Bucking under 3000 RPM!! (Mid year 911) Do I have to live with it??

CARRERIC · 07-19-2004, 12:30 PM

Is bucking under 3000 RPM just common for a mid-year 911's?

I have a 1975 911 Carrera (US) the car has been completely rebuilt. Engine slightly upgraded: webcam hard weld and regrind to SC cam profile, RS pistons, Fly cut heads 9:5 compression, crane optical ignition, SSI's with dansk muffler. Everything from complete cis fuel system to ignition wires are brand new. Car is very very fast and runs great, pulls very strong up to redline. However when I am cruising in slow areas and under 3000 rpm the car has some bucking. Can anything be done?

Eric

nullh2o · nullh2o's Garage

Rich mixture caused this for my SC

MotoSook · 07-19-2004, 12:37 PM

Part throttle buckling or just any throttle position buckling at less than 3000RPM?

Doug Steinel · Doug Steinel's Garage

When I set the mixture too rich for my SC, the idle surges and the car bucks at low RPM. How is the idle? I would check the mixture.

CARRERIC · 07-19-2004, 12:40 PM

Nullh 2o, Thats what I thout but the co is on spec.

Souk, It seems that sometimes when I let off the gas and feather it it is less apparent, and sometimes not. Also when I give it alot of gas it seems to be less appearant.

MotoSook · 07-19-2004, 12:58 PM

Check the vacuum...the 2.7 has a vacuum retard if memory serves me right.

SpeedracerIndy · 07-19-2004, 01:34 PM

I had this problem also. It was very bad when pulling away from a stop light unless I slipped the clutch above 3000 rpms. It turned out that the vacuum advance hose was not connected properly to the distributor. I still get a little bucking once in a while when the engine is very warm and I lug it. I also had someone tell me to check the advance unit in the distributor by pulling the cap and making sure the rotor will turn about 1/16 of a turn and then spring back. I'm sure someone can explain that procedure better, if it is even relevant...

tsuter · 07-19-2004, 01:46 PM

Lets see...... cold motor, 5th gear, cruising under 3000 like 2500??? Maybe up hill a little??? And you get some bucking???

CARRERIC · 07-19-2004, 01:55 PM

I live in Miami and the car alway's runs hot 200-220, with fender oil cooler. No hills in Miami. Vacum advance hose is new and on tight. Mechanic checked distributor on the machine and it checked out good.

I alway's thought I should switch to a crand MSD or something for better spark under 3K rpm.

Could this just be a characteristic of the mid-year 2.7's.

JTO · 07-19-2004, 02:00 PM

Carreric,
My '75 does not do this. There is a very slight "surging" when engine is lugging at low speed, say 2nd gear at 15 MPH.
Troy

MotoSook · 07-19-2004, 02:12 PM

"Could this just be a characteristic of the mid-year 2.7's."

NO!

Did the wrench check the dizzy on the bench with a vaccum source too? hmm..

Paulporsche · 07-19-2004, 02:13 PM

Carreric,

When you say "on spec" what % do you mean?

As others have said, usually this means your mixture is too rich.

Are all the other vacuum hoses new or checked out?

MotoSook · 07-19-2004, 02:26 PM

Gang, I have never ever gotten my car to idle well and then find out it was too rich or too lean on throttle such that it buckled like this car.

Yes, you can get the car to surge when to rich or lean, but lets not get our Pelican knowledge ahead of ourselves...

I don't think he's having problems at idle, so the surging at idle that would indicate rich or lean is not there....

it's happening under load at low rpm and throttle position seems to be a factor....what does that tell you?

Paulporsche · 07-19-2004, 02:30 PM

Souk,

I had this exact symptom some years ago. A slight leaning (1/32 turn)of the mix solved it. I didn't have any too rich @ idle problems, like hunting.

The car had points, but no DME, O2 or other emission control parts.

CARRERIC · 07-19-2004, 02:42 PM

I "believe" the mixture was 2.5%. It was done on the computer at the shop. All hoses on the motor where replaced. I spared no expense on the rebuild, everything new down to the new green engine shroud. The car idles perfect. It does not buck from starting off the line.

Paulporsche · 07-19-2004, 03:31 PM

2.5 to even as high as 3.5% shouldn't be a problem according to JW. But each car is unique, and you have a like new engine. You can try leaning the mixture in 1/32 or 1/16 increments and trying it out. If there is no change, you can easily put it back to where it was.

My car would buck at part throttle @ low revs. Leaning it slightly as I indicated solved it. This occurred when I had my new engine installed, which was a rebuilt , higher compression 3 enlarged to 3.2L

CARRERIC · 07-22-2004, 09:02 AM

Thanks for the reply's guys.

Do you think it can be caused by a bad Warm Up Regulator??

I am going to check my CO with my gunsen gas anaylyzer.

OldTee · 07-22-2004, 09:08 AM

air leak at injector. check carefully with carb cleaner.

CARRERIC · 07-22-2004, 12:05 PM

Just pulled the vacum line off the warm up regulator and plugged it, as recommended by JW in another post. Problem solved!! and I am a happy camper. Apparently the vacum was leaning out the car too much. It pulls straight to redline like butter. It has also caused my temps to drop by atleast 10 degrees.

Paulporsche · 02:32 PM

Carreric,
Congratulations. Which vacuum line did you pull? Do you have one on the top and the side? Did you plug just the line, or both the line and the opening @ the WUR by cutting the line , inserting a plug and then reattaching the plugged line to the WUR?