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Discussion Starter · #61 ·
This is where the carb was for the test drive above:
30 IFR's
70 IAB's
31 MAB
66/72 MJ
3/8 turn out on IMS

I'm thinking that I can probably fix the little lean hesitation by turning the IMS's out an 1/8 turn. That has worked before. Other than that little hesitation, it runs great, and this is probably the best economy (MPG) setup I could have on the idle/trans circuit. I need to figure out how to lean the 3000 rpm cruise without leaning the idle or transition circuits. I would like to see at least 14:1 at 3000 cruise. I might go ahead and swap out the mains for 64/70 just to see what happens. 64 is the smallest jet I have. I don't want to get this thing detonating. I am afraid I wouldn't hear it. What would a bigger MAB do in this situation?
I filled up the tank and recorded the mileage, so I can start keeping track of the MPG's. Before the rebuild it was in the 12-14 MPG range. I'm willing to bet it is better now.
 

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Try the 64 jet, or even 62 if you can, just in the primary and leave the secondary jet at 72. Just for cruise testing, if it helps you can then open up the size of the Power Valve Channel Restrictions(PVCR's) to get the combined jetting for WOT back to where it needs to be on the primary side.
Secondary side you aren't even using at 3000 rpm cruise, so jet change accomplishes nothing there..

You didn't happen to note the size of the primary block's PVCR's when you had the block off for the other mods??
 

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I think ? its in Dave Emanuele's book , he talks extensively about not only how 1 change can upset others but also the the " hole size ,length of bore , bore chamfer" affects fuel flow amounts . i.e. a .060 orifice .030 long will flow more fuel than an .030 orifice that's .060 long . then the taper of the chamfer changes flow again , pretty complicated stuff to be messing with , without a wet flow bench
 

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Discussion Starter · #65 ·
Try the 64 jet, or even 62 if you can, just in the primary and leave the secondary jet at 72. Just for cruise testing, if it helps you can then open up the size of the Power Valve Channel Restrictions(PVCR's) to get the combined jetting for WOT back to where it needs to be on the primary side.
Secondary side you aren't even using at 3000 rpm cruise, so jet change accomplishes nothing there..

You didn't happen to note the size of the primary block's PVCR's when you had the block off for the other mods??
Hi ericnova72. 64 is the smallest MJ I have. Summit has lots of them though. I will try my 64's in the primaries this morning.
The PVCR's are .052 in the primary. No secondary PV. How many steps in size should I use for tuning PVCR's?
I always appreciate your thoughts.

I think ? its in Dave Emanuele's book , he talks extensively about not only how 1 change can upset others but also the the " hole size ,length of bore , bore chamfer" affects fuel flow amounts . i.e. a .060 orifice .030 long will flow more fuel than an .030 orifice that's .060 long . then the taper of the chamfer changes flow again , pretty complicated stuff to be messing with , without a wet flow bench
I read somewhere that you could take the main jets out completely and still be able to drive the car on the idle and trans circuits. I read in several places that the main jets do not affect the idle. So I was not expecting that much change in the idle/ trans circuit. But there it is!
I am really leery of drilling my own jets for just the reason you stated. I have been using only factory drilled and size verified (by me) jets. That is why I have not moved the IFR to the lower position. I think the plan there is to get all my carb tuning done with factory drilled stuff first. Then do the low IFR mod with the drilled set screws. Then I will know where to go with the second try with drilled IFR's.

I'm getting closer and closer, and I am really enjoying tuning this carb. My carb knowledge has increased exponentially. Too bad I didn't learn all this 50 years ago.

It's going to rain in a few hours, so I think I will change that primary MJ quick and go for a ride before the rain.
 

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When I was tuning my old single carb 350 SBC with a 3310 holley , 64 primary was as small as I was able to use , a 62 would lean stumble off idle ,so there's another case where the main jets affected the overall mixture , enough that it affected drivability ..
As a side note , & I'm not discounting your effort in any way , USUALLY , holley set up from the factory is very close , AFA AFR & circuit timing , albeit , usually a bit rich . I will agree , that given the time & resources , it is fun to try to make them better !
 

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Discussion Starter · #67 ·
Back from another test ride. Only change was to 64 MJ's. Leaned everything again. Idle was a couple tenths higher. 1800 cruise was a few tenths higher. 3000 cruise really leaned out to 15 +. There is now a lean surge. I can tell it is running lean. So 64 is too far. There was a big jump in AFR by changing from 66 to 64 MJ's. I think I will put the 66's back in and go for another ride, just to see a back to back test with the same weather conditions. 66's performed well yesterday. Different weather. I will also richen the IMS's an 1/8 turn to see if that helps with the slight off idle hesitation I had with the 66's on the last test. I expect the 3000 cruise to be rich again though. Maybe I need to try 65 MJ's.
When I was tuning my old single carb 350 SBC with a 3310 holley , 64 primary was as small as I was able to use , a 62 would lean stumble off idle ,so there's another case where the main jets affected the overall mixture , enough that it affected drivability ..
As a side note , & I'm not discounting your effort in any way , USUALLY , holley set up from the factory is very close , AFA AFR & circuit timing , albeit , usually a bit rich . I will agree , that given the time & resources , it is fun to try to make them better !
Yep! I can testify to the MJ's affecting all ranges of carb operation.
I also agree that Holley carbs come setup pretty close. As many Holleys as I have had over the years (decades :) ), I have never done much more to them than change MJ's and adjust IMS's. And they all ran good. But these QFT carbs can be custom tailored to your specific application. I never got great MPG's out of them.
 

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I am glad your getting close to your new carb setup and very close to where you want to be. I for the first time found out just a few thousandths of an air bleed size can make a carb tune good or to lean. I had a nice carb put together last year which I use only during the cruising times but have a clunker carb to use during the winter months because of my truck sitting and still having something on it just in case I have to drive in an emergency situation.

I did not want my good carb to be just sitting on my truck for over three to four months without running and the fuel sitting inside and end up gumming up as I did not want to take the carb off and drain the fuel and have it sit dry and end up with the gaskets and needle and seat sticking. I know you can just undo the fuel bowl bolt at the bottom and drain it that way without taking it off but I actually torque my fuel bowl bolts and with my Hei in the back I can't get my hand torque screwdriver which is used on guns and stuff.

I had a .074 primary idle air bleed and on the secondary side I wrote down .049 and I thought the carb would run good or be very close but when I put the carb on I could hardly drive the truck and it would stumble and hesitate and seem like it was starving for air and was real low on power. I could not for the life of me figure out why it was running so bad when the clunker carb was setup all the same except on the secondary idle air bleed it was setup as .045 and the primary side was still the same and ran the same ifr sizes in the metering blocks.

I thought it must have been something ignition wise so I checked everything on my distributor and all checked out good and did not understand how it could be carb related. Well even after all these years of fine tuning, this was a first for me that an air bleed size made that much of a difference. I took out my drill bit gauge set and checked the rear idle air bleeds and it was about .052 or so and that was just enough to make my carb so lean that even changing out the ifr size up two sizes in the front from a .031 to a .033 still did not get rid of the stumble and hesitation and rough driving condition.

I had sold the original clunker carb on ebay to get my wife an alternator for her vehicle and the base plate throttle shaft went bad on the old carb and the cost of fixing it and other stuff I just decided to sell it and make some bucks to pay for her part as I already had plenty of extra holley parts laying around to put something else back together but that ended up as a whole other nightmare.


I finally took apart the carb and swapped out the main body with an identical one but with the same .074 primary idle air bleed and then with a .040 secondary idle air bleed and it then ran again and no more stumbles or gasping for air sort of deal and I swapped back to the .031 primary idle feed restrictor size and all is good and just a hair rich but running good again.

I ran a carb with a setup with the specs before on a 350 going back through my notes and did not think the 27 cubic inches of a difference of the engine would make that much of a difference but it did. What worked on my older 350 does not work on my 377 which is about to be gone. Hard to believe that when you get close with a good tune that just one air bleed change or a simple ifr change or jet change can make or break a tune just like that.

Normally on the air bleeds I have never had to change them but this was one rare case I did. Now I have to take that main body and drill out the holley pressed in air bleeds and replace them with brass set screws and drill them to size since I know what will work. Keep at it and you should be good to go.
 

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Discussion Starter · #69 ·
I made me a stubby nut driver this afternoon. Makes removing and installing fuel bowl bolts a lot easier.
IMG_0788.JPG
 

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Discussion Starter · #70 ·
Changed MJ's back to 66. Turned IMS's out to 1/2 turn out. Went for a test.
11.8 @ idle
14-15 @ 1800 rpm cruise
13 @ 3000 cruise
WOT is hard to take your eyes off the road to see :) . Around 10.5-11.
Different from the last test with 66 MJ's. Only other change is 1/8 turn out on the IMS's.
Going to have to drive it for a while like this. Start seeing what kind of averages I'm getting day to day. I'm wondering if going up on the 31 MAB's currently in it would lean the 3000 rpm cruise?
 

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Check at 3500, 4000, 4500 cruise where you are sure to be on the mains. If it stays pretty much the same AFR across the RPMs you have the correct main air bleeds and need different main jets to lean it out. If it gets richer with increased RPM, decrease the main air bleed size. If it gets leaner with increased RPM, increase the main air bleed size. Once you get consistent AFR across RPM, then adjust up/down to desired AFR with main jets. At least that's how I understand it.
 

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Discussion Starter · #72 ·
Check at 3500, 4000, 4500 cruise where you are sure to be on the mains. If it stays pretty much the same AFR across the RPMs you have the correct main air bleeds and need different main jets to lean it out. If it gets richer with increased RPM, decrease the main air bleed size. If it gets leaner with increased RPM, increase the main air bleed size. Once you get consistent AFR across RPM, then adjust up/down to desired AFR with main jets. At least that's how I understand it.
Thanks. I'll check that out on the next drive. It seems that I am running out of options to lean the 3000 cruise without running into lean surge.
 

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Discussion Starter · #73 ·
Spent yesterday afternoon working on the 600. I took the baseplate off and was able to even up the transition slots with a small file. I also drilled and tapped both metering blocks for the low IFR modification. I decided that it might be best to go ahead and do the mod as it seemed that I was at a dead end tuning wise.
Set up:
67/73 MJ's
28 IFR's (drilled 6-32 x 1/8" set screws)
70 IAB's
31 squirter
5/8 turn out on IMS

The changes from the previous test were 29 to 28 IFR's and the low IFR mod.
I could tell the low IFR mod at the IFR gauge. It seemed to steady out in all ranges. I liked that.
IDLE.....13.5-14
1800 cruise.....16-17, it would bury the peg at 18 on gentle acceleration and stumble slightly.
3000 cruise..... 14-14.5
WOT..... 13-13.5

Obviously pretty lean. But I think this is good news. I get to approach the problem from a different direction. This is as lean as I would ever like to get it. I would accept the idle and the 1800 and 3000 cruise numbers, although they could be slightly richer, but I need to get rid of the slight lean stumble. I also need to richen up the WOT.
It seems like a whole different carb by lowering the IFR's position. I'm really starting all over. I am debating whether to go back up 1 step on MJ's to 68/74 or try 29 IFR's. Knowing how the main jet affects all ranges, I'm kinda confused. I think I will try the MJ change first. I want to get rid of the stumble first. If the WOT is still lean, maybe I can get it back with bigger PVCR's.
Time for another change and another test
 

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Lean at 3000 RPM’s, a thought: first what is vacuum at the 3 grand RPM? What power valve and PVR? My though is at 3 grand you s/b in the main wells. Being that maybe a slight bigger primary main jet. While PV change to lower number and reduction PVR size to reduce flow and happen at wider open throttle. The idle passages s/b in action till something around 2300 to 2500. Then transaction to main well takes place.

Not saying this is the answer but worth a try.
 

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Discussion Starter · #75 ·
Changes made:
Up 1 step on MJ's from 67/73 to 68/74.

Current setup:
68/74 MJ's
28 IFR's (drilled 6-32 x 1/8" set screws)
70 IAB's
31 squirter
5/8 turn out on IMS

Results:
12.8 idle
16-17.5 @ 1200-1500 rpm cruise
15-16 @ 1800 rpm cruise
14 @ 3000 rpm cruise
WOT 12.8-13

Idles good, runs and transitions well. No stumbles. I could hear pinging before I rebuilt the engine with the aluminum heads. I am hearing no detonation now.
I would say I am good with the different cruise range AFR's.
The idle could be leaner, but I think I have given up on leaning that out. It just doesn't like it.
The WOT needs richened a little. I seems that at the initial hit of full throttle the power valve needs to open up quicker, too. 1800 cruise vacuum is about 15-16 inHg. Maybe take the power valve to 7.5 - 8 from 6.5. Not too sure on what size steps to take with the PV. I don't have any larger PV's, so will have to order. Maybe the PVCR's need to get bigger. Not sure on what size steps to take with the PVCR's either. I would like to see WOT AFR @ 11.5-12.
I have had this exact same setup in the carb before I made the low IFR mod. It is way different after the mod. It just seems to be better all the way around. More tunable. Glad I did it.
 

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Discussion Starter · #77 ·
I will have to go back over that part in Vizard's book. I have been spending a lot of time reading everything I can find on AFR tuning. It seems there are varying opinions on where WOT should be AFR wise. I was going by this chart I found on afrplus.com.

AFR Values & Characteristics in Four Stroke Engines
6.0 AFR - Rich Burn Limit (engine fully warm)
9.0 AFR - Black Smoke / Low Power
11.5 AFR - Best Rich Torque at Wide Open Throttle
12.2 AFR - Safe Best Power at Wide Open Throttle
13.3 AFR - Lean Best Torque
14.6 AFR - Stoichiometric AFR (Stoich)
15.5 AFR - Lean Cruise
16.5 AFR - Usual Best Economy
18.0 AFR - Carbureted Lean Burn Limit
22.0+ AFR - EEC / EFI Lean Burn Limit

I agree that I am pretty close. Maybe the best thing to do is to top off the tank and drive the car through a tank of fuel and check MPG's before deciding on any further changes. This car might go through 2-3 tanks of gas and not see WOT. It gets driven pretty easy, so I am not real worried about WOT AFR. I have it leaned out pretty good in my normal driving range. I'm anxious to see what kind of MPG's I'm getting now.
 

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Stop being concerned about the AFR meter readings and just go on how the car drives. If you like it.........leave it be. A carburetor is never going to be a compensator as with fuel injection. It has no ability to self adjust as you drive. You can only set it up as best one can test and change and retest till it is butt ass feels good to go. No 2 guys are going to like a set up the same. There for make the setting as you find it drives best. Ya might have to give and take to make the “favorite” set up work for the whole driving experience. When these cars were new the “setup” was a compromise. If you want it better than it is the whole engine (cam, compression, cubic inch, etc) may not be a perfect or even a good match to start with to get a perfect set up.

But we do enjoy sharing your experience and do not want to deter you in anyway. The journey is always better than just the destination. But in the end you will most likely be a compromise rather than the perfect setup.

I like the idea of parts rather than a computer program setting up an engine. Touch and feel is way better therapy. I will say again you are well out of the idle passages at 3k RPM’s and the main circuit is the main target since up til then everything else is fine. If you find it lean at that RPM perhaps it is running hotter as it would.
 

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I will have to go back over that part in Vizard's book. I have been spending a lot of time reading everything I can find on AFR tuning. It seems there are varying opinions on where WOT should be AFR wise. I was going by this chart I found on afrplus.com.

AFR Values & Characteristics in Four Stroke Engines
6.0 AFR - Rich Burn Limit (engine fully warm)
9.0 AFR - Black Smoke / Low Power
11.5 AFR - Best Rich Torque at Wide Open Throttle
12.2 AFR - Safe Best Power at Wide Open Throttle
13.3 AFR - Lean Best Torque
14.6 AFR - Stoichiometric AFR (Stoich)
15.5 AFR - Lean Cruise
16.5 AFR - Usual Best Economy
18.0 AFR - Carbureted Lean Burn Limit
22.0+ AFR - EEC / EFI Lean Burn Limit

I agree that I am pretty close. Maybe the best thing to do is to top off the tank and drive the car through a tank of fuel and check MPG's before deciding on any further changes. This car might go through 2-3 tanks of gas and not see WOT. It gets driven pretty easy, so I am not real worried about WOT AFR. I have it leaned out pretty good in my normal driving range. I'm anxious to see what kind of MPG's I'm getting now.
I've had mine on the dyno to tune carb a few times now. Mine makes best power at a little over 13.0 AFR on my gauge. However I can see by the spark plugs that mixture varies from cylinder to cylinder and I'm only reading the average of four. Who knows what it is on any one cylinder. All said and done, the only way to take out the guesswork and averages is port fuel injection - and it's just not worth the difference to me at this point.

Indicative of a Mixture Distibution Problem?

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Results:
12.8 idle
16-17.5 @ 1200-1500 rpm cruise
15-16 @ 1800 rpm cruise
14 @ 3000 rpm cruise
WOT 12.8-13
From that point right there, 3000 rpm ought to be up on the mains by then, so I would try leaning the primary main jets only to see if you can get the AFR to be more like the 1800 rpm readings. It may help your mileage.
With your vacuum readings I'd try a 9.5 or maybe even 10.5 power valve.
Once you get the 3000 rpm cruise and mild acceleration tip-in sorted out, then if it ends up lean at WOT you can increase the PVCR's to get it corrected.
Change in PVCR size is just a simple area calculation. PI x radius/squared.
Look up the drilled size of the Holley jets so you can calculate the area, note the change amount from the jet change and use that number to calculate how much bigger to drill whatever existing PVCR size is there now.
 
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