Tuning Street Demon carb

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MopaR&D

MopaR&D

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I bought a Summit brand kit of Holley carb jets for the secondaries on my Street Demon because I wanted to richen it up but something seems off. I went from the stock #86 to #88 jets and my AFR is about the same or possibly even leaner. Before that I had 54x38 metering rods and #87 jets in the primaries and at the same time I swapped the secondaries I went up to #91 primary jets. I started with 60x50 metering rods and that was still lean at WOT (high 13s) so went to 58x44 rods and it came down to low 13s, better but still lean. I find that strange because from the factory this carb comes with 56x40 metering rods, #89 primary jets and #86 secondary jets which I would think wouldn't be that lean.

I'm thinking A) some debris got into the carb when I reinstalled the fuel line as I have a homemade hard line going from the fuel filter to the carb and all I did for a "flare" was jam a tapered punch in the end and hammer it until the end opened up a bit; I'm afraid it sliced up the inside of the rubber line and some pieces got in. And B), my stock-replacement mechanical pump with 5/16" lines can't supply enough fuel for a 450-hp 360. It doesn't seem to be leaning out with RPM though, if I cruise in 2nd gear around 2000 RPM and then quickly floor it the AFR instantly goes to around 13.0-13.2 and stays there.

Should I upgrade the fuel supply before trying to tune the carb more? What would be the next step, adding an electric pump near the tank and a regulator near the carb? I'd prefer not to have to run a return line...
 
Bewy said:
Check the needles & seats in carb.
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I'm going to take the carb off and pull it apart, again... need to do that to change jets anyway. I'll try to blow out all the passages carefully with carb cleaner and see if any debris comes out.

This carb works nicely but every time I have to tune it a convention Holley double-pumper style carb seems more appealing. Such a pain.
 
MopaR&D said:
I bought a Summit brand kit of Holley carb jets for the secondaries on my Street Demon because I wanted to richen it up but something seems off. I went from the stock #86 to #88 jets and my AFR is about the same or possibly even leaner.
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Did you check the plugs? When you make jetting changes you should always check the plugs to make sure they correspond to what your 02's are telling you.
 
MopaR&D said:
my stock-replacement mechanical pump with 5/16" lines can't supply enough fuel for a 450-hp 360
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MopaR&D said:
Should I upgrade the fuel supply before trying to tune the carb more?
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I ran the 750 version on my stroker, in the 1/4 mile, with stock 5/16 line. At the time ran low 12’s @110ish no supply issues ever. Carter muscle pump, then a Carter electric (no regulator required version) simply because I wanted electric.
IMO, check all clean and Tune more.
 
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Hysteric said:
Did you check the plugs? When you make jetting changes you should always check the plugs to make sure they correspond to what your 02's are telling you.
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I didn't check them this time around but back in CO when I got the WOT tune dialed in to mid-12 AFR on the wideband readout I did check the plugs and they showed the tune was pretty much spot-on. I also ran the car on a chassis dyno where the shop plugged in their O2 sensor and the AFR readouts were the same as with mine. So I'm inclined to believe the O2 sensor is pretty accurate, I will still pull one or two plugs next time I drive it though. I just need to find a place where I can safely shut the engine off while at WOT and come to a stop, that way I make sure they aren't showing the ratio at part-throttle or idle.
 
12many said:
I ran the 750 version on my stroker, in the 1/4 mile, with stock 5/16 line. At the time ran low 12’s @110ish no supply issues ever. Carter muscle pump, then a Carter electric (no regulator required version) simply because I wanted electric.
IMO, check all clean and Tune more.
Click to expand...

I haven't ran my car at the strip since moving to sea level but I'd guess it runs mid-high 12s at 105-110 mph. Good to know the stock fuel supply should be sufficient. That also aligns with a video @318willrun did saying basically the same thing. It makes me wonder why the factory went to 3/8" line on engines that made less power like the 440 6-barrel, maybe because they wanted a margin of safety for people who would upgrade their engines?
 
Another issue that can change what would appear a lean to rich condition is a drop in output voltage from your alternator. The most important thing is to get the flame kernel started and the more "juice" you have to do that the better. I've seen a drop in only 0.5 volts change the combustion like I described above to what appeared to be a rich condition without changing any jetting whatso ever.
 
Hysteric said:
Another issue that can change what would appear a lean to rich condition is a drop in output voltage from your alternator. The most important thing is to get the flame kernel started and the more "juice" you have to do that the better. I've seen a drop in only 0.5 volts change the combustion like I described above to what appeared to be a rich condition without changing any jetting whatso ever.
Click to expand...

Duly noted, I am running a Mallory HyFire-6A box (bought when they were still their own company back in 2008) and Summit billet electronic distributor so I would think/hope that's not an issue but I'll keep an eye out for it.
 
MopaR&D said:
Duly noted, I am running a Mallory HyFire-6A box (bought when they were still their own company back in 2008) and Summit billet electronic distributor so I would think/hope that's not an issue but I'll keep an eye out for it.
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Find out what voltage it requires @ a minimum and make sure its getting it. Also use the coil they spec for it.
 
Hysteric said:
Find out what voltage it requires @ a minimum and make sure its getting it. Also use the coil they spec for it.
Click to expand...

IIRC it needs at least 11.5 volts or it won't fire, had it happen before with a bad battery and no-start condition even though it would crank over. Also when I installed the system I got the matching Mallory canister coil they spec'd for that box. When the car is running and alt is charging it's getting a bit over 14 volts according to my Auto Meter voltage gauge.
 
MopaR&D said:
IIRC it needs at least 11.5 volts or it won't fire, had it happen before with a bad battery and no-start condition even though it would crank over. Also when I installed the system I got the matching Mallory canister coil they spec'd for that box. When the car is running and alt is charging it's getting a bit over 14 volts according to my Auto Meter voltage gauge.
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I wouldn't trust 11.5 to be what it needs I'd be inclined to expect more like 14 volts. Is that 14V at Idle?

What alternator are you running?
 
MopaR&D said:
I haven't ran my car at the strip since moving to sea level but I'd guess it runs mid-high 12s at 105-110 mph. Good to know the stock fuel supply should be sufficient. That also aligns with a video @318willrun did saying basically the same thing. It makes me wonder why the factory went to 3/8" line on engines that made less power like the 440 6-barrel, maybe because they wanted a margin of safety for people who would upgrade their engines?
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there is a difference between running it wide open for 12 seconds or holding it wide open for a mile and a half. The factory didn't know how long the car would be wide open.
 
It is not a low voltage issue.
The instructions with the Mallory HyFire 6 state: 'This system delivers full voltage with a supply of 10-18v, & operates with a supply voltage as low as 8v.'

All of the 'box' igns [ Crane, MSD etc ] that I have seen work on the above voltages. I suspect they are internally regulated to 10v because some racers like to remove the alt & just use the battery as a power source. Preventing a problem before one is created...

These are CD igns & are not as sensitive to supply voltage as are inductive igns.
 
Bewy said:
It is not a low voltage issue.
The instructions with the Mallory HyFire 6 state: 'This system delivers full voltage with a supply of 10-18v, & operates with a supply voltage as low as 8v.'

All of the 'box' igns [ Crane, MSD etc ] that I have seen work on the above voltages. I suspect they are internally regulated to 10v because some racers like to remove the alt & just use the battery as a power source. Preventing a problem before one is created...

These are CD igns & are not as sensitive to supply voltage as are inductive igns.people far smarter than you beg to differ because they;ve tested it and
Click to expand...
I take it you've measured the flame kernel development with various ignition systems........people pushing the limits of this stuff wouldn't agree with you and I've seen a half a volt drop change combustion to the point where my 02's read rich and the plug read rich as well.

If you spent any time @ the Innovate forum before they shut it down it was discussed many times.
 
Hysteric,
He is not pushing any 'limits' here. His Mallory box could be faulty, but it has sufficient operating voltage.
CD ign has MUCH greater energy content than inductive ign.
O2 sensors need a power supply, which if it was low by 0.5 volt could be why your reading changed...or maybe it was just faulty.
The engine cranks & starts, an indication that there is more than sufficient ign voltage because voltage drops during cranking from the load of the starter.

I have installed two SD carbs. One, a 625 worked flawlessly. The other a 750 always had a flat spot. In desperation, the owner rang Holley [ from here ] but were unable to help. The problem was fixed by re-fitting the original QJ carb. Ign was HEI, which was not touched.
 
Bewy said:
It is not a low voltage issue.
The instructions with the Mallory HyFire 6 state: 'This system delivers full voltage with a supply of 10-18v, & operates with a supply voltage as low as 8v.'

All of the 'box' igns [ Crane, MSD etc ] that I have seen work on the above voltages. I suspect they are internally regulated to 10v because some racers like to remove the alt & just use the battery as a power source. Preventing a problem before one is created...

These are CD igns & are not as sensitive to supply voltage as are inductive igns.
Click to expand...

I found a manual for my box last night, this is correct. I'm not an electrical specialist but I imagine since the system is designed to operate on a range of supply voltages it must be regulated in a way to give the same output regardless, otherwise it would be kinda useless. The manual mentions the part about running 10V minimum supply specifically for applications where an alternator isn't used. I have a factory-type alternator that doesn't charge much at idle but gets going above 1,200 RPM or so like they pretty much all do.

I really have no reason to believe there's an issue with the ignition or electrical system. My maintenance log says I put fresh NGK plugs in about 2,000 miles ago so they should be fine.

Also my carb is performing great, there aren't any flat spots or other weirdness, never has been tbh. My sole issue is getting the WOT AFR where I want it. Something I did notice, if I would get into the secondaries without going fully wide-open the AFR would read in the low-mid 12s. I repeated this a couple times first going to about 3/4 throttle and then wide open and it did the same each time which to me rules out the ignition, at least for now. I'll hopefully be pulling the carb and checking it out this evening. I'm also not going to put it back together until I get a new fuel filter, that thing is long overdue to be changed from what I can remember and is likely also a potential source of my problems.

Trying to address the simple things first.
 
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MopaR&D said:
I found a manual for my box last night, this is correct. I'm not an electrical specialist but I imagine since the system is designed to operate on a range of supply voltages it must be regulated in a way to give the same output regardless, otherwise it would be kinda useless. The manual mentions the part about running 10V minimum supply specifically for applications where an alternator isn't used. I have a factory-type alternator that doesn't charge much at idle but gets going above 1,200 RPM or so like they pretty much all do.

I really have no reason to believe there's an issue with the ignition or electrical system. My maintenance log says I put fresh NGK plugs in about 2,000 miles ago so they should be fine.

Also my carb is performing great, there aren't any flat spots or other weirdness, never has been tbh. My sole issue is getting the WOT AFR where I want it. Something I did notice, if I would get into the secondaries without going fully wide-open the AFR would read in the low-mid 12s. I repeated this a couple times first going to about 3/4 throttle and then wide open and it did the same each time which to me rules out the ignition, at least for now. I'll hopefully be pulling the carb and checking it out this evening. I'm also not going to put it back together until I get a new fuel filter, that thing is long overdue to be changed from what I can remember and is likely also a potential source of my problems.

Trying to address the simple things first.
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There was more than on of those 6 boxes. Which part number do you have?
 
MopaR&D, I had to re-read some of your posts to find out exactly what the issue is. Am I correct concluding that you are tuning for a certain AFR reading, vs jetting for maximum WOT power? You mentioned the carb was performing great. While all combos will be different, I found mine to run best with about 12.7 AFR After much testing and tuning at the dragstrip. I take it you don’t have the tuning manual or the specific jetting kit for the carb correct?
 
12many said:
MopaR&D, I had to re-read some of your posts to find out exactly what the issue is. Am I correct concluding that you are tuning for a certain AFR reading, vs jetting for maximum WOT power? You mentioned the carb was performing great. While all combos will be different, I found mine to run best with about 12.7 AFR After much testing and tuning at the dragstrip. I take it you don’t have the tuning manual or the specific jetting kit for the carb correct?
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I'm tuning for around the AFR reading that gave me max power when I lived in CO. I do have the manual and the jetting kit for the carb, it's just a funky hybrid design that uses proprietary jets and metering rods on the primary side but the secondary jets are the same as a conventional Holley-style carb. Funny thing is out in CO with the power loss from thinner air and taller gearing at the time I could go WOT long enough on the street to get a seat-of-pants feel of power loss and gain. Here at sea level with shorter gearing I can't floor it more than a few seconds before I'm going too fast and need to hit the brakes so it's hard to tell any minute changes in power lol. 1st gear just spins the tires even from a roll.

This engine combo is also not very sensitive to tuning changes overall which I'm guessing is due to the low (9:1) static compression combined with heavily ported aluminum heads and 222* @ .050" on 110 LSA, .550" lift hydraulic roller cam.
 
Mopa,
What you describe in post #17 about going rich with less than full throttle is I believe a 'quirk'
with air valve style carbs. Part throttle, not enough air flow to open the AV, so AV acts like a choke & causes a rich mixture.
Carter foresaw this as a problem with the production TQ carbs & added a vacuum bleed arrangement to allow the AV to open slightly & lean the mixture. The Comp Series TQ did not have this. It could be that on a competition carb, it would be at WOT or it was added later on production models.
 
Update... I took the carb off and pulled it apart for the eleventy-billionth time lol. Blew out the secondary metering passages backwards with carb cleaner but didn't see any junk come out so I swapped the #88 secondary jets for #92s, cleaned it all and put it back together. I also pulled the old fuel filter and there was definitely some debris that dribbled out when I shook out the inlet end over the ground. Installed new fuel filter with new rubber lines between the metal lines; I got the rubber part onto the metal line going to the carb first and then blew it out with carb cleaner to make sure no rubber shavings were left in there. It's all back together now but we'll have to wait until tomorrow to fire it up as it's midnight and raining here and it's supposed to rain most of tomorrow morning, I'll report back after I go for a test drive. Trying not to drive the car in rain as there are a few water leaks into the trunk that I need to fix and remove the rust that's starting to form on the trunk pan.
 
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