Engine new engine troubles

Hi , looking, for some ideas on where to look for new engine troubles , i have a vibration or maybe a miss issue with new engine build. My engine is a new rebuilt 306 , it has TFS 170 heads , new scorpion 1.6 rockers, comp cams xe274hr-12 cam, .555 intake and .565 exhaust lift, 274, 282 dur, 112 lobe seperation. It has 93 cobra intake with new FMS 24# injectors, has new pro-m 75mm mass air for 24# injectors and cold air intake, new accufabb 70mm tb, bbk short headers, new stock distributor, new plugs and wires, and new stock replacement balancer. The engine starts right up , idles very good around 750 rpm, when I start to give it any throttle Im getting either a vibration or a miss , have gone over the vacuum lines cant find any leaks , the timinng is at 12-14 degrees .The valvetrain makes a little more noise than the old stock set up did but not really loud .I still have stock fuel pump. what do you guys use as a plug gap? Could the Valves be out of adjustment enough to cause this ?The compression works out about 10:1. I still have a stock a9p ecm .Sorry for such a long post just trying to give as much info as possible, Thanks in advance for any help.
 
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nickyb

WAIT,you now have a pair?
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Apr 3, 2009
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Plug gap ,stock ignition stock gap should be fine.
How does it drive?
Does vibration/miss go away at any point?
Have you had it tuned?
 
Only had for a 2min drive , the vibration or miss doesnt go away but doesnt seem to get worse with rpm, throttle response is very quick but dont seem to have the power it should have either. No has not been tuned , in my area very limited to whats available, can anything be done with stock ecm or do I need to go to aftermarket system? Im kinda new to efi always went with carbs before
 

Monkeybutt2000

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Only had for a 2min drive , the vibration or miss doesnt go away but doesnt seem to get worse with rpm, throttle response is very quick but dont seem to have the power it should have either. No has not been tuned , in my area very limited to whats available, can anything be done with stock ecm or do I need to go to aftermarket system? Im kinda new to efi always went with carbs before
Stock ecu will run that no problem.
 
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90sickfox

Wasn't a pretty sight...and I've got big hands
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Did you change the flywheel when you did the engine ? There have been a few mix ups with 28 and 50oz flywheels being switched in parts store boxes. It will cause a vibration that'll make you think the engine is about to come apart.
 

AeroCoupe

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Oct 28, 2001
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If you are questioning valve adjustment then rerun the valves. Have you watched the fuel pressure? Stock pump may be giving up the ghost or the hose between it and the hard line is cracked. Not saying that is the miss problem but if the fuel pressure is low it cannot help. You indicated the harmonic balancer was replaced with a new stock type so maybe look there?
 

Mustang5L5

Put lubricant all over the balls
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Have you checked for a misfire? Do you have a laser thermometer that you can point at each of the header primaries to see if they come up to temp evenly?

You can also perform a cylinder balance test to see if one of the cylinders is not firing. Running on 7 will produce a slight vibation, but be perfectly drivable.
 
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That cam requires 355 gears and 2500 stall converter, so if you ain't got those in place you probably losing power
That cam requires 355 gears and 2500 stall converter, so if you ain't got those in place you probably losing power
The car has a new 5speed t5z , and still has the 273 gears for now , just trying to decide to go with 355 or 373 or 410s
 
Did you change the flywheel when you did the engine ? There have been a few mix ups with 28 and 50oz flywheels being switched in parts store boxes. It will cause a vibration that'll make you think the engine is about to come apart.
no never changed flywheel just had my old one resurfaced , I have run into that before and my vibration or miss is not as bad as that.
 
If you are questioning valve adjustment then rerun the valves. Have you watched the fuel pressure? Stock pump may be giving up the ghost or the hose between it and the hard line is cracked. Not saying that is the miss problem but if the fuel pressure is low it cannot help. You indicated the harmonic balancer was replaced with a new stock type so maybe look there?
Thanks for the Ideas, the fuel pump is a new stock one , i may upgrade it to a better one , will check the fuel lines again but they looked fine when I replaced the pump, fuel pressure at Idle stays steady at 39-40 with vacumn line off and plugged , will have to hook up a fuel pressure gauge instead of tested and try it under load, compared my old balancer with new one the weight looks the same size , timing marks in same location
 
Have you checked for a misfire? Do you have a laser thermometer that you can point at each of the header primaries to see if they come up to temp evenly?

You can also perform a cylinder balance test to see if one of the cylinders is not firing. Running on 7 will produce a slight vibation, but be perfectly drivable.
I do have a laser thermometer all the tubes were pretty close in temp, in the cylinder test all were good , rehooked scan tool and now getting a code that says map /bar sensor out of range ,would that give me a miss? Also just realized the gas is prob over a year old , will drain tank and put in fresh gas , regap plugs , reset timing , reset base idle , recheck all grounds and replace map sensor . I know its something simple Im missing. Thanks for the ideas