Lean under load at low RPM

[kgrizard]

Guys -

I could use some thoughts from the collective on this problem I am having.

On hard acceleration from low RPM, I get a lot of stumbling and backfiring until the car gets to about 3K RPM. If I go easy on the throttle until 3K, there is noticibly less roughness (but still there). After 3K the car accelerates smoothly with maybe a bit of sluggishness. My LM1 shows AFR in the 17-18:1 area until 3K, then it goes rich at around 13.5:1, and gradually gets richer until about 12:1 at 6K.

I would really, really like to get the acceleration issue smoothed out, but don't know where to start.

Car specs are:
2.4 T bottom end (case,crank,rods)
2.2S top end (pistons, heads, cams, and MFI)
2.4 dizzy converted to mechanical advance
2in-2out muffler

If anyone can provide some insight, I would greatly appreciate it.

Thanks,

Kevin

[911pcars]

A couple of areas to check assuming "regular" jetting applies.

It's good you can check the A/F ratios. Check fuel level and accelerator pump operation. Avoid steady-state engine operation with lean A/F mixtures to avoid engine damage.

Sherwood

PS: Disregard my references to carbs. I overlooked the MFI info.

[Flieger]

How does "jetting" apply to MFI?

I would check your throttle bodies for proper flow first. Maybe the throttle shafts are worn or the butterflies wore grooves into their bores. This would allow extra air in. The S MFI cam would dump enough fuel in at the top end to use the extra air.

This is just my $.02, the safe, CMA route.

Check your plugs for carbon, oil, and color.

[edmayo]

You must verify that your cams and MFI pump timing are correct, have you done that? Do you have the hot air hose from the left heat exchanger to the thermostat on the pump, and do you have an intact hot air heater hose from the fan housing to the left heat exchanger? Have you set the pump control rod to 114 mm with all other throttle links from the crossbar disconnected? Once the pump control rod is set you will attach the linkage from the cross bar to the throttle bodies WITHOUT any pretension. How is your ignition timing set? about 10 advance at idle, then 32 at 6000? When all that is correct then start richening the idle adjustment on the pump. That's where your a/f is showing lean, you may have to also richen the load adjustment to help get the idle?low speed mixture up. At a warm idle, with today's alcohol enriched fuel you will need richer settings than were used in 1970. About 12 1/2 to 1 at idle and thru the RPM range.

[kgrizard]

Thanks for the info guys!

My throttle bodies only have about 5K on them since they were rebuilt by Eurometrix, and I have not made any adjustments to them since then. I will re-check the lengths and pre-tension settings.

I know the MFI pump timing is set correctly, but it sure wouldn't hurt to re-check along with the cam timing.

Are the distributor advance settings for a vacuum advance distributor? I ask because Henry at Supertech rebuilt my 2.4 dizzy and converted it to mechanical advance so that I could use it with my 2.2S throttle bodies that don't have the vacuum take-off for the distributor. I can't remember exactly the advance settings he recommended, but they were around 2 deg BTDC at idle, maxing at either 26 or 28 deg at 6000 rpm.

I really want to avoid lean detonation, as that and a high temp oil problem led to disaster with my old 2.2S motor.

[edmayo]

I'm not sure if I explained this well enough, you do not need to adjust the individual rod lengths on the throttle bodies, (if that was what you were saying) those are set when they are built. What you do need to do is to disconnect the main link that goes from the cross bar to each throttle body. Then disconnect the rod that comes from the bell crank back by #3 cylinder at the cross bar. Now remove the short link that goes from the cross bar to the MFI pump. This link is CRUCIAL to the operation of the system, if it is wrong you will ever get the system right. It must be 114 mm from center to center. Measure the total length end to end, now measure the diameter of of one end, subtract that from the total length, that is your center to to center distance. Reinstall that link from the pump to the cross bar. Back off the actuating screw to the micro switch. Now the only link attached to the cross bar is the pump control rod, it is supreme, all other linkage rods are set to attach to the cross bar without pretension. Adjust the microswitch screw to have one full turn after switch clicks. Check the MFI pump timing, it is timed at FE mark after TDC firing of cylinder 4, not cyl. 1 as many people seem to do. This may not solve your problems, but you must do all this first or you just dig a deep hole for yourself. I was giving you what I thought you MIGHT have for non vacuum retard timing, since vac. retard would be 5 ATDC. Use what specs you were given. How high an oil temp did you have? Is this not an S car with front cooler. Maybe you will need one, more horsepower = more heat, no way around it. When all this is correct then try to adjust the pump idle circuit richer to get your A/F closer to 12-13 to 1. Be sure to check for full throttle and adjust pedal stop to limit pedal travel.

[kgrizard]

Preliminary results are fantastic!

I checked the ingition timing first - the hold-down nut was loose and the timing was about 4 degrees too much on the retarded side. Emphasize the 'retarded' for not tightening correctly. Performance was much better, no more problem with on throttle acceleration, but idle was way too high.

Checked pump rod length, came in at 111 mm. Re-set at correct 114 mm length and re-set the throttle rods to eliminate pretension.

Now it doesn't go on acceleration - it leaps! Wow! It's almost scary fast out of corners.

Big grins!

Thanks again, guys!

Hope I will see you tonight at the Marriott so I can thank you in person.

Kevin