Thread: 350 lacking power

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i just rebuilt my 79 chevy 350 and it is lacking quite a bit of power. It is a 350 bored .030" over new comp cam .475 intake lift, hydraulic lifters and double roller timing chain , hypereutectic pistons edelbrock intake, edelbrock 650 carb, hei super coil distributor, headers, ect. It is lacking power, always smells like it is running rich and is sometimes hard to start(have to hold gas to the floor). The plugs have good color. I have adjusted the valves/ rockers, not it. I have played with the timing, not it. I put a new distributor and wires on, not it. i tried messing with the carb settings and the more you lean it out the less it likes to run and the no matter how rich you make it run it doesnt gain power. I put a new carb on it, no difference. The only thing i can think of is that the cam is not degreed right but the two marks on the cam gears were lined up when assembled and the lifters appeared to be in correct timing with the strokes. I would appreciate and thoughts or help.

thanks

whose cam?
part #?
Intake closing point @0.050" lift ?
deck height?
squish?
idle rpm's?
static compression ratio?

sorry im not exactly good with all the technical stuff but the sheet that came with the cam reads as follows:
Comp cams
part #: 12-242-2
grind#: CS XE268H-10
intake exhaust
valve adjustment: hyd hyd
gross valve lift .477 .480
duration at
.006 tappet lift 268 280
valve timing open close
at .006 int 26 btdc 61 abdc
exh 72 bbdc 27 atdc
these specs are for cam installed at 106.0 intake center line
intake exhaust
duration at .50 224 230
lobe lift .3180 .3200
rocker arm ratio .00 .00
lobe separation 110.0





this is the first engine i have built and i was just told to line up the marks on the two gears for the cam so that is what i did
the engine is running at 190 degrees
the timing is set at 8 degrees btdc at idle
idle is at around 800rpm
im not sure what compression it is but i can find out unless there is a way to find out by telling parts: heads etc
the headers are close to the starter but it only affects it over long periods of time
the engine will start right away almost all the time but sometimes it will almost instantly die they you have to hold the peddle to the floor to start it and it takes a while
i havent checked the floats but i have tried two carbs and one was brand new
i am almost positive the valves are right
i cant find any vaccuum leaks
they are 1.94 heads

hope i gave enough info

thanks

sorry the margins came out weird for the cam specs ........on each line the intake specs are on the left and the exhaust is on the right

IS YOU FUEL SUPPLY A-OK ? USUALLY FOR ME IT'S SOMETHING STUPID LIKE THAT THAT IS SIMPLE AND ALMOST UNRELATED.

I had a similar problem. Changed just about everything I could without taking the engine to bits and it ran just about OK, but not more. Not enough power, didn't respond to timing changes very well. Started each and every time, though, but ran much too rich. Sounds similar to your poblem. even the spark plugs were OK, only had a bit too much carbon on because of the rich mixture.
I did a compression test and found out what was wrong: The head gasket between #3 and #5 had blown, causing low comp on those cylinders and a poor burn rate. In addition, the passenger side head had a small crack and #4 cylinder was half full of water. You can fiddle with everything on top of the engine and it might get a little better or a little worse, but SBCs can run with quite a bit wrong down inside them. My guess is do a comp test, it's easy to do and if you find cylinder pressures differing by more then 10 or 15 psi, pull the heads off and have a look. Might cost you a day and some new head gaskets, but if the engine doesn't respond to outer accessories being changed it's probably got something to do with the innards. Especially if it's your first build it isn't even all that easy to get the head gaskets to seal properly. If you didn't tap the cylinder head bolt threads properly or some of the bolts were dirty or had rust or scale on them then your torque readings go all over the place and the cylinder heads are the #1 place to get the torque right exactly as specced. Of course it could be something simpler than that, but if it is a head gasket, the time to change it is now. Hope I could help a little, MadMax

i quickly did a compression test to day but did not have time to take headers off to test cylinders #5 and #6 (tester didnt have attachments) but all the rest seemed to be 145 to 150 psi.

for my double roller cam gears there are 3 keyways ...... square, circular, and i think triangle. which ones are advanced , retarded, ect?


thanks

350elcamino,

Just an idea, have you tried either a cranking vacuum test or an idle vacuum test? These can sometimes point you in the right direction...

For the cranking vacuum test, warm up the engine first, shut it off, connect a vacuum gauge to the manifold (not a ported one, pure manifold vacuum), connect a remote starter switch (or disable your ignition, you don't want the motor to crank), then crank the motor for about 15 seconds. What you are looking for in this test is a STEADY vacuum reading and a STEADY cranking speed. About 4-5" and 200 rpm are normal, but each motor is different. If your cranking speed or vacuum are NOT steady, then your cylinders are not pumping equally, probably some kind of leak past the rings, valves or head gasket. If the vacuum is steady but the rpm is not, then something is likely wrong with your flywheel/ring gear/starter (my guess, not your problem). If the rpm is steady but vacuum is low and slightly uneven, this indicates a likely case of low compression or improper valve timing (jumped timing chain or belt).



For the idle test, once again warm the motor and shut it down. Connect the vacuum gauge and then run the engine at idle. Note the vacuum (17-22 inches normal). Should be STEADY, note any fluctuations. Once done with that, run the motor at around 1900-2000 rpm, low cruise, for 15 seconds noting the vacuum readings. Then run it at 2800-3000 rpm for 15 seconds, noting the same. Here are some indications from your data:

Low idle vacuum (<17 inches): may indicate ignition or valve timing being too far retarded. Low compression, tight valves or a leak around your intake could also cause this. Clogged/restricted exhaust can also cause low vacuum, to check this open the throttle to 2500 for 15 seconds, if the vacuum drops during this time and does not increase when you close the throttle, then it may indicate a blocked exhaust.

Fluctuating vacuum within the normal range (bouncing needle) indicates uneven compression which could be caused by:
1) broken rings
2) head gasket leak near a couple of cylinders
3) valve leak
4) manifold leak near one or two cylinders
5) uneven A/F ratio or mix (dirty carb)

Increase your rpm and hold it for 15 seconds then watch the gauge as you release the throttle. The vacuum reading should initially increase as you release the throttle then gradually settle back to normal idle vacuum. If it does not increase at least 1-2 inches, then it is possible that you have worn rings/valves/cylinders.

High idle vacuum can indicate ignition timing too far advanced.

Hope this helps?

thank you I'll have to try that.