340 Chugging instead of Reving

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1969GTS340

1969GTS340

FABO Gold Member
FABO Gold Member
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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. 252/258@50. 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.
 
I chased an issue like that a couple years ago, turned out to be a bad coil. It idled fine but broke up at mid to high RPM's. On another note try running more initial timing and limit the total timing to 32. If you have an MSD distributor you can change the stop bushing, I think the smallest bushing is 18 degree advance?
 
dartindanno said:
I chased an issue like that a couple years ago, turned out to be a bad coil. It idled fine but broke up at mid to high RPM's. On another note try running more initial timing and limit the total timing to 32. If you have an MSD distributor you can change the stop bushing, I think the smallest bushing is 18 degree advance?
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Thank you. I have not tested the Coil yet but I'll add that to the list. Certainly sounds like a possibility. The coil is a Mallory promaster 29440 which I believe is specific to this ignition. Something else I need to check before swapping it.
 
Post pictures of the plugs. And EXACTLY what carb is it?

If you have 24 in the distributor you only have 11 or 12 initial and that’s not even close to enough. If I were doing it you have no more than 8 in the distributor and 24-26 initial.
 
Rat Bastid said:
Post pictures of the plugs. And EXACTLY what carb is it?

If you have 24 in the distributor you only have 11 or 12 initial and that’s not even close to enough. If I were doing it you have no more than 8 in the distributor and 24-26 initial.
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AJ/FormS said:
As to your Hi-fire, is that a CDI with Multi-strike?
Does that switch to Single-Fire at 3000, like the MSD?
lol........
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That... I don't know. I read that both have multi spark through 20* up to 3000 rpms. So I assume it does not multi spark beyond that level. I'd it did, I'm sure they'd be bragging g multi strike to 10k. lol
 
OK no you missed the point; if your engine begins to run stupid at the crossover point... maybe the box is bad.
I suggest to put a timing lite on it and see what happens to the timing as the rpm goes thru there.

However, seeing the plugs now, wow, they are all over the place.
looks like a couple are oil-fouled, a couple are rich fouled two are a lil lean not much, and two could be called typical; so that's quite a story.

Jus asking; why are you running 72s front and rear yet still with a PV.
 
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Either that 29440 or the 29625 coil works with that box.

Your plugs look PIG FAT but I’m not sure I’m buying that fully.

I think you have multiple issues happening, especially if this engine was running fine and then it started having issues.
 
AJ/FormS said:
OK no you missed the point; if your engine begins to run stupid at the crossover point... maybe the box is bad.
I suggest to put a timing lite on it and see what happens to the timing as the rpm goes thru there.

However, seeing the plugs now, wow, they are all over the place.
looks like a couple are oil-fouled, a couple are rich fouled two are a lil lean not much, and two could be called typical; so that's quite a story
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I was reading between the lines that you may be pointing out the similarity of the 3000rpm dead zone. But figured I'd answere your question regarding MSD. But... I do get what your saying and it's certainly another part to investigate. At one point (with a cheezy timing lite) it appeared to bounce around. Maybe it wasn't the lite.

The 72s were in it when I bought. After break in I figured the jets and power valve could be dialed in better.
 
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Rat Bastid said:
Either that 29440 or the 29625 coil works with that box.

Your plugs look PIG FAT but I’m not sure I’m buying that fully.

I think you have multiple issues happening, especially if this engine was running fine and then it started having issues.
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The engine has never ran right. It's a new built that has this issue since break in a couple weeks ago.

I read the manual and see where they call out the two coils you mentioned. It goes on to say that it is compatible with all OEM coils but recommend one if the above. I have an MSD 3 coil that I'll swap in to check off another variable.
 
1969GTS340 said:
The engine has never ran right. It's a new built that has this issue since break in a couple weeks ago.

I read the manual and see where they call out the two coils you mentioned. It goes on to say that it is compatible with all OEM coils but recommend one if the above. I have an MSD 3 coil that I'll swap in to check off another variable.
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Don’t change the coil just yet. See if you can shorten up the curve a bunch and get more initial on it. Then if you don’t mind, tell us the exact carb and what you have in it for a tune up.

If it’s run like **** since the jump there may be more to it than bad ignition parts.

BTW, I have to ask because it matters. Did you actually degree the cam or just line up the dots?
 
Rat Bastid said:
Don’t change the coil just yet. See if you can shorten up the curve a bunch and get more initial on it. Then if you don’t mind, tell us the exact carb and what you have in it for a tune up.

If it’s run like **** since the jump there may be more to it than bad ignition parts.

BTW, I have to ask because it matters. Did you actually degree the cam or just line up the dots?
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I'll postbthe carb modle number tomorrow when I'm in the shop.

I can shorten the curve. This has actually been recommended by others from my local track.

The cam was installed dot to dot. I felt it would be fine for breaking in the engine and get some test runs in. Then make improvements over the long winter we have here.
 
I noticed that while none of the plugs look great, the odd bank plugs seem noticeably darker. Seems odd, maybe just me. I was going to suggest a different set, or even a different brand. I had more of a high speed miss once. Changed out all my ignition parts before the plugs. Wound up being the plugs. Had switched from Champion to NGK. Put my old Champions in and guess what? Yep. I would have never thought either. Now with that being said, I use NGK plugs yet. Champion quality seems to be in the shitter these days.
 
How does it sound below 3000 rpm? Looking at the plugs I can’t imagine it revs cleanly up to where it dies off.
 
For testing purposes I would do two things right away if it were mine. I’d lock the timing at 32 degrees and I’d put a fresh set of plugs in it. Then continue diagnostics.
 
TT5.9mag said:
How does it sound below 3000 rpm? Looking at the plugs I can’t imagine it revs cleanly up to where it dies off.
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It starts easy, idles lumpy which i expected and reacts quickly to short < 3k throttle bursts.
 
1969GTS340 said:
I'll postbthe carb modle number tomorrow when I'm in the shop.

I can shorten the curve. This has actually been recommended by others from my local track.

The cam was installed dot to dot. I felt it would be fine for breaking in the engine and get some test runs in. Then make improvements over the long winter we have here.
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Since you didn’t degree the cam, take off the drivers side valve cover. Roll the engine over until you are TDC on over overlap (180 degrees from number 1 firing) and look at your valves.

If both valves are open the same, you are at split overlap and the cam is in straight up, whatever that is.

If the intake valve is open more than the exhaust valve, the cam is advanced from split overlap (straight up). You won’t know exactly how much, but you will know it’s advanced.

If the exhaust valve is open more than the intake valve, the cam is retarded from split overlap (straight up). You won’t know how much, but you will know its retarded.

Like I said, something is weird. I’m thinking cam timing is the first place I’d check.
 
Rat Bastid said:
Since you didn’t degree the cam, take off the drivers side valve cover. Roll the engine over until you are TDC on over overlap (180 degrees from number 1 firing) and look at your valves.

If both valves are open the same, you are at split overlap and the cam is in straight up, whatever that is.

If the intake valve is open more than the exhaust valve, the cam is advanced from split overlap (straight up). You won’t know exactly how much, but you will know it’s advanced.

If the exhaust valve is open more than the intake valve, the cam is retarded from split overlap (straight up). You won’t know how much, but you will know its retarded.

Like I said, something is weird. I’m thinking cam timing is the first place I’d check.
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Rat I agree about the cam timing should have been degreed in.
If the cam timing were way off wouldn't it run poorly through out the entire rpm range? Food for thought.
The plugs a pig rich but that again would not cause a sudden change at 3000 rpm.
Im inclined to point fingers at the ignition system like other members have suggested.
 
You already tried another carb and still the same. I’d revert back to a simple ignition, put in a points distributor and oem working coil temporarily and see what it does.
 
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