HEI Mechanical Advance Question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**pjmoreland** · 07-16-2005, 01:50 AM

I replaced my HEI cap and rotor this afternoon and noticed that if I twist my rotor clockwise, it moves about 30 degrees, and the mechanical advance weights wind up and stick out. Then if I let go of the rotor, the rotor very, very slowly returns about half way back. The mechanical advance mechanism seems to be sticky. Is this normal, or should the rotor spring back into position quickly?

Jeepster376 · 07-16-2005, 02:07 AM

Sounds gooey, it should snap back freely.

dwaite72lnghrn · 07-16-2005, 07:55 AM

It is probably rusty and week springs. Liquid wrench or PB blaster should loosen it up. The springs on the weights are what pulls it back. Later units had plastic bushings on the weight pivot arms. If they are still good then it is on the shaft lower down towards the pickup coil.

kaycee · 07-16-2005, 02:52 PM

Yes it should snap back something is binding or wore out you can get an advance curve kit for under $10. and it will come with those plastic bushings.plus you can adjust the springs to your spec.

rodnok1 · 07-16-2005, 03:21 PM

I usually find it's the shaft/bushings are gummed up. Take the dist apart and clean the shaft near the top. I had one out of a Vette that was soo gummed up I couldn't get the shaft out to clean.

**pjmoreland** · 07-16-2005, 05:02 PM

Thanks for the help. I'll go pull it apart and give it a thorough cleaning.

**pjmoreland** · 07-16-2005, 07:15 PM

After getting it cleaned up, it snaps back now. I have my timing set at 10 degrees. The mechanical advance gives me another 10 degrees of advance, and the vacuum gives another 10 degrees of advance for a total advance of around 30 degrees. Does this sound about right?

jhow66 · 07-16-2005, 11:52 PM

Quote:

Originally Posted by pjmoreland

After getting it cleaned up, it snaps back now. I have my timing set at 10 degrees. The mechanical advance gives me another 10 degrees of advance, and the vacuum gives another 10 degrees of advance for a total advance of around 30 degrees. Does this sound about right?

At what RPM are you checking it?

**pjmoreland** · 07-17-2005, 12:41 AM

I'm not totally sure what the rpm was. I just rev'd the engine up (with the vacuum advance hose disconnected) until the timing stopped advancing. I'm guessing it was at around 2000-2500 RPM. Then I hooked up the vacuum advance hose and did the same thing. It reached a point where no matter how much faster I rev'd the engine, the timing didn't advance anymore. At what RPM should the mechanical advance top out?

jhow66 · 07-17-2005, 12:51 AM

Get me the # off the dist. base and I will look it up for you tomorrow and get back with you. It will be a # such as -11113452 or 11014364 or the like.

**pjmoreland** · 07-17-2005, 01:06 AM

That would be awesome. Where should I look for that number?

How would I go about figuring out how much total advance is optimum for my engine? I have a 300 HP 350ci.

neonlarry · 07-17-2005, 01:25 AM

Most SBC like 36°-38° total mechanical timing. Thats initial plus mechanical, vacuum disconnected.

**pjmoreland** · 07-17-2005, 01:32 AM

Mine's way too low then. Can this be adjusted easily?

neonlarry · 07-17-2005, 01:36 AM

Yes, check out these articles:
http://www.sporttruck.com/techarticl...03/index5.html
http://www.73-87.com/7387garage/drivetrain/hei.htm

**pjmoreland** · 07-17-2005, 02:20 AM

Thank you neonlarry for the articles. They were very helpful. This is the part I found most interesting:

Quote:

Centrifugal advance assy. on the HEI is pretty darned good. The stock weights and advance plate are perfectly acceptable for all but the highest HP/RPM engines. ALL stock HEIs that were installed in V8s are designed to have a total centrifugal advance of 20 degrees, +-1 degree due to production line tolerances. This is as measured at the crankshaft (10 at the cam).

It is susceptible to old age, though. Typically the centrifugal advance weights wear their pivot holes into an "oval" or eat a trench into their pivot pins OR BOTH. This is bad and no attempt to change the advance curve should be made on a distributor that suffers from these problems- fix it first or get another HEI to start improvements on. Also, the centrifugal advance plate (that pivots around the main distributor shaft as the centrifugal advance moves it) up at the top of the distributor sometimes gets gummed up and sorta "sticky"- slowing the advance curve and generally preventing it from working correctly. If your centrifugal advance doesn't "snap" back to zero when you twist it with your hand and let it go then you have this problem. You need to pull the distributor shaft apart and clean everything out, especially up top, before you proceed with upgrades.

I'm thinking I might be better off just getting a new distributor.

jhow66 · 07-17-2005, 01:57 PM

Quote:

Originally Posted by pjmoreland

That would be awesome. Where should I look for that number?

How would I go about figuring out how much total advance is optimum for my engine? I have a 300 HP 350ci.

Look here on alum. housing:

07-16-2005, 01:50 AM	#1
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,780	HEI Mechanical Advance Question I replaced my HEI cap and rotor this afternoon and noticed that if I twist my rotor clockwise, it moves about 30 degrees, and the mechanical advance weights wind up and stick out. Then if I let go of the rotor, the rotor very, very slowly returns about half way back. The mechanical advance mechanism seems to be sticky. Is this normal, or should the rotor spring back into position quickly?

07-16-2005, 02:07 AM	#2
Jeepster376 Registered User Join Date: Feb 2004 Location: Massachusetts Posts: 269	Sounds gooey, it should snap back freely. __________________ 72 Cheyenne Super 4x4 SWB

07-16-2005, 07:55 AM	#3
dwaite72lnghrn Registered User Join Date: Jun 2002 Location: central pa Posts: 1,525	It is probably rusty and week springs. Liquid wrench or PB blaster should loosen it up. The springs on the weights are what pulls it back. Later units had plastic bushings on the weight pivot arms. If they are still good then it is on the shaft lower down towards the pickup coil. __________________ 72 longhorn c30 502BB/th400/fact air/4.11rear/ custom camper 72 bug(the better half's) under reconstruction 2009 HHR aqua blue 71 GMC k20 350/sm465 2009 Chevy hd3500 6.6 Victory Red crew cab dually Dave & Jeanne

07-16-2005, 02:52 PM	#4
kaycee Registered User Join Date: Jan 2000 Location: Wyoming USA Posts: 2,446	Yes it should snap back something is binding or wore out you can get an advance curve kit for under $10. and it will come with those plastic bushings.plus you can adjust the springs to your spec. __________________ 2001 Silverado K-1500 2005 FXDWG stage3 69 CHEVY Short/Step 327/4L60 373 posi 63 IMPALA 327

07-16-2005, 03:21 PM	#5
rodnok1 Registered User Join Date: Jun 2003 Location: Linden, North Carolina Posts: 617	I usually find it's the shaft/bushings are gummed up. Take the dist apart and clean the shaft near the top. I had one out of a Vette that was soo gummed up I couldn't get the shaft out to clean. __________________ 64 Panel Rebuild Pics Clutch Replacement and Borg Warner T10 Trans Rebuild 64 Panel 1/2 Ton 350cubes/400hp BorgWarner t-10 3:73 Auburn Posi

07-16-2005, 05:02 PM	#6
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,780	Thanks for the help. I'll go pull it apart and give it a thorough cleaning.

07-16-2005, 07:15 PM	#7
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,780	After getting it cleaned up, it snaps back now. I have my timing set at 10 degrees. The mechanical advance gives me another 10 degrees of advance, and the vacuum gives another 10 degrees of advance for a total advance of around 30 degrees. Does this sound about right?

07-17-2005, 12:41 AM	#9
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,780	I'm not totally sure what the rpm was. I just rev'd the engine up (with the vacuum advance hose disconnected) until the timing stopped advancing. I'm guessing it was at around 2000-2500 RPM. Then I hooked up the vacuum advance hose and did the same thing. It reached a point where no matter how much faster I rev'd the engine, the timing didn't advance anymore. At what RPM should the mechanical advance top out?

07-17-2005, 12:51 AM	#10
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Get me the # off the dist. base and I will look it up for you tomorrow and get back with you. It will be a # such as -11113452 or 11014364 or the like. __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

07-17-2005, 01:06 AM	#11
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,780	That would be awesome. Where should I look for that number? How would I go about figuring out how much total advance is optimum for my engine? I have a 300 HP 350ci.

07-17-2005, 01:25 AM	#12
neonlarry Registered User Join Date: Feb 2002 Location: Vacaville, CA Posts: 2,746	Most SBC like 36°-38° total mechanical timing. Thats initial plus mechanical, vacuum disconnected. __________________ 70 C/10 Light Red 350/TH350, HEI, Duals w/40 series Flows, 91 seat, LED taillights 99 Pontiac S/C GTP, SLP Ram Air hood, GMPP Konis & springs 95 Neon ACR, MP PCM, AFX UDP, 3.0 CAI

07-17-2005, 01:32 AM	#13
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,780	Mine's way too low then. Can this be adjusted easily?

07-17-2005, 01:36 AM	#14
neonlarry Registered User Join Date: Feb 2002 Location: Vacaville, CA Posts: 2,746	Yes, check out these articles: http://www.sporttruck.com/techarticl...03/index5.html http://www.73-87.com/7387garage/drivetrain/hei.htm __________________ 70 C/10 Light Red 350/TH350, HEI, Duals w/40 series Flows, 91 seat, LED taillights 99 Pontiac S/C GTP, SLP Ram Air hood, GMPP Konis & springs 95 Neon ACR, MP PCM, AFX UDP, 3.0 CAI



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