1:1 Linkage BG MIghty Demon 850, Calibration??

[ak49er]

Greetings.

I have a 1976 Hawaiian Jet Boat, powered by a Ford 460, (8.5:1), unknown cam, with a complete MSD Ignition, Barry Grant Mighty Demon 850, on top of an Offenhasuer 360 Tunnel Ram with a single carburetor top plate, breathing through Basset Classic, long collector headers with a Hardin control valve. I also am running a Holley Blue Electric Pump and regulator @ 6-6.5 psi.

A Raw Water Bypass Thermostat System cools the motor. Assuring low block pressure (12-16 psi) full block volume, and regulated temperature. It runs cool and full, as I like to say.

I am aware of the inherent fuel delivery problems of this type of induction setup, and have plans to remedy that by adding a lens of epoxy to the inside of the “outer” four intake runners, to effectively reduce the volume, while increasing the velocity to match the fuel charge of the “Inner” four intake runners. It is a lot of work, but I really like how this setup looks, and my drag racing days are long gone.

In the interest of performance and throttle response, I added a “1:1” linkage to the Mighty Demon, in place of the stock progressive linkage. As I did this I rebuilt the carburetor, adding new gaskets, 6.5-power valve, and resetting everything per the BG instruction manual, including float preset, transfer slot reveal, idle screw settings, and “Idle Ease” settings.

I called the BG tech department and asked if now, that I am making the Pri. And Sec, open at the same time, do I need to “balance” the carb, i.e. make both Pri. And Sec main jets the same, make both the accelerator pump cam the same, and either add another power valve to the Pri. Metering block, or just remove and plug the power valve on the sec. and also if I needed to make both the Pri. And Sec. transfer slots reval the same.

His answer was simply no, add the linkage and run it.

This however does not make mechanical sense to me, now that both sets of butterflies are opening at nearly the same time and rate, and flowing equal amounts of air, but getting different amounts of fuel.

1) Wouldn’t I need to assure that the main jets are all the same, all the way around?
2) Shouldn’t I assure that the accelerator pump cams are the same profile and setting?
3) I know exactly Jack **** about power valves, but wouldn’t that circuit need to matched by either adding or omitting another Power Valve? And what would the correct power valve be?
4) And wouldn’t I need to set the Pri. And sec. transfer slots to the same position?
5) The Pri. And sec. floats appear to be the same, but carry a different casting number , should these be matched too?

Everything I thought I understood about carburetors went out the window after talking to the tech. department rep., and I would like a second or third opinion.

Does anyone have any experience they could share?

 

[Flat Broke]

Greetings.

I have a 1976 Hawaiian Jet Boat, powered by a Ford 460, (8.5:1), unknown cam, with a complete MSD Ignition, Barry Grant Mighty Demon 850, on top of an Offenhasuer 360 Tunnel Ram with a single carburetor top plate, breathing through Basset Classic, long collector headers with a Hardin control valve. I also am running a Holley Blue Electric Pump and regulator @ 6-6.5 psi.

A Raw Water Bypass Thermostat System cools the motor. Assuring low block pressure (12-16 psi) full block volume, and regulated temperature. It runs cool and full, as I like to say.

I am aware of the inherent fuel delivery problems of this type of induction setup, and have plans to remedy that by adding a lens of epoxy to the inside of the “outer” four intake runners, to effectively reduce the volume, while increasing the velocity to match the fuel charge of the “Inner” four intake runners. It is a lot of work, but I really like how this setup looks, and my drag racing days are long gone.

In the interest of performance and throttle response, I added a “1:1” linkage to the Mighty Demon, in place of the stock progressive linkage. As I did this I rebuilt the carburetor, adding new gaskets, 6.5-power valve, and resetting everything per the BG instruction manual, including float preset, transfer slot reveal, idle screw settings, and “Idle Ease” settings.

I called the BG tech department and asked if now, that I am making the Pri. And Sec, open at the same time, do I need to “balance” the carb, i.e. make both Pri. And Sec main jets the same, make both the accelerator pump cam the same, and either add another power valve to the Pri. Metering block, or just remove and plug the power valve on the sec. and also if I needed to make both the Pri. And Sec. transfer slots reval the same.

His answer was simply no, add the linkage and run it.

This however does not make mechanical sense to me, now that both sets of butterflies are opening at nearly the same time and rate, and flowing equal amounts of air, but getting different amounts of fuel.

1) Wouldn’t I need to assure that the main jets are all the same, all the way around?
2) Shouldn’t I assure that the accelerator pump cams are the same profile and setting?
3) I know exactly Jack **** about power valves, but wouldn’t that circuit need to matched by either adding or omitting another Power Valve? And what would the correct power valve be?
4) And wouldn’t I need to set the Pri. And sec. transfer slots to the same position?
5) The Pri. And sec. floats appear to be the same, but carry a different casting number , should these be matched too?

Everything I thought I understood about carburetors went out the window after talking to the tech. department rep., and I would like a second or third opinion.

Does anyone have any experience they could share?

just because the secondaries are opening at the same rate/time as the primaries doesn't mean there is available vacum or demand to use the excess fuel. Lots of people have problems with demon carbs having too large of an air bleed orrifice which makes tuning them difficult. If you are having problems, I would look into that issue first.

I honestly don't know why you would opt for 1:1 linkage if you were already having throttle response issues. Unless there is a lot more going on on that 460 than you let on about, opening up the secondaries sooner is going to hurt your throttle response not help it.

I'd go back to the progressive linkage, then address whatever issues caused you to pursue the linkage change. You've mentioned that you aleady know about the issues with using that intake setup. The issues everyone raises about that setup are well founded and are certainly not helping your throttle response issue, even if you do restrict the runner diameter. Reason being is that becaue the runners are nowhere near the venturis, and as such the signal to the carb is not consistent.

Chris

 

[ak49er]

Thanks for your response Chris.

First off, I'll admit that the only problem was the fact that I never dialed the carb in till last week. I added a electric fuel pump and regulator and then was ready to eliminate more variables.

Adding the 1:1 prompted me to start from beginning, returning the carb to OEM config form my misguided attempt at tuning it two years ago. I then took the time to find a launch where I could leave the boat on the ramp, and tune the idle in, and then I had a whole weekday to make tuning passes.

Heck, it may have done just fine with the progressive, and I may even go back to that when the novelty wears off of a 5.00 go fast part. But right now it rips a$$ like never before in 8 years of ownership.

I'm not hiding anything, the boat came with a replacement 460, as it had been built with a 429 back in 1976, the heads are original, and I just have to assume it is a low compression model with a stock cam, as I have 20hg at idle and near that at cruise.

I plan a great rebuild but the wallet will decide that, and I like that dependability of it, I have never been left at the dock, unlike some of my buddies with more money than know-how.

But what about "squaring" up the carb, the original question, doesn't that make more mechanical sense than leaving it inequal?

 

[Flat Broke]

Thanks for your response Chris.

First off, I'll admit that the only problem was the fact that I never dialed the carb in till last week. I added a electric fuel pump and regulator and then was ready to eliminate more variables.

Adding the 1:1 prompted me to start from beginning, returning the carb to OEM config form my misguided attempt at tuning it two years ago. I then took the time to find a launch where I could leave the boat on the ramp, and tune the idle in, and then I had a whole weekday to make tuning passes.

Heck, it may have done just fine with the progressive, and I may even go back to that when the novelty wears off of a 5.00 go fast part. But right now it rips a$$ like never before in 8 years of ownership.

I'm not hiding anything, the boat came with a replacement 460, as it had been built with a 429 back in 1976, the heads are original, and I just have to assume it is a low compression model with a stock cam, as I have 20hg at idle and near that at cruise.

I plan a great rebuild but the wallet will decide that, and I like that dependability of it, I have never been left at the dock, unlike some of my buddies with more money than know-how.

But what about "squaring" up the carb, the original question, doesn't that make more mechanical sense than leaving it inequal?

To my way of thinking, definitely not in this application. In a single plane setup where each venturi is feeding two runners, or dual four where each venturi is effectively feeding it's own intake runner, that makes more sense, but you have about the farthest thing from that available in your setup. You've got the intake runners in front of and behind your carb's venturis, with a big, open, and turbulent space directly under the carb. So squaring from the standpoint of feeding individual runners or pairs of runners doesn't really have any merrit.

Next you have to consider the internal passages of the secondary circuits compared to those of the primary circuits. Are they the same (I'm genuinely asking as I don't remember off the top of my head)? How about the boosters? If the orrifice diameters are different in the internal passages of the secondary circuits vs the primary circuits, you can't expect both sides of the carb to respond to the same vacuum signal the same way. It'd be kind of like sucking milk out of a 4 oz glass with a coffee stirrer vs. with an In and Out straw. The 4 oz glass represents the jet size or available fuel, while the suction needed to pull 4 oz in a given period through the stirrer vs the In and Out straw represents the difference between the diameter of the internal passages of the primary and secondary side of the carb.

The other problem is that if you square your jetting, you'll likely have to compensate for a lower secondary jet with a slightly larger primary jet. In doing so, you open yourself up to afr issues at part throttle where the engine used to only be fed off the smaller primary jet size, but is now fed off a larger primary, and a sllightly smaller secondary jets (assuming the 1:1 linkage is still in the mix)

I am by no means a carb guru, I'm just giving you my opinion on the setup. Someone with more knowledge could post tomorrow with a squared jet and PV setup that works; but I know from first hand experience that Mighty Demon 850ADs are tough to setup for good idle and off idle, without setting them up with removeable air bleeds.

Good luck with it,
Chris

 

[ak49er]

Thanks again for your response, and laying it out in terms I can comprehend.

I emailed BG Tech and they stated empahtically, don't worry about the jets, no mention on the PV, but I should match the pump cams and cam position, and also set the transfer slots equal.

This week I will do the above, tune it in trying both the progressive and the 1:1, to gauge the differences.

Also, I would spend alot of time getting the idle just right, take it and rip it for awhile, only to have the idle go slightly erratic again, (lumpy, not smooth as I tuned it)

Any thoughts?

 

[Flat Broke]

Also, I would spend alot of time getting the idle just right, take it and rip it for awhile, only to have the idle go slightly erratic again, (lumpy, not smooth as I tuned it)

Any thoughts?

What does your engine temp do between those two states? Water temp in jets can be misleading. If you have an oil temp gauge it might more accurately reflect the internal temperature of the engine.

It could well be that your intial idle settings are fine when the engine is cool, but once you load it up and generate some heat, they're not optimum.

Now speaking of loading, the other issue could be that you're loading the plugs up and that's why the idle is deteriorating. Have you done any runs where you chop the throttle and kill the engine, then take a plug reading? Obviously you'd want to read a couple plugs to get an average, as that manifold is know for distribution issues; but you can do runs like that at various RPM levels to get an idea of what the mixture is doing since you likely don't have a wideband AFR meter to measure your mixture at various RPMs.

Outside the carb, have you verified your timing advance (if you're running any) always returns to the same spot after a run? It's a left field type question, but it never hurts to nail down the easy stuff while you're trouble shooting.

Also, what boosters are in that carb? Annular or downleg? I can say from personal experience, that the throttle response on the annular booster 850 was better than that of the downleg version, though it wasn't an exact apples to apples test with both carbs.

I'd definitely enlist the help of the guys who make a buck/living building motors/doing carbs by posting up in the custom engine forum. I'm just an enthusiast that hasn't had his boat in the water in longer than I'd care to comment

Chris

 

[ak49er]

Well the temp swings pretty far, any where from 160-185, depending on jet flow.

I like the idea of the oil temp gauge.

I did not check the plugs, as you mentioned the fuel distribution varies form cylinder group to cylinder group. But I didn't think of reading both groups and averaging, another fine idea.

Will check the timing as well.

Only maybe two more weekends of decent boating weather up here, then I have to put on my jacket and helmet as it gets kinda cold near 65mph.

I should know this answer, about the boosters, but I am too tired to look it up now, but I will refer to your advice there, I believe the BG Tech said I should swap out whatever I have, for the other.

Get in the water man, rip it.

Thanks again.

 

[Flat Broke]

Well the temp swings pretty far, any where from 160-185, depending on jet flow.

I like the idea of the oil temp gauge.

I did not check the plugs, as you mentioned the fuel distribution varies form cylinder group to cylinder group. But I didn't think of reading both groups and averaging, another fine idea.

Will check the timing as well.

Only maybe two more weekends of decent boating weather up here, then I have to put on my jacket and helmet as it gets kinda cold near 65mph.

I should know this answer, about the boosters, but I am too tired to look it up now, but I will refer to your advice there, I believe the BG Tech said I should swap out whatever I have, for the other.

Get in the water man, rip it.

Thanks again.

I'm not so sure that you can just swap the annular discharge boosters into a downleg booster MightyDemon and get the same results as a store bought 850 AD. I seem to remember reading something about the carb being tailored to the different boosters. Could be marketing fluff, but you might want to verify that if you end up on the BG techline again.

Chris

 

[rainmakermn]

850 is kind of excessive on a stock 460 ain't it?

 

[ak49er]

A little big yes, but when I bought it I had plans for the motor. Now not so much, I'm just down for the cruising aspect, not so much the racing anymore. Seems the high fuel prices of the last 8 or so years has made alot of my potential "competition" park their boats in favor of PWC.

 

[92 cole]

A little big yes, but when I bought it I had plans for the motor. Now not so much, I'm just down for the cruising aspect, not so much the racing anymore. Seems the high fuel prices of the last 8 or so years has made alot of my potential "competition" park their boats in favor of PWC.

LOT'S of good info from "Flat Broke" there. I have played with single tops on 460 Fords and the biggest problem I found that the plenunm area and squirters didn't match for a good "snap". I always stepped way up on the squirter size for a better A/F mixture on the hit, now you add a 1:1 linkage and add to the equasion! Butt load of air and a taste of feul add for a big bog and peddleing to make it get going. I don't know the altitude you are at but thin air tends to help were sea level air adds to the headache. Also an 850cfm is just what you want with the air volume of that intake. Also this is a personal thing and many will disagree but I wont run P.V.s on a tunnel ram or blown motor to save my life! good luck! Mark

 

[78_Tahiti]

Great info Flatbroke on the mighty Demon. I think I only spent 100 hours or so until it was just where I wanted it! I must say, after you get it set up where the motor is happy. Mine has been flawless. :rotflmao:

 

[SS 201]

Demons are a handful to dial, I would agree that carb is a little fat. However without the cam spec not knowing the duration @ .50 are tough to size in the Carb selection. Again I would check the timing in at least 3000 35 34 is a good number. Also with a vacuum secondary it eliminates the guess factory, works the secondary as needed. I put Holley 770 Avenger on my 460, runs like a bear.

 

[Flat Broke]

Demons are a handful to dial, I would agree that carb is a little fat. However without the cam spec not knowing the duration @ .50 are tough to size in the Carb selection. Again I would check the timing in at least 3000 35 34 is a good number. Also with a vacuum secondary it eliminates the guess factory, works the secondary as needed. I put Holley 770 Avenger on my 460, runs like a bear.

Not to be a contrary POS, but vacuum secondary carbs are by no means a panacea. If you're engine won't pull enough vacuum to overcome the secondary diaphram spring, you'll never see the secondaries open. Conversely, too light a spring, and you may be dumping more fuel than your motor can handle. You would have to have vacuum readings throughout the powerband to ensure the secondaries are opening at a rate that is perfect for the motor. Once you're going that route, you can pretty much just dial in a mechanical secondary carb to be just as efficient by selecting the proper power valve and jet combo.

Don't get me wrong, I'm not saying they don't work, but to make one work as efficiently as possible, you have to know enough to be able to also tune a mechanical to be just as efficient. But regardless of how well you tune a vacuum secondary carb, it won't have the same throttle response as a properly tuned mechanical secondary carb. I've run both in boats and cars, and can tell you that to get the best out of one type, you'll put in enough time to properly tune either type of carb.

Chris