guys i pulling my hair out i need help

[truckdude239]

i can not get my truck running right. timing with hei was base to run somewhat decent was 18degrees bftdc so we swaped the point distbotor back in. motor is a 305 out of a 78 c-10 its bored 30 over with stock cam. we can get it to run okay but it still pops and backfires timing with points set between 4-8 does the same thing.i need some ideas

[DavesRide]

That's how I have heard them act when I dropped the distributot in 180 degrees out. At this point I would start from scratch to find tdc on hole number one and align cap accordingly. If you used the number one piston on compression stroke and distibutor is in correctly, You have another issue. The question is basic so I kinda leaned towards a "basics" answer. In no whay am I questioning your ablility but I have messed myself up many times with something easy. By the way, is the engine fresh or used? David

[truckdude239]

i rebuilt the engine everything is within spec. i used a 4 barrel q-jet off a 350 would the carb be 2 big for the 305?

[DavesRide]

Originally Posted by truckdude239

i rebuilt the engine everything is within spec. i used a 4 barrel q-jet off a 350 would the carb be 2 big for the 305?

I have seen a 283 run beautifully on a big block marine Q-Jet (no choke). It makes me believe that the ignition or valve timimng is incorrect. It could also be lean from a vaccum leak or poor fuel flow. Let me put it to you like this. I have seen an engine run just fine only to look and see that I forgot to hook the fuel line to the carb. It just happened to be pointed at the top of the carb. Not perfect, but running. No fire Thank GOD. Another look was dropping the dizzy in in the wrong position and chased all sorts of defects. Take another look. Then dig deeper.

[special-K]

Are you absolutely sure the plug wires are right?

[truckdude239]

yes it idels good untill igive it gas, and it pops thought the exhaust. we check the plugs wires 4 times

[Longhorn Man]

I have had exactly what you are describing from the #5 and #7 wires crossed. They are side by side on the cap, and obviously on the engine... and missed it on a couple rechecks. Swapped them around, and it was all good after that.

[BLAZERMAN]

Have the heads also been rebuilt? Sounds like it could be weak valve springs to me.

[retselgerg]

sounds like a wire issue to me, the valve spring and being 180 out are also good ideas...id start all over and re do the ignition wiring

[truckdude239]

wires are not out check 3 times and instaled wires one by one i rebuilt the heads also i installed an set of used springs that tested good old ones i could push the valves open by hand.

[chucks70]

Check the cap really well for signs of arcing, had that happen to me before, ran fine while idling but at speed it backfired. Make sure your points are in good on the dizzy base also, could be fine at lower RPM then bounce out at high RPM

[truckdude239]

new points cap has less then 3k miles and looks fine inside had the same problem with the hei dist

[Richard H.]

1 or 2 valves to tight? Sounds like a compression check maybe needed.

[truckdude239]

guys still having trouble getting ti running right timed to 8 degrees btdc runs good untill warm then it starts missing. new coil and dis. new wires. truck will also flood out under load you think i have a bad carb???

[Wrenchbender Ret]

Is the cam new? Could it have some worn lobes?
Q-jet carbs work real good on 305,s. I'm thinking it needs a kit in it. The primary metering rods might be sticking.

[truckdude239]

i rebuilt the motor motor has less then 40 mins run time on it carb was a reman from advance auto

[70~BigBlock~Project]

Does your distributor have a vacuum advance on it? I had the same problem with the stock 350 in the truck I just bought. I set the timing, checked the plug gaps, cap, wires, etc and rebuilt the carb. Still does it. It runs fine at idle and you can rev the engine fine but when I try to drive it it backfires under load when I try to accelorate. I think I have narrowed it down to the vacuum assist on the distributor but have not bypassed it since the stock 350 is going to get replaced with a big block shortly.

I would look to bypass the vacuum advance if you have it or adjust the mechanical advance if I were you, assuming the wires are right and you don't have the distributor turned 180°.

[Billla]

Job one is to verify the TDC reference - if you don't have an accurate TDC reference, then you really have no idea what timing is being set. If you're completely sure the wires are right (and we've all done it it's also worth ensuring you have a full 12V going to the HEI.

If you reach a point where you're SURE the ignition is right and since the engine is fresh, I'd consider that possibly the cam is off a tooth or more :/

[Longhorn Man]

All this talk of verifying TDC made me think of something.
Did you use the stock damper? If you did, is there a chance the outter ring slipped? Bringing it to TDC and see if the line is on 0 degrees.

[Huck]

Did the motor run before you rebuilt it? did you use the same timing reference marker (some of the newer V8's had the marker vertically above the damper).
If you dumped it in drive way, I would:
1- pull the plugs--do a compression check to see how things look
2- I would double check that the dizzy is at TDC with #1 plug.
3- I would stick in new plugs and wires if old
4- I would put a vacuum gauge on it to see what it is doing at idle then adjust the idle mix--timing for max draw at idle
5- I would connect the vac advance to manifold vacuum!!
6- If all is good, it should run decent but if not, you have to check for correct spark at each plug, make sure coil is putting out and try swapping out carb.

I really prefer settin up a engine based on vacuum readings--it tells you a lot! Check out GMC Paul's web site on tuning up with a vacuum gauge. It is an excellent write up that may help.
Huck

[truckdude239]

=okay guys i installed a new damper with the number cylinder at top dead center on compression with both valves closed damper reads 0 degrees. i do not know if motor ran before i rebuilt it it was giving to me in real rough shape. i worked on it for another 4 hours then said to heck with it and towed it to my mechican who is very good

[truckdude239]

oh i installed an new point disabtor

[jhow66]

305 balancer has the timing mark that you read by looking down behind the water pump so the timing tab should be at 12:00 'clock on the timing cover.

[1969-08 chevy]

On a small block chevy after you line up the dots on the timing gear, you are 180 out. you have to rotate the motor one revolution until your timing marks on you balancer line up again on top dead. you should put the distributor in 180 out and see if it runs. you probably have bad back fire through the carb right now, hopefully you didn't mess up the pumps in the carb.

[6977chev]

i had something similar happen to a 250 i rebuilt one time. i had a 4 barrel intake on it and i had a rebuilt q-jet on it. it would idle fine when it got warm. but when i tried to take off it had absolutly no power an would pop and backfire to beat hell. i was sure it was a ignition problem, so i thought i put my distributer in 180 out, so i took it out and put it back in to what i thought was right. try to start it and it wont even hit, plus it turned over real hard. so i pulled the dizzy and put it back the way i had it originally, which was the right way. so i triple checked the plug wires, to no avail. so i finally went to the local speed shop a bought a edelbrock 500, thew it on and it fired right up, set my timing at 10 degress BTDC and it purred like a kitten and had good power. so with that experience if you have access to a carb that you know runs good, id try that and see what you got. if nothing else pans out for ya. hope this helps ya.