Still chasing an off idle bog
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Still chasing an off idle bog

  1. #1
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    Default Still chasing an off idle bog

    I've been trying to get this bog out for a few years. I've done everything to the Holley Dom that can be done short of reworking the internal fuel curve. I replaced it with a QFX carb and it helped some but it's still there. So I'm now looking outside the carb. This motor is a 468 with 12.5 comp, Brodix BB2 non cnc heads, super vic intake. The cam is a Comp 11-736-9. In dur. 286, Ex dur. 294 @.050, lift In .775 Ex .748, LSA 110, istalled on 108 ISL. This is in a Kurtis hydro. At 1500 rpm idle, it pulls 5in vacuum and at 3000 it pulls 10. As I roll the throttle from idle, it seems to run rough to 3000 rpm then starts to smooth out. I thought it might have been the 6al box and the multiple spark causing it. So, I installed a new digital 6al and it runs the same on the trailer. Haven't tried the new box on the water. I was told it could be that I'm geared to high, but I don't think so. I have 37 in the v-drive and a 31 split in the trans. Anyone have any ideas or suggestions as to what to try. Can't win races when you see your copetitor leaving you at the line. Once it goes, it runs fine.

    Tim

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  3. #2
    "Need For Speed" Gearhead's Avatar
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    Quote Originally Posted by River Rat 005 View Post
    I've been trying to get this bog out for a few years. I've done everything to the Holley Dom that can be done short of reworking the internal fuel curve. I replaced it with a QFX carb and it helped some but it's still there. So I'm now looking outside the carb. This motor is a 468 with 12.5 comp, Brodix BB2 non cnc heads, super vic intake. The cam is a Comp 11-736-9. In dur. 286, Ex dur. 294 @.050, lift In .775 Ex .748, LSA 110, istalled on 108 ISL. This is in a Kurtis hydro. At 1500 rpm idle, it pulls 5in vacuum and at 3000 it pulls 10. As I roll the throttle from idle, it seems to run rough to 3000 rpm then starts to smooth out. I thought it might have been the 6al box and the multiple spark causing it. So, I installed a new digital 6al and it runs the same on the trailer. Haven't tried the new box on the water. I was told it could be that I'm geared to high, but I don't think so. I have 37 in the v-drive and a 31 split in the trans. Anyone have any ideas or suggestions as to what to try. Can't win races when you see your copetitor leaving you at the line. Once it goes, it runs fine.

    Tim
    Just off the top of my head, that looks to be a fairly high duration @ .050 camshaft for the cubic inch as well as the gear ratio. What kind of RPM are you able to pull this gear/prop combination on the top? I would also be curious as to what your cranking cylinder pressure may be?

    Gear
    Last edited by Gearhead; 09-16-2012 at 12:37 PM.

  4. #3
    gn7
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    With a cam that size in an engine that size you think its going to run smooth under no load at 3000 on the trailer? Even underload that thing is not going to run all that smooth at that RPM. Running rough at 3000 and a bog are 2 different things.
    You may have posted this multiple times. But what exactly is "everything you can do to a dominator".
    You offer no discription of what it does, or what you have done to fix it.
    Read your post, and try to diagnose the problem with the info you gave.



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    Senior Member ap67et10's Avatar
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    Install a wideband.


    Andrew

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    Quote Originally Posted by ap67et10 View Post
    Install a wideband.
    Andrew
    I agree.

    What you already spent on the 2nd ignition box would have bought an Innovate LM-2 unit and you would have had datalogging too.

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    Quote Originally Posted by Gearhead View Post
    Just off the top of my head, that looks to be a fairly high duration @ .050 camshaft for the cubic inch as well as the gear ratio. What kind of RPM are you able to pull this gear/prop combination on the top? I would also be curious as to what your cranking cylinder pressure may be?

    Gear
    I agree with gear. Only way that engine would need that much duration if you wanted to make peak at 8200 to 8500 rpm. Engine hesitate when you hit it because "swallow" that much air/fuel.
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    Quote Originally Posted by ap67et10 View Post
    Install a wideband.


    Andrew
    Quote Originally Posted by Unchained View Post
    I agree.

    What you already spent on the 2nd ignition box would have bought an Innovate LM-2 unit and you would have had datalogging too.
    You guys are funny with your data logger tuning your engine. So he alreadfy knows the thing is dead ass lean on the hit. Does the data logger tell him what the cause is, like maybe too much cam for the engine and too plenum and no where near enough shot to cover for it. Does it tell him to change the sooter size or the pump cam?
    Data is only as good as the person reading it. Otherwise its totally useless.



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    Just for comparison 468HP peak power 7200 with BB2's untouched cam would be in the following range:

    750/720
    269/281
    109Sep
    Chris Straub
    Straub Technologies

    3HP is an A$$ Whooping!!! JW

  11. #9
    gn7
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    Wonder if the data logger will tell the guy that one problem is that between the T ram plenum and the camshaft that the fuel from the accelerater pumps arrives at the cylinders 25 revolutions after the shot, due to slow ass air velocity and no low RPM cylinder pressure . I am sure the logger explains that. What the thing probably really wants is 5* more timing, but then it won't want that once it hits.

    In some cases having a data logger is like having a set of encylopedias, but you can't read.



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    Last edited by gn7; 09-17-2012 at 09:11 AM.

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    The wideband gives actual, real data that has value, not "I've done everything to the Holley Dom that can be done short of reworking the internal fuel curve." that statement means nothing, it holds no value, or actual information because as you said GN7 there is no way to know the tuning ability or knowledge of the one posting. A wideband takes out guessing and assuming from this entire equation, and if he came on here giving info from his datalog...even if he doesn't know how to use it, someone else does. The parts about this that is so interesting, it was stated that he has been chasing this problem for a couple of YEARS! Are you kidding me? Years?? Go buy a $250 wideband/logger setup and figure out what is actually happening. A wideband won't fix your problem, but they just give you the information to know what will.


    Andrew

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    gn7
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    Andrew, you're smarter than that.

    Heres a hypothetical read out. 2500 rpm, WOT hit, and a read out of 15 to 1 at the hit. Now what do you suggest he does.

    On something like this, I could tell you more by looking at the boat and checking a few things and watching the boat run, than you and Unchained BOTH combined could EVER tell from a data read out. You'll know its lean, but you'll never know why!

    Nobody has asked a single thing except me and that was "what does every modification to a dominator mean?" I still don't know.

    Nor do YOU or Unchained know the timing, if its locked out, what RPM he is hitting it at, is the hestation immediate or aftert he hit, how long does the hestitation last, does it reoccure or just one hesitation. Does the domminator have annular boosters or standard, 2 or 3 circuit, dual 50cc pumps or only on the sprimary side. Current shooter size, cam color, etc etc etc.
    Of course NONE of the above means a damn thing if you have a data logger. All that is useless dripple.
    You can have your data looger. Id rather know the above info that your precious data logger will never take into account.
    Quote Originally Posted by ap67et10 View Post
    Are you kidding me? Years?? Go buy a $250 wideband/logger setup and figure out what is actually happening. A wideband won't fix your problem, but they just give you the information to know what will.


    Andrew
    No Andrew, I would ask are you kidding. The data logger will tell him there is a problem. Well duh!!!1

    Now push the magic little button on the data logger that tells you how to correct the problem.



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    Last edited by gn7; 09-17-2012 at 09:54 AM.

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    To me, the ignition timing is important, that's a good question. All the other carb questions mean nothing though. None of the questions tell me for certain exactly what the engine is seeing and doing. He can leave the carb crap alone, put in a wideband, take the data then report back. With that info alone it'd be easy to point to specific aspects of the carb to question and also make changes to.

    The difference between you and me is that we invest our ego into two different things. You invest in your own personal experience. I invest in knowing what to do with factual data on an individual basis. The funny thing is that sometimes I find myself leaning towards my own experiences rather than the data, and the more I do this the more I realize my experience means less.

    Quick story for you: this last weekend I was co-tuning an EMC engine. It's basically the final engine for the competition and only a few dyno runs were being made. It was running really well, then, on the very last run from 6k-6.5k power dropped off about 10hp and nosed over fast. The older guys immediately said it was mechanical so they ripped the covers off. I went to the run datalog and clearly at the end of the run one of the cylinders went very lean. They went right to that cylinder and leaked it down finding poor sealing on the intake valve. Within 30 min the head was off and it was easy to see the seal wasn't there on a portion of the valve causing leakage. The funny thing was that the older guys (lots of history in racing) were amazed that the AFR was so drastically changed because of the intake valve going away. The really funny part is that we saw the same thing on the previous 2 runs datalog at high rpm, but it wasn't as pronounced. Here's the craziest part...I didn't realize it till later, but I saw this happening about 5 runs earlier when I realized that that particular cylinder at idle was about 2 points richer than all the others. The engine still ran awesome and made amazing power at that point. No signs of ANY issues whatsoever and the AFR cleaned up immediately off idle. Thinking back I realize that I ignored the data, thinking it didn't mean anything, when really the data gave me more info than I even realized. Now luckly this wasnt too big of an issue, but the point is that the data told everything about the engine long before anybody knew anything. I'll take that kind of information over years of experience ANY day.

    We agree though on one thing, you still need to know what the info means. I've been doing this for a little while and I understand it better than most, but I'm still learning more about all the info I'm getting now (kind of like this weekend) and the biggest thing for me is ignoring some of my past experience of what I always "thought" was correct and relying on the data I have in front of me.

    Oh and if I were to offer a "gut" suggestion. I'd actually say the opposite of you. My experience tells me that 99% of the time carbs are too rich....especially on transition from primary only to primary and secondary operation. My guess is he has that hesitation around the 2500 mark as he gets into the throttle fully. However I would never actually make a suggestion on my gut...at least not anymore. Instead I say to get an O2 then report back when fewer egos and opinions can be involved....the results sure turn out better that way.

    Data is EVERYTHING....you can keep tuning based on your gut...ill stick with factual information


    Andrew
    Last edited by ap67et10; 09-17-2012 at 10:50 AM.

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    Bottom line Andrew, armed with nothing more than A/F and RPM , temperature read outs, your dead ass lost on something like this. You have NO way of knowing if the shooter is too small, the accel cam is to slow, if the floats are too low, if the boosters are the wrong type, if the carb is a 2 circuit or 3, if the timing is all wrong, where the cam is timed in etc etc etc. Your LOST with a data logger and NO additional knowledge of the setup. Totally 100% lost. You have NO CLUE where to start to fix the problem. You only knwo there is one.



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    Quote Originally Posted by ap67et10 View Post

    Oh and if I were to offer a "gut" suggestion. I'd actually say the opposite of you. My experience tells me that 99% of the time carbs are too rich....especially on transition from primary only to primary and secondary operation. My guess is he has that hesitation around the 2500 mark as he gets into the throttle fully. However I would never actually make a suggestion on my gut...at least not anymore. Instead I say to get an O2 then report back when fewer egos and opinions can be involved....the results sure turn out better that way.

    Data is EVERYTHING....you can keep tuning based on your gut...ill stick with factual information


    Andrew
    it doesn't sound like you have a great deal of experience, datawise or otherwise, dialing in carburetors in dragboat applications. which would result in your "gut" suggestion being the wrong direction. real world dragboat experience is that 99% of the time, carbs are WAY lean shortly after the hit during initial loading to wot. ask anybody. it's a leaning condition. it has NOTHING to do with transition from primary to secondary operation. dragracers go from closed throttle blades to wide open throttle blades - all 4 or 8. it is transition from closed blade idle to wot.

    i completely agree that data is extremely helpful. with it, a person can see things that are never felt. but looking at the data, and knowing what to do about it, are entirely different things.

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