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weak acceleration, poor mileage
https://slantsix.org/forum/viewtopic.php?t=58435		 Page 1 of 1
Author:  rich006 [ Sun Sep 27, 2015 4:44 pm ]
Post subject:  weak acceleration, poor mileage
Help a noob try to make his /6 run better! I have the original 225ci motor in my 1974 Swinger with 79k miles. I bought it a year ago. Everything is stock, but the guys I bought it from put a new carb in (the old one is in the trunk). Here's the info:

- always garage-kept, nothing visibly rusted or rotten under the hood
- all vacuum/air lines are connected except for a couple of short ones coming off the air cleaner--one hangs from the cylindrical cap on top of the air intake, and the other hangs from a small port just inside the air filter--I can't find anything that they should connect to--see picture below.
- starts right away with one pump on the gas pedal
- runs smooth and idles smooth

>Edit: I forgot to mention the spark plugs look good, with correct gaps, and I replaced the plug wires with some new-looking ones that I found in the trunk.<

So far so good, right? Here are the problems:

- bogs down badly when accelerating unless fully warmed up
- poor mileage, 11-12 mpg
- struggles to get over 60 mph on highway
- struggles to accelerate going uphill
- air filter smells like fuel and feels oily
- strong smell of unburnt fuel fills garage for a few hours after parking
- there are two short air lines that aren't connected--one hangs from the cylindrical cap on top of the air intake, and the other connects to a small port just inside the air filter--I can't find anything that they should connect to.

I'm not interested in upgrading anything (yet); I just want it to run like it should in the stock configuration.

Image
Author:  ESP47 [ Tue Sep 29, 2015 9:51 am ]
Post subject: 
First thing I would do is see if the carb in the trunk is the original one. If it is, put a rebuild kit in it and install it. The "new" carb might look all shiny and new but chances are it's a turd. I dealt with this over 10 years ago. Carb after carb while my car bounced between 10-12mpg. When I finally found a NOS card, mileage and performance skyrocketed.

If you don't want to go that route, check the choke and make sure it is working properly since it sounds like you're running very rich. Does the choke open fully once the engine is warmed up.
Author:  rich006 [ Tue Sep 29, 2015 1:49 pm ]
Post subject: 
The carb in the trunk is a Holley 1945 model 6724, which is the correct one. The one that is installed is model 6723, which according to the shop manual would be correct for a manual transmission. The differences are small and I doubt that is the main problem.

The two unconnected vacuum lines seem to be for the OSAC, which is probably not affecting anything important.

Today I ran the engine with the air cleaner removed and the crankcase air inlet open (covered by a rag). The vacuum advance line was open as well. It seemed to run fine--the choke opened up right away after starting (engine was cold but it's a warm day). Also, I didn't notice any fuel smell from the carb after shutting down. However, I did notice wisps of smoke coming from the crankcase air inlet when I removed the rag, and a strong smell of fuel in the smoke.

So I'm suspecting a failing fuel pump, as discussed here.

Of course, that wouldn't explain the power/mileage problem.
Author:  ESP47 [ Tue Sep 29, 2015 7:43 pm ]
Post subject: 
Just because the model is correct, doesn't mean the carb is good. It's well documented on here that remanufactured carbs are garbage. If your car has one, you may be chasing your tail where there is no end to it other than getting an original non remanufactured carb. I've done the dance before and it's a waste of time and money. There's a possibility that it could run fine, but chances are it'll never get you the mileage and power that is attainable.

Is the OSAC disconnected completely? The vacuum line should run right from the distributor to the carb. There's no need for the OSAC unless the emissions law in your area requires it. Make sure there are no uncapped vacuum ports. Do you have a vacuum gauge? They can be had at harbor freight for about $10.

Sounds like you're onto something with the fuel pump. I still suspect the carb as the main reason for poor economy and power.
Author:  Sam Powell [ Thu Oct 01, 2015 7:53 pm ]
Post subject: 
I say this to nearly everyone new to these old cars. Check the voltage at the large red output on the alternator, and then check the voltage on the high side of the ballast resistor. If there is more than .5 volts drop between the two, you will see enormous improvement from getting the wiring connectors cleaned up. The loss is in crimp connectors that are numerous in the system. The ameter can also be a high resistance element in the chain.

There are other causes for poor economy, but do not over look this one.

Sam
Author:  rich006 [ Sun Oct 11, 2015 4:40 pm ]
Post subject: 
Sam, I'll check out the electrical connections you mentioned. One thing I did this weekend is adjust the timing. Turns out it was retarded over 10 degrees. Setting it to TDC improved the acceleration problem quite a bit without seeming to hurt anything else--I hope the mileage will improve too.
Author:  wjajr [ Mon Oct 12, 2015 4:46 am ]
Post subject: 
Quote:
Sam, I'll check out the electrical connections you mentioned. One thing I did this weekend is adjust the timing. Turns out it was retarded over 10 degrees.
Retarded timing will sap power. By lighting off the air-fuel charge late or on the power down stroke after TDC a good percentage of heat of burn ends up going out exhaust, and not contributing to motivating the car.

When burn of A/F mixture begins at or before top dead center flame has time to fully consume all of the A/F charge and expand, its heat or force is exerted on piston before exhaust valve opens.

This why there is a mechanical and vacuum advance devices controlling firing off A/F charge BTDC when under way, to maximize force of combustion.

This burn timing is also controlled by lash setting to some extent, and is why it is important that lash be set correctly for best performance. Lets say one or several of exhaust valves are set a little too tight so that they open a split second quicker than they should, this acts like retarding timing on those cylinders effected and power drops off a bit because heat is prematurely dumped into exhaust. Same with intake lash being off spec., it changes the camshaft dimensions and timing slightly that the combustion process sees, and power drops off.
Author:  rich006 [ Mon Oct 12, 2015 5:05 pm ]
Post subject: 
Good info on the effect of timing and thanks for mentioning how the valve adjustment affects that as well. I guess adjusting the valves will be my next project.
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