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Rough idle on fresh rebuilt motor, Im lost.

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Old 06-14-2016, 08:10 PM
djc1978 djc1978 is offline
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Rough idle on fresh rebuilt motor, Im lost.

So long story short I just got my 302 completely rebuilt and now Im trying to get the truck to run. I am by no means a mechanic and am just learning as I go but here's what's happening. I changed literally everything in the engine compartment short of wiring. New holley street avenger 2 bbl, new dist, coil, plugs, wires, ect. I also blocked off the EGR valve ( if that could make a difference) I cannot get the truck to idle smoothly and the carb wants to either backfire or fall flat on its face. I timed it to 10* BTDC and then adjusted back and forth with the dist to get the highest vacuum, however the highest it gets is about 15". Any adjustment from there makes the truck idle even worse. Any idea on where to go from here? I also have tested for exhaust leaks and found nothing. The carb was working fine before the engine was rebuilt, only about a month old at that time. Thanks for any input.
DJ
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Old 06-14-2016, 11:06 PM
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4x4slik 4x4slik is offline
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Check for vacuum leaks. Also, tuning is a lot of separate adjustments. Adjust your timing to it's best performance, then adjust your carb to it's best performance, then retry timing, then retry carb, etc.
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Old 06-15-2016, 01:49 AM
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I would be thinking vacum leak as well...
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Old 06-15-2016, 01:57 AM
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EP145 EP145 is offline
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Check and recheck your firing ordered, are you using HO cam or early 302 cam there is a difference in the firing order. A friend of mine had the same problem as you described and we found the guy who wired it wired like a SB chevy. Here's a good reference.

Mini Tech - Ford 302 Firing Order Reference Sheet | The H.A.M.B.
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Old 06-15-2016, 02:35 PM
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Old 06-15-2016, 06:05 PM
djc1978 djc1978 is offline
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Thank you for the replies. I have checked for vacuum leaks using starter fluid with no rev ups. I know that isn't completely full proof but also with having 15" vacuum I thinks that's kinda high if I had a leak ( though like I said,I am no expert) The firing order is a sticking point with me. The motor is a 91 thunderbird which should be an HO. The shop that built it did it to the newer block, however, when I try to use the HO order I cant even get it to start but with the traditional order I can get it to start fine, just idles rough, but not necessarily like it is missing (if that makes sense). So all this is exactly why I am having the problems. It seems like everything that should work just sets me back. Thanks again for everything though and please keep the ideas coming. Hopefully ill have time this weekend to get back to it.
-DJ
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Old 06-15-2016, 08:31 PM
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usuallybowtie usuallybowtie is offline
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Okay...you've checked the firing order...BUT did you make sure you checked it in counter-clockwise order?
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Old 06-15-2016, 11:40 PM
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I really am just throwing this out there because I don't know for sure. But I believe your firing order is determined by the cam. If you replaced it, look it up and see if you can get the firing order. wrong firing order wouldn't necessarily miss at idle.

But again, just throwing ideas out there.
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Old 06-16-2016, 08:10 AM
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Use a timing light to set the timing and a vacuum gauge using manifold vacuum to set the carb and check for malfunctions. You wouldn't use a timing light to set the carb, would you? Don't use a vacuum gauge to set the timing.

15" hg vacuum is too low - you should be reading 20 - 21" hg. Suggests a vacuum leak or you're not runnin' on all eight.

Is the vacuum at 15" hg steady or fluctuating?

You said you blocked off the EGR - With a plate over the opening or just rubber nipples? If you used just rubber nipples, it could still be pulling vacuum - depress the EGR diaphragm - if the engine sounds different you have a vac leak in the EGR.

Check the hose from the manifold to the power brake booster for cracks, leaks, etc. Also check the swivel valve on the booster for leaks. If it leaks you'd also notice low brakes.

Check all rubber nubbies you have on the carb, vac tree, etc, for brittleness and/or cracks.

Vac advance diaphragm could be shot - apply suction at the vac can nipple. If you get air without the advance arm moving, the vac advance diaphragm crapped the bed.

On the rebuild - what kind of cam did they put in?
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Old 06-16-2016, 10:04 AM
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If the advice above doesn't resolve the issue, it may be necessary to take a look at the valve train. Assuming hydraulic flat tappet lifters, the builder may have applied too much preload. This could cause the symptoms you are observing.
This adjustment might best be done by the engine builder. They do stand behind their work, don't they?
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Old 06-16-2016, 10:51 AM
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Good call there, flowney
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Old 06-16-2016, 05:16 PM
djc1978 djc1978 is offline
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Ok, so as for the firing order I have been waiting for the builder to send me the parts list (been busy at work and unable to stop back in and keep pestering them) so I can lookup the cam. the plug wires are ran in counter clockwise direction. I did also time it with a timing light, forgot to mention that. Its at 15* no vacuum advance hooked up. the EGR valve is blocked off with a plate and I just plugged all the vacuum barbs at the engine to rule out any leaks in the lines and it didn't change the vacuum level. I also just did a compression test and all cylinders were between 118 and 125. I know that is slightly on the low side so could that and the low vacuum be indicative of something like late valve timing? The reason I haven't went right back to the builder is they don't do over the fender work and not being a mechanic, im trying to cover my end before I go through all the trouble to pull it again and it turn out fine. Again thanks for all the input, hopefully I can get something figured out and either have it back to the builder or running by next week.
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Old 06-16-2016, 06:29 PM
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Quote:
Originally Posted by djc1978 View Post
The reason I haven't went right back to the builder is they don't do over the fender work and not being a mechanic, im trying to cover my end before I go through all the trouble to pull it again and it turn out fine.
Sounds like you need a new mechanic. Its not like they didn't expect you to install the ****...

Edit- also, if I was a mechanic with a, "no over the fender work" policy, I would break that engine in on the bench to verify its running as its supposed to be before handing it back over to the person paying for my services.
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Old 06-16-2016, 07:39 PM
djc1978 djc1978 is offline
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I got it figured out! I had to play around with the timing just slightly to get her to fire easy every time. once I got that I switched the plug order to the HO order and she smoothed right out. Thank you all for the suggestions, its what kept me sane trying to get this. And Krazyness I agree with the firing it on a stand, I figured that's what they would do but wasn't the case, lesson learned if I ever get anything else built, but luckily they had everything right.
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Old 06-17-2016, 07:34 AM
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Great you got it figured out. One thing to add that may help going forward... It would be a good idea to check the timing pointer and verify true TDC.

There was some tolerance as originally built, parts changes can add error and sometimes the damper ring slips. Finding and marking true TDC on the damper or adjusting the timing pointer with the "Positive Stop Method" will make tuneups easier and may help you get her running better.
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