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Old 02-08-2010, 03:38 PM
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Engine tuning with BG 650V

Iím in the process of tuning my engine and Iíve come across a couple of questions. First I just want to say that am familiar with tuning my 700HP BBC running VP gas, using an open plenum intake on a 850cfm race carb, eehh emÖ back in the 80s. Now I have a 351w 10.5:1 compression, World Sr. ported steel heads, Comp cam around .550 with 235duration Hyd flat,, a speed demon 650 vacuum sec. Carb. Edlebrock RPM intake.
My questions:
I have stock jetting 70/78 , how do the plugs color with unleaded gas? Mine seemed white and clean with a very small hint of tan on a ten mile run on new plugs.(heat range 6)
Why do the secondaries have jets 8 steps up from primaries? Should I start to balance front to back? Would the rear cylinders get a richer dose of gas? The venturies are all the same.
And finally, what size jets is everyone using for this type of combination, realizing not all conditions are the same. Please specify if using Holley jets or BG jets as they flow differently.
Thanks in advance.
Lou

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Old 02-08-2010, 03:50 PM
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Quote:
Originally Posted by Cobra720
My questions:
I have stock jetting 70/78 , how do the plugs color with unleaded gas? Mine seemed white and clean with a very small hint of tan on a ten mile run on new plugs.
Like you're seeing, unleaded swill doesn't look much like the leaded of yesteryear. They will color, however. Look for color. Period. Yellowish/tannish will be about as good as it gets on a short run.

You can't idle at all, do the "chop" deal. A lighted magnifier is the only way to go, IMHO. You need to see way down to the base of the porcelain.

Quote:
Why do the secondaries have jets 8 steps up from primaries?
Prolly no power valve in the secondary side.

Last edited by cobalt327; 02-08-2010 at 04:31 PM. Reason: Fix quote tag.
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Old 02-08-2010, 04:23 PM
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Cobalt ahh stupid me, your right No PV in sec. forgot about that.
On the plug side, I never used WOT as I was not in an area to really do so. I was reading the middle of the porcelin. I thought you could read the cruise in the middle.
Thanks for your reply.
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Old 02-08-2010, 04:38 PM
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Quote:
Originally Posted by Cobra720
I thought you could read the cruise in the middle.
The tip and visible part of the porcelain above the shell will be fine for reading the plugs in most cases- it's just good to be able to see to the base of the porcelain, is all.

I'd like to be able to read a gasoline plug like the 'Cat in the Hat' (Jack Roush)- a lot of guys don't know he used to do all the carbs for his NASCAR teams before things got so hectic. The guy has forgotten more than I will ever know regarding plugs.
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Old 02-09-2010, 02:31 PM
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Word of advice here and do not base what jetting you run based on what other guys are using in similar combinations.

Every engine will be different and jet yours accordingly to what your motor wants. Our baseline jetting is what we feel is a good starting point and from there a plug reading will let you know what the motor wants.

When jetting make sure you add to the front and the rear and I usually like to start out by going up 2 numbers at a time as long as the motor is responding.
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Old 02-09-2010, 03:18 PM
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Thanks BG Tech it's nice to know your listening and helping, by the way the carb tunes really nice and the car runs great.
I'm running Ported Vacuum and I like it, no cooling issues and a nice deceleration when off the throttle at 14* initial timing. With my combo and tuning four corners my idle mixture screws ended up a 1/4 turn from bottom and barley respond. My throttle plates are about .015 in transfer secondaries and about .040 [maybe a little less] on primaries. Am I a candidate for drilling holes in the primary plates 1/16".
I will say that I can idle at 900 no problem and she pulls good, but should I get those plates closer to .020 with a little more air in the mix to lean it and have the mixture screws respond a little more?
Thanks
Lou
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Old 02-09-2010, 03:39 PM
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If you have a new vacuum secondary Speed Demon, it should have the idle eze set-up.

Are you using it to try to get the primaries closed up some?

Retro-fit baseplates are available if it doesn't have it. But drilling two holes (if done correctly) beats a $150 base plate.

Having said that, I'd be very surprised if you were not able to tune that carb w/o drilling any holes in the primaries, on a 235 degree @ 0.050" cammed engine.

If it were me, I'd start over w/the tuning, and try opening the secondary throttle shaft a bit more in order to close the primary side some.

Another thought- if you were using manifold vacuum to supply the vacuum advance, this would increase the idle speed- thus allowing you to close the primary side- possible enough to regain idle mixture screw control.

Something to consider.

Last edited by cobalt327; 02-09-2010 at 03:52 PM.
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Old 02-09-2010, 03:56 PM
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No ez tune unfortunately so I would have to drill the lower part of the plates if I go that route.
thanks for staying with me on this
Lou
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Old 02-09-2010, 04:02 PM
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Quote:
Originally Posted by Cobra720
No ez tune unfortunately so I would have to drill the lower part of the plates if I go that route.
thanks for staying with me on this
Lou
You drill the blades a bit out from the edge on the side w/the metering orifices.

But again- I seriously doubt that will even be necessary! That carb WILL tune in fine w/o drilling.
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Old 02-09-2010, 04:05 PM
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Cobalt I measured the sec transfer and I'm at .020, so when the weather gets better I will start over and balance the transfer slots a little more.
The manifold Vacuum would probably work, I just like the lopey idle and declaration on my 2600 lb cobra that 14* initial gives me
thanks, Lou
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Old 02-10-2010, 07:19 AM
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I'm with cobalt on this one and can't really see you needing to drill holes in the butterflies for the mild cam.

You already know this but the 14 degrees of base is low and should be brought up 2-4 degrees before you make any major tuning changes on the carb. I also like to run constant vacuum as well whenever possible for the hei.
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Old 02-10-2010, 04:26 PM
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I'll give the manifold vacuum a try and get those slots smaller.
Thanks again
Lou
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Old 02-11-2010, 06:04 PM
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This is an addition to the idea of drilling the primaries to allow more air/less transfer slot exposure.

While reading up on another subject, I happened across this- twice.

So in the interest of full disclosure, I'm going to reprint it here along w/a LINK to a site w/good photos and text regarding Holley tuning.

"This is a trial and error procedure, so start small, about 1/16 inch. You should not have to go much larger than 3/16 inch. Drill on the side opposite of the transfer slots, to help keep this added airflow away from the idle ports and transfer slots." (Bold & italics mine.)

I have not done it like this- but having done it but once, I am not in a position to argue the point!

That said, the results I got by drilling as I said above in post #9 worked as it was supposed to. I suspect the difference is minor, however it may be worthwhile to further research the subject.
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Old 02-12-2010, 06:26 PM
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Thanks again Cobalt. I'm getting my old knowledge back in gear again and you have been a great catalyst. I took my carb off and pulled my distributor apart [what else to do this time of year in this part of the country]. I measured the transfer slot and it was .045 sec. and the pri. was .035. This does not leave me alot of room to balance unless I change something. If I put the Black mechanical stop bushing in the distributor 18* [i have the blue now 21*] i'll be able to keep my total at 34* and advance my initial 3 more*. This will give me more idle rpm which then I can back down the throttle some, still using ported vacuum.
Ill then try manifold vacuum which I know will work.
So you are right I probably won't need to drill but I have better knowledge on where to drill if I do thanks to your help.
Tuning a hot rod is alot of fun.
Lou
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