Author Topic: Does a brand new distributor need re-curving?  (Read 5306 times)

Offline 6wt

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Does a brand new distributor need re-curving?
« on: March 30, 2015, 04:14:31 PM »
84 Chevy C10 / 350 crate engine, 50K miles / 700r4 trans / headers with glass packs.  This is my fishing truck, hopefully setup for dependability and gas mileage.

Just completed project to update my truck.  I think it runs well and idles really smooth.  But, when cruising around town, 40mph, or on the highway at 60, it sounds like the "power of the engine oscillates on and off."  It is a bit loud with the current exhaust setup.  But is sounds like the engine is coasting then not coasting; if that makes any sense.  Acceleration is good also.  No knock, no stutter, no hesitation.  I talked with a performance shop that rebuilds carbs and does dyno work.  They want to "recurve the distributor" even though the distributor is brand new...

Here is the background detail for the engine:
 The 350 crate GM engine had 50K miles on it.  Compression is 150 on seven cylinders, one at 140psi.  Eldelbrook 1406 carb came with engine.  Removed computer and computer related equipment from truck.  New HEI vacuum distributor from Summit.  New 8mm plug wires.  Headers and shorty plugs.  Timing set at 12degrees advance with vacuum hose plugged.  Mechanical advance in distributor unscrewed all the way, ie per the instructions this is no retard position.  Vacuum hose hooked up to manifold vacuum port on front of carb.  If i understand the timing situation, it is set to achieve 32degrees total advance.  ie. 12degrees advance manual set + 10degrees mechanical advance + 10degrees vacuum advance = 32degrees total.  I have not rebuilt the carb.  And vacuum testing on manifold pressure is rock solid at 20psi.  Here is the carb i purchased:  http://www.summitracing.com/parts/sum-850001-1/overview/make/chevrolet

Do you think the distributor needs to be recurved?  Am i correct in expecting the engine to sound smooth at speed without any up or down tone difference?

Thanks!

Offline VileZambonie

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Re: Does a brand new distributor need re-curving?
« Reply #1 on: March 30, 2015, 05:42:22 PM »
How is your TCC hooked up since you removed the ECM?

So you have your base timing set, now play around with it. Try advancing it but back off if it starts to ping.
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Offline 6wt

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Re: Does a brand new distributor need re-curving?
« Reply #2 on: March 31, 2015, 08:46:48 AM »
TCC - Is that the Transmission Control Connection?  If yes, i have the vacuum switch on the drivers side firewall connected to manifold vacuum.  Am i correct in understanding this switch controls transmission downshift?

Offline LTZ C20

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Re: Does a brand new distributor need re-curving?
« Reply #3 on: March 31, 2015, 09:08:18 AM »
The TCC stands for Torque Converter Clutch. It controls engaging the torque converter in sometimes 3rd but mostly 4th gear to create a direct power path between the engine and transmission. This increases fuel economy and relieves some load off the engine while cruising in high gear.
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Offline 6wt

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Re: Does a brand new distributor need re-curving?
« Reply #4 on: April 01, 2015, 09:54:21 AM »
After researching, here is what i was told:

Bottom line - Test any distributor, new or old, prior to installing to verify the advance operation properties for the two potential types of advance created by the distributor (mechanical and vacuum); ie.  a) when does advance start  b) how does it progress  and  c) what is the limit.

Advance results from testing are then combined with the initial advance created when you set the timing to achieve total timing advance for vehicle operation.

Every shop i talked with had horror stories of distributors creating advance that greatly exceeded or differed from specifications.  My 2cents... sounds great if my goal is performance, sounds like upselling/padding the shop bill if my goal is a daily driver with a brand new distributor selected with a documented advance curve and springs and driven no faster than 60-65 on the highway.  But then, i am a newbie with extremely limited experience.  Had i known this info prior to installing the distributor i probably would spend $25 to have the distributor tested prior to installation.  However, one shop wanted $100 to test and recurve the new distributor... sorry, i only spent $92 on the new distributor.

Offline 6wt

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Re: Does a brand new distributor need re-curving?
« Reply #5 on: April 04, 2015, 09:07:43 AM »
How is your TCC hooked up since you removed the ECM?

Did i research this correctly?  I removed the ECM, ie computer from under the dash.  I have the switch on the drivers side with vacuum connection hooked to manifold pressure.  This switch controls TCC, Torque Converter Control, for lockup of the 700R4 transmission for overdrive/4th gear.  Did i miss something?  Is the ECM performing the lockup or some other function(s)?

Thanks for any feedback!

Offline jaredts

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Re: Does a brand new distributor need re-curving?
« Reply #6 on: April 04, 2015, 04:37:09 PM »
I would see if this is potentially your torque converter clutch "hunting".  It's an annoying problem even in stock form on vehicles before this was computer controlled.  Try unplugging the TCC plug on the driver's side of your transmission and take it for a test drive.  If the problem goes away you can replace your vacuum switch with an adjustable one and make the problem much less evident, but it will still be there at times.  I don't think on an '84 you affected this by removing whatever ecm there is.  I don't think TCC was computer controlled until several years later.

Offline cvbear

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Re: Does a brand new distributor need re-curving?
« Reply #7 on: April 04, 2015, 07:23:04 PM »
Also you might check your vacuum advance.  If it wants 12 inches of vacuum to come in that may be about what your motor makes at idle (crate engine might be less than stock) and that will cause the vacuum advance to come "in and out" which can lead to the motor "surging" or running unevenly.  You can test by plugging vacuum advance and seeing if problem still occurs.
1984 SWB Silverado fleetside, 350 crate engine, 350 trans

Offline 6wt

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Re: Does a brand new distributor need re-curving?
« Reply #8 on: April 07, 2015, 07:02:59 PM »
Testing/Analysis update -
Spent the last couple of day testing Distributor and Timing to verify vacuum and vacuum advance prior to moving to the carb.  The new rubber vacuum line to the distributor had a very small split in the end allowing air in once vacuum built up.  The split was not large enough to affect vacuum at idle, registered a solid 20, and not enough to cause fluxuation at full vacuum, still registered a solid 19.5-20.  Cut the end of the hose, now distributor will hold vacuum.

Next focused on vacuum advance for the distributor.   I put in a new chevy hei distributor.  The directions said insert an allen key into the vacuum port on the distributor and turn counterclockwise to "eliminate retard."  The documentation says the distributor creates 12degrees of vacuum advance and 10degrees of mechanical advance.  I interpreted the counterclockwise turning to provide a full 12degrees.  That was incorrect.  Turning clockwise provides vacuum advance...  Now we have verified and consistant vacuum.  Set the initial timing to 6degrees and moved distributor advance from manifold vacuum to ported vacuum after discussing truck goals; gas mileage and easy driving.

Truck is running much better.  Put a sniffer on the tailpipe.  Engine is running very lean.  Will open the carb tomorrow to check condition and jetting.

Offline Stewart G Griffin

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Re: Does a brand new distributor need re-curving?
« Reply #9 on: December 05, 2015, 11:54:02 AM »
Did the truck originally come with 7004r?