big block chevy

i have 71 402 cbb.. lower end had rings put in.new cam 286,296 advertised duration 540-540 lift hyd flat tappets rotela t oil,,heads gone through,new valves,springs,guides etc etc.summit hei distributor 2 years old driven to car shows 4 per month,,rpm air gap intake,holley 670cfm carb..timming 12 initial,,38 total in by 3000..floats checked,new coil.modul 15"vacume at 800 rpm.vavles re adjusted..car runs good at 900 rpm after it heats up it starts running rough and backfiring through exhaust..get out and have to move distributor retarted like 10 inches then it starts i have to advance it to get it to run.then it runs decent for a while ..also double roller timming chain 1/4 in play any help,,, thank you much

maybe carb???

How old is the fuel???? Sometimes I forget on my occasional drivers and have some gas that is a couple years old and it gunks up the whole fuel system......

fuel is new, filters are new,carb was new but buddy took it off another car after 10 starts,, i did notice he didn't drain fuel out of the bowls,gumed up??? im going to try and swap carbs for a test ???thanks for your help guys!!! best way to clean that gum out of carb

Just pop the carb apart and clean it up good with some carb cleaner and compressed air... Pay particular attention to the idle passages and accelerator pump passages.... The fuel we're stuck with these days turns everything to varnish after a short time!!!!

thanks ill do that this week ill let you know what happens

i switched the carbs ran nice, in the end same result.. i did see a plug wire that looked like it grounded itself through the heat sheild around the # 8 spark plug boot to the header??? ill update asap .. any other ideas? thank you much

Hey man, what's your power source for the dist, do you have a msd box also.

no msd ...painless wire harness ,12 volts to hei..thanks for asking..think i found it..90% electric 10% fuel...put nice new taylor spiro pro plug wires thought i had it, went to fuel up for car show sunday,felt surge,stall,that was it.. check for fuel, tons of that (750 holley dp) no spark,the rotor which i had checked no less than 3 times in last 13 days had a burn hole through the center, the tip fell right off at the bend no way of seeing it,, thanks for all the help, this is the best site for car nuts,,,stainless nuts that is ,,,

hey guys im back again,went to car show today it sucks showing up looking like a miilion bucks, alright 990,000. and running like crap.. did the same thing backfiring through the exhaust,missing,stumbling,didnt want to run right,,,any ideas, please help this week ill check to make sure i have 12 volts at dis,i have a good 5 1/2 psi fuel ????????? at least i won an award and 2 others i painted took best in class,,, thank you much

The only reason I ask is I pulled a stupid on my 427,had 12vlts until she warmed up,then it dropped to 6ish, forgot about the resistor in line wire....It did exactly what yours sounds like, a ghost in the machine...
Also make sure your dist is supposed to have 12 and not knocked down to another voltage..

positive lead direct to the dist.

Sounds like a resistor wire from the ignition switch. They were used in point type setups to kick the voltage back to 8-9 volts. The HEI needs 12 volts.

Try the wire direct from battery positive terminal to dist. Bet it'll run like a charm.

If it does, just replace the wire from the ignition switch to the HEI. Good Luck

Oh, you will have to pull the wire off the battery to shut the car off when you run the hot lead.:D

thanks for the thoughts,, it has no resistor,,painless wiring harness 2 years old ran fine 1-2 years...ill will check that 12 volts in the next day or 2,,,thanks again guys

its about time,the car is running perfect...the deal was the distributor shaft had too much slack up and down,replaced gear and shim on distributor...the timming was thrown off by the rotor scraping the contacts on the cap....thanks for the thoughts...usa #1 these colors don't run! you dream it we'll build it! great site!