Well ive almost about had it with this motor. I rebuilt it and was running a quadrajet. The problem i have been having is that it will backfire through the carb when accelerating fast and it will not idle in drive. After calling a few carb people, i went with a holley 390. With my luck, I install the 390 and still have all the same problems. I set my timing to 8 degrees with the advance plugged and then 10 degrees and no luck. I also set the float just like the holley manual said and then i tried the idle mixture scews. Again no luck. I really doubt it is a carb problem yet again and i know the timing is ok, What else could cause this?

Anything would be helpfull at this point.


Motor:
250 with offenhauser intake, holley 390, and 307 pistons.

if it backfires thru the carb it most certainly is a timing/valve problem.
firing at the wrong time or on an open valve.
do a compression test 1st, with all the spark plugs out. if that is ok pull the valve cover and make sure all the valves are adjusted properly...you may have one too tight.

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You say you rebuilt it...
Did you change the cam? are the gears meshed in exactly the right position? Its been a while but I recall this was a sod of a job that took several attempts (engine was in the car at the time tho, and I had no way of holding the weight of the cam to align it). It only needs to be one tooth out...

the cam is real close to stock. The dots on the cam and the crank lined up perfect when i put it in. Im thinking i might have adjusted a valve to tight at this point. I wouldn't think its a timing issue because it runs perfect in park. Im gonna pull the cover off this weekend and redo the valves and see if that helps a little.

Just a suggestion: make sure the sparkplug wires are going to the right plugs.
I had a 350 that the mechanic (a "Chevy guy") mixed the 5 and 7 wires up on. He kept blaming the backfiring on the Holley and told me to get a Quadrajet. When that didn't work I brought it to one of the speed shops nearby where they got the firing order right. Boy did I feel stupid-I knew what the order should be, but never checked it.
BTW, it would idle okay (manual trans) but would backfire on accelleration.

make sure the timing is set right... if you have it at 8* go to 10* then 12*.. get it to run right at WOT.. then you can deal with it to run in park

[This message has been edited by 1981coupe400ci (edited June 11, 2005).]

Ok some good news today. I readjusted my valves and did some timing work and it runs descent down the road. It no longer pops through the carb. It could use a lil more gas when the pedal is to the floor but that is just some jetting work that I'll have to do. It goes down the road great with normal acceleration now http://www.nastyz28.com/ubb/biggrin.gif

After all this good news comes some bad. It still doesn't like idling in drive. It no longer stalls out but it idles at 400-500rpm. I need this increased by at least 200rpm for better vacuum and so it doesn't stall when I put on the AC. The timing and the idle mixtures are set for the best idle but its not good enough. The only thing I can think of is the torque converter. I was reading some info about them and from what I found out is that it could effect idle quality if the torque range is moved which is possible cuz of my mods. Stock stall is 1400 according to the manual and when I hold the brake and hit the gas, it is at 1000 exact. So I was thinking about getting the 2000 stall B&M.

So what are your thoughts on this and do you think it will help the problem?