Crazy Backfiring

[roysgreenstang]

Ok, Ive had my car for 5 years. Last year i put some money into the top end of the engine. I got a pair of CGM 60cc combustion chamber 200 cc intake runner heads with 2.05 valve double roller rocker hardend pushrods, gear driven timing gears, an msd blaster ignition coil, a 90 mm lincoln marc 3 maf with the 24 lb injectors that its calibrated to, a cold air intake, full length long tube headers, flowmasters, and a b 303 cam. I put all of this on after working out every kink i could find i still have a major backfiring problem under any, and all amounts of load. How ever under no load it seems to be running fine. Except for the terrible 5.0 surg that i also cant figure out. I have platium plug, that are the correct plugs for the heads, that i have changed several times, just to be sure, i changed my wires, cap, rotor, distributor, took out the salt aad pepper shaker wiring harness extention pigtails, checked all the grounds, insured that the calibration of the tps was on at 1 ohm. Even took the computer and put it on another mustang with similar upgrades and everything checked out. I am at a loss here. I did hone the cylinders before i put the engine together and a couple of my cylinders are not exactly on par with the rest, most are at 150 psi, which im not sure what that is for compression ratio, and 2 maybe 3 are at either 135-140 psi. Could that be causing this. The backfire is so bad that i cant drive it at all. It is backfiring through the exhaust, and possibly through the intake, though i cant be too sure of that. I was using the stock intake manifold, throttle body and fuel delivery system, i had 20-22 inches of mercury vaccuum, the valve timing was at 0 and the ignition timing was at 13 degrees advanced base timing, however i tried messing with the ignition timing, to no aveil. I have had ever mustang guy i know look at it, and it stumps them all, i was thinking of getting a good block and starting over with that, any advice on this situation would be highly appreciated. I also got the typhoon intake and a 70 mm throttle body now so if that helps please send advice, i would like to drive it this year. Another year without her would just be torture. Thanks

[goodyear1984]

i am assuming you have adi rockers..are they adjusted right.the 135 psi is more than anuf.are you sure you have the distributor in right?and the firing order right,13726548.not the other firing order..i am assuming you have a 302 right? if so thoes heads are way to big for it not that it will cause a backfire but a 200 intake runner and a 205 valve would be to big for a 347 stroker unless you were gonna rev it to 8000.

[roysgreenstang]

the rockers and distrubutor have been checked by an experienced mechanic and everything was perfect, firing order is as per the book says in the haynes manual under HO. Im sure its 13726548. Thats just from memory though, right now for 2 more weeks im in afghanistan so i cant check again, but its been checked so many times im sure its correct anyways. I have heard that gear drives can play havoc on a computerized car, but i cant figure out how, considering my mustang, i dont believe, has a knock sensor. I was told to replace them with the original type timing chain. Any insight on that would be helpful. Also the axhaust ports on the heads that i have got are at least twice as big as the exhaust ports on the heads that came off, would that in anyway produce the problems i am having, please keep in mind that the car at idle and no load seems to be running fine. Does the backpressure affect it that much. I changed the distributor to make sure that it wasnt the problem, again it wasnt. What problems exactly will i encounter from these heads being too big for the motor as you put it. Noone i have talked to said that it would cause anything other then a little less torque at low rpm. Is this going to cause me more headaches then the heads are worth. I was considering just taking another stock 5.0 and piece by piece changing everything that i changed from stock until the problem arises again, but thats alot of work and really im sure that it is something that can be fixed easy enough, i just cant seem to pin point it. The timing gear drive and the heads are the only things that i am thinking could be the problem, hoping for some feed back on this, i miss my car, and would like to drive it soon after i get home, thanks for the help guys.

[USMC302]

Was your timing chain gear adjustable? Meaning you are able to install it advanced or retard a few degrees? Was it installed straight up as in 0 degrees? Who set the valve lash on the adj rockers and were the pushrods checked for proper length?

[roysgreenstang]

yes the gears were set to 0 degrees. And yes they are adjustable, 4 degrees advanced or retarded. I just got the OEM length pushrods for a mustang, hardened steel. I compared them to the pushrods i took out of the engine and they were idintical. But also when i changed the pushrods i also changed the heads. Im not sure if those are exactly the right length pushrods for the new set up, but would that cause this outlandish backfire. Also the rockers were adjusted as per the books recomendation. The recommendation was and i dont remember the order and im in afghanistan right now so i cant check but it was 6 valves at #1 tdc not sure exactly which ones, turn the crank 360 six or four more valves turn 180 or 90 cant remember and then adjust the rest. I also adjusted, after that didnt working, use the 90 degree method. Start with # 1 Cylinder and tdc 0 degrees on the crank, adjust that, then 90 degree turn, #3 cylinder, then 90 degrees, etc. through the entire firing order, none of which worked for me. I heard that not having the proper length pushrod only affects the geometry of the engine making the valve guides wear a little sooner then they are suppose to but not affecting the engine performance as long as the rockers are adjusted correct. As for the adjustment, i would turn the adjusting polylock nut until the pushrod had drag when spinning it with my fingers then a quarter turn more. This is the only way i was told to adjust these rockers, in the book and by other people. If that helps, i hope to here your opinion on the matter, and thanks for the help. And if im wrong about the pushrod length and it not affecting the engine if the rockers are adjusted properly then please tell me. I dont know what i would do with her being on the road this year.

[USMC302]

Yes, pushrod length is very important, if they are too long, the valves aren't closing properly and it sounds to me like the valve adjustment will be the culprit. When you get back ( I just got back from Al Asad, Iraq myself) let me know and we can go through the valve adjustment again, it sounds exactly like what is causing the problem.

[roysgreenstang]

ok, that sounds good man, ummm, just curious though, the way i adjusted the rockers, is that the correct way to adjust them?

[USMC302]

Originally Posted by roysgreenstang

Start with # 1 Cylinder and tdc 0 degrees on the crank, adjust that, then 90 degree turn, #3 cylinder, then 90 degrees, etc. through the entire firing order, none of which worked for me.

Well, you said none worked for you and yes the book walks you through a rotation of turns to set lash, but if you get a remote starter, you just stand there and adjust the rockers once they are down and under no pressure, it's easier to see when they are down than rely on a rotation, this will ensure proper lash is set.

[roysgreenstang]

I will definately try that when i get home. The only thing is i have had many 302 experts come to look at my car and each did the rocker adjustment themselves the way they do for their own cars and all resulted with the same outcome. Also the rocker adjustment is what everyone says it is, but i just dont see how it could be that off. I have tried everything i could think of. Having them too loose, and the car sounds the same other then the tapping of the rockers because of them being too loose. i have tried not giving it the extra 1/4 turn. I have tried and extra 1/4, 1/2 3/4 and a full turn and it doesnt seem to change anything. I was curious about the gear driven timing gears if they could be the problem. All at once i changed the timing gears heads rockers and pushrods. What is the base timing gear time set to. Is it 0 degrees, advanced 4 or retarded 4. I have it set to 0. Oh i have upgreded valve spring too. That wouldnt cause any problems though would it. Oh yeah and the backpressure thing. Do 302's run well with little to no back pressure. Because the Exhaust ports are unusually large in comparison to the stock ports, at least 2 times as big, actually so are the intake ports. Im wondering if i should maybe just cut my looses and see if i can trade the heads for a set of gt 40 heads, but id prefer someone who has been building track cars give me their opinion on the matter before i do something so drastic considering these heads flow 500 hp. Id hate to get rid of something like that if i dont have to, but if i do, im close to the point that i dont care as long as the car is running good, and its got some gallop. Sorry for the long messages, i just dont like missing details, thanks.

[USMC302]

IMO the heads are not the problem. Are any of those "302 experts" ASE certified mechanics that set the valves? No offense, just curious. IMO it's def valve train related from what you describe, if it's not the rockers/push rod length, you need to check the timing gears and make double sure they are installed to 0 and not advanced or retarded. I have friends use the timing gears vise chain before with no issues, as long as it was installed correctly...You hear backfiring through the intake because the intake valves are open and/or not all the way closed when the combustion chamber ignites, which could point to a degreed cam, wrong cam, incorrectly installed timing chain/gears, rockers out of adjustment, length of pushrods, so on....Your valves springs are fine if they are double springs or upgrades for a 302. What are CGM heads? The back pressure should be fine with mufflers...you are running mufflers right? Are you running stock hyd roller lifters?

[roysgreenstang]

No offense taken. I myself as a trade choice am a mechanic. The mechanics that did the adjustmant other then myself are all licensed mechanics. I also had them look at the timing gears to ensure i wasnt incorrest installing them. The gears have been checked at least 10 times last year, and they are correct. But like is said, when i get home im ripping it all apart again, and starting over on an engine stand so i can take my time and ensure everything is properly put together. I will be trying everything you have told me. Yes i am running mufflers. They are flowmaster, and im sure that they arent giving me any amount of back pressure. The car has long tube headers too and a 2-1/2" H-pipe. the header do cover the exhaust ports on the heads, but its a close fit. And thats all i have for information about the car to offer. I cant wait to get home and try everything. Ill get her going 1 way or another.

[jovyarcher]

had the same problem with my mustang, as much as I want to try the suggestions given, what's that best way to fix this?

[goodyear1984]

its very easy to check and see if the push rods are the right size.just tighten the rocker just anuf so theres no lash,you want the roller tip to be centered on the valve stem.to adjust the rockers just put the pushrod in the tips of ur finger and roll the pushrod while u tighten the rocker as soon as u feel the push rod start to get just a little had to roll with the tips of ur fingers stop and turn 1/4-1/2.if u do that and it still does that its something else.make sure u are using the right firing order because there is 2.ps thoes heads way big for a 302