under heavy acelleration,or quick throttle response,I get a poppin out of carb.355cu.in, trickflow heads,msd6A,blaster2 coil,performer intake,700cfm holley double pump,intial timing is at 14degrees, total is 41 degrees all in by 2800rpm.please help??
Sounds like a burned valve, or ingition backing up into the carb like a leaking valve. If your ign timomg and balve timing are correct, it's the next check. Is the car misfiring at all? If it's a leaky valve it won't necessarily have a dead miss and may not be real noticable. Take a compression test to verify.
I have had bad cams act this way as well. They idle alright but you may want to check for a flat lobe. You don't have a Crane cam by chance do you?
Did you advance the cam timeing when you installed it? Most cams have advance "built in" and I found out the hard (expensive) way that you don't advance them unless you know what you are doing --I didn't!!
<BLOCKQUOTE>quote:</font><HR>Originally posted by kausboy: I have had bad cams act this way as well. They idle alright but you may want to check for a flat lobe. You don't have a Crane cam by chance do you?
Did you advance the cam timeing when you installed it? Most cams have advance "built in" and I found out the hard (expensive) way that you don't advance them unless you know what you are doing --I didn't!!<HR></BLOCKQUOTE>
the cam is a 280h comp cams w/480. lift,and it had 4degrees advance built in ,and yes I did degree the cam.to me it seems like a sticky valve or a vacuum leak.The engine has only 5 hours on it,is there a way to cure a sticky valve?
Throw money at it till it goes away!!. J/K
In my case I had to pull the heads and have the guides replaced --seems avgas is too dry for automotive engines.
It could be that the engine is way too lean as you suggested. Usually engines seem to backfire when the timing is retarded. The only other thing that comes to mind is the firing order is messed up or you have bad plug wires, carbon tracking or cracked dist. cap. or bad distributer. Maybe your timing marks are off? Possibly a bad grind on the cam? When my valves stuck in the head my studs actually started pulling out (pressed in studs) Are your lifters adjusted too tight? you don't have solid pushrods by chance. They won't let the lifter "bleed down" and they may be pumping up holding the valve open.
I think you should start simple like dist. timeing and lifter adjustment then work up to sticky valves and bad cam.
I am sure you have done this more then once but the basics are where to start. Firing order 18436572. Make sure the respective wire goes to the respective plug. You can use the rockover method to check for proper timing. Take the valve covers off and watch at the No.6 rocker arms. Slowly rotate the engine until you see one rocker moving up and the other just start moving down. At that point No. 1 should be on Compression stroke TDC and your timing marks should be near zero and your dist. should be pointing at or near no.1 (this is also the best way to adjust lifters so you know that both the intake and exhaust are on base circle on No.1) If that checks out adjust your lifters. May be too tight
If you write out the firing order with 1843 on top and 6572 below you will know the companion cylinder for each cylinder. For example when No. 8 is on rockover no. 5 will be on compression -adjust no. 5 lifters. I adjust my lifters till there is no play up or down in the pushrod --(don't do it by spinning the pushrod) then I go 1/2to5/8 turn to center the lifter.Rotate engine to next cylinder and continue till all lifters are adjusted on their base circle. Takes time but it is tried and true!!
Try this also --when you adjust the lifter loosen the rocker arm and tap on the valve with a dead blow (plastic) hammer and see if your valve "snaps" or "pops" shut like it should or if it hangs up. I guess if you are strong enough you could just push down on them gently and see if you can "feel' them sticking. I wouldn't advise hitting them too hard!! If the piston is up (and it will be on the compression stroke) you could do some damage. Maybe you shouldn't do that --I was just trying to figure a simple way to check for sticking valves without removing the head.
Anyone else out there with some advice?!?!?!
Hope this isn't too long and confusing
You could have some debri stuck on an intake valve from when it was built. I would pull the plugs and see if any are wet with fuel or just dont look like the others. Also run a compression test on all the cylinders. You want to catch this problem and fix it before it gets worse thats for sure. I would check all the easy stuff before you go taring apart the engine.
Its still popping up carb after checking valve adjustment, timing,and spark plugs look good, brownish/white.I did notice that it seems to pop up the carb when the secondarys are opened,but its a new carb,and the squirters are squirting and adjusted right.I`m stumped.....
Its still backfiring up carb after new plugs and timing.accel#416 plugs.50gap,14degrees intial, 31degrees total at 3000rpm.it seem to back fire when the primaries are just about fully open and the secondarys are starting to open I took a compression test 145-150 all cylinders,I also tried a bigger main jet83 seemed worseI went back to original 78,and tried a bigger and smaller nozzle shot no change .its 700cfm double pumper brand new out of box,now I`m starting to think its the primaries,what do you think?
I have same problem with 750 vac secondry. Have done search with varied results, it seems there is not one sure answer, but many. one person cured there problem by changing distributer shaft after finding it "sloppy" someone else cured theres by changing squirter size on carb. I've gone through the normal check list with mine & am at the pulling the cam stage, as suggested above, it seems the only thing left were problem might be. I shall let you know what i find.
<BLOCKQUOTE>quote:</font><HR>Originally posted by jhow66: What kind of dist. are you using? does it have a vac. advance? if so be sure and disconect it when checking time. It sounds like you are getting a low time backfire. Hpoe this helps.<HR></BLOCKQUOTE>msd electric dist. blaster2 coil,vacuum advance,what is a low time backfire?
What that means is if you set your timing at, for example 12 degrees below top dead centre with vac advance connected, when vac advance is disconnected your timing is actuarlly set at 0 degrees. Vac advance servs no purpose when under exceleration.
too lean and/or too retarded. jets seem ok. I have the same heads on a 350 w/ dp 650 cfm. I have the initial advance set at 18* BTC. I would check the mechanccal adv. for wear, as you may have enough vacuume adv. till you open the secondarys. After you open them, advance is totally mechanical. If you dont have any mechanical, you'll pop as the advance drops back to the initial setting!
Make sure you check initial timing with the vacuume advance attached to the timed vacuume port. The one with NO vacuume at idle! If you set it up to the manifold vacuume port, your pulling vaccume advance at idle, this could result in an AFTER top DEAD center timing when vaccume drops at wide open throttle. That .050" spark plug gap isn't helping things either.
<BLOCKQUOTE>quote:</font><HR>Originally posted by joe clance: too lean and/or too retarded. jets seem ok. I have the same heads on a 350 w/ dp 650 cfm. I have the initial advance set at 18* BTC. I would check the mechanccal adv. for wear, as you may have enough vacuume adv. till you open the secondarys. After you open them, advance is totally mechanical. If you dont have any mechanical, you'll pop as the advance drops back to the initial setting!<HR></BLOCKQUOTE>mechanical isworking 26degrees @ 2000rpm and 31degrees @ 3000,It was at41degrees at 3000rpm till I lowered it.