Changing 4.0 timing......

[Cjmartz2k]

Does choping the ears off the distributor and manualy turning it actaully change the timing? On my '98 Sahara, I choped the ears off, but I couldn't turn it more than 10* off 0 before I got a check engine light. As far as I know, the only way to set it is with the diagnostic reader thingy from the dealership because of the OBDII computer timing advance--is that right? Will the timing gun be lying to me? Also, the FSM says that turning the distributer doesn't even affect timing, just "fuel synchronazation" (whatever that is) and that the timing is soley a function of the CPS (I think they are just trying to stop you from playing with it, but what do I know). Is this the case? I was getting backfires out the intake a while ago with the "distributor timing" set to 10*, so I had the dealership set it back to 0*, but it seems weeker now. I WANT MY POWER BACK!!! I'm just not sure it was the timing that did it.

 

[old_man]

Rotating the distributor has no effect on the timing. That is set by the computer using the CPS. If you want a few more degrees of advance, get the high altitude CPS.

 

[Ed A. Stevens]

Moving the distributor alters the rotor to cap alignment past the point where they can efficently conduct secondary coil voltage to the plugs (you get a mid-throttle missfire). With aftermarket (and some factory) camshafts you have to index (adjust) the distributor to assure the rotor and cap are alligned correctly. Most XJ's do not need this adjustment.

Timing changes can be accomplished through resetting it within the computer program (on OBD1 and OBD2 systems) and can be changed by relocating the Crank Position Sensor (on all systems, including the pre-OBD Renix). The Renix systems had a factory high altitude CPS as an option to advance the timing (and Turbo City had one, or you can build your own).

 

[CRASH]

Moving the distributor alters the rotor to cap alignment past the point where they can efficently conduct secondary coil voltage to the plugs (you get a mid-throttle missfire). With aftermarket (and some factory) camshafts you have to index (adjust) the distributor to assure the rotor and cap are alligned correctly. Most XJ's do not need this adjustment.

Timing changes can be accomplished through resetting it within the computer program (on OBD1 and OBD2 systems) and can be changed by relocating the Crank Position Sensor (on all systems, including the pre-OBD Renix). The Renix systems had a factory high altitude CPS as an option to advance the timing (and Turbo City had one, or you can build your own).

Ed, it seems like there is a limited range of adjustability in my renix distributor. How would I go about checking and indexing the distributor in my Renix stroker? It seems to run fine, timing wise, but I am lacking power in the low end. Could be my cam, as I mentioned in my ModTech thread (I went with your suggestion of the Isky 133125/26 cam), but it could be a timing advance thing also.

Considering the advanced CPS as an experiment.......as well as advancing the cam 4 degrees to get some low end back in it.

CRASH

 

[Ed A. Stevens]

The ignition timing off-idle may be too slow, or out of index.

I would read the ignition timing with a light to determine what you are starting with. Mark down the advance at idle and at a constant 3000 rpm. The idle should be about 10 degrees BTDC, and (if you can index the balancer) about 32 degrees at 3000 rpm.

If the light reads like the timing is not stable (the timing reading jumps in a non-repeatable manner at a constant rpm or when blipping the throttle) the distributor indexing may be off. Read (look at) the burn pattern on the rotor tip. If it looks like the arc is running off the end of the tip it's a good indicator the distributor needs some rotation adjustment to center the contact path.

There is also a diagram in the Strokers Group files section to show how to index the distributor with a cut cap and feeler gages (but check the rotation, because IIRC there is some doubt on the marking and distributor rotation).

Check if the knock sensor is working. Run a timing light at a constant rpm (2000 or a stable reading) and have a friend tap the block with a light hammer on either side of the distributor. The timing should back off at the tap and then restabilize. If it's not backing off, the knock sensor may be sending a false signal that is always backing off the timing (this happens sometimes if the knock sensor was overtorqued into the hole).

Advancing the cam timing will rock the torque down the rpm range. If you have the time and keyway (or a Cloyes chain set) to make changes to get the results you want it can make a significant difference in the torque/rpm performance.

 

[CRASH]

The ignition timing off-idle may be too slow, or out of index.

I would read the ignition timing with a light to determine what you are starting with. Mark down the advance at idle and at a constant 3000 rpm. The idle should be about 10 degrees BTDC, and (if you can index the balancer) about 32 degrees at 3000 rpm.

If the light reads like the timing is not stable (the timing reading jumps in a non-repeatable manner at a constant rpm or when blipping the throttle) the distributor indexing may be off. Read (look at) the burn pattern on the rotor tip. If it looks like the arc is running off the end of the tip it's a good indicator the distributor needs some rotation adjustment to center the contact path.

There is also a diagram in the Strokers Group files section to show how to index the distributor with a cut cap and feeler gages (but check the rotation, because IIRC there is some doubt on the marking and distributor rotation).

Check if the knock sensor is working. Run a timing light at a constant rpm (2000 or a stable reading) and have a friend tap the block with a light hammer on either side of the distributor. The timing should back off at the tap and then restabilize. If it's not backing off, the knock sensor may be sending a false signal that is always backing off the timing (this happens sometimes if the knock sensor was overtorqued into the hole).

Advancing the cam timing will rock the torque down the rpm range. If you have the time and keyway (or a Cloyes chain set) to make changes to get the results you want it can make a significant difference in the torque/rpm performance.

Good info. I have an adjustable timing gear set, so it's no problem to adjust the cam.

I'll pull the cap this weekend and check the burn pattern. I'll check the knock sensor as well. I guess I could always unplug it as well, plug it into a knock sensor that is not connected to the engine, and see if that helps.

CRASH

 

[markaboo929]

i just simply advanced my 88 4.0 renix distributor by slotting the plastic on the dist cap and advancing the cap mechanically from the distributor housing.i also am going to cut and grind where the rotor sits on the plate to further advance the timing-the cap slotting only goes so far-ill let you now

 

[markaboo929]

oh yeah only cut on 1 side of the cap where the hold down screws go so you still have plastic on both sides of the screw head and use a slightly bigger washer to help retain what is left.if it doesnt help you still can put the cap back to where it is supposed to sit from the factory

 

[palmrose2]

I never did put a light on mine but I never got any spark knock so I slotted the CPS as much as I could. It is now held on by just one bolt because the other bolt had nothing to grab. Still no knocking with 87 octane. I put a bunch of silicone around it after cleaning the area so as not to let water and mud get up on the CPS and flywheel. This was on an 87 2.5L but I see no reason why the 4.0 would be different.

 

[Xjcowboy]

which way do u slot it to, is the cps sensor supposed to move to the right or to the left.

 

[Dr. Dyno]

If you're looking at the CPS from the front, you'll want to move it to the left (counterclockwise) to get more timing advance.

 

[BlueCuda]

I cut the tabs off of my dizzy and advanced the fuel synch a little bit. Helped low end quite a bit, as well as throttle response. Thinking of getting the cloyes timing chain kit and advancing the cam. My stroker has great power but lacks a little at low RPM IMO. I really want to build a long rod stroker to help cure that

 

[Xjcowboy]

Well guys i found an easier way to move the cps sensor. I put the cps sensor in a vise clamp and then just hit the medal strip a bit til it moved the desired amount. that way u dont have to elongate the holes or nothing. pretty easy and worked perfect for me. ihave an 89 xj so it helped cuz i had put the bigger injectors on it of a 96xj and im going to put the msd.