Bad MPG. Timing off and No advance on my HEI - The 1947 - Present Chevrolet & GMC Truck Message Board Network

rscam · 04-29-2009, 05:06 PM

I posted a dissapointing MPG thread a few days ago. Just to let you know. I checked my timing and it has moved to about 20 degrees BTDC. I have no advance on my dist. I guess it should have about 34 degrees @ 2800 rpm with the advance unhooked. I reset the timing to 8 degrees BTDC and I have no advance with the vacuum hooked up or not. It stays at 8 degrees no matter the rpm. I have the L-79 cam which is not known for MPG. How do I fix the advance problem. Its an HEI dist.

68gmsee · 04-29-2009, 05:41 PM

First thing is you need to check your vacuum advance to make sure it works. I sucked (I know it sounds weird...) on the rubber hose to see if mine worked. It should move fairly easy. If it doesn't, it's bad. If it does, then you may not have it on the correct carb port.

Next is to make sure your harmonic balancer mark is in fact at TDC. Insure your #1 piston is TDC and check to make sure the marks align. If they don't scribe a mark on the balancer. It is possible the balancer is moving so you just have to keep an eye on that.

BADSAM · 04-29-2009, 06:27 PM

How old is your HEI? There are some springs in the that work with your centrifugal advance. You should be able to slightly move the rotor and it spring back, if it doesnt do this you have issues either the springs or with the distributor if its on the old side. It can be fixed though.

KB71C10BBSUPER · 04-29-2009, 06:32 PM

The one thing you need to check is to pull the dist. cap and see if your mech. advance is working on the dist. They will freeze up from time to time. Simply turn the rotor clockwise a lttle. if it moves and springs back oK if not it is frozen. You can remove the rotor & lube it with penatrant if it is frozen and work it back and forth
Hope this helps, K.B.

1969CST-10 · 04-29-2009, 07:40 PM

Just like these guys said.
I have been working on my new "toy", it had bad mpg and running poorly.
Checked mechanical timing (it was off) readjusted at idle, ok.
Then, no vacuum advance (bad diaphram) needs replace.
Checked centrifigal advance in distributer, like they said it was frozen and didn't move. Needs to be replaced also.
Minor maintenance stuff but makes a huge difference in performance!
Worth checking.

rscam · 05-05-2009, 06:13 PM

Well it turns out the vacuum advance was bad and the bushings on the centrifigul weights were worn so bad the weight were hung on top of each other. I had to order the bushings from NAPA. I am going to put it all together tommorrow. My timing was way off. The nut must have not been tight and it walked. It always started ok to be off that bad. It was like 20 degrees off the scale.

04-29-2009, 05:41 PM	#2
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Bad MPG. Timing off and No advance on my HEI First thing is you need to check your vacuum advance to make sure it works. I sucked (I know it sounds weird...) on the rubber hose to see if mine worked. It should move fairly easy. If it doesn't, it's bad. If it does, then you may not have it on the correct carb port. Next is to make sure your harmonic balancer mark is in fact at TDC. Insure your #1 piston is TDC and check to make sure the marks align. If they don't scribe a mark on the balancer. It is possible the balancer is moving so you just have to keep an eye on that.

04-29-2009, 06:27 PM	#3
BADSAM J'ville Junkman Join Date: Mar 2006 Location: Pomeroy, Ohio Posts: 572	Re: Bad MPG. Timing off and No advance on my HEI How old is your HEI? There are some springs in the that work with your centrifugal advance. You should be able to slightly move the rotor and it spring back, if it doesnt do this you have issues either the springs or with the distributor if its on the old side. It can be fixed though. __________________ 72 3/4t Suburban 402BB (Daily driver for 8 years) 68 GTO (never ending proj) 91 Kawasaki ZX-7R 57 Chevy 210 Wagon - 9 Passenger (Wife's project-working on floorpans) 73 Corvette (17 yr old daughter's: Another project - what's wrong with me?) 53 Willys M38

04-29-2009, 06:32 PM	#4
KB71C10BBSUPER Registered User Join Date: Apr 2009 Location: Valley Center, Ca. Posts: 37	Re: Bad MPG. Timing off and No advance on my HEI The one thing you need to check is to pull the dist. cap and see if your mech. advance is working on the dist. They will freeze up from time to time. Simply turn the rotor clockwise a lttle. if it moves and springs back oK if not it is frozen. You can remove the rotor & lube it with penatrant if it is frozen and work it back and forth Hope this helps, K.B.

04-29-2009, 07:40 PM	#5
1969CST-10 Registered User Join Date: Jan 2009 Location: FAIR OAKS CA Posts: 9	Re: Bad MPG. Timing off and No advance on my HEI Just like these guys said. I have been working on my new "toy", it had bad mpg and running poorly. Checked mechanical timing (it was off) readjusted at idle, ok. Then, no vacuum advance (bad diaphram) needs replace. Checked centrifigal advance in distributer, like they said it was frozen and didn't move. Needs to be replaced also. Minor maintenance stuff but makes a huge difference in performance! Worth checking. Attached Images

05-05-2009, 06:13 PM	#6
rscam Registered User Join Date: Nov 2005 Location: Monroe,Georgia Posts: 313	Re: Bad MPG. Timing off and No advance on my HEI Well it turns out the vacuum advance was bad and the bushings on the centrifigul weights were worn so bad the weight were hung on top of each other. I had to order the bushings from NAPA. I am going to put it all together tommorrow. My timing was way off. The nut must have not been tight and it walked. It always started ok to be off that bad. It was like 20 degrees off the scale.

04-29-2009, 05:06 PM	#1
rscam Registered User Join Date: Nov 2005 Location: Monroe,Georgia Posts: 313	Bad MPG. Timing off and No advance on my HEI I posted a dissapointing MPG thread a few days ago. Just to let you know. I checked my timing and it has moved to about 20 degrees BTDC. I have no advance on my dist. I guess it should have about 34 degrees @ 2800 rpm with the advance unhooked. I reset the timing to 8 degrees BTDC and I have no advance with the vacuum hooked up or not. It stays at 8 degrees no matter the rpm. I have the L-79 cam which is not known for MPG. How do I fix the advance problem. Its an HEI dist.



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