360 hesitation and

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KJoeZ61

KJoeZ61

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I just broke in the cam on my newly rebuilt 360 and it starts right up, idles beautifully and will run a high RPM no problem. I was told to set the timing at 10*. Today was the first time I attempted to actually drive it which was a test drive around my development, didn't go far before returning. Its sputtering and backfiring from the carb under load. The car had the same carburetor and distributor prior to the rebuild.
Details.
1971 Plymouth Scamp
360 engine with close to factory spec cam
1406 Edelbrock carburetor
Proform Distributor
 
Mine is set in the low 30s and I haven't noticed anything out of the ordinary...oh by the way this was set by a professional engine builder!
 
leblanc said:
Mine is set in the low 30s and I haven't noticed anything out of the ordinary...oh by the way this was set by a professional engine builder!
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And your build is like his build?
 
Joe Zartler said:
I just broke in the cam on my newly rebuilt 360 and it starts right up, idles beautifully and will run a high RPM no problem. I was told to set the timing at 10*. Today was the first time I attempted to actually drive it which was a test drive around my development, didn't go far before returning. Its sputtering and backfiring from the carb under load. The car had the same carburetor and distributor prior to the rebuild.
Click to expand...
So this is long after the accelerator pump shot is all used up?
And only under WOT,and full load?
Does it quit as soon as you back out of the throttle?
If yes, pull a couple of plugs and see what they say.
If no check for water in the gas.
If no water, clean the carb
 
@Joe Zartler

AJ nailed the first steps with a hammer of advancing the timing didn’t help enough or at all.

AJ is sharp in this. There are so many things going on at once. Basic trouble shooting first!

Do so report back Joe!
Let’s get this bad boy up and running!
 
Well I added some dry gas thinking possibly moisture in the gas, no luck. Below is the picture of one of the plugs, they look fine. No matter where I set the timing 0*-5*-10*- 20* it runs best around 10* but I still am getting the back fire under load. Could the carb be bad from sitting 2 months dry? I know AJ is saying clean it, does that mean rebuild it? \ \
IMG_1523.JPG
 
comments in the quote
I just broke in the cam on my newly rebuilt 360 and it
starts right up,
idles beautifully and
will run a high RPM no problem.
These comments indicate that;
the gas is probably OK,and
the low-speed circuit is fine,and
that the power system is also functioning reasonably well.
And that the rotor phasing is OK,
and that the Ignition system MAY be ok,as well.
I was told to set the timing at 10*. Today was the first time I attempted to actually drive it which was a test drive around my development, didn't go far before returning.
Its sputtering and backfiring from the carb under load.
This tells us that;
the gas may be contaminated or oxygenated or just stale,or
the power system is faulty,
or the plugs are overheating,
or the ignition is crapping out.
The pic of the one plug seems to indicate that the fuel is OK, that the heat-range is close,SO, to rule the plugs out; What heatrange are they?, or what's the plug number.
The car had the same carburetor and distributor prior to the rebuild.
Details.
1971 Plymouth Scamp
360 engine with close to factory spec cam
1406 Edelbrock carburetor
Proform Distributor
Click to expand...

Those plugs do not look fine. But they appear almost new. Are you sure the gas is pure gas, the same fresh stuff you ran with the old engine? Is it oxygenated?
Since there is no black on them whatsoever, I suspect the secondaries are just lean.
So now we just need to determine; is it fuel or is it ignition.
To do that, you can kill two birds with one stone, by disconnecting and blocking the secondaries closed, then see what the primaries do.
If it does the same thing, back out of the throttle just a little until it stops. If the car surges ahead ,then it is very definitely lean, she wants more gas on the primaries, and you need to fix it. You may have to try it several times to learn this trick. If it surges ahead, then it wants more gas. How far you had to back out gives a hint as to how much more gas it wants. Try this at various rpms starting no lower than 3500rpm, and no more than 4500 for now.
 
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The plugs are Champion RN12YC. And yes its the same fuel plus I added about 5 gallons that I just bought. Your saying to disconnect the secondaries, that would be the two rear barrels, correct? W
IMG_1524.JPG
IMG_1524.JPG
IMG_1524.JPG
ould disconnecting the pin I'm pointing at accomplish that?
 
Joe Zartler said:
Well I added some dry gas thinking possibly moisture in the gas, no luck. Below is the picture of one of the plugs, they look fine. No matter where I set the timing 0*-5*-10*- 20* it runs best around 10* but I still am getting the back fire under load. Could the carb be bad from sitting 2 months dry? I know AJ is saying clean it, does that mean rebuild it? \ \View attachment 1715113456
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That thing looks lean and looks like it has too much timing.

Did you verify TDC when you assembled the engine?
 
Post 12 edited
12s are the right plugs
Yes the back two. Just remove the link rod and put a spring on there or a wire and anchor such that the secondaries cannot open. Make careful note of the links orientation and then take it right out. If you changed the cam, installed new rings and gave it a valve job, then it's gonna be lean,and this is normal.

YR has a good point; you should check the power-timing. I know you said it's the same distributor, and it used to run fine. But check it anyway and limit the timing to 34* for now. Do that before monkeying with the carb.
Just disable the Vcan,and rev it up to 4000, and reset the D to 34*. Then observe only, the idle timing and report that here.
 
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AJ/FormS said:
Post 12 edited
12s are the right plugs
Yes the back two. Just remove the link rod and put a spring on there or a wire and anchor such that the secondaries cannot open. Make careful note of the links orientation and then take it right out.
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So you are saying that you believe the problem is in the carburetor, correct? If it does what you are saying how would I fix it? And to the other reply, yes it was at TDC when it returned from the machine shop.
 
Yes and no
Both YR and I are on the same page. I'm just leading you down the path of diagnoses. I could tell you what I would do, but would you learn from it? Better it is, IMO, that you learn what does what, so you that when you see the results, you can anticipate the next move.
At the beginning of the thread, we didn't know if it was the gas itself, or a faulty fuel system,or a weak ignition, or detonation,or even a plugged exhaust.
So far, we have ruled out bad gas. And the plug is pointing to the carb.
Next we are gonna get a handle on the AFR.And close in on the correct AFR. If the crapping out goes away, then we will know that we are on the right track.
 
What was at TDC?
I think what YR is asking is; Is the TDC mark on the balancer known to be correct.If you do not know, then you should prove it, because if you don't, then the ignition timing could be in error, and there is no way of estimating how far out it is.
And one of the comments you made earlier pointed to a possible erroneous timing setting.
This one from post #10
Well I added some dry gas thinking possibly moisture in the gas, no luck. Below is the picture of one of the plugs, they look fine. No matter where I set the timing 0*-5*-10*- 20* it runs best around 10* but I still am getting the back fire under load. Could the carb be bad from sitting 2 months dry? I know AJ is saying clean it, does that mean rebuild it? \ \
This is unusual.
Are you saying it idles best at 10*
or backfires the least at 10*
or something else?
 
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AJ/FormS said:
What was at TDC?
I think what YR is asking is; Is the TDC mark on the balancer known to be correct.If you do not know, then you should prove it, because if you don't, then the ignition timing could be in error, and there is no way of estimating how far out it is.
And one of the comments you made earlier pointed to a possible erroneous timing setting.
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Okay thank you, I appreciate it. By the way the entire exhaust is new 2-1/4 with DYNOMAX mufflers, and it sounds nice!
 
I think I really did
AJ/FormS said:
Well that rules out another possibility then,


YR;
hell-yeah that was funny..... to you, and to me..................... But Rumble ain't saying much......................yet.
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I think I can rule out another one! I did something really stupid, 5 & 7 wires are on the distributor backwards, my oh my, this car hates me! Could this have messed up the break in as well?
IMG_1525.JPG
 
heeheeheeheeheehee
No, If the Rs were up at 2500ish, then the cam will be fine.
But I'm a little confused, lol. You said
I just broke in the cam on my newly rebuilt 360 and it starts right up, idles beautifully and will run a high RPM no problem. I was told to set the timing at 10*. Today was the first time I attempted to actually drive it which was a test drive around my development, didn't go far before returning. Its sputtering and backfiring from the carb under load. The car had the same carburetor and distributor prior to the rebuild.
Click to expand...

What plug did you show us, back in post #10?
 
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While anyone is looking at my wire picture above. Anyone know a good wire retainer to use here? What you see is needless to say temporary.
 
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