This newbie needs some help. This is the first engine I have "rebuilt".

I have a 75 vette with the anemic smog motor. So, I decided I would pull and beef it up a little. I left the bottom end untouched. Here is what I did:

Heads - 2.02/1.94 valves 180CC intake runner
Carb - Edelbrock Performer 600CFM (unchanged from old motor)
Intake - Edelbrock Performer
Camshaft - CompCams ***** Thumpr (yes I know, it's a car show cam!)

Here is my dilemma. I am having major misfire on the motor now. I have changed plug/wires and brand new distributor.

Timing - ~35 BTDC full advance. Around 15 at idle with the vac line plugged. I have not even messed with the vac advance....as I don't want more variables in the equation. Seems like it should run ok with only the mech advance?

Here is what I don't understand. It seems to be running really rich right now. This perplexes me as I changed nothing in the carb from old motor to motor with new heads. These heads should outflow the old smog heads hands down. Therefore, it seems to me that all things be equal, the motor should not be running very lean.

I took the carb apart last night and set the floats. They seemed to be way out of adjustment. I took it for a drive and it seemed to be doing ok then it was like it just fouled the plugs and started missing badly. When I removed the plugs, they had black soot on them....seems like this means it is running rich???

So, most recently I went and bought new wires/plugs and stuck them on. Now it is running like complete crap.

I am very discouraged right now because I was so excited to get this running. It just doesn't have near the power that I thought it would. I know that I will probably be ridiculed for using the ***** Thumpr cam, but I read an article in carcraft where they made ~400hp with that cam in a mild motor with vortec heads.

Any help from all you veterans would be great appreciated.

 

I wonder if your float valve is stuck? Perhaps you got some crud in it in the process of swapping it around?

 

Black soot is a usual sign of rich. Does the exhaust stink or make your eyes burn? It's also possible you got a bad set of plug wires- even new ones. Check the resistance of each wire.

 

Check for a vacuum leak, Manifold to heads.

Gary

 

So, Did you clean the plugs before you took it out for a drive? Could the soot be residual from before the carb rebuild last night? What is the current compression on the cylinders? What plugs and what plug gap are you running?

 

Thanks for the quick replies all! I haven't done a "good" vacuum leak check. Would this really cause such problems with it running?

I believe plugs I just put in were AC/Delco R45TX. I did not gap them :0

I completely agree with all of you....all signs point to way too rich of a mixture. The part I just can get my head wrapped around is why is it a rich mixture now....I have to believe that these heads outflow my old heads by a considerable margin making me think it should be lean now??? It seemed to be running way to rich from the beginning of this new engine's life and all I did was slap the carb on it....I did nothing to the carb.

One thing I forgot to mention...my old heads were 76cc combustion chamber and new heads are 64cc. I have no idea what my compress ratio is now as I didn't check, but I would guess it is in the neighborhood of 9:1...maybe a bit more. I believe this motor was 8.5:1 stock. I haven't put a pressure gauge on it yet.

 

How did you set the valves/ Could be to tight. How does it idle? Vacuum leak will cause a poor idle.

Gary

 

Car has flat tappet hydraulic lifters. I set them by spinning the pushrods while tightening the rocker arm ( I have roller rockers). Once I felt resistance, I put another 3/4 turn on them. One thing I didn't do and don't think I had to was pump up the lifters. I didn't think you had to do that, but I am new to this, so maybe I screwed up. Any thoughts on this?

As a side note, I did put the ZDP (I think that's the name) additive in the oil for break in and ran the car at around 2500rpm's for 30 minutes. So, I don't think I have a flattened lobe issue. When I look at the rockers, they seem to have a lot of movement just eyeballing it. I watched a video of a flat cam, and it seemed obvious that the cam had flattened just by watching the rocker arm movement.

 

you do NOT have to pump up the lifters....sounds like carb trouble...keep posting....good luck....

 

You set the valves starting at TDC then rotating the crank 90 degrees to set the next one in order etc?

Gary

 

Duke94....maybe I need to check my valve settings again. What I did was started with cylinder 1. I had the motor on the stand at this point without intake on it. I didn't do the 90 deg piece, but with the intake off of it, I was pretty confident that I was on the base circle of the cam when I adjusted the valves. I will run through that again. It is just such a pain to turn the motor over. I don't like using the crank snout bolt for fear of stripping. Any tricks on this. What I did in the past was removed crank pulley and then stuck 3 bolts in the balancer where you would use the puller. I then just used a pry bar to turn it over wth the 3 bolts.

 

it the rocker tips have rollers ... most heads require guide plates ... and those plates usually need adjusting so as to properly align the roller with top of valve stem. Adjust gp by loosening rocker studs & shifting gp.

otherwise, it's fairly common to have rollers running off the side of stems & can pop a retainer loose then drop a valve ... make sure your valve train is properly aligned.

as said ... check for vac leaks

 

also ... depending on gasket thickness, a stock L48 block & pistons w/ 64cc chambers can be UNDER 9:1 ... but no more than 9.3:1. Did you put too much cam in with too little compression?

 

Did you already have 'premium' fuel in the tank? Or did you put in an octane booster (if you had 'regular' gas in there)? The increased compression of the new heads will require higher octane to run well.

I suspect that your timing got messed up during the change. What have you done with regard to timing?

 

I usually put a 15/16 socket on the alternator and turn the engine with it. If the pulley slips on the belt, just use your hand to put tension on the belt below the alternator as you turn it clockwise. It's extremely easy.

 

Here's the procedure I use. Source is "How to rebuild your SBC" by David Vizard.

Gary.

 

Is my cam too big? Maybe, I have no idea. The thing that makes me think maybe it isn't is that the cam I took out was basically same lift. I do realize that this cam I have chosen does have different characteristics like the LSA. I hope I have not chosen too big of a cam, as I am not excited about jerking the motor back out!

Here is my new theory...can somebody verify. So, I readjusted the float levels in my carb. They were way off. Is it possible that the reason it was running so rich before was because it was flooding the motor due to float level being incorrect. Now that I readjusted, the carb is actually performing as it should and is creating a lean condition that is causing all of my missing?

I messed with it again last night and I had it running (terribly), and when it shut down it had a gigantic fireball come out the carb. That points me back to timing

 

You're getting close. Check and double check that the wires are on the dizzy in the correct order. Make sure the one you think is # 3 actually goes to #3 etc. Been there done that more than once!

Gary

 

Did you degree in the cam? sounds like the cam is not in correctly. Those Thumpr cams have alot of overlap that could accout for the soot you found.

 

As far as degreeing the cam, I didn't degree it. The instructions on my cam said that I could just put it straight up because the comp cams are ground 4 (or 6, don't remember) advanced already. It said of course you could degree it for maximum performance. Am I wrong to assume that I should be able to get it to run well, just with reduced performance since I didn't degree the cam?

I was looking to get 350 hp out of my setup. Maybe I am overly optomistic!

Which one of you all lives close and can come over and help!!! Thanks for all the input...I really appreciate everyone's help.