Carb choice for a mutt motor.

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Airmonger

Airmonger

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Hi crew. I'm a little green when it comes the venerable 318 LA motor.
I tortured a couple of 340 motors in my misguided youth, however a huge volume of water has passed under that bridge.
I recently acquired a 67 Barracuda. Mix-n-match 318 of unknown bloodline. Factory 273 exhaust manifolds w/duals. 4 speed, 8.75 diff w/3.73 Sure-grip.

The known:
Originally 273 273 2bbl.
68 318 block.
? Year 273 heads.
65 cast iron 4bbl intake.
600 CFM Edelbrock carb w/vacuum secondaries.

The unknown: Bore size.
Stock, oversize?

Compression ratio.
Low, I assume.

Cam specs.
Stock, I assume.

In knowing that the 273 2bbl heads are really hindering flow, I feel that the 600CFM carb might be overkill. I understand that the motor will only pull so much air. Only as much as it can process. I just feel that because of the lack of secondary response when under full throttle. I have just cause for questioning whether or not this particular mismatched engine would work better with a smaller (500cfm)
carburetor?
In using the standard formula for CFM configuration, I come up with 440 CFM. I don't spin the motor past 4800 RPM, simply because it seams to have ran it's race by then. When in a lower gear and standing on it from say, 2500 RPM, the result is very
uneventful.
With funding is still in the recovery mode from the purchase, it's not feasible to just slap another carb on it. Not to mention that it seems the price of smaller carbs are grossly overinflated. I have researched the CFM rating of 273 HiPo carbs, but feel like that engine had a more balanced approach with hotter cam, etc.

Just looking to gather some insight from someone who has experienced similar issues. All input is welcomed, just member this. I'm not making any changes anytime soon, although I know the engine would benefit from them. From what I'm learning, a 360 may lay ahead in the distant future. For now, I just want to see if I can get this little mutt to be a better driver. Enjoying the forum. Learning tons.

PINKS ALL OUT - TOPEKA - JULY 2010 013.JPG
 
What carburetor is on it now, brand, part number, etc? We can probably help you more with any information other than what you have given so far.
 
600 sounds fine. If it doesn’t stumble at wot, it’s not too big. I suspect it is effectively a stock 318.
 
273s and 318s used basically the same heads, so they're not the stumbling block with the stock cam (hydraulic/solid?).
A carb change may require a carb adapter, since the 273 manifold uses the early "small" AFB bolt pattern, some aftermarket (ie Holley) carbs don't come with both patterns. You may also run into some interference with the butterflies and the manifold openings, requiring either some grinder work to the manifold or a small spacer. I wouldn't go over a 600/625 (depending what flavor you prefer- Holley or Carter/Edelbrock) cfm carb. Stay with vacuum secondaries if a Holley. I would confirm satisfactory function of your current carb first, though; whatever it is.
More importantly at this point, get the setup and tune right. Sort out your timing/advance curve, that will pay more dividends than anything else right now. Something like a 10-15* initial with about 36* all in- experiment to see where it's happiest (remember to set your initial with the vacuum can disconnected and the port plugged). And make sure all your connections are correct- vac adv. to PORTED vacuum source (not manifold vacuum), coil polarity is correct,
Make sure your throttle cable is set so you're getting full throttle travel and opening all the way. If you have an aftermarket carb already, it may require a Mopar linkage adapter (if it doesn't already have one) in order to get correct travel.
What ignition is on it? Points? Chrysler electronic? Some type of aftermarket?
If points (single or dual), make sure the points are clean and gapped correctly and the rubbing block is lubed and not excessively worn- points ignitions are becoming somewhat of a lost art nowadays, so who knows how it may be set up.
Getting the tune and function right should be the first priority and may be all you need for now.
 
A '68 318 with closed chamber early 273 heads shouldn't be low compression unless it's got a felpro 521SD (4.18 bore x usually .0625 minimum compressed thickness) head gasket or similar on it. Run a compression test for us. A well tuned 650 AVS2 is as close to perfect as you'll find for most nearly stock 318's.
 
Demonracer said:
What carburetor is on it now, brand, part number, etc? We can probably help you more with any information other than what you have given so far.
Click to expand...
Listed under the known caption.
Edelbrock 600 CFM standard bore, vacuum secondaries. Electric choke. I believe the model number is 1406. Not home to check it visually.
 
Garrett Ellison said:
A '68 318 with closed chamber early 273 heads shouldn't be low compression unless it's got a felpro 521SD (4.18 bore x usually .0625 minimum compressed thickness) head gasket or similar on it. Run a compression test for us. A well tuned 650 AVS2 is as close to perfect as you'll find for most nearly stock 318's.
Click to expand...
Unsure of the details. The engine was put together by the PO's friend. He has long since passed on. Hard to say what happened 20 years ago, or what was even available parts wise back then. I don't really want to disturb this set up because it is a really nice driver. A little underwhelming, but nice.
 
The 600 is fine. Just get it tuned right and it should run fine.
 
What RRR said, plus take the air cleaner and check the vacuum line to the vacuum advance on the distributor. It sounds to me like it just needs a good going over and check out the timing.
 
I'm leaning towards timing as well. The PO gifted some upgrades after the sale. Mopar electronic distributor, new in box. Along with a disc brake kit for the front. That's going to be another box of surprises, I'm sure.
I'm having fun with this, but it has a lot that needs to be "undone". Only the finest hillbilly engineering was applied to some things from all I can see.
 
Tune what you got, don't worry what that formula says.
 
Professor Fate said:
Make sure your throttle cable is set so you're getting full throttle travel and opening all the way. If you have an aftermarket carb already, it may require a Mopar linkage adapter (if it doesn't already have one) in order to get correct travel.
Click to expand...
I would double and triple emphasize this. Have seen too many setups that someone thought they could just hook the throttle cable up anywhere and it would work OK. That would be the first thing I would check. Then follow up with all the other suggestions.
 
Another add-on is a Adiovox cruise control. Before my next drive, I'm going to plug the vacuum port that it pulls from. I have a on-off throttle transition surge that occurs when coasting down hills, and I think that this old CC may have a ruptured cannister. It isn't working at this time so eliminating that is simple. Just wading thru this one piece at a time.
 
Wow '68 318, 230 hp 9:2:1 cr, short block, with 273 closed chamber heads.

Light free floating rods and flat top pistons with valve relief (factory in '68) .040 ths down in the hole.

318 3.jpg


I am liking this, 600 cfm 1406 perfect carb for that 318.

Even if the 318 stock short block has the original 2 bbl cam, they still benefit hugely by giving the engine the fuel that it needs with the 600 cfm carb.

Proof in the pudding > stock '69 318 with added 600 cfm 4 bbl. Huge improvement over the stock BBD gas leaking 2 bbl carb. ('69 318)

20230124_110028.jpg


69 StPaul 2019.jpg



☆☆☆☆☆
 
Airmonger said:
I'm leaning towards timing as well. The PO gifted some upgrades after the sale. Mopar electronic distributor, new in box. Along with a disc brake kit for the front. That's going to be another box of surprises, I'm sure.
I'm having fun with this, but it has a lot that needs to be "undone". Only the finest hillbilly engineering was applied to some things from all I can see.
Click to expand...
It actually looks pretty nicely done. Only things that stand out in the photo are the battery cables showing some bare copper, and that is not exactly uncommon.

tripple on the suggestions above regarding the throttle cable and timing.
Spend any money you have avail on a timing light, tach, and timing tape. With any replacement distributor, the timing curve will be a mystery until you measure and plot it. If its a somewhat retarded compared to what it wants, the engine will respond to throttle input smooth but slow. The original 4 bbl 273 advance, and the DC/MP small block distributor advance are right in the ballpark.

Once that's done, then you can work on tuning that carb jetting/rod combo. Enrichment and jetting then can be tuned at the track. All for the cost of tools, some fuel and your time, plus whatever your local dragrstrip charges for T&T entry.
 
I appreciate the input. Let it be known that I have all the resources for tuning and such. In another chapter of my life, I was a certified BMW tech. I transitioned from cars to motorcycles because that was where my heart was at the time. I still buy and sell bikes as a hobby, but due to heart health and surgeries, I've returned to square one. I also took the misguided turn and became deeply involved with building and racing Cleveland powered Fords. Again, many years and careers have passed since those days. Some things need to be learned and some unlearned. One of my first discoveries was this 318 has a 273 harmonic balancer on it. I've already installed the correct timing tab on the block. There are simply so many unknown factors with the engine, I really wished the builder would have documented everything done to it. Since that didn't happen, I will systematically figure out what I am working with.
As stated, I am actually enjoying solving the puzzle. I haven't been challenged like this in a long time. It's good for the soul.
 
Oh, I've already replaced the cables, but I do need to get the correct battery back in it. The battery in the picture died and I'm running the one from my CJ5 in it.
 
Mattax said:
The original 4 bbl 273 advance, and the DC/MP small block distributor advance are right in the ballpark.
Click to expand...
Factory range of acceptable timing on engine with initial set at 10* 600 to 650 rpm.
This is a good starting point for a non-smog 4 bbl setup.
1717208615192.png


The Chrysler built MP magnetic pickup distributors have a similar curve. Here's one I measured.
1717208985756.png

If the curve is close to these, then its mostly a matter of trying a little more initial which will shift the whole curve up.

Unfortunately the majority of magnetic pickup distributors will be setup for earlier emissions and 2 bbl engines so without additional info its best to measure and plot.
 
I certainly appreciate that info. I am a little too advanced I believe. It doesn't seem to surge or buck on trailing throttle when retarded to 7° initial.
I won't be doing anything until I get a mechanical oip pressure guage installed. Drove to the local non-ethanol fuel supplier this morning. The dash oil pressure guage dropped out. I stopped, messed with the sending unit. It came back up momentarily then dropped off again. The coolant temp guage remained consistent, I had already driven far enough that if there was any damage to be done, it would have already happened. Gotta love 50+ year old wiring on anything.
 
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