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-   -   Blowback through carb (http://www.hotrodders.com/forum/blowback-through-carb-227219.html)

QD1956 12-16-2012 06:20 AM

Blowback through carb
 
Having difficulty tuning my rebuilt 327. Engine is knocked up with race performance internals and is the first time I've ever tried tuning a motor this hot. Whenever I try to start the engine, mostly when it's already warm, I have to crank it over and over and play around with pumping the gas pedal. Sometimes I have to pump more than other times and sometimes I don't have to pump at all and it will fire up. But most the time when it tries to start, burnt gas fumes and un-burned gas come jumping back out the top of the carb. Lots of smoke and really embarrassing! Whenever i use a shot of starter fluid then crank it over, it fires right up. I've adjusted timing and even main jets on carb. which is a QFT 680. Could I possibly have the wrong aluminum head spark plugs or maybe not enough spark? I have a 12:1 compression, mallory distributor w petronix II ignitor and a 40,000 V coil.

454C10 12-16-2012 07:28 AM

When it "fires up" on starter fluid, does it run after that or does it shut down after a few seconds?

What is the ignition timing set at?

What pistons and heads are you using?

Infomaniac 12-16-2012 08:56 AM

Sounds a lot like a blown powervalve.

cdminter59 12-16-2012 09:22 AM

Blowback through carb
 
Post all the information you can about your motor. Brand of crankshaft, rods, pistons, head gasket compressed thickness or part number, brand and type heads, , brand of spark wires, type of spark plugs AC R45, part number of camshaft and type, hydraulic or solid tappet or Roller cam. Who built the motor? Who setup heads. Have you checked for vacuum leaks? Spray starting fluid all around intake flange and carburetor base at gasket. If idle goes up you have a leaking gasket. Tools you need are a vacuum gauge and timing light with advance. (has a knob on back end) Check spark plug wires with multimeter to see if you have continuity. What kind of car or truck (year and make) is the motor in? Read this article on HEI distributors and timing. http://www.crankshaftcoalition.com/w...EI_distributor Use the vacuum gauge to set idle mixture screws. Make sure your fuel pump pressure to carburetor is about 5-5.5 psi. How or what adjustments did you make to the jets on the QFT 680? Has the engine back fired thru the carburetor if so you may need to replace power valve. Get a vacuum reading at idle. If the reading is say 10 divide by 2 = 5. Use a 4.5 power valve. Here is the instructions for your carburetor. http://www.quickfueltechnology.com/I...ons_Rev._1.pdf


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