Timing & Gas Milage
 

So I popped into the "What kind of gas mileage do you get with your 67-72?" and after I posted what I was getting it went south from there. I feel a bit bad since it became about me and my timing. So I am starting a new thread, mostly to see what the out come is. Or I hope there is one ;-)

Summary:
Engine rebuilt to stock with a mild RV Cam, still a bit under 1k miles on the engine (say reaching 700 miles). I got 10 mpg this past Sunday for the 1st time with reasonable mixed driving. My timing is set to 12° but I found that my Vacuum Canister on my distributor leaks, takes it under 25 seconds to drop to 0 after pumping it to 15~20hg. I am trying to learn how to properly find both mechanical and vacuum timing - I am thinking I didn't do that right last night.

Currently I have what the previous own put in (he bought it from Ebay), a Top Street Performance JM6701. Not much you can do with it, other than set timing. The Vacuum Canister is a odd thing that will not take a replacement off the shelf, though they are sending me one for free. During the rebuild my bother and I did talk about replacing it - just to have a known brand.

I finally decided to buy from Summit this morning a PerTronix Ignitor III D7100700 with their matching coil. This unit looks very similar to the Top Street Performance distributor, its black, small-ish, and has female connections on the cap - which from a visual stand point what I am wanting. I know a lot of guys say go with a GM HEI, even go get one from a junk yard. But those things are monsters and I don't like the look of them. The deed is done and at least I should be able to support this one better than the TSP distributor.

I also have a 72 Quadrajet 4BBL at Quadajet Power but I don't expect it back until March. My installed Quadrajet is from a 78 Truck that the previous owner put on and while it was rebuilt, I am not sure how well it was jetted during the rebuild (there was a lot of questions around its rebuild when my brother & buddies had it apart a few times).

As I get stats from timing and MPG changes I'll try and keep this tread updated.

PS:
As for how the truck is running, outside of MPG, I have no complains. She takes off like a bandit, even spinning the tires if I dog it. At any speed I can give it WOT and she immediately goes. No hesitation or mis-firing that I can notice. I have had it in two different shops for various things (O'Ring replacements/Transmission leak fix) neither offered up "hey, you have timing problems". They actually both said in one way or there other, she is a nice truck and drives great.

PSS:

• SBC 350 Rebuilt less than 1k on engine at this posting.
• TH350 Rebuilt at same time as engine.
• 307 Gears
• Power Brakes and A/C (off during all timing checks)
• 12° Initial timing
• 30° ~3500 Max Mechanical (by offsetting light ° button to read 0 on the timing mark)
• 45° ~3500 Max Vacuum (by offsetting light to read 0 on the timing mark)
• 17 hg Vacuum @idle
• Park/Neutral Idle 650
• Tires 275 60 15
• 60 MPH = ~2500 RPMs

Update for Real World MPG with my rebuilt 72 Carb from Cliff and Progression Ignition:
March 25th 2022, 14mpg on a 275 mile trip on Texas Hwys from Kyle to Kilgore Texas. This is mix driving were a lot (most) is 65~75 state Hwys and little towns on the way.

Re: Timing & Gas Milage
 

Update Pertronix 02/09/21: Canceled my Pertronix Ignitor III D7100700. Seems the Ignitor III does not work well with older OEM Tachometers - something about multiple spark. Though it appears using the Ignitor II D100700 is okay with OEM in-dash Tach. I am now looking directly at Pertronix site and have submitted a tech question.

Changed Ported to Manifold Vacuum 02/09/21: Idle went up by ~150 RPMs, so lowered it back down to ~650 RPMs. Truck is puncher off idle/full stop and preforms just as well, if not better when WOT. Matter of fact, I was able to spin the wheels at 3/4 peddle from the stop sign. No ticking / knocking that I can hear. But this was only getting her up to 50 and testing WOT between 35~40 MPH. There is only so much I'd speed around town LOL

Changed Ported to Manifold Vacuum 02/09/21 Part II: I put about 60 miles on last night. 80% 55~70 MPH and the rest town driving. I got 12 MPG

Installed PerTronix Ignitor II D100700 & Flame-Thrower II Coil 45011 02/13/21: Now getting a full 38° Max Mechanical Advance @30k, with a 12° Initial timing @650 RPMs. Also no apparent issues with my OEM Tach that I see so far. Vacuum canister on PerTronix actually works (even the TSP replacement canister I got in the mail will not hold vacuum).

The changes are: New Pertronix Flame-Thrower II, New Flame-Thrower Coil & Wires. 12° initial timing, ~900 RPM idle (Park) and ~640 RPM idle (Drive).

Update 10/31/21
With my newly rebuilt 72 Quadrajet done by Cliff High Performance and my newly installed Progression Ignition I got 18.5 MPG!

Timing setup can be seen with Progression Ignition app (it is controlled by your phone).

https://67-72chevytrucks.com/vboard/...1&d=1635725621

Re: Timing & Gas Milage
 

Here's a good timing article written by old GM engineer John Hinckley. He helped develop the systems and he explains it very well.

http://www.camaros.org/pdf/timing101.pdf

Re: Timing & Gas Milage
 

Here's another article by Hinckley that goes over the vac advance.

http://www.stl-vettes.com/65Vette/co...ng_Advance.pdf

Re: Timing & Gas Milage
 

thank you, downloaded and saved into my Timing Folder locally!

Re: Timing & Gas Milage
 

You're welcome. What type of distributor do you have? I didn't notice it in the other thread.

Re: Timing & Gas Milage
 

I listed that up in my summery - Top Street Performance JM6701

I am now looking at a Pertronix Ignitor II D100700 but I want to know make 100% sure it will work with my stock/OEM Chevy Tach (seems I read the III does not).

Re: Timing & Gas Milage
 

Jeez, I missed that twice :dohh: I looked it up and they give you almost no info on that distributor. You need to find out how many degrees of mechanical advance and vacuum advance it provides. Also, at what rpm does it allow max mech advance.

Re: Timing & Gas Milage
 

I think I am done with the TSP JM6701. I more or less got from them on a call, you need to go to the "Pro" for whatever I was asking about. So I rather just go with someone else that others have actually bought and installed for better "user" help!

Re: Timing & Gas Milage
 

That doesn't sound like very good customer service. I would ditch them too.

Since you have a mild build, a HEI will work just fine and is an upgrade from the points distributor. I've used the Accel 59107 HEI on different vehicles with zero issues. They're a reliable OEM replacement, reasonably priced, and work great.

I've never used one so I have no first hand experience (which means I may be talking out of my ass), but I've read from multiple sources, the Pertronix units have a high rate of failure. Lots of folks here on the board have used them with no problems, so I wouldn't really be too worried, but from what I've read on them, I'll never use one. Regardless, it sounds like it will be much better than that TSP. Since I only use HEIs, I have no recommendations for a points distributor.

Cheers

Re: Timing & Gas Milage
 

Called up PerTronix's support and I have to say, that was enjoyable. The support guy (Mitch I think it was) was very easily understood, helpful, and knowledgeable. He says between the Ignitor II & III either one could likely give me a Tachometer issues, where it would read up to 1k to high. Though he says it only needs a resister mod (10K 1/2 RESISTOR) added between the negative post of the coil to the tachometer wire (but he has seen many customers not need it either with my setup). He also says, the III is overkill for my application and the II is what he would recommend.

On another positive note, their website gave me a coupon code and I saved $28 bucks on my order for the Ignitor II and matching Coil.

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Very nice, sounds much better than the support at TSP.

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I would not go as far as TSP support was bad, they just reported the fact my options were limited (none) with the distributor (which I did not even buy, it came with the truck). While I am sure I had a bit of luck, it was easier to get through PerTronix.

Now, if I have problems with the PerTronix distributor, at least I know its history and it is a choice I made (for good or bad).

Re: Timing & Gas Milage
 

One thing I learned from the docs, I am 95% sure my vacuum can is connected to ported vacuum and not manifold vacuum. My vacuum line comes from the can, to the front of my Qjet to the port above the right adjustment screw.

I "think" this location is the original factory connection and from the reading this was for emissions. While this is the factory distributor vacuum can source, it is not a proper source?? Gee I hope I read that right!

Edit:
yes, it does make a difference - the clearly reacts to manifold Vacuum as apposed to the connect I have it on the Carb that is ported. When on the Carb ported Vacuum, there isn't a difference when I disconnect it. When I moved to manifold vacuum, I gained roughly 150 RPMs and when I disconnect it I bounce back to my idle set speed from the carb's ported vacuum.

I also read here on this web page basically the same, distributor should use full manifold vacuum: Everything You Wanted to Know About Vacuum Advance and Ignition Timing

Edit 2:
Man, there is so much on Manifold vs Port Cab Vacuum ... talk about having a line in the sand with groups of people on either side!

Re: Timing & Gas Milage
 

There were, I saw here, some vehicles with ported spark before it became an emissions requirement. Some people get pretty vociferous about their interpretation of the truth, is my take. :lol: I'm running manifold vacuum to my distributor. The engine has always run hot, and I made many incremental changes to get it to run "normally". I've used different temp gauges to verify the results, so it's not a gauge problem. The biggest help was using a high-flow thermostat.

BoT, your truck came with ported spark, and yes it was connected to the vacuum nipple on the driver's side front of the carburetor. My truck was originally ported spark, but like I said, I routed full manifold vacuum to it.

Re: Timing & Gas Milage
 

With my very limited test drive, I see improvement. I shall keep an open mind and eye on it. It's an easy change either way. I'll do a 20~40 mile drive on the back roads to see how this holds up. Plus it will be a few days or more before I'll have anything to work on/replace the distributor!

Thank you for your input/information Steeveedee!

Re: Timing & Gas Milage
 

At the level of detail you are working, I would wait til you had 5k miles or more on the engine. I would be concerned that your motor is "tight" causing part of your gas mileage issue. The other items that will affect gas milage:

-what is the diameter of your tires?
-does your starter turn slower when the engine is hot vs cold start? (ie engine is tight)
-do you have excessive "slippage" of your torque converter (assuming it is a non lock up?)

Re: Timing & Gas Milage
 

Do you have a wideband? I think leaning out the cruise AFR will get you the most bang for the buck. Leaner mixtures require more timing, so basically you wind up trying to lean out cruise and add in advance until you reach the point of diminishing returns or where you can’t realistically lean out the carb more without affecting part-throttle performance negatively.

I started to go down this road to get decent fuel economy out of my 396 K10, but eventually accepted the fact that, with two small kids, I was going to spend a lot more time in my 2500HD crew cab than in my K10. Not sure how to lean out cruise with a quadrajet but I believe it’s doable.

Re: Timing & Gas Milage
 

Just set the timing to high idle and be done with it. Disconnect the vac advance. Rev the motor until it doesn't advance anymore then set to 36 degrees while holding the rpms up. Lock the distributor back down. This will get you through the engine break in. You can rehook the vac advance after recording the new idle speed advance.

Setting timing to factory spec will not work on a non factory distributor. You have no way to know what wieghts are in there other than doing the timing the way I said and subtracting the new base timing from the 36 degrees. The speed of the curve is also unknown. I have never owned a dialback timing light that was overly accurate. As a side note some of the factory HEIs had around 40 degrees of advance from the factory on the later emmissions motors with base timing set to 0 degrees. These are not desirable but to prove my point on timing.

Every motor is different but SBC tend to like 36 degrees of overall mechanical advance ( base + mechanical from wiwghts) and around 20 degrees of vac advance for a total of around 48-52 degrees of total timing on a stockist motor. High performance tend to want the same mechanical advance with more base timing but less vac advance. Stock motors want the tighter heavy advance springs and lighter performance cars will take the lighter springs for the wieghts. In other words you want a slow advance curve.

Lastly you just have to try vac advance both ways. Most motors like full advance but some like ported.

Good luck.

Re: Timing & Gas Milage
 

You change the secondary metering rods. The taper of the rod coming out of the fixed office make it act like a different sized jet. Cruise should be on the primary side of the carb so not much is going to affect the milage other than proper mixture screw settings. The throttle blades are about the size of a dime on the front barrels so they run real lean as it is.

Re: Timing & Gas Milage
 

thank, i did not know that!

Re: Timing & Gas Milage
 

That makes some sense, though I don't think I went wrong getting a different distributor that didn't come from eBay that no one seems to have heard of before.

1. 275 60 15
2. Not at all. My starter has no problems, turns over well (cold or hot).
3. All I know it is a stock TH350 that was overhauled when the engine was done.

Sorry, I cannot say either way - need to find out what wideband means.

Noted, though sounds complicated! I need to print that out, read over it to insure I understand what you have suggested. Personally, I have all the time in the word to try things out at least once! Though to remember @cj847 comment, perhaps I need to let it break in a bit more...

Re: Timing & Gas Milage
 

This information is wrong.
The air fuel mixture at cruise (and anytime the throttle is open above the off idle transfer slots) is controlled by the primary jets and the primary rods. The farther the primary throttle plates are open the stronger the signal developed by the venturies and the more fuel that pulled through the primary jets. The size of the jet sets the maximum amount of fuel and the rods fine tune that amount . There is also a throttle tip in circuit that uses senses manifold vacuum to richen the primary mixture when the throttle is moved open from a cruise. This circuit has a spring that overcomes the vacuum and lifts the rods to richen the mixture to prevent stumbling.
This is the condensed Cliff Notes explanation.

Poor fuel economy can be caused by the wrong jets or the wrong rod or a combination of the two.
Too strong of a spring on the tip in circuit or low vacuum or both will hurt fuel economy.
Improper vacuum advance operation will kill milage. The whole purpose of the vacuum advance canister is fuel economy.
Leakng plugs on the bottom of the carburetor center section will hurt economy. As will at least a dozen other simple things.

The mixture screws ONLY affect the fuel mixture at idle!
The primary bores are 1 3/8" in diameter. Far larger than a dime.
The secondary rods only control the fuel mixture of the air thar passes through the secondary bores of the carburetor. The secondaries are closed in a cruise situation. (Unless your cruising at 4500 plus rpm.under a heavy load)
There are dozens of good books on carburetors out there and some of them are Quadrajet specific. They are much better at explaining all of this than I am able to.
Get a book that explains carburetor operation in a manner that works for you. Read it and ask specific questions.
Your truck should be getting 12 mpg around town and at least 14 mpg at a steady 60 mph on the highway.

Here is a link to my Burban repair thread and there is tuning information of a similar combination to yours in various places.

http://67-72chevytrucks.com/vboard/s...698377&page=19

Re: Timing & Gas Milage
 

The carb that I have on now is just temp, until my replacement is back from Quadrajet Power, which funny thing they are just down the road from me in Waring, TX :-)

Edit:
When my brother & friends rebuilt the carb, I seem to recall they didn't like the jets in the kit. I'm pretty sure they did some funny stuff with them, though if I'd ask he'll give me the same answer I generally get "that was too long ago, I don't remember" line. Regardless of that, the plan was always to switch out the previous owner's O'Riley's 78 Qjet for a 72 matching numbers Carb. It has just taken an extra long time to get that plan finished.

Re: Timing & Gas Milage
 

On one of my trucks I built for dd 20 odd years ago. I added a vacuum guage so I could monitor mpg performance.

Little things that help mpg.

A air cleaner that is above the carb. The opposite of a drop base air cleaner. This helps air flow into the carb.

A small spacer or grinding out the divider in your plenum. Sonthe carb can see both side of the plenum. Thus equalizing vacuum on both sides.

Get a afr guage. I am not sure which is a good brand anymore, but they help you dial in cruise afr. While keeping full throttle where you need it. Roadkill Garage Idaho tater truck episode has a good tutorial on how to tune step by step, but you have to have some carb knowledge first.

A cowl indication hood with the back end open. This allows air through the grille to escape, and reduces drag. While allowing the carb to get cooler air with out a ducted system. Which is not a bad idea either.

While they won't make a 12mpg truck get 22mpg. That can help you achieve your trucks fullest potential. Which is pretty well baked into the fact it is a truck, the engine, stuff in the bed, height of truck, and heavy wheel/tire combo's. All of which effect mpg.

Recently I purchased a used Decked system for my truck. It dropped my pure highway mpg to 22 pretty much all the time. From the 24-26 mpg I usually got depending on how fast I was driving. I still like it, and the extra storage works for the way I live my life. So longer drives cost a bit more which is acceptable for what it does for me. Especially fishing or camping.

Re: Timing & Gas Milage
 

@HO455 - I have saved your link to your thread to my browser Chevy folder ... gee that is all kinds of information that for things that are on my list! Like the Alternator (I have a M.A.D kit in my garage collecting dust)

Re: Timing & Gas Milage
 

There’s a section in here on the apt adjustment.
First carb I tuned had the older style apt. Adjusted it to the point where there was no slight stumble when I cracked the throttle open. That mod gained me 2 mpg over stock.



http://www.crankshaftcoalition.com/w...4MV_carburetor

Re: Timing & Gas Milage
 

Good tip!

The Qjet that I have on now, I had to open to get all the junk out because of my fuel tank (I ended up having a radiator shop clean/seal the tank). I rather never have to open the carb again LOL

Re: Timing & Gas Milage
 

You are correct that the carb has all the circuits in there like pretty much all others and those circuits all build up off the idle circuit. My point was simplified for a reason. The air flow the primary is very limited and there isn't much adjustment to be made easily. A qjet mix and fuel consumption will stay reasonable due to the small primaries. These carbs are not user friendly as far as people learning so it was a leave it be thing. There is just a bigger chance to booger it up compared to other carbs. The OP is learning timing curves so modding a qjet is further along in his future. We all start somewhere and build on that knowledge. It has been a long time since I have been inside a qjet personally. What makes them great for this application makes them bad for my usage. They are alot harder to mod for performance than a holley and I need all those CFMs on the primary side. I don't tend to keep stock SBC around.

Re: Timing & Gas Milage
 

I do appreciate all the replies!

"...The OP is learning timing curves so modding a qjet is further along in his future. We all start somewhere and build on that knowledge...."

Yes, I am totally learning and even if I don't end up doing one thing or another - having knowledge on how things operate is never a bad thing!

I did update my 2nd post, I am roughly getting 12 MPG with my full manifold vacuum after my 60 mile test drive last night.

Re: Timing & Gas Milage
 

One of the best distributers you can buy is a GM HEI out of a mid 80's vehicle. No vacuum advance. They are easy to set up and will provide you with great service.

I had one in my 71 C-10 the whole life of the 305 TPI motor with 300,000 miles on it and I put 1 module in it. They are the best and cheapest for a stock motor. My truck had the 305/700-R4 in it with 4:10 gears and I got 18 MPG out of it running 70-80 MPH.
For a street truck that is used for just driving it and not hot rodding it you want beat the setup.
All these boxes and stuff are hype to sell you the product but @ the end of the day your paying for nothing but fancy electrical gadgets to fail and leave you on the side of the road.
One thing I always kept in mind for my trucks is drivability. I didn't want anything I couldn't get at a local parts house if I was on the road. That is something to keep in mind.:chevy:

Re: Timing & Gas Milage
 

Hi Andy,

"I didn't want anything I couldn't get at a local parts house if I was on the road."

Ya, that one statement rings true for me, which is why I really dislike my current Top Street Performance JM6701 distributor. Though what outweighs the GM HEI for me is that I cannot stand the look of it, I really can't. There might be time I'll reach a point where tying out the PerTronix ends up being a PITA that I'll be forced to a GM HEI or even back to points (but I'd have to be really pissed off and frustrated).

I actually have an extra module for the TSP distributor in my glove box, I got it months ago. I just did not expect to find the Vacuum Can not holding vacuum the other day. I think it is holding enough vacuum to operate but no telling how long it will hold up.

Re: Timing & Gas Milage
 

That HEI the advance was controlled by the ecu, That is why it had no vacuum advance can.
It will run like garbage on an engine without the ecu controll.

Re: Timing & Gas Milage
 

Good point, but I would want the GM HEI with vacuum advance over any aftermarket one for the easy ness to find one when on the road.
The point was the GM HEI's are some of the best distributors you can run for a stock motor are mild built motor. ;)

Re: Timing & Gas Milage
 

^^^This. A street driven daily driver needs vacuum advance for performance, mileage, drivability, idle cooling, etc.

Re: Timing & Gas Milage
 

So I tried out my total timings again by going up in RPMs until the it stopped advancing (before in another thread I just stopped my RPMs at 2500).

• 12° Initial idle timing @650 RPMs (worked my light button to 12° to read 0 on the timing mark). This hasn't changed since it was set months ago.
• 30° ~3500 Max Mechanical (worked the light offset button to 30° to read 0 on the timing mark while holding the RPMs)
• 45° ~3500 Max Vacuum (worked the light offset button to 45° to read 0 on the timing mark while holding the RPMs)

So I assume that means if I now adjust my distributor to 36° @3500, my initial idle timing will move up from 12°. I assume that won't be so good? I was watching a video on YouTube and this is because the crankshaft has X degrees of timing in it? Or it is a rubbish distributor?

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Was 12 deg with or without the vac adv hooked up. The vac adv should be around 15 deg added to your initial timing. If you are using manifold vac.

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12° was set for Initial idle timing with no vacuum connected to distributor.

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That sounds like a lot. What is your timing at idle with the vac hooked up? What year engine do you have? Both 70 and 71 350 CID call for 4 deg adv at idle with dist vac adv disconected.

Re: Timing & Gas Milage
 

72 350 with a mild RV Cam after being rebuilt. I keep seeing on the internets, 12° for Initial idle timing with no vacuum advance.... though it was set that way before me going all over the place to confirm we did it "correctly" months ago.