Main Jet Size Question

CraigD · 11-20-2001, 09:59 AM

My 71 911T SEEMS to lack power at full throttle at higher rpms. It accelerates nicely thru about 4000 to 4500 rpm then seems to taper off. I checked the timing and dwell both are correct and I recently adjusted the valves. This is the first time I have really pushed the car so I don't have pre valve adjustment feel to compare. I suspect the main jets are too small as the idle jets were too small prior to my replacing them with larger 55's. I pulled a plug after the drive and it is grey/white in color. I didn't have tools with me to pull it right after a hard run so it has some low rpm cruising time on it. I will take tools and pull a plug right after a full throttle run to see what it looks like. I am curious if anyone with Zenith carbs on a 2.2 T can tell me what size main jet they are using. Also, does this sound like something needs to be corrected or is it normal. I know the torque drops off after about 4500 rpm but the hp max is closer to 5500 rpm.

Thanks in advance for any and all responses,

Craig

Early_S_Man · 11-20-2001, 10:39 AM

Craig,

It is probably something simple, and nothing to do with carb jetting.

Did you check the ignition advance through the whole rpm range up to 6000 rpm?

Have you checked to see if the carbs are actually opening all the way when the pedal is down fully? Often on older cars, wear at all of the pivot points ... bushings, and ball-cup sockets on the pedal pushrod, lever-arm, and throttle rod, etc. can mean that the carbs aren't even opening fully. All of the ball-cup ends are adjustable, and the lever-arm bushings at pedal-box and on the transaxle stud are easily replaced, and readily available.

In the following diagram, the bushings are #28 and #15 in the lower half of the drawing:

CraigD · 11-20-2001, 10:19 PM

Warren,

I checked the timing up to what I think is max advance at 35 BTDC. I have to admit I didn't rev it up to 6000 as it seemed to stop advancing at about 4K. Also, it kind of freaked me out to rev the engine to 6k. I guess I better recheck the timing. If it advances further over 4k then I have it set to far advanced. At idle it is about 2 ATDC.

I just replaced the bushing # 15 on the diagram and yes the butterflies are vertical when the accelerator pedal is on the floor.

Thanks,

Craig

CraigD · 11-24-2001, 05:05 PM

I just finished checking the timing. I am at maximum advance by about 3500 rpm. I ran it all the way up to 6000 and all that changed from 3500 to 6000 was I got a little back and forth of the timing mark by 1 or 2 degrees rather than a stationary mark. I'm not sure how to tell what distributor I have but this seems like the advance comes on a little too quick. Looking at the manual it should be somewhere between 22 and 27 BTDC at 3500. At idle it is approximately 2-3 degree ATDC.

Also, I took the car out to warm it up prior to checking timing. This time it ran well up to 5800 or so where I backed off. Tried a repeat and it seems to have lost power, tried again after cruising easily for a while then ran it up and it ran well again. Maybe my problem is as simple as a clogging fuel filter. Hopefully the filter kept any crude from making it into the carbs.