Hello all,

Whew! It's been a while since my last visit, and I've had a bit of a problem after installing the Edelbrock Performer RPM intake manifold and Edelbrock 4 bbl on my 1977 Caprice's 305. From a standstill, it hops up pretty good, but at around 20 MPH, or about when 2nd gear engages, the torque plummets, and it feels like the car is standing still until about 30 MPH, then it begins to pull again with no problems as far as I've been willing to take it RPM-wise. This problem is especially bad up hills. After reading the tuning guide in the Edelbrock manual, and reading a few forums, I think the problem is most likely the carb running too lean, too rich, or the timing is screwed up. It doesn't smell rich at idle, but I haven't yet figured out a way to smell the exhaust when cruising at 20+ MPH! I didn't reset the timing when I replaced the intake manifold, because the stupid timing marks are directly on top of the balancer, about 1/2 in. away from the fan blade, so I couldn't figure out a way to do it without getting my face chopped off. If anyone has advice in that department, that would also be greatly appreciated. I'm waiting for the SA book "How To Rebuild & Modify Carter/Edelbrock Carbs" to see what it says, and I'll probably be getting a calibration Kit fairly soon as well, but in the meantime, I wanted to get some knowledgeable opinions on the issue.

 

Point the timing light between the water pump and front cover. That's where the timing tab is.

Get the timing set properly. Until you do that, you can't properly tune a carburetor.

Can we assume you're using a vacuum/mechanical advance distributor?

The way to smell the exhaust at 20+ MPH is with an O2 meter.

 

The problem with the timing tab is that I can just barely see it when the engine is off, and it's sort of corroded, so I cant really discern the marks without using White-Out, but I can't really reach it to mark it. So I'd have to get way close to it to see, and that's when the fan would disfigure me. I'll have to try to give it another shot this weekend. The distributor is the stock HEI. I wish I had an O2 sensor, but I'm a long way from getting into EFI territory, and a seperate gauge with a wideband isn't in the budget... yet.

 

I think this might be related to another problem I've been having with the car. In park, the engine runs real smooth, but when I put it in gear (especially reverse), the RPMs drop (no tach, but I can hear it), the engine starts to shudder, and unless I goose the gas, it stalls. I think I really have to perform a carb, ignition tune up, but I can't seem to find a good website with a calculator for determining optimum ignition advance for a certain RPM. Would putting the car in reverse and lowering/raising the idle speed adjust on the carb be the best way to determine the optimum idle RPM? I'll also have to get a tach. Does anyone know off the top of their heads what diameter harmonic damper an LG3 305 uses? I wanted to buy a timing tape to set the advance points using a Crane HEI timing kit. My procedure is as follows:

1. Set the idle RPM
2. Set initial timing advance at idle
3. Determine some RPM points to calculate optimum advance curve (suggestions?)

For my essentially stock engine, if I discover HEI problems, would you guys suggest a rebuild, recon stock, Autozone-special cheapie Accel, or something else? Any plug/wire suggestions?

 

there is a timing tape kit at jegs that comes with all size tape for the balancer and it is cheap. you need to put that on the balancer and then set the timing. set the carb by the instructions in this section. it works. as for the timing my 305 is running best with 12* at idle. get all that set up after the tune up and then worry about getting the advance right. get a tach. make sure you get it running smooth first or it will keep driving you nuts.

 

What year is your car? If it's a 305 HO, it might have somewhat smaller/better shaped combustion chambers than the LG3. Do you still have a bone-stock HEI? Did a little more research, and it seems that achieving the MBT (apparently can stand for all sorts of things, but in this context, I mean Minimum timing for Best Torque) is dependent on so many nuances of a specific car/engine combination, the only way to find it is a dyno. The procedure seems to be to set some fixed RPM data points, and tweak the initial, the vacuum, and the mechanical advance at each RPM point to achieve the best torque output, then retard the ignition timing by 1 degree as a safety margin. The point is to dial in tuning so that maximum cylinder pressure is attained around 15deg ATDC (which is deceptively simple sounding, but one has to take into account AFR, compression ratio, manifold vacuum, ignition timing, quality of spark, location of spark plug, size/shape of chamber/bore, octane/quality of fuel, and a dozen other parameters that affect the speed of the flame front in each chamber, which makes it basically impossible to calculate on paper). I also imagine that your carb and ignition system should already be pretty close to optimal, to remove as many variables as possible. Also, practically any change made to the vehicle that affected combustion in any way would require the process to be done over. If only I had a Nextel Cup budget...

Just looked over my copy of "How to build High Performance Chevy Msall Blocks on a Budget" by the great David Vizard, and it describes a cheap way of getting pretty close to MBT by using the strip, rather than the dyno.

 

Lots of edits on my last post, so I figure I might as well bump.

 

mine is the 305 tbi motor. not it is carbed. prolly same as yours. and yes i have read that and most if his chevy issues. still read them from time to time. but. we are not david vizard. wish i knew a 1/4 of what he does. but no matter what anyone said about perfect timing. you will need to get the engine running smoothly. and instead of using a dyno(unless you have the time and money) the best way to tune it is on the road. you can get it damn close with a lot of patience. i have mine running great. but it did take some time and a lot of reading.

 

Been busy moving, so I haven't had any available daylight hours to check the timing, but when I started up the car to move it, I had to put some starting fluid in it (starting to get colder in CT), so I had the top of the air cleaner removed, and I think I saw my first carburetor backfire. A fireball shot out of the carb with a "pop!". I just looked up some of the causes of this, and vacuum leaks and timing too retarded are likely culprits. Looks like I'll be getting a vacuum gauge as well.

 

Checked the timing today, 5 deg. BTDC, as per stock spec. So, now what? Advance timing due to different requirements for the new carb/intake combination? Try to tune the carb?