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-   -   Cam Question (http://www.hotrodders.com/forum/cam-question-229360.html)

pilona42 02-09-2013 12:20 PM

Cam Question
 
Ive been pulling my hair out for a couple weeks now because of a SBC Ive put together from some parts I had laying around. Its a crate motor short block with a set of 2.02 stock heads. I cant get the thing to run and it is spitting up fuel from the carb. My instinct was a timing issue so I checked my dist. at TDC and I was dead on. Ive checked compression and all cylinders were at 150. When I was turning the motor over by hand I noticed I could hear a pretty big hissing sound coming from the intake. I was convinced I had leaking intake valves and that was the culprit for her spitting up so I pulled the heads and tested them. There was no leaks at all!! After scratching my head some more I turned the motor over with the heads off and noticed that my intake lifter was not down all the way before the piston was on its way up. As a matter of fact it is coming up about an inch before the lifter is fully seated. My question is "Is this normal?" I know different cams have different lift and durration but shouldnt the intake valve be fully seated before the compression stroke starts? I appreciate any insight you guys might have on this.

Thanks,
Andre

F-BIRD'88 02-09-2013 12:38 PM

The intake valve does not close until after BDC. even on a stock cam.
The bigger the cam duration the latter the intake valve closes past BDC.

set the timing using a timing light. 34 to 36deg BTDC @ max mechanical advance.
The best initial timing will depend on the cam used in the motor.

generally 12 deg +/- for a stock OEM camshaft. You cannot set it by ear.

Did you line up the cam gear timing dot when installing the cam?

pilona42 02-09-2013 12:45 PM

I did set them dot to dot. I even double checked it yesterday. So a hissing sound coming up through the intake is normal?

F-BIRD'88 02-09-2013 01:01 PM

Quote:

Originally Posted by pilona42 (Post 1644423)
I did set them dot to dot. I even double checked it yesterday. So a hissing sound coming up through the intake is normal?

You are likely hearing it leak past the piston rings. If the cranking compression is 150 its good.

On a stock OEM cam the intake is not fully closed until 40deg ABDC +/- The piston is approx 1/3 past BDC

The bigger the cam the later the intake closes (generally)

Did you verify accurate timing tab TDC location, using a piston stop to find true TDC?
often a issue with non stock aftermarket parts. inaccurate timing setting is the result.

what cam?

hcompton 02-09-2013 01:05 PM

Sounds like the timing chain is off either 180 out or a tooth off.

Might want to pull the cover and make sure the cam is timed right.

Also double check the wiring it is the most common start up issues plug wires crossed. Even seasoned mechanics do it from time to time. With the hold downs and all the wire close to each other its not hard to get one mixed up. I always make it a point to check it agaist the chart on my wall for firing order and which cylinder is which and cap position for the cyl. One wire off it runs fine one wire crossed it could stop the engine from starting if its always firing at the wrong time.

pilona42 02-09-2013 01:11 PM

Thanks, it must be somthing stupid. Ive tried a couple different carbs because I noticed excessive fuel in the intake when I removed it. Got a edelbrock off a running vehicle and installed it with no better result. When I took the heads off a few of the cylinders were flooded with fuel! I cant remember if I had the plugs out and turned the motor over after I pulled my bad holley so Im hoping it was just extremely flooded.

F-BIRD'88 02-09-2013 01:14 PM

after installing the cam and timing gears dot to dot. You must rotate the crankshaft 1 revolution, before
installing the distributor. just like the cam install instructions (that you did not read) tell you. or the spark timing is 180deg out.
and it will not run.
it is fireing on the overlap . make it fire at TDC on the power stroke by installing the distributor correctly.

F-BIRD'88 02-09-2013 01:15 PM

fuel pressure 5psi

pilona42 02-09-2013 01:18 PM

Quote:

Originally Posted by F-BIRD'88 (Post 1644444)
after installing the cam and timing gears dot to dot. You must rotate the crankshaft 1 revolution, before
installing the distributor. just like the cam install instructions (that you did not read) tell you. or the spark timing is 180deg out.
and it will not run.
it is fireing on the overlap . make it fire at TDC on the power stroke by installing the distributor correctly.

I understand that the dot to dot position is the compression stroke for #6 and I have verified that I am on the compression stroke for #1 when dropping in my dist.

F-BIRD'88 02-09-2013 01:18 PM

If you filled the cylinder with gasoline remove all the spark plugs and disable the spark power.
Crank it over and all the fuel will come flying out.
reinstall the spark plugs and CHANGE THE OIL. the oil is gas deluted now.

get the spark set up on the TDC ont he fireing stroke not the overlap.
5psi fuel pressure. more is not better. it will flood with more than 7psi.
this must be your first time.

F-BIRD'88 02-09-2013 01:19 PM

large non stock duration cams need a lot of spark advance at idle.
stock timing and a stock timing curve will not work with big cams.
Post all the engine specs.

pilona42 02-09-2013 01:21 PM

Quote:

Originally Posted by F-BIRD'88 (Post 1644437)
You are likely hearing it leak past the piston rings. If the cranking compression is 150 its good.

On a stock OEM cam the intake is not fully closed until 40deg ABDC +/- The piston is approx 1/3 past BDC

The bigger the cam the later the intake closes (generally)

Did you verify accurate timing tab TDC location, using a piston stop to find true TDC?
often a issue with non stock aftermarket parts. inaccurate timing setting is the result.

what cam?

It is a stock cam from the crate motor short block.

pilona42 02-09-2013 01:30 PM

Quote:

Originally Posted by F-BIRD'88 (Post 1644444)
after installing the cam and timing gears dot to dot. You must rotate the crankshaft 1 revolution, before
installing the distributor. just like the cam install instructions (that you did not read) tell you. or the spark timing is 180deg out.
and it will not run.
it is fireing on the overlap . make it fire at TDC on the power stroke by installing the distributor correctly.


I never uninstalled the cam. Just replaced the chain and gears. Ive been keeping an eye on the oil and It is contaminated. Not my first time. Ive put a sbc together many times on my racecar but it has been about 5 years. Thats why this is so frustrating!

F-BIRD'88 02-09-2013 01:32 PM

what crate motor? they do not all have the same cam in them?

what else did you change.. everything...

F-BIRD'88 02-09-2013 01:34 PM

There is NFW you could build your own race motor and not know that the intake valve closes after BDC.

the basics
http://www.iskycams.com/degreeing.php


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