Need help with the 3.7L

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Slowpro69

Slowpro69

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Hey guys, I just came across a 69 dart 4 door with the 3.7 L slant 6 all original matching numbers with 31k original miles. After getting it home and puting new tires on it I took it for a drive and it runs decent but when I get up to around 50 to 60 it falls on its face down to about 40. Does anyone have any clue on what might be causing this? I have installed new cap and rotor plugs and wires coil.
Any help will be greatly appreciated!
 
Based on your symptoms, I would start looking at the fuel system. Any car this old will be subject to problems there.
- Rust or crud in the fuel tank can be clogging the fuel filter. This will become more evident as the fuel flow increases at the higher engine power levels needed for higher speeds. Change the filter but expect to have to pull and drain and clean the tank. Any car of this age seriously needs to have the tank cleaned and inspected.
- Rotted rubber fuel lines from the tank to the main metal fuel line under the car or from that line into the fuel pump. These can crack and allow air to be sucked into the fuel supply and limit fuel flow in that manner.
- Old fuel pump failing and falling on its face, that will starve the enigne of fuel.

Did you get all of the old gas out of the tank when you got the car? If not, then if the gas iss indeed old and 'varnished up', then that can get in the engines valve guides, cause them to stick, and that can bend pushrods. So, that is another reason to pull the tank and clean it all out and inspect it.
 
That was one thing I did not mention replaced tye fuel filter and the gas in it is clean. I also isolated the tank to the pump and ran a line to a gas jug and it still does it. Will be changing the fule pump tomorrow. If that doesn't work would you think the carb would be an issue?
 
Mmmmm, OK, did you run the gas from the jug and actually drive (brave or foolish man LOL!), or just run the gas from the jug in the driveway? The flow rate while driving is a lot more.

Can you describe 'falling on it's face' a bit better? Does just drop out instantly or does it falter and sputter?

And the gas being clean does not mean that the tank is not full of crud that is being sucked into the system. It'll keep plaguing you unless you get past that.

Sure, the carb could be it. But the initial symptoms don't seem to indicate that since it sounds like you can drive around at 40-45 mph OK. If you are driving at 30-35 mph, can you accelerate just fine up to 45-50 mph?

Since it could be a number of things, then a process of elimination is needed (like you sound like you are doing). Do you have acccess to a fuel pressure gauge? (And a voltmeter just in case?)
 
check fuel float drop. my truck would bog badly after 4000 rpm, just poop out until it was down to an idle or I let off the gas. The filter was stuffed. Could be pump or float now that you changed filter. A bog is typical of a lean condition (or a fouled plug on a 2 stroke!)


>> That is a sick feeling, your WOT on your bike out in the desert, trailing the pack, and your bike bogs and dies. You got gas, so you pull the plug and clean it with sand and the edge of your shirt, screw it back in and she starts right back up, you can relax now....,,
 
Could there be alot of slop/play in the timing chain?
 
krazykuda said:
Could there be alot of slop/play in the timing chain?
Click to expand...
That wouldn't cause it to 'fall on it's face' unless this just happened once and the chain happened to jump at that time. Since it sounds like this problem is persistent, then does not seem at all likely in my mind. And a retarded cam would not hurt higher RPM power and casue sudden drops in power.

Your point does bring up a good question though: has the OP checked ignition timing? Just a point to cover in being complete in looking at things. Does not seem likely to cause these symptoms, but easy to check.
 
I ran the can in the drive way. It also happens when I rev it up in the upper rpm range. Checked the ignition timming when I replaced the cap rotor and points. Aso went through the carb as well float looks good. The falling on its face I mean there is a loss of power with some sputtering if I put the pedel to the floor it will die if I feather the throttle it will stay running but go no faster than like 45mph. I understand if the tank is dirty it would still clog but since I isolated the tank and it still falls off at the upper rpm range I would think the tank would not be the problem. Aj are you talking about the flaper in the exhaust manifold... could that be an issue?
 
Just to throw in some ideas, make sure your choke is in working and adjusted order. Check fuel mixture and cold/hot idle rpm's. What kind of carb is it? Vacuum leaks? These old slant sixes don't need a lot once everything is fixed up. Tuning and adjusting multiple times come with any carbureted engine.
 
If you're running the Holley single barrel 1920, I'd take it apart in the very least. It has a few places to get clogged up. The accelerator pump could also be leaking out the back, or the economiser on top. You could also have bought it and the previous owner put too big of jets on it, or way small depending on when exactly the issue occurs. That carb is super easy to take apart and clean as there are only 3 pieces - the carb body, metering thingy (block?) and the float bowl cover.

Another thing to check would be the float and needle and seat. Once you open the cover you can see the float, needle/seat and jet, accessible with the carb still on the car - easy.
 
Fuel system. Start with the tank. Take it out. Look inside and inspect it closely. If it's not rusted all to hell, it has crap in the bottom of it probably 1/2" deep. Rinse it out or take it to a radiator shop and get it professionally cleaned and coated on the inside. Blow out the fuel lines, get a new fuel pump, filter and be prepared to rebuild the carburetor. These cars are OLD. Gas is not filtered great out of the pumps at the station. Over time, sediment builds up and contaminates the fuel system. Unless you do as I outlined, you will keep having trouble with it. Fix it right now and you will have years of trouble free service.
 
Your symptoms do indeed sound most likley to be fuel system related at this point. Since you ran out of the jug and it did similar things as on the road, then it does seem that you have isolated the issue to not be in the tank but keep in mind that crud from the tank may have caused the problem in the first place. There have been oodles and oodles of threads on this problem and the old tank is the most common issue.

If you have the fuel pressure gauge, tee it into the line just before the carb and see if the pressure is steady at idle and on up to the problem RPM's. Or just put on the new pump and see what happens. Beyond that, it could only be the carb (if the diagnosis of a fuel problem is right....).

The heat flapper would not cause this by itself in general. But the other possibility is that this a cold driveability problem. Was the engine fully warmed up when you tried this? I have had a similar problem when the engine was not fully warmed up, the choke was pulled off (I have it set lean), and it just would bog and stumble at above 1500-1800 rpm in the first mile of driving or so. But it would warm up fast and go away in another half mile or so, so this is not likely your issue.

One other idea: At what gap did you set the points in the distributor? Did you change and set them, and change out the condensor? If you have not done that, go back and do that first, as you have missed an important part of the ignition system that can cause severe driveability problems when bad/set wrong. (Sorry I did not re-read your initial post sooner and catch that you did not mention them being changed.)
 
Sounds just like my 69 Dart on a cross-country drive in 1991. It would start stumbling around 55 mph on a flat highway, especially when warmer outside. The fuel hose at the fuel pump inlet was soft and collapsing. That was about when they started adding ethanol everywhere. I replaced it in NM and purred the rest of the way to CA. Change all your hoses to the better "fuel injection" type (you can find the SAE number in a search). It is much stiffer and ethanol doesn't attack it. Don't fool with old spools of "carburetor hose" out there.

If that doesn't work, try blowing air back thru the gas line (remove gas cap first). There could be crud on the pickup sock, if it is even still there (usually not). Next step, remove it and see, or just replace if erratic output since only $45 on ebay (a few tricks, read here first).
 
Thanks BillGrissom,nm9stheham,RustyRatRod,Serj22,1970Dart198,pishta. All info is being considered and I will be working the issue all weekend!!!
 
If you find it's not a fuel delivery issue check the advance mechanism on the distributor to see if it's even working. They fail quite often especially if they're old.
 
After a few busy weeks I finally found the time to work on the dart. Thanks to all for the great info. She is back up and running better than ever after replacing the float and spring!
 
20 years ago I had a 71 duster that I drove everyday 130 mi trip to work. ran great. sold it to mechanic friend, it kept having problem getting good fuel supply . he changed tanks 3 times. he finally found a dead bug in the long line from tank to pump, it would get caught at a curve in the line and plug it and and then work its way back from that curve and it would get enough fuel to run>LOL
 
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