I'm running a Holley 600 double pumper set up for boost. The fuel system is a mech. fuel pump referenced to the bonnet. The engine is a 305 sbc. I have the jets at 64/80 with the PVCR's drilled to 1/16". I'm using a digital A/F ratio gauge to try and tune it. The problem is that it is running too rich at cruise (~13.5). This causes the engine to load up and slowly stall.

I tried putting a piece of MIG wire in the idle feed restrictions and at first it looked like it helped out. I took it for a drive and would stay around 14.5 during cruise. Then after taking it out on the highway and doing some hard accelerations, the ratio went back to 13.5 and stalled on the highway after turning around. I'm guessing that it is just too rich since it loads up after a while. Also, I checked a spark plug and it was black. Can someone verify the too rich condition and offer some advise on what to do to fix it?

Another thing is that under boost at 4500rpm (full boost at 3500), it starts to sputter and lose power. Is that a too lean condition under WOT? If so, should I up the sec. jets or drill the PVCR's a little more?

Thanks for any help.

 

Fuel delivery. Your carb is running dry after a period of high flow, and the fuel system is not keeping up.

Fix that ASAP, or you WILL (not "may") melt down your motor.

As far as the rich condition, check your bonnet for leaks; and if it still has a power valve, check that. One instance of the dry-bowl lean-out you describe so perfectly, is capable of popping a PV.

 

For the lean problem, I'm going to drill the PVCR's some more and up the sec. jets a little.

The loading up/rich problem is more ellusive. I checked out how much vacuum I'm pulling to see if it was a PV opening too soon issue. It reads 15psi under no load and stays around 10 during cruising. The PV is a 6.5 and I've changed it to make sure it wasn't bad.

Also, to clarify a little about what the problem is. During city driving, it almost never loads up, even if I gas it every now and then. It gets bad when driving on the highway. It will load up about every 15min and I'll have to put it from OD to drive to try to blow it out a little. Once I do that it's fine for another 15 or so min.

It's the worst when I get off the highway driving stoplight to stoplight. It will load up almost immediately if I give more throttle than the bare minimum to get up to speed. Once it loads up, I have to put it in neutral and rev to 3000 and hold it there for a min. till it clears up.

Maybe this extra info. will help diagnose the problem to help me fix it.

Also jets are at 68 and 80 now.

Thanks.

 

Cruising rich is still a problem. If I only have 65's in the primaries, the problem has to be the power valve, right? Is there a possibility that the boost could be causing to make it stick open at times? It's just sometimes the car will have problems and others it drives perfect. Could it be something besides the carb?

 

Nobody has any ideas? The guys at turbomustangs couldn't help me at all either. I tried installing a lower power valve in case it might have been opening too early. I went from a 6.5 to a 2.5 and the engine is still flooding during cruise speeds.

 

float levels too high??

too much fuel pressure for the specific seat??

... just throwing out ideas here.

 

I believe it was the float level being too high. I lowered it about 1/4" from the hole. Ever since then, it hasn't loaded up from normal driving.

Now, under medium to hard acceleration, it breaks up really bad and when I let off, the carb floods the engine and it dies. I pop the hood and the top of the carb is saturated. This carb stuff is starting to get frustrating.

 

Just curious, have you asked on the Power Adder forum here?

Maybe I'm trying to read this thread too fast, but is this a blow-through or suck through system?

 

I posted here because my carb is messing up even without boost. Also, not many people on the power adder forum are carbureted (or if they are, they're not posting their problems). I could try over there too.

Also, my setup is blow-through. I modified it following the instructions atBlowthrough Carb How-To.

 

sounds like your fuel pump isn't making enough volume to keep the bowls full at WOT.

I've done a lot of carb work, but not with blowers.
I am wondering, does your blower have a wastegate (or whatever they call em for blowers) to vent the boost when you lift on the throttle, because if you slam the throttle blades shut, but still have residual boost, it might be physically "blowing" the fuel out of the vents and boosters....

I remember seeing a 240z video that had a centrifugal blower and it had a wastegate.

I was going to post a 2 meg .wmv file, but it won't let me.
...no videos allowed here??

 

You're thinking of a blow-off valve and no, I don't have one (wastegate vents the exhaust, bov vents the compressed intake air). Others have ran blow-through without one, but maybe I do need one?

Also, it is an overly rich condition. I even disconnected the secondaries and it still did it. Plugs are black as oil and black smoke out the exhaust when it loads up. I'm about to try blocking off the power valve, just to see what it does. The fuel pump could be pushing too much and causing the float to stick, maybe? There are just so many variables, I don't know where to start.

 

like I said above in #6, it could be too much pressure....
AND also not enough volume.

they are different.

check the pressure at idle, for a carb it shouldn't be more than 6 psi for a regular needle and seat. there are some racing applications that will allow 9 or 12 psi, so you need to know what the pressure is first, and what needle and seats it has in the bowls.

then do a volume check, how fast will it fill a gallon jug?

then compare that to how much fuel it needs at WOT, then you could tell if something is wrong there.

do you have a good, (small micron) fuel filter or screen that is high-flow?-- (restriction in the path)

I have seen some quadrajets that will suck the fuel out of the bowl vent at higher throttle openings.

what is the bonnett that covers your carb? if it is closer than 1/2 inch from the top of the bowl vents, it can suck fuel out there due to high airflow....

 

Fuel pressure at idle is 6psi. You're right about the pressure/volume relationship.

How do you do the volume test and have the engine running at the same time?

For the fuel filter, I'm using what came with my dual feed fuel line.


I fabricated my carb bonnet. It is 2 3/4" tall. I think the primary vent tube is around 2".


I'm going to be going home this weekend (live at college), so I'll be able to try more stuff. First, since the spark plugs are black, I'm going to replace those with a heat range hotter (R43TS to R45TS). I'll see what that does first.

I"m also going to do a test to see if the fuel pump is putting out too much pressure at WOT and pushing fuel past the float (since it is boost referenced to increase fuel pressure with boost pressure). I'm just going to get rid of the reference, so is operating normally, and see if the fuel is still coming out of the top of the carb.

I'll post my findings and hopefully I can get this thing to run right.

 

do you have a mechanical fuel pump or an electric one?

electric pumps obviously don't need the motor running.
mech ones should pump at least a quart in 15 seconds at idle. the bowls in the carb will allow at least that amount of running time before it drains them dry and it dies.
there should be no bubbles in the line(hard to see if you have a black rubber line, you can use a clear one if you can find it.)
mech ones will pump a BIG stream every other crank revolution (one cam revolution), kinda' like having an orgasm, if it just dribbles out, it's getting old. it might be making enough pressure, just not enough volume.

I would do away with the sintered brass filters you have and buy a good (read-"expensive") in line filter. I have seen too many of those get plugged up.

be careful with the hotter plugs, be sure you're not fixing a symptom instead of the cause. if you have raw fuel on top of the carb and the plugs are black, I would say it's not the plug's fault. hotter plugs can cause detonation ESPECIALLY in a pressure fed motor-- you don't want to fry a piston or rings..... I just had a friend who did some messing with his egr valve on a supercharged regal and it broke ring lands on like 4 of 6 pistons, and couldn't even hear the detonation... has the computer thing going and it didn't even register knock sensor increase.

good job fabricating your bonnett, but you may want to invest in a flowed, tested, proven design, that has had many hours of dyno research and development that might have the air flow from top to bottom, and not around in a circle -- there's a reason they're expensive....
(I'm not saying yours does that, it's just a possibility...)

 

My fuel pump is a new Carter strip mechanical fuel pump. I like your analogy to the worn out fuel pump.

The plugs have to go anyways, because a set can only take so many over-rich cycles. Last year (pre-turbo even), my car just wouldn't start one day due to wore out plugs. So, I figured I could try a heat range hotter while I'm replacing them anyways, and watch for detonation.

As for the filter and bonnet, they are going to have to stay for now. Once I graduate and get a job, I'll be able to make some upgrades.

 

I appreciate all your help. Some of your ideas were an improvement, but all of the conditions were linked to one main problem. I've been getting some blow-by for some time now and wasn't sure where it was coming from until I did a compression test and another test that pressurized the cylinders to determine where the air was escaping. It turns out that I have three bad cylinders in which I either have broken rings or cracked pistons. My last drive uncovered a knocking sound coming from one cylinder that is definitely a cracked or broken piston.

I believe this is what was causing most or all of my tuning problems. I now have to replace the engine and I am curious to see how many problems go away just by putting in an engine with even compression on all cylinders. So, after that is done, I'll probably post what improvements were made by replacing the engine (and if I have any more questions). Thanks again for trying to help me out.

 

Being a carb must be sorta like having a little brother. Even though it wasn't your fault, you're still the one that gets blamed.

Well, hope you can get it back up & running soon.