Hi All,

I've got a 1966 Plymouth Valiant with a 1980 production year 255 slant six engine (I know that from casting numbers).

I'm a very green noob here, and do not really have any experience working on engines, or the necessary tools. On top that, my living situation is that I have a covered space, but not a garage of my own. So while my Valiant is very well protected from the elements, I can't do much work on it myself. The HOA for where I live specifically prohibits working on cars in the carport/parking area.

I got this Valiant around the end of July, and at first it got only 10mpg.
I took it to a shop where they overhauled the carb (a Holley 1920 that turns out was missing the throttle return spring), and installed a Pertronix electronic ignition.
They also did a leak down test, here are the results (cylinder compression is good):

cyl %
1 35%
2 30%
3 25%
4 25%
5 34%
6 25%

I know those are not good.

With the carb overhaul and the electronic ignition, the mpg went up to 12, but that's still not great.

I posted all this in the "Slant 6" section for forabodiesonly.com, and Slantsixdan suggested that I adjust the valves. Unfortunately I don't know how to do that. If anyone in the Portland, OR area is willing to help me learn, I'm willing to supply pizzas, six-packs, or whatever I can do to make your time spent teaching a noob worthwhile.

One other thing: this engine idles fine when cold, but once the engine warms up, it makes an odd sound kind of like engine knock, but is not actually engine knock. Being the noob that I am, I don't know how to describe this sound well. It does not happen when the engine revs.

Thanks in advance.

Welcome here. Unlike "that other board" this board is full of slant-6 freaks, geeks, and experts—many of whom are in or reasonably near Portland. You'll generally get better advice here, because "over there" most of the guys are focused on V8s and don't know as much as they think they do about slant-6s. Just sayin'!


Any time you see red text in a post on this site, click it; it's a link. For example, see the valve adjustment article (that's the same link I provided you before -- go ahead and click it!).

I know you've been talking (and hearing) about taking the engine all apart, but I don't think you're at that stage yet. High leakdown can indicate a worn-out engine due (or overdue) for a rebuild, but it doesn't necessarily mean that. Faulty valve adjustment is a prime suspect where high leakdown is detected, and fixing it is cheap, simple, fast, and easy.

Moreover, a leakdown test must be done under specifically-defined conditions in order to be valid, and sometimes people -- even professional mechanics -- don't do it right, which makes the results invalid. And there are other basic tests that ought to be done to get a handle on the overall health of the engine. For example, check for timing chain slack.

I know from our past conversation that you have already got the Stockel and Peterson books, but you should also have the factory service manual described in this thread.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I live just across the river in Vancouver and can show you how to set the valve lash, but with a busy schedule and with the lack of daylight in the evening it may be tough to find the time. I'll check my schedule and get back to you, but it's probably going to be a week before I have time.

Welcome to the board.

_________________
Joshua

Joshua,

Thanks!

Weekdays are probably not going to work. With it getting dark so early, there's no way I can make it up to Vancouver before dark.

Would a weekend work for you?

SlantSixDan,

Once again, thanks, and double thanks for suggesting I sign up here on slantsix.org.
I should've posted here before "that other board"!

And I think my mechanic is in the V8 camp.
When I asked him about the low mileage I'm getting, I get the impression from his responses that rather than mess with a slant six he'd sooner just drop a V8 in there and be done with it.

OK... I will ask:
Have you calibrated your odometer?

Also,
What size main jet is in that Holley 1920??

I will take a guess at this answer:
"I don't know what size jet it has... the shop that did the carb rebuild, did not tell me"

If that is the case, get a known # 56, 57 or 58 Holley main jet and a float bowl gasket, then remove the float bowl and replace the main jet.
(if you want the best mpg, go with the #56)

As a side note... a common "trick" used by carb rebuilders is to put a rich (big) main jet into a rebuilt carb. Doing that tends to hide other problems the carb may have and makes it "feel" powerful and that means fewer returns... until the customer figures-out the bad mpg.
DD

That's a good thought. Pull a spark plug or two and take a look at the tips. What do they look like -- are they light tan to greyish-white? Or are they closer to black?

Also, it occurs to me that we don't know which manifolds were used in this engine swap. A 1980 exhaust manifold would not accept a 1966 choke thermostat, but the 1980 choke thermostat would have been designed to work with a heater control circuit not present on a 1966 car. The low mileage could easily be explained by improper choke operation. Some clear, sharp photos of the engine compartment, with the air cleaner housing installed and removed, would help us get an idea of your underhood setup and spot any easy fixes that might be lurking. See here for how to post photos on this forum.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Doc,

I have calibrated the odometer.

And you guessed right, I don't know what size jet the carb has and the shop that did the rebuild, did not tell me.

SlantSixDan,

Here are the pics:

with air cleaner housing:




w/o air cleaner housing:




I'm going to have to get an extension for my socket set to check the spark plugs.
And as you've probably noticed, this Valiant was originally white. Someone painted it red about 8 or so years ago.

Hi Renato.
Welcome to the slant 6 forum.
I hope i did not freak you out on the other forum!
I was giving my opinion there based on my experience with similar symptoms you described.
So it looks like its time to get your hands dirty,and learn some mechanical stuff!!

Please follow the advice of the experts here.
Grab the manual as Dan advises.
Do the valve lash (preferably with help).
Do all the other things these guys suggest.
If after all these procedures your mileage is still poor then an engine problem may exist.
hopefully that noise will dissapear after following thier advice.
I realise Im jumping the gun here, but
As I stated on the other forum ,I suspect a bad cam/lifters.
Im not trying to freak you out ,I sincerely hope Im wrong(happens all the time)

but the noise you describe is exactly as mine was.
Only noisy when warmed up ,and only at idle.not a consistant deep knock but a lighter knock that would come and go a bit and disappear when the revs rise.
sound familiar?

just relax and "try" to enjoy the learning experience.
These are simple engines ,therefore they are great to learn on.
Im not sure if you guys have car clubs over there,but if you do it may be time to join one ,a mopar /slant six one would be ideal!
Once again good luck .
Brendan.

I suppose I should go introduce myself properly on another part of the site!

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1963 AP5 Valiant 225 PB 904 Stock except extractors & 2 1/4 inch exhaust.


1962 S series valiant 225 PB auto. factory every thing. Full restoration job.
Currently on hold till AP5 is on the road.

Good pics. Looks like the '66 intake manifold, carburetor, choke, and possibly exhaust manifold were used all together, so that's fine - probably no mismatches in this area to cause problems. You'll want to check that your manifold heat control valve is working correctly. You'll find its counterweight on the front face of the central collector part of the exhaust manifold; if you were to draw a straight line directly downward from the front of the carburetor where it says "HOLLEY", straight down through the intake manifold, it would touch the counterweight. The counterweight is a rectangular flag-shaped dingus about 1½" by about 2" (later exhaust manifolds have a different counterweight which is circular and about 3" diameter — let us know which kind you find). With the engine cold, grasp the counterweight and attempt to rotate it counterclockwise (as viewed from the front of the car). It should turn easily. When you let go of it, it should spring briskly back to the clockwise position. If it behaves otherwise—doesn't turn, turn stiffly, doesn't spring back—let us know.

The carburetor doesn't look as though it's been grossly abused. I'm not seeing a sandblasted piece-of-junk "remanufactured" item, so there's hope it's not been ruined. That's good.

Looks as if there have been some underhood modifications. That brake booster and master cylinder appear to have come from a General Motors product—does the car have front disc brakes? And the ignition coil is not mounted in its usual location; perhaps there is some kind of a nonstandard ignition system or modification to the distributor. That'll warrant close attention including checking whether the vacuum advance is in good shape; distributor faults could account for at least part of your poor mileage. To check the vacuum advance, with the engine idling unplug the carburetor end of the small-diameter hose that runs from the carburetor to the distributor. Suck on the end of the hose; you should be able to pull a vacuum and the engine speed should increase. Putting your tongue over the end of the hose should hold the vacuum and maintain the speed increase. If you can't pull or hold a vacuum or the speed doesn't increase, the vacuum advance unit or its hose is faulty.

The black plastic box at the rear of the inner fenderwell on the driver's side is probably a voltage regulator (moved off the firewall to make room for that brake booster). The silver box next to it is???? Another, spare voltage regulator? Can't see, photo not big or clear enough. Windshield washer fluid tank and pump removed to make room for relocated voltage regulator and ???box.

Later-model ('70s) radiator swapped in.

Glass-body fuel filter -- not very safe, should go away. Metal-body fuel filter in this same location would be okeh; I prefer to do the Fuel line mod.

So all in all, someone's been enthusiastically changing things under the hood of this car. Some of those mods might not have been done all the way the way they were supposed to be, which could be accounting for your poor mileage. And if this car has an automatic transmission, the 1968-or-later engine in front of the 1967-or-earlier transmission required a 1/8" spacer ring that might well not have been added because this is not common knowledge. Without the spacer ring, the transmission front pump and bushing will wear. Are you able to get in contact with whoever did the modifications (previous owner or ???)

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

After I get home from work this evening I'll check the manifold heat control valve.
The car has drum brakes and that is a GM drum brake booster.
The guy I bought that car from told me about that.
That's the Pertronix electronic ignition I had installed about a month ago.

I'm not sure what the black and silver boxes on the drivers side inner fenderwell are, but later I'll look for markings/numbers and get an up-close pic.
This car does have an automatic transmission.
Is there anyway I can check on that spacer ring, or will I have to take it in to a shop for that?

I think most of the mods were done by whoever owned it before the guy I bought it from. I'll contact him and see what I can find out.

Those leak down figures aren't very good at all... Like was mentioned it has to be done properly, did they tell you where the leakage was? Eg exhaust, rings etc? The beauty of a leak down is you can do that sort of diagnosis.hopefully the valve lash adjustments will help. It's not really a hard procedure,I read that you are not allowed to work on cars where you park, maybe you can go to a friends place? Once you have seen how its done you'll be able to tackle the job yourself. There is a good article on this site, perhaps lift off the cover yourself, have a look and then go through the article, if it makes sense then grab some feeler gauges and give it a try? Good luck with it, as a new guy here I have found this site a wealth of great info. The thing about old cars is that it gets expensive to get the required maintenance performed by a suitably talented guy, there's plenty who will take your $$, but not many who know their stuff and show the professionalism and pride to do good work on old cars, at least that's how it is in Oz. i have fallen victim of so called specialists doing work on my folks car while i was living interstate and im still finding stuff now that im working to fix.for me part of the "fun" of old car ownership is doing the work myself and learning.the driving is the icing on the cake. If you could find a place to do some work yourself I'm sure you will always get plenty of help form the guys here. Steve

It sounds to me like you need to get this car into the hands of a slant six guru who can sort out what has and hasn't ben done to the motor (and vehicle in general) and who will be able to check things like valve lash and carb settings.

The good news is that it is still running and driving. Most slants that still run and drive can be saved.

Make sure to have a valve cover gasket ready when you do the valve lash. You'll want to change that out when you remove the cover.

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69 Plymouth Valiant

Finally getting back to a few other things that SlantSixDan suggested I check above.
The counterweight is missing, so I guess that means that the manifold heat control valve is not working properly.
Here's a pic of where it should be:

Is this the kind of thing I can fix with the help of local slant six guru? or am I going to have to take the car in to a shop?
And If I have to take it in to a shop, I'm not going back to the place I've already been to twice.
I did this, and when I sucked on the carburetor end of the hose, engine speed did not increase. Maybe I wasn't sucking hard enough (no jokes, please), but I'm a healthy guy, non-smoker, in decent shape, so I doubt that. When I put my tongue on the end of the hose, though, it did hold the vacuum.
SlantSixDan, what's your diagnosis here?

Here's a close-up of those two boxes:

The black box looks brand-spanking-new and looks like a voltage regulator. The white sticker says: TRANSPO-CN C8010-2711-12V
I did some googling, and as best I can tell, this is an after-market Mopar-compatible voltage regulator. Is this something that would've been installed with the Pertronix electronic ignition?
I have no idea what that smaller silver box is, but it looks like its been there a while. There are no numbers, letters, or markings on it.