OT Slant Six Help!

[PhilJohnson]

I bought me a 74 Valiant to replace the 98 Crown Vic that I had been using as a DD. The Vic's oil pan has a huge leak among other things. I figured that I'd be getting at least low 20s for fuel mileage. Last time I checked the car was getting slightly less than 16 mpg. It's a 225 with the automatic. I consider this to be horrible in a light car doing 90 percent country/highway driving. I had a Ramcharger 4x4 that I got an honest 20 mpg with a 318 and a 3 speed automatic. I did your basic tuneup. Car is still running bias ply tires, no fuel leaks, carb doesn't leak either and seems to be in good working order. The engine does sound like a small diesel. Am I missing something or is there some sort of trick to wringing out a few extra miles out of a slant six?

[greg g]

is the choke pulling all the way off? Any black smoke from the exhaust? Is it pinging? You said diesel noise, check the timing should br tdc or a couple advanced. What temp is it running at. /6 like 180 to 200 .

[PhilJohnson]

is the choke pulling all the way off? Any black smoke from the exhaust? Is it pinging? You said diesel noise, check the timing should br tdc or a couple advanced. What temp is it running at. /6 like 180 to 200 .

No black smoke however I noticed above the exhaust pipe there is quite a bit of black sooty buildup on the bumper (can see it in the one picture). It does seem to ping some under load. The diesel noise is from a clattery valve train. It sounds like the valves need some serious adjusting. I have no idea what it's running for a temp, it does have great heat so I'd assume it's good.

[greg g]

That soot is a very good indication its running way rich. The carbs on those are pretty simple, not much more complicated than a B and B. Kits are cheap. Id go throughthe carb, calibrate the choke, get a new air filter, and check the timing. I never got less than 22 with my /6 and it was in a early Dart about 700 lbs heavier than the Valiant.

[Guest P15-D24]

auto and air. Never got above the mid teens unless I was only going 55.

Three speed auto tranny, realtively heavy car for a compact, just don't think your going to see much better.

[randroid]

Phil,

I honestly think that what Greg said should be the first things to do but just to put another variable into the equation is that your Ramcharger probably had the A727 transmission, which I've always thought was a fine one. Some of the difference (albeit a small amount) in mileage could be due to that alone.

-Randy

[JerseyHarold]

Check to see that the PCV valve is operating and that the hose and its connection to the base of the carb isn't clogged. Excessive crankcase pressure will contribute to blow-by and leave an oil film on your bumper.

[Rusty O'Toole]

How many miles on it? Did you adjust the valves?

The diesel sound is an indication that the car has 100,000 miles on it and the valves have never been adjusted. If they were adjusted every 20,000 miles the car would be as quiet as a Cadillac. Not a 1974 Cadillac with 100,000 mile on it, a brand new Cadillac.

If the valves have not been adjusted the rocker arms and valve stems are pounded and peened over. This will not prevent adjusting them but it will mean they will never be totally silent.

And, if it has that kind of miles on it you should buy a rebuilt carb.If there is any wear in the throttle shaft replace the carb.

While you are at it check the automatic choke, and heat stove on the exhaust manifold if there is one, and the carb heat pipe and control valve. If all these things are in place and working right it will start and run like a car with fuel injection.

You say you did a tuneup so I assume the plugs are new, properly gapped and of the right heat range. But did you test the plug wires and inspect the cap and rotor?

If the engine has good compression and in tune, you should get over 20 MPG. I used to get 22 with a similar car. If you got 20 with a 318 powered 4X4 you should get 30 with the Valiant.

Edited January 13, 2012 by Rusty O'Toole

[oldodge41]

I have a '69 Dart with the 225/6, Auto and 2:90 something rear. Always struggled to get much over 15mpg until I went to a supersix setup with the two barrel carb from a newer model. I gained mpg and performance but doubt I get 20. Maybe 18, haven't checked in years. I love these motors but economy has never been great on the few I have owned. Setting the valve clearances will help the sound and performance, possibly gain a little economy but don't expect too much. My dads 383 auto '68 Chrysler Newport got better mileage than my brothers '68 Dart slant back in the day, and that car was way heavier.

[Flatie46]

There are other things that'll help mileage. A free flowing exaust with a header. It doesn't have to be loud. I've heard an electric fan will help. Ignition is important, it does have an electronic setup doesn't it.

[oldmopar]

For some additional answers try the /6 site.

http://www.slantsix.org/

[PhilJohnson]

Thanks for all the suggestions. Hopefully I can tinker with it this Sunday

How many miles on it? Did you adjust the valves?

No' date=' supposedly the car has 25,000 miles on it. I bought it from an agency disposing of a very elderly lady who was the original owner's estate. I have my doubts as to the 25,000 mile claim, it seems a bit too rusty for that low of mileage. Also either Granny was portly, Valiants have really weak seat springs, or they are weak from 125,000 miles worth of driving. I do agree the valves need to be adjusted which makes me wonder why Chrysler just didn't go with hydraulic lifters right from the start. I've never adjusted valves before so it'll be a new experience for me.

And, if it has that kind of miles on it you should buy a rebuilt carb.If there is any wear in the throttle shaft replace the carb.

It seems to run well, no leaks, and the accelerator pump seems to be working great. I hate tearing something apart that doesn't have any obvious signs of issues. I'll leave that for last if all else fails. Plus I have property taxes due so a 200 dollar carb is out of the question.

While you are at it check the automatic choke' date=' and heat stove on the exhaust manifold if there is one, and the carb heat pipe and control valve. If all these things are in place and working right it will start and run like a car with fuel injection. [/quote']

Will do. The choke seems to work well but I'll double check.

You say you did a tuneup so I assume the plugs are new' date=' properly gapped and of the right heat range. But did you test the plug wires and inspect the cap and rotor? [/quote']

Wires are new, cap and rotor were cleaned up. I bought a new cap and rotor but it would appear that slant sixes must be made differently in China

If the engine has good compression and in tune' date=' you should get over 20 MPG. I used to get 22 with a similar car. If you got 20 with a 318 powered 4X4 you should get 30 with the Valiant.[/quote']

One would think. I think a 318 would get better mileage in this car than the slant six. It just seems sorta doggy. Oh well, I have what I have to work with. The Ramcharger I'd merely rest my foot on the gas to cruise. I have no idea what the final drive ratio was but it must have been pretty high. It got the best mileage at a steady 60 mph. I almost never used more than a quarter throttle to take off from a dead stop. It had dual exhaust and headers but other than that it was pretty much stock. My friend got rid of it after complaining how it would only get 12 mpg. He has a lot heavier foot than I do.

Edited January 17, 2012 by PhilJohnson

[JerseyHarold]

RE: Odometer mileage

Mopars from that era had a 'wiper' for the 10,000-mile number wheel as a way to deter rollbacks. If the mileage went, for example, from 59999 to 60000, the '5' got smudged by the wiper. So, if the '2' in your 25000 mile reading is smudged, it's the second time around.

[Suddensix]

Have you checked the general condition of the engine: compression test etc., checked for vacuum leaks or dragging brakes or a slipping transmission? You may be trying to breath life into a dead horse. Not having owned a slanter for many years I don't remember them giving great gas mileage. They are a great little engine though. Good luck.

[PhilJohnson]

RE: Odometer mileage

Mopars from that era had a 'wiper' for the 10,000-mile number wheel as a way to deter rollbacks. If the mileage went, for example, from 59999 to 60000, the '5' got smudged by the wiper. So, if the '2' in your 25000 mile reading is smudged, it's the second time around.

Interesting, will look

Have you checked the general condition of the engine: compression test etc., checked for vacuum leaks or dragging brakes or a slipping transmission? You may be trying to breath life into a dead horse. Not having owned a slanter for many years I don't remember them giving great gas mileage. They are a great little engine though. Good luck.

Well the transmission seems to be good, no slipping. Unsure of the rest of it. Worn out it may be but I still think it should be able to obtain 20 mpg. My wore out no compression F100 with a 300 six got 20-22. It does seem to require a lot of throttle to maintain speed. I'm gonna tinker with it some more tomorrow.

[JerseyHarold]

One more thing to check: Make sure the TV rod from the carb linkage to the trans is not bound up. It will affect the shift points and drop your MPG's. This is something to look for if the car sat around quite a bit.

We bought a '76 Valiant to flip from an auction years ago and it shifted terribly. Expecting the worst, I took it to our friend at a trans shop. He zeroed in on the TV rod, and with a couple of shots of WD40 and a whack or two it freed right up.

[plymouthcranbrook]

I have a 1980 Volare with the one barrel 6 that usually gets about 20-22 on the highway at 60 or so. Gutless of course and I have a light foot. The Volare is somewhat heavier as well.(17,000 miles on it)

[PhilJohnson]

Didn't do too much to the Valiant today. I worked overnight so energy was an issue. Still I managed to put on a different cap and rotor. Motor seems to run smoother but nowhere near as smooth as the old 50 Coronet. Also hooked up a vacuum line to the hot air thingee on the air cleaner. It seems to work. I noticed the car makes a strange rattle noise under load. I'm going to check the ignition timing tomorrow. Before I turned down the idle it used to diesel pretty bad. Probably going to try and adjust the valves tomorrow too if I can find my feeler gauges.

[james49ply]

Dieseling, sure sign of to far advanced timing, unless there is so much carbon on the pistons that the compression went way high.

[Rusty O'Toole]

Slant sixes get good mileage, for their times, and have adequate power if tuned up right. By that I mean you could tour the United States at the legal speed limit with a full load of passengers and baggage and climb any main road hill.

When the slant six was designed it was for an economy car. Even in 1960, hydraulic lifters were the norm so I don't know why they were not included. Eventually they did go to hydraulics but this was just a few years before they went out of production.

Solid lifters are not a problem. In fact I prefer them. You only have to adjust them every few years, it is not hard to do, and they will be perfectly silent for the life of the car which is more than you can say for hydraulics.

Remember to get a new valve cover gasket and follow the instructions in the manual. I find it best to loosen all the jam nuts and adjusters before starting the engine then adjusting the valves with the engine idling, just like the book says. Do not disturb the grease and sludge, or try to clean it off.

It sounds like your car needs a complete tuneup. This is not hard to do if you go by the book and do one step at a time.

Ignition first, then valve adjustment, adjust the carburetor last.

Get it running nice and leave it alone, it will run great for 10,000 to 20,000 miles without further attention except oil changes.

By the way if your engine really had 25,000 miles on it you would not be able to hear it run. If it sounds like a diesel it has 125,000 with no valve adjustments. Do the valves and it will be quieter but not totally quiet.

Some years ago I bought a Dodge farm truck with 160,000 miles on it. The engine was dead silent. The truck had led a hard life and the outside was beat up but it had always been maintained, oil changed, engine tuned and the valves adjusted. It was as quiet as a brand new Cadillac. I have had many old slant sixes but this was the first one that was totally silent, also the first one that had been maintained by the book.

With a few hours work and a few parts you should be able to have your car running like new. Those motors are very simple to fix, and long lived if you give them a chance.

Edited January 18, 2012 by Rusty O'Toole

[PhilJohnson]

Those motors are very simple to fix' date=' and long lived if you give them a chance.[/quote']

I'm afraid no one told my motor about being long lived I went to start it today, it was wicked cold. No oil pressure. It started to knock so I shut it off although to be honest it always seemed to have a slight noise in the bottom end. I'm not impressed, this is the second slant six to crap out on me I guess I could put in a different oil pump but I'm tempted just to say screw it and throw in an AMC 258 which in my opinion is a much better motor. I had better luck out of those old flatheads and better fuel mileage too

[Rusty O'Toole]

Well it is 38 years old which is 266 in dog years. It passed its best before date with the Bee Gees. For a car, it has already had a very very long life.

Having made that crack, all is not lost. There is a possibility you can fix the motor in an hour or less for no money.

It is possible the bypass valve on the oil pump went wide open and got stuck. The oil pump is on the outside of the motor and the valve is on the outside of the oil pump. There is a screw in plug that gives access to the valve. It is like a small piston with a spring behind it. Take it out and clean off the sludge and you may get your oil pressure back.

Or, take a look at the oil filter. If the valve stuck closed it can blow the oil filter wide open.

This is too obvious but did you check the oil?

Edited January 20, 2012 by Rusty O'Toole

[Tim Keith]

Slant sixes are good motors and can get better than 25 MPG, especially in a mid '60s Valiant. I had a '65 Fury with 225 that got better than 20 MPG. In my experience a high mileage slant won't have bottom end problems when the oil level is kept up with reasonable change schedules. I had a worn out Valiant with 225 that leaked oil and it got 24 MPG with a 3 speed manual transmission. I drove that Valiant for 120 miles a day for 2 years, four years ago. On www.slantsix.org you can find help to achieve close to 30 MPG in a Valiant. The big benefit is the slant six is easy to maintain. 225s are long lasting motors in part because the torque output means that most of them rarely exceed 3,000 RPM.