucking/miss problem

[GBStu]

I have a 1979 911SC with CIS, I’m having a problem with the car bucking or missing on part throttle when under load at all rpm's up to about 3000. Once over 3000rpm and WOT the engine is fine. Here's what I’ve done so far:

1) Clean all the injectors, soaked them in Techron and blew them through with compressed air several times
2) Cleaned the distributor internals
3) Set the timing at 35deg BTDC at 6000rpm
4) Set the idle mixture
5) Measured the WUR (48psi, just where it should be)

I have a DE in two weeks time, I'm getting a bit worried about this problem!!!

What else should I check?

Cheers, Stu

[pa911]

Sounds like it's running lean,You adjusted the fuel distributor?

[stlrj]

How do your spark plugs look?

[Nine9six]

How did it run before you started cleaning and adjusting?

[GBStu]

I haven’t pulled the plugs yet, I'll do that tomorrow.

The car ran fine last year.....however, ever since i pulled it out of "hibernation" I’ve had this problem, I must say though; it has gotten worse the last couple weeks.

Thanks for your replies

Stu

[Cooper911SC]

You might also check out your ground connection at the battery and chassis.

I had a 914 that would buck and surge something terrible...turned out to be a bad connection between the chassis and neg cable. The terminal at the battery was pristine though.

my .02

Good luck, Cooper

[oregonmon]

I have the same problem on same year on same car. Ha! Currently my engine is dropped for a clutch change and to try to fix that problem along with other maintanence. My mechanic pulled the plugs and found the Bosch Platinum plugs. He says these do not work well in this generation and they had a lot of unburnt fuel on them as well. We are also doing a valve adjustment and fuel filter. Wires and distributor are newer and shouldn't be an issue.
I'm hoping this might fix the problem and if it does or doesn't I will post it so.

[oregonmon]

Got the car back and this is what the mechanic did and the bucking is gone.
Check this other link.
Light throttle miss

[scottb]

How long was the car in "hibernation?" Have you tried a fresh tank of gas?

[Porsche_monkey]

It's too lean. Fuel pressure or mixture screw.

[GBStu]

Thanks for your replies.

I did remove the fuel tank to repair a leak and then sealed with a POR product over the winter. The car was off the road for about 4-5 months

Other things I’ve checked
1) The cold and warm pressure, they were both where they should be (34psi at 20C cold, 48psi warm, pump pressure 70psi)
2) Installed a new fuel filter.
3) Run about 3 tanks through the car since the start of the season

I ordered new injectors from Pelican yesterday; they should be here by Friday.

I'm going to check the valves this weekend.

Any other thoughts?

Thanks, Stu

[Porsche_monkey]

Are you 100% sure of the mixture? Sounds like it's lean. Do you have a modified WUR?

[GBStu]

I'm not 100% sure of anything anymore!!!

I did check the cold and warm pressure a couple of days ago. They were within spec.

I will check for vacuum leaks tonight.

Thanks for the help, Stu

[Porsche_monkey]

An airbox leak could cause a lean condition. As could a leaky blow-off valve.

[Paulporsche]

When you come to a stop, say @ a light, does the engine want to stall, do the revs drop or fluctuate?

Have you verified your fuel level?

[OZCarrera3]

Stu,

I have had a similar problem with my CIS Carrera 3.0 for some 9 months now (sorry to be the bearer of bas news)! Mine idles badly, bucks an surges on light-to-medium throttle opening and seems to get worse as the engine warms and gets to operating temperature.

So far I've had the car to 5 (yes 5) specialists including the local Porsche centre with no resolution to date. This is what I have found so far trying to resolve this issue, without (yet) fixing the problem:

Tuned (on dyno and oscilloscope)
Compression tested (5 @ roughly 175psi, 1 at 160 psi - No. 1 cylinder)
Fuel pressure tested (Ok)
New fuel filter
New plugs
Repaired cracked air box
Rebuilt fuel distributor
Rebuilt WUR
Replaced all hoses and gaskets to CIS
Run fuel and injector cleaner repeatedly
Valve mis-adjusted (No. 1 cylinder)
Full valve adjustment
Valve badly 'coked' (carbon build-up - re-seated on No. 1 cylinder)
Broken compression ring (in process of replacing on No. 1 cylinder)

The last two items have only just been identified, so I'm hoping they will be the cause! I wish you luck, deep pockets and persistence.

[nhromyak]

Check timing at idle, check if you distributor weights are moving freely.

Good luck.

[Hawks911]

Long shot here, but have you checked the transmission mounts? On our recent (last August) '78 SC purchase, they were both missing their bolts! Car bucked off the line in kind of a resonance situation that went through the accelerator. New mounts and bolts cured the problem. Unlikely in your case but worth a peek.

Good luck.

Mike

[Mookster]

Ignition coil?

If you have friend with a similar vehicle it is a quick and easy swap to test the coil.

Good luck.

[al lkosmal]

ditto on the distributor. I have been troubleshooting a rough running engine. Turned out that the distributor weights were not moving/working. I swapped in another distributor and problem gone.