bog/backfire

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.:mad: :mad: :mad:

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 :LOL: :LOL:
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.

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Originally Posted by thesals

propane?! i like to just cover the engine in gasoline :LOL: :LOL:
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

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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????:LOL: :LOL: :LOL:

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 :LOL:

hey im back, fuel press is 6lbs and i also adjusted valves(back off until starts tickin and tighten till quits then went 3/4 turn more into it)and it runs a little better. and let me say they were pretty tight. but anyway still waiting on fed ex to deliver intake(they bite). i also blew thru all fuel lines from front too back and cleaned out fuel cell,didnt find anything.plugs are black but i havent had chance to drive it very far just around the block. carb has 65 jets in primary meter block and 6.5 power valve. Can anyone tell how too make piston stop out of old sparkplug??

Drop the jets to 62's, 65's are too rich for that engine combo.

62-s ok (just in primaries?) well i played with it again today and i think i found true tdc but not perfect i just did it with screw driver . the difference was about 5deg.(to the left of mark on balancer) so i marked it and retimed it retarded it to 34deg. total and 18initial and readjusted air-fuel. i als backed vac adv screw in pot 4 turns. it ran pretty good @ idle, no bog or spit.went to drive it and BLAHHHH!! doin same thing runs like s--t! acts like way too slow. still has low vac and i srayed for vac leaks again, around throttle side it(base plate) it kinda dropped a little??STILL WAITING ON INTAKE FROM FED EX!! well let you know after new intake ,, and ill try the smaller jets.. thanx

My money is on the vacuum leak probably intake gasket. ;)

got intake gonna put on tommorow. man edelbrock has a way better intake than the junk thats on there!! im hoping it was just the intake gaskets, that would be easy fix,, well see. also gonna adj valves the right way too.. let ya know how it runs...???>>><<

hey, changed intake yesterday and set valves(by marking balancer then turning in 1/8 turns and adjusted them 1/4 turn once i started feeling pressure on the pushrod, done all by the chart in "how to build 350"book)then put on tdc and dropped dist in cranked it up , put 34deg total timing and 18initial. readjusted floats(again) and a/f ratio. wellllllll, it does not bog or spit at all!!!!!!!! but i still have low vacuum (14-15in.)in park?????wonder what i did too fix it, i dont know but my money is on the intake, this one is better suited for my motor. but i still wish it ran better, next i think ill buy a piston stop and check true tdc the right way(instead of screw driver)But i sure would like to know why so low of vac?no leaks ;this intake fit like it was made for it, not like the other cheapo knock off.

What kind of shape was the old intake gasket in???

gasket was good but i noticed a littletrace of black stain on base plate gasket (from backfiring or leak??)but anyway couldnt find any evidence of leak from looking at intake gasket, only base plate gasket.when i put first intake(air gap knockoff) on i set it on engine(before gaskets and dry ) just to see how it fit, the front and back(where end gaskets or silicone go) had about 1/4 in gap, i did the same with the new one(perf.-eps.) and it had no gap. it fit perfectly. ill probably know more when i can work on it again and run it longer and check plugs.do you think i set valves correct? and one more STUPID QUESTION before i go , how do i raise or drop initial timing? i think you do it with the springs to weights in dist right?because my total is where i want it(may change when i get piston stop).thanks again for the help. I THINK IM HEADED IN RIGHT DIRECTION. DOESNT POP ANYMORE===YEAH YEAH!! one more thing i adjusted choke cap two notches leaner also.