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Hope somebody can help!Just rebuilt a 1972 chevy 350 with a new
crankshaft,edelbrock torquer plus cam,torker manifold,holley carb,
summit 1.6 ratio rocker arms,HEI distributor.This motor is in a 1979
nova and ran well before.Now when trying to start it backfires
through the carb something terrible.Have tried readjusting the
valves,and thetiming looks good for starting(checked with timing light).
Any ideas?I'm dying over here.
First, go back to basics and recheck for proper firing order, Again, and again until you're sick of doing it. Second, check for enough fuel going into the beast. A lean condition will cause this always.
Those are the two biggest proponents of carb backfiring.
you could be wipping off a cam lobe
Firing order first, intake valve riding open second, vacuum leak third thing to check. My money is on crossed wires.
That's assuming the cam is timed right.
I've checked the firing order countless times and adjusted the valves according to
edelbrocks instructions.I've rebuilt several motors(fords,chevys,and pontiacs)and
they've always started right up.I will check for vacuum leaks and check fuel
pressure as soon as I can.Thanks for the tips.
re check 5 and 7 wire
It sounds like a timing issue. I know that you’ve probably looked at this until you’re blue in the face. The sketch below represents what you should be looking at from the top. This is real basic – but unless you’re building a race motor, the following will get you where you need to be.
Start all over with number one at TDC. Make sure that when the piston is at TDC (as indicated by the timing marks on the harmonic damper) that both valves are closed on number one. If not, your cam is out of time and there may be some big problems. You didn’t mention any bad “crunching noises” so we trust the cam is in time with the crank.
Remove the distributor cap and the spark plug wires. The distributor rotor should be in the approximate position shown. If not, you’re off and need to take the distributor out and re-insert so the rotor matches the sketch. Snug the distributor hold-down bolt but not so tight that the base of the distributor will not turn by hand. Reinstall the wires. I always follow the firing order, i.e., 1-8-4-3-6-5-7-2. This might sound basic – but do it.
Now install your timing light on number one and start the engine – it will start. The little things are really the big things
Good Luck,
Glenn
Hi peoples,I'm back.I've checked for vacuum leaks and have plenty of gas,
even replaced the fuel pump.I took the valve covers off and while cranking
looked for any sticking valves,but there were none.When trying to start
this thing it will backfire and start the inside of the carb on fire.Has anyone ever heard of a camshaft being ground wrong?I'm at my wits end here
and have never seen anything like this.Any ideas?
I believe your 180* out.
Remove #1 spark plug and rotate the crank clockwise until you feel pressure building against your thumb while holding your thumb, or your buddy's thumb, over the spark plug hole. Watch the harmonic damper ring and bring the TDC mark on the inertia ring up to match the timing tab on the front cover. Now you are at TDC firing #1.
Remove the distributor cap and verify the rotor is pointing at about 5:30 O'Clock as you stand in front of the motor. (more or less pointing at #1 cylinder). If not, remove the distributor and re-stab it, positioning the vacuum advance can so that you will have room at the manifold runners to turn the distributor housing clockwise and/or counter-clockwise to change initial timing later. Plug #1 plug wire in at the 5:30 position, matching up with the rotor. Going clockwise around the cap, plug in the remainder of wires in firing order....1-8-4-3-6-5-7-2.
On the front of the crank pulley, make a mark or attach a little piece of masking tape at 12:00 O'Clock, 3:00 O'Clock, 6:00 O'Clock and 9:00 O'Clock.
Back off both rocker nuts so that you have a little play. Grasping the pushrod with your thumb and forefinger, jiggle it up and down while you hold the rocker arm tip down against the valve stem tip. Adjust the nut until all jiggle room is taken up and you have zero lash. Give the nut an additional 1/2 to 3/4 of a turn. Do both the intake and exhaust on #1 while you are there.
Rotate the crank clockwise 1/4 turn (90 degrees) so that your 9:00 O'Clock piece of tape is now at 12:00 O'Clock. Back off the intake and exhaust nuts on #8 cylinder and adjust both valves the same as you did on #1.
Rotate the crank another 1/4 turn so that the 6:00 O'Clock piece of tape is now at 12:00 O'Clock and adjust both valves on #4 cylinder.
Keep rotating the crank 1/4 turn at a time and continue through the firing order until you have all valves adjusted. 1-8-4-3-6-5-7-2. Driver's side front to rear, 1-3-5-7. Passenger side front to rear 2-4-6-8.
It takes a lot of experience to get the valves adjusted correctly if you are using the "spin the pushrod" method of removing lash at the rockers. I would guess that 99% of the fellows doing it this way will get the valves too tight. This "jiggle up and down" method was passed on to me by Denny W and is bulletproof.
how old are the sparkplug wires? rotor and cap? do you have points? does it idle at 2500rpm? more info please?
I think he's 180 out also, great info from tech and all.
It doesn't matter where you put the #1 wire as long as the dist cap matches, wires may be short in some cases.
Also make sure the firing order is in the correct direction with the rotation of the dist.
Good luck.
are you using a COMP.CAM? the reason is ask is because i got a bad cam from them for my 455 t/a when they first came out was the first and last time i used there products.long story
I dont know a whole lot about engines, BUT I had this same exact thing happen to my chevy 355 engine. I found out it was the + battery wire to the distributor.. It had a break in the wire and only a few strands holding it together... maybe something to check out...Quote:
Originally Posted by sd455
This may sound stupid, but is the fuel line connected to the right port on the carb? I learned this the hard way. When I changed a carb once the mechanic put the fuel line on a vaccuum port because that is where the fuel line was originally. So then, everytime I would try to start it, it would catch on fire.
If you are all done laughing at me, it was my first "hot rod" and I dutifully learn from my mistakes.
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 :)
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.:3dSMILE:Quote:
Originally Posted by DogKrapFace
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.
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......Quote:
Originally Posted by Flamedchevy
Now don't we feel silly! :o:LOL::LOL:
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:D) However, when we restarted it this time it sounded like crap and was popping through the open headers......flames coming out too. :eek:
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.:confused:
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:cool:
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.
I'm writing in this old thread hoping that someone will chime in. I yanked a 350 V8 Vortec engine out of a chevy truck and set it up in my boat. I replaced the head gaskets and lapped the valves, after which it started and idled nice (for a few seconds, running dry), but when I connected the cooling water, the two center cylinders on the right side filled with water. I then replaced the right side head, same procedure again, valves adjusted and ready to fire, but now it just backfires through the carb, I've triple checked the timing, wires, ect, but to no avail, fresh gas out of an external tank, fuel pump working, nothing happens other than the backfiring. Need a little help here