Nice, glad to hear you have it running right. From here on out, I'd suggest that whenever you make a change, it's one at a time. That way you know what affects what.

With the 600cfm I had on there, I was getting 15 in town and 17 - 19 on the highway going 65. I had some more work to do on it though as it stumbled off the line a lot and performance was soggy. But the mileage made me happy.

That's on a nice 80 degree summer day.

On average though, 12 - 13 around town. 15 - 17 highway.

 

Second update. well after a spring pin break on the unilite dizzy which chewed up some teeth on the cam gear and a compression check I pulled the motor and did a complete rebuild. I bought a used 600 cfm (holley 1850-4) and rebuilt it as well. the motor is great but the carb sucks I could never get it to die when running af screws in so I think someone messed with internals. I finally went back to the 390cfm and put 56 jets in the primarys and it works well except for some runon after the key is turned off occansionally. I plan to get a a/f guage and fine tune the mix.

 

You might want to ask these guys about ignition re-curve kits and tips. You've made a bunch of upgrades some of which will improve cylinder-filling (dynamic compression), so the stock spark curve worked out by the factory is no longer correct for your engine. This involves a lot more than just checking timing with a blink light. In fact, the initial timing spec you get out of the manual is probably sub-optimal.

Without developing a custom curve for your modified engine, you aren't getting all the power and fuel efficiency that you've built into your engine. This is extremely common, and is why lots of low-grade hot rodders (like me) are sometimes disappointed when the changes they make and the aftermarket parts they buy don't perform as advertised. And getting the timing curve spot-on is not so easy to do when you don't have your own dyno and test track, unfortunately for us all. This subject is sort of a hobby-horse of mine, because getting timing right is a PITA, and therefore we tend to either avoid it entirely or do some inadequate, quicky tests and then put off and put off doing it right (guilty as charged, and I know better!!).

The timing re-curve kits are surely a lot better than nothing, but not as good as optimizing YOUR individual engine. And when you get to doing the road-testing, and then the endless diddling with springs and weights and such, you'll understand why some guys go for those systems that you re-program with a laptop computer. As much as I distrust computers (I'm OLD), I've about reached the point of wanting to learn how to do this. One thing I haven't tried yet is MSD's dash-mounted timing adjuster, a box with a **** that lets you advance/retard timing while you drive. Can any of you smart guys give me a review on this product?

 

if you can't get the run on cured by adjusting the timing
An old trick is to use a" dashpot" which is a carburetor mounted solenoid used to control idle for vehicles with air conditioning.
. you could rig one for your base
idle to the ignition switch to shut off with the key.

 

I think I know what fixed it.
The #37 squirter along with turning the carb around.
I am one of the few dissenting opinions that feels exhaust manifold heat is not needed with EFI exhausts.

The smaller runners in the dual port intake are on the bottom, nearest the radiated heat of the exhausts. When you had the primary side of the carb mounted away from the valve cover the primaries were feeding the larger top half runners which are less likely to get as much heat off the headers. Now that the primaries are flowing on the bottom of the intake voila, the charge is warmed and life is good.

 

Look at the overlapping warm-up systems available even without using exhaust gasses to heat the intake:

-- On start-up, hot air drawn up the flex-tube and into the air cleaner snorkel, if you have that system (and you should), . . . and . . .

-- By the time the thermostat in the air-cleaner is shutting off the hot-air snorkel door, the engine coolant is now warm enough to put some heat into the circuit in the bottom of the intake manifold (if it has one), . . . and . . .

-- Eventually the heat pouring off the surfaces of the exhaust system warms up the whole engine-well.

I'm with you on this, Flyer, unless you have real cold winters. The older factory log intake manifold could get extremely hot, even when the warm-up valve had closed; early Econolines were well-known for vapor-lock in the summer.

And if you want to have the benefits quick, add a plug-in engine heater system.

 

As far as I understand it, the primary runners are on top. The primaries are situated closer to the valve cover, which means that for them to be on the bottom, they'd have to wrap around the upper runners.

As for underside heat, from my experience, it really isn't having to do with making the intake warm. It's keeping it a stable temperature. Also, I've put my hand on the underside of the intake on a hot day after driving about 50 miles, and it really isn't that warm. Uncomfortable, sure, but I could leave my hand there. Certainly not the 190* that the coolant would keep it at, or hotter with the stock setup.

Also, knowing how my carb drives on a cold morning vs. a warm one, trying to tune for different temperatures is a pain. Using some underside heat to always know it's going to be one temperature (if it gets warmer, it's cooled back down. If it gets colder, it warms back up) makes my tuning job a lot easier. FWIW

 

It's been a long time since I looked at one. I think the theory behind them was at low airflow and higher intake vacuums the smaller lower runners are fed by the primaries. Then at high loads the secondaries open and the high airflow can be directed along the roof of the runners for best cylinder filling.

Can anybody find a photo of the DP port to confirm this? DP intakes are not my favorite.

 

The runners on my dp.

 

Hrmmm... Interesting.
So the smaller runners ARE on the bottom. I thought the secondaries went through the bottom runners.

I wish I had one in hand to look it over. Don't the two carb holes closest to the head go through the upper runners and the two further away go through the lower?

Thank you for posting that up.

 

Nope, the ones closest to the head are the deepest. I remember that from the last DP I owned.

 

Oh okay, I was getting nervous i'd been telling everyone to put it on backward.
That just didn't make sense to me. It would seem so much easier to make the holes further back drop down to the bottom. Engineering-wise.

 

Nothing about the DP intake makes any sense to me!! That's why I don't recommend it if anyone ask me. The C-series all the way! Why pay that much money for an intake with a restriction built into it? The 300 has big enough cylinders to not have to worry about the intake vacuum signal/velocity with the available intakes.

 

Yet early in the smog era, Offy stated that if you combined the Dual Port and Holley 390 on a 300/six with a good set of headers (and probably a mild cam, I forget now), and re-tuned for the new combination, you could gain, AS I RECALL, something slightly over one hundred horsepower. The 300 gave them one of the best outcomes with this manifold design, as I remember (and I think I save the article).

I don't think they are liars, but I always wondered about that.

The Dual Port has a better shape than the old log manifold, but the thickness and drag of the splitter in the runners has to have a lot of drag when the engine is wound out. It should give you some nice crisp throttle response, that you might not ordinarily get with a biggish carburetor. Seems like it might work pretty good on a work truck, the kind of rig you build, Fordman. I actually have one, traded something for it, but have never cleaned it up and put it on anything. I figure to try it on the 300/six in my old bread truck someday, when I rebuild that engine.

The Dual Port was a clever idea that really needed to be combined with a cylinder head specifically designed for it. Then the port sizes could be got right, and the head also could produce a very strong swirl effect. If one of you with a lot of foundry experience got a wild hair to try something, this Dual-Port-plus-new-heads combo could really wake up certain old engines that had awful heads to start with. I could name some examples of such engines, but people would get defensive . . . .

Speaking of wild hairs: there was a young gal sitting at a folding table in front of a pet store recently with a couple of cages of pet rabbits. These bunnies had been abandoned by their owners, and the gal was trying to find them new homes. I said, "You have all these domestic rabbits. Do you ever get a wild hare?"

She was too young. When you get to my age, the kids don't get your jokes.

 

No thanks. I've been there done that. I've ran both the dp & C-series on the same vehicle. I saw enough of an improvement in power with the C-series over the dp ( with the old butt-o-meter dyno, aka seat of the pants ) that I've never ran one again.

Most of the guys bragging about how great the dp intakes are usually haven't tried the C-series.

Oh yea and my "work" trucks ( well at least the ones that run on gas ) are all getting my own fabricated intakes with multiple carb set ups.