Donki

[Donki]

I am getting a lot of air sucking threw top of carb 80 Datsun 720 backfires really bad out of carb won't stay running it's getting fuel spray in main need help stock carb is rebuilt don't no what's going on 

[Lockleaf]

Since these are down draft carbs, that are supposed to suck air through the top, so we don't know what you mean by your concern.  Give us more, clear detail and you will get responses.

[bottomwatcher]

Yep there is definitely more to this. What was done recently? Sounds like someone was screwing with the timing or a broken vacuum hose.

[datzenmike]

On 7/3/2019 at 12:16 PM, Donki said:

I am getting a lot of air sucking threw top of carb 80 Datsun 720 backfires really bad out of carb won't stay running it's getting fuel spray in main need help stock carb is rebuilt don't no what's going on 

 

Slow down an punctuate your run on sentence.

 

Air is supposed to suck through the top of the carburetor.

 

Backfire can be from too lean a mixture, possibly a blocked primary jet. Is the carb bolted down snug with a good gasket?

Check the plug wires are going to the correct plugs. The order is 1342 in a counter clockwise direction. Usually 2 and 3 are reversed. 

Ignition timing incorrect. Should be 12 degrees BTDC.

Too tight a valve, specially an intake. Should be 0.012" exhaust and 0.010" intake set on a hot engine.

[Jon's AD3 720]

On 7/16/2019 at 4:53 PM, datzenmike said:

Should be 0.012" exhaust and 0.010" intake set on a hot engine

are you talking about valve lash or the amount of acceptable movement, side to side if you will, between the valve guide and the valve?