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Running Like a Dog Today


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I drove my Z to work today and man was it running like crap! It was sputtering and backfiring (from the exhaust) on acceleration like crazy, not even hard acceleration just average.

If I pull the choke up then it drove better...killed any power I have though and made passing tougher. However, I would have to push the choke all the way down when I came to a stop otherwise the car would stall.

I drove it a 1 1/2 weeks ago and it was running fairly well all things considered. Now today the temp was 5-10 degrees less and the humidity was higher as it has been raining here on and off for the past several days. I did have it running on the 4th showing it to a few friends but did not drive it anywhere.

Any ideas or help would be great!! I'm at a loss as to what's going on. :ermm:

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When I first bought my Z it used to do the same thing when and after rain, and would sometimes just die in very heavy rain. This made me think it had to do with moisture in the electrical system.

It turned out to be the rubber cap/grommet from the coil wire that's connected to coil had hardened up and wouldn't seal correctly. I have also had this problem on another old car, 1967 Mustang, and it turned out moisture was getting under the distributer cap.

I would definitely check all wires coming to and from the distributer and coil, dry out the connections, then see if the problem is cured by running it. If it is replace faulty wire/wires. Check to see if there is moisture under distributer cap, if there is replace it.

The moisture might not be that visible, use some nondemaging heat to dry things out.

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Fuel filter is less than a month old and still looks brand new.

A couple guys I work with and I just took a break and walked out to see the car (one had not seen it yet) and they both agreed that the problem has to do with the fuel system in some way. Suggestions were to spray carb & choke cleaner into the carbs to try and clean them up a bit. Another thought was maybe about a vaccume leak somewhere (but I have no clue how to check for this).

At lunch I checked for moisture on all the plug and coil wires and everyting looked dry. It's in the mid to upper 90's today and the car was parked out in the sun so it has heated up nicely so anything that may have been damp should be dry by now. Yet it still runs like crap.

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I would pull 1 spark plug from the front three and 1 from the back three cylinders and have a look, it's amazing how much this will actually tell you. Most shop manuals have a spark plug diagnosis guide showing pictures. Plugs should be toasty brown. You will at least be able to tell if your getting to much fuel or to little. It will give you a better idea where to look next, and only takes a few minutes to check. This is almost always the first thing I check when my car starts acting up.

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I don't know if this appilies in this case, but, I just had a mystery miss-poor performance issue. I have the ZX EI distributor, so I thought "at least I know it's not the points"! Turned out the plug to the EI module was almost completly disconnected. I would not have seen it. I thought my coil wire had gone bad. Went to pull the coil lead off and jostled the signal wire to the module and it fell off. AhHA!

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Guess you gotta be careful doing it but. . . We're always careful right? I have detected vac. leaks by spraying small burst of starting fluid or ether around the vac. hoses, lines and fittings. When I notice an RPM change or difference in the way the motor ran there was a possible location where the vac. was taking in the ether. When you find a good leak you can see the spray evaporate very quick and a definite change in RPM.

Also the deal with the moisture getting in sounds like the other place I would look too.

Good luck.



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Ok, so if you read the original post you know what the car is/is not doing.

1 - I pulled the plugs and they were ALL black as could be. Looked this up and the car is running rich. This was supportive of the fact that the car is backfiring like crazy. Damn, these plugs are less than two weeks old. I wiped them down and put them back in.

2 - I pulled the air cleaner and while the car was running sprayed each carb with carb & choke cleaner. Oddly enough when I sprayed the front carb the car would bog down as expected but when I sprayed the rear carb it would not bog down no matter how much I sprayed in there. I put my hand in front of the rear carb and it was pulling air.

3 - Using a scynrometer I tested both carbs and the meter shot up to 30 (the highest reading). This is one of those nifty meters that MSA sells to balance and tune your carbs. Now I do not know what a reading of 30 means nor how to adjust them to be where they should be.

4 - While the car was running I turned the adjusting screws on both carbs counterclockwise a couple turns and this did not seem to make any difference. I turned the adjusting screw on both carbs all the way clockwise then counterclockwise 1.5 turns. This seemed to make no difference either.

??? I'm lost!! Now what??? :disappoin

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The first thing you need to do is either clean the plugs, buy new plugs or they will keep fouling. Which is why you have power problems.

The problem is definitely with your carbs, their adjustments are out of sync. I would first make sure the chokes aren't stuck open. You check this by feeling directly on the bottom of the carbs. They should both be up if the choke is off. If one is stuck open don't force it shut. You need to loosen the cable first. Also check to see if you have the correct amount of oil in them.

The next thing I would do is go to this attachment on tuning carbs. It is from Zhome.com.

If you get stuck I can answer so specific questions, but is a very lenghty process explaining the procedure in type. Only takes about 10 minutes to complete the tune though.

The best thing I ever did for myself in regards to tuning SU's was buy the video from Ztharapy. I would still be guessing about this stuff if it wasn't for that tape.su tune

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biker, I forgot to mention that someone along the way has converted this car to elctronic ignition so there are no points.

GTZZZ, is the video really that helpful? I guess maybe I should buy a copy of it.

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SU tuning is just about as easy as anything gets on a car. There are dozens of sites on the web that will tell you how. Search for SU carb tuning or Skinners Union carb tuning. It doesn't matter if the site is a Datsun site or a Triumph site, it's all the same. Sorry to blaspheme here, but I never found those cheap a$$ unisyn flow meters to help. I trust that my own senses are more accurate than those things. You also should learn to "read" plugs. There is a fundamental old school skill that is necessary to learn how to tune carbs.

The reason when you shot stuff in the rear carb and nothing happened is because the last three cylinders aren't firing, most likely because of plug fouling. Wiping them off doesn't clean them well enough, they need to be bead blasted to get that carbon off.

If the carbs were purely the issue, it wouldn't matter if they were synced or rich or what not with WOT. You will have power if your ignition is good. Too rich, and the carbs may be fouling the plugs, or you may have an ignition problem that is contributing, but until you get some good plugs in there, you really aren't going to know if the carbs are right or not. Like somebody said here, spend $10 on some new plugs.


'73 FP 240Z

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  • 4 weeks later...

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