360 trouble. please help

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MoparOrNokar

HammerTime
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I have a 74 340 with a mild cam and 4 barrel eldlbrock intake i bought last year. It hada holley vac secondary that was giving me alot of flooding problems and back fire so i put on a new edlebrock mech secondary. Still having back fire issues so im thinking it wasnt the carb. If the car is in park it will rev up no problem. If i attempt to drive down the road the motor leans out and pops at high rpm like right before it shifts.
 
What's your timing? Poorly set timing generally causes backfires. It's probably too retarded
 
Get the timing right for your combo.Prove theres no vacuum leaks.Install a set of cooler plugs.Tune the carb.
If youre unfamiliar with how the timing systems interplay, spend a little time punching the search button.
If youre unfamiliar with carbs, I recommend you disable the secondaries, so you can tune the front first.
If/when you have more questions,or if you are familiar with carbs and dizzys; please give us some details as to your engine,such as; C/R,cam sz,heads/springs,trans/reargears/TCstall,and exhaust.

At what rpm/load setting does it pop. I mean if your springs are mismatched or done,they could be limiting your revs.
 
Thats right, timing first.Actually, maybe the fuel is first.
Getting a tune done is like trying to hit a moving target.First you take a shot at the timing. Then you sneak up on the carb tune. Then you take another shot at the timing. If it responds favorably, maybe take another crack at the carb. And so it goes, until one day, you decide, enough.
And then they go and change the fuel, or you decide to try a different fuel; and its back to square one.Or you change the cam,or intake,or headers,gears,etc
Of course experience and equipment can whittle down the time factor from all summer to; a couple,a few,or several hours, at most.
 
If it only acts up right before the shift it sounds like the valve springs are shot and letting the valves float. When the aftermarket cam was installed were new matching valve springs installed and the height checked/set?
 
also take out the spark plugs and look at them. do a cranking compression test. the 340 was made from 1968-1973
 
Alot of what is inside the motor i am unsure about. Actually anything inside the motor i am unsure about. I didnt mean to say 340. Its a 360. Last night we disconnected the vac from from the dist and set initial time at 5degrees btdc. I looked up the stock timing for an auto trans in 74. Also found a vac leak on both sides of the intake manifold. I just did gaskets at the end of last summer so maybe my the intake is warped? I will be tearing that off today and checking and replacing. I wish i knew more about the engine but unfortunatly i dont. 727 trans with shift kit 391 rearend bone stock heads.
 
MoparOrNokar said:
What is the power timing? I dont mean to open up a can of worms. Just kinda new to all this
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The more you increase your initial, you need to limit mechanical advance in the distributor.


FYI: 5* initial is way too low. I would shoot for at least 14* to 15* for starters. Just keep it to where you don't have spark knock (detonation).

I have my 318 set at 16* initial.
 
if you gonna run a performance engine you hafta have a recurved distributor, you havta find out where it's timed and know where its been, and where it ends 15-20 to start, fast advance curve, ends at 34-38 depending on engine. new hemis are much much different. blown /turboed is much much different
 
What RPM & how much throttle (WOT?) is it at when it acts up?. It does it from first to second & second to third or just second to third? check plugs/plug wires. need several psi at carb inlet at WOT. rotor phasing. reluctor gap (.008"). "power" timing, 35 total (checked with vac adv capped) is a good tried and true number for a SB. If you make a mark on the dampener 2&7/32" CW from the TDC slit & you want it there when the slots are maxed out (high RPM) AFTER you set your initial & I'd go with 15 BTDC & maybe 18. You will need to weld/file the slots to get it spot on. holler how it goes/progresses/any Q's What I'd do right now is set the dist so you get 35 (2&7/32") then see if that gives you enough initial to function & if so lets find the backfire then later dial in the timing perfect.
 
5 degrees initial timing is what the factory set the initial timing to. According to you it has a much bigger cam than stock and that requires a lot more initial timing to run it's best (mainly affects idle and low speed performance) so as the others said the initial timing needs to be a bare minimum of 15 degrees. Personally I'd set up the distributor for 20-22 initial with 12-14 degrees of centrifugal advance in by 2500 rpm which would give you 34 degrees total timing. It's possible that when you got the car someone jacked up the initial timing without re-curving the distributor in which case the total timing would be wayyyy too much and possibly cause the misfiring you mentioned. It can also damage the engine

I remember reading an article that shows step by step how to re-curve your distributor but now I can't locate it. Anybody else remember that article to post the link. It was one of the magazine's that put it out. Possibly Hot Rod??
 
Verify your plug wires are in the right order! I had this happen to me when a friend had some "fun" with me and swapped a few cylinders!!! Revved like normal until in gear then POP POP chug....Funny now but.......
 
AJ/FormS said:
Re the pics; are those metal gaskets?
What do you see?
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Don't look like steel shim gaskets. I see oil on the bottoms of the ports like it's sucking oil in there.

MoparorNokar, when you replaced the intake gaskets did you use the cork end seals on the intake? If so throw those things away when you put it back together and use a 1/4" bead of high temp RTV (make sure the surface is spotlessly clean and free of oil before you apply the RTV). I've seen cases where those cork gaskets wouldn't let the intake clamp down on the intake gaskets enough to seal properly
 
Yes i used the cork end seals. My buddy just told me thats why it leaked too. The good new is there is a bunch of metal chunks and paste in the valley. Out comes the motor i guess. Pulling this motor with headers is a pain i see now. Drivers side is packed in there lol
 
Man that's a bummer. I generally just unbolt the left header and leave it in there. Plenty of room to get the motor out with it in there and lots less work than taking the steering linkage apart to get it out
 
greenswinger said:
Verify your plug wires are in the right order! .
Click to expand...


Or check #5 & #7 for arcing. If they got too close to the exhaust manifold and burned the jackets, they can sometimes cause arcing and mis-fires.

Start the car at night and open the hood and look for sparks jumping (it's easier to see at night), or test the resistance of the plug wires...
 
I unbokted both headers and im just leaving them in. I need a cheap rebuild option and opinions. Build this 360 again or just leave the car off the road and build a 408 out of it. Block is punched 10thousandths. I also have a 340 thats around 390hp i could buy around july. It has 9.5 compression, forged rotating assembly, mopar purple cam i think. He also has a set of brand new aluminum heads that need to be milled to clear the dome pistons. Throwing in a set of hooker headers for $4k. What do u guys think?
 
I took my 360 punched it out 0.040 and used kb 107 with rebuilt iron heads with 2.02 1.60 valves ported and polished. My crank is a scat 4" stroke and stock rodsI used a comp thumpr cam 279/296 with a Holley strip dominator high rise intake with a 660 street avenger carb I bought a Chevy style HEI distributor off eBay actually all parts were off eBay, everything shipped was around 2500$ CAD and the machining was 400$. I dyno this combo 458 hp at 5400rpm and 489 ft lbs torque at 3800 rpm great combo with he right fuel compression and timing
 
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