Hi all. This question is for my buddy who has a 91 TA. Heres the engine breakdown....355 chevy with trick flow heads, comp cam with .494 lift, int and exh, edelbrock air gap intake, mighty demon blow through 750 cfm carb, and a custom turbo kit with a 77mm turbo and 3 inch back exhaust. The ignition system consists of taylor wires, autolite plugs, an accel super coil, and a cheap new no-name brand distributer. Now the problem is that no matter what the timing is set at, the car constantly runs like complete garbage! We recently changed the plugs to the correct autolites because the originals didn't have enough thread on them to get the spark into the combustion chamber, but that didnt seem to help much. Seems like no matter what settings we fool with, it never gets any better. It seems like its running pretty rich too, even when we play with the pressure, and bowl levels. I am almost positive it may be the cheap quality distributer, but I'd like to see if any of you have any input before we go spending more money. It idles like crap....if it idles at all, and while driving, it pops and seems like it has a pretty noticable misfire

 

hope he isnt running much boost through it at this point

what is the wide-band telling you?

what are the plugs looking like...

what is the duration at .050

needs alot more info to even have a shoot in the dark with a setup like that

lots of specifics not just it doesnt idle and pops

 

OK, plugs look VERY black and charred...not that nice brownish color you want. Only has 5 psi for now, and it runs shitty even when the boost pipe is off the carb. Unfortunately i'm not sure what the duration is at .050, but I would have to say it's right around .230. We checked the plug wires 3 times to make sure there on the correct terminals. I just want to try to get it narrowed down to an ignition problem or fuel problem. He has a 255 lph in tank pump, with an aeromotive regulator with a return line, set at 6psi at idle. We have played with the pressure and carb bowls, but this thing always burns your eyes while it's running, and the gas drips out the exhaust. Sucks gas down as well, even though it should be a 500 horse motor.

 

personally i think a cam with 230* at .050 is a little big for a Turbo... a split pattern would also be better was it ground on a 114-112 LSA?

seems like it is a carb problem... when you are running this without the carb hat... you arent pressurizing the bowls right? also what is the plug gap you are running... i would think it should be around .035

 

First off, leave the boost pipe off until you figure out what's going on with the motor. Boost on a bad-running motor is going to be nothing but quick death (although a blow-through carb probably won't run well at WOT without the boost). But it should at least idle and drive around at part throttle reasonably well, even with that cam.

MIGHT be the carb, might not. Borrow a known-good-running carb from a friend and bolt it on temporarily to see if the situation suddenly improves dramatically. If it does, you've got carb issues.

Beyond that you're down to valvetrain and ignition-related issues. If it was me I'd start with a compression test on all 8. One valve adjusted down a smidge too far and holding a valve off the seat can make it idle like a top fuel car, plus miss and pop at cruise. If you've got one or more cylinders that are definitely low on compression a valve adjustment is the first thing I check (becuase it's one of the few things you can do anything about without ripping the motor apart to see what's broken). Check for bent pushrods and broken valve springs visually while you've got the valve covers off to adjust the valves.

Then it would be on to the ignition system. Could be a bad distributor or module- who knows? I keep a known-good junkyard HEI distributor laying around for just such circumstances. You can stab it in, hook up one wire for power (even straight back to the battery if you think the power feed from the ignition switch to the distributor could be faulty) and you've just replaced basically the whole ignition system except for the plugs and wires.

Gotta be a little like Sherlock Holmes when diagnosing a bad-running engine- eliminate everything it CAN'T be and what you're left with is where the problem is. Change only one thing at a time as you go through this process.

FYI- don't go by what the exhaust smells like. It's always going to smell "heavy" when you've got a misfire/bad-run situation going on. Even when you correct the problem that cam is still going to put out some nasty fumes because of how much overlap it has. You've grown up with cars that have little cams, EFI and catalytic converters- they don't make smells like old carbureted muscle car engines with carbs, big cams and no emissions. But that's what you've built here, so expect different smells than maybe you're used to, even when it's running like a top.

 

Cam has a 110 lobe seperation. Plugs are gapped right at .035. The swapping carb idea is pretty good actually, and my brother has one that runs perfect on his camaro, so I will definately try that, and let you know the outcome. As for the valvetrain, we already removed the valve covers and looked at everything, and it all checks out fine. Even fired up the engine and did a running adjustment of the rockers so we KNOW they are all perfect.

I dont have a compression tester, but I would say that would be a last resort, because I've built 4 engines so far(all fuel injected thank god!), and this one is brand new....definately seems like a fuel or spark problem by the symptoms.

I've had cars running rich before because of a bad ignition or such, but trust me, 5 seconds after this thing fires up, if you light a match, bye bye garage. It's pretty bad. I've heard demon carbs are total garbage, and actually, I'm kinda leaning towords that for now.

Like I said though, were gonna swap carbs and I'll let you know what happens, but this is a problem that has had us stumped for 2 months now, so if you guys can help me get it figured out, A+++++++ to you! Thanks for the input so far.

 

what are you running for timing?

and how is your FPR set up? where is it getting its boost reference from?

 

If this thing is running as rich as you say, the plugs are probably (?) in bad shape now. Also, if there's gas dripping out the pipes I would be concerned about gas in the oil. Not that it is causing the miss problem but you don't want to wipe a bearing out. Good luck.

 

Not sure what we are running for timing because we dont own a timing light yet. The FPR has a vacuum port which is hooked up to the carb for the boost reference. Plugs are new as of yesterday, so not too worried about them right now. Also, there actually was gas in the oil, and we changed it only about a week ago, so that shouldn't be a problem right now. Definately gonna try the carb swap and then let you know how that turns out.

 

that port is a boost reference port it should never see vacuum...

5.) What is the fitting in the regulator cap for, how /where should it be connected if it’s used and what should be done if it isn’t?
All Aeromotive, carburetor bypass regulators incorporate the necessary design to allow the regulated fuel pressure to be boost referenced, on a 1:1 ratio. For “blow through” forced induction applications, where a turbo or centrifugal supercharger pressurizes the carburetor or a carburetor box, the regulator boost port should reference to positive pressure only, not vacuum, and be connected to the carburetor box or bonnet rather than the intake manifold. For carbureted, natural aspirated engines, with or without nitrous injection, and for roots supercharged engines where the blower draws through the carburetor, the boost/vacuum reference port should be left open to atmosphere, never plugged.

 

Hi guys. Just thought I'd let you know, we found the problem. Turned out to be the demon carb.....we bolted a holley 750 on from my brothers car and it ran like a charm!! Thanks a lot for the help all. Now for my next question, anyone know who makes a good blow through carb? Haha