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TOPIC: 1985 F-150 with 300 cid Inline 6

1985 F-150 with 300 cid Inline 6 17 Nov 2012 13:48 #1

  • Ron
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I recently performed the mod on this 85 F-150. Let me tell you, it was a learning experience. So much a veteran am I that I just jumped right into the mod without asking about the performance before.

This vehicle has the first gen electronics. A carburetor with a air valve and a TPS. I had ever seen such a thing. So, I just did the mod and reinstalled it. When I restarted the engine, it was spitting and sputtering and shaking like a dog in cold bath water.

I spent three days with this truck.

Here's the scoop: In (partially) solving for this issue, I made a RE-discovery. It turns out the intake manifold gasket was S-H-O-T. When I sprayed it with carb spray, I got only a very slight uptake in idle speed. It was only after going to three shops to ask for advice, an old-timer (like me) got his gas can with a tiny spout and poured a slight dribble in the area.

The engine sprang to life!

I have told many of you time and again that when something isn't right after performing the mod, vac leaks can occur anywhere. If, after you've fixed all the vac line and diaphragm leaks and the engine still doesn't respond correctly, the brake booster AND the intake gasket should be checked. The booster can be pumped down. The intake cannot.

So, how do you check an intake gasket?

You put a ratchet (in this case a wrench) on the bolts and tighten them. When I did this, in the area where the gas was poured (behind the carb), I found two bolts that weren't even hand tight, with those at the ends being perfect.

What happens during normal operation is the gasket decays over time, and part of the material falls away. This leaves a gap between the intake manifold and the head. This gap causes the bolts to appear loose.

Anyway, once the gasket was tightened the engine immediately smoothed out and began purring like a kitten in your lap. The emissions dropped like a rock and when I took it out for a spin, spin I did! It will rip the tires and accelerates like no tomorrow.

And the moral of this story, my friends, is this: No matter how many mods someone has done (I've done more than 1500 now!) you must ALWAYS remember the basics.

VACUUM VACUUM VACUUM is almost ALWAYS the cause of failure to respond appropriately.

Just because you haven't been able to find it doesn't mean it ain't there!

And always remember that if you can't figure it out, there is always someone to turn to for advice. The old-timers are your best resource so make friends with some of the older mechanics in the area.

60+ is the best range. Younger mechanics tend to like being the authority. The older guys tend to be more laid back, and will be happy to pass along some of their knowledge to you, young whippersnapper!

PS: The customer has been advised the intake manifold gasket still needs to be replaced. I also recommended replacing the electronic ignition and carb to an old-style. From there, I will take that truck to more than 25 mpgs.

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Last edit: by Ron.

Re: 1985 F-150 with 300 cid Inline 6 23 Nov 2012 00:08 #2

  • Tracy Gallaway
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Thanks for that, Ron!

I once did a bunch of work on an identical truck as this. Leaded gas was used- this clogged the cat, causing the exhaust to plug up. That in turn got the exhaust man. hot enough to first warp it, then caused an engine fire! I had to replace the exhaust man., electronic feedback carb, sensors, wiring harnesses, it was a big mess!

If emissions regs there allow it, then replcing the feedback carb and going back one generation on carb. and ignition would be cool...could also clean up those little thin plastic vac. hoses Ford was so fond of back then!!


Something easily missed on any old carb.(including Re-manufactured ones from the Parts store) is a vac. leak caused by a worn throttle body at the throttle shaft. I have had carb's saved by having the throttle shaft bushings re-worked. I know the owner of Competition Carburetors here in Reno, he's done this job for me many times.

Anyhow glad you got to the bottom of this one, Ron!

TracyG Gadgetman Reno
Tracy Gallaway
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Gadgetman Reno, NV

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Re: 1985 F-150 with 300 cid Inline 6 16 Dec 2012 03:15 #3

  • Tracy Gallaway
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I just gotta chime in again on this one.

This is about my '74 Dodge Dart Sport w/ the famous 225 slant six. This engine has been modified to a factory Super Six intake/carb combo, that is to a 2-bbl Carter BBD from a 1-BBL Holley. Also the exhaust has been changed to Dutra duals, the rear axle changed to 3.23:1 from 2.74:1, and on and on...

Point being: I've had these mods done for years now. IT ran great on the road, but the idle Sucked!! I went round and round trying to improve idle quality doing most anything one could mention to smooth it out...


I haven't Grooved the carb yet, but I did re-route the PCV hose off the carb base into the air cleaner, and capped that PCV port on the carb base. I had also tried running a home-made water vapor setup, involving air bubbled thru water in a jar then into a vac. hose to a manifold vac. port on the carb. This 2-bbl Carter has several vacuum ports. Anyhow- I removed my vapor jar setup and capped that port, along with capping the PCV port...

And the clouds Parted, a ray of Light shone down upon my Slant, and a Mighty Voice Thundered: "IT'S ABOUT TIME YOU DID THAT!!!"

And...and...my Slant was HEALED--HEALED I TELL YA!!:woohoo:

Guess what, Batman?? no more rough idle quality. I reduced idle speed to 800RPM, it Smoothed right out, it idles in gear docilely, it finally acts like any good Slant Six should. All because the Vacuum Leaks were fixed...after all these Years:blush:

Maybe we should start a new 12-Step Program: Vacuum Leaks Anonymous...Well I got up and told my Slant's Story, who wants to go Next?

TracyG Gadgetman Reno
Tracy Gallaway
Carburetor Coach
Mood Elevator
Gadgetman Reno, NV

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