Small Block & Big Block Chevy Specific - sbc with running issue-low vacuum and needle is bouncing-engine Gurus HELP plz
04-24-2012, 01:26 PM
I have a SBC that I just got running - mild solid cam 10:1 domed pistons when idling this engine is running about 3-4" of vacuum and the guage is jumping around like a valve issue. heads were pulled sent to known good shop nothing major wrong- 2 guides were a little tight no bent valves but spring needed shimmed as exhaust had low seat pressure. reasembled and runs almost same, with this cam it should idle at 10-11" of vacuum. no leaks found, did comp test befor heads sent off 140-160psi range. while heads were off inspected all lifters to see if a lobe went down all looked good. I have to admit I'm stumped it seems to run better when more initial timing is used - had it set at 12 degrees , its been running rich due to vacuum and the 6.5" power valves when up around 2 k it sounds alot better - vacuum is up to 12-14" so power valves shut off the extra gas. The timing cover is an orig GM with welded mark on it and balancer was new GM for this motor anyone have any ideas where to start?
stock point distributor 30degree dwell
holley 780 70 pri 74 sec jets and 6.5" power valves float level set and OK
stock chevy alum intake
cam 254 duration @ .050" (intake & exhaust) .485" lift (with 1.5 rockers)
114 deg. lobe separation
186 heads 202/160 stainless valves
4.030 bore forged pistons - basically a 302 engine
cant remember if my hair is turning grey or just falling out because of this
04-24-2012, 02:46 PM
Gotta change that power valve. I always take my max vacumme at idle and cut it in half.
you will never get it set right if its dumping fuel. Then with the vac gauge hooked up to a constaint vac source set the idle mixture. i do this by turning the mixture screws till i get max vac. then take that and cut it in half for your power valves. I also do not run a power valve in the secondires, just a delete plug and up the jetting.
04-24-2012, 03:15 PM
I doubt they make a 1.5" power valve, and I have swapped power valves once, this engine ought to have 10" of vacuum at idle and I dont know why it doesnt AND the guage is jumping around like it has valve issues. this engine was rebuilt to stock specs
04-24-2012, 04:00 PM
They make one lol I run a 2.5 in my car and it sees 7lbs at idle. It also manages 15 mpg on the freeway.
Check valve lash, hot and cold. A valve could be hanging due to not enough lash. 10lb's at idle is a lot for a solid tappet cam.
04-24-2012, 04:15 PM
I've been studying these alot and found this thread with good sound info on this combo http://www.camaros.org/forum/index.php?topic=2196.15
it should idle higher than 3" I assembled this engine 12+ years ago and second guessing if the cam timing was set corectly? driving me crazy second guessing it.
any suggestions or thoughts on why vacuum guage would fluctuate so much at any speed?
cam is a Crane copy of the GM 30-30 cam and its what GM put in these cars stock
04-25-2012, 11:45 PM
I would double check valve lash, that can cause what you describe.
an out of tune carb will also cause that, try fixing timing at 12-14 degrees with idle at 900 rpm then lean out the idle circuit of the carb and you should get it running fairly smooth.
the power valve and main jets have no affect at idle because air flow is too low through the carb, all the fuel is pulled through the idle circuit. only when fuel is flowing through the main jets and out the boosters will the power valve have any affect.
04-26-2012, 11:11 AM
lash has been checked prior and of course after the heads were reinstalled
the power valves are still dumping extra fuel at the lower vacuum levels that this tries to idle at , when RPM gets up to around 2k vacuum is suffecient that they close and the correct amount of fuel is delivered from what I can tell. I will check what lash is when hot and see if they tighten up or? being engine is all iron it shouldnt change alot
04-26-2012, 11:49 AM
Dude go back and re read both my posts. You have to go back and start from step one. Also you cant go off what worked for other people, esp on carb jetting and idle tuning. Every motor is different in some way. its a place to start but not ideal.
Yes your motor is all iron...... But your lifters, push rods, rockers and valves are not. Steel will get marginally larger with heat and we are talkin a few thou it could be off to hang a valve. All lash is set hot check your cam card.
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