74 US Carrera Bucking-Jerking 2000-3000 RPM
 

I have been in process of restoring my early production 74 US Carrera (#57) and have had it running now. Part of the restoration was going through the entire fuel and ignition system either cleaning or replacing components that I deemed were not working properly.
I continue to have an annoying problem with engine bucking/jerking in the 2000 to 3000 RPM range. This bucking occurs at light throttle and in all gears except 5th which is probably due to the higher car speed and gear ratio dampening the bucking effect. If I apply throttle the bucking stops. It is not due to driving at too low of speed for the gear (lugging). I've reviewed other posts about this but I don't see where anyone came up with final answer(s) to this problem.
I have conducted CIS pressure testing several times now. Here are my current values in PSIG:
*System Pressure: 70 (fuel to WUR shut off and throttle pressure regulator (TPR) return clamped) recommended range from manual is 65-75 psig
*Cold Control Pressure: 35 which is at the 35 psig upper limit at the ambient temp measured. Still using the original 001 WUR, still works so no reason to replace.
*Warm Control Pressure: IDLE 42 (recommended range 40.5 to 43.5 psig) this is measured with throttle in idle position (TPR affects warm pressures)
*Warm Control Pressure Part Load: 51 (recommended range 49-55 psig) this is measured with throttle partially open to achieve highest pressure
*Warm Control Pressure Full Load: 38 (recommended range is 37-43 psig) this is with throttle wide open.
*Leak down test I have 22psig at 20 minutes. I have verified the accuracy of my test gauge to be good.
As noted, this car has a throttle pressure regulator that works along with the WUR. As throttle position changes the warm control pressure changes.
I had a Pertronix ignition module installed when I first ran the car. Thought this could be the culprit so I have points installed currently with no change to the problem. Points, dwell, plug gaps are set to spec. New Beru wires installed, firing order checked several times. New Bosch injectors. I have checked timing several times also, spec is 5ATDC with vac hose connected and 35BTDC @ 6000 PRM with vac hose disconnected. My altitude is 4500 ft so I could have a little leeway to advance timing but for now I'm trying to stay at spec. I installed an AEM air fuel ratio (AFR) meter to allow adjustment of fuel mixture and real time monitoring. I currently have at idle an AFR of about 13.7 which equates to about 2% CO (spec is 1.0 to 2.5) Unknown if my AFR is accurate but when run rich I can tell it is by the exhaust odor being more pungent.
I have conducted a smoke test of the intake and find no leaks. No pop off valve installed. I have checked the air flow plate to be correctly centered in the venturi, at the correct height and free movement.
Has anyone resolved this bucking/jerking problem in early CIS cars?
If you have let me know, if you have other suggestions or thoughts also let me know. Thanks for your time. JK

JK,

PM sent.

Rex

Rex,
Received your message, I replied a hopeful you got that, thank you.
I’ll continue to make adjustments for fuel and timing outside of the specification in trial and error to determine if changes will resolve this issue. The bucking problem occurs at any engine temperature cold, warm, hot. In saying this out loud I begin to wonder if the issue is ignition related? Something further to investigate.
Jk

Hi JK,

I fought a similar issue with my 74 and the same 001 WUR and TPCV valve. I replaced every fuel related component including the airbox (w/o a pop-off) and still had the issue. It ended up being a coil despite the original one passing the basic ohm and spark tests. Putting an oscilloscope on the car and running under load finally revealed the problem with the coil. I would also check out the KlassikATS site if you haven't already. Lots of good information on ignition related stuff for 3-pin CDI cars.

I still have very minor parking lot speed bucking (1400-2200 rpm) that I just cannot seem to tune out. The only thing that has helped is taking the TPCV off of the throttle body so it does not lean out the mixture at low throttle input. But this makes the car run pretty rich at cruise. I think it's just the nature of this CIS.

Justin

Justin
I’ll put the coil issue on my list. What coil did you use as a replacement? I’ve heard some should be avoided.
Thanks!! Jk

JK,

I got your message. I used the Parts Klassik coil and swapped out my braided Beru wires for a standard non-braided set because of the lower resistance. Those were recommended to me by Kurt Donahue.

The bucking I experienced was in the same RPM range as yours and at all engine temps. At full throttle, it was fine. Anything less I would get the bucking. Cruising in 5th was fine though.

Your coil should be able to produce a bright blue spark with a loud snapping sound from at least 3/8” away from ground. Mine would jump that gap but with a weak orange-ish spark. The new one was night and day.

Justin

I have had the same issue with a fresh engine in my '75 911S. Engine builder threw his hands in the air. I sense it is in the fuel distributor/plunger somewhere. These days I have been bitten good by the 914 bug and only take the 911 out a couple times a year. Fuel pressures check out. Starts perfect, pulls nice, but she bucks in the same rpm range

Enriched the fuel and went for a drive. Bucking still the same. Had engine in the mid 4000 rpm range and with a light throttle the bucking was still noticeable almost like a high speed miss. Going to search for a coil, gut feeling this might be the problem.
I’ll post after the new coil experiment
Jk

My coil wire to the distributor was causing my bucking at the rpm ranges you stated. Changed it and solved the problem.

Boy does this sound all too familiar. I also own an early production (#63) US Carrera so it also had the early 001 WUR. After finishing the restoration back in 2012 I had several mechanics try to fix this same problem. So flipping annoying! I was about at my wits end and had almost resigned myself to accepting the fact that this was just something I would have to live with after a full engine rebuild in which we increased displacement to 2.9 ltrs. In the end I finally worked with a very experienced mechanic who suspected my original WUR was the likely culprit, but since they can’t be adjusted he suggested trying a company that makes programmable WUR’s. They make ones specifically for our model years. Here is a link to their website. Ever since this was installed now more than 10 years ago the car has run great with no such hesitation at low RPM’s anymore. I have no affiliation with this company, just a very grateful customer.

UnwiredTools UTCISâ„¢

Resolved bucking and jerking!!
 

I changed the coil as suggested and this has resolved probably 90% of the bucking problems and the engine is MUCH more smooth both idling and accelerating. I can live with the outcome car is more enjoyable to drive now.
Thanks everyone for your input.