I am sorry about your problem. There are so many things that can happen under boost, especially when it goes way lean. If you broke a ring land or burned a piston it would be smoking and would show up under leak down. Boost also puts a strain on valve springs and it is always wise to install stiffer springs. So I recommend you pull the valve covers and look for bent or broken components and then do a leak down test.
So did you do the compression test? If only one or two cylinders have low compression, it sounds like a bent/burned valve or stuck valve/ bent pushrod to me. If all the cylinders have low compression, it might be a timing chain issue, if none have low compression, I'd be looking for fuel/carburetor issues.
Besides the fuel delivery problem Mart3406 talked about,I believe your compression ratio may be a little to high for a blown street engine(especially with todays gas). Your probably around 9 1/2 to 1 and should be more like 8 to 1. Then there's the matter of engine timing,most blower engines need some kind of boost/retard system. A stock distributor would give more advance with more rpm's. It all can lead to bad things happening.
-------- If the carb is enclosed in a pressure box, the float bowl is now operating under blower pressure rather that normal atmospheric pressure. . But I give up If you want to believe - in spite of all known laws of physics and common sense - that you can somehow get fuel at the same or a lower pressure to flow into a float bowl or be drawn into a venturi that's operating at the same or a higher pressure, you're welcome to your beliefs I made my suggestions in response to Scoto's original queries about the likely cause of his suspected and probable 'lean-out' condition - which no one else has offered any explanation for the way'. I didn't post my suggestions to get into s "theoretical' pissing match. I've stated what I believe likely led to Scotto's lean-out condition and have offered up what I think the solution for the problem is, based on personal knowledge and 40-odd years of actual experience. People are free to think and believe what they want and to take what I've suggested or not. If someone's got some better ideas or suggestions,, than post them. But I've made my points and stated what I believe and know That being said, I'm done with this thread.and happily now leave the 'theoretical' pissing matches to the armchair and keyboard-driving "experts". Mart3406 ==========================================
now your post is correct '''by inclosing the carb in a hat it is seeing blower pressure in the float bowls'' my correction was just so those that read this thread ................
I can't find the post that says whether this is a draw-thru-the-carb or a blow-thru situation. Yes, with a blow-through-the-carb you do indeed need the fuel pressure to stay higher than the boost pressure or the fuel flow shuts off, giving a super-lean condition. You can also crush brass or plastic floats with high boost and go super-rich. Some plastic foam floats will survive high boost and some will soak with fuel if you "pressure it in". That can drastically change the float level. We used Holley "nitrophyl" floats on our high boost carbs (no not the goofy tire-filling gimmics. different stuff) to avoid float-collapse. But that doesn't apply to a draw-through carb. And I didn't see any post that said whether this was a draw thru or blow thru situation..... Did I miss one? The plugs do not look like they ran rich, unless he still drove for a while after the event.. If so, he didn't read the plugs in time to get a true picture.... If you shut down immediately and read them right away, then it means something. If you drive home before reading the plugs, then they tell you very little. Right now I have an inclination to agree with the ones who think he blew off a vacuum hose or gasket or something. Check to see if a vacuum line is missing? Vac advance still in good order? Brake booster still ok? No busted diaphragm? Carb gaskets intact?
Just my two cents, I've run a blown V-6 for 10 years, and all of your pictures look like a lean condition to me, red plugs, white valves= running very lean. So I'd have to agree with all the others, a vacuum leak somewhere.
This is correct. One thing that one might bear in mind. Maybe o not fot this problem but just in general is that 10 PSI is sort of a magic number. Above 10 pounds your fuel requirements take a big jump. Most of the fellas aren't running near that but it is something to bear in mind. We have a twin turbo SBC (blow through) that we run around here that regularly runs 14 PSI and we can adjust the waste gates to run 18 on the strip. Originally we had a nitrous nozzle poked directly into the intake. The soloniod was on a boost sensative switch so when we reached 10 PSI it would blow raw fuel into the intake to over come the problem. Now we have a more high tech setup. We also water spray it to cool the charge. Getting back to the problem at hand the exhaust ports look like there may have been a leaking gasket on the header. That will make a hot spot that will show up white. The plugs look to me like richer jets are in order. Maybe I'm wrong but if there wasn't a problem I would look into richer jets. I'm still leaning towards a blown gasket. If the intake isn't the problem then it may be time to take a look at the head gasket. I've had then break between the cylinders and not touch a water or oil hole. It won't act like a normal blown gasket (no water in oil or oil in water and niether in the cylinders) but it does happen. Sometimes it doesn't even cross fire it just doesn't run right. Even just seeping when combined with boost will cause a problem that you wouldn't normally notice. A compression and leakdown test would help with the diagnosis.
Sorry guys, it's a blow-through supercharger with a carb hat from a 1955-56 Ford McCulloch kit. I did drive it a couple miles home, so you're right, the spark plug reading is going to be useless. Didn't even think about that when I was looking at the plugs. I've already checked the carb and vacuum lines twice, maybe I'll have a second pair of eyes look at them just in case I'm missing something, but externally everything looks okay. I guess next a compression/leakdown test is in order. I'll have to borrow a buddy's so I can look into it, hopefully this weekend. As far as the fuel system goes, I thought I had it set up pretty well, but maybe not. I've got a Holley 14 psi/140GPH electric pump with a 3/8 fuel line feeding the regulator on the firewall and a 3/8 return line that is T-ed back into the fuel line at the back of the car between the fuel tank and the fuel pump. The gauge on the feed line to carb shows 9 psi when running. Is that not sufficient to feed the engine under full throttle at say, 7 psi? I know the fuel pressure has to be higher than the supercharger pressure or it won't allow fuel into the carb, right? How much pressure does it take to crush/sink a float? I figured at the low level of pressure I had I was pretty safe.
just to put this in the mix on the blow through subject. remember how people still say that carbs are better fuel mixers than efi? To this day, if you dont need cold startup/warmup, and ability to change tune up and down mountains and atmospheric pressure and such (conveniences) the carb still is better. because of the air bleeds. fuel going into the throat, is already emulsified with air from the high speed air bleeds. SOOO much better than spraying droplets on the back of the intake valve.... other than sealing it up and nytrophil (sp?) crush proof floats and jetting- one of the most important mods to make a carb blow through is to resize the bleeds....... rick
YES. You will need several pounds of extra fuel pressure when the bost goes up. Like someone else said here, it needs to be several pounds more than the boost level. I turned a Holley fuel regulator into a boost-referenced regulator. I used a small copper tube that ran to the carb enclosure for the boost signal to tell the regulator to increase the pressure. I don't have my old drawings, but here are the basics... The Holley regulator used a diaphragm that was exposed to the atmosphere on one side (by a tiny notch in the case under the adjuster screw), and the other side of the diaphragm was exposed to the fuel pressure. At the bottom of the regulator case was a setscrew with a locknut where you could adjust fuel pressure by turning the screw against an internal spring. It used the very same principle that a mechanical fuel pump uses for regulating it's own pressure, only the spring is adjustable. In a normal mech pump, the diaphragm spring keeps the fuel pressure about 5 lbs over atmosphere. When you stiffen up the spring, the pressure goes up. The regulator does exactly that in the same way. In fact, the Holley regulator we used is built just like a mechanical fuel pump, but without the cam lever. Basically a diaphragm, valves, and spring in a can, but without the lever. If you send boost pressure to the "atmosphere side" of the diaphragm, it tricks the regulator into raising fuel pressure to 5-7 lbs (or whatever you set it to) over BOOST pressure instead of 5-7 over atmosphere pressure. That is also the exact method Studebaker used to modify their mechanical fuel pumps for their supercharged models- They simply took the stock mechanical fuel pump, drilled the tiny vent hole in the pump case to accept a screw-in fitting, and then ran a small line to the carb in order to send boost pressure to the pump diaphragm so the pump would always stay a few pounds ahead of the boost pressure. We did exactly the same thing with a Holley regulator that regulated our high-pressure electric pump. That way the fuel pressure will follow the boost pressure and keep you supplied. Without it, the fuel flow will simply stop when boost goes up enough to counter the low fuel pressure. I wish I could find my old drawings of what we used on our race car. If you have 9 lbs fuel pressure, then reach 7 lbs boost, you are most likely starving it of fuel. You need a bigger pressure differential than that to keep it flowing fast enough to feed the engine, especially when it gets thirsty.
I run 20PSI on my Turbo 2.3 Ford. It is EFI, but the fuel pressure rises one PSI for every pound of boost. I too think you are going lean under boost and need to reference your fuel pressure to boost pressure.
It is a Kaiser VS57 that has had front half of case or snout removed (crudely, too, I might add!) and a fixed aluminum pulley added. I bought it from the estate sale of a gentleman who used it on his "racing Studebaker". That's about all I know about it. Dare-to-Be-Different, a big thanks for the run down on boost referencing the regulator, I'm going to do that right after I get this thing running correctly again! What great information!!
You're welcome. Here are a few pics of regulators of the type we used. I will have to describe what we did to them. I can't find my old pics. The fuel fittings are the normal "IN" and "OUT" with a gauge port that you can either plug or stick a gauge into. You can see the fuel pressure adjuster screw with the locknut on it. That is how you set the pressure. Turn the screw against a spring inside the case. What I did was to take the adjuster screw out and replace it with a drilled, hollow screw that I could attach a hose or fitting to. On one car, I simply drilled the screw (it was made for an allen wrench, so I drilled right into where the allen wrench was supposed to go) and brazed a copper tube right to the screw, then put the screw back in. That became the boost-port to tell the regulator what to do. The "boost-port" hose must be connected to the carb bonnet where the boost is, or to the carb enclosure, but NOT TO THE MANIFOLD. You MUST get rid of the small opening or small passage that the regulator uses to reference the atmosphere. If you look very closely at the first picture where the locknut touches the case, you can see what looks like a small notch in the case right under the locknut. It's on the right side of the picture under the nut. (click to enlarge) That is the tiny passage that lets the regulator compare itself to the atmospheric pressure. You need to completely block that small notch/passage, and hook it to boost pressure. On mine, I filed the case surface under the nut so it became flat, then used a soft copper washer/gasket between the case and the locknut. That kept it from sampling the atmosphere pressure. You could probably use a small dab of epoxy to fill the notch if you dont want to modify the case. The drilled adjuster screw with the copper boost-tube attached became the new "vent" that told the regulator to regulate the fuel pressure a little bit higher than "THIS BOOST PRESSURE AMOUNT" when I connected the tube to boost pressure. Instead of only a few pounds over atmosphere, it was several pounds over BOOST pressure. Next, you need to find a pump that will deliver at least 8-10 pounds above whatever boost you expect to reach at the highest levels, then adjust the regulator to deliver only about 4 or 5 lbs over, or 6-9 lbs over whatever the boost happens to be at any given time. With experimenting, you will find the fuel pressure you think works best with your usage. (4 lbs over boost, or 7 lbs over boost, or 8lbs over boost, etc). Just remember that it has to stay at least a few pounds over boost level at all times or the fuel won't flow. With ours, I think it liked just a few pounds fuel pressure over the boost pressure at the lower boost levels, and more like 8-9 or so pounds over boost level when we reached 12 lbs boost and higher. When we eventually pumped in boosts of 22 lbs and higher, the fuel pressure skyrocketed, and we had to come up with a modified version, but that is another story that you probably won't need quite yet. This was all a number of years ago when I couldn't get enough of racing. I wouldn't be surprised if I hear that they now sell boost-referenced regulators to the turbo people, and you don't have to modify them like we had to.
I agree with Mart. Also hollow brass floats can collapse under boost and cause rich mixture with pressure box blow through set up.
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