please help, sbc not running right

Well...I would definately go through all these external poss. before the mixed up lifter theory...but...I was beginning to think the same thing...If they are mixed up...they WILL fail...QUICK! As I was reading again, your post, I noticed that they were used...and a little light went on...but...before ya' take it apart, go through all yer other external poss. I had a 351 C. motor...it did this FOR WEEKS! we could not get it to run, idle...just crackin' poppin', stumblin'...went through the ignition system, timing...everything...in the end, we found that the intake was not sealing with the spacer ( nitrous plate,in this case, actually ) the intake was OLD and not true across the plenum, and also, needed to be heli-coiled...and the resulting vaccum leak was massive! It was almost the entire problem...That would be a nice, simple fix...

 

I had what sounds like the exact same problem just a month ago... the car ran fine and i accidently moved the distributor a bit when changing the cap. (hod down clamp wasn't tight) I thought, no biggie, just moved it back to where i thought it was, and timed it by ear... now mind you, this thing ran fine before i touched it, and i didnt even lift the distributor up and out, just moved it slightly while in position.... Anyhow, i got it running ok, shut it off and forgot about it. A week later, the thing barked and popped through the exhaust so bad i couldn't believe it. Nothing i did would fix it. I was convinced it decided at that very moment to wipe out a cam lobe or 2 (it was a GM crate with just break in time on it) Turns out all it was, was that all 8 plugs had fouled themselves out so badly from being out of time for just that short period of 5 min or so, that even when the distributor was right, it wouldn't run. Long story short, take out all 8 plugs, if theyre all black, sooty, and wet with gas, toss em... or at least put in a breand new set and clean the old ones later... then recheck your timing with a light, and i bet its fine. Make sure you hooked the vaccuum advance to full intake vaccuum or your advance won't work right. Seriously, check the plugs! Sounds too easy, but it worked for me.

 

Are you doing your idle tests under load? or is it in park and still hissing and popping like a sumbitch. If you are doing it under load what happens after 2,000RPM? My old Mustang had way too much cam for anything less than a 2500 RPM Stall converter, and it spit and died and backfired out the damn Holey a bunch before I changed the stall converter.

If it is not under load then the vaccum advance would be my first place to look. Then Timing of the cam. I have also heard of similar issues when overtightened rocker arms were an issue. But that is rare in most cases. and honestly I doubt it has anything to do with your situation.

The cam is a bit big but should be liveable, the Edelbrock RPM package .325'' Intake/.340'' Exhaust, It is good after 1500RPM, before that and it feels like it is missing and chug on my BBC (obviously different specs but same expected outcome for power range). Your Intake was tuned for cam lift .280" Int./.295" Exh.), (@ Valve .420" Int./.442" Exh.) Timing @ .050. I wonder if your Fancy new heads need more time to scavenge the exhaust?

 

I ran into a similar problem when I pulled the top end of my 283 apart and swaped to a TH350 at the same time. I pulled the top end apart first and then pulled the tranny. Not realizing, I was rotating the crank when removing the torque converter bolts. When I put it all back together I aligned the distributor back to the original position. When I started it, it ran like shit. So I thought I was 180 off. Nope! About a n hour and a half later I figured out I was actually one tooth off. Ran perfect once I figured it out. I'd keep thinking timing.

 

Sounds like a bad combo of parts and a cam that's off a tooth. Changing to an rv cam, new lifters, lots of break in lube will cure it. Lots of work, but an off the shelf grind for 8-8.5:1 compression 350 will work better than a funky grind, and it gives you a reason to check cam timing.

 

thanks for all of the input guys. I played with it all day and could not get it to run much better. I finally decided to pull the cam. I'm gonna get something a little better matched for my setup. Which eldelbrock cam and lifter package will work well for my setup?

 

Better yet buy the intake matching Edelbrock Performer cam for fairly cheap and you know you have that much matched up. Cam and lifter set 2102 or 2103

 

well, I'm done pulling the cam out and I just ordered the 2102 cam and lifter set. Guess we will find out sunday if that is the issue.

 

This time, adjust the valves properly. Adjust em with it running, that's foolproof. 20 small block chevys under my belt, and still get em wrong sometimes.

I've run more mismatched combos than yours without the kinds of problems you describe.

 

How do the bottoms of the lifters look?

 

Well, too late now, but that big cam and the lower compression ratio makes a lazy engine. I know, I have the 253 @.050 duration with 8.5 to 1 compression. I advanced my cam 4 degrees and also run a lot of initial timing. Mine runs fine, but it is lazy to rev when you just smack the throttle. Just a consequence of the low compression and big cam. Good side is I can run the cheapest shit gas and no problems. Runs good once above 2500 rpm.

All this has nothing to do with the popping and other isues you have, but it seems like you had an ignition issue to me.

 

sounds like you may have a couple of plug wires crossed. a cracked dist. cap will give the same symptoms..

 

another thought...have the heads been milled ? is it possible that the valves hit the pistons, at one time...causing bent valves ? that will also make it pop out the exhaust. are they stock length pushrods ? have you made sure that there are no bent ones ?

 

Curious if the new cam helped you. I think you were off a tooth on the dist but that's only my guess. I remember fighting with my 64 Elky for two weekends after I rebuilt the 283 and finally a friend came over and picked up the dist and set it back in and it took off.

 

O I gotta do it !!!! Pull it and replace with a Flathead!

 

Is there a chance that you have a fuel pump with way too much pressure on this car, so the carb's needle and seats are leaking and fuel is gushing down the throat through the vents.[/quote]

good call rusty, i have a 350 on a run in stand that ran like home made shit. same thing , move the dist no change ,popping thru the exhaust etc. plugs were always dripping with fuel , turns out the needle valve wasnt seating correctly and dumping a snot load of gas down the intake. fixed it and now it runs like a champ ....just wish i had a cool car to put it in

 

i dont think its timing if its backfiring out the exhaust, usually it would backfire out the carb,

 

check the intake for leaks

 

I was reading some post today about engines running bad and started thinking how much I wanted to know what the end result was with them. Then I remebered all you guys helped me pin poin the problem with mine.

The verdict is in: It was the cam. I replaced it with the suggested edelbrock cam and all is great for the last 4 months. Runs fantastic. Thanks to all that helped, and I learned a lot.

 

I think I am running into the same exact situation with my motor...I put the motor at TDC compression stroke on #1 crank dampner at 0 mark and it would not fire...so took the front cover off and found my engine assembler guy had put the crank gear with the triangle up, not the dot, and the cam gear was at 2 oclock...so...HOW DID YOU FIX IT after you found it? Details please.

 

You line up the marks. Pull timing chain, Rotate crank dot to 12 oclock. Rotate cam to 6 oclock, put chain back on.


(You do realize this is a 7 YO thread?)

 

Several years ago I swapped cams in a sbc chevy and since I'm old enough to remember them I decided to use a 3863151 chevy cam (350 hp 327) in the engine. It really didn't run as I expected then I remembered the (lower) compression ratio of the engine I put it in and figured out why it didn't run right. Big overlap plus low static compression make for low cylinder pressures. Today's gas probably doesn't help much, either.

 

yep I realize that, Keep. You say rotate the crank dot to 12 oclock. Thats all well an good...but...the engine is currently already at TDC #1 compression stroke both valves closed ready to fire. If I rotate the crank as you say, wont that take it off firing #1 position? I heard you are supposed to put the dots at 12 and 6 (cam & crank respectively, then rotate the crank one full turn which is suppose to put the #1 piston at firing position. I was told if #1 is up at TDC and the cam & crank dots are together (6 & 12 respectively) that puts the firing at #6 cylinder. To me, it now seems like the assembler had the crank in the wrong spot to begin with, and has left me to deal with it...and I would agree with your method IF the crank and the cam gears were found to be in different positions other than where I found them when I pulled the cover off with # 1 up ready to fire. (i.e. cam dot at 2, crank 3-position gear lined up with triange, not dot. What do you think??

 

Keep, when you said rotate the crank gear....did you mean rotate the crank to put it in position, or to pull the gear off and rotate it leaving the crank alone....I just re-read what you wrote, so my bad for mis interpreting if thats the case.....I JUST WANT THIS SUCKER TO RUN RIGHT!! Thanks for your reply, by the way.

 

I mean rotate the crank around until the dot is at 12 oclock.

BTW when the dots are at this position it is ready to fire on #6!! So remember that when you stab the distributor back in.

Unless the builder advanced the cam for some reason? Just line up the dots as described, point the distributor rotor to #6 and fire it up!

BTW TDC on #1 would be both dots at 12 oclock.

 

To fix your problem -
First - Remember the cam is loaded with spring pressure it may either be hard to move or want to rotate once chain is off.

Second - the piston TDC location happens 2 times in one cycle. Once at cam over lap or top of exhaust stroke changing to intake stroke and once at the top of the compression stroke changing to power stroke.

Third - the timing marks are correct when lined up vertically thru the center lines. The crank always at 12 o'clock and the cam can be at 12:00 or 6:00. Both cam locations are correct but the distributor rotor location will change. With the cam dot at 6:00 #1 cylinder is in its over lap position and thru the firing order that means #6 is ready to fire and that's where the rotor should point. ( see how that works 6:00 for #6 ). With the cam dot at 12:00 #1 cylinder is ready to fire and the rotor should point there.

Understand all that ?

Here we go.
1- with chain on Rotate crank until cam dot is at 6:00.

2- Remove cam gear and chain, make sure cam doesn't rotate from spring pressure.

3 - rotate crank so timing mark is at 12:00

4 - reinstall timing chain and gear.

5- check that rotor is set to fire # 6 cylinder.

6 - rotate till timing marks are both at 12:00 and check to see that rotor is set to fire #1

Hope that helps you