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Author Topic:   KP Lugnut ??
chief4car
Member
posted February 03, 2003 08:09 PM
I saw on another post, you were asking about air bleed holes in the butter flies. We have a 750 alcohol carb that was a bugger to tune. We had a bog off of idle. Our carb does not have any holes in the butterfly. We could not get it to idle unless we exposed alot of the transfer slot. We thought this was causing the hesitation. Our solution was to opes the secondaries more so we could close the primaries. This seemed to help as the hesitation was all but eliminated.
My question is is this a good solution or does the carb maybe need the holes in the butterflies.

I think I asked this next question before but want to be sure. We are running this carb on a 406 SBC with a Victor Jr intake. We are going to try swithch from a 1 inch spacer to a 2 inch. I think you recomended we up the jetting by two sizes. We are currently running 80 jets front and back (2 power valves) this carb has the auxilliary jet holes. Do you think this will be sufficient jet change. With it being alcohol I dont want to burn a piston.

Thanks
Jim


KPLugnut
Member
posted February 03, 2003 09:02 PM
That sounds a little on the lean side for jet but it depends on air quality, too.
It's not out of the ordinary to see 84's or 86's in a carb set up like that.
As for the bypass holes drilled in the butterflies, I wholeheartedly recommend you drill them at least in the primary side in order to get the butterflies closed down on the transfer slots again. Your bog is because you're not even on the idle circuit anymore with not holes and having to just crank down on the fast idle ***** .
Doing that will bring in the midrange and send premature signal to the boosters (main circuit) and possibly even introduce a wicked amount of booster pullover (dribbling).
You'll need to verify, too, that there is no play in the pump arms, and that you have at least a 50cc primary pump and a fast ramp 30cc secondary pump/cam combo if not a 50cc setup back there as well.
Alky needs a lot more volume than gas, but it still has the same needs as far as setting up the idle circuit to insure quick throttle response.

Hope that helps,
Dayton Umholtz, aka "KPLugnut"
Kinetic Performance


tilley88
Member
posted February 04, 2003 12:37 AM
Over my head!


chief4car
Member
posted February 04, 2003 08:30 AM
Thnaks for the quick response KP.

On the jetting that is what came in the carb and like you we thought it was on the lean side. We tried several combos even up to 86 or 87 and it just ran to rich and rough. We went back to the 80's and it ran much better, we have not seen any sign of detonation, etc. and the plugs look good.

On the idle circuit. We were able to close the primaries and cover the transfer slot, so that we only had about .030 showing, but to do this we had to open the primaries more and in turn exposs this transfer slot. I assume we didnt acctually fix the problem just covered it up?? Will drilling these holes also improve throttle response. Also, what size holes should we start with.

What about the change in Carb spacer do you think two jet sizes will be sufficient for a change from a 1 inch to a two inch.

KPLugnut
Member
posted February 04, 2003 10:27 AM
yes, drilling the idle bypass holes in the butterflies will allow you to close back down on the secondary butterflies and still be able to have the primaries closed down on the idle transfer slot. That right there, once the mixture screws are properly re-set, will greatly improve throttle response.
In fact, THAT is the key to a responsive carb - the idle circuit MUST be properly tuned.

On the carb spacer, keep in mind that the most significant thing you'll notice by adding that much more volume under the carb is that it'll change your torque curve and the driver will feel THAT more than the need for a jet change.
I'd suggest 2 jet sizes and see if you really even need THAT, since if the carb is dial'd in properly, and air quality has not changed, then you're not flowing MORE air thru the carb just because you added more plenum volume underneath the carb.
If it's an open 2" spacer, then all that is doing is adding plenum volume and will raise the torque curve up in the rpm range (at the expense of low rpm throttle response.
If it's a 4 hole spacer, then it does the same as the open one, only the lower end isn't quite effected as much.
The open spacer will possibly make the carb "seem" like it needs to be rejetted in some way simply because of that bit of sluggishness off the bottom end that you won't have with only the 1" spacer.
The 4 hole one will also be that way but not necessarily as pronounced.

Hope that helps...

KPLugnut


chief4car
Member
posted February 04, 2003 03:23 PM
Thanks for the help KP. I appreciate you expertise.

Can you give me a starting size for the by pass hole in the butterflies.

Jim

chief4car
Member
posted February 07, 2003 02:02 PM
^^^ ????

Can you give me a size on the by pass hole in the butterflies.

Jim

KPLugnut
Member
posted February 07, 2003 02:49 PM
Sorry, I missed your earlier reply to this thread. :-)

In your case, I'd start with not less than about a .090" hole and not larger than about .120". I know that's a wide margin, but a lot depends on the actual vacuum reading at idle (which is one reason why we ask for so much engine info from our customers when building carbs).
I supposed you could narrow it down to starting with a .100" hole in the primaries at least, and then testing it by re-setting idle mixture screws and see how far you have to expose the idle transfer slot.
The goal is to drill whatever size hole it takes in order to have the idle transfer slot "squared". By this, I mean expose as much length as there is width to the slot. Generally, .020"-.030" is the "sweet spot".

So start out with a .100" hole and adjust as necessary, OK?

Again, sorry for missing your earlier post.

KPLugnut

chief4car
Member
posted February 07, 2003 03:44 PM
No problem.

Thanks for you insight and expertise.

Jim

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