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Old 06-27-2013, 08:51 PM
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Tracking down a miss?

I just got the motor i built in between the frame rails a few days ago. On the first fire up I noted a miss, Like its running on 7 cylinders. Shut it down, and started to track down the miss. Even though everything is new, I checked the coil, Cap, Rotor, Module, wires and plugs to be sure. Its getting spark to the plugs. Is there anything else that would cause a miss in a sbc?

Its worth noting that this motor does have a set of roller rockers, and I have never set rollers before. I did it just like you would a normal set. 1/2 turn past lash at the base circle of the cam lobe. Cam is a hydro flat tappet. Any help with this would be great. I really hate to keep starting it. I haven't got the 20 min break in done because its not running on all cylinders, Or so it seems.
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Old 06-27-2013, 10:11 PM
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If its just a slight miss and not backfiring or popping or knocking or pinging run it for the break in it could be just a lifter slow in pumping up or the rings need time to seat a little better ! a little carb adjustment during cam break in might be needed or even a sluggish valve or vacuum leak get the cam break in time in and the oil to running temp and see if the miss dissipates. If the miss scares you and your not confident it won't damage your engine don't start it till you track it down! But its much easier to find what cylinder is missing while its running after cam break in

Jester

Last edited by painted jester; 06-27-2013 at 10:17 PM.
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Old 06-27-2013, 10:19 PM
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do you have an inductive timing light? use that on each wire.If you have headers,look for a cooler pipe
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Old 06-28-2013, 06:29 AM
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Quote:
Originally Posted by painted jester View Post
If its just a slight miss and not backfiring or popping or knocking or pinging run it for the break in it could be just a lifter slow in pumping up or the rings need time to seat a little better ! a little carb adjustment during cam break in might be needed or even a sluggish valve or vacuum leak get the cam break in time in and the oil to running temp and see if the miss dissipates. If the miss scares you and your not confident it won't damage your engine don't start it till you track it down! But its much easier to find what cylinder is missing while its running after cam break in

Jester
The miss is noticable but not back firing or poping. It will idle at 900+/-. Are you saying that even with this slight miss it won't hurt to continue the break in?
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Old 06-28-2013, 07:40 AM
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Originally Posted by vinniekq2 View Post
do you have an inductive timing light? use that on each wire.If you have headers,look for a cooler pipe
Vennie, I do have a inductive timing light. Is it going to hurt running it long enough to do that test?
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Old 06-28-2013, 02:56 PM
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Quote:
Originally Posted by Slammedsi View Post
The miss is noticable but not back firing or poping. It will idle at 900+/-. Are you saying that even with this slight miss it won't hurt to continue the break in?
Unless the miss is already a sign that a lobe and lifter are failing.

Bogie
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Old 06-28-2013, 05:19 PM
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Quote:
Originally Posted by oldbogie View Post
Unless the miss is already a sign that a lobe and lifter are failing.

Bogie
I agree

You could also back off the rockers to 1/4 turn and adjust them after the break in to spec.s! Even tightening down .003 for 20 minutes at 2000 rpm would be OK as long as plungers are off (not touching) the lifter plunger lock ring or spring clip.

Jester

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Old 06-28-2013, 09:28 PM
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You did make sure you have the firing order correct?
You have checked it for vacuum leaks?
Was one of the spark plugs dropped during installation and it closed the gap and nobody noticed?
Look at all the valve springs, make sure there's not a broke one.
You are checking to make sure a plug wire isn't arking and you can't see it? If in doubt start it in total darkness, if you see a blue streak you found it.
It's easy to go over on the valve adjustment. I always set them with the engine running and why is because you can hear what's going on when it's did this way. With the engine at idle, I simply tighten them until the tick stops, then I really slowly back off until the tick starts, then very carefully tighten it until the tick just stops, then I give it a quarter turn and on to the next one. Keep in mind if the cam, lifters, rockers or heads are new it's not uncommon to have to go back several times and adjust the rockers as things change quickly as they break in.
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Old 06-28-2013, 09:37 PM
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Quote:
Originally Posted by oldbogie View Post
Unless the miss is already a sign that a lobe and lifter are failing.

Bogie
I wouldnt think thats possible. It had the miss on the very first fire up.

Quote:
Originally Posted by painted jester View Post
I agree

You could also back off the rockers to 1/4 turn and adjust them after the break in to spec.s! Even tightening down .003 for 20 minutes at 2000 rpm would be OK as long as plungers are off (not touching) the lifter plunger lock ring or spring clip.

Jester
Thanks jester.

Quote:
Originally Posted by DoubleVision View Post
You did make sure you have the firing order correct?
You have checked it for vacuum leaks?
Was one of the spark plugs dropped during installation and it closed the gap and nobody noticed?
Look at all the valve springs, make sure there's not a broke one.
You are checking to make sure a plug wire isn't arking and you can't see it? If in doubt start it in total darkness, if you see a blue streak you found it.
It's easy to go over on the valve adjustment. I always set them with the engine running and why is because you can hear what's going on when it's did this way. With the engine at idle, I simply tighten them until the tick stops, then I really slowly back off until the tick starts, then very carefully tighten it until the tick just stops, then I give it a quarter turn and on to the next one. Keep in mind if the cam, lifters, rockers or heads are new it's not uncommon to have to go back several times and adjust the rockers as things change quickly as they break in.
I check and doubke checked the firing order. Its correct, everything is brand new including the assimbled heads. I pulled the plugs and checked the gaps on all of them. There all set to .040
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Old 06-28-2013, 10:05 PM
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Being you've checked then, go back over the valve adjustment.
Or if you want to feel safer about it. Yank all the plugs and do a compression check. If you find a cylinder that's dead or low, then readjust the valves on that cylinder.
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Old 06-29-2013, 02:22 AM
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Crank the engine.
Using an insulated tool, pull one plug wire at a time and reconnect if the idle drops.
Continue this until you come to a plug/wire that, when pulled, the idle does not change. That is the bad cylinder. Diagnose from there.
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Old 06-29-2013, 06:59 AM
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Quote:
Originally Posted by DoubleVision View Post
Being you've checked then, go back over the valve adjustment.
Or if you want to feel safer about it. Yank all the plugs and do a compression check. If you find a cylinder that's dead or low, then readjust the valves on that cylinder.
Thanks. I think I'm going to start there this weekend.. will update once I get the time.
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Old 06-29-2013, 12:30 PM
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Would a excessively tight valve lash cause something like a miss.
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Old 06-29-2013, 03:20 PM
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Quote:
Originally Posted by Slammedsi View Post
Would a excessively tight valve lash cause something like a miss.
Yes!! That's why I suggested a looser lash like 1/4 turn 0r .003 down off the lifter lock clip.

DoubleVision mentioned a 1/4 turn too!

Jester.
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Old 06-29-2013, 03:49 PM
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I think I would fire the motor, bring r's up to 2500 steady, let the motor warm up and adjust the valves hot. Back off the adjusting nut until the rocker clatters, then tighten down until the rocker gets quiet, then another 1/4 turn.....done.

If you do this sort of thing often enough, you will procure a couple of old OEM valve covers and use a hole saw to cut 8 holes for a socket to go down through and engage the rocker nuts to do the hot lash. Bolt the covers on with old gaskets and tape over the access holes in the covers. Pull back the tape one rocker at a time to adjust, then replace tape to prevent an oily mess in the engine compartment and go to the next rocker.

For the rest of you, here is the static valve adjust procedure......
http://www.crankshaftcoalition.com/w...stment_SBC/BBC
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