340 Chugging instead of Reving

fishmens67

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Is it possible it's firing when the valves are off the seat.
as in valves not closing totally.
 

408 swinger

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Reading back through, I don't see mention of valve springs. New? Used? Shitty bad new ones?
 

12many

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You say you swapped carbs for a known good running 850 and the issue is still present.
You really should swap in a points ignition. Nice, basic and simple, quick too. See if it does the same or not.
 

1969GTS340

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Reading back through, I don't see mention of valve springs. New? Used? Shitty bad new ones?
This is a brand new build, the engine was broke in with only the outer springs then inserted inner springs after 20 - 30 min at 1800-2000 rpms
 

1969GTS340

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Is it possible it's firing when the valves are off the seat.
as in valves not closing totally.
Yes that is possible. I am planning g a leakdown when I can coordinate the equipment with a buddy. I did verify lift of each valve and cam lob with a dial indicator. Once peak lift was reached, I walked the valve back down.to zero while looking for a sticky spot on the indicator which may indicate the valve was sticking. None were found during that test.
 

1969GTS340

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You say you swapped carbs for a known good running 850 and the issue is still present.
You really should swap in a points ignition. Nice, basic and simple, quick too. See if it does the same or not.
I agree but don't have a set up available.
 

TT5.9mag

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It might be time to drill a big hole in a spare distributor cap and really check the phasing
 

1969GTS340

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It might be time to drill a big hole in a spare distributor cap and really check the phasing
That's interesting. If I understand correctly, this would provide a site window to inspect the alignment of the rotor to the cap #1 firing position? If it was found to be off, how would it be corrected?
 

TT5.9mag

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The easiest way is an adjustable rotor if Mallory makes one for your distributor.
 

1969GTS340

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Here's a question for the ignition guys. My understanding is that the reluctor blade should be lined up with the center of the pickup when firing? With my timing set at 32*, the blade on the reluctor is full blade width away from the pickup. Should I adjust the pickup to center blade and regap at .008 ?

20220927_135444.jpg
 

TT5.9mag

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The question is, where does the rotor point when the pole piece on the reluctor is lined up dead center with the pickup?
 

1969GTS340

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The question is, where does the rotor point when the pole piece on the reluctor is lined up dead center with the pickup?

Might be difficult to see in this pic (attached), The rotor is between 8 and 1, I'd say closer to 1, which may be correct at 32btdc. If I rotate the engine to bring the reluctor dead on, the rotor is darn close to dead on and the timing mark is at TDC. So basically, as I advance the timing, both rotor and coil off shift the same amount. Since my rotor is not adjustable, I just tried moving the pickup. It's obvious that it doesn't move in all directions like points. There is virtually nothing to gain in trying to move the pickup.
Edit... I am adding a second pic, trying to show where the rotor is in relation to my TDC mark on the back of the intake below.

20220927_135553.jpg


20220927_154022.jpg
 

TT5.9mag

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Ok try this, with the distributor advance locked at 32, MAKE the pole on the reluctor line up with the pickup dead nuts on center AND the rotor line up dead nuts on center on the number one terminal in the cap. You might have to remove the distributor to do this. Do NOT line them up at TDC put your balancer at 32 degrees btdc
 

fishmens67

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If the reluctor is lined up and the rotor is pointing to #1 at TDC, it should line up when distributor is twisted to 32btdc
in firing phase.
It should be pretty easy to figure out.
 

1969GTS340

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Ok try this, with the distributor advance locked at 32, MAKE the pole on the reluctor line up with the pickup dead nuts on center AND the rotor line up dead nuts on center on the number one terminal in the cap. You might have to remove the distributor to do this. Do NOT line them up at TDC put your balancer at 32 degrees btdc

That is what I tried to describe that I did above. With the dist locked and engine timed at 32btdc. I pulled the cap and turned the engine over by hand until I got to 32btdc. The pics above show the misalignment of the pole / pickup. and the rotor was before the #1 button on the cap.
I'm learning this as I go but it seems to me that the location of the rotor is correct when set to 32 btdc. I didn't calculate the radius of the cap x 3.14 bla bla bla.... but it's close. In addition, when the rotor is at 32* shouldn't the reluctor pole be dead nuts over the pickup/firing?
 

1969GTS340

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Compression test complete.

1. 149
2. 149
3. 149
4. 148
5. 149
6. 151
7. 148
8. 149

The new plugs (30 minutes old) came out wet and black.

When I turned the engine over to do the compression test, gas was spaying out the remaining open plug holes onto the tops of the fenders.

I'm taking a shot in the dark here but, with the rotor and pickup being out of phase, isn't this another indication the phase needs ro be corrected?
 

12many

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When I turned the engine over to do the compression test, gas was spaying out the remaining open plug holes onto the tops of the fenders.
Ever look down the carb while running at the rpm’s you say is starts chugging? Tried lowering the fuel pressure a little and run it?
 
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1969GTS340

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Ever look down the carb while running at the rpm’s you say is starts chugging? Tried lowering the fuel pressure a little and

I've ran pressures from 7 down to 4.5 while running the engine. It didn't make a change in performance. I have not looked down the carb when it starts to chug or pop out the exhaust. I did change carbs to one from a.running car with no change. I can say there is a lot of unburned fuel.
 

TT5.9mag

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What does the timing do, not just the number on the balancer, but what does the strobe look like when the engine starts to break up?
 

12many

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I've ran pressures from 7 down to 4.5 while running the engine. It didn't make a change in performance. I have not looked down the carb when it starts to chug or pop out the exhaust. I did change carbs to one from a.running car with no change. I can say there is a lot of unburned fuel.
I’d run it to warm the engine up a little then turn off the pump and then bring up the revs to (where you had the chugging) and see what it does, before the bowls empty.
 

Phreakish

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I think you're onto something with the reluctor.
I'm not familiar with that distributor, but is it possible the pickup wiring is backwards? I seem to remember if it's flipped, you'll get more "advanced" spark when looking at the rotor and reluctor relationships to the cap and pickup. The spark will also be shitty - but no idea if that applies to your Dist?
Typically, a magnetic pickup should be wired in the direction that results in the least advance (most retard).

Figured I'd mention it, if it applies.
 

MOPAROFFICIAL

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Hey guys, I'm turning this mystery over to this forum after 2 weeks of troubleshooting with no change. I'll try to keep this brief and to the point and provide some details on the build.

340 block +.060, stock forged crank and rods with KB pistons, Oregon Cam solid 572/569. 252/[email protected] 106 lobe seperation. Howards oil thru lifters, EQ Magnum heads 1.6 Crane roller rockers. Edlebrock RPM Airgap with Holley 750 dpdf and 1 inch spacer. Ignition is all Mallory, HyFire VI 685, Coil and Comp SS distributor. Accel 8mm wires, Holley blue fuel pump regulated at 7psi. Hooker super comp headers. The car is intended for drag use only, running 93 octane currently.

Problem - at approximately 3000rpm and up, the engine chugs and struggles.

The following changes have been made with no improvement.

Carb rebuilt, jets 72 front and rear. Power valve at 450. Eventantually changed the carb to an 850 from a running car, no change.

Distributor set with 24* mech advance, springs changed to be all in at 2500rpm

Rechecked valve lash .016 Intake .018
Exhaust. A-OK.

Rechecked timing mark with piston stop. Off by 3* remarked with new degree tape.

Disconnected the tachometer.

Made sure the ignition box Rev limiter is set at 6500. Two step is not activated.

Checked firing order, cap and rotor condition.

Adjusted initial timing from 5 to 15 (29 to 39 all in) with no improvement.

Plugs indicate running rich.

I am more then willing to share any more information needed, in exchange for clues that solves this issue.
Adjust the total/full advance timing down to 28 degrees... then test drive it.
It may only take 6 to 10 degrees mech advance with that head, say .. if you're running a higher initial like 20 degrees btw @idle
Do a cranking compression test 'carb open' and let us know if you can.
Also ..it only needs about 5 psi. 7psi is testing the needle n seats imo. Thanks for considering this board for help, you're in good hands.
 
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