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?

Ummm...
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.

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70 camaro 307 (350soon) /350th

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?

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.

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.
T.P.DONAGHY.

<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.

Popping through the carb is an ignited fuel mixure that somehow happened OUTSIDE the combustion chamber in the intake runner..This has to be due to one of 2 things..a leaking intake valve as in a burnt valve, or a spark timing that is so far off that the plugs are firing while the intake valves open still like a way to far advanced timing seting. If the popping is consistant,like smooth...pop...smooth..pop..it's a single cylinder problem..I think this is coming down to your interpretation of a "POP"...is this a crackling, or an actual back-fire through the carb???


-gary

<BLOCKQUOTE>quote:</font><HR>Originally posted by cavemate:
Popping through the carb is an ignited fuel mixure that somehow happened OUTSIDE the combustion chamber in the intake runner..This has to be due to one of 2 things..a leaking intake valve as in a burnt valve, or a spark timing that is so far off that the plugs are firing while the intake valves open still like a way to far advanced timing seting. If the popping is consistant,like smooth...pop...smooth..pop..it's a single cylinder problem..I think this is coming down to your interpretation of a "POP"...is this a crackling, or an actual back-fire through the carb???


-gary<HR></BLOCKQUOTE>I would say more of a back fire through carb.

i have been having this problem also. when i crack the throttle it will backfire through the carb. will an internal vacume leak cause this. such as shaved heads (milled) and non shaved intake manifold.

669Dennis,
I had a similar problem on a MPI engine, backfired throught the throttle body above 2000 rpm, idled fine. My exhasut lobe was gone on one cylinder. I highly recomend you do a dynamic compression check. I assume yours was a static or cranking compression check that everyone does and then stops there. That does check out the rings, valves and pistons, but not the cam or port restrictions.
1. warm the engine and then remove one plug and ground it out, also disconnect the fuel injector if an injected motor.
2. install your compression gage on that cylinder and remove the shcraeder valve or leave it in the gage and just bump the bleed button every once in a while.
3. Start the engine and write down the dynamic compression readin on the gage.
4. Now using the throttle plate not the gas pedal snap the throttle and release it as quickly as possible while watching the gage. You want the engine to take in a big gulp of air, but not accelerate.
5. compare your static, dynamic running and dynamic snapped throttle readings for each cylinder. the dynamic one should be about half of the static and the dynamic snapped one should be about 80% of the static reading.

If you are well below the 80% on the snap you have an intake air restriction could be a worn intake lobe, rocker problem or severe carbon buildup on the intake valve.

If you are well above the 80% on the snap you have an exhaust air restriction could be a worn exhaust lobe, collapsed lifter or clogged cat converter if all are high.

This REALLY works and will save you a lot of time and cash. Like I said it worked this Fall for me when the car ran fine but backfired as soon as I loaded the engine.

If you want the science behind it here goes, otherwise just go do the test and you'll be on your way soon.

Normal comp tests test sealing of the combustion chamber not engine breathing. engine vacum tests the breathing of the entire engine when checked at say the manifold plenum. You need to test the breathing of a single cylinder in your case. If you pull 18" of vacum atmospheric is about 30" so 30-18=12, the 12 is what your engine is drawing in. 12"mercury = about 6psi absolute. Compress it say 9 to 1 you get 54psi if all the charge makes it into the cylinder and back out again. so you'd expect to see this 54 or 55 on your compression gage. when you sanp the throttle the manifold vacum drops and so the absolute air pressure going into the cylinder increases. So if one cylinder reads low the air didn't make it in, if it reads high the air didn't make it out and the next charge of air raised it even further.
Good Luck, let us know how you make out,
Dave


[This message has been edited by SY1 (edited 12-05-2001).]

I put in 73 primary jets and its still popping when I rev it above 3000rpm.I noticed when I too k my valve covers off that there is hardley any oil in the valve train area.I have a set of crane, gold roller rockers was thinking if the hole in the roller rocker doesn`t line up with the push rod no oil woul squirt out,there for keeping the lifter pumped up i.e keeping the valve open.I have trick flow heads 23 degree.what do you think?

Did you figure your problem out?

SY1, I'd never heard of a dynamic compression test. That's a great idea.

joe

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69 Rat-Infested Resti-Clone
WCA Member
Badboatdude@CS.com
69 Pics http://hobbystage.net/camaro/joeg/index.html

For what it's worth...I add my 2 cents..
Make sure the plug wires are neatly routed and of proper length...make sure to run the plug wires through the wire seperator hold-downs.... Poorly routed wires can cross-couple and cause popping from wacky ignition occuring at the wrong time..
Also I would re-lash the valves ...I have seen popping occur from the lash being off..
Sometimes a big cam installed can lift the stock press-in studs from the head..causing all sorts of timing problems..
I do not agree with the normal lashing procedure as outlined in many text and cam instructions..there is room for error..
The correct way ( I think) to lash the hydraulic lifters is one by one ...you start with one lifter and turn motor till the lifter is at MAX lift (approx by eye and hand) then mark the balancer..then spin the balancer 360 around till you reach this same mark again....now you are at the base circle of that cam lobe... now do a 3/4 turn past 0 lash.. now move to the next rocker..
Regards
Chris

i have had this happen before when i had the #5 and #7 plug wires switched. it is actually quite easy to do this, and can be easy to miss. the popping is from the #7 cylinder firing 90 degrees before it should.

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1971 Nova(looks like 69 camaro from underneath!)
355sb, vortec heads, HOT cam,T-10 tranny, 3.70 gears 16X8" IROC wheels
see pics here http://community.webshots.com/user/novaderrik

I had this same issue with my car and I ended up eliminating everything!

I don't understand how too lean would cause backfire. This would cause a bog. In my case, the carb was way too rich because the fuel level in my secondary bowl was set way too high. You could smell the gas in the exhaust and see the gas stains in the concrete where I ran the car.

Check out a thread I started to help me with my carb backfiring.
http://www.camaros.net/forum/Forum5/HTML/000877.html

Good luck.

Dave


[This message has been edited by Scooby Doo (edited 01-07-2002).]

I changed my name from 669dennis to 55dennis.Well i did fix the problem I changed to comp cams 274xtreme energy cam what a nice cam that is way better than 280h.More little more bottom and torque and alot more top end power.No more popping great street motor 6000rpm no problem,getting it dynoed in a couple of weeks.thanks for all the help.