Last post on Oct 25, 2013 at 11:53 AM
You are in the Speed Shop Tuning and Modification
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Chrysler Le Baron, Engine
#1 of 11 1978 Chrysler LeBaron~ Stuck and I need help!!!
Aug 30, 2011 (11:08 am)
I am working on a 1978 Lebaron with a 318 rebuilt to factory specs, the only modification that has been made is the addition of a 4 barrel carbeurator in place of the 2 barrel. Now, we can get her to run strong on the warm up and the idle, but while test driving there seems to be way too much gas and not enough spark. Adjustment of the distributer as well as carbeurator seem futile. Also, there is an electric ignition module that was sciliconed to the firewall that has since melted off. This distributor is foriegn to me as it is electronically controlled and not controlled by vacuum lines. We can afford to fix the electric module (new ones are not attainable) or get a vacuum controlled distributor but not both.
NEED this question answered first: Should the spark conrol computer be compatible with an aftermarket 4 barrel carb?
Then this question: If the spark control computer will not be compatible with the 4 barrel can I replace the distributor with a vacuum control distributor and have no computer?
#2 of 11 Re: 1978 Chrysler LeBaron~ Stuck and I need help!!! [lunaray42]
Aug 30, 2011 (2:48 pm)
In order to appropriately answer your questions; I need to know whether this car will have to go through a smog inspection.
The vehicle you are working on was originally built as a "lean burn" model; which (unless my memory fails me) has the ignition module built into the spark control computer on the air cleaner. Again; if I remember correctly; the module on the firewall is not an ignition module. It is an ambient air temperature sensor; which controls the fuel enrichment, and/or the EGR system activation. You might be able to find a used or an unsold new one from an Internet source. Depending on the parts that are damaged, it may not be repairable. Somebody may have mounted an ignition module from a non-lean burn Chrysler on the firewall, and attempted to use it to bypass the module in the spark control computer; but I don't believe there was one there originally on the lean burn model.
There is a vacuum diaphragm on the spark control computer; which for all practical purposes is equivalent to the vacuum advance diaphragm on a distributor. However; I do not recall whether this diaphragm is intended to be driven by ported vacuum or full manifold vacuum. A little experimentation should sort that out. But hooking up that diaphragm to a suitable vacuum source might make a great big difference in the running quality.
The potential compatibility of the 4 barrel carb you have will partly depend on the brand and type of intake manifold you are using; as well as the brand, CFM rating, and model of aftermarket carb. Please post that information in your response.
Some of the richness you are experiencing may come from a choke pull off diaphragm (also called a choke vacuum break) which is not connected to a vacuum hose, or is not adjusted properly; or has a ruptured diaphragm.
The 1978 Le Baron 318 was available with a choice of 2 barrel or 4 barrel carb. The stock replacement Carter Thermo Quad 4 barrel is available rebuilt from rock auto for about $262 plus shipping. I'll say more about carbs after hearing from you in answer to the questions I raised.
#5 of 11 Re: 1978 Chrysler LeBaron~ Stuck and I need help!!! [zaken1]
Sep 01, 2011 (11:29 am)
It is an eidelbrock performer intake cam and carburator tuned, its a 600 cfm carb. . Ported vacuum seems to be more effective, but still the serious lack of power remains. There was no aftermarket ignition alteration, theres been no augmentation to the original design (thankfully).Also I hooked the factory throttle position sensor to the carb, I seem to be experiencing a slight advance in timing but still a severe lack in power. No emissions inpection, either. Will it be better to find a computer designed for the 4 barrell instead, and if so does anyone know a part number?
#6 of 11 Re: 1978 Chrysler LeBaron~ Stuck and I need help!!! [lunaray42]
by MrShift@Edmunds HOST
Sep 01, 2011 (12:20 pm)
Oh now wait a minute. You didn't say anything before about a performance cam. What are the specs on this cam?
#7 of 11 Re: 1978 Chrysler LeBaron~ Stuck and I need help!!! [lunaray42]
Sep 01, 2011 (1:45 pm)
OK, now that I know more about what we're dealing with; I can get more specific: The Edelbrock performer cam is a mild upgrade; and is not radical enough to make it incompatible. The Performer manifold and carb are tuned; but they are tuned to work with a high compression engine and a free flowing exhaust system; they are not tuned to work with a low compression, restricted flow, lean burn engine. There is no off the shelf computer that will work with this combination; you need to change the balance of the system.
The problem here is that your car was manufactured with a low compression ratio, along with air injection, plus lots of exhaust gas recirculation and a severely restricted exhaust system, and in addition has stock timing sprockets that are machined with retarded keyways (which was a 1970's Chrysler trick to reduce emissions at the expense of performance). ALL of these things should be changed; if you hope to make it run any better than a stock motor.
The most important changes are to disconnect the air injection pump, but leave the check valve at the exhaust manifolds in place. If the intake manifold has an EGR valve; disconnect and plug the vacuum hose to that valve.
The stock catalytic converter has surely become very clogged in 33 years of use; and if it is still there; is now killing your power and messing up the fuel mixture. I would strongly recommend either replacing the catalytic converter with a straight pipe; or installing a high flow converter in its place. You can keep a single exhaust system; but I would recommend using a low restriction muffler. I'll give you a muffler recommendation, if you tell me your preference of quiet, mellow, or radical sounding.
Buy a Clevite/Perfect Circle # 93103 timing set with 3 keyways and a heavy duty double row roller chain ($56.79 from Rock Auto). Set the cam timing to one of the more advanced settings. It will take some experimentation to find the best keyway to use; but it definitely will not be on the retarded side.
The carb cannot be adjusted for performance by just turning the idle mixture screws; that does not affect the mixture in the speed and load ranges you need. For real performance improvement; it will require changing the metering rods and jets. Edelbrock makes a calibration kit for the 600cfm performer; which has a full assortment of metering needles, springs, and main jets. If you don't have one; buy it. You'll need to know the carb number to get the right kit. (http://www.summitracing.com/search/Brand/Edelbrock/Carburetor-Model/Performer/P- art-Type/Carburetor-Calibration-Kits/?Ns=Rank|Asc) That will be one of the most effective things you can do; but all the other things I mentioned above will each change the fuel mixture; so there is little point in trying to tune the carb until after you get all the mechanical settings right.
#8 of 11 Re: 1978 Chrysler LeBaron~ Stuck and I need help!!! [zaken1]
Sep 01, 2011 (2:01 pm)
Thank you SO much for all your info, its been a long road, but I think we're close to her becoming a daily driver. We aren't too concerned with aftermarket performancing-out this vehicle, she isn't even breaking 35 mph! (reguardless of how far you push the pedal). This is my first time asking for help this way, a cry out into the interworld, hoping to hear a cry back... Thank you!
#9 of 11 Re: 1978 Chrysler LeBaron~ Stuck and I need help!!! [lunaray42]
Sep 01, 2011 (2:23 pm)
Please replace the fuel filter; and see how much difference that makes. Also stand behind the exhaust pipe and have someone in the car step hard on the accelerator; while you feel the exhaust pressure coming out of the pipe. There should be a blast of exhaust pressure and increase in sound from the pipe when the accelerator is pressed. If there is little change in exhaust pressure or sound; the catalytic converter is clogged. If you ever want to go faster than 35mph, or not burn out the valves in the new motor; it will be necessary to remove or replace the converter. If there is a big change in exhaust sound and pressure; than the converter is OK, and you have other problems. Let me know, and we can go from there.
#10 of 11 Re: 1978 Chrysler LeBaron~ Stuck and I need help!!! [lunaray42]
Mar 31, 2013 (4:50 am)
We have to start with where the original design went wrong. Put in a points condenser ignition. These never fail and run poorly long before they fail. They are easy to adjust, just set dwell between 26 to 32 degrees depending on city vs highway driving. Get rid of the carb and buy a manifold adapter plate and out on either a holley 2300 ( 2 barrell) or 4150 ( 4 barrell ) carb with a manual choke. Plug the EGR valve. Plug the valve comming off the passenger side of the exhaust manifold. Put in a new power brake booster, it probably has a vacuum leak. Change the timing chain to a double roller chain and check for cam to crank true zero. Thus car should run like a top!