Bad Back fire, it got fixed!!!

So as all the post you read, the book and most people you talk too, the RENIX really cant be timed. You bring the motor to TDC, line the rotor up to the #1 in on the cap and there you go. I just installed a new 4.0 in my 89 and at first had some troubles but fix them 1 by 1. The last was a bad back fire through the exhaust. So I moved the rotor at TDC one cap pin # 5. The motor just pures. No vibration, shaking or missing. Hits 4500 with no effort at all. I dont know if anyone else has ever had this problem but it seems to have worked

kenny811 said:

So I moved the rotor at TDC one cap pin # 5.

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What do you mean by this? I don't have a Renix engine, so maybe the distributor is different on mine.

Instead of the rotor being lined up with the #1 pin on the cap (the pin with the wire going to the #1 plug) , I had to move it a tooth tward the #5 pin.

Was your TDC on the compression stroke or exhaust stroke? Im not sure if that would have the same effect you're describing or not. Seems like you advanced the timing quite a bit. How dose it feel in the power department?

kenny811 said:

Instead of the rotor being lined up with the #1 pin on the cap (the pin with the wire going to the #1 plug) , I had to move it a tooth tward the #5 pin.

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Got it. Thanks.

By "moving the rotor", did you pull the distributor out and put it back in rotated one tooth off where you had it before. Or did you turn the distributor housing?

I moved the distributor (by the tooth) but not the housing. It has the little fork that holds the housing, yeah it did seem like a lot to turn it and that could be because the distributor it self might be off in some way.

In reality, you did not change the timing. Somehow rotating the shaft (also rotor) a little bit made the rotor closer to the distributor cap post that it needed to be at. The spark is still triggered by the flex plate notches passing the CKP.

In a way that is true, I didnt change the timming per say but it didnt fire on the #1 positsion from TDC like it was suppose to. And in this instance the CPS did not not come into play with the timming being out or the fix. because the jeep started and idles prefect at 850 rpm. If the CPS was out it would not have started at all.

i think redneck is on the right track, it might not have been on the compression stroke when you lined up the rotor. that or it wasnt quite at TDC and #5 truly was, since the firing order is 1-5-3-6-2-4. but whatever it was, its fixed and thats all that matters

TDC on #1 is when the timing mark on the harmonic balancer lines up with the mark on the engine. This is when the rotor in the distributor needs to be lined up with the #1 post inside the distributor cap. If the mark is lined up on the harmonic balancer, you are on the correct firing point for either the #1 or the #6. This is why you verify that you feel air coming out of the #1 spark plug hole or check the valves (remove valve cover) to know that you are on the compression stroke or the exhaust stroke. If this is on the wrong stroke, the distributor will be 180 degrees off, not at the #5 point. But hey, if it worked, go with it? That is the main point, but some people also like to know why. Another possibility is turning the motor to line up the marks backwards could let timing chain slop enter into the equation. Got to make sure to only turn it in the normal running rotation.

If it were the timing chain I would have been very bummed out. Every part in this motor exept for the block it self is brand new. And I was 100% sure I was at TDC, it just really puzzeled me that the timing would not have worked out like it was suppose too. I went back and double checked it, its 1 tooth advanced from the #1 pin. I guess you can say in the middle of # 1 & #5. But on the other hand it runs so smooth and with out a cluch fan or AC to watch it, looks so effortless. Now new control arms top & bottom, New D44 in the rear and it should be ready.

If its working for you, run with it! :lickout: Im still curious as to what the heck is going on but, whatever... If it works, it works! :cheers:

winterbeater said:

TDC on #1 is when the timing mark on the harmonic balancer lines up with the mark on the engine. This is when the rotor in the distributor needs to be lined up with the #1 post inside the distributor cap. If the mark is lined up on the harmonic balancer, you are on the correct firing point for either the #1 or the #6. This is why you verify that you feel air coming out of the #1 spark plug hole or check the valves (remove valve cover) to know that you are on the compression stroke or the exhaust stroke. If this is on the wrong stroke, the distributor will be 180 degrees off, not at the #5 point. But hey, if it worked, go with it? That is the main point, but some people also like to know why. Another possibility is turning the motor to line up the marks backwards could let timing chain slop enter into the equation. Got to make sure to only turn it in the normal running rotation.

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You are right about finding the compression stroke. The issue lies with setting the motor at TDC. At TDC, the rotor should not point directly at the #1 position. The rotor should point directly at the #1 with the motor sitting at around 14 degrees BTDC. His distributor was simply indexed off by one tooth.