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74 620 backfires out the carb


sanyantho

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well if carb is new, dist is tight I really dont know, beside ck the valve lash.

 

Now if you opened up the new carb and fucked with it anything can change. but they really pretty simple.

 

 

I had bad things like plug wires arching or spark plug electrod smashed before but most people catch these and we just dont mention it.

 

reground the condensor or get a new one. inspect it closely

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well if carb is new, dist is tight I really dont know, beside ck the valve lash.

 

Now if you opened up the new carb and fucked with it anything can change. but they really pretty simple.

 

 

I had bad things like plug wires arching or spark plug electrod smashed before but most people cat these and we just dont mention it.

 

reground the condensor or get a new one. inspect it closely

Thanks Hainz, I did not mess with the carb. I will check the valve lash in a bit.I will pull out the plug and check those as well. I will diffidently check the condenser and replace.

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Hey Mike,

    I took the cover off the top of the carb to check and it was running dry. So I adjusted the float level, it running correct now. But that did not stop the backfiring out the carb. In fact, it seem to have gotten worst. Now while I am idling, and I push on the gas, it starts to backfire. Before it would only backfiring as I take off on first gear.

 

I am going to check the valve lashes again, as you suggested. Anything else you can think off? Thanks so much for your help everybody!

 

You should have pulled the main jet out while the cover was off to see if part blocked. The float was probably out of whack from all the backfires.

 

Check valve lash. A tight intake won't close properly.

 

Also an intake valve may be burned/cracked and not sealing.

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You should have pulled the main jet out while the cover was off to see if part blocked. The float was probably out of whack from all the backfires.

 

Check valve lash. A tight intake won't close properly.

 

Also an intake valve may be burned/cracked and not sealing.

So I was checking the valve lash and found that the valve spring was not set correct or loose on the first intake valve. This is the valve that I had to shave off the adjusting bolt to give more pay to get the clearness. When I put the post, shaved adjustment bolt, and spring back together, I did not set the spring correct because it had way more pay than the other valve springs. Now that I set the spring in correctly and tighten it the best I can, the truck runs like a beast! No more backfiring, seem to have more power also! Does anybody know where I can get the valve spring? I know I going to need to replace it, because it is so weak compare to the other valve springs.  Thanks you Mike and Hainz!

image_zpsuzaxplga.jpeg

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The little clip on spring? We call them the mouse trap. Not essential for closing the valve but helps position and hold the rocker on the adjustment post
 
 

Check valve lash. A tight intake won't close properly.

 
The intake clearance was likely too tight and the valve not sealing tight. When the cylinder fires the explosion got past the valve seat and ignited the intake

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The number one spark plug on the right compared to the rest on the left.

image_zps5dqmro8y.jpeg

Check for leaks in the intake manifold gasket where it bolts into head for #1 cylinder....when it's running spray carburetor cleaner in that area and listen for changes in idle....try not to do it when engine is very hot.

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I have tons of those Rat trap springs, see if anybody has some locally.

glad its working out OK. You will need that head redone soon as the valve is sinking. Usually its the exhaust valve that does this.

 

 

spark plugs are mostl likely fine. cant really do much with it anyways.

 

Buy a timming chain tool from amazon. datsun L series timming wedge.

My valve is already sinking. But mines is an intake valve. Where do I take the head, to get them redone? I will buy the timming wedge asap. Thanks

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My valve is already sinking. But mines is an intake valve. Where do I take the head, to get them redone? I will buy the timming wedge asap. Thanks

 

Might be easier to just source a U67 head from any L20B. Larger intake ports and larger exhaust valves with square ports to match your L18 exhaust manifold.

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I had a later U67 head and the seats are the harder type but  good Head shp needs to be found that does Datsun heads and can install seats well.

I always have a extra Head that I can install and then research this at a more leasurely pace. not to give it to the wrong shop.  They have to shave the head then install the seats recut them so the same lash pad can be installed ect.

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Hi All,

      I bought an u67 head from a member for $335 including shipping. Should I install right away or wait until the stock head(one currently running) goes completely dead? I already order the timing wedge tool from Amazon, as well ask the head gasket. I still looking to order the intake/exhaust gasket. Does anybody know where I can order that from? Thanks in advance!

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