where to look next?very sluggish 390 - Ford Muscle Forums : Ford Muscle Cars Tech Forum
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post #1 of 24 (permalink) Old 12-17-2018, 05:18 AM Thread Starter
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where to look next?very sluggish 390

we put a 390 c6 combo in my sons 67 galaxie .it replaced the 289 that ran perfect but he just wanted a bigblock.the 390 ran great in the po vehicle and we witnessed it too.lots of power.it has a lunati voodoo cam and a new timing gear set.the gears had 3 keyways but it was set at the stock location.it also has a msd ready to run 8595 distributer.we first ran it with the stock coil from the 289 and it idled fine but did not have any power at all on acceleration.we thought it may be the old edelbrock carb so we bought a brand new holley 670 street avenger.same results though.then we bought a msd blaster 3 coil thinking the coil may be bad.it would not run at all with the blaster coil.it would try to start but cut right out once the key went to run position.the stock coil said "use with external resistor"on it so we figured somewhere the voltage was dropping.we ran a lind directly from the pos side of the batt right to the + side on the coil.it runs now but it still has no power at all on acceleration.it sounds like the 4bbl isn't even opening at all.we checked the stroke of the throttle cable and it is getting a full pull on the throttle lever.we have the timing set at about 10 deg btc at idle but we have played with it and turning the dist either direction has not helped very much.
any ideas as to where to look next?could we have burnt something out in the dist bu running it with the stock coil?
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post #2 of 24 (permalink) Old 12-17-2018, 07:23 AM
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Re: where to look next?very sluggish 390

My guess would be a vacuum leak. Does it have power brakes? Disconnect and plug the hose and see if it runs better. Check the vacuum modulator for the transmission. Is the distributor hooked to manifold or ported vacuum? Some people swear by manifold vacuum but I've always used ported vacuum and would suggest that as a starting point at least. Spray carb cleaner around the base of the carb while the engine is running. Any change in RPM (up or down) indicates a leak.


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post #3 of 24 (permalink) Old 12-17-2018, 06:35 PM
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Re: where to look next?very sluggish 390

Yes, check the amount of vacuum the engine is making. I have a cam with more lift and duration and it makes 15 inches at idle (900 rpms). I recently installed a MSD ready to run distributor and coil on my 302. I set the timing to 14 degrees BTDC. The distributor adds 19 degrees of mechanical advance. My dist came with 2 medium springs installed. I replaced one med spring with one small spring. The advance comes in at 2400 rpms now.

I think your engine should make 18 or more inches of vacuum at idle. I think you could increase the timing of the engine. If you have two of the heavy springs in the distributor, the full advance will come in very slowly.


You should also run a 14 gauge wire to the distributor. It needs a full 12 volts.

Last edited by 65merccyclone; 12-17-2018 at 07:50 PM. Reason: add more data
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post #4 of 24 (permalink) Old 12-18-2018, 08:58 AM Thread Starter
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Re: where to look next?very sluggish 390

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Originally Posted by 65merccyclone View Post
Yes, check the amount of vacuum the engine is making. I have a cam with more lift and duration and it makes 15 inches at idle (900 rpms). I recently installed a MSD ready to run distributor and coil on my 302. I set the timing to 14 degrees BTDC. The distributor adds 19 degrees of mechanical advance. My dist came with 2 medium springs installed. I replaced one med spring with one small spring. The advance comes in at 2400 rpms now.

I think your engine should make 18 or more inches of vacuum at idle. I think you could increase the timing of the engine. If you have two of the heavy springs in the distributor, the full advance will come in very slowly.


You should also run a 14 gauge wire to the distributor. It needs a full 12 volts.
do you mean to the coil or distributer?
the dist came with the engine and I don't have the spring set to try.i do have the stock points dist.i think I'm going to pop it in to see if there is a difference.
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post #5 of 24 (permalink) Old 12-18-2018, 01:24 PM
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Re: where to look next?very sluggish 390

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Originally Posted by cherokeechief7 View Post
do you mean to the coil or distributer?
the dist came with the engine and I don't have the spring set to try.i do have the stock points dist.i think I'm going to pop it in to see if there is a difference.

Maybe first make sure you have 12V to the coil when it is running, before you swap distributors. The car may still have the original resistor wire in the system that knocks back the voltage to the coil in stock applications. It is probably a fat pink wire under the dash, coming off the ignition switch.

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post #6 of 24 (permalink) Old 12-18-2018, 03:33 PM Thread Starter
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Re: where to look next?very sluggish 390

thank you....yes we tried running a wire directly from the pos batt terminal to the + on the coil.in the old vehicle we took it from it could smoke the tires no problem.now it doesn't even seem as if the 4bbl is opening.were gona try putting a points distributer we got with it in to see if anything changes.of course we will have to go back to the stock coil with the resistor wire too.this is so frustrating!it ran so good just before the transplant.
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post #7 of 24 (permalink) Old 12-18-2018, 04:29 PM
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Re: where to look next?very sluggish 390

i bet you have a tall rear gear
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post #8 of 24 (permalink) Old 12-18-2018, 08:00 PM
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Re: where to look next?very sluggish 390

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Originally Posted by cherokeechief7 View Post
do you mean to the coil or distributer?
the dist came with the engine and I don't have the spring set to try.i do have the stock points dist.i think I'm going to pop it in to see if there is a difference.

Coil. Sorry about the typo error
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post #9 of 24 (permalink) Old 12-19-2018, 04:01 AM
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Re: where to look next?very sluggish 390

I would start with getting some real numbers and more info

- What is initial timing, total timing and what rpm does total come in at. Aftermarket FE distributors are notorious for slow curves
- What is the cranking compression, best with all plugs out and carb wide open. My thought here is you may have accidentally misaligned the gears and retarded the cam
- Which voodoo cam is in it?
- Did you check pushrod length when you swapped the cam in? or was the cam in it in the last vehicle? 390s have multiple pushrod lengths from the factory and many cam kits use SBF lifters which can change preload
- Any chance you have any wires crossed?
- Can you open the secondaries by hand? Engine off, hold throttle wide open from drivers side linkage with left hand, with right hand, grab small secondary linkage and see if it will turn. You will see how the little link bar gets out of the way as you move the throttle.

The coil swap stuff may be a red herring, however, running a coil that says it needs a resistor can hurt the distributor, not the coil. When the coil says that, it is telling you it's got low resistance, and as the field collapses, it can spike the distributor through the trigger wire.

My initial guesses are

- Distributor advance is stuck for some reason
- Timing is off due to slipped balancer or incompatible pointer / balancer
- Bad vacuum leak as discussed
- Cam not degreed properly if new cam

Just generating ideas
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post #10 of 24 (permalink) Old 12-19-2018, 06:14 AM Thread Starter
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Re: where to look next?very sluggish 390

thanks ill look into all of these things.we have 3.0 gears in a ford 9 3/8 rear from a Lincoln.
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post #11 of 24 (permalink) Old 12-19-2018, 06:48 AM
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Re: where to look next?very sluggish 390

As I've understood it the PO did the engine mods before you bought it-all you did was swap it into your car. MSD provides several checks to help diagnose problems on their site. You may also want to perform a continuity test-not given on their site. If your RTR module has gone to ground its toast. The engine may run but extremely rough with no power. Just a thought.
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post #12 of 24 (permalink) Old 12-19-2018, 07:17 AM
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Re: where to look next?very sluggish 390

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thanks ill look into all of these things.we have 3.0 gears in a ford 9 3/8 rear from a Lincoln.
aha what gear was in the previous car?
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post #13 of 24 (permalink) Old 12-19-2018, 11:51 AM Thread Starter
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Re: where to look next?very sluggish 390

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Originally Posted by JimGTA View Post
As I've understood it the PO did the engine mods before you bought it-all you did was swap it into your car. MSD provides several checks to help diagnose problems on their site. You may also want to perform a continuity test-not given on their site. If your RTR module has gone to ground its toast. The engine may run but extremely rough with no power. Just a thought.
what is the rtr module and can I check it?i went to the msd site like you said and saw some good info too.
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post #14 of 24 (permalink) Old 12-19-2018, 03:05 PM
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Re: where to look next?very sluggish 390

Sorry if this seems silly, what are you running for exhaust?


You may not have big enough pipe, I had some restrictive mufflers in Side Pipes that made a 560hp BBC seem really lazy (this was after Engine Dyno where the only change was the Exhaust, shop Headers vs Side Pipe Headers/Muffs). I put new high flow Muffs. in and it woke up
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post #15 of 24 (permalink) Old 12-20-2018, 05:28 AM
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Re: where to look next?very sluggish 390

RTR = Ready To Run distributor. The module is the internal "brains". You need a meter that can measure ohms-borrow one if you need to. Key off, one lead to the orange wire in the RTR loom, the other to the distributor body. A low ohm reading indicates grounding. At least this will rule out or confirm a problem. Good luck.
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