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  #31 (permalink)  
Old 03-28-2012, 01:31 PM
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MB = Metering block, almost any carburetor adjustment can be made with a vacuum gauge and you can be confident that it helped the engine run better. I used to keep a vacuum gauge hooked up to all of my carbs and laying in the floor or seat of the vehicle so I could monitor where vacuum was when I was driving. That allowed me to tune the Power Valves for increased drivability and mileage.

PV accounts for about 10 jet sizes in fuel coming in when the vacuum drops to the point of the PV Opening. I used to use the 2 stage valves in most of my stuff so I could get a 5 jet pop when I needed it.

I had a 400 SBC in a Suburban that would get almost 20 mpg around town and 23 on the highway. I built a 650 Vacuum secondary with front and rear metering blocks and ran 60 jets in the front 65 jets in the rear and 4.5 / 7.5 PV front and 3.5 / 6.5 PV rear. It made 500ft# TQ and 425RWHP. I was running 305 heads on it with a Dish top 4VR TRW forged piston, pistons milled about .100" to get Block deck close to Zero. Made about 9.8 CR and I ran it on 87 octane with 30* total timing. I drove it for almost 10 years, pulling towing trailers, back and forth to Charlotte and one weekend I was at a race pulling with the dually and the shop got robbed and Suburban got stolen.

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  #32 (permalink)  
Old 03-29-2012, 06:38 AM
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Quote:
Originally Posted by cobalt327
The ported vacuum port should be just that- ported, unless the butterflies were open so far that the ported orifice in the throttle bore was "seeing" vacuum. But I wouldn't think that would be the case, given the fairly healthy amount of initial timing you have.

If the idle mixture screws are responsive, that about negates the possibility of the butterflies being open too far. That and if the idle is otherwise OK and there's no smelly exhaust, etc. that would indicate you were into the transition circuit at idle.

Sometimes (depending on exactly where the pick up point in the throttle bore the vacuum port is fed from) there can be a small amount of vacuum present at idle from a ported vacuum port. If the Ecurve is sensing just the presence of ANY vacuum, and not how much vacuum, I suppose that could do it.

I'd just use a vacuum gauge on both ports and see what the difference is. If they were the exact same, you'd pretty well know there was a bad gasket, or some other problem w/the carb.
Ya, I did some playing last night and I found that there is a difference between the two ports. The ported vacuum port had about 4" and the full vacuum source was bouncing at 15-16". The higher the idle 1200 or more it is just a vibration of the needle and if I part throttle it to bring the rpms up, the needle is as steady as a rock. The lower I bring the idle the large the bounce becomes (like 3-4" vacuum). Is this the cam causing this large fluctuation in the vacuum at idle (seeming to fluctuate with every pulse of the engine)? The more I adjusted the idle and mixture screws I could get the ported vacuum down to 0 at idle but the timing and the throttle response suffered. I need to do some more playing. Hmmm
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Old 03-29-2012, 01:00 PM
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Sounds like a vacuum leak somewhere because as you turn the idle screw out you should increase RPM and Vacuum until it starts going the other way from getting over lean.

Most cars will run in the 12"-16" range at idle and then vacuum drops as you open the throttle under load, driving down the road.

The vacuum signal that is constant is for stuff like your power brakes so they are not ever lacking. The port that moves is the one you use to measure engine performance.
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Old 03-29-2012, 02:58 PM
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Quote:
Originally Posted by raceman14
Sounds like a vacuum leak somewhere because as you turn the idle screw out you should increase RPM and Vacuum until it starts going the other way from getting over lean.

Most cars will run in the 12"-16" range at idle and then vacuum drops as you open the throttle under load, driving down the road.

The vacuum signal that is constant is for stuff like your power brakes so they are not ever lacking. The port that moves is the one you use to measure engine performance.
Did you look back in the thread at my cam specs? I do not believe that the cam I have will ever reach that kind of vacuum at 700-800 rpm. The only time I can achieve that amount of vacuum is at 1200 rpm on the full vacuum port. If i drop the idle down to 700-800 rpm I am bouncing from something like 7"-10" (at full vacuum port) with every pulse of the engine as the idle is incredibly choppy. Do very choppy cams cause choppy vacuum readings at idle?
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Old 03-29-2012, 05:15 PM
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ok, I'm so over tuning this like a retard... I wish someone could walk me through tuning the carb and timing with a dyno or at the track (I'm not asking anyone for this but i wish I could around here). I threw on my 750 DP Holley completely unsetup just to see what would happen. The engine still stumbles and misses but seems to do this higher into the vacuum even at 16"-17" idling at 1500 rpm. I'm really thinking that the vacuum gauge fluctuating is due to the carb being mis adjusted. I have single wire O2s in the collectors of my headers but I don't have gauges for them yet. I waited until the engine/exhaust was good and hot (idle/reving in driveway) and took some readings from the O2s with a DVOM. I am idling around .800-.850 mv which is lean. I rev the engine up hold it at like 3000 rpm and no real distinct change is noticed. I realize it's not WOT or even accelerating just a really high idle but I'm beginning to get very discouraged in tuning the carb (1406 edelbrock or 750 holley dp) this is just not a strong suit of mine Could the stumble/miss at idle be a combination of big chamber heads and short/cooler plugs (R43T), and/or a mal adjusted carburetor and timing which is at 12 degrees and manifold vacuum bouncing around 10" (bouncing I'm assuming is due to the miss/stumble at idle)? Sorry for this thread getting ridiculously long but there is LOTS of good information here! Thanks guys!
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Old 03-30-2012, 10:09 AM
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The motor wouldn't even start if you are @ 26* inital.
Maybe if you used a jumper engine it would.
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Old 03-30-2012, 10:10 AM
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Do a comp check. Sounds like a valve problem.
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Old 04-01-2012, 02:38 PM
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a little update. I found the heat riser block off on my edelbrock intake loose and the gasket was extremely hard and brittle. Replaced the gasket and tightened down the bolts thinking this may be the reason the vacuum gauge is bouncing around at idle.... Nope. Still hunting for whatever may be causing this. Now the engine is running really fat, bogging and popping on hard acceleration.
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Old 04-01-2012, 03:46 PM
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I'm surprised that no one has mentioned that with a cam lobe separation of 106 degrees you will get a choppy idle, and an unsteady vacuum reading because of it! Most stock cams have a lobe sep. around 112 degrees or more for a smooth idle and steady vacuum.
If your vac is 12 degrees or less at idle your carburetor may need a power valve with a lower opening value.
FWIW
ssmonty

Last edited by ssmonty; 04-01-2012 at 03:53 PM.
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Old 04-01-2012, 04:19 PM
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SSmonty, ya... I knew with the 106 LSA it would be a choppy idle. I wanted that, but I wasn't sure if the vacuum would bounce as well. I guess it only makes sense. Since I now have the Holley on there, you're prlly right about the power valve. I will look into a lower number power valve. What is it... 2 numbers lower than your lowest vacuum reading?
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Old 04-01-2012, 04:55 PM
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I'm certain its lower than that. More like divide the lowest vacuum reading by 2, then subtract 2 from the results, but I can't say for sure. I never had to replace one unless it was blown.
Perhaps someone that's more qualified than myself will chime in?
ssmonty
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  #42 (permalink)  
Old 04-01-2012, 05:35 PM
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Quote:
Originally Posted by Caballerokid
SSmonty, ya... I knew with the 106 LSA it would be a choppy idle. I wanted that, but I wasn't sure if the vacuum would bounce as well. I guess it only makes sense. Since I now have the Holley on there, you're prlly right about the power valve. I will look into a lower number power valve. What is it... 2 numbers lower than your lowest vacuum reading?
The PV need to be lower than the vacuum you see in gear at an idle. 2 numbers lower is fine.
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Old 04-02-2012, 12:40 PM
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Ok... so I replaced the R43T delco plugs with R44TS plugs and they barely fit with the pistons. I made sure to double check. I also retimed the engine and ran a different curve. I was at 14 degrees initial and 34 total all in by 3500 rpm. I changed it to 17 degrees initial and 32 total all in by 3000 but I still get the pop and bogg on hard acceleration. It has to be the carb giving me troubles now. I pulled the holley off and blew it apart. Here's some info, let me know if anything sounds wacked. It is a 4777-2 750 DP mechanical secondaries. Main jets are 76 secondary jets are 67, main and secondary squirters are 25. Using yellow/orange pump cam in top hole. Also the PV is 6.5. Since my idle vacuum is about 10" I was thinking I should get a 4.5 or a 3.5 PV. But this is what it is right now. Does anyone see a reason I would be getting a bogg, pop and low power on hard acceleration? The old plugs (R43T) look carboned up like too much fuel so going off of that it seems the carb is way too rich. Can anyone suggest a better setup for this carb? Here are some numbers off of the metering block if it helps anyone. Main metering block L47772, 8539, 8520 and 5. Secondary metering block L47772, 6497 and 650. Thanks in advance!

Last edited by Caballerokid; 04-02-2012 at 12:46 PM. Reason: Pump cam info
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Old 04-02-2012, 01:25 PM
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Broken valve spring, bad vavle, bad cam???????
Have you done a comp. check?
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Old 04-02-2012, 02:13 PM
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I have not done a compression test yet. None of the plugs I took out look like there is anything wrong other than too much fuel. It is a brand new engine with maybe a couple hours of run time on it. Less then 20 miles driven even and everything is brand new. The vacuum needle only bounces at idle. Part throttle/high idle the vacuum needle is solid as a rock. I may need to adjust the valves slightly but I'm really doubting that as I was very meticulous in assembling this engine.
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