Carburetor experts only please
 

Twin small blocks had big"1100 cfm holleys on them ran good but last 1/8 throttle boat nosed over and lost 3-4 mph unless you pumped the throttles hard and squirted the excelarator pumps? then it would pick right back up. Was told they were 2 big for the motors which i thought sounded right. Bought 2 brand new holley 750's double pumper and ran tonight and runs better does not fall of at all but if you pump the throttles at WOT it pick up about 1-2 mph then stop pumping it settles right back down? Is it possible that they are pulling vacuum and the power valves are closing?

Carburetor experts only please

First off check your vacum at idle
divided it in half add one, that is the size you need for your power valve .#2 sounds like you need to go to a larger jet size . Have you checked your spark plugs after w.o.t. sounds like its lean. #3 fuel pressure and volume. Hope this helps

Not an expert but I know one.

Call Nickerson. Everything they've ever done for me has been downright amazing.

Vacume @ idle good start. Went to larger jets today helped maybe a little but running 74 primary 87 secondary I think that should be big enough. Fuel psi is 6-7 new pumps and regulators.

Could tell you what's going on, but since I don't consider myself a carb expert, I'll just shut up.

Thanks for all of your help. You rock

How aggressive are the small blocks? 350/s or 400 plus cid. You may be losing vacum signal at the venturis due to cfm still being too much. Venturi air speed is what creates the the draw to pull fuel thru the jets and into the motor.

Thanks I guess from what I was told today that it is normal to gain some speed from pumping the throttles at WOT and hitting the accelrator pumps.
Thanks for the help

you really ought to consider putting jet extensions on the secondary jets. fuel tends to run to the back of the float bowl under accelaration.

did you ck the fuel pressure at WOT throttle ? first carbs seemed to big....but sounds more like a delivery issue to me, sometimes you can set that pressure 5-6 psi at idle but anti-siph. valves and restrictive marine fuel inlets choke out flow at wot, Rob

Hey Rob,

I checked the psi at WOT and it stayed steady. the speed does not drop off at WOT anymore and I guess it is normal for the boat to pick up when pumping the $hit out of the throttles and hitting the accelerator pumps?

Thanks

Mike

pumping the throttles at wot won't get any accel shot..you are way past the cams for them to work, need to run wide open on fresh plugs and do a plug test, sounds like the jetting is off...not supercharged are they ? Rob

Rob,

no they are not supercharged. I checked the plugs and they looked ok a little gray not white. the new carbs came with 71/83 and I went up to 74/85 which I think should be enough jet. Someone else mentioned jet extensions which makes sense to me, I think I am going to try them?

74/87 sounds like a big gap to me. I would try to keep the primary/secondary size gap around 6-10 jet sizes. The reason primaries have smaller jets than secondaries is most carbs don't have power valves in the secondaries, so the secondaries get larger jets to make up for it. Then again, if they came stock with a 12 size gap...

If it were me, I would try 79/87 and read the plugs. Always better to start rich and work down, then the alternative. Also, check here on how to read plugs:

http://www.4secondsflat.com/Spark_plug_reading.html

This is 100% false you should not gain anything from pumping the throttle at WOT. If that were the case every race boat, drag car, and cup car out there would have the driver (throttle man) pumping the throttle at WOT. This is usually an indication that you are running lean!

Jet extensions should not be required on a offshore boat since they are designed for high G-force loads away from the rear metering block which would happen as you're starting to accelerate, NOT once you're already up at WOT.

You can install a vacuum gauge to check to see how much vacuum the engine is producing at WOT, but with a carburetor of that size with a mild engine if the butterflies are all open it is highly unlikely that you'd be building anywhere near enough vacuum to shut the power valve.

Your tune up on the carburetor is out in left field most Holley carburetors will have about a 6 to 10 number spread (at most) if your only running a Power Valve in one side of the carburetor and haven't made major calibration changes internally.

If you're having to jet up the smaller carburetor that is generally an indication that the fuel system is not keeping up with the demand of the engine. Remember pressure and volume are not the same just because you have the proper amount of pressure doesn't mean the fuel system is keeping up with the demand of your engine.

You should probably flow test your fuel system to make sure it is flowing what it is supposed to, and possibly reset your float levels.

There you heard it from the expert.

:food-smiley-007:

YOu still haven't said how nasty these smallies are. 1100's are way big for a small block and for most applications a 750 is still to big. Don't listen to whoever is telling you that pumping the throttles will net more speed, they are idiots...Sorry, I am not an expert either. ;)

Thanks for all of the advice sounds like we are way of track with our thinking and will try the advice. I installed 2 new fuel pumps and regulators and new fuel line from the pickup's and checked there are no restrictions in the pick ups? Hmmmmm

Wellllll they are 350's with 10.5 comp and bow tie heads. Rollers cams with a little bit of lift. Nothing crazy breating thru stainless marine exhaust.

I would run a vacume gauge at this point see what vacume you are running at wot, compair to your power valves, are you running power valves in primary only ? if you are running ft and rear your jetting is to far apart...should be close to sqaure with 2 or no power valves, if only in primary about 8 sizes richer without the valve. jet extensions won't help a bit on a boat like yours...if you checked plugs after any idling you cannot tell much at all....needs to be fresh plugs and a wot run shut right off and pull the plugs then..Rob

Have you actually flowed the fuel system to see if it's flowing enough fuel?

This combination is not going to make enough vacuum at WOT to worry about closing the Power Valves.

No I did not check volume? WHat is the best way to do that?

Check with your fuel pump manufacture so they can tell you how they want it flowed to make sure it's flowing within the parameters it was designed to.

RumRunner. Please explain how you can have good fuel pressure at WOT and yet be running out of volume. I'm having trouble comprehending that. Seems like if the volume was insufficient the pressure would fall off.

Volume and pressure are independent of each other. You can flow 60 Gallons Per Hour at 6 PSI, just as easily as flowing 600Gallons Per Hours at 6 PSI. Just because you have X amount of fuel pressure doesn't mean it is flowing ENOUGH volume for the engines demands.

I understand that. But, wouldn't the pressure drop if the volume was inadequate? Isn't pressure generated because you have more volume going in than is being used? If the engine is using more fuel than can be supplied then the pressure should fall, right? I always felt if the fuel pressure was good there was not a supply problem, or if there was a supply problem the pressure would drop at WOT. I'm not trying to dispute what you are saying as I totally respect you as the most knowledgable carb guy around. Just trying to understand and learn something.

You're right and wrong... Within a given "Known" situation that was running properly if the pressure dropped it CAN be an indication that the volume isn't keeping up with the engine, however there are a couple of things to look at.

First the fact that you can use a 100 GPH block mounted fuel pump, and have 6 lbs of fuel pressure, but you can use a 300 GPH electric pump and still have 6 lbs going to the carburetor. So with which one will you get more fuel to the carburetor?

Next there is actual an inverse in pressure and volume meaning the more pressure you have the less volume you're actually flowing. Take the little Holley Blue fuel pumps they flow 110 GPH Free flow (no pressure against them), but they are like 70 GPH at 9 PSI. The same is true of the bigger pumps as well.

Now we get into Pressure at the N&S you can actually raise the pressure going to the N&S and flow more fuel through it (due to pressure differential), but you also run the risk of aerating the fuel.

If you were to take two 5 Gallon buckets, and cut a 1" hole in the center of the bottom of both of them and fill one with 1 gallon, and the other with 5 gallons, and put them over the top of a 1 gallon pale the buck with more volume with flow faster through the same hole and fill the 1 gallon pale quicker.

When it comes to running fuel pressure I prefer to run lower pressure (to the carburetor) with larger N&S so I can control it better.

Real life head scratcher (at that time) issue

Fue System In this order - Pump/fuel flow meter/psi gauge / 3 ft of -8 line/psi gauge / carb.
Psi read fine and exact same on the 2 different gauges.
Carb bowl fuel turned into frothy mess than dropped out of view.
Motor ran f'd up / A/F ratios went up thru the moon.
Removed fuel flow meter.
Carb kept constant fuel level w/ no frothing.
Same psi.
Engine ran good without missing a beat and a/f ratios right on target

So,
Not enough volume - plenty of psi

Real life example that Dyno company called BS too......until after we solved it and reported our exact findings.

Thank god for clear sight glass on some Carbs. It proved what was happening and threw some people's thoughts and theories right out the window.

So the frothing was something restricting the volume, yet psi was good? Interesting. I'm still having a hard time with this. If the volume is too low, the needle and seat will stay open because the fuel bowl is not full. Therefore the pressure should drop. What am I missing in this picture? Keep the discussion coming.

Wow this is getting deep I should have "paid" more attention in school :eek:

Gofast -
Tell us, in detail about your fuel system.
Pick up line ID
Any anti-siphon valve ?
Fuel line size throughout - all parts
fuel fitting type and size ID
Fuel pump man/part#/type
If you have a regulator the man/part#/type
Water seperator - housing fitting size and where placed, before or after fuel pump.
Any other filter in system ? Where ? What man/part"/type

If you have an electric pump how far below the top of the tank is it ?

So............every little thing that has to do with the fuel.

You seem to definately have a problem with fuel supply and/or carb so let's attack this. If the system is all in check then you can get good avice from Mr Carburetor ie: Rumrunner.

The pressure comes from the restriction of the open N&S assemblty, not JUST from the volume. Since either the fuel pump is internally regulated (to control the pressure) or it will have an external regulator it sets the pressure not the volume.

I am on the road in Canada right now and will not be home untill the next race next week in Patchoug bu there is what I can remember about the fuel system.

Pick up line ID
Any anti-siphon valve ?
Fuel line size throughout - all parts
fuel fitting type and size ID
Fuel pump man/part#/type
If you have a regulator the man/part#/type
Water seperator - housing fitting size and where placed, before or after fuel pump.
Any other filter in system ? Where ? What man/part"/type

If you have an electric pump how far below the top of the tank is it ?
Fuel line all new 1/2 i think it is the big stuff from west marine, No anti-siphon valves,All line the same including holley daul feed lines stainless to carbs,Pumps are holley middle of the road price not the cheep ones LOL regulator are ne from holley along with psi gauges Water seperators are spin on style before the pump and regultor about 1 foot from the pickup tube. no other filters. no electric pumps either

Rum -

6 psi from a 1/2 inch line will produce more volume, than 6 psi from a smaller line, such as 3/8 inch line. Or, the larger fuel line will pizz out a greater volume of fuel than the smaller line, if the psi is the same at both lines.

That what you're saying, Rum ?

Yup.