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| Symptoms of too Large a Carburetor? https://slantsix.org/forum/viewtopic.php?t=16246 |
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| Author: | bwhitejr [ Sun Feb 19, 2006 3:03 pm ] |
| Post subject: | Symptoms of too Large a Carburetor? |
What are the symptoms of too large a carburetor? I am almost convinced that is the case I have. I have read two Holley books and they all state that the carburetor should match the application. This is a moderately modified 225, a Holley 390 cfm, a 4800 GVWR truck 3.55 rear end and A-833OD transmission. Do I have a problem? bwhitejr |
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| Author: | CStryker [ Sun Feb 19, 2006 3:30 pm ] |
| Post subject: | |
Stock cam and valves? If so you might be pushing it (a little bit), but it should be close enough. There is a lot of bad vibe towards the 390 Holley on this board, not becaues of size, but because of the carb itself. I've never worked with one, but I'm sure someone will soon reply that it's part of your problem. They symptoms of overcarburation are generally a bog and lack of low end power. Once you get up in the higher rev ranges it will feel 'ok' but there is still more power to be had from a properly sized carb. What power valve and jets are you running? |
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| Author: | emsvitil [ Sun Feb 19, 2006 3:33 pm ] |
| Post subject: | |
Does a holley 390 have vacuum or mechanical secondaries? I don't see a problem with vacuum secondaries as they won't open up until there's sufficient airflow. Mechanical secondaries are harder to tune if the carb is too big. |
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| Author: | imnoisydart [ Sun Feb 19, 2006 3:59 pm ] |
| Post subject: | |
All 390's except the special HP one (not common) have vacuum secondaries if this helps. |
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| Author: | DusterIdiot [ Sun Feb 19, 2006 10:03 pm ] |
| Post subject: | Uh, no... |
Quote: Do I have a problem?
Considering I run your cam, a hyperpak intake, stock valves, 3.55 gears, same tranny in a duster...no, you're not over carburated...in fact on my car I've found it's slightly under carbed and should have a 450-500 on it for 'best results' (600 was a bit big, but got great response)... something tells me it just needs to be tuned better... -D.idiot |
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| Author: | bwhitejr [ Mon Feb 20, 2006 4:29 am ] |
| Post subject: | Symptoms of Carburetor too large |
I have read two Holley Books, and they both point to the case of the idle slot is covered too much. The throttle plates are almost all the way closed at idle. The idle speed is 850 RPM. I think it might need to go up, just to be closer to the slots for off-idle response. I am confused about the role of the secondary idle stop. Why would it be on the bottom of the carburetor where you can't get to it. And I am not even sure of what it does. bwhitejr |
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| Author: | DusterIdiot [ Mon Feb 20, 2006 9:01 am ] |
| Post subject: | heh... |
Quote: Why would it be on the bottom of the carburetor where you can't get to it. And I am not even sure of what it does.
This is one of the big 'ooops' on Holley's part, it's a real pain...The adjustment is for two reasons...one you can adjust where the plates are in relation to the transfer slot (if not lined up right, when the secondaries kick in you'll also have a neat 'bog' like you have now), also it can be used to adjust the secondary plates so you have a bit more adjustment on the front plates.... Stock the 390 won't run on my Hpak...if I open the primaries so they are into the transfer slots about .12 it's great but my low idle is 1500-1800 rpm...(not a good setting)....If I tip the rear plates so they just barely touch the bottom side of the transfer slots, and close the front ones a bit...I get a nice idle of 850-1000 and I get a wider range of tuning from the more accessible primary idle screw... I also find changing to a quick change secondary spring kit to make secondary tip in tuning much easier.... -D.Idiot |
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| Author: | slantzilla [ Mon Feb 20, 2006 10:51 am ] |
| Post subject: | Re: heh... |
Quote:
I also find changing to a quick change secondary spring kit to make secondary tip in tuning much easier....
Yep, that is some of the best money you'll ever spend on a vac. secondary Holley.-D.Idiot One simple test for too big a carburetor is run it as fast as it will go wide open, then let up on the throttle a little and see if it picks up speed. Hard to believe that a 390 will be too big. 
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| Author: | bwhitejr [ Mon Mar 06, 2006 4:37 pm ] |
| Post subject: | Too Large? |
An update. I put the screw in the 2nd hole for the cam (the holes are not very far off) adjusted the idle mixture screws - driver side is 5/8 of a turn out from seated and the passenger side is 1 5/8 out from seated. Should they be the same? bwhitejr |
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| Author: | Slant Cecil [ Mon Mar 06, 2006 7:12 pm ] |
| Post subject: | |
I always adjust the idle screws the same # of turns out. This may not be perfect, but without an A/F meter and maybe 6 EGT gauges, I think it is the best way to set it. The secondary idle screw in a vacuum secondary carb is to expose just a bit of the transfer slot to pull a small amout of fuel from the bowl. This is to keep the fuel from going stale if the secondaries are seldom opened (what's the purpose of adding a 4bbl if your not going to open them up?!). I find it's easier to get a smooth idle with the screw backed out, closing the secondary butterflies. If you have a bog, make sure, the float level is right the jetting close PV with the correct vacuum opening point clean idle and main air bleeds Then tune acc pump with cams and squirters to eliminate bog. Secondary opening points have two parts, opening point and opening rate. Rate is with different springs. Lighter spring has a fast opening rate. Rate does not control when opening begins. Point is with kill bleed sizes. This required drilling and tapping bleed port and using set screws with different size holes in them. The bigger the hole, the later the opening point. So a smaller bleed hole and a lighter spring will have the secondaries start opening sooner and be fully open quicker. You'll know the carb is too big when driveability and low end power drop off. The 340 Six Packs had 980cfms for 340 cu in and they drive OK, 390 cfms on a 225 cu in motor so do OK also. |
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| Author: | bwhitejr [ Tue Mar 14, 2006 7:11 am ] |
| Post subject: | Symptoms of Carburetor too large |
Well...I reset the secondary idle stop to where it is open and maybe 0.040 from getting into the transition slots. I have more power now and the car feels "lighter" the by-product of the above is that the idle is rougher. I can turn the RPM's down, but the quality of idle is worse. I like the new added power, but I would like a better quality idle. I still have the original problem of any acceleration ...like at a stop sign, normal start... it bogs and then if I keep pushing the accelerator it picks up and goes. Holley Tech Support says to change the #25 shooter to a #35 shooter and it should work. I will be picking up my new #35 shooter this afternoon. What are the chances that the larger shooter is the fix for the off-idle stumble? bwhitejr |
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| Author: | 1969ron [ Thu Mar 16, 2006 4:41 am ] |
| Post subject: | |
hi, while your waiting for new #35/.035 to come get a 1/32 drill bit and drill out your old #25/.025 to 1/32/.032. it will give you a idea if the bigger pump shot will help you out of the hole. ron |
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