1972 Demon 340 engine mods

[nm9stheham]

I don't see any notes about ignition timing or cam timing, but I suspect this is more due to the cam and rear axle gearing and having 3 speed manual than anything in the carb. But it is not clear if you are having sluggish response at low RPM's or high PRM's or at part throttle or WOT. If it is sluggish at part throttle response, then look elsewhere than the carb type as neither type would be in the secondaries at that time. (And of course any carb can be faulty...)

 

[Dartswinger70]

I always figgered if I was around in 72-73 and bought one of those detuned 340's that I would just swap a earlier cam and top end over onto the bottom end and be done, Weekend job wrenching and tuning.

 

[Marcohotrod]

I always figgered if I was around in 72-73 and bought one of those detuned 340's that I would just swap a earlier cam and top end over onto the bottom end and be done, Weekend job wrenching and tuning.

problem with this idea is all 1969-73 340s had the same cam. the lower compression was .100" shorter pistons. and 1971-3 had bigger carbs

 

[Miszny]

OK so here is an update.

Since I had contact with this car I noticed erratic idle. Two weeks before heads swap I noticed that spark plugs looked coal mine black. I checked timing and it had 15 degrees advance.
The owner told me that he has an issue with driving longer on highways. He said that after some time car wont go over 30mph, looses power and everything goes back to normal when it cools down.

We swapped the heads. I did not touch the distributor in the process, what I noticed was that nr1 cylinder plug was directed towards the driver, as if distributor was 180 degrees out. Also spark plug wires seemed stretched. I started the engine after heads swap and it started right up. Idled as it used to(erratic). I took the car for a ride and only after 10-15 miles spark plugs looked just like the ones he used for a few months. All black. I heard explosions in the exhaust at crusing speed also.
I set the engine to 1st tdc, turned the distributor 180 over and reconnected the wires. I also had to clean the cable terminals because half of them were badly corroded. It started right up, so I went out for a ride, nothing was changed, still idled badly, still felt underpowered. Basically heads swap gave zero power change.
After this I decided to play with timing advance. This made me think as I was able to advance it to 30 degrees at idle and engine speed was getting higher even past this value. Also 30 degrees made it react to gas better(?!)... It started revving to 6000+. Before that 4000 rpm felt like a struggle. I am suspecting some electrical issue, weak spark due to bad coil or something. This car acts really strange.
I will also double check my distributor installation. I verified markings on the balancer are correct with piston stop.

Any guesses? Suggestions?

 

[Miszny]

I see no change in behavior with coil change. I was able to set it to idle at 20 initial advance but it does not run as it should, shakes all over the place.

 

[nm9stheham]

For ignitions checks:
- Measure the resistance of all your spark wires. The 1 to the distrbutor should be 3k ohms or less. The other 8 should be under 10k ohm each.
- Measure the resistance through the distributor pickup with it disconnected; it should be under 500 ohms.
- Measure the resistance through the ballast resistor. Put the meter at the owest resistance scale, and touch the meter leads together several times and note the resistance readings of the leads alone. Then measure the ballast while disconnected and subtract the average resistance of the leads. The ballast resistance value should be under 1 ohm.
- Place the end of the coil's spark wire about 6-8 mm from metal and check to see that a good blue spark jumps that large gap.
- With the ignition connected and the engine idling, check the voltage to the ballast resistor at the blue wire; it should be no more than 1 volt lower than the voltage across the battery terminals. Then check the voltage at the other end of the ballast; it should be between 6 and 9 volts.
- How did you set #1 TDC? You need to make sure that it really is at TDC with a piston stop and then check to see if the damper ring is correctly at 0 on the timing marks. If the damper ring has slipped, then all your timing readings will be retarded.

If this is all OK, then you have done most of the possible easy ignition checks. (There are a few more possible).

- Have you checked the float level in the carb? Is this the Holley that you mentioned that came on the car?
- Next I would pull out the power valve and check for a rupture in the diaphragm.
- While doing that, clean out the bowl and metering block. You can also check the needle valve; with a hose connected to the inlet, blow air into the hose and then gently push the float up; it should stop the air completely when the needled valve closes.
- The idle mixture screws ought to be set between 1 and 2 turns CW from fully closed.

The above are just the basic carb checks.

 

[Miszny]

Ok so distributor wire is like 11 inches long and it’s 5kOhm, 6 out of spark plug wires are over 10kOhm, where 3 are over 12kOhm (not the longest).
I stopped here now and bought new wires + new plugs. Will come back to it after I get them.
If there is anything else I can check please let me know.

 

[Miszny]

This is what spark plugs look like...

 

[Miszny]

Ballast restor measurements:

 

[yellow rose]

That's a carb issue. You ain't fixing that with ignition.

You need to evaluate the carb. It's getting way more fuel than it needs.

 

[Miszny]

This car had this carb for a few years, the way it ran deteriorated over time.
I put 64 jets in place of 72 jets without change. I also put new power valve, reset fuel level, put all set of new seals on the carb, cleaned it and still no change. It shakes all over the place. Heck I even put old 600cfm Holley fully rebuilt and it acted all the same.
This is why I’m looking for source of problems elsewhere.

 

[Miszny]

Is this what I should be getting?

 

[nm9stheham]

Yes it is..... What do you read on the ohmmeter when you connect the alligator clips together? Do that and subtract the amount of the lead resistance from the actual reading. With these low resistance values, you need to correct the resistance readings to remove the lead resistance.

The value of the resistance reading on the lower half of the ballast looks too high, even with lead resistance subtracted out. This is the main power feed to the coil and it should be < 1 ohm, cold. Factory cold resistance is actual close to 0.6 ohms, cold, and around 2 ohms hot. When the ballast heats up, that resistance goes up, and a 3 ohm cold ballast resistance will probably be 6-8 ohms hot. That high resistance will kill your spark energy, and may explain why the car craps out when it warms up; the ballast takes 30 seconds to 2 minutes to get warmed up and increase its resistance.

Not all replacement ballast resistors are right so I always try for an OEM one. I can understand why this may be hard to get in Poland. Can you get the single MSD 0.8 ohm ballast resistors over there? That is a good backup part for the lower half of that ballast.

And the spark plug wire resistances I gave you are general numbers that I use for troubleshooting. But when the resistance wires get old, their resistance goes up and it eats up spark energy.

Did you check the resistance into the distributor pickup yet for less than 500 ohms?

That's a carb issue. You ain't fixing that with ignition.

You need to evaluate the carb. It's getting way more fuel than it needs.

The plugs look like carb as you say. But the general description of 'craps out after for a while' points to the ignition failing when warm/hot. This thing has been serviced with who-knows-what-parts, so a thorough ignition system check is needed. Don't tune the carb until the ignition is right.

 

[Miszny]

Yes it is..... What do you read on the ohmmeter when you connect the alligator clips together? Do that and subtract the amount of the lead resistance from the actual reading. With these low resistance values, you need to correct the resistance readings to remove the lead resistance.

The value of the resistance reading on the lower half of the ballast looks too high, even with lead resistance subtracted out. This is the main power feed to the coil and it should be < 1 ohm, cold. Factory cold resistance is actual close to 0.6 ohms, cold, and around 2 ohms hot. When the ballast heats up, that resistance goes up, and a 3 ohm cold ballast resistance will probably be 6-8 ohms hot. That high resistance will kill your spark energy, and may explain why the car craps out when it warms up; the ballast takes 30 seconds to 2 minutes to get warmed up and increase its resistance.

Not all replacement ballast resistors are right so I always try for an OEM one. I can understand why this may be hard to get in Poland. Can you get the single MSD 0.8 ohm ballast resistors over there? That is a good backup part for the lower half of that ballast.

And the spark plug wire resistances I gave you are general numbers that I use for troubleshooting. But when the resistance wires get old, their resistance goes up and it eats up spark energy.

Did you check the resistance into the distributor pickup yet for less than 500 ohms?

The plugs look like carb as you say. But the general description of 'craps out after for a while' points to the ignition failing when warm/hot. This thing has been serviced with who-knows-what-parts, so a thorough ignition system check is needed. Don't tune the carb until the ignition is right.

I will check the pickup tomorrow.
As for ballast resistor availability, yes its a problem but a buddy of mine has 2 or 3 so there is hope.
Connecting clips together shows value below 0.0 so not really impacting this measurement.
The worst plug wire had 14.3 kOhm resistance, but it was hard and tip was badly coroded.

Carb tuning will be done with o2 sensor from my car.

 

[nm9stheham]

OK, well you can put one of the standard single section ballasts in place of the lower half of your dual ballast, and then keep the upper half connected. Good on having some access to spares. And very good on the meter lead resistance being 0.0. That means that lower ballast half is definitely not right.

And OK on the plug wires being hard... that is not a good sign for them. They sound like they are shot.

By the way, you can check the coil primary resistance too. The coil + to coil - is typically around 1.4 or 1.5 ohms for a stock type coil.

And I cannot remember: Have you measured compression on all 8 cylinders?

 

[AJ/FormS]

I think it would be a tuff thing to tune a 284 hydraulic-lifter 340, for a smooth idle,lol; but with solid lifters, what is the net duration after lashing?
IMO, I would be looking into the mechanical end of things before you spend a week tearing your hair out. Especially since it floods the plugs with more than 1 carb, and with multiple timings, and wants very large amounts of initial timing. To me it kindof sounds like the engine is losing cylinder pressure at higher engine temps, or it's pushing gas out the boosters.

What is the idle vacuum at what rpm with what timing.
As you know, those three are intricately linked together by; the T-port sync,the operating PCV, and by even cylinder pressure. Cam timing will raise and lower the absolute vacuum reading, depending on the LSA.

Headers will pull fuel-charge across the pistons at lower rpms, where it will ignite and burn..... in the presence of fresh air, causing a popping out the exhaust. The culprit is usually a header leak right at the heads. The popping diminishes or goes away with increasing rpms,
but often increases during compression braking, especially with a too-far closed throttle.... usually the result of too much idle-timing. With the throttle now closed, the pistons are gonna pull real hard, and they will try to pull spent exhaust gasses back into the chambers during the overlap period. But the headers will be trying to create a negative pressure at that time as well. If the headers are not sealed at the heads, oxygen will find it's way in there, and pop-pop-pop is the result as it ignites. And of course at closed throttle, the intake too is under negative pressure.... so any air getting in there that did not get in there past the throttles, is going straight into the chambers...... with no fuel in it......

 

[Miszny]

I think it would be a tuff thing to tune a 284 hydraulic-lifter 340, for a smooth idle,lol; but with solid lifters, what is the net duration after lashing?
IMO, I would be looking into the mechanical end of things before you spend a week tearing your hair out. Especially since it floods the plugs with more than 1 carb, and with multiple timings, and wants very large amounts of initial timing. To me it kindof sounds like the engine is losing cylinder pressure at higher engine temps, or it's pushing gas out the boosters.

What is the idle vacuum at what rpm with what timing.
As you know, those three are intricately linked together by; the T-port sync,the operating PCV, and by even cylinder pressure. Cam timing will raise and lower the absolute vacuum reading, depending on the LSA.

Headers will pull fuel-charge across the pistons at lower rpms, where it will ignite and burn..... in the presence of fresh air, causing a popping out the exhaust. The culprit is usually a header leak right at the heads. The popping diminishes or goes away with increasing rpms,
but often increases during compression braking, especially with a too-far closed throttle.... usually the result of too much idle-timing. With the throttle now closed, the pistons are gonna pull real hard, and they will try to pull spent exhaust gasses back into the chambers during the overlap period. But the headers will be trying to create a negative pressure at that time as well. If the headers are not sealed at the heads, oxygen will find it's way in there, and pop-pop-pop is the result as it ignites. And of course at closed throttle, the intake too is under negative pressure.... so any air getting in there that did not get in there past the throttles, is going straight into the chambers...... with no fuel in it......

Sooth idle is relative. My car has big block with solid roller cam .640 lift and 285 duration and comparing to this 340 with hydraulic cam it idles super smooth for what it is. I'm realistic.
This 340 shakes all over the place. I set the lash to zero + 1/4 turn.

I will replace all bad electrical stuff that needs fixing and go from there. Will have the parts this week.
I did not check the vacuum but I managed to set it to 850rpm idle and 20 degrees advance. It ran the best this way, also gas pedal reaction became much better without hesitation that it had before.
I will of course check vacuum and report back as soon as it runs again.

 

[Miszny]

OK, well you can put one of the standard single section ballasts in place of the lower half of your dual ballast, and then keep the upper half connected. Good on having some access to spares. And very good on the meter lead resistance being 0.0. That means that lower ballast half is definitely not right.

And OK on the plug wires being hard... that is not a good sign for them. They sound like they are shot.

By the way, you can check the coil primary resistance too. The coil + to coil - is typically around 1.4 or 1.5 ohms for a stock type coil.

And I cannot remember: Have you measured compression on all 8 cylinders?

I will measure coil, I have good used MSD Blaster 2 coil that I'm using now on this car.
Leads resistance is actually 0.02, so its low enough to skip it.

I took exact measurements of plug wires for all 8 cylinders and coil wire again:
Coil wire: 5.6kOhm
1: 12.3kOhm
2: 11.3kOhm
3: 9.8kOhm
4: 13.7kOhm
5: 12.6kOhm
6: 11.2kOhm
7: 8.7kOhm
8: 8.6kOhm

Compression will be checked again today. I measured but did not take notes.

 

[AJ/FormS]

Sooth idle is relative. My car has big block with solid roller cam .640 lift and 285 duration and comparing to this 340 with hydraulic cam it idles super smooth for what it is. I'm realistic.
This 340 shakes all over the place.
I set the lash to zero + 1/4 turn. Ohchit, for some reason I thought you had a solid 284 in there.
Well if the vibration is as bad as you say, I would suspect an engine balance problem.
I will replace all bad electrical stuff that needs fixing and go from there. Will have the parts this week.
I did not check the vacuum but I managed to set it to 850rpm idle and 20 degrees advance. It ran the best this way, also gas pedal reaction became much better without hesitation that it had before.
I will of course check vacuum and report back as soon as it runs again.

I had a 292/508 cam in my balanced 360,(I know not apples to apples), at 11.3Scr,(also with Eddies), and at 14* initial. It hardly shook at at all, just the typical big-cam pulsation, which smoothed right out when given throttle.At 750rpm it idled a tic under 10". I drove that combo with 4 different carbs,at various times.
I'm guessing you have multiple problems

Make sure the power-timing is staying in the ball-park of 32* to 36*.
Those Eddies on my 10.9Scr 360 are quite happy at 14 initial, 28 at 2800,32 at 3400. With 3.55s, and an A833, it will roast the 295s to well past the legal speed limit here, with any cam I have run it with. No specialty ignition,and nearly 50 year old 750DP. My current cam is a Hughes [email protected], and is also set to zero plus 1/4 turn.This just for comparison.

 

[Wyrmrider]

Keep at it
those plugs could be cleaned up and reused- I do not see any wear just too much gas
but how old are your valve stem seals and are they Viton- worth a change if not
are new plug wires spiral mag wire- high resistance nickel wire or carbon core?

 

[nm9stheham]

That MSD Blaster 2 is a decent unit and has slightly lower coil primary resistance (< 1 ohm) than the stock ones at around 1.4 ohms. It can help the coil's magnetic field get to a bit higher state of charge each cycle so it'll have a bit more spark energy. But the wrong or bad ballast will just kill that....

 

[Miszny]

OK so magnetic pickup is like 420 Ohms. Coil checks out also. Will report on compression today, too much work yesterday to play with cars, but heads are fresh out of machine shop with new seals, valve job and all the work.

 

[Mattax]

I heard explosions in the exhaust at crusing speed also.

Late timing for the conditions.
Ideal timing is based on rpm, and load. (However if the fuel mix is too far off, the burn rate will change - so its hand in glove relationship.)

This made me think as I was able to advance it to 30 degrees at idle and engine speed was getting higher even past this value.

In neutral, the engine will take a lean mix - which burns slower and likes more advance. But its inefficient and makes less power when the drivetrain is engaged than if a richer mix and less advance was used.

Also 30 degrees made it react to gas better(?!)... It started revving to 6000+...Before that 4000 rpm felt like a struggle. I am suspecting some electrical issue, weak spark due to bad coil or something.

Upper end under load should need more energy to initiate and burn.
Sounds like its the advance may be used up. If running an advance like used in the MP race distributors, set timing at 2000 - 3000 rpm. If running a normal advance mechanism, the advance needs to be measured at every couple degrees from 600 rpm until it stops, and then a few high rpm. If its the original distributor, it may have a long but fast primary advance like was used on the early smogged engines. If that's what you find, set the timing at 2800 to 3400 rpm and live with the idle timing until you have a chance to shorten the slots on the inside.
See the explanations posted here:
CAP vs Non- CAP High Perf Timing Curves
Timing with a MP "Tach Drive" Race distributor

I did not check the vacuum but I managed to set it to 850rpm idle and 20 degrees advance. It ran the best this way, also gas pedal reaction became much better without hesitation that it had before.

Obviously getting closer - its time to measure the curve.
Vacuum is a useful indicator of efficiency under a given load - if you can hook up a gage or MAP sensor

Carb tuning will be done with o2 sensor from my car.

Tune for performance. The WBO2 is just a way to keep track of how things change. At low rpm, with plugs that fouled, I wouldn't fully trust the WBO2 to be interpreting AFR correctly. Bottom half of post #81 is super good suggestion.

 

[yellow rose]

Late timing for the conditions.
Ideal timing is based on rpm, and load. (However if the fuel mix is too far off, the burn rate will change - so its hand in glove relationship.)
In neutral, the engine will take a lean mix - which burns slower and likes more advance. But its inefficient and makes less power when the drivetrain is engaged than if a richer mix and less advance was used.

Upper end under load should need more energy to initiate and burn.
Sounds like its the advance may be used up. If running an advance like used in the MP race distributors, set timing at 2000 - 3000 rpm. If running a normal advance mechanism, the advance needs to be measured at every couple degrees from 600 rpm umntil it stops, and then a few high rpm. If its the original distributor, it may have a long but fast primary advance like was used on the early smogged engines. If that's what you find, set the timing at 2800 to 3400 rpm and live with the idle timing until you have a chance to shorten the slots on the inside.
See the explanations posted here:
CAP vs Non- CAP High Perf Timing Curves
Timing with a MP "Tach Drive" Race distributor

Obviously getting closer - its time to measure the curve.
Vacuum is a useful of efficiency under a given load - if you can hook up a gage or MAP sensor


Tune for performance. The WBO2 is just a way to keep track of how things change. At low rpm, with plugs that fouled, I wouldn't fully trust the WBO2 to be interpreting AFR correctly. Bottom half of post #81 is super good suggestion.

Most excellent post.

 

[Miszny]

Compression test shows all cylinders between 140 and 150 psi. Driver side closer to 150, passenger to 140. Before heads swap situation was similar, 120sh on passenger side and also 130 on drivers. Small change for better I guess.

Distributor cap looks seriously oxidated, 5 out of 8 spark plug terminals were green and looked really bad. I used small wire brush on dremel and bring it to decent shape, but also ordered a new cap just in case my cleanup won't be enough. Shame I did not take a photo before, but here is how it looks after cleanup. One cylinder looks the worst of all.