High speed air bleeds [Archive]

bryanm

05-07-2009, 07:33 AM

I just got my motor back in my car and running but now it has a little stumble from about 3500-4200 rpm when you ease into the throttle. If I just whack the throttle hard it revs clean. It did not do this on the dyno but I had my high speed air bleeds on my Demon carb drilled out to supposedly the same size that they were on the dyno. I am thinking that they got drilled out to big. What do you guys think, is that the cause?

TopspeedLowet

05-07-2009, 08:42 AM

The biggest advantage of using a dyno to tune is that you prove the parts you are going to race with. If you are not using the same carb that you dyno'ed with you know nothing about the mixture now and no doubt have possibly a huge fuel curve change by changing the bleeds after the dyno.
Get the bleeds back and carb set up you dyno'ed with and then look for where the problem is if you still have it then. Ignition and other things could be at fault if you know that the carb is working well which you can not be sure of now.
The demon carb is not the best choice for racing but if this is what you dyno'ed with successfully, then set it back where you were it and change step by step so you know what change the engine hates.
Bruce

itsabird

05-07-2009, 08:42 AM

could be, i would try a size smaller bleed, and see if that fattened it enough, i would go in small steps, to maintain dyno numbers. jmo,

itsabird

05-07-2009, 08:46 AM

topspeed type faster than me, ( what he said).

bryanm

05-07-2009, 10:42 AM

I think I confused topseed, everything is the same as it was on the dyno. We used the shops air bleeds while on the dyno and then I had mine drilled to supposedly the same size as the ones were on the dyno. The problem is that demon does not make air bleeds like a Holley or Quickfuel bleed, you can not buy them sized already, you have to buy blanks and then drill them out. I thought you could just by them sized like a jet but you cant.

TopspeedLowet

05-07-2009, 12:15 PM

Yes you can drill them out to match them. No confusion there, but if you radius the entry hole it will act much leaner or bigger if everything is not identical. The depth of the hole will effect the bleed as well. some bleeds have to be drilled all the way through about 1/4" total depth, and some only require about .030 depth to get to the other side of the bleed. No biggie either way that creates a minor change. You may look at to see of you are running less fuel pressure than on the dyno too. The lower fuel level can make a lean hole when the booster tips in off the idle / transition circuit. Go about 4 hundredths smaller on the bleed to test, that is about the same as a jet change on holleys and enough to tell if you have the bleed too large quickly. I have never done part throttle pulls on the dyno before but you may have had this condition you are fighting present on the dyno, but did not show itself and behaved like you said it behaves when you slap the throttle and runs fine. The power valve may be too low a number and causing a lean condition in the car that the dyno would have never measured or known because it was wide open flowing fuel the whole time the test was run, not part time like on the street........... Good luck hunting, if you have reason to believe that the bleed is drilled bigger than what you dyno'ed with than you are likely correct on the fix but driveability problems are common on dynoed engines that do not run at wot like they are tested.
Bruce

dparker

05-07-2009, 01:19 PM

I just got my motor back in my car and running but now it has a little stumble from about 3500-4200 rpm when you ease into the throttle. If I just whack the throttle hard it revs clean. It did not do this on the dyno but I had my high speed air bleeds on my Demon carb drilled out to supposedly the same size that they were on the dyno. I am thinking that they got drilled out to big. What do you guys think, is that the cause?

Is this a drag car or street car? Are you running racing fuel or pump gas?What kind of HP numbers on the dyno?

bryanm

05-07-2009, 01:24 PM

It is a drag car and runs on race fuel. It is a sbf, it made 580hp and 540 ft/lbs torque. I am wondering if this will even show itself on a full blown pass. I will start with the air bleeds and go from there.

dparker

05-07-2009, 01:35 PM

I really don't think the air bleed is the problem. Those demon carbs seem to never be clean from idle to wot. They usually have a bad spot somewhere. I usually change plugs after I break in an engine. My advice would be examine and change plugs. And change carb.....If you still have the problem you can eliminate the carb as the source.

bryanm

05-07-2009, 06:56 PM

I have already changed plugs. The carb worked fine on the motor last year with the old heads, it has never been an issue.

TopspeedLowet

05-08-2009, 03:01 AM

The head change may be giving a better vacuum signal than previous which again points to a power valve tip in point if you run one. Otherwise you will have to go over everything again a find out what is different that you think is the same as last year. The timing curve need may be different on the street now if you run centrifugal and vacuum advance with the head change as well.
Food for thought
Bruce

bryanm

05-08-2009, 07:10 AM

The timing is locked out now and has always been. I am starting to wonder if this will even be a problem when I am making passes.

dparker

05-08-2009, 08:01 AM

The timing is locked out now and has always been. I am starting to wonder if this will even be a problem when I am making passes.

What fuel pressure are you running? I've seen the mist from the needle seat as fuel goes into the bowl allow fuel in the vent in bowl. Make sure your fuel pressure is 7 or under. If it doesn't miss at wot I don't think it will be a problem.
Something I just thought of. Does your carb have progressive linkage? If so, that may be the point the back butterflies are starting to open. May need bigger squirters etc......

TopspeedLowet

05-08-2009, 10:56 AM

Go to the track and kick some ass. My 1200HP big chevy acts like it's unhappy on the return road at part throttle after a 7 second 184 MPH pass.
I used to let that bother me and I would tune it away, but by the time I get the intermediate circuit tuned perfect to prevent the burble, I would change the jets for weather and the cycle begins again. Who gives a crap what the engine runs like on the return road as long as its running back to the pit on it's own power? You know what I mean......
Bruce

jmarksdragster

05-08-2009, 05:38 PM

How about some info, what Demon carb, what engine and engine specs, what is it going in. The main air bleeds are not what you want to open up without knowing what you are trying to accomplish.

bryanm

05-11-2009, 07:39 AM

The air bleeds were opened up on the dyno when we were trying to get the a/f mixture that we wanted so I knew exactly what I was trying to accomplish. It is a sbf 408ci, mighty demon 800 carb, afr 205cc heads, victor jr intake, 680 lift roller cam, 266 duration at 050, 12:1 compression. I took it to the track this weekend and it is falling on its face as soon as I let off the brake and then it comes out of it and really hauls ass. It ran great on the dyno and I have never had any problems with the carb before. I also noticed that the fron float bowl keeps filling up even after I adjust it. I have another carb I borrowed that I am going to put on and try. I just dont know what else it could be.

dynaguy03

05-11-2009, 11:35 AM

Demon carb..my friend i can only say i struggled with a demon .. always adjusting on it .. once i thought i had it right... next weekend -- there i was adjusting,jetting, idle mix.. ugg.. i sold it and bought a PRO SYSTEMS gas carb...all i did there was adjust 2 sizes jet and it was good to go!.. now im on Alcohol with an APD whole different animal.
FYI my Demon Dyno'd great but sucked at the track!!

dparker

05-11-2009, 01:14 PM

The air bleeds were opened up on the dyno when we were trying to get the a/f mixture that we wanted so I knew exactly what I was trying to accomplish. It is a sbf 408ci, mighty demon 800 carb, afr 205cc heads, victor jr intake, 680 lift roller cam, 266 duration at 050, 12:1 compression. I took it to the track this weekend and it is falling on its face as soon as I let off the brake and then it comes out of it and really hauls ass. It ran great on the dyno and I have never had any problems with the carb before. I also noticed that the fron float bowl keeps filling up even after I adjust it. I have another carb I borrowed that I am going to put on and try. I just dont know what else it could be.

Bryan, you do have jet extensions in don't you?

bryanm

05-11-2009, 02:09 PM

Yeah, I have jet extensions in it. I just put a 750 Quick Fuel of my dads on it and it ran great, never missed or loaded up at all. I am going to order a rebuild kit for the Demon and go through it. Something must have gotten in it it or something, I dont know.

dparker

05-11-2009, 04:45 PM

Yeah, I have jet extensions in it. I just put a 750 Quick Fuel of my dads on it and it ran great, never missed or loaded up at all. I am going to order a rebuild kit for the Demon and go through it. Something must have gotten in it it or something, I dont know.

Thats a good idea. Also be looking for a new carb. I hate to bash something, but that's a never ending story with those carbs. good luck.

bryanm

05-12-2009, 03:12 PM

Well, I went through my Demon and cleaned it all out and it stil does the exact same thing. I called Demon and they were absolutely no help at all, so I guess I will be getting a new carb.

TopspeedLowet

05-12-2009, 03:49 PM

Well, I went through my Demon and cleaned it all out and it stil does the exact same thing. I called Demon and they were absolutely no help at all, so I guess I will be getting a new carb.

You are not the first buyer to be taken by the mighty demon. I hope you are able to steer other race junkers from buying one of those gems that ole Barry peddles.
Sorry that you have to re buy something that you already paid for once.
Bruce

jmarksdragster

05-12-2009, 03:53 PM

If the front bowl keeps filling you have to correct that. It will spill over when you launch and flood it. Either the N&S is bad, a casting imperfection in the bowl, or the float is bad or hanging up. You don't have g-forces on the dyno adding to the problem, possibly just enough to make the difference.

bryanm

05-13-2009, 06:34 AM

It has a bog in my garage even when it isnt on a pass. If I ease into the throttle it loads up at around 3500 RPM, and it is even worse on a launch. Basically I thnk the carb is junk and with how rude and unprofessional the Demon tech line was I dont really care to do business with them or promote there product anymore. From what I have heard from other people this will not be the last thing to go wrong with the carb even if I fixed it now.

dynaguy03

05-14-2009, 08:08 AM

It a shame ... but dont feel bad.. i went thru it. By moving onto another carb you will be rid of that demon headache .. trust me ..i beat on the demon for sometime... But when you get a good carb you will know it and just by covering the basics it will make the demon look like the junk it is.
I like Pro systems for gas and APD for Alky carbs...

just my 2 cents