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Topic Review (Newest First)
11-18-2012 07:10 PM
632Mantis
Quote:
Originally Posted by Stubbs82 View Post
Found my problem i believe. The #8 exhaust valve lobe on the cam is gone. doesnt look like enough left to even open that valve. And the fun begins now. Thanks for the help though.

Ouch. Looks like you will be buying parts.

One more thing about ignition timing: You don't always need to run the vacuum advance. Mine is plugged, I don't even use it.

When you run a larger cam, they like more initial timing. If your mechanical curve is enough, then you are all set.
My distributor is set for 18 at idle, and around 36 max when I rev it up.
Vacuum advance is plugged.
11-17-2012 11:09 AM
Stubbs82 Found my problem i believe. The #8 exhaust valve lobe on the cam is gone. doesnt look like enough left to even open that valve. And the fun begins now. Thanks for the help though.
11-11-2012 06:06 AM
454C10 There is a 2:1 ratio between the cam and the crank rpm. Every other rotation the marks will line up at 6 and 12 and the other time at 12 and 12. The 2:1 ratio is from the difference in the diameters of the timing gears. The block doesn't care where to install the marks (6/12 or 12/12), as long as they line up.

BBC's like timing. 17 at idle is good. 38 -40 total mechanical is good. The problem is with your vacuum advance (too much advance). Get an adjustment vacuum advance canister from summit racing and set it to add only 10 degrees.

You can use ported vacuum or manifold vacuum on the vacuum advance. Ported vacuum doesn't have vacuum at idle. Or at least it shouldn't have vacuum at idle, but sometimes does if the throttle blades are open past the transfer slots at idle (which is a bad thing).

Ported vacuum is an emission device so I prefer to use manifold vacuum. So, 17 at idle mechanical, then 10 more from the vacuum advance will give you 27 at idle. And around 50 when you rev it up in park.
11-10-2012 07:24 PM
Stubbs82 That makes sense. I understood what said before, but was thinking that the cam would have to rotate another 180 again to get it back right. But, with the distributor turning half the crank revs, i was really confusing myself. Like i said, enjoy learning and doing hands on myself, but i have a tendency to over think everything I try to figure out on my own. I will adjust distributor 180 and post results. Thanks to u guys that replied. very helpful and appreciated. Thanks again.
11-10-2012 06:27 PM
techinspector1
Quote:
Originally Posted by Stubbs82 View Post
when i brought up #1 TDC and adjusted the cam and sprocket so that it was at 6 and the crank was at 12, did i cause the timing to be off by not setting the cam and crank at 6 & 12 with the #6 TDC? I wouldnt think it would run at all if that was to happen.
When you brought #1 to TDC, you also brought #6 to TDC. They are companion cylinders, except that they are 360 degrees apart. Remember, there are 720 degrees in a complete cycle, on a 4-cycle motor, so if you put the cam sprocket at 6PM and the crank sprocket at 12PM, the motor is ready to fire #6. The cam lobes for #6 will be on the backside of the lobe, more or less on the base circle, which means both valves are closed, ready to fire. Meanwhile, #1 cylinder is 360 crankshaft degrees away from what #6 is doing, even though the piston is at TDC just like #6. The difference is that #1 is at TDC overlap, where both valves are open.

Let me explain it again. Install the crank gear with the dot at 12PM, then install the cam gear with the dot at 6PM. Pay no attention to where any of the pistons are in their respective bores. Bring the dots together where a straight edge would run between the centerline of the crankshaft, up through the crankshaft dot, up through the camshaft dot and on through the centerline of the camshaft. When you have this part of the motor installed with chain and the dots lined up, rotate the crankshaft one complete turn clockwise so that the camshaft dot is at 12PM and the crankshaft dot is at 12PM. Now, the motor is ready to install the distributor and run the wiring because both valves are closed on #1 and #6 is on overlap.
11-10-2012 06:26 PM
cdminter59
BBC timing and idle problems

When you set the crank at 12:00 and the camshaft at 6:00 the #6 piston is firing. You can drop the distributor in and have the rotor pointing to the tower that #6 plug wire is in. You can also rotate the crank 1 revolution and drop the distributor in pointing to the tower that #1 plug wire is in. It is easier to just drop the distributor in with the rotor pointing to #6. Check your timing again, if it is off your balancer may have slipped. Here is an article to read to determine TDC if you suspect the balancer is off. http://www.crankshaftcoalition.com/w...op_dead_center
11-10-2012 02:57 PM
Stubbs82 when i brought up #1 TDC and adjusted the cam and sprocket so that it was at 6 and the crank was at 12, did i cause the timing to be off by not setting the cam and crank at 6 & 12 with the #6 TDC? I wouldnt think it would run at all if that was to happen.
11-10-2012 02:29 PM
techinspector1 When you put the cam sprocket on the cam, as you are building the motor, you phase the dot on the cam sprocket at 6PM and the dot on the crank sprocket at 12PM. This sets up cylinder #6 to fire at TDC. After getting the whole mess buttoned up, you must turn the crankshaft one revolution clockwise. This puts the cam sprocket dot at 12pm and the crank sprocket dot at 12PM and sets up cylinder #1 to fire at TDC. I have no idea why the GM engineers set it up that way, but they did. Seems to me that they could just as easily punched the dot on the other side of the cam sprocket and had 6PM/12PM be TDC for #1 instead of #6.
11-10-2012 02:15 PM
Stubbs82
BBC timing and idle problems

I'm having issues with timing and idle on BBC in my beat up blazer. I purchased a motor that was supposed to be rebuilt with less than 5000 miles on it and built to run strong. I plan to use as an operate daily with the occasional play time toy. You will have to bear with me as I am no master mechanic but like to learn and do the work myself. I bought from the second owner after rebuild. Here is what i know about it without tearing it down:

It is a 72-74 454 block with 65-66 396 heads. I ran the numbers and believe it said they were 97cc closed chamber heads. I could be wrong. It has a dual plane intake and Edelbrock carb. Has a "mild" cam is what i was told but no specs on it. It has stock damper on it. Seems to have slightly domed pistons. Accel billet dist w/ vac advance. MSD 6A ignition and accel 8.8mm performce wires. The plugs are NGK. Also has headers. When I first brought home and dropped in, the timing seamed way off. I tried to adjust to no good ending coming from it. Noticed the pullies didnt line quite right to find the damper bolt to be stripped and not on all the way. I retapped the hole and put on all the way. Did not seem quite right again, so i decided to check the timing on the chain and gears. I found that when #1 at TDC the crank gear was line correctly and the cam sprocket was 180 deg out. Havent heard of any reason why it would be that far out intentionally. Timing marks not even close. I spun the cam around and proceeded to turn motor slowly to make sure no valves hitting or stuck and appeared ok. I have since tried getting it right numerous times to nothing that seems to fall into place all at once. Got it to idle at 750 at 17deg. Total mechanical time was set at 37deg. when i hooked up the vac adv, the timing appears to jump off the chart on light. way past the 60deg guage at 3200rpm. Then it seems to have either early detonation or valve tapping or something on passenger side. not noticed until around 2500 rpm.

Any suggestions on things to try would be appreciated. Like i said, i am very green and ignorant to some of the things, but want to learn and get it right. Thanks in advance.

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