73 Tuning issues

[RaysK20]

I dropped in a new good wrench 275 horse 350 in my 73. In the process I added a edelbrock performer intake and hedman headers and a new HEI to be rid of the points. Other then that the motor is stock. I also added a kit to the stock quadrajet carb to freshen it up. It had been sitting for several years. It fired up real easy and I broke in the cam for 30 minutes. I set the initial timing at 10 degrees btdc. off the vac. It stumbles real bad just off idle and or will bark threw the carb like the timing is retarded, then runs like hell to about 5000 rpm. I have check rechecked then checked another ten times that my timing is in line with the marks and the distributor is lined up properly. I cant find any vac leaks anyplace and it idles smooth as glass. I am at a loss. Should I dump the quad and go buy a new edelbrock carb? Any help would be great.

[GMC Guy]

did you make sure you were on TDC before planting the distributor? your timing could be 180* off. i could be wrong, but it's something to check

[RaysK20]

Yes I am dead positive I have it on the mark at top dead center. It runs perfect except that studder just off idle. But once it clears up you take off like a bat outta hell.

[Kixwy2]

What lines do you have running to your carb? is everything else blocked off, ie, no open ports? I ditched my EGR and put in a block plate, and blocked the line to the carb. I have mine timed at 11.5* BTDC, and slightly lean. (this is after lots of fiddling.)

I usually make a small change, check the result, and then make another small change - just so I don't foul anything up too much. lol

Once my engine is warm, I have no stumble off idle at all.

[LONGHAIR]

If it was 180 degrees out, it would not run at all.

I would suspect a carburetor issue with the stumble. The easiest test it to try another carburetor. Even if that is not your "perminent" cure, it will get you pointed in the right direction.

[OleBlue79]

Are you flooring the gas when you feel the hesiation? Are you already rolling when you do it or from a dead stop? Q- jets are famous for the air flaps on the secondaries opening too fast and sucking too much air in at once. Then your engine catches up and off you go! This is an easy fix. Q-jets are really good carbs, just appearently a little misunderstood.

[69BBsuburban]

Where did you pull the 12V from when hooking up the HEI? Alot of the older GM rigs used a resistor wire in the "run" position going to the coil to drop voltage to it.

[73chevyman]

set your timing at 3500 rpms it will put you dead on the mark then adjust your carb with a vacuum tester if that don't help then I go and try another carb there is a video out adjusting your timing this way..........

[73chevyman]

heres the video i was talking about............

YouTube- How to adjust a Chevy Edelbrock...

[Oilbrnr]

Did you re-use the harmonic balancer or put in a new one? I was having crazy issues setting my timing until I realize there were TWO timing marks. Actually a 'tit' and a notch, and there is about ten degrees difference between the two.

[73chevyman]

after i set my timing with the vac back on and the mark on my truck was at 12 oclock and mine runs fine but i need a new carb mines way to big for it.............

[GMC Guy]

Originally Posted by Oilbrnr

Did you re-use the harmonic balancer or put in a new one? I was having crazy issues setting my timing until I realize there were TWO timing marks. Actually a 'tit' and a notch, and there is about ten degrees difference between the two.

do you know what the other mark is for? mine has it too

[RaysK20]

Ok, The balancer is new and came with the motor. It does have two timing marks that are Identical. My timing tab is off to the side around the one oclock position. when at top dead center the other mark is strait up for the other style timing pointer. I have tried both marks and it will only run with the first mark in the rotation set at tdc. Ok, the vac being pulled for the distributor is off the timed port. the only other port went to the air cleaner housing and its currently plugged off. There was never egr that I am aware of. I am also running a new pcv so its not leaking and all my rubber lines including the one on the trans is new. The bog is there if you slowly depress or stab the gas. If you floor it alot of times it will bark threw the carb. The Hei is wired with a new 12 gauge wire from the bulk head. I deleted the factory wire so its getting full voltage. I am at a loss at this point. A guy I work with is going to loan me a known runner edelbrock 750 to try out. I will keep you all posted. This is my only other option that I can think of.

[73chevyman]

750 are to big thats whats on my truck right now and it dumps gas out the tail pipes but it is a start....but if you do get a new carb stay in the range of 600 and 650.....

[wheels67]

Bad carb part. Probably accellerator pump, hence the hesitation. I think maybe a power valve is blown...not sure. A 600 cfm carb will fix it.

[Doorslammer]

what happens when you add more timing ? say another 4 degrees

[RaysK20]

The accel pump is new. I just put a kit in the carb. I am not a pro, but the quadrajet is not to bad in my book. Could just be worn out to. I can add more timing upward of 15-16 degrees but it gets choppy running at that point. I dont plan on staying with a 750, its just to help see if the carb is the problem. I will bolt it on this weekend. If it works a 600 edelbrock will find it home under the hood.

[silverstreak]

If it idles fine, and smooths out at full throttle, I'm thinking it's something in that carb on the primary side. Just for kicks, I'd pull the top off and make sure the metering rods on the primary side aren't stuck. As someone else mentioned, make sure the secondary air door is'nt opening prematurely.
Have you peeked at the advance weights in the dist to be sure they aren't binding or sticking?

[OleBlue79]

A flat on lobe on the camshaft can make them bark through the carb at certain RPMs also. I had a 402 BBC that would bark and pop at about 45MPH. Before 45MPH and past 45 MPH it ran great. I also installed a cam and didn't break it in properly once and it acted like that. When I pulled it out one lobe was nearly round and the lifter had a small hole rubbed in the bottom. Reinstalled the factory cam and that fixed it. It was a hard lesson learned. I hope it's you carb and not the camshaft.

[73chevyman]

i'm with you there oleblue i had the same problemwith my 73 #4 exhuast valve was rounded off and it idles fine but when you hit the gas it barked and poped change the cam and lifters and everything was fine now the carb is to big it's pretty much dumping gas out the tail pipes