CRG Discussion Forum
Camaro Research Group Discussion => Mild Modifications => Topic started by: sdkar on May 30, 2007, 01:58:42 AM
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I installed my new engine and it ran fine. I have never removed the plug wires since new. During trying to time the engine I had a huge backfire after. Now I have to set the timing to where the vacuum cone hits the carb just to get it to start. It still runs rough and backfires slightly. The engine timing acts like the distributor still needs to move even closer toward the carb just to run right, but O can't move it any more as it is at its limit and hits the manifold. This is a brand new crate motor. I don't think I could jump the timing chain on a new 502 with no miles on it and only about 10 startups (no?). It ran great before the backfire. Anything I should check? I am sure if I moved all of the wires over one on the distributor it will run fine again...but this they are now set at the factory plug ins and ran fine before. Any ideas.
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When did the backfire occur? Was it on idle or during higher rpm:s? To me it sounds like one valve (or more) have been hit by a piston and are now worn. They simply don't seal against the valve seat. I should do a compression test and find out. The reason can be that there is a quality issue on a piston or valve or simple the combination of parts don't match together. It may be a clearance problem.
Good luck.
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Did you break the cam in? If it's not a roller you may well have wiped out a cam lobe. Far-fetched sure, but if you didn't run the engine at at least 2500 rpm for the first 20-30 minutes it can happen with a flat-tappet cam if the valvesprings have a high load. Been guilty of this myself. Just out of curiosity, try taking the distributor hold-down loose and turning the rotor counterclockwise until you feel the distributor gear slip over the next cam gear notch and see what happens. That will advance the timing relative to the distributor position. Good luck!
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If the motor is equipped with a Holley carburetor model that incorporates a power valve, the diaphragm in the power valve may have been ruptured as a result of the backfire. If that is the case, the engine would typically exhibit symptoms of a very rich mixture.
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It is a holley 850 dp. What should I check to find out if the problem is the power valve?
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The Holley website note regarding this problem is located at:
http://www.holley.com/data/TechService/Technical/power_valves.pdf
Apparently, carburetors made since 1992 have been equipped with a safeguard against rupture of the powervalve diaphragm during severe engine backfire. If the carburetor is older than 1992 and/or you wish to trace this possible source of the problem, the diagram on that page shows the location of the valve in the secondary metering block. I don't know what the modern, "approved" method for checking is but back in the day we used to take them out and blow through them to see of the diaphragm held air but then again, we did a lot of stuff "back in the day" that would probably be frowned on by today's technology. I suppose that the most prudent thing to do would be to determine beyond a shadow of a doubt that your carburetor is equipped with a power valve (not all models were) and when it was manufactured before removing the secondary float bowl and metering plate. In any case, having bowl/plate gaskets on hand would be a good idea before proceeding.
Good luck.
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Sounds like the distributor is out a tooth or two. I have seen this problem in the past.
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Here is the latest. Brand new ZZ502 with brand new Holley 850 cfm Model 4150 (part # 12366996) that supposedly has blow out protection on the power valve. While trying to time the engine, I had one very loud back fire followed by a couple of minor ones. The wax in my ears fell out on the first BANG. The engine ran horribly immediately and the plugs were badly fouled. Finally, the engine would not even start and would just sputter and backfire. I cleaned the plugs and put them back in and was able to get the engine to run again, though still rough. As stated above, the power valve in the carb has the blow out protection. How good is this protection. Could the power valve still be messed up and need to be replaced? How do I check the power valve to make sure this is the problem? Finally, any other ideas on what could be wrong and what I should check or try to do?
Thanks guys,
Steve
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Also, how could a distributor be out a tooth or two. I find that it can only be exactly right or 180 degrees out. Sure the teeth will let you skip one at a time, but there is a "bar" or whatever that allows the distribtor to only fully seat in these two positions and will not lock down all of the way if you are only a tooth or so out. Any opinions?
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Steve,
The symptoms you describe (ran rough immediately after backfire, fouled plugs) certainly make it sound like at least a part of the problem is the power valve. I don't know how effective the blow-out protection really is but the engine would be dead rich and the plugs would foul quickly if it is ruptured.
I hope that others will add suggestions based on their techniques to my comments because it's been a long time since I dealt with this problem. I would remove the carburetor from the engine in order to work on it. Check the diagram on the Holley website to locate the he power valve in the secondary metering block. Be sure to drain the gasoline from the carburetor. Remove the floar bowl screws. Be careful to note the positioning of the float bowl and metering block gaskets. The cork ones I remember were fairly delicate and it's important to put them back exactly the way they came out. They can be put on upside-down.
When you have the metering plate in your hand you will see the power valve threaded into it. If you're not sure what it looks like, check the pictures of the various valves on the Holley site. It will thread out of the metering plate with little difficulty (I recall that it took a fairly large open-end wrench to fit it). Carefully unscrew the power valve. You won't necessarily be able to visually determine if it is ruptured. The way we used to check them was to try to draw air through them from the flat side. I know that this will sound relatively primitive but we did this by holding the flat side to our lips to see if we could suck air through the valve. If the valve is broken there will be no resistance to air movement in either direction. If the valve is good, the diaphragm will hold air and the plunger will open and close when you suck on it. No doubt there are vacuum testers available that would be able to give you a much more scientific analysis. This was just the way we did it track-side years ago.
If you suspect that the valve is ruptured, check the flats around the sides of the valve for the number that designates the level of vacuum required to activate the valve on your application. As you can see from the Holley site, there are many different ones available. Your carburetor was designed with a particular one based on the needs of that engine/cam combination. Replacement with another of the same specification would be important.
Disclaimer: If there's a better way to do this I hope others will contribute their own techniques. I don't want to lead you in the wrong direction but that's were I would start. You can remove the carburetor and check this in a few moments, much quicker than checking for timing chains or distributor gears.
Good luck.
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the "bar" you speak of is the oil pump drive pin. It can be at any position. It has nothing to do with the distributor being exactly right or out 180 degrees. Believe me, I've been through this may times. If your distributor is pointing in the wrong direction to get the timing even close to right, it is out by teeth.
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Why not borrow a carbutator from someone and try if this make a difference. This will make things clear.
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You can check the timing chain by turning the crank back and forth and watch the dist. rotor also listen for chain slapping the cover that would be really loose but it happens. just because it is new doesn't mean the chain or timing gear bolts or crank key doesn't have a problem. It sounds like the cam timing jumped a tooth or two if you rotate the dist. around and make it run better and the headers get red idling it's jumped I had a friend's 302 do that it ran after moving the distributor, but not good and he couldn't figure out why the headers got red. It would rev up and sounded a little flat but ran. It had very little time on a double roller chain and it stretched or wore out really fast. maybe it wasn't preoiled? I always put mine in a can full of engine oil and heat it to preoil. I also would drain the oil and check for debris maybe cut the filter open. That engine is very costly I wouldn't take any chances.
Good Luck
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Pulled the carb apart and the power valves are okay...so that is not the problem. I cleaned the plugs but they foul in minutes. I put the #1 cylinder at TDC and the rotor points exactly to #1, so I do not think the timing has jumped. During initial first startups, the motor got hot as I did not have the radiator full with coolant and water. I don't have the temp gauge installed yet and don't know if it just seemed really hot because I was working on it at the time or if I got it too hot and blew a head gasket. Could these symptoms be caused by a blown gasket? I want to check the compression in the cylinders...what would be the correct pressure?
If you have any ideas that have not been mintioned, let me know as I will test anything to see what the problem is.
As always, thanks for the help.
Steve
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I compression tested all of the cylinders and got a consistent 190 psi from all eight of them. So I am guessing that the head gaskets and heads are okay. So what do I try next?
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I am wondering if my problem is just that I flooded/fouled/burned up a set of plugs and all I have to do is install a new set of plugs. Is this possible. I will try this next but I would hate to put in a new set of plugs and foul them up as well at a cost of about $4 ea. Also, the spark plugs have writing on them saying "AC Delco" and "4" underneath that. How do I find out what plug this is or will the parts store know if I bring them the plug.
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What kind of fuel pump are you using? A Holley needle & seat can't take much over 7 psi before it gets overwhelmed and starts to flood. Also, how did you set the floats? All the lip service about taking out the sight plugs in the bowls and adjusting the float up until gas begins to dribble out is absolute b-s. The float is always too high that way. You take the bowl off, invert it, and set the top of the float just even or a little below the top bowl screw holes. I feel for you, man--this has to be driving you nuts! Good luck!