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My new motor in my truck backfires thru the intake when i rev it up. But only if the fuel pressure is below 40psi with the vacum hose disconected. It also seems real low on power. For starters i have a rebuilt 302 shortblock balanced. gt40-p heads with trickflow springs, 1.7 roller rockers, e303 cam, 24lb injectors, 75mm pro-m meter, a9p computer, and stock 95 intake and throttle body. In a 4x4 ranger with a aod tranny and 2400 stall converter. could the stock intake be restricting it that much? I want to get a cobra intake and a 65mm throttle body. Will this get this thing moving? Also I am only getting around 10mpg. I think I need to recheck my valve lash. I didn't use any shims on the rockers. just bolted them on and they seemed to be fine. But I did it before the motor was ever ran and the lifters weren't pumped up. Could this be my main problem? It seems to run okay with the pressure cranked up to 45psi. Timing is set at 12 degrees.
 

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It sounds like a timming issue. Have you checked the firing order, the H.O. 302's have a different firing order than regular 302's. I made that mistake on my friends car when we were putting it back together and it was back firing through the intake also.

302 H.O. firing order: 1-3-7-2-6-5-4-8
 

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Ryan99 said:
I think I need to recheck my valve lash. I didn't use any shims on the rockers. just bolted them on and they seemed to be fine. But I did it before the motor was ever ran and the lifters weren't pumped up. Could this be my main problem? It seems to run okay with the pressure cranked up to 45psi. Timing is set at 12 degrees.
yes, most definitely go back and adjust. you will most likely need to. i did after mt rebuild. your valves might be hanging open.

take care,
adam
 
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