rooster287 Posted March 4, 2009 Share Posted March 4, 2009 Hi, I'm having problems with my freshly rebuilt 350 in my 74 Z. While parked the car idles well and has good throttle response with no misses or backfires. As soon as I put the car into drive and attempt to accelerate, I get a nasty backfire through the carb and the car wants to stall. I have to feather the accelerator to get the car up to speed. At speed the engine feels like it has no power. The engine has a 270H cam, an edelbrock 1406 carb, edelbrock air gap manifold, edelbrock alum. heads, headers and an HEI distributor. We've checked the timing without vacuum at 8 deg advanced, readjusted all of the rocker arms and adjusted the carb mixture screws to 2 turns. We've also verified the wires are going to the correct cylinders and the distributor isn't turned 180 degrees. I've check the plugs and they seem to be on the rich side (black and sooty). The plugs haven't been changed since I rebuilt the motor so I don't have any idea how many miles are on them. One strange thing is happening when we try to get a read on the RPMs. I just bought a digital timing light and we connect it to the number one cylinder. The gun won't show a consistent RPM reading at idle; the number jumps around. I don't know if this is the root cause of the engine problems but i'm suspecting that there is something going on with the distributor. The distributor is only about three months old... Any help or suggestions would be great. thank you. Quote Link to comment Share on other sites More sharing options...
saxfiend1967 Posted March 4, 2009 Share Posted March 4, 2009 sounds like it is 180 degrees out of time. they will run just not good Quote Link to comment Share on other sites More sharing options...
saxfiend1967 Posted March 4, 2009 Share Posted March 4, 2009 i would change the plugs and rejet carb if you are certain it is timed correctly i like accel plugs Quote Link to comment Share on other sites More sharing options...
jt1 Posted March 4, 2009 Share Posted March 4, 2009 If the dist is 180 out, it won't run at all, only backfire, stumble, and sneeze. Make sure the accelerator pump on the carb is working. You should see it squirt fuel from both squirters with only a slight movement of the throttle. Eddies are known to have the pump seal dry out and not squirt properly. Also try some more advance. Your spark curve is a whole new topic, but you can probably use some more. If the carb is squirting properly, I would try another coil in the dist. jt Quote Link to comment Share on other sites More sharing options...
saxfiend1967 Posted March 4, 2009 Share Posted March 4, 2009 If the dist is 180 out, it won't run at all, only backfire, stumble, and sneeze. Make sure the accelerator pump on the carb is working. You should see it squirt fuel from both squirters with only a slight movement of the throttle. Eddies are known to have the pump seal dry out and not squirt properly. Also try some more advance. Your spark curve is a whole new topic, but you can probably use some more. If the carb is squirting properly, I would try another coil in the dist. jt if a small chevy is 180 out of time and distributor is in the rightplace it WILL run .I bought a trans am from a guy several years ago with a 350 and drove it for about 4 months it didnt sputter once and it sounded great but didnt have any power. I sold to a friend who is a chevy guru and he moved dist., It ran like a scalded ape after that. I do agree with your advice though Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted March 4, 2009 Share Posted March 4, 2009 if a small chevy is 180 out of time and distributor is in the rightplace it WILL run .I bought a trans am from a guy several years ago with a 350 and drove it for about 4 months it didnt sputter once and it sounded great but didnt have any power. I sold to a friend who is a chevy guru and he moved dist., It ran like a scalded ape after that. I do agree with your advice though 1-8-4-3-6-5-7-2 normal starting with tdc #1 6-5-7-2-1-8-4-3 180 degrees out starting with tdc #1. As you can see, No it will not run unless you rotate the dizzy 180 degrees or remove and reposition the rotor 180 degrees. I think your confusing being 180 degrees out with being a tooth off or two. Quote Link to comment Share on other sites More sharing options...
rooster287 Posted March 4, 2009 Author Share Posted March 4, 2009 thanks for all of the responses jt1 - i didn't list it in my original post but we did check the accelerator pump to verify it was squirting fuel. the more i think about this, the more i'm suspecting a problem with the distributor. last night after I posted i put my timining light on my 87 jeep 6 cylinder to see what the RPM reading was. It was steady at 730. It's an HEI distributor so there isn't any external coil. Is there an internal coil which can be replaced on an HEI? Also, could a problem with the distributor cause the RPM readings to fluctuate like crazy at idle? Also, i'm wondering if there could be a grounding problem? The car has had a lot of hacks electrically (previous owners) and there are wires everywhere. I've got a ground from the back of the engine to the firewall. I'd like to avoid buying another distributor to prove this one is working OK if I can. I'm also going to replace the two 3/8 vacuum lines going to the distributor and transmission with new hose. The existing hose looks OK but it's rock hard. We had to do this on the transmission hoses because they were spewing tranny fluid under pressure. Quote Link to comment Share on other sites More sharing options...
fastzcars Posted March 4, 2009 Share Posted March 4, 2009 I would check the weights and springs under the distributor cap to make sure they are sliding freely. Plus I would put a little more timing in say from 8 to 12 degrees. I also found when I was running the HEI dist that my hood latch was touching one of my spark plug wire boots, causing it to ground out! I didn't find this out until one day my car died on the road and when I opened my hood to investigate, my car would start ok . But when I slammed my hood close it instantly shut down! Quote Link to comment Share on other sites More sharing options...
Pyro Posted March 4, 2009 Share Posted March 4, 2009 back fire out of carb is lean and/or retarded ignition timing. you don't have a stock cam anymore, so stock ignition timing will not work. try 15 degrees advance at idle, then plug in the vacuum advance to the manifold vacuum for another 15. should be idling around 30. Adjust the idle to 800 or 900 rpm and set the idle mixtures screws for best/highest idle (highest vacuum). You should have a 2400 to 2800 stall converter with that cam, right? If that doesn't fix it, then add more accelerator pump. Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted March 4, 2009 Share Posted March 4, 2009 (edited) We've checked the timing without vacuum at 8 deg advanced, readjusted all of the rocker arms and adjusted the carb mixture screws to 2 turns. If all else fails I believe your problem is in valve adjustment. You said you readjusted all of the rocker arms. Were they out of adjustment? How did you adjust them? Backfiring through the carb indicates that the cylinder is sparking before the valve is closed or the valve is not adjusted right. Firing before the valve is closed is timing too far advanced. Your going to want about 35 degrees total advance at WOT which does not inlcude vacuum advance. On any performance engine vacuum advance is pretty useless. Edited March 4, 2009 by dr_hunt Quote Link to comment Share on other sites More sharing options...
rytherwr Posted March 4, 2009 Share Posted March 4, 2009 Don't forget to check for vacuum leaks while you are wandering around under the hood...bend the hoses to make sure they aren't cracked as well... Quote Link to comment Share on other sites More sharing options...
grumpyvette Posted March 4, 2009 Share Posted March 4, 2009 silly question, But,have you tried to used a DIFFERANT timing light to time the ignition timing and check the advance at both idle and at 3200rpm,and have you verified the TDC on the damper and timing tab are accurate? did you degree in the cam? Quote Link to comment Share on other sites More sharing options...
jt1 Posted March 4, 2009 Share Posted March 4, 2009 Wayne's suggestion of a vacumn leak is a good one. That will cause the idle to jump around and could cause your backfire. A can of carb cleaner or an unlit propane torch are good tools to use to check. Remember the big nipple on the base of the carb. Pull a plug, put it back on the wire, ground it, and spin the motor over. If you get a good blue white spark, chances are the coil is OK. If the spark is reddish orange you've got problems. On a hei dist the coil is located in the dist cap. Sometimes on a freshly painted motor the grounds don't make good connection. You can always run a ground directly to the battery to check. jt Quote Link to comment Share on other sites More sharing options...
rooster287 Posted March 5, 2009 Author Share Posted March 5, 2009 UPDATE: Well I decided to go back to basics this morning and simply replace the plugs. The original plugs were NGKs and they didn't look too bad. Funny thing was after checking the NGK website, they didn't appear to be the right plugs for the chevy. So this afternoon I went to my local Kragen and bought eight new Champion plugs. After getting home I pulled an old plug to compare to a new one. The new plug is almost 3/8" of an inch LONGER! Well, after replacing all of the plugs and reattaching my timing light, my idle finally settled down on the gun. The real proof was my first spin around the block! Wow no hesitation and no backfires. In any case, it appears that the simplest fix is all it took. I really appreciate all the suggestions that were posted. Quote Link to comment Share on other sites More sharing options...
jt1 Posted March 5, 2009 Share Posted March 5, 2009 Glad you found it. Did the first "good" spin around the block put a big grin on your face for a while? jt Quote Link to comment Share on other sites More sharing options...
Pyro Posted March 5, 2009 Share Posted March 5, 2009 you still need more timing. 8 isn't nearly enough. and you still need to use the vacuum advance if you plan on getting any MPG. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.