Team Camaro Tech banner
Cancel
Create thread New Garage Forums

30* advance?

Jump to Latest
1 - 13 of 13 Posts
A

apprentice

· Registered
Joined
·
128 Posts
Discussion starter · #1 ·
hello all,
i finally got the engine to run and idle (see "rough run rebuild"), but only if i advance the spark timing 30-35*. The quality of the idle is pretty good, and it sound great, but the advance scares me. When i built the engine, i installed the cam 4*.
204/214 dur
107/117 lobe centres
0.422/0.444 valve lift
it also only makes 12mmHg of vacuum which kinda bites. Any ideas?

thanks,
apprentice
 
#2 ·
I hate to say this but advancing a cam 4* is usually done to a cam designed for high rpm use to get a little more low end out of it. It also makes it idle rough. A cam with 204-214*@.050 doesn't really need more low end. I think if you run that cam straight up you'll have all the low end torque and vacuum you need.

------------------
'69 RS Camaro
355 5-speed 4.11
 
Discussion starter · #3 ·
thanks 1 2run, I'b believing more and more that my advancing the cam was a moment of weakness. I caved-in to the peer pressure and bench-racing mumbo jumbo. I think i'll check again for vacuum leaks and try to lash the valves while running before i mess up my new engine bay and engine. Any more ideas on what else i try before i reset the cam? Would that account for having to run 30* advance at base?
 
#7 ·
Do you have the vacuum advance disconnected when you read 35* advance? Did you TDC-verify the balancer index mark against the timing tab? Do you have a post-'69 balancer with a pre-'69 timing cover and tab or vice versa (8* difference)?

------------------
JohnZ
CRG
'69 Z28 Fathom Green
 
Discussion starter · #8 ·
hey guys,
here is some more info; It will idle at 8* advance at 750rpm but very roughly. Every now and then at 8* it'll stumble so badly that it dies, but it'll go for 5-10 min. or more. I marked the dampener using a piston stop in #1, turning the engine both ways until the piston hit the stop, made marks, then split the difference for the TDC mark. This was all done using my new bolt-on timing tab. I did NOT disconnect the vacuum advance this last time (though i did the previous times) I just plain forgot. But with only 12mmHg vacuum, could it throw things off by much? by the way, how does one convert from mmHg to inches of vacuum?
 
#10 ·
Does your vacuum gauge really read in mm Hg? That's how sphygmomanometers (blood pressure) are calibrated. I guess I'm used to mine, that reads in inches of mercury (not water). Is your gauge connected to a full manifold vacuum source?

------------------
JohnZ
CRG
'69 Z28 Fathom Green
 
#11 ·
I think total timing is more important than base (idle) timing. You have to use a dial back timing light to measure total advance or you can do it with a regular timing light and a timing tape.

Anyway, you want 34-36* total timing - that is with the engine rev'd up to 3000 RPM or more until all the advance is in. You measure this with the vac line to your distributor disconnected. Then once you have the total timing set, let it idle down and look at where your base timing is at.

Actually, advancing a cam should make the engine idle better, it would also make more low end torque. 4* is not a problem, but most cam manufacturers grind 4* into their cams already - in other words if you install it straight up its really 4* advanced. well, if you advance it another 4* and the cam already has 4* ground in then you are at 8* which IS a problem.

I'd also check for a major vacuum leak - if the port under the back of the carb is left off you will heave real trouble getting it to idle and your vac reading would read low also. (pls don't ask me how I know
Image
)

Hope this helps

ps. Look on the bright side . . . at least you have one he11 of an alternator bracket setup
Image
 
Discussion starter · #12 ·
Hey all,
yup, my guage reads in mmHg. I used to do a lot of lab work in University. Anyway, if "inches" refers to inches of Hg then i'm in even worse shape than i thought!!
Image

i'd be at something like 4 inches (snicker snicker!!) . I couldn't find any vacuum leaks when checking with either a stethescope ( a mechanics unit, not another hold-over from my lab days!) or with a spray bottle. The vacuum port out the back of the carb is plugged off. I'm measuring from the line going to the booster which is coming out of the intake 't' (also goes to vac. advance). And yes my alternator setup sure is trick (thanks Eric68!!), i'm hoping this battle doesn't wage on as long as that one. But thanks to you all, i got that one beat....eventually!!!
Image
 
1 - 13 of 13 Posts
About this Discussion
12 Replies
6 Participants
Last post:
apprentice
Team Camaro Tech
posts
2.4M
members
90K
Since
1998
A forum community dedicated to 1st generation Chevy Camaros owners and enthusiasts. Come join the discussion about performance, builds, restoration, modifications, classifieds, troubleshooting, reviews, and more!
Full Forum Listing
Explore Our Forums
Bench Racing Engine Body Shop Performance Brakes, Suspension & Steering