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Old 04-08-2004, 05:58 PM
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Carb Tuning Input/Thoughts (blower)

Ok here's the deal. I drove my car to a burger run, and there was another gentleman there with a blown 350 (6-71). He was running Edelbrock carbs and could not get the thing to run right at all. He was amazed at how well my car (started, ran, idled, sounded), so he runs out and buys the same Holley carbs I have (I gave him the name and number of the place I bought them). BDS also told him to ditch the Edelbrocks. He calls me up after he gets the new carbs and still can't get the thing to run right. I offer to help him out (just thought I would give a little background).

It has a mild hyd roller 228/230 @ .050 .474 lift. I pulled the carbs off (Holley blower double pumpers 750CFM) adjust the idle screws set the floats. The carbs come with 8.5 power valves, 70 primary jets, 80 secondary jets. After I set the carbs up they started and idled fine (really had to lean out the idle mixture screws) they are about 1/2 - 3/4 turn out.

Now he is running a Hunt Magneto (I know), with the timing set at (are you sitting down?) 42* I tried backing down the timing some and the car didn't like it (I am assuming the timing tape is correct, he swears it is). Vacuum is 16 inches at idle.

With the carbs set up as mentioned above it idles fine starts fine, he takes it down the street rolls into the throttle and bang backfire, he had changed the jets to 68's, I put the 70's back in and he took it out again, ran a little better but still surged and popped, so I put 72's in it, same thing runs a little better but still popped. So I say lets just jump up to 82's and see what happens, BINGO it runs like a scalded ape (no smoke at all just perfect). This bugs me, there is no way it should need that much jet in the primary side. The power valves are boost referenced. I am thinking he needs a higher rated PV. What do you guys think? Could it be that with that HUGE amount of timing he actually needs that much jet? I think I have him talked into getting rid of the mag (this is a street rod and will never see the track)another project for me but, I don't mind he is an older gentleman and very nice, plus he actually pays even though I refused.

Now for comparison I have a 388 with an 8-71, same carbs as above and WAY more cam, I am currently tuned very conservative 13* initial and 33* total (all mechanical). My car runs fine with the 70's pri and the 80's secondaries. I have the same 8.5 power valves, runs absolutely perfect. More blower, larger engine, more radical cam and needs less jet? Oh I need to double check where his compression is too.

He is a happy camper right now, it runs better than it ever did (actually runs real good). I just don't see why it wanted so much jet on the primary side. The power valve must not be opening. Before I go out and buy a set of higher rated PV's and stick them in, I thought I would run this by you guys.

Sorry for the long post but, I wanted to make sure I included all the details.

Let me know what you think.
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Old 04-09-2004, 03:22 AM
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anyone??
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Old 04-09-2004, 05:29 AM
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Hey Royce,

I think that the jetting is not that strange....I'll explain. I have had this conversation on this site before and was told I am an idiot by some so you decide......

When a carb is grossly over sized(which I think two 750's is), it will take more jet to get the desired AFR. Myh thinking on this is that it is due to poor signal in the venturis. As air rushes through the carb, the air movment causes a pressure low in the veturis that in turn pulls fuel from the bowl through the jet and out the venturi. When velocity is very low in the carb so is venturi signal. Normally this can be okay because the engine is at cruise during this condition and lean mixture is good. When the carb is oversized, the main jets usually have to be somwhat larger to get the disired AFR.

You said it happens when you roll into the throttle. This is the point where the engine will really show a lean mixture since mixture has to be overly rich for smooth acceration. If you are running on the edge, it will cough and backfire because pump shot is just not enough.

The problem may not get much better as the engine revs because the carbs are really not seeing a high percentaage of their total flow. The air bleeds also help tune fuel curve. These are just as dependant on total airflow throught the carb. If the carb is oversized, you will not have a good curve. Of course this can ve fixed with new tunable airbleed body.

Just some thoughts.....dont throw stones,

Chris
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Old 04-09-2004, 06:41 AM
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What color accelerator pump cam was on his carbs, I'll bet it was the black cam. The pump cam is a very important tuning aid and likely the thing that should have been changed first before playing with the jetting.

What you have done is richen the mixture enough that the pump can keep up. I cannot stress enough how important the correct pump cam is to carb setup, especially in an application like yours.
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Old 04-09-2004, 11:07 AM
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OK guys, thanks for the input.

Chris,
I know two 750's seems like a lot but, for a blown application most manufactures will recommend two 750's on any blower that is a 6-71 or larger IF performance is a goal. Most street rods run smaller carbs because they only cruise and performance is not a goal. You theory does make sense though and I would never throw stones at anyone that is trying to help, I appreciate it.


4Jaw,
I do see your point but, in my experience if it was the pump shot the problem would happen when you mashed the gas. This problem occurred when you slowly rolled onto the throttle. I am not saying your theory is wrong, and that is something I can check. Now lets say you are right, why wouldn't the car be running rich when once on the main circuit? With 82's in there I expected to see black smoke for sure. This is the reason I was leaning towards the power valves. Also if you rev the car fast (mash the throttle) it didn't pop initially, it took a few seconds, this to me points to the main circuit as well. Now if the power valves are opening and with the 82" wouldn;'t you expect to see a VERY rich condition if the problem was only the pump shot? You could also feel the car surging while on the main circuit, this also tells me it was lean. I have plenty of pump cams and this is something I can play with. Trust me I was shocked to see it take the 82's and run like it does.

One other point when he had the Edelbrocks on the car he had the jetted REAL fat as well, I think he aid he had 80's or 84's in those. I am not big on the Edelbrocks so I don't know how that jetting relates to the Holleys.


What do you guys think about that much advance?

Thanks for the input guys, keep it coming if anything else comes to mind.


Royce
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Old 04-09-2004, 11:23 AM
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Yeah but Royce Im sure he still gets better gas mileage with a blower.


hehehe sorry couldnt resist

Just kidding buddy,


Which 6-71 does he have large or small bore? I had 2 650's on mine running with 70-73 jets. I had made several of the common changes for holleys with blowers before I even bolted the carbs on. Just as a suggestion one of the problems I had was that at idle and crusing my blower seemed to push the fuel to the rear. Infact letting it idle I would check the plugs and the fronts were lean and rears were rich. I had to run my front carb richer then the rear carb. It might be possible that this is happening with his.

Let us know what you find

Ben

Last edited by brainsboy; 04-09-2004 at 11:41 AM.
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Old 04-09-2004, 12:34 PM
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LOL, Ben

I will let you know what I find. Hopefully I will get a chance to tinker with it some more this weekend. It is a small 6-71 I am pretty sure, it is a BDS how do I tell by looking?

I will pull a few plugs next time I am over there. I am going to go ahead and get a couple power valves to see if that is the problem.

These are "blower" carbs that were set up by a carb shop. So we did not need to make any modifications. Only tuning and with mine I had them set-up in a matter of minutes. The exact same carbs but, I knew exactly what I had in my engine, and I knew where the timing was for sure and everything else about my engine (since I built it). With his car I only know what he tells me or what I can see.

Royce
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Old 04-09-2004, 12:48 PM
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Royce,

try letting it idle for a few min, then check a front and rear plug. I would be curious to see if his is pushing the fuel towards the back too.. I dont think you can check if it is a small or large unless you pull the blower off. Good luck this weekend

Ben
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Old 04-09-2004, 12:55 PM
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Royce,

Is that total timing.....surely it is not initial. If so, that is a bunch!!!. I run 20 initial with the S-10 and it is perfectly happy. I dont really know alot about blower setups, but those two carbs still seem they are very large for part throttle application. While they might not be oversized for all out performance, I think that they could attribute to the problem when rolling in the throttle. I also agree with your conclusion that the main circuit is lean. I dont think that the PV's should be the problem with 8.5, but if when you roll into the throttle the vacuum stays above that point, you could be on to something. On your engine, vacuum may drop faster to below the PV open point than it does with your friends. If you have a piece of vacuum hose and a gauge you could watch it when driving and see how it is acting. Vacuum is strange to watch. My engine in the S-10 idles at about 8 inches of vacuum, but jumps to around 15 while cruising. It pulls 2x the vacuum under load at a higher RPM.......it was strange to me to watch that happen. Of course the needed rich idle and leaner cruise RPM kindof explains it. Just something else to watch and think about when you get to drive it.

Chris
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Old 04-09-2004, 02:03 PM
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I once pulled a set of #100 jets out of a SBC and after putting the right cam in went back to a #67 jet, the pump affects everything while the throttle is in motion that is why you adjust it first before adjusting jetting at constant throttle.

Jet for the plugs, adjust the pump cam for acceleration and throttle response. If the PV is opening at idle or during cruise you will chase your tail that is why I recommend putting a 6.5 in for testing.

Here is a quote from a Holley tuning book I have;

Quote:
Pump Cams
Another way to tune the accelerator pump is via the replaceable accelerator pump cams. Much like the engine camshaft, the carburetor pump cams have varied lift and duration profiles. The specific cams are color-coded, and each cam has a pair of mounting positions. Brown and yellow pump cams are designed for use with 50cc pumps, while the other color-coded cams can be use with standard accelerator pumps. Like shooters, the pump cams require experimentation to determine which cam is best suited for the application. Most sharp tuners will first tailor the pump discharge nozzle (shooter) and then utilize the pump cam to fine-tune the system.
I happen to think that is backwards and tuning should start with the cam since in my experience the shooter is right on the money 99.9% of the time.
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Old 04-09-2004, 03:38 PM
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Ben,
I will let you know about the plugs, I hope to work on this some more this weekend or early next week. He might not want to touch it because it is running great but, I am not happy with the way it is set up.

Chris,
With the mag what you set is what you get. No advance built in, they are designed for WOT use only. If you have a drag car with bo alternator they are great (you don't need and power for them), they have no place on a street car. I explained it to him and he agrees and wants me to swap it out over the winter. I will hook up the vacuum gauge with a longer hose and watch it from the drivers seat.

4Jaw,
I hear where you are coming from and I have fixed may a problem by adjusting the pump shot. This is really acting a little different but, since I asked the question I must not have it figured out on this car. I will honestly try to adjust the pump shot and see what happens. I didn't really give it a thought because it doesn't act like that at all. When cruising at a steady speed with the 70's in the carb, the car was still surging (not accelerating just a steady cruise) in that case that indicates a lean condition and the acc pump is not in the picture, do you agree with that? The pump cams are pink not black.

I just got off the phone with him and we are going to play with it a little this afternoon, I will let you know what I find.


Royce
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Old 04-09-2004, 07:28 PM
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You can determine whether you have a large bore or small bore 6-71 by pulling the carbs and the carb adapter plate. There is a difference in the size of the support that crosses the middle of the opening. I don't remember the dimensions, but BDS can give them to you. No need to pull the entire blower.

I'm running 2 Edelbrock 750s (#1407) on a 350 with a small bore 6-71 at 8 lbs of boost. I kept the standard size primary metering rods ( .071" x .047") and put in smaller jets. I put .104" jets in both the primaries and secondaries. The standard (as delivered) for the primary was .113" and the secondary was .107". These are not blower carbs or anything set up special. Took them out of the box and bolted them on.

I determined my carb set-up by using data from the Edelbrock website. They recommend using 2 - 600 cfm (#1405) carbs for blower applications and also list the jets, metering rods, step up spring and needle & seat as a starting point. Using the jet and metering rod dimensions, I calculated the effective area of the jet in both power and cruise modes. Since the 750 should flow approximately 25% more (750 = 1.25 x 600) than the 600, I increased the effective jet size for cruise and power mode by 25% from the Edelbrock 600 cfm recommendation. I built a spreadsheet of all the jet and metering rods sizes available, calculated the effective jet size for each combination and then picked the combination that gave me the 25% increase. And then I got cheap and didn't want to buy both jets and metering rods. So I found a combination that was close by just changing the jets.

Idles good, cruises good, light & heavy throttle acceleration are responsive, no back fire and no bogs. I just checked the plugs and they look text book. They're not Holleys, but they run just fine.
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Old 04-09-2004, 08:21 PM
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all im going to say is im glad i found this web site,. if i had someone like ya'll living around me i would never have a carb problem. i have learned alot by asking and reading every ones post .. robbie
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Old 04-09-2004, 08:52 PM
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If the brown cam and 50cc pump does not make your engine run better...I'll eat my hat. You guys are killing me here...I'm giving away all my tuning secrets on the internet of all places...sheeez!

Brown cam, REO pump...every hot rod I have seen has benefitted from this mod.

Here's a great article...but leave the shooter sizing till last...just my opinion.
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Old 04-09-2004, 10:21 PM
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Well lets see if i can add something to this. Several guys had some good information espesaily S10. I put one of my engines
on the dyno last fall and this is some of the results. This is a 604 ci.
but some of the results can be compaired. Started with 850 holly
carbs that flowed 930. At a 6500 rpm pull the 930's were so rich
at top rpm there was a explosion when backed down to idle and
cut the ignition. Too much fuel and not enough AIR. Next we changed carb addaptors and installed 1150's. First pull with 95
square jets was way to leane so changed to drilled out 100 sq.
still to leane. Now to much air and not enough FUEL. Next step was to add fuel blocks with power valves and ended up with 95 sq. jets. Power valves fixed the problem but at that time carbs
were not boost referenced so i used 2.5 pv. temporaly. Now for the timeing test. Started at 30 and ended at 38.
30 made 996
32 -------1002
33--------1011
34--------1008
38--------1003
Blower motors do not need a lot of timeing.
oy
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