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 Post subject: Let's discuss my carb
PostPosted: Sun Mar 31, 2019 7:49 am 
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Location: Orange County, NC
I've gone back to running my trusty old, and I mean OLD, Barry Grant alcohol carb. It's consistent, but has a couple of small issues. The first one is a bog off the line. If I go wide open off idle, it stumbles. Always has. I've tried about every combo of squirter and cam and can only make it worse. I even had a buddy who used to run a record holding super stocker messed with it one day and he couldnt make it any better either, and he was a really smart carb guy. If I take the idle up to around 1800-2000 before going wide open it's fine, but from idle (about 1400), it stumbles. I dont know off the top of my head which cams are on it now, but it has 50's in the front and back along with 50cc accelerator pumps.
The second is there is a flat spot during acceleration. It only lasts about a second or so, but I can feel it in the seat. It's in the middle of high gear when it happens. The converter is spragless so it's not like I am feeling the converter lock up. Changing float levels or pump pressure has zero effect on it. When I borrowed my brother in law's 850 alchy QuickFuel carb last year, it didnt do either of those things. And because it didnt have the flat spot in high gear it was a touch quicker.
At this point I'm starting to wonder if it's in the metering block circuits. But I dont really want to spend $180 on new metering blocks to find out it still does it.
Neither issue is enough to keep me from running it and going rounds, but I know it's not as "right" as it should be.


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PostPosted: Sun Mar 31, 2019 3:53 pm 
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G day Rob,
A couple ive had with alcohol carbs have been how the idle has been set.
Have a look and see where the primary throttle plate is sitting in relationship to the trànsition slot.
Should be just near the bottom of it, if it is too far up the slot you will get a flat spot, you can close the primary down so you have 10 or 20 thou of transition slot exposed under the plate and then set the idle speed using the secondary idle speed screw.
Everyone says front and rear should be even, maybe in a perfect world.
With 50 cc pumps, make sure you use the proper 50 cc pump cams ( brown? Not home at the moment)
And 55 or 60 shooters.
Richen the idle mixtures untill it doesnt stumble.

Your other issue, is the fuel pressure constant through the pass? Correct floats?
What size needle and seats?
What fuel system?
Cheers


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PostPosted: Sun Mar 31, 2019 4:27 pm 
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Thanks Paul. I'll have to take a look at the idle slot and see where it sits. That is one thing I havent fooled with. Honestly didnt even think about it.

I dont know if the fuel pressure is constant or not as I cant see the gauge from inside the car. It has .150 needle and seats in it and the correct floats. It's always done the flat spot thing (and I've had a couple of different electric pumps on the car), but I just didnt notice it as much until I ran the injection which didnt do it at all.

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1969 mustang, 377C/powerglide
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PostPosted: Sun Mar 31, 2019 4:58 pm 
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Gday Rob,
I'd be trying to get a handle on fuel pressure through the pass if i could.
Sounds like a supply issue to me, you would think it couldnt be if another carb worked fine but the way two carbs are setup there fuel usage can be very different and if you are bordeline on supply you'll maybe have issues.
What pump are you using? What pressure?

Also silly question but how do you set your floats, if you are using an electric pump with 150 thou needles and decent fuel pressure than chances are you have your floats lower than where they should be.
Very easy to lose float drop when setting by the sight glass with an electric pump and 150 thou needle and seats.
When the float drop is reduced so is the fuel into the bowl.
Fuel supply or fuel into the carb issues through the needle and seats usually show just into top gear in my car.
Cheers


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PostPosted: Sun Mar 31, 2019 7:12 pm 
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Location: Orange County, NC
My carb has float bowl extensions, the other carb does not. I run the pressure just under 8 lbs. The pump is a Magnafuel 300.

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PostPosted: Sun Mar 31, 2019 7:47 pm 
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Gday Rob,
You run 1/8 or 1/4?
You might get away with it on the 1/8,?
IMHO on the 1/4 you don't have enough fuel supply.
Fuel bowl extensions are an old school way of trying to crutch inadequate fuel supply and you introduce a lot more fuel slosh in the bowls.
A 300 pump with a front tank and say a bg throttle bypass where you set up the idle pressure to 3 to 4 psi then have the main reg set to 9.5 - 10 psi when you full throttle is a good way to do it with an electric pump.
There are some 150 needle and seats that flow a lot more fuel than others.
Cheers


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PostPosted: Mon Apr 01, 2019 9:36 am 
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My gas carb (ProForm center + QFT parts) did the same thing. Would not leave clean until staged on the brake @ 3100. I haven't tried the alky carb with low RPM leave yet so no help there.

Now heres one - last week had all kinds of issues with the 650DP gas carb stumble leaving @ 2200 on Friday. Upped the shooters - no change. Saturday I brought the old-old check ball style bowls and used those to replace the fancy QFT aluminum bowls that use the little "umbrella" as a check valve. All new parts BTW. Replace bowls - problem gone.

I don't know.

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PostPosted: Mon Apr 01, 2019 9:38 am 
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I'd say that your carburetor has what's called a "lean hole" that you run through and or drop into when going down the track. What happens is that the fuel curve of the carburetor isn't calibrated very well and in order to get a mixture that's reasonably close for max HP at high speed the engine ends up with an area that's too lean down low. I bet that if you had the engine on the dyno you could see it easily by just throwing some jet at the engine. High speed power would probably go down but somewhere at lower RPM the engine would come alive even though up high it is way too rich. This was really common to see in the old pre HP 390 Holleys that were built for some Nascar racing. You can jet it to get the best torque or the best HP but not both...

Most likely the problem is caused by the booster having the wrong discharge channel size for methanol which is making it difficult for the circuit to "get going" at lower demands.


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PostPosted: Wed Apr 03, 2019 4:48 pm 
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Thanks guys.
I messed with the carb a little tonight.
Paul, I'm currently running 1/8 mile. I honestly dont think it's a supply issue as I used to run 1/4 back when the car had a 3 speed in it, and only had a Volumax 250 pump, and it never laid down during a pass. Not sure if it still had this flat spot after the shift as it generally wound up a lot quicker with the gear ratios being closer then they are now with the glide, but when I was logging passes and printing them out it never showed up on a graph. It could be a supply issue with this pump versus the Holley pump, but I would think the glide would be easier on consumption than the 3 speed. I could be, and probably am, wrong on that assumption though.

I did check the idle slot tonight, and it's near the bottom on both. I didnt pull it off to check from the under side, but when I opened the blades slowly it seemed like there is a lot more above the blade than below.
So then I checked the squirters. I guess in the midst of all the trying to get the stumble out we changed a few things. It had 45s in front and rear, and the pump cams had been changed as well. I put 50s back in it as well as the correct pump cams. It seems better, but it's kind of hard to tell while it's on the trailer. I raised the front float a touch as well, and bumped the fuel pressure a little. My experience has been if I go any higher on it with this carb she will start pushing fuel out the boosters at idle.
Like I had said before, it's not something that will keep me from running it (last time out it was pretty consistent and went several rounds), I just dont think it's optimum for ease of racing. I'll find out saturday (hopefully) if I made any progress or not.

That could be it too Dave. It's an old, and I do mean OLD, BG carb. The original owner said when he first got it back from BG after they converted it, fuel would run out the headers while the car ran. He sent it back and they fixed that issue, but then it wouldnt go down the track clean. He sent it back again and it worked so he stopped fooling with it. I've always though the jet spread in it was rather odd, but since it worked I never dug in to it too deep. It has holes in the main wells right above the main jets to richen it up. Because of that it runs best with either .136 (maybe a .135, cant remember off the top of my head) or .140 jets in the front and with either .180 or .185 in the back, and that's the way BG said to run it.


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PostPosted: Wed Apr 03, 2019 8:51 pm 
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That's how they did methanol conversions in the old days by drilling a constant feed hole above the jet. Most of the time they are between about .125 and .150. This works but it doesn't work as well as having all of the fuel enter lower in the main well through a single jet. One problem with the constant feed hole setup is that since they are higher on the block than the jet they are more susceptible to being uncovered during racing maneuvers.

It probably has a power valve in the front and none in the rear. If that's the case many times the jet spread with and without can be about 40 numbers with methanol.

On methanol conversions if the main well and the booster hole are not sized correctly they can have a tendency to "slug" where solid fuel, then air, then fuel comes out of the booster.


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