L26 blowing back through the carburator

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Hi all.

 

I recently bought a 260Z as a project car to work on with my son, and I'm having some engine issues. The car definitely needs a paint job and needs a good bit of interior work, but the body is great and has very little rust to speak of. The engine wasn't running when I got it, mostly because of electrical issues with the ignition.  The ignition has been corrected and it working properly now. 

 

My main problem at this point is that the engine is "blowing back" through the carb at times. I say blowing back because it doesn't really seem like a backfire, although it has backfired through the carb before when the ignition timing was way off.  The engine will crank, and it will stay running, but only if I feather the throttle. It will run ok for a second or two then either surges like crazy from too much fuel (my own fault), or sort of sputters and stalls if I just try to let it idle. Sooner or later (within seconds usually) it will start blowing fuel and air back out of the top of the carb. I'm not 100% sure if it's happening during compression or exhaust, or both, but it did briefly ignite the fuel in the carb once when it had flooded a bit.

I forgot to mention that the old Hitachi carbs were replaced by the previous owner with a 390 CFM Holley 4 bbl.

 

I assumed that this problem was caused by a timing issue, or possibly bad valves.  I don't really have a whole lot of time to work on the car right now, but I had a few minutes yesterday, so I pulled the valve cover off to take a look.

 

These are some of the things that I did/checked/observed:

 

-Put the cam lobes for #1 intake and exhaust at 10 and 2.

 

 

-Made sure that #1 was at TDC.  It was.

 

 

-Checked the timing mark on the crank pulley. Looked fine

 

 

-Checked the position of the distributor rotor to make sure that it was pointing at #1. Probably not perfect, but close.

 

 

-Attempted to verify that the cam sprocket mark lined up with the reference mark.

As you can see by the picture below, NONE of the sprocket  holes were in the correct position to be able to see the reference mark.  This is obviously wrong, but I'm not sure if that alone would cause the issue that I'm having. Also, the timing chain seemed to be a bit looser than I think it should be.  Again, I don't know if that alone could cause my blow back problem.  Also, for whatever reason, someone had set the valve clearances to .005!  All of them.  Intake and exhaust.  Easy enough to fix, but it makes me wonder what else the previous owners may have done incorrectly.

 

 

Has anyone ever had a similar issue?  Any suggestions on what is the most likely cause?

Thanks in advance for any and all comments.

[johnc]

Adjust the valves, set the timing, sync the carbs, adjust the mixture.

[NewZed]

He said it has a Holley 390.

 

Is that your distributor or a picture from the internet?  Looks like a ZX distributor.  If you can't check the timing at least make sure the triggering points are aligned in the distributor.  Or about 10 degrees retarded with everything at TDC.  That will give you clue about timing.

 

I've seen Chevy small blocks pop back through the carb when a cam lobe was flat.  I'd imagine a burnt/warped intake valve might pop back also.  Valve lash will decrease as valves sink in to their seats from wear.  The old unleaded gas problem.  Focus on what the valves are doing.

 

The sprocket problem is odd.  Maybe it's on backward.  Some of the aftermarket sprockets don't have the mark.

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Thanks for the replies.

 

I'll definitely be adjusting the valves. The mechanical timing seems OK from what I can tell so far. I guess I need check the crank sprocket to be 100% sure since I have no idea whether the pulley is the original one or not. The ignition timing would be really hard to do given that the car won't idle. That's pretty much my goal at this point - to get the car to idle enough to check things out with a timing light.

The distributor is from a 280ZX. I guess I forgot to mention that. I'm going to check the compression as soon as I get a chance. That should at least allow me to rule out a valve if the compression is OK. As for the lobes, I don't see anything that jumps out at me as far as wear goes, but I'm not ruling out anything at this point.

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Update :

Just checked the compression and it looked pretty good, so I'm inclined to think that the valves are OK. I guess the next step is to get the cam sprocket lined up where it's supposed to be. Guess I need to make one of those chain tensioner wedges that I've seen mentioned.

These are all dry numbers

#1 170

#2 170

#3 170

#4 172

#5 172

#6 175

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Turns out that the chain tension is fine, so no issue there.  The cam gear and timing chain are both aftermarket.  No triangle notch on the gear, so no real way to check the timing based on the reference mark.  I still need to adjust the valves to get the proper clearance,  but that's pretty much it as far as the mechanical timing goes.