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Carbies & Exhaust Backfire. Timing ?


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Hey guys I have been going around in circles for a long time now. I have read all the books and searched the net high and low with nothing that has really stood out.

I seem to have what I think is a timing issue but cant seem to find what is causing it. The car starts and idles but when the throttle is pushed it backfires from the exhaust and also the carbies and stumbles like its misfiring. It doesnt matter what I do to the carbies ot just keeps happening.

Things I have checked and seems to be ok

-Valve clearances.

-Carbies. Floats, needles, 2.5 tuns out on jets, dashpots drop with a clunk ATF oil in them.

-Distributor is 280zx. Matching base wired to diagram on the net

-Coil is pertronix 3ohm. Bench tested with multimeter and is within specs

-New leads, plugs, cap and rotor

-checked for vacuum leaks and have nothing I can find.

-Compression check 150psi over all cylinders

-Checked Timing marks and cant see an issue but maybe overlooking something. Both cam lobes are both up. Sprocket is set on 1. Chain doesnt seem to have stretch according to the notch. Crank pulley is zero'd at TDC and cylinder 1 piston is at TDC.

I have included a few pics of the timing marks at TDC can anyone see any issues.

Does the rotor look like its not pointing at exactly TDC 1







Edited by blu260z
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OK that's good. Have you made sure that No1 lead lines up with the rotor button while at TDC and they aren't out by one hole? I mark the dizzy body with a texta where No 1 lead goes so I can see when the cap is off at TDC.


The rotor button must cover the post in the cap for all the timing range say between 0 and about 40 deg. If the metal part of the rotor button runs past the edge of the post then it may spark to the adjacent post and fire in the wrong cylinder. If you whack the timing light on No one lead and give it a rev, what happens? It should be a relatively smooth and uninterrupted move around the pulley to about 30 deg and back. Do that for all leads and see if they move the same roughly. It shouldn't make big jumps around the other side of the pulley. If it does you are getting cross firing which could be dirty or damaged leads or tracking in the dizzy cap due to dirt or a crack. If it doesn't move from your 10 deg at idle then the mechanical and vacuum advances are seized up. Did it do this before you replaced he leads and cap? What you have, to me, sounds like it is firing way out at the wrong time while inlet and exhaust valves are open or less likely extremely lean. 


Also grab the rotor button and give it a gentle wiggle to make sure it is solidly mounted. There should be a slight springy feel to it and it should always move back to the same spot. Give the vacuum advance a suck and see the plate rotate then return. Make sure the inside of the dizzy cap is in good condition, pins are clean and the carbon brush is located in the center and springs in and out.


The last thing I can think of is the star wheel thing (6 points) sitting under the rotor button where each point should be lined up with the corresponding point on the center shaft. This triggers the spark so with zero timing and TDC they should exactly line up. The top part (rotor button) needs to be aligned with the bottom part holding those points on the shaft. I can't remember if the top part is on a spline or not - been a while since I stuffed around with one of those.


If all that lines up then maybe have a look at the carbs. If it only misfires when you raise the revs quickly but runs fine while holding the revs up then it could be running lean plus insufficient dampening in the pistons making it extremely lean under acceleration. Check the valve is on the end of the dampening rods and there is an amount of resistance when you push them into the oil. I'm assuming the springs are in the top of the carbs pushing the pistons down? Don't laugh I've done that before.

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Going by your pic where the crank is at zero & your piston is at TDC I would  try setting the crank at 10* advance instead of zero as is now , just mark the 10* segment of the plate at the Harmonic balancer with a white spot also & use the timing light to match them up whilst you adjust the dissy , try that . Works fine for me .

                                   Just out of curiosity what did you change with the motor to arrive at this situation .


                                                                                    Regards: Alan.

Edited by zeds4ever
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Howdy in the pictures on my first post the distributor has a white texter mark where the No1 lead points at TDC. The cap is in good condition and distributor vacuum moves and holds when sucked on and springs back.


Scott I think I have the pump drive correct as per the picture in the manual.


Thanks Alan thats something I will keep in mind.


I did get somewhere over the last couple of days and mannaged to get the engine to run without a missfire. I re checked plug gaps and made 2 plug gaps smaller, must have got caught and bent them out a little. However I messed around with the distributor advance and found if I set it lower than 20deg it causes the backfire and rough idle. When the advance is set to 25-30deg the car runs and revs quite well minus a small stumble on initial acceleration. What could cause it to need so much advance ?


I have included a few videos so hopefully it can explain it a little easier


First is with the timing light. As you can see it is running off the scale and the last mark is 20deg



Second is the engine running at the 25-30deg advance



Thanks for everyones help it is much appreciated


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Just a thought but if the harmonic balancer is old the outer ring can slip on the rubber and move the timing mark. This could account for the high initial timing.


To check this, set the engine up to TDC on number 1 firing. This can be done using a screw driver down No. 1 plug hole, then look at the timing marks on the balancer to confirm the pointer is aligned with the TDC mark.

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Hi Chris , in the video clip I can't see a white marker on the timing plate itself ? , or are my old eyes deceiving me .


                                                     Regards: Alan .

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Ok guys a couple more videos first hopefully showing the white timing mark on the crank pulley better and how far its off the scale. The bottom mark on the timing scale is 20deg so its upto 40deg advance at idle.



Second video is of me trying to adjust the timing back to around 17deg with the distributor only before it runs rough and stalls.


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well the engine runs, if the timing was that far out it would not start let alone run so it might be the pully as suggested. put a piece of paper wad in no 1 plug hole and wind the engine over till the paper blows out, stop turning as it is coming up on the compression stroke, place a wooden dowl in cyl 1 resting on top of the piston and watch it as you line up your pully it should be rocking the piston when the pully is at TDC recheck all your timing marks

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G'day Chris , to set your timing properly with your timing light you would be better to have a white dot on the 10* mark prior to the Zero mark  on the timing plate at the crank area as the timing light is of a strobe concept & will show  these clearly when the engine is at the point when both white marks line up , be sure to have your timing light at right angles to this so as to avoid a parallax error situation , once you have this part right then adjust your dissy with motor running with the revs about two to three thousand rpm , find a happy medium with this method by listening to motor response & hopefully all will be OK  .

                                                                                Worth trying mate , it works for me !!!.


                                                                            Regards : Alan  

Edited by zeds4ever
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I really think the rotor button is running off the post in the cap when you retard it. That's why it runs crap. The small 8mm bolt at the bottom of the dizzy might have enough adjustment to re-align the rotor button with the dizzy shaft but you may need to move the shaft one tooth retard.

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If you found a ball bearing in the dissy that means your vacuum advance plate that contains three of these has partly disintegrated as it is made of a type of plastic material which over the years with heat & wear & turn causes this . Would suggest that you make up a aluminium plate similar to what I & Groundhog made up it works fine .

Pity you didn't mentioned the bit about finding the ball bearing the other day as I would say that this is where your major issue probably is & could have saved you a lot of time maybe! .  


                                         Regards : Alan .

Edited by zeds4ever
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Hi Aircobra, when I started to read your post I did think that. However over the last week I checked all my timing marks again for what felt like the 1000th time, and guess what it was correct !! So I decided to rebuild the 280zx dizzy especially that I found ball bearings in the bottom from the vacuum plate.


Everything got stripped, parts all cleaned. The body got a sand blasting to look like new aswell as the reluctor and stator as the we're rusty. I had to make a brand new ball bearing holder/breaker plate component as that had fallen appart.

Then finally re-build with some grease, put on a new ignition module and a new MSD master blaster coil.


The car now runs beautifully at 17deg advance. I now just need to fix the stumble/ flatspot as soon as you open the butterflies and everything should be sweet.


Thanks for everyones help

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