Bucking again...

[tadel001]

This is a weird problem that is hard to chase... The car is bucking. It doesn't do it right away. The car warms up and then starts bucking. I have MSD ignition. I have swapped it out with another unit. That unit worked for a while and then started up again. I have already checked the fuel system and it is not that. Pretty sure it is an ignition. Any ideas on trouble shooting this?

[ossiblue]

You will need to provide much more information regarding your car before anyone can offer help. What year and model do you have? What engine and fuel system? You may wish to include the basic information--car model and year as well as your location--in your signature or avatar profile so all will know immediately what you have

[tadel001]

I thought that was in the signature. Opps. 1976 2.7 911s with CIS and everything stock. Car has 83,000 miles on the engine. Runs strong except this bucking.

[Trog]

Lean?

[longhornchris04]

1: Check the ignition again - timing... and coil (you mentioned you swapped MSD boxes, but what about the coil
2: Fuel System - if it behaves fine cold - then bucks when warm it sounds like fuel to me. CIS starts rich then leans to operating as the engine warms (like all fuel systems)... If it is going too lean then you will get bucking. Have you run an exhaust gas check while warm?
3: Vacuum leaks - the rich mix at startup can mask this - again leading to a lean condition... leading to the bucking.

[ossiblue]

You say you checked the fuel system, but what exactly did you do?

It does sound like a lean condition to me, as mentioned above by longhornchris04 and Trog. Have you checked your fuel pressures? If you are doing any trouble shooting on CIS, the first step (assuming ignition is in specs) is to check the fuel pressures--system, cold regulated, and warm regulated. A lean condition when warm can be due to too high fuel pressure or vacuum leaks. If the pressures are in spec, then proceed to check for vacuum leaks.

This should get you started. Ask any and all questions that you have, and report any findings or symptoms you discover, no matter how innocuous they may seem. Before you start digging in, try the search button at the top of the page--there are literally hundreds of threads on CIS problems, many exactly like yours.

[DRACO A5OG]

When my baby started to buck, it was a failed Distributer. The shims and inner snap bushing/bearing discinergrated as a result the rotor was skipping.

Replaced distributor never bucked again.

[Gemballa2006]

I have an 86 carrera coupe that was doing the same thing, turned out to be a faulty connection at the cylinder head temperature sensor. I don't know too much about your car, or even if yours has this, but mine runs perfect now, and it was bucking so bad I could barely get 30 feet before

[tadel001]

I checked the mixture last night. It was 6%. I adjusted back to 3%. The bucking is still the same. My next course of action was to check timing, swamp the coil and then check WUR pressure. Any recommendations for a WUR pressure gauge?

[Paulporsche]

Many posters have used the CIS fp gauge from JC Whitney.

I had some bucking issues, along w/ some other symptoms, this spring. My control pressures were way off, and sometimes (depending on which WUR I was using) the cps weren't even changing at all. I also discovered an emerging vac leak in the metal pipe to the AAR and AAV, an open throttle body vac port, and an atmo, rather than vac, line to the thrmovalve.

I corrected all these and now have a sweet running engine. I have a couple very long and (to some) boring threads on this.

Your post brings up the first thing to do before tackling the FI: ensure all ignition components are good.

Do you have points? Have you checked the gap/dwell? Are they opening and closing?

Have you checked the plugs and wires? As others have said, how about the diz and timing?

[pete3799]

When does this bucking happen?
Does it do it when accelerating,decelerating,coasting, at what speeds?
I'm thinking it may not be ignition but could be bad motor and or tranny mounts.
Some cars tend to buck when just feathering the throttle at low RPM.

[47silver]

mine started to buck, blown air box, even had a pop off valve, but a big back fire may have done it.

[john walker's workshop]

pull the side vacuum hose off the warm up regulator and plug the hose. see what that does.

[MotoSook]

Do this before you do anything else.

pull the side vacuum hose off the warm up regulator and plug the hose. see what that does.