Timing & Gas Milage

[Rich72C10]

@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)

[geezer#99]

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

[Rich72C10]

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

[burnin oil]

Originally Posted by HO455

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

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.

[Rich72C10]

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.

[Andy4639]

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.

[Rich72C10]

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.

[ChevyRacefan]

Originally Posted by Andy4639

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.

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.

[Andy4639]

Originally Posted by ChevyRacefan

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.

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.

[68 P.O.S.]

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

[Rich72C10]

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?

[BigBird05]

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.

[Rich72C10]

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

[BigBird05]

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.

[Rich72C10]

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.

[BigBird05]

I looked it up in the 70 chevy truck manual and it says 4 deg with out vac adv.

[68 P.O.S.]

Here’s how I usually go about setting my timing.
First I pull off the carb and look underneath at the 2 transfer slots in the primary throttle bores. There’s 1 in each primary throttle bore and they’re probably over exposed and look like a rectangle. Using the idle adjustment screw, close the throttle blades all the way, then slowly open it to expose the transfer slots again. Once exposed, adjust them to look like a square, not a rectangle. This ensures you’re only on the idle circuit when actually idling. Once the slots are squared, reinstall the carb and don’t touch the idle adjustment screw again. Then I open the distributor cap and remove the advance springs so the weights move freely and it takes little rpm to reach full advance. This also keeps you from having to nearly redline your engine with your head under the hood lol. Disconnect and plug the vacuum advance. Start her up, bust out the timing light and rev it to like 2000 rpm so the advance weights are fully extended (fully advanced). Then set the total timing to 36 degrees btdc. Lock the distributor down and reinstall the advance springs. Ideally you want springs that allow full advance around 2500-3000 rpm. Then you can take a reading and see what your timing is at during idle as a mental note (this is your initial timing), but do not adjust it, or you’ll screw up your total timing and have to redo it. Next, reconnect your vacuum advance to manifold vacuum and listen for pinging. Your vac advance should be limited to provide between 10-15 degrees of advance. Anymore and its ping city. Check your idle speed and adjustment if needed. Mine is always right on after setting timing this way and never needs adjustment. Boom, you’re done!

[Andy4639]

Are you sure the timing scale is the right one for the motor? If the motor is a 72 and they used a newer timing cover then the timing marks may be off.
Depending on what balancer you have also.
Just asking because I ran into this on the 68 step I was building. It took a bit to figure out.

[Rich72C10]

They did not use a new timing cover, that I am sure. They only painted it pretty and put in white painted lines on the marks

[68 P.O.S.]

Originally Posted by Andy4639

Are you sure the timing scale is the right one for the motor? If the motor is a 72 and they used a newer timing cover then the timing marks may be off.
Depending on what balancer you have also.
Just asking because I ran into this on the 68 step I was building. It took a bit to figure out.

Good call Andy

[Rich72C10]

Originally Posted by 68 P.O.S.

Here’s how I usually go about setting my timing.
First I pull off the carb and look underneath at the 2 transfer slots in the primary throttle bores. There’s 1 in each primary throttle bore and they’re probably over exposed and look like a rectangle. Using the idle adjustment screw, close the throttle blades all the way, then slowly open it to expose the transfer slots again. Once exposed, adjust them to look like a square, not a rectangle. This ensures you’re only on the idle circuit when actually idling. Once the slots are squared, reinstall the carb and don’t touch the idle adjustment screw again. Then I open the distributor cap and remove the advance springs so the weights move freely and it takes little rpm to reach full advance. This also keeps you from having to nearly redline your engine with your head under the hood lol. Disconnect and plug the vacuum advance. Start her up, bust out the timing light and rev it to like 2000 rpm so the advance weights are fully extended (fully advanced). Then set the total timing to 36 degrees btdc. Lock the distributor down and reinstall the advance springs. Ideally you want springs that allow full advance around 2500-3000 rpm. Then you can take a reading and see what your timing is at during idle as a mental note (this is your initial timing), but do not adjust it, or you’ll screw up your total timing and have to redo it. Next, reconnect your vacuum advance to manifold vacuum and listen for pinging. Your vac advance should be limited to provide between 10-15 degrees of advance. Anymore and its ping city. Check your idle speed and adjustment if needed. Mine is always right on after setting timing this way and never needs adjustment. Boom, you’re done!

Well, that is interesting - seems like one does not fuss over initial timing but total timing (I have read that a few times). I also like to note, I hate rev'ing up the engine so I like the "remove springs" idea.

[Rich72C10]

for the record, this is TSP most basic Read to Run distributor ... far as I can tell there are NO springs / adjustments.

[BigBird05]

Does the 72 BBC have Transmission spark control like the 72 SBC? The TSC on the 72 SBC uses a temp sender on the passenger side of the block.

[68 P.O.S.]

Originally Posted by BigBird05

Does the 72 BBC have Transmission spark control like the 72 SBC? The TSC on the 72 SBC uses a temp sender on the passenger side of the block.

Wrong thread

[Rich72C10]

I haven't any idea and I am not sure if my SBC has that (it is a 72 with a TH350). Oh, looked up "Transmission spark control" - nope don't have one of those! I see that perhaps you posted in the wrong tread but you made me look that stuff up LOL