billybob6110

hey guys im back again with another question for my 355 sbc. the car runs really good awesome throttle runs 180* 200* in traffic or at idle. the prob or concern I have is that when im at cruising speeds 60-75mph the car has a sort of buh buh buh sound through the exhaust. kind of like hitting a rev limiter sound, but its not real loud but I notice it through the exhaust system. I have hooker headers to 2.5 inch pipe to dynomax 3 inch bullet mufflers. could it be exhaust, timing or carb leaning out?

-=Z28=-

iTrader: (3)

Or is it your carb loading up? How much pressure in your fuel line?

sofakingdom

iTrader: (1)

Pretty easy to tell.

Make ONE CHANGE. Pick your field of battle. (timing, jets, etc.)

Is it better? You did the right thing. Do it some more. Keep doing it until something else gets worse.

Is it worse? You worked on the right thing but went the wrong way. Put it back where it was, plus a bit more the other way. Keep going, same as above.

Did it have no effect at all? You messed with the rong thing. Put it back where it was, then attack something else.

One thing is FOR SURE, it's not the exhaust.

If I was the betting kind, I'd bet it's carb related. Probably wants a tweeeeek more jet. If it's a Holley, check the fuel level, make sure it dribbles out the pri sight plug when you just barely bump the fender while it's running. If that's correct, increase the jets 2 #s while making NO OTHER changes.

ONE change at a time.

AlkyIROC

Always start with timing. Any other changes such as jetting, plug gap etc won't affect where the engine likes the timing unless you're going major component changes (heads, cam intake etc). Once the best timing is determined, don't change it.

Base timing depends on the cam grind. Total timing really depends on the engine but generally for best performance, the timing should be at full advance by 3000 rpm and is usually anywhere from 32-38 degrees for most cars although it's not uncommon to have it as low as 28 or as high as 45 degrees. Changing your base timing will change your total unless your distributor gets recurved to move it back to the desired total.

Typically, it's recommend 10- to 12-degrees of initial (base) timing when the duration of the camshaft is less than 220-degrees @ 0.050” of valve lift; 14- to 16-degrees of initial timing with a camshaft duration of less than 240-degrees @ 0.050”; and 18- to 20-degrees of initial timing when the camshaft duration is less than 260-degrees @ 0.050” of valve lift.

After timing is sorted out, play with jetting, air bleeds, idle mixture screws etc to fine tune the engine to make more power.

billybob6110

I have a 600 edelbrock carb with 6 psi. im thinking of putting the 750 edelbrock on and go from there

Night rider327

iTrader: (13)

Drop that fuel pressure down! Edelbrock carbs do not like anything over 5.5 psi and that is very max. I set all of mine in the 4.5 to 5.25 psi range.

billybob6110

is 4.25 enough for the carb to perform correctly? ill give it a shot and see what happens

-=Z28=-

iTrader: (3)

YES! thats about perfect. . 6pis IMO is too much for a carb. Unless you are WOT 100% of the time....

-=Z28=-

iTrader: (3)

Also, that 750 is pushing it. Remember, bigger is not always better. I think that 600 is a perfect size. Sounds like you need to adjust fuel pressure and get a tune up. New plugs, check wires, rotor, ect. And maybe your 600 edelbrock needs a rebuild/adjusting. Id start there before putting another mystery carb/problem on your engine. All that will do is either work, or multiply your problems. I vote for the latter. GL

-D

sofakingdom

iTrader: (1)

People always obsess over having as much fuel pressure as possible. Not sure why. But I think a clue is, it didn't start until EFI became commonplace.

Carbs don't work off of fuel PRESSURE, the way FI does. If your carb could be kept full at all times with .0001 psi, then .0001 psi is "enough". All the fuel pressure in a carb setup has to do is, keep the bowls full; that is, fill them up as fast as the engine drains them. The actual pressure in the system DOES NOT MATTER, as long as this simple flow requirement is met.

In reality of course, it takes some pressure to force the requisite amount of fuel through the needle & seat. But once you've met that, you have "enough".

There is also such a thing as TOO MUCH. The pressure tends to force the needle valve open against the very weeeeeek force available from the float floating on the gas in the bowl. If that happens, the carb loses control of the fuel altogether, the bowl overfills and spills over, and anything resembling "calibration" or "tuning" of the carb, goes right straight down the drain.

A bigger carb will not solve your problem. A non-defective one, or a better tuned one, of WHATEVER size, might run better than a defective or poorly tuned one of WHATEVER OTHER SIZE; but that's not the same thing as saying that "bigger is better". Learn to TUNE your car instead of just swapping parts.

To Alky's point about the timing, it is IMPOSSIBLE to set a carb up right, if the spark curve isn't at least somewhat close to WHAT THE ENGINE WANTS. This can be something ENTIRELY different from whatever "spec" some team of emissions engineers caused to be printed in some "book" for some other engine that was originally wrapped in your shape of sheet metal that you happen to have YOUR engine wrapped in. His curve suggestion is a good starting point; problem being of course, that you have VIRTUALLY NO WAY WHATSOEVER of actually "measuring" it and applying a "number" to whatever your ignition system is doing. Instead, find the timing curve YOUR engine wants by TUNING it (making ONE change, evaluating the effects, and making further changes to THAT ONE parameter in the same manner until no change you can make to it makes THE ENGINE run any better, then making ONE change to some other parameter, and so on through all available parameters) and not by allowing some "number" to pop into your head and pretending that you know better than the engine what it's going to like and trying to force it on it. THE ONLY use for "numbers" is to know what parts to buy next, and to find out what your settings are AFTER you get them PERFECTLY OPTIMIZED so you can duplicate them in the future.

billybob6110

tpook car out multiple times and still has exhaust noise. it more sounds like if the cars running out of fuel but its not. give it a little more throttle and it goes away. kinda baffled at what it is. the mufflers im running are only 9 inches long and dump behind rear seat. could it just be the noise from exhaust rather then a problem with car? it has all kinds of power, just noise is real annoying when cruising.

billybob6110

also I noticed my timing is suppose to be set at 38 degrees all by 3000rpm, but if I go higher in rpms the timing will slowly creep up to about 45 degrees. is that supposed to do that? and could I possibly be hearing some detenation?

billybob6110

im leaning more toward a cruise mode lean cause if im cruising Is only when I notice it. give it a little more gas pedal and it stops. if let out of gas itstops. so maybe a lean surge im getting into. do I just jet up or do I need to do the metering rods also?

AlkyIROC

You need to know how much base timing you have and how much mechanical advance is in the distributor.

Base timing is adjusted by you. Not sure what distributor you're running but I'll guess at a large cap HEI. Mechanical advance is built into the distributor. It can be modified to adjust total timing and the timing curve. With vacuum advance disconnected, your total timing may be at 38* however with vacuum advance connected, it can add even more advance based on the engines light loads or deceleration. If the vacuum advance is disconnected and your total timing goes above 38* then the distributor has more mechanical advance than you need and needs to be recurved with proper weights and cam to limit how much mechanical advance it has. That's all part of doing a proper distributor recurve.

Always adjust base timing and check total timing with the vacuum advance disconnected. Once it's connected back up the timing can be anywhere. That's why the majority of race cars don't use the vacuum advance. My engine doesn't even have a timing curve other than the start retard. Once the engine is started, timing is at total advance. With a crank trigger, timing gear drive and locked out distributor, my timing mark is rock solid through the rpm range.

billybob6110

im running hei correct. I don't have vac advance on the distributor. recurve would be the springs and weights correct?

AlkyIROC

Weights and center cam determine how much mechanical advance is in the distributor. Springs control how fast or slow the curve is. Light springs will allow the weights to extend quicker. All parts can be mixed and matched to achieve the proper results. 2 different size weights, 2 different springs etc.

billybob6110

still having issue. curve is coming in properly like it should. timing is good.
I pull plug and there little blackish but nothing major. the noise only happens at 55-65 mph (2500-3000rpm). I don't have a lockup convertor would that have anything to do with it? im At a loss with the noise. its more irritating then anything. I adjusted carb to try richer and it still does it. I though of going less timing to see what happens....my dizzy curve is set to all be in by 3000 rpm. is it to much curve for when cruising? cars a wekkend driver show driver maybe sees track couple times a year