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240 backfires and has erratic power

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Hello all, I have been searching and tuning and searching some more and I'm throwing in the towel and asking for help. I have a stock 73 240 with the exceptions of a lightened flywheel (5lbs taken off), roundtopps, a turbo oil pump, and just recently a zx dizzy upgrade. I rebuilt my l24 600 miles ago and have been tinkering with tuning carbs and ignition timing and it just doesn’t seem right. I just replaced the oil today from the 30w break-in oil to 10w-30 valvoline vr1. Compression test showed 150psi in all cylinders. Ignition timing is set to about 10.5. I am a newb and this was my first engine rebuild.

Alright done with the introduction now to my symptoms:

1) When accelerating hard at higher rpms (4000+) the power seems erratic like it’s there and then it’s gone, back and forth. Not to an extreme degree, but it seems to vary at revs within 100+/- of each other just going by seat of the pants.

2) It isn’t a lean pop as far as I can tell, but if I hold rpms (in neutral) at about 4000 and then let off there is a backfire or 2 on the way down that I believe is through the exhaust.

3) While I was tuning carbs I was going to rev after idling for a few seconds and as soon as I started to open the throttle both carbs backfired white smoke (I haven't been able to replicate this event)

4) When tuning the carbs with the front disabled I can’t seem to get the car to die even with the adjustor fully leaned on the back (bad jets?)

5) When I let the engine brake the car it backfires, I am unsure if it is through the exhaust or the carbs.

6) With the new dizzy the idle has smoothed, but still seems a little rough

7) On the initial break-in trip the car had normal oil pressure, but on the 80 mile trip home I lost the pressure and it has been non existent since, I replaced the sensor. I know all connections are good because the sensor sends the needle all the way up and then down again when the key is turned to on. I am unsure if the oil pump is truly a turbo pump because it was an eBay purchase. I have pulled the valve cover to make sure that oil was still circulating

Also, I don’t have much to compare to, but the power seems to be lacking

I don’t know what a slipping clutch feels like, but could the erratic power be contributed to that?

On my to do list tomorrow is re-checking valves, plugs, float settings, and ordering a video from Ztherapy. For the video do you recommend “ZTV09 Just SU's ” or “ZTV01 240Z-280X TuneUp”

Thanks for the help in advance and I’m sorry if this has been posted before, but I cant find it.

Also I once found a great write up on what caused backfire where once and I cant find it, if anyone can help with that it would be appreciated too

Thanks again, roger

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I'm having a similiar problem. The car sort of like.. bucks like the clutch is slipping it seems. The car makes lots of noise and revs to 6k, but it doesnt seem like it keeps pulling. I haven't tuned the carbs or anything but I did remove some of the smog stuff and it seems to run a little better except for idling is a little rough sometimes. Anyone know what the cures are?

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First thing , you need to cure the oil pressure situation. You can pick up a cheap manual oil pressure gage for about $15.00. Just remove the sender and screw in the line there. this will tell you what is going on. My Turbo pump puts out 25 at idle and 50 psi at 2500 rpm. Compare to that. I also use the ZX dist. but set my timing at 16* btdc. #1. Do you have water running through the intake manifold? At a Show last weekend , we were able to put some of our Zs on the dyno. The Zs without water cooling the intakes , once the engine was run on one pull , even though allowed to cool down . They predetonated at the higher revs. I run with coolant in the intake and did not have that problem. This might be what you are finding above 4K.

Because today's oils are formulated for the new cars. The oil companies have removed the zinc from there oils for emission reasons. Our engines and engines with flat tappets suffer early cam and rocker failure because of this. Valvoline Racing oil has the correct formulation. You can contact Hagerty Ins. to verify this among several other sources. I mention this because you have a new engine. #4 When allowing the engine to slow the car on compression and you have back fire, a little of this is normal , more like a light popping not a cherry bomb bang. Have you checked or adjusted the floats in the carbs? If the float is set too high it will cause raw gas to enter the air stream in the carb just as if the choke was pulled on. this could account for. #3 This is generally referred to as a lean pop. Unless the timing is not set correctly. All the best. Gary

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but set my timing at 16* btdc.

Does this mean that when the strobe lights the timing mark is at 16 on the timing guage thing?? going by the harmonic balancer

also any reccomendations on which ztherapy video to go with

Edited by HxC240z
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When the strobe fires the mark on the engine dampener is pointing to 16* on the degree plate mounted on the engine. Does your timing light have a adjustment knob on it? If it does be sure it is set at zero. I haven't seen either video.


Edited by beandip
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Get the Ztherapy Tuneup, as it covers more stuff than just the carbs. It'll teach you how to do your valve adjustments, set your timing and then do your carbs.

The backfiring is probably related to your ignition timing, which if I understand you right is 10.5 advanced? Try lowering that a lot. You might even set it to about 5 before. If you can't get it that far retarded, its likely because you put that dizzy in wrong. There's only two ways to install it, but the wrong way will cause your timing problems.

The timing problems will also explain the hesitation at high RPM. Another thing explained by having that timing so far advanced is if you are having problems starting it on the first crank of the day. It'll crank slowly, like the battery is low.

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tried getting a pressure guage, but advaced/autozone don't know what I'm talking about and napa's are 50$

it was set to 10.5* btdc by haynes's reccomendations, but I have heard 16 allot, so I'll reset it to that. 5* sounds way too late I've never heard anyone say that late, any reason why it should be there???

I'm pulling the pump apart today and checking for clogs/problems

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where can you get a good cheap guage, the pump seemed good, where should I check next, the springs??

if it loses prime can it refill itself okay??

I took it out and seemed to be running great, did allot of little things while I was in there... help, I wanna dive my z:cry:

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