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.

[siteunseen]

If that's TDC and it looks like it is, your cam sprocket is way off. One of those 3 windows should be about 11 o'clock. I can post some very helpful info from atlanticz's tech tips when I'm back home later today. Sitting in a waiting room right now, sorry.

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Yeah, I agree. No idea how everything else can seemingly be in the correct position (cam lobes, piston, crank pulley, distributor rotor) and the cam sprocket be so far off.

I'm glad to have any and all info I can get! Thanks for the reply.

[beermanpete]

Have you done a compression test? Or bettter yet, a cylinder leakage test? You need to veryify you don't have a bad valve before chasing the cam timing.

[siteunseen]

I don't see a spray bar for the cam so that heads been off before, might have missed a tooth or two when they put one back on.  Just a guess though but it happened to mine.

 

I tried and tried to get mine right.  I knew my car had never been worked on, it was unmolested or so I thought.    I couldn't get the distributor's tang exactly right, it was full advance and out of adjustment.  I spent a day messing with it, dropping the oil pump to realign the distributors drive gear.  Never got it right and drove it that way for a few months.  Took the motor apart and saw where the sprocket wasn't in time with TDC, but I wanted to rebuild it anyway so parts are scattered all over my house now.  But I enjoy working on them so it's no big deal to me and I'm learning.

 

Could the firing order be off?  I had a rod cap come off a 280 and had to replace the block.  My Dad is a great mechanic and said a straight 6 is a straight 6.  But he's a Chevy guy and had the firing order set for a Chevrolet, wouldn't start.  Went to work the next day and one of his buddies said the order was different for Datsuns (late 80s no internet).  Came home switched them around, fired right up and I drove that car for years.

 

 

http://atlanticz.ca/zclub/techtips/timing/mechtime.html

 

 

 

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Gonna do a compression test today to try and rule out the valves. The head has been off before. The previous owner had the head shaved, ported and polished, so there is a chance it was reassembled incorrectly. The firing order is correct according to the service manual. I checked it several times when I was trying to sort out the electrical issues the ignition was having when I first bought the car.

Thanks for the replies.

[gundee]

Gonna do a compression test today to try and rule out the valves. The head has been off before. The previous owner had the head shaved, ported and polished, so there is a chance it was reassembled incorrectly. The firing order is correct according to the service manual. I checked it several times when I was trying to sort out the electrical issues the ignition was having when I first bought the car.

Thanks for the replies.

 

Yes, you can see the head has been off as with the valve cover off, is real clean.

I also see its a E88 head with no spray bar. Internal oiled cam?

[gundee]

What is under that distributor rotor? Points? Electronic pick up?

[rossiz]

it does sound like a timing issues. the cam gear should have one of the numbered marks lining up with the bright link on the timing chain. there are 3 numbered marks on the cam gear which you can line up with the bright link to advance the cam and compensate for chain stretch/wear. there are 2 bright links on the chain, one is set to a mark on the crank gear - of course this assumes that the second bright link was correctly set on the crank gear (which you can't see unless you take off the timing cover)... 

 

if the chain has been installed wrong (bright link not lined up w/crank gear mark) you can still set the cam gear correctly, it just takes a little more work. with #1 at TDC (be sure) you can set the cam gear so that the timing mark (v groove in base of cam gear) lines up with the stamped mark in the thrust washer on the first cam tower. before you take off the cam gear though, be sure to wedge the cam chain to avoid losing your cam chain tensioner (which turns your 1 hr. job into a 6 hr. job).

[siteunseen]

Here's a good video showing how to remove and rotate the cam sprocket.

 

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Thanks for the helpful info guys.

I'm not sure about the spray bar for the cam. This is the first time I've had the valve cover off, and aside from the other day when I took the photos, I haven't had much time to work on the car to see what all is going on. I'll definitely watch the video, and try to get the cam sprocket lined up with the correct hole. I assume that the #1 hole is preferable?

Thanks again.

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Upon further inspection, it looks like the cam is oiled by the towers. The back side of the lobes are drilled, and the end of the cam is plugged.