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Discussion Starter #1
I've run into a few things with my engine after I got it all broken in.

I've got the idle roughly set around 1100rpm, but the engine will idle anywhere from 1000 to sometimes 1400rpm without touching the pedal, and sometimes it will high-idle for a while and won't go down without a quick blip of the throttle.

The other issue is I think it has a slight miss. At idle the engine shakes around a good bit, but not consistent enough for it to be the cam I don't think. When revved up to around 2500-3000rpm I can hear a booming noise like a cylinder is dropping off and on.

Its also very gaseous, the exhaust fumes burn my eyes
 

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Well of course more info, jets, age of carb, any mods done. Mine doesn't idle at the same speed all the time either. I set mine at 800 but it seems to stick at 1000 quite often.
 

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Discussion Starter #3
Jets are 73/73 (they probably should be changed, thats just what came in it before I rebuilt it)

It was rebuilt last year with a Holley rebuild kit, everything is stock in it besides that. 6.5" power valve, and I turned the secondary idle screw in a bit.

I have no idea how old the carb is, I got it from my uncle who used it way back in the 70's I think.
 

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Do you by chance still have the O2s? May help to tell if you are way rich or lean. I think that the "factory" jets may have varied through the years and models. Mine is pretty new, maybe 5 months old. Came with 67/75 jets if I remember right. I kind of agree with the "all 4 corners the same" theory for racing, but maybe try 67/75 and just see what happens. Hopefully others will chime in with their experiences as well. I'm kind of thinking that your primaries are way too rich though. Especially with a 351. You may even try 65 jets in the front and leave the rear alone.
 

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Discussion Starter #5
No O2's, I don't know that the car had them in the first place, since it was a 4cyl carb engine.

I would certainly think the primaries are way to big, as bad as the fumes are. Its almost as bad as alcohol. I only have about 10" of vacuum at idle, but I'm running the 6.5" power valve that came with the rebuild kit. Should I worry about changing that out?
 

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All 4 corners should NOT be jetted the same on a carb that has only a front power valve. The rear will end up being WAY leaner than the front. The reason is that when the power valve opens, it adds the amount of fuel equivilent to 8 jet sizes. So, if you had for example 72 fronts and 72 rear jets, it would be like having and 80/72 split under WOT causing the rear to be much leaner.

Now on an HP or Demon carb that has 2 power valves, jetting the carb equally in both front and rear would be the thing to do.


paul351w,

Since that is an older Holley you are using, it does not have "power valve protection" built into it. Maybe the power valve has blown already. That would cause it to run 'pig rich' at idle. Older Holleys are notorious for blowing them. Check the float level also.
 

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One more thought:

Remove the carb and check to see if the throttle blades are still in the transition slot. Should be no more than .040" (I believe, someone correct me if I'm wrong). It's possible that the plates (throttle blades) are open to far to get an idle out of it and it is pulling on the main metering system to early. The cure for this would be to drill small holes (start with 1/16" and go up in 32nd" increments until it's right) in the blades on the metering block side.
 

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Discussion Starter #8 (Edited)
Yeah, I guess it could've blown, but it does have the newer style valve in it with the sheetmetal cover. Maybe that doesn't matter.

It does smell rich at idle, but it was run above idle for a good share of the time and it was still quite rich smelling.

I think when I cracked the secondaries open more it brought things back closer to where they should be
 

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paul351w said:
Yeah, I guess it could've blown, but it does have the newer style valve in it with the sheetmetal cover. Maybe that doesn't matter.

The power valve protection is actually built into the throttle base. You can add a retrofit kit, if it doesn't have one already.
 

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By the way you're describing your idle, I doubt the power valve has blown. When you change the front jets, you can suck on the power valve with a finger on the back to see if it's moving. Also, if you have 10 inches of vacuum at idle, the 6.5 is fine.
 

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Discussion Starter #11
Could the over-rich jetting have something to do with it missing possibly? Its not a constant miss, just enough to be noticeable
 

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Paul put that power valve checkball in sometime soon man, I need to get one in my 750 as well


Don't worry too much about the idle yet, you'll need to adjust it after you install the c4 anyway ;) Concentrate on that miss or whatever is going on.

Order your converter, pull that carb to install the pv protection and check the jetting, double check your timing, fuel pressure, check the plugs and do a compression check for your records....worry about tuning it further when you can get it out on the road :D
 

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I am going thru some tuning on the 650 DP on my 85 GT. I had the carb jetted kinda like a 750, I had 71 jets in the front and 80 jets in the back, it ran fine around town, but the plugs were carbon black and when I took it to the track I could see a swirl of black smoke trailing the car when I would launch it. I got a Holley tuning book for Christmas and decided to put the stock jetting back in (67 jets in the front, 73 jets in the rear) , but it caused a lean stutter when accelerating, and my plugs were still carbon black unless I checked the plugs after WOT. I bumped the front jets up to a 69 and most of the lean stuttering went away, I'm almost thinking that the 6.5 power valve might need to be changed to a lower valve. I'm thinking of buying a 4.5 and trying it.
BTW the idle mixture screws on the sides of the front metering plates are supposed to be turned out 2-2 1/2 turns according to the Holley manual. That might be some of your idling problems.
 
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