Hi This question is for the carb tunners ,
I am trying to tune my mighty Demon 850 GC carb .
right now the engine set up is pretty good , but i would like to ask how could i tune the very light throtle circut of the carb , because the idle is clean can adjust all 4 mixtue screws to pretty much any a/f ratio i want currantly the engine is running like this .
Idle set at 14.0 a/f ratio @ 185 deg water temp
when you open the throtle the a/f gos to 11.5 11.7 this seams to rich
if u drive the car at steedy speed it will run 11.5 all the time
the faster you go with out flooring it i will start to lean out to 13.0 .
When you floor/mash it in forth gear i have it at 12.9 -13.2 a/f

i have tried smaller main jets on the primary side down to 77 from
stock factory 80 jet and it seams to run sluggish with that
jet .so went back to box stock form and runs good just pig rich at say 2500 rpm in fourth up to 3000 rpm .

what can i do to lean the transition circut ? should i change the IFR
to a smaller one?

or put smaller main jet and open up pcvr to say 72 from 65?

what do you guys think ?

may not be much help,but,i have ran a few 750 speed demons,straight out of the box,on mild to healthy built 440's, and my experience,was that they had to be fat at idle,and they absolutely LOVED alot of initial timing.

In that type of carb, the IFR's handle the fuel in the transition circuit.

""when you open the throtle the a/f goes to 11.5 11.7 this seams to rich""

""if u drive the car at steedy speed it will run 11.5 all the time""

""pig rich at say 2500 rpm in fourth up to 3000 rpm""

It depends on how your carb is setup, but going by your above description, your mains might not be starting until around the 3000rpm mark. If this is correct, than the IFR's will make a big difference in A/F ratio and the IAB's will make a smaller difference for tweaking.

I am currently having the opposite problem as you are. I am very lean on the transition circuit (15.6-16.6 A/F) and I am waiting on some larger IFR's to come in UPS so that I can tune that problem out.
I can watch the A/F ratio start to richen up as the rpms got into the 2700-2800rpm range and then by 3000-3100rpms, the A/F ratio richened up into the 13.8 range where I believe my main circuit was starting in. My mains appear to be a tad rich while my transition was 1.2+ too lean on the A/F gauge.

Search for the "Tuning resuts with an O2 kit" thread, or a very similar title. I talked about one possibility in great detail with DaveNC.

If at cruise your throttle plates are barely open and in the transition slot area, and you have an engine with low idle vacuum/high cruise vacuum, you're seeing the common result.

Assuming that's true:

Use the smallest Idle Feed Restrictiors you can get away with.

Try lowering the primary float level below that of the top emulsion bleed. You'll need to take te front float bowl and metering block off and observe where the sight plug is in relation to the top e-bleed.

How many emulsion bleeds do you have in each side of your primary metering block?

Transition slot restrictors as a last resort.

If I misread the conditions under which the rich condition occurs, shout back.

J

Yea i know what you mean about the weather Jim .
I allways tune to the best idle vacuum but now that i have the lm2 meter it seams to be easer to just set the intial timming at about 20 deg and go for about 13.5 or 14.o a/f .
the most i have ever seen from this 493 is about 8 or 9inhg at at idle .
i do try and keep the butterflies almost closed and ajust the idle with mixtures and the idle ease valve that demon has .

the car idles like i said pretty good and smooth for a solid roller
at 1000rpm .
just trying to get that tranition ciruit to come in .
the car sees more street cruising than the track these days so i would like to get it where the plugs arnt sooty black all the time .
thanks for your suggestions i think two minds are better than one on this and i will let you know what i find in fewdays or so i had uncle pass so i will be attnding his wake and funeral for the rest of the week .
Cj.

ok i finally got back to tuning this 850 demon carb here is a list of what has been done since last post .
1. installed smaller IFR down to 28 from 35 in attempt to lean out transition circuit .
2. dropped main jet to 78 from 80 not sure that made any difference or not because i was trying to hit a home run and did two changes instead of one (i know BAD)
3. opened up the pvrc from 59 to 70
Started engine reset idle mixtures to get 7 to 8 inch of vacuum.
Test drove car while monitoring air fuel .
First of now the idle speed a/f has to be richend up to get the car to idle .
The idle a/f is at 13.0 to get the car to not go to lean when opening the throttle .
From 2500rpm down to 1500 rpm( deacell) the engine wants to lean out to 17 or 18% and car bucks alot more between idle and 2500 rpm cruise .
I think i over shot the IFR size to small and the transition is way to lean .
but i am now starting to see dry spark plugs and not wet blackened plugs so i am getting closer .

think i might do the following before opening the carb back up
1. decrease the idle air bleed some from 70 to 65
2. Raise float level to start mains sooner .
3. open up secondary mixtures scerws some more .
What is your thoughts on thus so far ?

.035 to .028 is a rather large jump in IFR size. I'd first return the float level to normal and see if that fixes the problem. My guess is it won't, but "normal" is a great place to start.

Try get a good idle, while keeping the fuel pinched as much as possible. I'd suggest returning to the stock IAB size - whatever that was - getting the IFR as close as possible, then fine tuning with IABs. If it works out that the best idle is with the screws 1 turn out, there are probably more benefits to be realized with smaller IFRs. If the best idle is obtained with the screws so far out that further enrichment is impossible, you went too far.

Once that's done, if it's rich at light throttle you can once again try lowering the float level to delay the start of the mains.

Are you pretty sure the primary jetting is good now? The fact that you opened your PVCR might allow you to drop the main jet size. To test, stay out of the secondaries and never allow the power valve to open, and drive in every conceivable condition while meeting these two requirements. That will give you a good idea. It will also tell you if you've selected the proper power valve.

Basically, it's a balancing act as airflow increases. The idle circuit hands off to the transition circuit, and that hands off to the main circuit. Too much overlap or not enough...that's what we're fighting. The very hardest thing to get right is the very light throttle cruise, as we try to make a high performance engine behave itself while making 20 horsepower. Hang in there - the results are worth the work!

every thing so far what you said makes sense .
the only thing is the main jets seam OK car is cruising around mostly at 13.5%a/f up to 2000 from 2000 up to 3000 it is now going a touch lean .
i think the IFR is slightly to small because i have to open the mixture screws more now than before and that is what i have to do to keep her out of the 15%a/f danger zone at-very light cruise .
the box stock idle bleeds are 70 i helped the situation by putting the 65s in today seems better than 70's on the idle bleed .
Here's the thing the power valve is a 5.0 but i can not open the valve when cruising on the transition circuit of the carb because the cruise vacuum is so high 18 inches at 2800 rpm have to almost floor it to get it down to 5.0 inches to see the power valve open .
Can i try a 10.0 it seems that would be easy to get to with out flooring the throttle and then maybe i could go to a 76 main jet ? will the pv open at idle say 900 rpm? if idle vacuum is only 7 or 8 inches?

CJS, the idea is to get the IFRs as small as possible (to lean out the transition circuit) without negatively affecting the idle quality and without creating an off-idle lean spot. It is expected that you'll have to adjust the idle mixture screws counter clockwise after this change, so don't sweat that. Good idle quality while still having some adjustment remaining on the screws is the goal. The more you can successfully choke down the idle circuit, the more you'll help the light throttle/high vacuum cruise you're encountering. You might have to slowly sneak up on it. Don't go crazy all at once because you might get it "on the hairy edge of just right" today, and tomorrow's atmospheric conditions won't support it. That's why I said to get close with IFRs beginning with stock size IABs, then you can fine tune with IABs without having to get too far off the reservation.

An open power valve at idle won't affect a thing. Think about it - the PVCR feeds into the main well - the same passage as the main jet. Think of the PVCR as being in parallel with the main jet. If the mains aren't flowing, no fuel from the main jet enters the venturi. The same is true for the PVCRs with an open power valve. Another way to think about it - in any situation where a MJ change will not affect the mixture, neither will an open PV. You should have a vacuum gauge in the car as you drive. If the AFR begins going lean under load at, say, 8" hg, you might want to try a 75 or 80 power valve. This might allow you to go leaner on the MJ if you had previously jetted it up to cover this high load situation. Obviously, this strategy won't work if there is a second situation where you have 8" hg and the main circuit is working, but it isn't lean.

Shout back!

J

hey Jim , Just wanted to thank you for your time and helping me understand what needed to happen in the transition circuits on my Demon .
I think i have finally got the car running like it should.
The transition circuit is now starting to show results . I opened up the IFR to 31 from 28 and installed a 6.5 power valve this has helped the driveabllity. I may try a 7.5 or 8 to see if it would be better but it seams to open much sooner than the 5.0 i had .
The main thing that i have come to understand is that every motor has a different air fuel requirement and trying to tune for 14.7 may not work on a big engine with a cam with alot of overlap and big single plane intake .
My motor is not happy when air fuel ratios go to 14.0 and leaner
it surges so much you would think that the engine has a arcing out spark plug wire. Not the case 3 sets of wires later Ha Ha.
But if i keep it 12.5 to 13.0 a/f cruise and in the power circuits the engine performace is seamless and smooth. EYE OPENER!
So now that i understand the fuel required by the motor has a target value,I can easily make adjustments for the demand .
Thanks again for your help .Cj.

Good deal CJ!

Glad you made improvements. Always glad to help!

Jim