Thread: 350 backfires through carb

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I just had the exact same thing happen to me.....backfiring through carb after a rebuild, wouldn't start no matter what I did, adjust valves, set timing, new distributor, wires plugs, new heads, rebuilt carb, no intake vacuum leaks, good compression, plenty of fuel, spark and great oil psi. NOW for the trick. When I took the crank out, and rebearinged the bottom end, one thing I should have checked for which I didn't and here it is:

The jerk who owned the 350 I bought who had put a brand new double roller chain in it.
He was a dogkrapface and did not find TDC when reinstalling the crankshaft timing gear.
At TDC I had the arrow on the gear facing up, not the circle.
Then when installing your timing chain, you think everything is ok but it is not.

Circle to circle was NOT tdc on number 6(cam circle at 6 oclock crank circle at 12 is TDC on # 6)( cam circle at 12 and crank at 12 is tdc on # 1.

So by him not installing crank gear correctly, and my own stupidity for assuming he did, when adjusting the valves, I was and you are actually on the ramps of the camshaft and you will never get the valves or the timing correct.

I took the cover off, ripped down the front of motor, removed chain and gear, found TDC #1 reinstalled gear and chain with circle to circle(TDC#6) then rotated engine so circles are at 12 and 12 for tdc#1, dropped in distibutor etc etc, set valves normal way when all back together, with timing set at 8 btdc, it fired immediately not even a crank. All backfiring gone, running incredible.

Moral of the story, dont second guess yourself, just double check yourself, hope this helps.( rotate to TDC#1 THEN install crank gear with circle at 12)

Double check your timing gears. I had this issue where my motor would not run. Wouldn't even start no matter what I did. Checked everything 10 times over, find tdc #1, check plug wires, check plugs, had fuel, oil pressure, new distributor, new heads etc....

Adjusted valves 15 times at least with the EOIC method and every other method known to mankind.

When I rebearinged this motor, I should have checked a vital thing but I didn't and it bit me in the uknowwhat. The guy who had the motor before me had installed a brand new double roller timing chain. When he installed the CRANKSHAFT gear, he used the wrong slot in the gear on the keyway.

When I went to install the chain, circle to circle(12on crank 6 on cam)=TDC on cylinder 6, was not really tdc on any cylinder. CrankCrankCrank no run, nothing but backfiring through the carb, spitting gas 2 feet in the air.

I finally figured out what the problem could be, so I took timing cover off, removed chain, removed number 1 plug, physically looked inside plug hole while turning the engine by hand with a 5/8 socket, until # 1 was at TDC(very easy to see with flashlight), Looked at crank gear, and bingo, the arrow was at 12 oclock and the circle was at 3oclock. Setting the valves was a waste due to the basic initial timing being way off. See where im going?

I pulled the crank gear, reinstalled it with circle mark at 12, and #1 piston at TDC. Reinstalled timing chain with both circles at 12 oclock position. This is #1 TDC timing. Dropped the distributor back in( wherever the rotor is now pointing is your number 1 plug post. Using this method it is impossible to be "180" out Play with the distributor until you have it where you want it and can have the room to advance or retard if you choose.

My motor fired IMMEDIATELY not even 2 cranks of the starter, and runs sick. I set the timing at 8 btdc and have not touched it. An easy way to cut off vacuum advance is to leave the hose attached to both carb and distributor and use a pair of needlenose pliers to pinch vacuum line when setting timing. Hope this helps someone out

Originally Posted by DogKrapFace

An easy way to cut off vacuum advance is to leave the hose attached to both carb and distributor and use a pair of needlenose pliers to pinch vacuum line when setting timing.

If the hose it left in place, when you pinch the hose, any vacuum in the diaphragm will be trapped and possibly cause an erroneous timing check/adjustment. Always remove & plug the vacuum supply hose to the dizzy.

Years ago I used a point & condensor style distributor but did not use a resistor to drop the voltage from 12 to 9 volts. This clause the points to ark and timiming to jump all over. It would run for 5 minutes and then overheat. What type of distributor are you using?

Firing order is probably right it's just that your top dead center is on the exhaust stroke and not on the intake stroke, which would cause it to back fire through the carb. You're 180 degrees out. Trust me I being there.

Great suggestions. Starting from scratch is the best. Generally the problem is simple and plain. sd455 have you found the back firing problem? I feel timing is the issue. 180 out.

Originally Posted by Flamedchevy

Great suggestions. Starting from scratch is the best. Generally the problem is simple and plain. sd455 have you found the back firing problem? I feel timing is the issue. 180 out.

The dude hasn't even been on the site since the 2nd of July, and only made 3 posts.......... Doubt he's still fighting the backfire problem......

Now don't we feel silly!

Don

We? you gotta mouse in yer pocket?.....that's funny Don.

Well fellas,I've tried everything you've given me,even replaced timing chain with
gears from summit.I'm going to pull this motor back out,take it back apart,and
start all over again.Thanks for all the info.

I want to thank you fellas for all the info. I was having the same problem,I just installed a new Edelbrock manifold & 1406 carb. found that the distibutor was "off" (HEI) brought up #1,then moved the dist. and DAMN the ol'girl fired & idled! YeeHa! Although, tell me if I'm right, half-way through slowly pushin' the fuel pedal down, I get a small (much smaller) backfire through the carb. Could I still be 1 tooth off on the distributor???

Here is one which I learned the hard way today on a 350 chevy block in a boat. The distributor shaft has a shear pin located near the bottom. If this pin breaks the rotor arm will not properly align and there is no way you can accurately time the spark. If you turn the rotor while the distributor is in place and the rotor turns along with the upper part of the shaft holding the rotor then remove the distributor and check for a broken shear pin. Good luck.

I thought I would just toss something into this old thread. The other night we did some work on Dan's rpu, like hooking up the Lokar throttle and other minor stuff. We had started the car up the day before and it ran like it always did, like a clock (well, a very cammed up clock) However, when we restarted it this time it sounded like crap and was popping through the open headers......flames coming out too.

We immediately started thinking the worst, wiped cam lobe, bent pushrod, etc. We pulled the valve covers and everything looked great and the rockers all moved like they should. We pulled the plugs and did a compression test, and it was 180 across the board.

To get another opinion we called my Son Don and he asked how the plugs looked. They were black as we hadn't adjusted the fuel pressure down from 6 to the recommened 4.5 (Rochester tripower setup), so he suggested we clean them. I tossed them in the blast cabinet and reinstalled them. The motor fired like is should and the popping was totally gone. I would have never thought carboned up plugs could cause that kind of miss, but maybe the 10.5 compression is a factor. So this week I'll pick up a new set of plugs, and we have already bumped the fuel pressure down to 4 psi.

Sometimes we assume the worst has happened and it is just a minor problem.

Don

there is a thing called a ajustacap,I'm sure you dont have one but at this point it sounds like your looking for any possibility.really sounds like its 180% out.but nune the less,here goes.a ajustacap looks like a reg. dist cap from the outside.but if you pull it off & look at the inside it looks like black glass was poured in it.If this is the case,it will not work with the reg.firing order.check the inside of the cape if it looks like glass replace the cap and wire as normal.I have a ajustacap on my street rod,never heard of one befor, never seen one before.almost drove me nuts trying to put a set of wires on the car.good luck

Wouldn't that suck if he only had a bad spark plug.
Check, recheck is what I would do again.
I don't think it would back fire like that with a bad plug. Maybe..
I know you have to run the motor around 2 grand when you first start them. That's what I did to my 455 for about 20 minutes. You need the cam greased up pretty good. (Comp cam on mine). Coil? Enough spark? 180 off sounds right. Is it exhaust valve up when timing mark set or intake valve? I forget.
I just don't know what to tell him either.
Hope for the best cause that really sucks.