just rebuilt runs bad

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6tfo

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Just got 340 engine back from the machine shop with pretty much a full overhaul. Set up includes mild Comp cam, KB 10:1 pistons, new RHS heads, Edelbrock RPM air gap manifold w/1806 thunder series carb (650cfm). Installed the motor in 71 Duster and configured MSD 6AL, MSD 8203 blaster coil, MSD 8534 distributor. We set initial timing by findingTDC on the compression stroke, rotor lined to number one plug, then distributor turned about a quarter inch counter clockwise to add 12-15 degrees advance. Starting with number one, wired plugs 1,8,4,3,6,5,7,2. Problem is, when started, the engine backfires through the carb and we hear tapping noises. Verified TDC on compression stroke and correct firing order several times. The machine shop believes TDC is right but thinks the issue has something to do with the ignition system. Timing light connected to #1 indicates 12 BTDC, twisting distributor a little either way doesn't help.

Any thoughts on why this engine would be backfiring through the carb and tapping?
 
5/7 spark plug wires not swapped.
 
intake valve hanging open....either misadjusted lifter....take the valve cover off...and turn engine over....look for maybe a bent pushrod or something...look for a valve not going up and down...
 
when you say rotor lined to number 1 plug, this could be wrong? i think it should point to the first intake bolt on drivers side. maybee you need to turn that gear a little to get it straight? good luck
 
possibly 180 out. If the distributor was dropped in with timing marks pointing at each other then it was at TDC for #6 cylinder. Has happened before.
 
Time to diagnose.Call the engine builder.Did they set the valves? If not,did they set lifter preload properly? Most yahoos bolt it together,let it fly.Ask them first.
 
Abodybomber said:
Time to diagnose..................Did they set the valves? If not,did they set lifter preload properly? .............................
Click to expand...

^^^THIS^^^^



CHECK THE COMPRESSION or better yet just CHECK valve lash. CHECK for an open plug wire or broken plug. With engine running use a probe at the distributor cap to GROUND OUT the plugs one at a time. This will tell you quickly which cylinder(s) have a problem. You DO HAVE the plug wires going CW around the cap?

This nonsense of "guessing" the ignition timing is BS

SET THE TIMING so that it WILL RUN

TO DO THAT:

EITHER remove the valve cover(s) so you can see the valves, bring the timing marks up so that THEY ARE SET at about 12-15* BTC, that is, ALIGN the marks WHERE YOU WANT the timing to be set, then look at the valves to see if no1 valves are both closed. If not, rotate the engine ONE turn, and reset the marks

ALTERNATELY pull no 1 plug, stick your finger in the hole, and bump the engine until you START to feel compression. Now pay attention to the marks, and bring the marks up to (AGAIN) where you want the timing set

SCRIBE the position of the no1 plug tower in the cap onto the top rim of the distributor, and drop the dist. in with the rotor pointing to that mark, and now RETARD the distributor some, and slowly ADVANCE it back until, (if you have points) the points just open, or until the reluctor tip is centered in the pickup coil core.

USING THIS METHOD of setting the timing is close enough that you do not need to screw with it. If the engine won't run there, something else wrong.


discharger1332 said:
when you say rotor lined to number 1 plug, this could be wrong? i think it should point to the first intake bolt on drivers side. maybee you need to turn that gear a little to get it straight? good luck
Click to expand...

The intermediate gear position is not an issue. You can time any Mopar no matter the gear position.

63dartman said:
possibly 180 out. If the distributor was dropped in with timing marks pointing at each other then it was at TDC for #6 cylinder. Has happened before.
Click to expand...

He indicated the engine starts and runs. If it runs at all, the dist cannot be 180* out
 
You let somebody else build the thing. Let them diagnose it.
 
The machine shop installed the distributor as part of the build. Before installing the motor I removed the distributor, marking it carefully. After install I put the distributor back in, engine started after hard cranking. Checked timing, and it looked good but was bouncing a couple of degrees and the mark appeared very dim under the light. I ran the motor at around 1500 rpm for about 45 minutes. It actually sounded pretty good but idled a little rough. A test drive indicated a lack of power. I found that the timing mark that I was triggering off of was actually a scratch located about 60 degrees from the correct timing mark. At this point I figured I did not put the distributor back properly. So I found the compression stroke of #1, which lined up with the true timing mark, reset the distributor/rotor to #1, then followed the firing order. Started motor, and it ran bad, light triggering on the real timing mark about 12 BTDC, backfire through the carb and pinging. I went back to the scratch and the engine sounds good, but lacks power. This isn't right and I called the machine shop. They say the real timing mark was set 12 BTDC when they gave it back to me, this I didn't verify during install.

Since then I've verified and re-verified my TDC set up. I've used my thumb to feel #1 compression, and I've removed the valve cover to make sure both #1 valves closed. Compression is between 135 and 145 on all cylinders when set. To add flame to the fire, when I delivered the motor to the machine shop for rebuild I included a Mancini double roller timing chain kit. When I picked the motor up the machine shop said the kit had a problem and they were unable to degree the cam properly using it and gave it back to me.

Thanks to all for your response so far. It's appreciated. This all is very frustrating for me.
 
Sounds to me like you need to CONFIRM that the timing mark is actually correct. This is easy to do.

Make or buy a 'piston stop' like this:

http://www.jerrybramlett.net/images/pic_installation.jpg

Remove no1 plug, make sure it's "down a ways," then remove the battery ground. You may have to play with the length of the stop.

Wrench the engine around until the piston stops. You don't want it "at the top" but at some random distance down a little. At this point, make a temporary mark on the balancer right under TDC on the timing tab.

Do the same thing after rotating the engine CCW against the stop. You will now have TWO temporary marks, and true TDC will be halfway in between. If the original mark is correct, that is where it will be
 
Is the system well grounded? My 340 is real picky and absolutely requires a very good ground. After that it likes 36 degrees of total advance, you do not mention mechanical versus vacume advance. If it runs it is not 180 out, and with the compression you indicated it sounds like the valve train is operating properly. I would look electrical based on symptoms you have indicated.
 
Are they new plug wires? an old or damaged set could leak spark to another cyl. just throwing it out there..

AND as MarksMopar and others have stated,, a distributor installed 180 degrees out... WILL NOT RUN,, sure will backfire though.... lol

I think 70aarcuda's on the right track,, that noise being the vital clue,, possibly valve sticking open,, then snapping shut,. the backfire coming from the spark, and the valve being snapped shut by piston contact.

If you can run it with the valve covers off,, put your finger on the side of the valve retainer or on the rocker tip, of each valve,, trying to sense one that feels different.. this is how I find the loose valves at adjustment,, you can feel the loose one by the tap. you'll be able to feel it snapping shut. hope it helps, if not now, in the future... cheers
.
 
Did the shop degree the cam at all? Sounds very similar to the 360 I just put together. I had installed the timing chain one tooth off. I have no idea how I managed that, but the engine turned over kind of hard on break in but fired up fairly quick and was able to run for a full 30 min and sounded decent at 2000-2500rpm. At idle my mild cam (edelbrock performer rpm cam) sounded alot more choppy than it should have. When I tried to drive it it had almost no power. I had to feather the gas and even then half the time it would backfire and die.
After looking over everything I could think of-carb, ignition, rocker adjustment, etc. I drained the antifreeze and oil and popped the front cover off and saw the chain a tooth off.
Hope this isn't your problem, but its something else to consider checking out and at least its fairly easy and quick to do.
 
Yep,, tooth out would have all those symptons,, cept the tapping..

To check for tooth out on timing chain,...turn engine over by hand until you get #6 cyl valves in overlap, ie. just as the one valve closes, the other opens,, rock it back and forth, till you're sure it in dead overlap.. look at timing mark,, it should be dead on tdc,, if it's off by about 1/2 inch or more(varies with chain wear,, your's is new),, it is a tooth out.. hope it helps.. grant
 
inertia said:
..turn engine over by hand until you get #6 cyl valves in overlap, t
Click to expand...

It's a little difficult to tell with modern cams, because they are not centered in "split overlap" like some older and stock cams were
 
I haven't found a modern cam to be more than 1/2 inch off,at the crank, and his cam isn't that radical ,, but I will defer to your experience,, thnx
 
inertia said:
I haven't found a modern cam to be more than 1/2 inch off,at the crank, and his cam isn't that radical ,, but I will defer to your experience,, thnx
Click to expand...

"1/2" I can buy that.
 
knew I shouln't a used metric ... lol ...cheers... my point was, I would expect him to see, with being off a tooth,, is that the mark would be beyond the degree marks on the damper, like about 1 or more inches, (more metric) ;-)... lol.. just a quick check for his cam, specifically..
 
18436572..CW or CCW??? waa waa waa!
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If it's got the "10:1 pistons and RHS heads" with a small cam, I think 145psi is low. If it was me, I'd talk to the machine shop to see if it's ok for you to pull the intake to inspect the cam. 1500rpm is not really high enough to splash enough oil up to the cam, which is why the rpm is high when you break in the cam. I use 1800-2000 rpm. Otherwise there's not enough oil pressure to cause a lot of splash off the crank and rods... I'd check the cam visually before any more running or turning over. if the shop does not want you to, they should send someone over to do it.
 
I agree, I have a 70 stock, except for 234/.484 purple, with 40000 miles on it. 155 psi.
 
I tried to answer all new questions below. Thanks again for your thoughts and suggestions.


"Time to diagnose.Call the engine builder.Did they set the valves? If not,did they set lifter preload properly?"
I'll have to ask (I'm not sure I know what set the valves means). The new head ports were angled and polished, the valves, rockers, lifters are all new.


"You let somebody else build the thing. Let them diagnose it."

The machinist is spending a lot of time with me on the phone. The last thing he had me check was the compression and that both valves were closed on #1 compression stroke. This was Friday, their shop is not open over the weekend.


"Sounds to me like you need to CONFIRM that the timing mark is actually correct."

I'm going to use a "piston stop" tomorrow.


"Is the system well grounded? My 340 is real picky and absolutely requires a very good ground. After that it likes 36 degrees of total advance, you do not mention mechanical versus vacume advance."

I'll double check the ground tomorrow. The MSD distributor has magnetic pick up and mechanical advance. I tried locking the mechanical advance just to establish initial timing. No help.

"Are they new plug wires?"

Yes (new MSD wires), along with new MSD 6AL box and new MSD Blaster Coil, the MSD distributor is used.

"Did the shop degree the cam at all?"

Yes they did, what bothers me is they said they were unable to degree it with the Mancini timing chain set. I brought this up with the machinist. He explained why it didn't work (I wasn't able to follow him) he seemed confident that the Mancini chain kit was bad.


"18436572..CW or CCW???"

Clockwise.

"1500 RPM is not really high enough to splash enough oil up to the cam"

On second thought I think break in was at 2000 ~45 min. 80-90 psi oil pressure. Temp steady ~180.
 
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