What did I do??!!

[flyboyjack]

I just completed the HEI conversion on our 71 "Super" w/original 350 SBC. I bragged to my wife that the HEI, new fine wire iridium plugs, (gapped @ .045 +/- a little), and new plug wires would pay for themselves w/i a year of driving. Only problem is the MPG went DOWN to 9.9 MPG vs 11 MPG with points, condenser and OLD wires. I'll run another tank and recheck. No misses, timing is "on the nuts". Oh well,....still a great truck!!

[dtkarst]

Well the way I see it is, if everything is right and how it should be, you’re good. I don’t think you bought it to save on fuel economy lol

[Ironhorse]

From reading the up grades you did I don't see anything that would cause the gas mileage to decrease with this caution:

By running the HEI, new plugs and plug wires is it possible that because the truck is running much better you have a bit "heavier" foot on the go pedal enjoying the better performance? You said timing was on the money so that is ruled out.

Try doing as my driver's ed teacher taught us way back in the dark ages when they actually still taught driver's ed in school. Drive as if there is an egg between your foot and the accelerator pedal if you want maximum gas mileage. Not much "fun" and no sound of a SBC & carb opening up with a burst of the go pedal BUT it will increase your gas mileage. Give it a try with a tank of gas, just have to remember to remind your self about the "egg"

[hjewell2]

Get rid of the plugs and go back to standard plugs as you had before.

[weim55]

While your initial timing set at idle speed may be correct, your advance curve at higher RPM with the vacuum-centrifical is probably different with the HEI. You can taylor (tune) The advance curve for maximum mileage but it's a little bit of a pain in the butt. To simplify it here's what I would do : kick your initial timing up an additional 5° from where you have it set now. If the truck starts OK and you cannot hear any detonation, drive The truck for a tank of gas and check the mileage again. If it gets worse go 5 degrees the other direction and check it again. Important the driving style and city-highway driving remain totally consistent for apples to apples comparison.

Steve weim55 Colorado

[cleszkie]

X2 on differences in vacuum advance and mechanical advance curves between your old points dizzy and the new HEI. Rarely are HEI curves perfect for your ride right out of the box. A little tweaking is usually needed to dial it in.

[firedemon]

hei distributor were never made for fuel efficiency they were made for emissions . the stock curve is always horrible then people tune for performance . performance is usually more efficient but its hard to keep the foot from being heavy . have you tried playing with the carb for better fuel mileage ? get an aem wideband air fuel guage along with a good vacume guage and tune that carb in as best as it will go nothing beats real world numbers as you drive everyday not even a dyno

[Sparky dave]

Have you set total timing? And is vacum advance piped in?

[72kool]

I know the problem..anytime you change a part on these trucks, it causes a problem called lphs. Which stands for left pedal happy syndrom. Happens to me every time ��

[Sparky dave]

Originally Posted by 72kool

I know the problem..anytime you change a part on these trucks, it causes a problem called lphs. Which stands for left pedal happy syndrom. Happens to me every time ��

Not right pedal happy? I know I'm English and we're all backwards here but am I gettin me pedals mixed up 😂

[flyboyjack]

Hey guys,

thanks for all the inputs. I did not do this for MPG's. Just figgered it'd be a side product of all the effort. I know I'm not gonna win any fuel economy contest with a Prius,... I just thought,.....

Still, as y'all know these are great trucks and my affection hasn't waivered whatsoever. I'll keep tinkerin' and post any updates/improvements. I've been wanting an air fuel and vacuum gauge. (Father's Day is coming!!)


Jack

[72kool]

Originally Posted by Sparky dave

Not right pedal happy? I know I'm English and we're all backwards here but am I gettin me pedals mixed up 😂

Hahaha it's a Monday thing.

[3757chevy]

No need for an iridium plug . I use NGK V-Power myself , but... fuel mileage is not why I drive MY TRUCKS...

[burnin oil]

You didn't say which HEI you used but if it is factory I will gaurentee that the timing curve is really lazy. You will at the very least need a spring kit to bring in the curve quicker. The Spectre advance curve kits are junk but do have some decent springs! Figure out how much advance the motor is seeing through the mechanical advance, ie not including vacuum. Easiest way is to put some light springs on it, rev the motor to 3500RPM and lock the timing down at 36*. Let initial fall where it falls. Now switch the springs out for some medium springs. You just wanted to make sure that you were getting full advance. It is also a good idea to double check it as the looser springs will tend to give an extra degree or two.

Now hook up the vacuum advance can to manifold vacuum so it always has vacuum applied at idle. 99% of small blocks run smoother and make more vacuum this way. When you hit the throttle the vacuum drops and the vacuum advance drops out leaving the mechanical curve for acceleration. This is the way it was done before emmisions.

Now you can experiment with the vacuum can. It should be stamped with a number on it. An internet search should tell you how many degrees it adds and what vacuum is needed for it to engauge. I never had good luck with the aftermarket adjustable cans but Echlin and stock GM canisters always seem to be fine.

As others have said, junk the plugs. I have always ran autolites or Delcos in stock heads. Electrically all we care about is that the plug has a good ground strap. Multi strap spark plugs are a gimic since the spark will jump to whatever path to ground is easiest. All the extra strapping does is shrowd the spark kernal. Also try to close the gap up a little on the plugs. 35 seems to work really well for me. Even on my MSD boxed truck I run the plug gap tighter. It is just easier for it to jump the gap. I also see less deteriation in the distributor cap and rotor.

Lastly. When you installed the HEI did you reuse the coil wire for power to the HEI? If so you need to switch the wire out. That is a resistor wire and only feeds about 9v to the ignition after you let go of the key from starting. This was common place back in the day to help the points live longer but HEIs do not like being under volted.

[flyboyjack]

I installed a Proform HEI, says it's specifically made for SBC and BBC chevy motors. Model 141-681, with a yellow cap so I could get a yellow spark instead of a red or black spark, (truck is ochre and white). I did indeed change out the reduced voltage coil wire and installed the YearOne wire as it was depicted elsewhere on this forum. The plugs are Autolite iridium fine wire and only have one grounding strap so I think I should be OK there. I will get new/stronger springs and start tinkering. I really appreciate your help and expertise. Thanks,... more updates to follow.

Jack

[68 P.O.S.]

Have you done any tuning to the HEI or did you install it out of the box? Did you tune the carb as well? How did you set your timing?

[68 P.O.S.]

I like that yellow cap, looks pretty cool and will definitely match the truck well. Do you know how many degrees of mechanical advance and vacuum advance the HEI provides? List each one out separately if you can. I doubt you’ll need stronger springs for it because they are likely too strong already. You’ll have to test it with a timing light to see what RPM it stops advancing at. Ideally you want you’re mechanical advance all in between 2500-3000 RPM. As I mentioned in your “74 HEI” thread, the Crane 99600-1 adjustable vacuum advance kit has the springs you want to get it in the 2500-3000 RPM range and the vac advance can has the most adjustability to dial it in to the 10-15 degrees you need. You have the plugs gapped right at spec where they need to be at 0.045 so that's not an issue. Still may need different plugs tho. What is your timing set at?

[flyboyjack]

I've rechecked everything EXCEPT for the plugs, (obviously). Mechanical advance all in around 2700 rpm at 34*. If the fine wire autolites are the problem,... then I'll be "hornswaggled"! It ran great with the old dizzy, points, etc.. I'll keep thinking on this. Didn't figure I'd need to mess with the carb as it was running things pretty well before the HEI install.

[cypressbog]

Try switching back to copper core plugs.

https://shop.advanceautoparts.com/r/...en-spark-plugs

[demian5]

Finally a topic where EVERYONE can flex their expertise about ignition timing!

[garyd1961]

I would up the total timing.

[firedemon]

if your looking for better fuel economy timing will only gain you a mile or two assuming its not off by a mile . that weird clunky looking thing that actually holds gasoline in it between the air cleaner and intake manifold can make huge differences . you can get a fairly decent vacume guage for around 20 bucks get the carb extremely close to perfect and it will help with the timing too

[burnin oil]

As said, get those other 2 degrees of timing in there. You may be surprised in what it does for the motor. As an engine rule of thumb the more inefficient the combustion chamber is the more timing is needed. Also a heavy ride with interstate gears needs a slower timing curve than a T bucket. With total timing set at 36* what is the base timing set at with no vacuum advance? Plug the advance back in and whats the new timing?

My truck is nowhere near stock and does not have to pass emmisions. My timing is 24 degrees initial with 36 total by 2k RPM. This is not a normal curve but it is what the truck likes so that is what she gets. As of now it has no vacuum advance since it has never liked it. One of these days I will play with it again and see if I can get a little in there through the computerized ignition (programmable MSD). This motor is beyond what most people call streetable so don't take the numbers to heart for your ride. My point is to give the motor what it likes.

When you have the distributer cap off double check all the advance mechanisms inside the dizzy. Make sure none are sticking or binding and that it returns to its neutral state easily. Proform makes some really good OEM parts and some that are not. I have no idea of the quality of the unit you have. I have in the past ran a proform dizzy in the above motor and it ate a module in a few months. I chocked it up as a that sucked and still carry it in the tool box as a spare for the MSD setup. I wired the truck to run the bling ignition or HEI. When you run on the edge of streetable, you always keep the options open for roadside repairs.

Quick story for ya. I broke down in TX a few days before heading back to TN. I shattered a small diameter rotor button and cap from MSD. O well. You know what happens and this is why I normally carry the old ones as spares. Not so lucky this time so I call all over North Texas and coundnt find a cap anywhere and I needed to roll out of town before I could mail order one. I bought the bullet and went to the store and bought a cheap MSD HEI, carry it back to the truck and fit the install for 2 hours. It wont seat down all the way. It was engauging the oil pump drive but was sticking up about an 1/8". In frustration I pulled it back out and looked at the drive gear real close and see that it was mismachined from the factory. One side of the slot had a slight bevel towards the center, you guessed it. 1/8" from seating all the way down on the pump drive. Some times parts arn't always what they are cracked up to be.

[68 P.O.S.]

Did you ever figure this out?

[flyboyjack]

Originally Posted by 68 P.O.S.

Did you ever figure this out?

I have not found anything yet. As with most of us, I've had several "irons in the fire". Just sold one of them. So, I'm hoping to possibly start all over. Remove and re-gap the plugs. Check/reset timing to 36* around 2500-2700 rpm. And lastly take a hard look at the carb. It's still hard starting, just the same as before the conversion to HEI. (It's not running rich to my eye, but,...…). It may also just be one of those motors that no matter what, is locked in to 12.5 MPG. Highway 13-13.2.


I'm almost to where I don't care too much. It runs so smoothly. And I didn't buy this truck have a "look at me" ride. But it sure gets looks and comments. Even with the original paint and little rust starting to show. I've honestly had FIVE offers to buy it in the last five days. These are just great trucks!

Jack