Well, as I said, it has been a while. I thought I remembered the Crane cams I used called for more on the intake, but I may very well be wrong here.
If I remember correctly, the valve is accelerating away from the piston, the relative motion is greater..... wait a minute here, if the valve floats.... the piston will catch it.......
OK,, you are right, the exhaust needs more than the intake.....
Of course, always use the grinders specs, which they will gladly supply.
Should have cleared that particular cobweb better, shouldn't I??
Shep, you are no doubt more current than I am, I willingly accept the correction. Thanks!
Yes, usually when you float a valve it is the exhaust valve the the piston eats if there is not enough clearance. It is not a problem with intakes because of the operation relative to piston motion. When they flot you will get some bad poping through intake that can get ugly.
This is probably the way I'd have to go, although making a temporary change to solid lifters would probably require a good deal of disassembly wouldn't it?
Also...would you think Chevy would be able to provide info as to how much I could advance the cam without suffering damage?
Interestingly enough, I forgot to mention that the Edelbrock tech dude said that unburned fumes are not a good indicator of a rich condition, that only fouled plugs are.
I read this on earlier...
"On the other hand, if the cam keeps the valves open for longer periods of time, like with a racing cam, problems start to occur at the lower engine speeds. This will cause unburnt fuel to exit the engine since the valves are still open. This leads to lower engine performance and increased emissions."
Unburnt fuel...smells like a gas factory...Hmm...
So...if this all adds up, then the engine truly isn't "running" rich, it's just wasting a lot of gas at lower speeds due to the cam profile...?
Info per Google searches show that the vacuum probably won't ever get beyond 15" at idle.
Also, George...if you read this. Concerning our discussion of "venturi" vacuum. I was just flipping through an 2yr old issue of Four Wheeler, looking for the ad that I bought the engine from. I ran across an ad for the Holley Truck Avenger series of carbs.
One of the highlights in the ad mentions that "H.V.S. Annular Boosters provide an ultra High Vacuum Signal for incredible low & mid throttle response and torque".
Doing a Google search on the H.V.S. also shows a variation in wording, calling the H.V.S. the "high VELOCITY signal". Would this support my theory that the lower airspeed, which is affected by the vacuum at idle, could be not helping matters?
At this point, I'm not looking for a carb solution, more or less looking into cam solutions. But I need to satisfy my own curiousity here.
You would only need one solid lifter, you would only check one cam lobe at a time. On a small block Chevy you will have to pull the intake to change a lifter. If I am wrong on this, let a Chevy guy chime in here and set me straight. On my 350 horse 327 I had to remove the intake to get to the lifters, the head design may have changed from then, but I am not sure on that.
I doubt if Chevy will comment on how much you can advance the cam, that is an experimental situation, well beyond what they will participate in. The only possible damage is piston / valve contact and you can easily check that yourself in the engine.
Yeah, Hmmmmm.... sounds like what you are living with doesn't it?
Right. Wonder what you around town mileage would be with this setup, care to buy stock in a major oil company now??
With the cam indexed in an advanced position, you may get more vacuum, (probably will but not sure how much) but there is no way to know without doing it, unless you can find an identical engine that has had this very change and then check with the owner.
We used to call these "boost" venturis. Back before we used the new politically correct terminology. If you look down the throat of one of the carbs, you will find the center bore of the "boost" venturi (small venturi hanging in the middle of the large venturi) is quite small, provides better velocity with lower overall flow. This is where the off idle and midrange mixture is entering the air column.
Carb technology has come a long way, but the overall technology is still somewhat like a flush toilet compared to fuel injection.
Again, you are right, the operative solution here is to get better vacuum (airflow through the venturi) at idle and low speed. Actually, the problem most likely is due to a pulsing of the intake charge due to the cam overlap. The average vacuum / airspeed is just too low to support proper venturi action. Carbs work best with smooth constant airflow, we are talking midrange and high speed here.
You are on the right trail here. Perhaps if you reach the point of changing the cam you might want to consider the Crane RV cams, they are really potent and provide gobs of low RPM response and midrange torque that will bend your mind. A real good value, and I own no stock in Crane, just have used their products and found them to be excellent.
You are doing a really good job of getting a handle on this, when you are done you will have a real good feeling of self pride, just before you add all the tickets and figure out what this education has cost you. After that, all bets are off.
I have read through the Edelbrock manual many times, but had been concentrating on the calibration section exclusively for some time now. This low vacuum thing got me thinking, so I started asking all these questions here as well as doing a lot of searching through Google.
By slightly changing my search criteria, I ran across a thread on a forum. Another person was having the same problem with the same engine and same carb.
He had this to say:
"I bought the Crane curve kit on Sat. My base trimming was 12 degrees B=2ET.D.C and total was 36 degrees B.T.D.C with the vacuum can disconnected. I changed the vacuum advance from the "timed" (ported ) side to the full manifold vacuum side (drivers side), and this seemed to help a lot."
I then ran across another thread mentioning manifold as opposed to ported on the vacuum advance and made reference to it in the manual. After scouring the manual, I found this: =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D Long Duration Camshaft
If the engine has a fairly radical camshaft it may require an excessive amount of throttle opening for idle and/or have low idle vacuum levels. Either condition can lead to poor levels of adjustability and erratic idles.
Another fix for the above condition is to run as much spark advance as possible at idle. If the distributor is fitted with a vacuum advance unit, connect it directly to manifold vacuum. =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D The thing is, I've seen that before, but it didn't hit home for some reason.
What would this accomplish other than a lot of initial advance? As soon as I hit the gas, would the loss of manifold vacuum actually retard timing or would the centrifugal advance "catch up" as the vacuum advance was releasing?
Oh, by the way...in that thread, the dude that seemed to be the most help recommended advancing the cam by 7=B0.
Using manifold vacuum would provide maximum (or near to it) advance of the ignition timing at idle. Throttle application of course kills manifold vacuum so timing reverts to base timing, until RPM's begin to increase at which time the mechanical (centrifugal) curve begins to advance the timing. Would probably provide much of what you have been looking for.
Well, I have always found that it took at least 8 degrees to provide any real change, and in your case I felt 12 would be better, but I may have been a bit too agressive. My ego is not involved here, your driveablilty is what is involved.
Always check piston to valve clearance, even if someone else has done the same thing. Manufacturing variances can be a real devil to deal with, it doesn't take long and provides a great deal of peace of mind.
You may have found the answer here. Best of all to you.
Well... I pulled the carb last night. I had bought some new jets, but apparently they gave me the stock sizes. That's fine, as my stock jets were in my toolbox that was stolen from the truck months back. I installed them and reinstalled the stock metering rods, thus putting the carb back to factory calibration.
I pulled it off to give me a chance to swap out the carb adapter. I switched from an open, single-plane style adapter to a proper dual-plane style adapter, which actually has 4 holes which correspond to the square bore on top, and Q-Jet style spread-bore on the bottom.
I reset timing to about 8=B0 BTDC and then plugged the vacuum advance to a manifold vacuum source, which brought it up to about 18=B0 BTDC. I was totally guessing here.
I didn't like the way it ran. I thought it felt kind of shaky (that's the best word I can think of) at off-idle and cruise settings. I reset timing back to 16-18=B0 initial and plugged the vacuum line back into the ported source and liked it better.
Only thing is, now I'm right back where I was 6 months ago. Vacuum still holding around 14-15" Hg at idle. Now that I'm actually driving with the vacuum gauge full time, I can tell that the throttle response in relation to the carb is different. After recalibrating the carb, it seems to want to jump to "power" mode (vacuum dropping below 4") with less "foot". With the leaned-out setting I was running before, there seemed to be a bigger area between idle and Power mode.
Well, that dude was just giving a recommendation to the guy that has my same setup (although in a different vehicle far as I can tell). I don't know if he'd actually advanced that cam in that engine before.
Rest assured, nothing will be done without triple-checking all the clearances.
I'm more or less stalling and playing around. I know what's coming. I just need to speak to a shop or two and get price quotes on installing a cam. Then I'll decide whether the price of paying someone else to do it outweighs the effort involved in me doing it...which would involve me screwing something up, figuring it out, undoing the whole thing, and starting from scratch, starting a 40+ message thread here on Usenet, etc...
Oddly enough, I woke up this morning with the desire to get a 2nd vehicle ;-)
Putting in a cam is EASY compared to adjusting a carb to get it just right. DIY and save some BIG money. If you need help just ask, but it isn't that hard on a small block.
I am definitely considering that route. Knowing what all has to come off the engine and how much labor would be involved in installing a $100 part definitely has me looking for online instructions.
I'm going to be looking for a step-by-step guide today.
The main thing is coordinating so that I won't put myself out of driving to work and that I have access to my girlfriend's truck if and when I need to make a run to a parts store mid-project.
Not much to it. Pull radiator to gain room, remover water pump, remove dampner pullys and dampner, remove timing chain cover, mark position and remove distributor, remove intake, remove valve covers, loosen rocker ball studs to remove push roads, remove lifters, remove cam drive gear and remove chain and then remove cam. Do replace the lifters and do coat them with something like STP (and cam lobes) when you install new one and when you first fire it up, keep at at about
1200 to 1500 RPM or better for first 10 or 15 minutes and lifters break in some.
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