Got the wideband hooked up and need some help adjusting it. While just using the primaries it runs rich about 11.8-12.5. When I hit the secondaries it goes lean real quick to about 15 and stays in the upper 14s. I'm shooting for 13.5, cause that’s what I've been told is safe although 14.7 is perfect. I've already lowered the primaries to a 73 jet down from 75. I haven’t done the secondaires yet (laptop connection problems) but I think I have something like 82-85 jets. Can I go lower on the primaries so its not so rich under normal driving (you can hear its rich) and raise the secondaries? I have #31 squirters, which came with the carb. Do I need a bigger squirter on the secondaries because it goes lean right when I mash it? 750 Demon mech. secondaries. When I went from 75 down to 73 it hesitates more when the secondaries kick in.
Carb jetting for A/F ratio
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ddonaca351
Active Member
Not too sure about deamons (never owned one) but if they are like holley's (what the basic design is from) you are on the right track with the primaries... keep droping jets to what you want for a/f
OH!!!!!! by the way, 14.7 IS NOT perfect... it is theoretic perfect, but in actual practice 15s are good for emissions (in the 14s is good too) but BAD for power and detonation, for optimum power you want to be about 12.5-13:1 n/a, or down around 11.5-12:1 to be safe with a power adder.
Now back to the symptom, to get over the initial lean condition see if the accelerator pumps have an external cam like the holleys do (little plastic fella on the side of the linkage) they can be changed to either of two positions (marked with a 1 and a 2) on the cam, or they can be swapped to another cam all together. This allows you to tailor the profile of the cam (how much how soon, versus over how long) the accel pump shot comes in. If you get a big lean ****e right away that tapers off quickly to an overall average lean level (caused by too small of secondary jets) then try a cam with a more agressive profile. More shot at the start of movement, but shorter duration. This will overcome the initial lean ****e, then by fattening up the secondary jets you'll cure the overall lean condition under heavier loads.
NOTE: also check the accel cam linkage to make sure it is adjusted properly, very little (VERY LITTLE) if any play, before it starts to actuate the accel pump. If the pump is delayed in actuating by loose linkage it can cause a lean ****e too.
IMHO... I would aim for about 14-14.5-1 while light throtle cruise (for good emissions and mpg), and quickly changing to 13.5 as throtle is applied to avoid "tip in" detonation in hot weather, to about 12.5-13:1 at WOT. This is more or less the profile that I aim for on my computer in my 5.0 (the 70 doesnt have wideband) and it seems to work well.
NOTE: iron versus alloy heads, static compression, cam profile and how much ignition lead you run will also play a major role in avoiding detonation.
Hope this helps...
Dave-
OH!!!!!! by the way, 14.7 IS NOT perfect... it is theoretic perfect, but in actual practice 15s are good for emissions (in the 14s is good too) but BAD for power and detonation, for optimum power you want to be about 12.5-13:1 n/a, or down around 11.5-12:1 to be safe with a power adder.
Now back to the symptom, to get over the initial lean condition see if the accelerator pumps have an external cam like the holleys do (little plastic fella on the side of the linkage) they can be changed to either of two positions (marked with a 1 and a 2) on the cam, or they can be swapped to another cam all together. This allows you to tailor the profile of the cam (how much how soon, versus over how long) the accel pump shot comes in. If you get a big lean ****e right away that tapers off quickly to an overall average lean level (caused by too small of secondary jets) then try a cam with a more agressive profile. More shot at the start of movement, but shorter duration. This will overcome the initial lean ****e, then by fattening up the secondary jets you'll cure the overall lean condition under heavier loads.
NOTE: also check the accel cam linkage to make sure it is adjusted properly, very little (VERY LITTLE) if any play, before it starts to actuate the accel pump. If the pump is delayed in actuating by loose linkage it can cause a lean ****e too.
IMHO... I would aim for about 14-14.5-1 while light throtle cruise (for good emissions and mpg), and quickly changing to 13.5 as throtle is applied to avoid "tip in" detonation in hot weather, to about 12.5-13:1 at WOT. This is more or less the profile that I aim for on my computer in my 5.0 (the 70 doesnt have wideband) and it seems to work well.
NOTE: iron versus alloy heads, static compression, cam profile and how much ignition lead you run will also play a major role in avoiding detonation.
Hope this helps...
Dave-
I changed the secondaries from 82 up to 85 and didn’t notice much of a difference in the ratio. The hesitation is much better but it is still is lean when I mash wide open. Also my vacuum dropped from 14 to 11 when I changed the jets this time. Is this normal? From what I read it sounds like the accelerator pump cam only delays it in other spots. I'm not sure what number mine is on but its red. So I guess I need a different cam or squirter. I also can adjust the butterflies on the secondaries and am not sure where that should be set. This is harder than I thought it was going to be but kinda fun. I don’t have time to work on it this weekend but maybe I can post a graph so you can see what’s going on.
ddonaca351
Active Member
We just did a thread on here like two weeks ago explaining tuning a holley, so try a search...
Some general things to remember:
- If you move on to step Y before step X is tuned correctly, you are only covering up the problem, not fixing it, and it never will run just right that way....
But here goes some quick stuff.
- For ease of tuning the secondary butterflies should be just cracked a hair (get a holley tuning book for initial setup of your particular carb), the primaries should be set inbetween the idle circuit discharge port and the transfer port, if they are open too far, you will be pulling fuel through the jets at idle (NOT GOOD)
- Start with the idle mixture screws, (about 1.5-2 turns out is agood place to start), and float levels (fuel juuuuust to the bottom of the sight plug), BEFORE ANYTHING ELSE
once these are good (and only after these are good) then move on to:
- primarie jet size while light throtle cruise and the accel pump linkage and cam profile (remember there is a 1 and a 2 setting on each pump cam) to eliminate the stumble on throtle opening. IF... thats IF you try to use too big a jet to overcome the wrong accel pump settings your mileage will SUCK, and the engine vacuum will go to hell, (sound like any engine you know?)
- once the primaries are ok at light cruise, and the stumble is tuned out with accel cams and possibly squirters (although this is not usually needed), then move on to the secondaries.
- If as you tune the secondary jet sizes up to get the desired a/f ratio, if you develop a stumble on accel, then you may need to back the accel pump cam off just a bit, but this is rare.
Oh usually (not always, but usually) if they say "lays down" it means its lean, if they say "stumbles" then its rich... just fyi.
And most of the time, more vacuum = more efficient (better tune)
Ok, so
1 get a book,
2 do a search on here,
3 go tuning!!!!!!!
Hope this helps,
Dave-
Some general things to remember:
- If you move on to step Y before step X is tuned correctly, you are only covering up the problem, not fixing it, and it never will run just right that way....
But here goes some quick stuff.
- For ease of tuning the secondary butterflies should be just cracked a hair (get a holley tuning book for initial setup of your particular carb), the primaries should be set inbetween the idle circuit discharge port and the transfer port, if they are open too far, you will be pulling fuel through the jets at idle (NOT GOOD)
- Start with the idle mixture screws, (about 1.5-2 turns out is agood place to start), and float levels (fuel juuuuust to the bottom of the sight plug), BEFORE ANYTHING ELSE
once these are good (and only after these are good) then move on to:
- primarie jet size while light throtle cruise and the accel pump linkage and cam profile (remember there is a 1 and a 2 setting on each pump cam) to eliminate the stumble on throtle opening. IF... thats IF you try to use too big a jet to overcome the wrong accel pump settings your mileage will SUCK, and the engine vacuum will go to hell, (sound like any engine you know?)
- once the primaries are ok at light cruise, and the stumble is tuned out with accel cams and possibly squirters (although this is not usually needed), then move on to the secondaries.
- If as you tune the secondary jet sizes up to get the desired a/f ratio, if you develop a stumble on accel, then you may need to back the accel pump cam off just a bit, but this is rare.
Oh usually (not always, but usually) if they say "lays down" it means its lean, if they say "stumbles" then its rich... just fyi.
And most of the time, more vacuum = more efficient (better tune)
Ok, so
1 get a book,
2 do a search on here,
3 go tuning!!!!!!!
Hope this helps,
Dave-
TFOB
Founding Member
Demon has a book on how to tune the Demon Carbs.
With a hesitation when secondaries kick in Make sure your rear butterfly is set so the transfer slot underneith looks like a little square.
With a hesitation when secondaries kick in Make sure your rear butterfly is set so the transfer slot underneith looks like a little square.
BlueMonster65
Founding Member
start at the beginning, check float level, fuel pressure then after you set the idle transfer slot opening to .020" per demon specs. and adjust the idle mixture screws to obtain the best vacume reading, make sure your idle is where you need it to be say 900rpm by using the idle eaze circuit., then you can start to mess with the accel pump cam. demons come with the pink cam which when on position 1 delays the discharge the most. you can try switching to position 2 but you will most likely need an orange or green cam. these cams provide a greater initial discharge and taper off. where the pink is more gradual.
you may need to change to a different value powervalve to make up for the time it takes to transition from idle circuit to main circuit. you could need to go lower or higher it all depends on what vacumme your engine is making. in theory the secondary jets should be 8 sizes more than the primarys. the powervalve along withthe accel pump will provide the increase in fuel until the carb completes the transition to the main circuit where the jetting takes over.
you may need to change to a different value powervalve to make up for the time it takes to transition from idle circuit to main circuit. you could need to go lower or higher it all depends on what vacumme your engine is making. in theory the secondary jets should be 8 sizes more than the primarys. the powervalve along withthe accel pump will provide the increase in fuel until the carb completes the transition to the main circuit where the jetting takes over.
thehueypilot
Active Member
I have a wide band "Innovate" air/fuel meter and it takes the guess work out of choosing the jets sizes. I have a 600 vac sec that I use just to cruise with and I bring the idle up in the garage to 3k and read the meter. For cruise I like about 13.5:1 (anything higher will cause high speed surging on my engine) on the primaries and add eight to that for the secondary jet size. The jet size for the 600 is 62 pri and 70 sec. For racing I do the same thing but I am looking for about 12.0:1 in the primaries and add eight to the secondaries. My meter will record multiple passes so I can fine tune from there to average 12.0:1 throughout the entire run. The 850 w/vac sec is 74 pri and 82 sec. The 650 DP is 66 pri and 74 sec. Every engine will be different but it sure makes adjusting the carb a lot easier.
I am using the Innovate meter also. I have never heard of the difference between the primaries and secondaries being 8 jet sizes. I do not think I will be able to even come close to that. Right now I am at 73/85 which is 12 different. Looking back at the graphs I guess its okay right now, about 13.8 at wide open. I want to go leaner on the primaries (11.9 cruising) but just the slightest acceleration, only using the primaries, from a constant speed it goes to 13.5, is this normal? Ill have to go richer on the secondaires if I bring down the primaries. I know how to do all the first steps like set the fuel bowls, pressure is at about 6.2, adjust mixture screws to max vacuum, my butterflies are at the right place at idle (I think) I know a lot of people say that a 750 is too big for a 351 is that why I cant get only 8 jets different?
thehueypilot
Active Member
You should be adjusting your A/F at idle to the meter readout and not to vacuum. At idle I adjust mine until I get about 13.5:1 and 900 rpm. Raise the engine speed to 3k to get your primary A/F reading to get your primary jet size. The eight numbers is a rule of thumb but with the A/F meter you can dial your sec in by reviewing your readout. Don't try to read the A/F during a hard run. Use the record function to review your progress. It may take a few runs to get dialed in but the record fuction make it easy. I also use the iductive clamp to read rpm's during the run so I can pin point my shifts to the A/F readings.
Well I've played with it some more and am now at 73/85 with the jets but still have problems. I tuned it during the day when the engine is hot and air temp is high 70s. During the day when I mash on it, it goes, hesitates, and then goes. At night when the secondaries are barely open and hold it there it lightly pops (out the exhaust) and sputters and the A/F is at 14.7-15. Mine does not like anything above 14.5. But at night when I mash it it is awsome! The A/F is 13.5-13.7. It is defiantly faster with this setup, day and night. I am going to raise the primaries again but under a steady cruise with the 73s in the A/F gets as low as 10.8. I tune it to about 12.5-13 at idle, in gear. Any leaner and it wont idle that well. It just seems like once I get either the low-end dialed in then the top end sucks and same with the other way around. From day to night it acts totally different if I tune it to the wideband.
thehueypilot
Active Member
I have no experience with Demon carbs, but from what I understand they are basically a Holley design. When your car takes off good then hesitates it sounds like the accel pump nozzle is too small to keep up with the demand. You might want to increase the primary squriter by two numbers. The accel pump cam may need a shorter duration profile. What color is it? At cruise 10.8 is way too rich. It may be the power valve coming in too soon causing too rich of a mixture. It should be sized for cruise vacuum minus two numbers. Make sure your float level is adjusted correctly because if it is too high the heat will make it expand and overflow into the vent opening causing a rich mixture. Carbs........you got to love them!
I think I got it right this time. All I did from yesterday is increase the primary jet size from 73 to 75 and all my problems went away and the A/F ratio is good. It works well in the day and at night when it gets pretty cool. I guess I ended pretty close to the 8 jet difference rule any way, I'm at 10. The only thing is that it surges at idle at night although it’s always done this but it used to do it when it got hot, not cold. Thanks for all the help!!
Now if I could only get the RPM thing of the reader to work correctly I could have a nice graph. Have you had any problems with it Huey? The RPM gets all jumpy when I get into the higher RPMs like it has a bad ground. I have it soldered in to my MSD/tach pickup and my tach works fine. When It does work its off by about 500 RPM on the top end. When I calibrate the RPM I have to go to 1500, not 2000 like it says or else its way off. The RPM reading will also freeze on a regular basis when I have to laptop in the car with me.
Now if I could only get the RPM thing of the reader to work correctly I could have a nice graph. Have you had any problems with it Huey? The RPM gets all jumpy when I get into the higher RPMs like it has a bad ground. I have it soldered in to my MSD/tach pickup and my tach works fine. When It does work its off by about 500 RPM on the top end. When I calibrate the RPM I have to go to 1500, not 2000 like it says or else its way off. The RPM reading will also freeze on a regular basis when I have to laptop in the car with me.
thehueypilot
Active Member
I am using the inductive clamp for reading rpm from an HEI system. I wired my meter directly to the battery because of the electrical noise I was getting though the cig. lighter. My graphs are pretty clean after I did this with only occasional spikes. I was told that a lap top would not calibrate the rpm's though their preferred method. I calibrate mine on the car by raising the RPM's to 2K while it senses it from the inductive clamp. Mine is very accurate and I can measure each shift point within about +/- 10 rpms. The Innovate forum was helpful for any issues as the company answers most questions. See attached chart for sample run. I record each run during a race to evaluate later.
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