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Old 06-20-2007, 10:14 PM
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Odd Carb Problem...

I am having a carb issue I have not seen before and I want to see if you guys can help. I am using a holley 350 cfm 2bbl on my 300cid 4cyl. antique pulling tractor. Calculating the engine at 70% VE naturally aspirated it shows to need around 300cfm, so the sizing should be about right.

The problem is that the carb floods badly at a fast idle around 2000-2500RPM. I pulled the power valve and upped the jet size thinking I probably had a power valve issue. This only made the problem worse. After investigating more I found that when the throttle blades begin to open it begins to drip fuel from the main fuel circuit.

I checked float levels, needle and seat holding pressure, and blew air through the metering block and air bleeds in the main body. Everything I have checked seems to be okay. Jet size does definately effect the issue and the reaction is as expected; worse with larger jets.

Any ideas would be appreciated.

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Old 06-20-2007, 11:46 PM
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Any idea what your fuel pressure is at? My best guess is too much fuel pressure. What kind of fuel pump are you using?
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Old 06-21-2007, 01:28 AM
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I went through this years ago turbo on my cutlass, and the usual things that cause this can be several things. Too much fuel pressure (many swear holley`s can handle up to 9psi and I don`t doubt it but I never had one handle anything past 6)
Timing set too low so the idle has to be moved up to compensate so the main boosters come in since it`s past the point of the idle slots cause the carb "thinks" it`s off idle. Lack of a vent in the fuel system, pressure builds up, and has no where to go and the only outlet is the fuel line to the carb so the pressure over comes the needle.
Lastly just as a precaution, Check the needle, make sure the rubber tip is still intact. good luck.
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Old 06-21-2007, 04:41 AM
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Carb

Ive had same problem It was usually dirt in the carb and gas was commin out all over the place had them rebuilt professionally 150 bucks and problem solved I now have 2 big fuel filters on it Havent had a problem since.
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Old 06-21-2007, 06:55 PM
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what jet are you running? its possible it just has too much jet you didnt say what cid it is but as a comparison i run a circle track deal 355 inch 500 cfm holley and it has a 73 jet with a power valve. as a guess with a small cfm small inch deal a 50 is probably close. try putting in the jet you took out but with out the power valve and see. it will be lazy with no valve but its worth a try
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Old 06-21-2007, 08:18 PM
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After reading all of this, I agree with most of it. First, 3-3 1/2 lbs is plenty of fuel pressure for that carb, and motor. Next, double check the needle seat for a burr, or something showing the needle is not seating. Third, install your stock jet. Afdter that, is the power piston spring ok ? And, finally, at 2000 rpm's, you are no longer on the idle circuit, but you are on the transition circuit, so you will see a fuel drip. Also, check the throttle shaft for excessive wear. There are a few other things also. But do those checks first. Oh, and is the base gasket good ?
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Old 06-21-2007, 10:20 PM
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Thanks for all the ideas. To answer a few questions....

The fuel pressure is right at 6psi and the pump is a high pressure holley EFI with a boost referenced 4309 mallory regulator. I checked for the needle and seat by removing the site plug and running the pump. I have not checked the bowl vent, but I am going to.

The engine is 298cid and timing is locked at 24 degrees. The engine has a big cam(for a tractor engine) that is very comparible to a 270-280 duration range crane, and it is solid flat tappet.

I am running a 74 jet and have tryed a smaller one which does help the problem. However, running 20psi of boost, I want the main fuel nice and fat to start and I will lean it back.

I actually called Holley today and they pretty well said what Denny and DV did; at 2000RPM and cracked throttle, it should start dropping fuel.

So, I have come to the conclusion that this little carb with the big jets is just going to be very fat off idle untill I send it off for "tuning" at a carb shop. I am going to get it set up with adjustable bleeds along with the usually blow through tricks.

I am going to pull it tomorrow night, so I will find out then how it works. I am planning to come out of the hole around 3000 RPM, so I should be able to spool the turbo when I start loading the engine with the clutch(2 disc slipper). My hope it is will clean up at that point and come alive.

Chris
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Old 06-21-2007, 10:36 PM
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It sounds to me like you need to drill a hole in each throttle blade so you can close them back down and get it back on the idle circuit instead of the transition slot, then you can tune the air/fuel mixture screws for a cleaner idle.
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Old 06-22-2007, 05:07 AM
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Thanks for The help

I'e noticed a lot of gas going back in the return line from the mechanical pump (stock) Dont know if that is normal .The idle came down a bit but still too high ,not knocking when I urn it off now Ive put all new vacumn lines on and they are all tight seems to be lots of vacum , The intake is an older Torker and had to ream out center bolt holes . I still have to try some of the ideas but been raining for a week here and truck is outside.
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Old 06-22-2007, 09:00 AM
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Another thing to keep in mind is vibration. 4 Cylinder 2bl carburetor calibrations are usually pretty tough on racecars because of this. Not sure how much vibration you're getting on this engine, but it may be leading to the problem. Is there any fuel being drawn out of the squirters?
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Old 06-22-2007, 12:01 PM
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Gas

The only gas I see is whats going in the return line, Itys going in a Gas Can.
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Old 06-22-2007, 06:15 PM
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Quote:
Originally Posted by TurboS10
I am having a carb issue I have not seen before and I want to see if you guys can help. I am using a holley 350 cfm 2bbl on my 300cid 4cyl. antique pulling tractor. Calculating the engine at 70% VE naturally aspirated it shows to need around 300cfm, so the sizing should be about right.

The problem is that the carb floods badly at a fast idle around 2000-2500RPM. I pulled the power valve and upped the jet size thinking I probably had a power valve issue. This only made the problem worse. After investigating more I found that when the throttle blades begin to open it begins to drip fuel from the main fuel circuit.

I checked float levels, needle and seat holding pressure, and blew air through the metering block and air bleeds in the main body. Everything I have checked seems to be okay. Jet size does definitely effect the issue and the reaction is as expected; worse with larger jets.

Any ideas would be appreciated.
I'm a little confused, you said "I pulled the power valve and upped the jet size thinking I probably had a power valve issue."

Is this:
- I removed the power valve and ----
- left it out?
- replaced it with the 1) same vacuum rating, 2) a different vacuum rating?
- drilled the PV jets to a larger size and did one of the above?

- I did something with the power valve and the main jets?

It doesn't take much throttle to get a drip off the main venturis, so I'm not too sure how critical that piece of info is. I guess the first things are to get the float level and fuel pressure right, assuming the fuel valve can shut off incoming fuel without leaking.

- Fuel pressure should be only 3 to 6 psi, more than that will need a regulator.

- float level set to manufacturers recommendations or a bit less, a 300 inch 4 banger has a lot of surges in the manifold from a few large cylinders so the fuel in carb can get into a hydraulic oscillation that the emulsion tubes aren't damping, a larger emulsion jet size would be required, but don't go there till we talk a lot more. First get the main jet size down to about 60 and the float level at least no higher than minimum spec.

- The power valve needs to be a low number, the number on the valve is the manifold vacuum where it opens. I'd say you want less than 10 maybe a 6 so the pumping of this engine doesn't cause it to leak too early.

- Get the idle correct, the best idle on the least amount of throttle opening. Don't forget that the idle fuel screws will also affect the main feed as well so you don't want to feed this thing a lot of idle fuel.

- The throttle bore includes a round hole that should be under the throttle plate at idle. This is where the idle mixture enters under the high vacuum load of a closed throttle. There is a slot, this is the mid range transfer, it needs to be above the throttle plate at idle or it will add fuel before the engine is ready for it.

- While the idle and transfer ports don't add fuel when the main is in operation, it's important to remember that they and the main jets all connect to a central fuel transfer passage (the main well) and affect the total fuel available to the engine all the way thru WOT.

- Then there's the general inspection for failed gaskets and cracks that can allow uncontrolled amounts of fuel into the engine.

- First things first: -fuel pressure, -float setting, -effective shut off by the float valve, -the float isn't saturated or sunk; then report back.

Bogie
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