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Discussion Starter #1
Well, I got tired of getting 10 mpg with my Blueprint Engines 306 in my 65 Merc Cyclone. I just received a AEM A/F gauge kit from Summit Racing. A exhaust shop welded a bung to the exhaust pipe on Friday. I'll be installing the parts today.

It seems there are three areas to focus on, idle/transition, cruise and wide open throttle. The engine has stock exhaust manifolds on it now so it's chocked a little bit. On a side note, I install a K&N Substack a week ago. It must help airflow because I noticed a distinct off idle stumble with it on the engine.

Here are the baseline of all current settings car, engine, carb and tuning. I'll try to make one change at a time.

3.50 gears with trutrac in the rear end. P215/70R15 tires. Toploader trans, not close ratio.

Vacuum at idle (900) 15 inches, 20 inches at 2000 rpm (40 mph), 22 inches at 2500 rpms (55 mph), 20 at 3000 rpms (60 mph), 18 inches at 32-3300 rpms (65 mph)

Timing is set at 16 at idle, lightest springs installed in dist so it's all in at 2500 rpms. MSD distributor 8582. Plugs are Champion C61YC gap is .044. Wires are Moroso 8.8 ultra 40.

idle screws set at 7/8 turn out. Main jets are demon 66, PV is 6.5, nozzle is a 33, has a blue cam in it (not sure which position).

I'd appreciate any suggestions on which circuit to work on first or recommendations.
 

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Discussion Starter #4

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Discussion Starter #6
I installed the parts and took some baseline numbers.

idle - on choke engine cold - 11.4
idle - choke off engine cold, 11.9

2000 rpm, 40 mph, A/F ratio 13
2500 rpm, 50 mph, A/F ratio 13.2
3000 rpm, 60 mph, A/F ratio 14.2

As I accelerated with part throttle, the A/F ratio would hit 15, showing the off idle stumble.

Wide open throttle test in 2nd gear showed A/F ratio of 12 - 13.

Came back and tuned the idle screws.

Checked the current settings and found the passenger side was way out, over 2 turns while the driver side was at 7/8 like I thought it was. I'm wondering if the passenger side screw is opening up because of vibration? I'm ususallt pretty diligent about getting both sides matched up the same.

Anyway, I turned the screws to 1/2 + 1/16 turns where the engine started to stumble. A/F ratio fluctuates between 13.3 - 13.7. Vacuum increased from 15 to 16.

Went for another test drive. The engine ran better and was more responsive. Cruise A/F ratio between 2000 and 3000 rpms was between 14.1 and 14.5. This includes the part throttle driving in busy traffic. The off idle stumble (15 reading) pretty much disappeared.

Next step.

Since I can't turn the idle screws in any farther to get the idle to lean out any more, it seems the next step is to open the secondary blade very slightly so I can open the idle screws a little more. (let the secondary side give it some air).

Next would be to install smaller mains.

Am I on the right track? So far I really like this gauge and what it's telling me.
 

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I installed the parts and took some baseline numbers.

idle - on choke engine cold - 11.4
idle - choke off engine cold, 11.9

2000 rpm, 40 mph, A/F ratio 13
2500 rpm, 50 mph, A/F ratio 13.2
3000 rpm, 60 mph, A/F ratio 14.2

As I accelerated with part throttle, the A/F ratio would hit 15, showing the off idle stumble.

Wide open throttle test in 2nd gear showed A/F ratio of 12 - 13.

Came back and tuned the idle screws.

Checked the current settings and found the passenger side was way out, over 2 turns while the driver side was at 7/8 like I thought it was. I'm wondering if the passenger side screw is opening up because of vibration? I'm ususallt pretty diligent about getting both sides matched up the same.

Anyway, I turned the screws to 1/2 + 1/16 turns where the engine started to stumble. A/F ratio fluctuates between 13.3 - 13.7. Vacuum increased from 15 to 16.

Went for another test drive. The engine ran better and was more responsive. Cruise A/F ratio between 2000 and 3000 rpms was between 14.1 and 14.5. This includes the part throttle driving in busy traffic. The off idle stumble (15 reading) pretty much disappeared.

Next step.

Since I can't turn the idle screws in any farther to get the idle to lean out any more, it seems the next step is to open the secondary blade very slightly so I can open the idle screws a little more. (let the secondary side give it some air).

Next would be to install smaller mains.

Am I on the right track? So far I really like this gauge and what it's telling me.
Your 4V, is it a vacuum advance or are we talking double pumper?

If vacuum advance what I planned on doing with mine is using my a/f ratio gauge I built into a tuning tool to set the primary jets to be slightly on the lean side around 16:1 when cruising with no load. I then planned on setting up the secondaries to enrich to the point where I am slightly rich on the load side to be around 12:1 to 13:1 which from all the reading I did is the range for best power/torque.

That is the plan for myself but I will be going through the secondary as well playing with that as I purchased a secondary spring quick change kit to try different secondaries to fine tune the kick in for my specific application.

I personally might be going about it wrong but this is what I think would be the way to shoot for as you are cruising on just 2 of the 4 barrels so if you set up cruise to be slightly lean when you are not under a load then you can improve fuel economy.
 

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Discussion Starter #8
Your 4V, is it a vacuum advance or are we talking double pumper?

If vacuum advance what I planned on doing with mine is using my a/f ratio gauge I built into a tuning tool to set the primary jets to be slightly on the lean side around 16:1 when cruising with no load. I then planned on setting up the secondaries to enrich to the point where I am slightly rich on the load side to be around 12:1 to 13:1 which from all the reading I did is the range for best power/torque.

That is the plan for myself but I will be going through the secondary as well playing with that as I purchased a secondary spring quick change kit to try different secondaries to fine tune the kick in for my specific application.

I personally might be going about it wrong but this is what I think would be the way to shoot for as you are cruising on just 2 of the 4 barrels so if you set up cruise to be slightly lean when you are not under a load then you can improve fuel economy.
I should have posted the carb model. It's a 4160 4v with vacuum advance and electric choke. I am trying to increase the fuel economy since this is a street car and will probably never see the track even though it has all the right stuff for drag racing (slow drag racing that is).

I installed the secondary spring kit too. My notes from last year indicate I put the yellow spring in it. I'll take a look tomorrow to confirm which one is in it.

I want to avoid lean conditions and would like to see something in the 14:1 ratio in the idle, off idle and cruise areas. I think being a little rich at wide open throttle will be okay as long as it's not too rich. I wasn't sure where the sweet spot is. I think you mentioned 12-13. I think that's what I'll shoot for. Thanks for the post.
 

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idle mix ussually doesn't have a lot to do with cruise mix. i'm sure your mains are too big. probably in the range of 48 or so. keep checking plugs
 

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I should have posted the carb model. It's a 4160 4v with vacuum advance and electric choke. I am trying to increase the fuel economy since this is a street car and will probably never see the track even though it has all the right stuff for drag racing (slow drag racing that is).

I installed the secondary spring kit too. My notes from last year indicate I put the yellow spring in it. I'll take a look tomorrow to confirm which one is in it.

I want to avoid lean conditions and would like to see something in the 14:1 ratio in the idle, off idle and cruise areas. I think being a little rich at wide open throttle will be okay as long as it's not too rich. I wasn't sure where the sweet spot is. I think you mentioned 12-13. I think that's what I'll shoot for. Thanks for the post.
Yep all the charts Ive seen on Air/fuel ratio lists 12:1 to 13:1 is typically the ratio for max power. 16:1 is listed by the same charts as being the best fuel economy. Lean you have to be careful with as it will be easy to go too lean and run high temps. I am going to try and slowly lean it out and start off with a goal of 14:1 to 14.7:1 then shoot for 15:1 to see how it goes I might try to push it to 16:1 but I have to be careful on my build as the 306 short block im getting uses Hypereutectic Pistons, from all the reading I have done, they are from what I heard more stable and doesn't expand/contract as much so they can be fitted with lower clearance to the cylinder, but at the same time they don't like spark knock and can easily damage them. So for me I have to try and play my going lean as careful as I can. Lean wise I will be running my engine for a month or so at 14:1 then check the plugs see how they look. Then bump it down to 15:1 and then do the test again then check again. I don't know of any way to tell spark knock as from my research on it everything I found says you can have spark knock and not even feel it or know it.

Carb I have to run is the 600cfm summit carb, Ive heard great things about it, reviews are great. Its basically a holley built carb for summit based off the old holley design which was based off the ford 4100 carb. For me I like the old style one piece base where you don't have bowls that can leak like on traditional holleys.
 

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Discussion Starter #11
idle mix ussually doesn't have a lot to do with cruise mix. i'm sure your mains are too big. probably in the range of 48 or so. keep checking plugs

I agree, that's why I want to open the secondary a little to help improve idle characteristics. The engine made 390 hp on the dyno and came with #66 mains. Last year I put in 64s and it didn't like them but the plugs were really black back them. Thanks for the info.
 

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Discussion Starter #12
Yep all the charts Ive seen on Air/fuel ratio lists 12:1 to 13:1 is typically the ratio for max power. 16:1 is listed by the same charts as being the best fuel economy. Lean you have to be careful with as it will be easy to go too lean and run high temps. I am going to try and slowly lean it out and start off with a goal of 14:1 to 14.7:1 then shoot for 15:1 to see how it goes I might try to push it to 16:1 but I have to be careful on my build as the 306 short block im getting uses Hypereutectic Pistons, from all the reading I have done, they are from what I heard more stable and doesn't expand/contract as much so they can be fitted with lower clearance to the cylinder, but at the same time they don't like spark knock and can easily damage them. So for me I have to try and play my going lean as careful as I can. Lean wise I will be running my engine for a month or so at 14:1 then check the plugs see how they look. Then bump it down to 15:1 and then do the test again then check again. I don't know of any way to tell spark knock as from my research on it everything I found says you can have spark knock and not even feel it or know it.

Carb I have to run is the 600cfm summit carb, Ive heard great things about it, reviews are great. Its basically a holley built carb for summit based off the old holley design which was based off the ford 4100 carb. For me I like the old style one piece base where you don't have bowls that can leak like on traditional holleys.

Sounds like you have a good plan. Conservative is good. I've heard the same thing about knock. That Summit carb seems to be a good one. I hope it works well for you. I'll keep posting my changes. Maybe it'll help you and some others.
 

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Discussion Starter #13
I pulled the carb off the engine today and found several problems.

1. the primary transfer slot was open far more than it should be. I tried opening the curb idle screw to set it to the square position with no luck. Problem #2

2. The curb idle screw will not allow the primary throttle plate to close enough to make the transfer slot a square. I had to adjust the choke screw (1/4 screw on opposite side of carb)

In looking at the mechanisms, I don't understand why there isn't more adjustment. It seems the plates on the primary throttle are in the wrong position. Might have a bad carb?

I haven't got to the secondaries yet.
 

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the plates are not cut square on the edges. make sure they match the bore. also loosening the screws holing the plates, backing off the speed screw, tap the plates to get them to settle in the bore.
fast idle speed screw should not be involved unless the choke is on
 

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Discussion Starter #15
the plates are not cut square on the edges. make sure they match the bore. also loosening the screws holing the plates, backing off the speed screw, tap the plates to get them to settle in the bore.
fast idle speed screw should not be involved unless the choke is on

That's great advice. I'll do that tomorrow.

Early on I had tightened the fast idle speed screw when adjusting the choke. I think I over adjusted it. Loosening this screw allowed the curb idle screw to close the throttle plate a little.

Since I had closed the transfer slot a little bit, the engine ran a little richer when I reinstalled everything. Since I had a substack in the garage, I reinstalled it. It actually helped a lot.

Idle A/F went from 13.2 - 13.5 to 13.9 - 14.0
Cruise A/F (2000 and 2500 rpms went from 13.5 - 13.9 to 13.9 - 14.4

Due to heavy traffic, I couldn't really give it a good test. Idle screws still at 1/2 + 1/16. Vacuum went up to 16.5. The choke didn't work too well as it didn't step down the cams. I might not need the choke at this time of year.

Thanks for the great advice.
 

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you need to use a vacuum gauge to set idle mix. not typical for both screws to be the same. without vac. gauge, turn each in to a rpm drop. (lean drop). then backout 1/2 turn. do for each side over and over. don' t worry about them being the same
 

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Discussion Starter #17
Had an interesting day today. Pulled the carb off and backed the curb idle screw off thottle plate. Loosened the screws for the blades on the primary throttle shaft. The throttle shaft would not move enough to cover the transition slot. Tightened the 4 screws. I looked at the linkages and the culprit seemed to be the choke adjustment screw or a linkage problem with the choke mechanism. I also installed 65 demon mains since the wide open A/F was a little high. Another change I made was to install a bolt for the secondary throttle adjustment so I could adjust that if necessary.

I pulled the choke assembly off the carb and the throttle plates would cover the transition slot. Started to reinstall the choke mechanism but found the throttle shaft/assembly moved as I inserted the second screw meaning something was in contact with the primary throttle shaft and causing it to move. The throttle plate moved .029. The only thing I could find was the white bushing the adjustment screw goes through for the choke adjustment. I removed it and filed it down.

Reinstalled the parts and the transition slot was covered. I then adjusted it using the curb idle screw to the square position and set the idle bleed screws to the recommended 1.5 turns out.

Reinstalled carb and started it up. It wouldn't idle with the curb screw set at this position. Turned in in until I got the recommended 850 rpms. Started adjusting the idle mixture screws and ended up turning them in to where they were before 1/2 turn + 1/16. I'm afraid the transition slot will be right where it was before. The idle bleed screws were much more responsive than they were before.

Took several test runs and the A/F is a little worse than it was before. The off idle bog returned but not as bad.

Decided to adjust the secondary screw I put in. I was expecting this to increase the idle rpms by letting some air in from the secondary side. there wasn't much change at all. I ended up reversing the changes I made to this setting.

Tomorrow, I'll pull the carb to see where the transition slot is and put in some new plugs.
 

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when you opened the secondaries to increase idle speed you didn't realize that the fuel was increased also through the tip in circuit in the secondaries. after opening the secondaries you need to turn in the idle mix screws to make a change. sometimes the tip in circuit is too rich(wrong booster assembly). nothing is as it seems with a carb
 

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Discussion Starter #19
when you opened the secondaries to increase idle speed you didn't realize that the fuel was increased also through the tip in circuit in the secondaries. after opening the secondaries you need to turn in the idle mix screws to make a change. sometimes the tip in circuit is too rich(wrong booster assembly). nothing is as it seems with a carb

'nothing is as it seems with a carb' The most truthful statement all day.

Hmmm. I got it wrong again. I though opening the secondary screw gave it more air. I wasn't aware it gave it more fuel too. The idle screws are already set at 1/2 + 1/16 (pretty tight) so I don't see any benefit to opening the secondaries if that add more fuel. I don't understand why this engine needs the transitions slots open so much to idle at 850.

Thanks for the great comments.
 

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Discussion Starter #20
you need to use a vacuum gauge to set idle mix. not typical for both screws to be the same. without vac. gauge, turn each in to a rpm drop. (lean drop). then backout 1/2 turn. do for each side over and over. don' t worry about them being the same

I have been using the vacuum gauge. The highest reading I get is 15 inches. I can't really turn them back out a 1/2 turn from the stumble point. The A/F ratio keeps getting richer with every small turn I make. Thanks again.
 
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