Thread: bog/backfire

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hey im having bad problem with carb, sounds common, though ive never had it before. 350/290 horse crate motor(new). EVERYTHING NEW. have professional products crosswind intake(dual plane airgap copy) msd hei streetfire dist. and holley 600 vac sec #80457. when i try to get on gas quickly it wont rev(go) unless i do it slow or feather it. if iget past that spot it does fine! sometimes it would spit out carb. no vac leaks 10-13in of vac and it is set to highest vac in on gauge. timing is right and adv is working (vac and mechanical) this flat spot off idle is driving me crazy can anyone help me?? it has 31 squirter and i tried 35 but it got worse.

If you're running a stock cam, 10-13 inches of vac sounds low to me. most stock or nearly stock motors I've measured will idle at 18-20 inches. The 383 I'm running now has a Comp 262XE cam(218 degrees @.050 duration) and it idles at 18 inches. I always run straight manifold vac. on the advance. I think you MAY have a manifold leak.

dennyw<> the timing is at 36 total and around 16 initial if i come off that any it really runs bad . it backfires worse when it is cold then when you warm it up it mainly bogs but it will sometime give a spit, but definatly worse when cold. havent checked fuel press yet but it has new holly 80gph pump. floats are set to where if you rock truck a little it wil run out,(bottom of threads).distributor has 12.42 volts(not running) i have it wired to toggle switch(i bypassed column keyswitch.)and using a pushbutton start.also i took the truck to good mechanic friend and he switched vac hose to manifold i had it on ported,he set timing at 36total (he says)and readjusted air fuel it runs better but still does the bog thing.ordered new intake performer eps see if that helps

after i got it back from mechanic i played with the vac adv hose(switched it back to ported just to see what it would do it was just like before it would spit out carb so i put it back to man vac and reset air fuel because he set it to where it was running really rough when i did it ran way smoother but spit and hesitated again.choke is set at middle notch on housing, it comes off late.i havent checked where he has timing but i think he set it by ear.something isnt right just havent found it also i checked for vac leak there is not one.intake thats on it is 1500-6500rpm i ordered a idle-5500 performer eps something more around low end torque could the int on it now be too much??it has no offline torque , by the way its in s-10 it should hall!!!sorry so long but im really frustrated?!i really aprreciate any advice yall can give. thanks

Sounds like the Holley carb needs some tweaking... Changing the vacuum spring will change the rate and time at which the secondaries kick in. Get a timing light and see where the timing is set at, and make sure the vacuum and mechanical advance is coming in correctly. Could be it's set with too low of intial timing and/or mechanical advance is coming in too soon.... To me, I think the engine should be producing a lot more vacuum, which could indicate the timing being off or perhaps a valve out of adjustment....

Sounds to me that the floats are set too high and I'd drop the squirters down another notch or two and the primary jets might need to go down a size or two. Holley's from the factory are generally jetted a little rich.

"Bog" is almost always either retarded timing or lean mixture. Or a vacuum leak. Your vacuum is definitely too low for a basically stock engine. I check for leaks with a small propane torch, just blow propane along all the gasket edges, if she revs up, there's the leak.

P.S. Dont light the tourch, just spray the propane.

cool guys you all have givin me alot of info ! ill try some of this stuff when im off monday and let you know where i stand then. thanks again.

propane?! i like to just cover the engine in gasoline
btw ASE says using propane is dangerous and is not suitable... you're supposed to use a smoke machine... now if everyone just had the $1200 to buy one of those babies.... heck, if only the shop i work in even had the money to buy one

Should have mentioned that the propane thing is for outdoors only, don't do it in the shop. The amounts used aren't dangerous but your insurance guy would have a fit! Much like using ether to seat a tubeless tire, no sense of adventure, these guys.
BTW, I tried acetylene once.(For a vacuum leak, not seating a tire.) It worked, but the noise was unbelievable. That stuff sure cracks.

Did this engine come with a harmonic balancer and timing tab or did you have to supply one? There are several different locations for the timing marks on Chevy balancers depending on where the timing tab is located on the engine. You need to verify that the TDC indication you have is correct. On an assembled engine you can use a piston stop in #1 cylinder. Turn the engine by hand in both directions until it stops, mark the balancer in line with TDC on your timing tab in both directions. Halfway between these two marks should be true TDC. If you have the wrong pointer for your engine or the balancer hub was marked wrong, your timing can be off a LOT and still look right with a light. A quick way to check would be to just crank in some more initial timing and see if it helps.

Originally Posted by thesals

propane?! i like to just cover the engine in gasoline
btw ASE says using propane is dangerous and is not suitable... you're supposed to use a smoke machine... now if everyone just had the $1200 to buy one of those babies.... heck, if only the shop i work in even had the money to buy one

Good thing them guys don't hang out in my shop, they probably wouldn't like a lot of things that I do!!!! I do my best to keep things safe, but sometimes you just have to rely on good old common sense. Heck, if you bought every piece of equipment and supplies they think you should have you'd be broke before you ever got around to buying any tools to do the job with!!!!! Would be interesting to watch some of these fellas work on their own stuff, huh????

sorry i was wrong vaccum is even lower with it in gear it is 9in. of vac and in park it is 13in.. 24deg. initial -- 38deg. total all in about 2500rpm .(vac hose disconnected)with it connected it idles @ 40deg initial!!! didnt think i had vac leak but i remembered i checked it with carb cleaner?!? that probably isnt flammable enough, propane sounds better. ill keep trying things tommorow, and im gonna set valves and check fuel press.anyone wanna buy an S-10 lol lol. let you know what i find asap. thanx sooooo much!

sometimes if you used every single piece of specialized equipment they make for everything you do... you'd spend more time dragging equipment out and hooking it up than you could spent just using plyers and a screwdriver