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Not trying to get off track, but in your 1st post you said the float level is set at just below 1/2 way on the sight window. The float level should be set at the bottom of the window with gas just dribbling out when the window is removed. I think your float level is too high.
Duane
Not trying to get off track, but in your 1st post you said the float level is set at just below 1/2 way on the sight window. The float level should be set at the bottom of the window with gas just dribbling out when the window is removed. I think your float level is too high.
Duane
Carbs with sight holes are to the bottom of the hole.
Carbs with a sight glass are halfway up the porthole….the porthole glass is bigger in diameter and slightly lower if you were to compare the two.
Most fluids in a container have a "meniscus" caused by surface tension where it looks like the fluid has crept up the side of the container. This effect is not the true level. The sight glass in the carb is no different. So when the level is half way up the glass, the true level inside the bowl is lower and thus set correctly.
Some really interesting tuning tips with this thread and other research.. Be interested if the pros agree these are correct.
1) You don't chase a near stoich (14.7) afr for idle with fuel screws. Have never had much luck with vacuum or rpm as never could really determine true changes in either with larger than expected turns on four screws. Have been playing with one simple tip which is you turn them down (lean) to make your engine "angry," then back them off eighth turns equally until off idle throttle response when reving and driving is strong. Simple idea but has helped me. So many opinions on this.
2) Idle air bleeds don't necessarily determine fuel screw turns at idle. Was worried about running them with only 3/4 out and went from a 71 idle air bleeds to 75. Noticed a very lean spot right at light throttle, just maintaining coasting speed. So afr of 12.5 at idle/true coasting, up to 15-16 at throttle to maintain coasting speed, then 12.8 to 13.5 or so when going to 1/4 throttle to mid. Learned on another forum the lean spot is the air bleeds, with recommendation to take air bleeds for this carb down to as low as 62s and start from there. Will check afr at WOT for secondary jets this weekend.
Next steps:
Air bleeds from the new 75s back to the stock 71s (per advice here) - check afr again at "coasting" throttle. Won't worry about going smaller (richer) if all works OK.
Stop losing sleep over 12.5 afr at idle. BTW.. even with this, no significant black smoke, smell, eyes burning, etc.. Seems OK.
Swap out pink cam on primary with orange or green. Head scratcher the orange has a lip the others don't for some reason. See if I can keep play out of acc pump arm but still get .012 (not .015) clearance both sides.
Keep stock 70 jets in primary
Test 76s now in secondary with 3/4 and WOT runs
Take out 35 pump nozzles replace with stock 31s. Will prolong shot, which could help the transition time esp. in secondary side.
Will report back on status. Know this is a lot of detail, but perhaps it will help someone else as well.
Can you stall the engine by turning in any one of the idle mixture screws? If you can't, your transition fuel circuit (IFR (idle feed restrictor) being too big maybe and needs to be reduced using a "paper staple" for example) needs attention which is top of the tuning list as it is part of the idle circuit. Even when the transition slot is set correctly, too much fuel can still be pulled affecting the idle quality. This will affect your idle afr. Idle afr is not worth fussing over really (except for the IFR) but it shouldn't be below 14, imo.
Air bleeds should be the last thing you adjust and they don't go up or down a size or two like fuel jets. They go up and down like 10 sizes to fine tune and will effect the entire fuel curve which may not be desirable after all the other tuning is done. But if your a stickler type of guy, have at it. I haven't found any need to play with the air bleeds. The only afr's you really need to be concerned with are cruise and wot. Transitory afr's like acceleration are just that and should be checked for going too rich or lean.
Can you stall the engine by turning in any one of the idle mixture screws? If you can't, your transition fuel circuit (IFR (idle feed restrictor) being too big maybe and needs to be reduced using a "paper staple" for example) needs attention which is top of the tuning list as it is part of the idle circuit. Even when the transition slot is set correctly, too much fuel can still be pulled affecting the idle quality. This will affect your idle afr. Idle afr is not worth fussing over really (except for the IFR) but it shouldn't be below 14, imo.
Air bleeds should be the last thing you adjust and they go up or down a size or two like fuel jets. They go up and down like 10 sizes to fine tune and will effect the entire fuel curve which may not be desirable after all the other tuning is done. But if your a stickler type of guy, have at it. I haven't found any need to play with the air bleeds. The only afr's you really need to be concerned with are cruise and wot. Transitory afr's like acceleration are just that and should be checked for going too rich or lean.
Thanks. On this carb, the IFRs are pressed in.. but will try your test. Yes, air bleeds back to stock, and think it should run well. Feels pretty close now.
Yes, they are pressed in on mine also. I had some idle issues too like the eye watering syndrome. So I used a small strand of wire about 0.020" diameter to put in the restrictor. A common fix for Holley's.
I had a hesitation also and thought I had it licked with a 37 squirter and pink cam but it still seemed a bit sluggish or not a crisp throttle response. I put the orange cam in #1 hole and started going down in squirter size to #28 which started hesitate again. Went back up to a #30 or 31. Just nice!
Can you stall the engine by turning in any one of the idle mixture screws? If you can't, your transition fuel circuit (IFR (idle feed restrictor) being too big maybe and needs to be reduced using a "paper staple" for example) needs attention which is top of the tuning list as it is part of the idle circuit. Even when the transition slot is set correctly, too much fuel can still be pulled affecting the idle quality. This will affect your idle afr. Idle afr is not worth fussing over really (except for the IFR) but it shouldn't be below 14, imo.
Air bleeds should be the last thing you adjust and they don't go up or down a size or two like fuel jets. They go up and down like 10 sizes to fine tune and will effect the entire fuel curve which may not be desirable after all the other tuning is done. But if your a stickler type of guy, have at it. I haven't found any need to play with the air bleeds. The only afr's you really need to be concerned with are cruise and wot. Transitory afr's like acceleration are just that and should be checked for going too rich or lean.
If you have a friend who plays the guitar, ask him/her if you can have their old guitar strings next time they put new ones on. They are great for cleaning those tiny little passages in carburetors, among other devices, and you can also use them to change the effective area of the idle feed restriction in the metering block, since there are a variety of different diameters for the strings.
I am referring to over-all throttle action. From idle to WOT. Primaries open then LATER the secondaries. So to me, that is not a faster rate. That is a delay.
What you are referring to is ratio, not rate.
LOL! I think I understand what's going on here. I thought you were serious! Ya got me, man, hook line and sinker!
OP here. Well, the car runs pretty good now. Also got the timing curve dialed in, so now back to the carb. All relevant engine info in the first post.
To recap, ended up with:
70 primary jets (stock size)
74 main jets (stock)
Idle air bleeds back to the 71s (stock)
Nozzles back to 31s (stock)
Power valve is 6.5 (stock)
Primary acc pump cam is orange, secondary is pink
Transfer slots look “square” or .020
Idle mixture screws are just above ¾ turn out
Fixed the bog with proper acc pump linkage adj.
AFR meter says:
Idle: 12.4 to 13
Mid throttle with load: 12.8 to 13.8
Heavy throttle: 12.5 to 13.2
Only issue is lean condition at light throttle cruising. Just touching the pedal to maintain 20-30 mph or so the AFR goes to 15-16. Little throttle pressure and goes to ~13 or so.
Assuming this is the IFR transition… Confusing as the low number of turns out on the fuel screws would imply a rich IFR condition-so restrict them. But lean at very light throttle cruise would suggest opening them up. These are fixed in this carb.
Should I drill these out and not worry about the turns on the screws? Or put some wire segments in them to restrict them now - just to see? Thoughts?
OP here. Well, the car runs pretty good now. Also got the timing curve dialed in, so now back to the carb. All relevant engine info in the first post.
To recap, ended up with:
70 primary jets (stock size)
74 main jets (stock)
Idle air bleeds back to the 71s (stock)
Nozzles back to 31s (stock)
Power valve is 6.5 (stock)
Primary acc pump cam is orange, secondary is pink
Transfer slots look “square” or .020
Idle mixture screws are just above ¾ turn out
Fixed the bog with proper acc pump linkage adj.
AFR meter says:
Idle: 12.4 to 13
Mid throttle with load: 12.8 to 13.8
Heavy throttle: 12.5 to 13.2
Only issue is lean condition at light throttle cruising. Just touching the pedal to maintain 20-30 mph or so the AFR goes to 15-16. Little throttle pressure and goes to ~13 or so.
Assuming this is the IFR transition… Confusing as the low number of turns out on the fuel screws would imply a rich IFR condition-so restrict them. But lean at very light throttle cruise would suggest opening them up. These are fixed in this carb.
Should I drill these out and not worry about the turns on the screws? Or put some wire segments in them to restrict them now - just to see? Thoughts?
Where did you set the secondary butterfly stop screw at? You want the secondary butterflies set with the same visual light gap as the primaries once they are set right. (when looking at the transition slot). Set the primaries first then hold the bottom of the carb up to the light and set the secondary stop screw so the light gap looks the same on primary and secondary. This is the base line setup.
OP again with an update. After much wrestling, was still not happy with the tune. Issues:
Must always pump the throttle a few times when starting fully warm
Was running really lean at tip in. Tiny bit of stumble. AFR showed a lean spike at tip in, but then right back down to 12.5 or so.
On a mission to solve these. Found two interesting things.
Further playing with the idle mixture screws, hooked up vacuum gauge and found my vacuum doesn't change much with the primaries, which still like to be about 3/4 turn out. But, on the secondaries, it was making a big difference at approx. 1.25 turns out. Got up to around 18-19" of mercury from 13-14.
Worst mistake - checked transfer slots again and looked fine until i realized that my upside down carb had the e-choke on and fast idle engaged.. Ugh. So disengaged the fast idle and no transfer slots on either set of butterflies. Had initially opened up the secondaries a bit to try to uncover those t-fer slots, but that created an idle so high that had to close the primary butterflies to lower the idle speed and then covered those t-fer slots. The fact that idle speed and t-fer slot adjustment is one in the same is a challenge for me. Automatic means two idle speeds to make happy, one in N and the other in D.
So, backed off secondary butterflies to where the adjustment screw is hardly touching the arm with no transfer slots showing, but now have good t-fer slots in the primaries and good idle speed. That fixed the light stumble on tip in. But, now really rich at tip in.
Chasing the rich issue:
Primaries jets down to 69 which feels small for a cammed 383. Still sitting at mid 12s AFR even at 1/4 throttle, etc. which still seems rich
Replaced the 71 idle air bleeds with 75s to lean out the idle circuit. Didn't really do anything.
Still playing with fuel screws and thinking having the secondaries about 1.25 turns out, where my idle speed and vacuum is highest, could be creating the rich condition
Questions:
Should the four corners idle mixture screws need to be the same? All guidance is to do them all the same, but my vacuum gauge tells me different.
Would the main air bleeds have any effect on idle or tip in? Or even through 1/8-1/4 throttle AFR?
Is it time to revisit the Idle Fuel Restrictors, which on this carb are pressed. Not sure if opening them up by drilling, or putting in wire and closing them down would be a good next step.
To recap, ended up with:
69 primary jets (one below stock size)
74 main jets (stock)
Idle air bleeds up to 75s from stock 71s
Nozzles at 31s (stock)
Power valve is 6.5 (stock)
Primary acc pump cam is orange, secondary is pink
Transfer slots "square” in primaries, no transfer slots in secondaries
Idle mixture screws are ¾ turns out, just over 1 turn out secondaries
Timing is 15 at idle, 25 with 10* of manifold vac adv, all in at 36 (no vacuum).
Trying to solve for overly rich tip in and hard starting.
Your Holley has a lot more adjustments available as compared to the older HP carbs. Some of the mods people do are to make changes on carbs without changeable bleeds. I have used wires in the transition circuits before on carbs that were jetted close but ran pig rich at idle or cruise. The carb idle air fuel ratio might be fine but the cruise can be fat. The wires in the block can bring back adjustability if the circuit is just too rich for your application. Old school mod.
You have 4 corner idle adjustment in your setup. You want to have all 4 of the mixture screws pretty close to each other. You dont want 2 fat ones in front and 2 lean ones on the rear - might idle well because OVERALL fuel total is good BUT but you can go rich at cruise when you are pulling cruise fuel from the primary slots (for example). If you start with all of them baselined and have all 4 butterflies set the same on the transition slots EDIT (primary butterfly set to form a square transition slot - secondary butterfly stop screw set so you can match the light gap to the primaries when looking up through the butterflies). You can set the idle mixtures first. Your primary transition slots are where much of the additional fuel will be pulled from at tip-in and cruise. You can lean out that circuit if needed with ALL 4 of the mix screws together. This will affect idle and cruise as well. If you end up going lean at tip-in, you may need to work on filling in the hole with the accelerator pump shot. Bigger Nozzles shoot all the gas quicker. Smaller nozzles shoot for a longer time. If it goes fat then lean when you accelerate you can do a smaller nozzle to stretch out the length of the shot.
The secondaries are less of an issue. Spend more time on the primary side getting the idle and off-idle right first. After that it gets much easier to work on the secondary side.
Leave the butterflies showing a square transition slot.
Tiny wire (staple) in the idle fuel restrictors and reset the mixture screws. Less fuel pulled through the transition slots at tip-in.
You are supposed to pump the gas once or twice before starting a cold carbed engine. Hot start should be easy without a pump.
I sort of agree with this diagram. Maybe it helps.
"Power" valves should probably be called something else like maybe "load" valves. They are very misunderstood because of the name.
Put a copper wire strand in each of these 2 holes. A staple works too.
I made this suggestion in an earlier post, but it was several months ago, so I'll re-iterate it here: if you can get your hands on a set of cast off guitar strings, they come in different diameters. You can measure the width of the orifice you're trying to tune and using some simple math, figure out what its area is, and by measuring the diameter of the guitar string, you can figure out how much you are changing its effective area by inserting the wire into it. It's a tuning trick that I have found very useful over the years. Also, they are really great for cleaning all those tiny little passages in a carburetor when you are doing a rebuild. That right there makes it worth getting your hands on a set. Guitar players usually have no use for old strings, and will be happy to give them to you.
What MPH & RPM are you measuring your cruise AFR at?
I had a lean tip in that took me a while to resolve : PV wasn't opening soon enough, I had to go up to a 10.5 PV. I have about 20hg vacuum cruising like you do as well.
04-26-2022, 08:51 PM
