I have an HO 5.0 from a '91 MK VII. I re-ringed it and put on an Edelbrock performer rpm intake, a 570 cfm Holley street avenger carb, Heddman short headers with 2.25 inch exhaust. Duraspark distributor for '85 mustang with an e-core coil. I'm using a t5 from an '85 Capri and manual brakes. Cranking compression is 155 to 165 psi It runs rough and misses intermittently at idle and low rpm. It's much worse when cold. The vacuum at 700 rpm is about 18 in. and drops about 1 inch when it misses noticeably. Timing is set to 14 btdc and total of 34 degrees total at 3500 rpm. I did the cylinder contribution test. Shorting out 1 cylinder at a time, each cyl. dropped 35 to 40 rpms and the intermittent miss continued. I think it acts like the carb base gasket is leaking air but I can't find any vac leaks on top of the engine. I tried smoke testing the intake. The smoke did come out the valve cover openings but I think it may have come past the rings in the cylinders with open intake valves. What do I do next ? I don't want to pull this intake off for no reason. What's wrong with my engine?
At 18" it doesn't have a leak if it did would likely be some smoking. A leak in lifter valley will suck oil into intake. Getting a RPM drop for each cylinder means it isn't missing, likely a uneven fuel mixture or maybe ignition problem. Have you checked to be sure power valve in carb isn't leaking?? Isn't as common in Holley as in past but still happens.
Are the intake and carburetor both known quality items? In other words, are they used and have performed well on a different engine or are they new or new to you?
Thank you, Tom and Hotrock. I got the carb and manifold both used but in like new condition. I rebuilt the carb like I always do with a used one. I went through it again a couple of weeks ago because of this problem. I've had mystery trash get in them before. The power valve is good and every passage and restriction is clean. I put it back on with the same results. I then swapped carbs with my Mercury. The problem stayed with the Maverick, the Merc ran great still. The manifold doesn't look like it was modified. It looks like the sealing surfaces at the heads are parallel. I checked the spark with my tester set at 40,000 volts and the spark is loud and blue. It even arced out of the tester to the upper shock mount more than an inch away. The plug wires are new cheap house brand from one of the parts chains. I don't remember which. They check out ok at about 7,000 ohms per foot. I've made sure they are separated and I sprayed them with water and got no change. I have new copper core plugs that I installed to replace the platinum ones that had fouling carbon on the insulators from previous tuning sessions. I checked the new plugs after putting about 50 miles on them and they still look new. The timing marks show rock steady with a linear advance to maximum. Is it possible that the manifold is too big ?
Due to many different different engine combo's and potential calibrations, I would've probably asked WHICH specific 570 model #.. but it seem's you've narrowed it down to ignition if that's truly the case. Where's the mixture screw adjustment as it currently sits? Maybe weak coil that gets aggravated when cold? Process of elimination.. swap in another. Inline factory voltage resister has been removed and confirmed all primary and secondary voltages along with ground/s continuity?
Thanks groberts101. It's the 4150 model and has the original main jets and no mods to the air bleeds. Now that you mention it. There is a discrepancy in the mixture screws of less than 1/4 turn. Driver side is at 1 3/4 and pass is at about 1 9/16 to get highest vac. I usually see smaller differences if any. Is that too much ? I did swap coils when I had no spark one day. Turned out to be a connection I had pulled loose. I ran it with each of the coils and it was the same. I'm using a new 21 circuit wire harness so I have a 1 ohm ballast resistor in circuit. I've bypassed it with no change. Another thing is the exhaust sounds different on each side. The pass side has more pop to it I think. I have an h pipe right behind the trans cross member. I thought that would make sound the same on both sides.
Is it common to trigger a E-coil with a Duraspark? I've never tried. I know they have lower resistance than most tower type coils and the resistor is not used with TFI system. I have a E-coil on my Comet but am driving it with a TFI module, triggered with points. Resistor is bypassed. TFI module handles current and dwell, no current across points.
I've never used an e coil with the duraspark before. It never occurred to me that it might be incompatible. I'll do some research. The coil is 1 ohm. I have no problem using other options for ignition.
Not sure what you mean by too big? Many a 302 Ford stock or modified run Performer RPM or their equivalent manifolds with no issues. Including mine. You can check that off your list of possible problem areas. Are you able to maintain a good curb idle with your engine or does it idle too fast?
Thank you rthomas771, It looks like lots of folks have used these coils with duraspark. Some with , some without the ballast. Thank you Hotrock, I was thinking that the runners might be too big to maintain charge velocity at low rpms. I can idle it down to less than 500 rpms. It's very ragged but it doesn't stall even with the throttle fully closed. I thought that was a problem because i don't remember being able to close the throttle on anything before and keep it running, but my carb guy says that is normal. Thank you baddad457. I started with a 2 inch 4 hole, then a 1 inch 4 hole. Then just the carb and 1 gasket right to the manifold. Now i have just a heat shield and gaskets. There were no differences that I could measure.
You seem to know your stuff in hashing out the details. In my experience, anything under quarter turn is more than decent enough considering the firing order/air demand bunches up on on one half the engine banks occasionally and the manifold design itself is not perfectly idealized from a flow balanced standpoint. Lot's of variables there. Is this the model# you have? https://www.holley.com/products/fuel_systems/carburetors/avenger/street_avenger/parts/0-80570 Since you seem to have isolated it away from a fueling issue it may be slightly moot at this point but I will add this. Looked at that 1st carb linked below and it's crazy jet spread spec's on holley's site. Don't even know.. what cam's in it?, stock 5.0 roller?.. but either way the 1st carb linked below is waayyy wrong performance calibration for this little motor. 11 number front to rear jet spread equates towards MPG scenarios of small primary/large secondary spreadbore booster designs. Accelerator pump nozzle #31 is crazy small for a slightly bigger dual plane like this one too. 35 or 36 would be closer to ideal for that manifold design. All really dependent on your specific parts combo and intended performance goals. Or this one? https://www.holley.com/products/fuel_systems/carburetors/avenger/street_avenger/parts/0-83570 Accelerator pump nozzles still too small but the 2nd model has preferable spec's for what this motor should want.
IMO, that's a dead clear indicator that the rear is exposed too much and the front not quite enough while trying to avoid too high idle speeds. Front transition slot exposure should be at least .040" to get the primary barrels moving more air, increase demand and help all the front circuits on transition and pump shot.. moving fuel quicker. IOW, adjust the front to specified .040" exposure.. then adjust rear to desired idle speeds. Primary side enrichment always rules on a vac sec street carb. If your guy's good it should already be setup damned close to that.
Ok, I was wondering if maybe you were trying to use a stock 1" PCV spacer under it. That will get you a vacuum leak on most aftermarket manifolds. I ran a 570 Street Avenger carb on an Explorer motor and it was fine as it came out of the box. Are those spark wires new ? The best for the price I've found so far are Ford Racing's 9mm sets. Still about $40 a set and they last forever, even in an E series van's engine box.
