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  #16 (permalink)  
Old 09-14-2010, 07:49 AM
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I have the same problem with my BMT cam, SUPER rich idle with my Mighty Demon 750DP.. If I do 3 cold starts, my plugs are toast.. I called BG and they told me to drop down one size on the IFR's.. I have some on order and will let you know if that solves the problem.. I dont think its the cam because I know a handfull of guys running the same cam and they have no issues at all with a rich idle BUT I am always reading about rich idle issues with the Mighty Demon's.. If you notice most rich idle condition problems with the Thumper cams, its usually with a Demon on top..

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  #17 (permalink)  
Old 09-14-2010, 01:21 PM
Technical Support Barry Grant
 
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Lots of good advice from Cobalt.


I'm glad I saw this thread because nowhere in the 3 emails from the op that have been exchanged regarding this did he bother to tell us he had jetted the carburetor up 10 numbers front and rear and it wasn't until the 3rd that we were informed us the pv had been plugged!

Definitely some bad advice from the dyno guy.


I have forwarded your emails to one of my techs who will be contacting you for a phone number so he can talk directly with you and get to the bottom of what is going on here.

While you are waiting for this my advice would be to take the 6.5 power valve and install back in the carburetor along with the original jetting for a starting point. Set both transfer slots at .020 and the idle-eze at 2 turns open. Run all the mixture screws in and back them out 3/4 of a turn. This will give you a base line to restart and adjust the carburetor for which we can then tune from.

Once you have done all of this take another vacuum reading to see if a power valve change will be needed or not as I suspect the vacuum will rise with the proper jetting back in it.

Your emails indicated you had 21 degrees of timing at idle which may be ok but if more is needed you will need to recurve the distributor to keep your total where you want it but my techs can get more into that by phone if needed.
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  #18 (permalink)  
Old 09-14-2010, 01:23 PM
Technical Support Barry Grant
 
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Quote:
Originally Posted by bubbahotep
What about the air bleed sizes? Bigger to lean it out?

Since you had good results with someone elses carb it has to be all carb related issues, no?
With no pv and 10 numbers of jet added front and rear you could pull the air bleeds out and it still wouldn't correct it.
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  #19 (permalink)  
Old 09-14-2010, 02:32 PM
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Kudos to Tech @ BG and their team. If THAT'S not good customer service, I don't know what is.
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  #20 (permalink)  
Old 09-14-2010, 04:19 PM
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I am running MDS dist. and 6AL On Thursday I am going to put the carb back to factory jets along with power valve then set the butterflies like they should be and see what happens and then go from there.
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  #21 (permalink)  
Old 09-15-2010, 01:03 PM
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Hey Redride

Please keep us updated on your progress, especially if you get it fixed please let us know how it was accomplished.

I'm playing the same game at the moment with a brand new Street Avenger 870 on my zz502 with the smallest thumpr. Although not as severe as your issue I'm really rich at idle even after adjusting transfer slots, changing power valves, locking timing, trying hotter plugs, playing with the idle mixture, and drilling some holes in the butterfly plates. :sigh:

considering yanking the thumpr but really dont want to..

Last edited by SS66chevelle; 09-15-2010 at 01:14 PM.
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  #22 (permalink)  
Old 09-15-2010, 01:18 PM
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Quote:
Originally Posted by SS66chevelle
Hey Redride

Please keep us updated on your progress, especially if you get it fixed please let us know how it was accomplished.

I'm playing the same game at the moment with a brand new Street Avenger 870 on my zz502 with the smallest thumpr. Although not as severe as your issue I'm really rich at idle even after adjusting transfer slots, changing power valves, locking timing, trying hotter plugs, playing with the idle mixture, and drilling some holes in the butterfly plates. :sigh:

considering yanking the thumpr but really dont want to..
Where do you have your base timing at? I realize its not our carb but the timing info applies to everyones stuff really and one of the biggest issues we run into here with ZZ motors is guys trying to run the GM spec for base timing. Its too low.
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  #23 (permalink)  
Old 09-15-2010, 04:36 PM
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I did talk to the BG tech guy on the phone today and told him that I had decided to start over and put the carb back as it was from the factory and start from there to which he agreed. I thought I was doing right by doing what I was told by the guy at the dyno shop but it appears that may not be the case. I will post the results as soon as I get the carb set back up. One thing for sure is that it can not get worse and I really feel that is where I should have started.
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  #24 (permalink)  
Old 09-16-2010, 12:44 PM
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At the moment I have unlocked the timing and I am running at 26 initial 36 all in by 2500. It doesn't want to start with anything less than 20 initial because of the amount of overlap in the thumpr cam I am assuming.

I made some progress last night by increasing the size of the holes in the primary butterflies to 7/64 and resetting them so that the transfer slot exposure is back to .020 - I was then able to lean the idle mixture until stumble then back out about 1/8 and It's finally to the point where it's not billowing black and blue unburned fuel out the pipes but its still too rich (smell)

Tonight I am going to try and lean it out further while at the same time opening the secondary butterflies slightly to see if I can go leaner with more air as a test.. I've also been reading that I may need to change the Idle Flow Restrictors to something in the .03x range and air bleeds in the .06x range but I am waiting for a holley carb tuning book to arrive first (don't laugh it's the only way I can learn this stuff)

My last resort is that we've got a Mr. Norms speed shop in town and they can tune it on a chassis dyno for $450 but.. I need to learn how to do it myself, or at least how to get it close - I am tired of the mystery surrounding these things and the fact that there is so much misinformation out on the forums about how to do it right..





Quote:
Originally Posted by Tech @ BG
Where do you have your base timing at? I realize its not our carb but the timing info applies to everyones stuff really and one of the biggest issues we run into here with ZZ motors is guys trying to run the GM spec for base timing. Its too low.
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  #25 (permalink)  
Old 09-16-2010, 01:46 PM
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Quote:
Originally Posted by SS66chevelle
I've also been reading that I may need to change the Idle Flow Restrictors to something in the .03x range and air bleeds in the .06x range but I am waiting for a holley carb tuning book to arrive first (don't laugh it's the only way I can learn this stuff)
Air bleeds are the opposite of IFR's, you go bigger instead of smaller to lean it out.. My downsized IFR's are on backorder with Jegs so it may be a while before I can give you any feedback but your condition is exactly like mine.
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  #26 (permalink)  
Old 09-16-2010, 02:57 PM
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Reading this short article in CHP makes me think that we are on the right trail.. finding the right combo should be lots of fun.. hopefully the tuning book will provide some general guidance on both IFR and Air bleed sizing

maybe I will try the wire strand trick for testing purposes then replace with proper size IFR if the results are good...

http://www.chevyhiperformance.com/ho...498/index.html




Quote:
Originally Posted by My68Vette
Air bleeds are the opposite of IFR's, you go bigger instead of smaller to lean it out.. My downsized IFR's are on backorder with Jegs so it may be a while before I can give you any feedback but your condition is exactly like mine.
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  #27 (permalink)  
Old 09-16-2010, 03:56 PM
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Quote:
Originally Posted by redride
Ok, The tuner at the dyno shop told me to pull out the power valve and put a block off in it and then go up 10 jet sizes front and back.
This is completely wrong information. When you delete the power valve you only go up jet sizes on the secondary side of the carb. If you increase the primary side your going to run WAY rich under part throttle conditions. Remember under part throttle conditions the power valve will remain closed. Also it isn't 10 jets sizes all the way around, it's 10 jet sizes on the rear jets, with the front jets remaining the same. And this is just a "rule of thumb", your probably going to only have to go up about 7-8 jet sizes in reality. Now this only stands for carbs with one power valve that is on the Primary side of the carb. If the carb as a power valve on the secondaries, then you may have to go up more than 10 jet sizes. Your best bet is to get a LM-1 or some other type of A/F meter and log your fuel ratios. Other than that your just going to be stabbing in the dark.
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  #28 (permalink)  
Old 09-16-2010, 04:09 PM
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Quote:
Originally Posted by SS66chevelle
Reading this short article in CHP makes me think that we are on the right trail.. finding the right combo should be lots of fun.. hopefully the tuning book will provide some general guidance on both IFR and Air bleed sizing

maybe I will try the wire strand trick for testing purposes then replace with proper size IFR if the results are good...

http://www.chevyhiperformance.com/ho...498/index.html
I tried the wire strand (staple) trick and didnt have any success at all, it ran really rough after I did that, actually to rough to get an accurate result.. The problem with the wire is that it blocks about 50% of the hole because its roughly .017 going into a .034 hole.. I think we only need to drop one or two sizes .002-.004 to have success.. From what I understand, changing the Air Bleeds can also effect the transition off idle so I wont go that direction unless I dont have any success with the IFR's.. My carb actually runs great and I can stomp on it from a dead 1000rpm idle and it will launch hard.. I just dont want any changes I make to effect anything else but the rich idle condition.

I assume the wire strand worked for them in that article becasue someone had already pre-drilled the IFR's out to .055 so when they stuck a .017 wire strand in it, it only reduced it to .038, which is almost where is was stock..

Last edited by My68Vette; 09-16-2010 at 04:18 PM.
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  #29 (permalink)  
Old 09-16-2010, 04:26 PM
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Ok I pulled the carb and put it back to the origional jeds and put the power valve back in along with setting the butterflys where they needed to be up the transition slot along with setting the Idle-Eze at 1 1/2. Same problem, it will not idle at all and turning the Idle-Eze does nothing. When I say it will not idle I mean it is so low that it dies (below 200 rpm). So in order to get it to idle I will either have to turn the primary idle screw in about 2 turns which will move the butterflys way up the transition slot or start drilling holes in the butterflys so I will not have to open them so far. What next???
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Old 09-16-2010, 04:34 PM
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How about opening the rear throttle blades a little??

There should be an adjustment screw that you can turn to open them.

If that doesn't do it, then yes you may have to drill holes in the throttle plates.
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