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Old 10-18-2007, 07:57 AM
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Backfiring out of drivers side header?

Hello once again to everyone.
Ok, this is what is going on with my engine which is a rebuilt 350 4-bolt main sb bored .040. I personally rebuilt it and torqued every bolt. It only has about 2.5 hours of run time on it, and the car that it is in ' 81 El Camino has never been on the road since the rebuild. Everything in the motor is new.
If I put my hand behind the driver side header after the motor has warmed up, and choke fully open, I can feel an unusual "puff" of air hit my hand at the same time it backfires. It does not do this on the passenger side. I have harland sharp roller rockers with screw in studs, Crower pro street solid cam w/ .504 lift on the exhaust, and .492 lift on the intake. The cylinder heads are just iron 76cc chamber 1.94intake, 1.5exhaust that have received a mild port and polish job by me. I used a shim gasket on the heads to get my cr at about 9.99:1. The valves, seals, etc are also all new. To hold the rocker arms in place I am using what the manufacturer sent me. They are black, about an inch long with a little allen set screw inside it to make sure it does not come loose. However, I am thinking that one may have worked its way loose, hence the backfire problem I have suddently inherited, but I am not sure. The guys at NAPA who rebuilt the heads, set the spring pressure said this is what may have happened.

I have been working with getting my timing right also. I think I have the right spring combination on the centrifugal weights inside the distributor. My timing mark appears to stop advancing right at the 3000rpm point. Should be betweeen 2500 and 3000 rpm if I am not mistaken. I also noticed what sounded like a backfire when I was rechecking my timing. This happened at about 2500rpm.

Also, in a possible unrelated problem, I pulled all eight plugs a few days ago do discover all were black, not sooty oil black, just black. Frisco thinks this may be related to a possible rich condition that I may also have . I am running a holley 750 with vacuum secondaries. I am not sure what the jet size on the primary is, but the secondary has #83. I thought since the block was bored .040 and the heads were ported and polished that a little "extra" gas might help make a little more power. Anyway, I just wanted to mention this for its related possibilities to my backfire problem.

I have come to the conclusion that putting the motor together was something that I always wanted to do, However; the real fun happens when everything is together and you try to get it to all work together.

All advice and recommendations are welcomed and very much appreciated.
Thanks,
Robert D

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Old 10-18-2007, 05:41 PM
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One of the 1st things you should do is run a compression test across the board then I would look for a bad plug ~wire ~carbon track in the rotor~

If its not in the electric then I would look to see if you have a sticking or not opening valve

if still nothing look for a loose rocker arm~
bent push rod ~collapsed lifter~ wiped out lobe on cam.

That should keep you busy & answer a lot of questions , hopefully its something as simple as one of the listed as above.





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Old 10-19-2007, 03:07 PM
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How do you check for a collapsed lifter and/or wiped out lobe on the cam without removing the cam?
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Old 10-19-2007, 04:10 PM
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lifter.......and cam.

if the cam has gone flat,pull a valve cover,turn over engine and look for the rocker arm to move up and down,no movement,bad cam, or bent pushrod.
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Old 10-20-2007, 09:03 AM
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Quote:
Originally Posted by newdad1
How do you check for a collapsed lifter and/or wiped out lobe on the cam without removing the cam?
boatbob2 replied concerning a flattened cam lobe and/or bent pushrod.

As to a collapsed lifter or incorrectly adjusted lifter (too tight); you will see the rocker move up and down as normal. However if the lifter has been adjusted too tight the valve will not be closing. Re-adjustment is the "fix". Rotate the engine so that the lifter to be adjusted is on the heel of the cam. The lifter will be at it's lowest point. Back off the rocker arm adjustment nut until the pushrod can be moved in an up and down direction. Slowly re-tighten the adjusting nut until there is no up and down movement of the pushrod. This is zero lash. For hydraulic lifters only; tighten down an additional 1/4 to 1/2 turn.

For solid lifters there should be a gap specified by the cam manufacturer between the valve stem and the rocker arm tip.

Spinning the pushrod will give a false indication of the zero lash.

You can still spin the pushrod even if the lifter is completely collapsed.

Last edited by Frisco; 10-21-2007 at 07:57 AM. Reason: this is for hydraulic lifter adjustment
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Old 10-20-2007, 09:26 AM
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The black plugs is a indication of running too rich...you need to re adjust your air/fuel mixture on your carb as well as check your float level for proper adjustment. The back fire could be one of several things....you did not mention what you set your initial timing at. It should be between 10 and 12 degrees BTC. with the vacum advance hose removed and pluged.
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Old 10-20-2007, 11:44 AM
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You may also want to doublecheck the firing order is correct. Cylinders 5 and 7 are on the drivers side and are right next to each other in the firing order. It's very easy to get them crossed if you're not careful.
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Old 10-20-2007, 01:11 PM
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Quote:
As to a collapsed lifter or incorrectly adjusted lifter (too tight); you will see the rocker move up and down as normal. However if the lifter has been adjusted too tight the valve will not be closing. Re-adjustment is the "fix". Rotate the engine so that the lifter to be adjusted is on the heel of the cam. The lifter will be at it's lowest point. Back off the rocker arm adjustment nut until the pushrod can be moved in an up and down direction. Slowly re-tighten the adjusting nut until there is no up and down movement of the pushrod. This is zero lash. Tighten down an additional 1/4 to 1/2 turn.
Hey Bill(Frisco), do I perform this adjustment procedure for solid lifters and roller rockers also, as in my case? I am running a Crower Pro Street Solid Cam with .504 lift on the exhaust side and .493 lift on the intake, I think my spring pressure is about 110.

As long as we are on the subject when I was putting the motor together, I initally adjusted the clearance between the roller rocker tip and valve tip as follows ;
Tools used; boxed in wrench, feeler gauge, allen wrench.
-Bring cylinder no1 to TDC of compression stroke aka to fire no 1 plug.
-adjust 4 intake and 4 exhaust valves to a specified clearance.
-rotate engine 1 complete revolution
-adjust remaining 8 intake and exhaust valves.

Note: NAPA machine shop made a copy of the exact procedure and what valves on what stroke to adjust. The idea was that I only had to turn the engine over twice to adjust all 16 valves. I am not exactly sure what the spec was for the clearance, something like .016 for the exhaust and .018 for the intake, but not entirely sure. Anyway, thats another thing, Now that the engine has a little run time on it, what clearace betweeen roller tip and vave tip should I adjust to for my intake and exhaust valves?Hey Henry Highrise, I have my initial timing set at exactly 12 degrees BTC. My vacuum advance hose was removed and plugged. Also, my mechanical advance is set as per the spring and weight combination inside the HEI to come in at about 3000RPM and 32-38 degrees BTDC. In other words, the timing mark stops rotating at 3000rpm as viewed with the timing light.

I am using Autolite 26 plugs, not sure what I gapped them at. Is this a decent plug to run with my combination. It has been suggested that I may want to consider running a hotter plug to help with the rich condition. BTW not that this really matters the carb was completely rebuilt by a trusted person. Float level and fuel/air mixture were all set, however, I am going to ask him about possibly adjusting the fuel/mix.

Many thanks again for the help.



Thanks again for all your responses.
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Old 10-20-2007, 04:46 PM
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A popping in the exhaust is caused by a tight valve adjustment or spark plug gap, and like earlier stated crossed plug wires between cylinders 5 & 7. This will cause a major miss-fire all of the time though.

MSD recommends a starting plug gap of .050
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Old 10-20-2007, 04:55 PM
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Quote:
Originally Posted by newdad1
If I put my hand behind the driver side header after the motor has warmed up, and choke fully open, I can feel an unusual "puff" of air hit my hand at the same time it backfires.
If you can feel a "puff of air" behind the headers.. wouldn't that indicate you've got a problem with the header gasket?
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Old 10-20-2007, 06:37 PM
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backfireing

I don't understand why it doesn't back fire until it is warm. Backfiring would be coming out the intake ( if its back firing)
I'd start pulling plug wires and watch the rpm or a vacuum gage for a drop. That would at least tell you what cyl. its on.
Running a compression test would tell you if all the cyl. are the same.
I'd try narrowing it down before I start tearing in to it.
I know how you feel. The first 350 eng. I built, the shaft from the dist. to the oil pump was 3/8 to long. It was in a 65 chevelle. I couldn't get the pan off even after jacking the eng. up. Had to pull the eng. to get the pan off, then the pump and change the shaft. I spend more time trying to pull the pan than it took to pull the eng. Read all the thread then go fishing. You're more likely to catch something with the right bait.
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Old 10-20-2007, 10:47 PM
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I don't have the entire exhaust system hooked up yet. I am running it with open headers for the time being, so the "puff" of air is not from the header gasket, but I have had that problem in the past and it is annoying.I haven't tagged it yet to run on the street but need to do so soon to help seat the rings.

Ok, I will be spending my Sunday checking these things.

Stay tuned.
Thanks.
Robert
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Old 10-21-2007, 07:52 AM
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Quote:
Originally Posted by newdad1
Hey Bill(Frisco), do I perform this adjustment procedure for solid lifters and roller rockers also, as in my case?
Yes and NO. The lifter should still be on the heel of the cam. For solid lifters you will set the rockers so that you have the correct feeler gauge between the valve stem and the rocker. The figures you list below seem correct. See new gap figures in my latest post below. Your adjustment at present is too tight.

I edited my post above because I incorrectly assumed you were running a hydraulic lifters and cam.

Quote:
Originally Posted by newdad1
I am running a Crower Pro Street Solid Cam with .504 lift on the exhaust side and .493 lift on the intake, I think my spring pressure is about 110.

As long as we are on the subject when I was putting the motor together, I initally adjusted the clearance between the roller rocker tip and valve tip as follows ;
Tools used; boxed in wrench, feeler gauge, allen wrench.
-Bring cylinder no1 to TDC of compression stroke aka to fire no 1 plug.
-adjust 4 intake and 4 exhaust valves to a specified clearance.
-rotate engine 1 complete revolution
-adjust remaining 8 intake and exhaust valves.

Note: NAPA machine shop made a copy of the exact procedure and what valves on what stroke to adjust. The idea was that I only had to turn the engine over twice to adjust all 16 valves. I am not exactly sure what the spec was for the clearance, something like .016 for the exhaust and .018 for the intake, but not entirely sure. Anyway, thats another thing, Now that the engine has a little run time on it, what clearace betweeen roller tip and vave tip should I adjust to for my intake and exhaust valves?
I prefer to adjust each lifter separately. This involves rotating the engine many times but is more accurate. I am assuming that the clearances you mention above are for a "cold" engine adjustment.


Quote:
Originally Posted by newdad1
I am using Autolite 26 plugs, not sure what I gapped them at. Is this a decent plug to run with my combination. It has been suggested that I may want to consider running a hotter plug to help with the rich condition.
I like NGK Platinum plugs.

Last edited by Frisco; 10-21-2007 at 09:07 AM.
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Old 10-21-2007, 08:37 AM
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I just looked up the cam you are running. It appears to be Crower Pro Street model 00321. The intake lift is .482 and the exhaust lift is .504.

The gaps are .022 for the intake and .024 for the exhaust. See pictures below. Look at the upper right.

If you have the gaps set at what you posted (.016 exhaust and .018 intake) they are all too tight. Re-set to the above figures and you should be "good to go".

Crower Cams Scroll down to page 35.
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Last edited by Frisco; 10-21-2007 at 08:56 AM. Reason: added link and pictures
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Old 10-25-2007, 08:53 AM
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Back To The Top.

Waiting for results from newdad1.
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