small backfire
 

im looking for a few suggestions

last week i fired up my chevy for the first time after a rebuild its running ok but i am getting a slight backfire through the carb its enough to make the idle shaky. at start up it gives a good shot through the carb .

now the motor is a 327 with a comp cam an hei distributor, performer intake and a edlebrock 1405 carb. and headers.
the machine shop assembled the block and cam i installed the lifters pushrods etc.. i also rebuilt the carb it was only put on a car once but supposedly it was flooding right out of the box so i used a rebuild kit and replaced the needle, cleaned the jets and adjusted the floats (they did seem to be a bit out of adjustment) for initially adjusting the rockers i used the method of rotating the motor untill a cyl was at tdc (both rockers down) then slowly tightened them until the pushrods would not spin in the lifter seat then went a quarter turn to set them. upon start up i let it run 20 min to break in the cam when we went to adjust the rockers while it was running for the most part they were very close but about three of them were very tight maybe a full turn before they would start clacking. i really hope i didn't screw up the cam. as for the timing its advanced about 8 to ten degrees (with the vacuum advance disconnected) that seems to be where the motor likes it the best. i figure to check out the cam im going to have to do a compression test here are two videos of the motor running well one is us starting it up this is before we adjusted the rockers and it has open headers inside my portable garage so it was very loud and the audio was picking up all kinds of noise so you cant use that to diagnose anything. http://youtu.be/0R1eFynyS8U http://youtu.be/oxLqrGHvVtY so what do you think???

Generally what you describe is due to a badly-tuned carb or timing issues, but I'm confused about the cam now. Certainly if your method of finding TDC was off a bit, you'd probably wind up with loose rockers instead of overtight ones. But I don't understand why you wanted to reset them with the motor running. Are you using solids? In that case you would never do the 1/4-turn thing at the end, you'd set them with zero lash and then adjust them while running (hot) with a feeler gauge. Assuming you have hydraulics, it is bothersome that you had some that wouldn't show lash until a full turn, but that still doesn't mean you trashed the cam necessarily.

All of this is why I went to roller rockers long ago. You can fingertighten the polylocks, then give 'em the additional turn (I go 1/2 turn). But you don't zero-lash them by going down until the pushrod no longer rotates, just until the rocker is seated. You can force the plunger down too far otherwise.

The fact that you just did 1/4 turn at the end helps somewhat. I would check my timing again, and try another carb to be sure that's the cause. As for the cam, try the method I suggested (again assuming hydraulics). Loosen them all up, go down until the lash is gone (the pushrod can still be turned usually), then go another 1/2 turn. You should have no problem with overtightening, though you can check for undertightened, which could happen if your cam wasn't totally on the base circle when you made the adjustment.

Another advantage of aluminum rollers... they restrict oil flow so you can watch 'em running without taking a shower in hot oil.

So, just to be orderly...
1. Reset the valve lash as suggested to be sure you aren't grinding yourself a new cam
2. If it still burps, look for easy stuff like crossed-up plug wires, timing etc.
3. If it still burps, swap carbs and see if that works.

Good luck!

thanks for the reply, they are hydraulic and i was always taught to check the adjustment with the motor running. i don't do this every day so if i didn't double check it i wouldn't know if i set them up correctly in the first place. and seeing that a few were tight i guess i wasn't spot on

the first thing i am going to do is double check the firing order

as for the timing i used a timing light to set it so i dont think thats an issue.

Had that problem once. Spider egg's in the dist cap!

Sounds like an intake valve too tight or cross firing cables like #5 & #7. :D

ding ding ding........we have a winner 5 and 7 were crossed and some how i managed to put the drivers side valve cover on backwards...... the rockers almost wore holes in it :sweat:

and there is a problem with the carb it suddenly started idling real fast and i couldn't tune it back so it runs real nice now but i do need to have the carb looked at that part is beyond me. thanks for the help guys. :thumbup:

Maybe the carb problem is just a vacuum leak? Could be at the carb gasket, an unblocked or loosely-fitted vac port, or even the intake gaskets. If no, and if the idle mixture screws aren't at fault, might just be a bad carb.

Are you running manual choke?

I think your rockers are not set properly. I adjust the rockers same way you did except i turn the crank by hand until I see the exhaust valve closes then bring the piston close to TDC. Then I tighten to push rod until there is no play then 1/2 turn for the final setting. Then go to the next cyclinder do the same thing again. And I never adjust it when it's running.

Not to harp on it, but I must agree with this. It's the prescribed method, and the standard thread size on normal 3/8 rocker studs makes between 1/2 and 3/4 turns after zero lash the correct preload on the lifter plunger. Less and it's loose. More and it can (or will) bottom. I know having the stock-style cross-threaded locknuts makes it a total pain, but it can be done anyway. Just be sure you gauge zero-lash at the point where the rocker no longer lifts off the pushrod, not when you can't turn the pushrod at all. Depending on whether the lifter is fully primed with oil or some has bled off, you could go way too deep trying to get the pushrod so tight that it won't turn at all before you do that last 1/2 turn to preload the lifter. Then it could be bye-bye cam lobe.

I suppose that rechecking them with the motor running eliminates any worry about how well you found the lobe's base circle during initial static adjustment (along with accounting for any other inaccuracies that could exist like pushrod length etc.), but it's awfully messy with standard-type rockers. With roller rockers it would be a snap. They don't squirt oil at you.

thanks guys the rockers are all good now my main problem was that wires 5 and 7 were crossed as soon as i fixed that it ran 100% better i did check for vacuum leaks and all is tight it may just be a simple adjustment on the carb im going to get someone whos really good at carbs to take a look while its running and get there opinion. its just a little rough at idle the skip is gone so we'll see what happens

Now that it is, you probably need to re-check the initial timing. Might explain the fast idle speed. :D

i did recheck the timing its about 12 -14 degrees advanced the motor seems to like it best there. as or the idle i know its in the carb when i put a little pressure on the throttle linkage the idle will drop down and the idle screw isnt even touching the cam on the linkage also the pass side idle mix screw stopped working so i may have some dirt in there i will at the least have to pull it off and re check it all and clean it out i have a glass fuel filter and you can see a lot of junk in there.

one other question i was checking some pics of timing tabs and found a pick that showed the top of the tab scale was marked before and below zero was marked after http://www.hotrodders.com/forum/chev...to-178514.html mine is marked a on top and b on the bottom just wondering what the difference is