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Bog under acceleration
I have a Barry Grant Speed Demon Vac Secondary on a Mowtown 415. Crower 290 grind level 4 cam - solid roller.
I am experiencing a big big bog under acceleration. Tried changing vac secondary springs - no real difference. Obviously getting stavation when accelerating hard. Your advice as to what are the most likely causes and remedies would be appreciated. Thanks Guys. Mike |
Re: Bog under acceleration
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I am running 6.5 psi from an electric pump.
Vacuum at tickover is around 10 - I have a vacuum gauge and see what it does when I floor the accelaerator. It is a radical roller cam though so I guess the vacuum is not brilliant. The bog sems to last quite a while and it feels like you have slammed your brakes on when you floor the throttle, so I am guessing that when the secondaries kick in it is not getting the fuel to go with the addition al air flow. Mike |
Is the secondary squirter working? Take it off and check for dirt. Blow it out with air to make sure.
Did you check the float levels? Dumb question by I gotta ask. |
Floats are fine. Does the vac secondary have a squirter?
Mike |
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It will have a version of the squirters, but not like an acclerator pump. In a double pumper, even with the engine off when you open the secondaries you get gas shot into the carb just like from the primaries. He could still have some dirt in there. Is this a carb change or did it just start?
Ron :) |
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BTW, I like your sig. Obviously borrowed from one of Voltaire's quotes that "common sense is not so common." |
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If you look at the attached photo, the speed demon I have is the one with the vacuum secondaries (red cannister). as you can see in the photo it dosent have a secondary accelerator pump like the one in the top image. Maybe I should have gone for the double pumper.
Any ideas. Mike |
What size carb is it?
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Just a simple question... when you are getting "bog" what gear are you in and what is the engine's RPM?
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Contact FBO Systems
I had experienced difficulties with my setup using a BG SpeedDemon carburetor. My problems disappeared after working with Don Gould from FBO Systems, http://www.4secondsflat.com/. Ultimately, I had to sell my 750 and replace with a 650 baselined for my application by Don. What a difference! No tip-in bog, it fires up and idles on its own within seconds, throttle response is incredible, power band definitely opened up and no bogging. I can't express how happy I am with what Don was able to do for me.
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69Mach,
No, I wasn't saying that he has a double pumper with vacuum secondaries. Bad wording on my part. What I was trying to say was that when the vacuum pulls the secondaries open, he could have some dirt blocking the fuel path into the carb. I have 850 Double Pumpers on both of my cars and like them. But I am awful at getting my meaning across in posts a lot of the time. Only problem is the one with no choke is cold blooded as the devil. By the way, my signature I don't have a clue as to who wrote it originally. I have a small daily saying leaflet from my church that I read each mornings saying and it was the signature on one of those pages but they never give the people who wrote them. Ron :) |
I have a similar problem. I have a holley classic, 650 four bbl with vacuum secondaries (80783C). The secondaries never really opened up under WOT so the dyno tuner just bolted them open. This is not an ideal solution as it hurts your bottom end. My solution is to ditch the vacuum secondary carb and put in a 700 cfm double pumper, also a holley classic (4778C). Maybe I have a vacuum issue somewhere. Whatever the cause, the carb ain't pulling its weight and it is gone.
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Just checking the set up as I bolted it straight on. Butterflies were !0 thou primary and zero secondary. Adjusted to 20 thou both primary and secondary so tickover and metering should improve.
The bog comes in at around 2500 to 3000 RPM which coincides approx with when the secondaries should be kicking in with the purple spring on a 415. Its a violent bog when accelerating hard and seems to last a long tine but i guess its about half a second(which seems an eternity) before it goes and acceleration comes in again. In response to the question its a 750 vac secondary which should be OK with the 415 - I didnt want to overcarb and reduce airflow speed. And hey guys thanks for any Advice given, I ve come to respect and value your replies over the times I have posted. Can misadjusted secondary idle adjustment affect bog. I wouldn't have thought so, but I am sure the setting at all corners is not right yet. Thanks again Mike By the way whats the weather like over there. Its actually a heat wave over here which if you have visited in the past you will come to realise is a rare occurance. |
Mikey, my BG was doing the same thing and I took the bowels and the meetering plates off and blew them out with an air hose, put it back together and it runs great now. Guess there was something cloging it up somerwhere in there. Give it a try just be carefull not to tear the gaskets up when taking it apart.
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My 2 cents worth is that the secondaries are opening too soon, creating the bog. You can slow down the point at which the secondaries open by changing the spring inside the canister. Go buy the Holley kit that has all of the springs, then put in the heaviest. If it does not bog, swap spring for the next size down until it bogs, and then go back one size.
If it still bogs with the heaviest spring, something else is wrong, or you have overcarburated. |
Try some
Mike,
Dig out your timing light and check your initial advance. If it's not enough the engine will act exactly like your bog. Say you have 10* initial now, go up 4 * to 14 initial and try again, if it helps then go to 16* iniatial and recurve your mechanical advance back 6 * so your total timing is correct. Perry.:cool: |
I had exactly the same symptom, using a holley 750 vac; using the proform center body.
Roll on works fine, 2200 slam to floor; heavy boog then takes off. Springs from heaviest/lightess no difference. I had a cleve expert ride in it ; he said sounds lean as the secondaries open; recommended changing the squitter cam to smaller intially and more later. Nozzles are #32. Changed cams from the orange to one that had less early and more "later" lift. Trace the cams on some paper to see the different profiles; they is only about .010 in differnces in many of them. Bottom line, and much to my surprise, the new cam; I think it was the green or purple fixed the problem. Make sure there is no clearance under the cam follower; as soon as the throttle moves the squirt should start; even moving the curb idle screw changes this relationship. I havent felt it stubble once since the cam change; so I'm leaving it alone. Although I was thinking of leaning out the mains because my plugs are dark. gn |
Thanks guys as always excellent response
Mike |
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