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Old 04-15-2004, 06:50 PM
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Exclamation supercharged question

I have two tuning questions.
1. I have a small bog problem at idle to wot transition falls on its face for about 1/2 sec then it rips. what gives?
2. I have a 174 b&m blower with a under hood boost guage when free reving it from under hood it shows vacuum only no boost pressure is this a free rev thing or should I be showing boost?
heres my specs.......406cid sbc that has a lunati crank, gm X rods, lunati forged blower pistons, angle plug bowtie heads, comp cams 509 cam, b&m 174 blower, 2 edelbrock 650s, hooker super comp headers to a 3in pipes with a X over with borla xr1 mufflers, carbs are man. secondaries accel pump in middle slot on both carbs rear carb opens 1/2 before front carb starts.
It runs too good to be off far, just need that final touch. if ya need more info let me know.
Thanks
Ed

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Old 04-15-2004, 06:56 PM
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Typically a bog would be from a rich condition. Lean would typically cause backfire. But, I am not sure how you are going to tune that with the edelbrocks. Every one I have worked with has a flat spot........

Chris
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Old 04-15-2004, 07:22 PM
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Thanks Chris , I am just hoping to get a positive (its a rich) conclusion I then think I can lean the front carb out enough to fix it, I hope. As of now they are setup identical and I have room to go one way or the other.
Ed
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Old 04-15-2004, 11:07 PM
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Well I think it is probably lean. Try increasing your pump shot. Since it is only a momentary problem that tells me it is the pump shot not the jetting. If it was a stumble with black smoke I would say it is rich. With a blower if you are gong to make a mistake, make it to the rich side. You can always lean it out a little if needed.


Royce
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Old 04-15-2004, 11:58 PM
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2 carbs on a 174....?

I think we need a picture of this setup!
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Old 04-16-2004, 05:44 AM
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442 Ill post some this weekend your gonna like it its in a 79 cutlass.
Royce increase the pump shot on both or just the front (secondary) carb.
Ed
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Old 04-16-2004, 06:36 AM
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Royce may be right on the lean condition. To be quiet honest, I have never been able to tune an edlebrock carb to have a good idle to WOT transistion. I have also never riden in a car that someone else has tuned that did any better. That is why I dont run them. I really can tell you if it is a lean or rich condition that causes it, but pump shot is the place to start. With two carbs, you are certainly amplifying whatever the problem is. I would bet you will have to tune them both. If you try richening and it does not help, try leaning and see what happens. I am not familiar with wether or not edlebrocks have any kind of transfer slots, but that might be another place to look.

Chris
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Old 04-16-2004, 06:44 AM
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Im going out to increase the pump shots soon as its late enough for the nieghbors to tolerate the noise. lol. What about bringing the front carb in later? ah transfer slots you mean pump adjustment?
also after reading the knowledge pages I belive I will show no boost until there is a load on the engine is this correct?
Ed
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Old 04-16-2004, 09:07 AM
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Yes, I would increase the pump shot on both carbs. I know it is challenging to get the Edlebrocks to worj well in a blown application. People have done it so don't give up. If all you have is one flat spot that should be easy to tune.

Yes, you will only show boost when the butterflies are open fairly wide and there is a load on the engine. I can drive around all day with my 388ci 8-71 SBC and not make any boost at all. Once you mash the pedal then the butterflies open and the blower will then have enough air to compress and make boost.

Royce
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Old 04-16-2004, 09:13 AM
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blown....booom

It's been along time since I worked with the Edelbrock type carb. They are very similar to the Carter AVS or AFB of olden days. The only one I really had a great deal of time with came from Gary Dyer. This set up I put on a 6-71 blown 454 bbc in a Camaro with a 4sp and 456 gears which I did the entire motor and installation. I got all of the tuning info from Dyer and it worked great. There was a minor stumble as you note but I surely would go for richening and try to make it worse this way rather than lean!! Most assuredly you will back fire and really bang the blower going lean. I richened it up a bit according to Dyer and it flat turned into the next to wildest street beast I ever worked on. The AVS style has all of the same circuits as Holley but they just look different. I'm not sure if there is a manifold reference for the power valve or not. This usually cures the problem. The blower pulls so hard against the bottom of the carb that even under full throttle there can still be a vac signal under the carb thus causing the PV to close or shut off. This the why for the manifold reference. Also why race FI work so well. IE there is no power valve..just your foot.
here is dyers site www.dyersblowers.com/

He also has a trouble shooting column and an email tech.
good luck
bentwings
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Old 04-16-2004, 09:28 AM
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I havent used an edlebrock carb with a blower but I do remember reading that it was common to have to buy a heavier weight butterfly for the secondaries, as they tend to lean out. Another problem for it running lean is that with two 650's and a small blower size you may not have enough velocity at low rpms, you may have corrected it with the idle jets, so the idle is ok, but when you give it the gas you run into a lean pocket. Try running a larger needle valve and I would try going 1 step heavier on the secondary weights.

Ben
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Old 04-16-2004, 10:20 AM
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ok stepped both pumps up and took care of alot of the bog. almost perfect now very small spot I can feel . now Im gonna try a bigger squirt nozzle on both.
As for the edlebrocks so far I love them I love holleys too dont get me wrong I like the tunabilty of my holley but its just cuase I know how to tune them. edlebrocks are new to me.
thanks for the tips guys sure I will have more questions soon.
Ed
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Old 04-16-2004, 09:06 PM
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here ya go

pic of the engine.
http://img51.photobucket.com/albums/...9/AUT_1503.jpg
<img src=http://img51.photobucket.com/albums/v156/Blown79/AUT_1503.jpg>


Ed
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Old 04-17-2004, 11:07 AM
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Ed, Very cool picture. I don't think I have ever seen a 174 with dual carbs. Sounds like you are getting close on your tune up as well.

Royce
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Old 04-17-2004, 11:30 AM
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Thanks, Royce
the rest of the car is in my album here now.
yes getting closer I will get back on it this evening after the bbq.
Royce whats a baseline timing setting for this motor?
thanks
Ed
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