Early 273 intake problems

[Richard Sweet]

Maistrelli,
Was that a 2 or 4 barrel from the 340?

 

[toolmanmike]

Maistrelli,
Was that a 2 or 4 barrel from the 340?

Nope! A 2 barrel 340?

 

[Maistrelli]

Maistrelli,
Was that a 2 or 4 barrel from the 340?

4 barrel

 

[dibbons]

Before bolting on the factory Thermo-quad, I'll be breaking in my current 340 build using a 318 '72 factory small block two-barrel intake manifold and a factory 318 1972 NOS Carter BBD carb, should be interesting.

I did the same thing back in the '70's when I built up my 273 with the 600 CFM double pumper, first put the stock 273 Carter BBD two-barrel on the Tarantula manifold using a fabricated carb spacer/adaptor to break in the motor on the street.

 

[pishta]

If the 2bbl is good, nothing tells the motor its topped with a 2, it doesnt know until it starts wheezing at the top end. Heck. block off the secondaries (just pull clip on vacuum pod) of a 4bbl vacuum secondary carb. bingo, a 2bbl. a 500 cfm 2bbl is not the same as a 500cfm 4bbl in flow. They are measured using a different pressure drop. "The 500cfm 2bbl was equivalent to a 350-400cfm [email protected]" (4bbl spec) vac drop..."

 

[66fs]

Jed,
I'd still advise looking for a 2 barrel carb rated at 500cfm, which is what you'd use with a 4 barrel. I don't have a specific reference but they are available. Remember that most 318's and many 340's and 360 engines also had 2 barrels stock. Also, 2 barrels are easier to rebuild, but I'd look for a new one that is tunable. It might be just as expensive as a 4 barrel however.
There are also other things that you can easily do to your engine to help: dual exhaust, blocking off the exhaust heat riser into the intake manifold, going to electric or mechanical choke, and making sure that the distributor is set and timed right and of course correct valve settings (very important with solid lifters).
What problems did you have with your 2 barrel carbs?

No 340's came with a 2 barrel carb. 340's were high Performance only.

 

[jedrattle]

Sorry I’ve been absent. Work and family stuff. Thank you all the responses so far.
This is where I’m at today (with y’alls help)...

I feel that the Offy is for a later model and I definitely have 315’s heads (273). I think I’m going to sell the Offy. It’s soo nice, I don’t want to be the one to mod it. Plus everyone is saying that the quadajet is a pain to tune. I’m waiting for a phone call to go get a performer intake. Yes I know I have to mod it.

Now as for the carb problem. I have had 3 different carter bbd’s on it. They run fine for a short time and then go to hell. Won’t hold an idle once hot is the most common problem. The last one I had the idle mixture screws out 3 1/2-4 turns just to get it idle cold. Forget about it idling once hot. Yes I checked for vaccum leaks. I’m done with 50+yr old carbs. I’m buying a brand new carb today. But which one. I’m debating on putting a small cam in it. I plan to drive the hell out of once it runs correctly. I was thinking a 500cfm Edelbrock AVS 2. Is that enough cfm with a small cam? The problem is the AVS 2 jumps to 650cfm which I feel is too much. I’m not sure I can jet it down enough to run correctly. Thoughts?

The specs are:
273 2bbl engine with 2 1/2in dual exhaust
4 spd trans
2:94 rear end
25.4 tall rear tire

 

[65Val]

Sorry I’ve been absent. Work and family stuff. Thank you all the responses so far.
This is where I’m at today (with y’alls help)...

I feel that the Offy is for a later model and I definitely have 315’s heads (273). I think I’m going to sell the Offy. It’s soo nice, I don’t want to be the one to mod it. Plus everyone is saying that the quadajet is a pain to tune. I’m waiting for a phone call to go get a performer intake. Yes I know I have to mod it.

Now as for the carb problem. I have had 3 different carter bbd’s on it. They run fine for a short time and then go to hell. Won’t hold an idle once hot is the most common problem. The last one I had the idle mixture screws out 3 1/2-4 turns just to get it idle cold. Forget about it idling once hot. Yes I checked for vaccum leaks. I’m done with 50+yr old carbs. I’m buying a brand new carb today. But which one. I’m debating on putting a small cam in it. I plan to drive the hell out of once it runs correctly. I was thinking a 500cfm Edelbrock AVS 2. It’s that enough cfm with a small cam? The problem is the AVS 2 jumps to 650cfm which I feel is too much. I’m not sure I can jet it down enough to run correctly. Thoughts?

The specs are:
273 2bbl engine with 2 1/2in dual exhaust
4 spd trans
2:94 rear end
25.4 tall rear tire

Sounds like a great plan! A 500cfm carb is perfect for a 273. I'm a Carter man, but a Holley 500-600 will be just fine as well.

 

[toolmanmike]

Sorry I’ve been absent. Work and family stuff. Thank you all the responses so far.
This is where I’m at today (with y’alls help)...

I feel that the Offy is for a later model and I definitely have 315’s heads (273). I think I’m going to sell the Offy. It’s soo nice, I don’t want to be the one to mod it. Plus everyone is saying that the quadajet is a pain to tune. I’m waiting for a phone call to go get a performer intake. Yes I know I have to mod it.

Now as for the carb problem. I have had 3 different carter bbd’s on it. They run fine for a short time and then go to hell. Won’t hold an idle once hot is the most common problem. The last one I had the idle mixture screws out 3 1/2-4 turns just to get it idle cold. Forget about it idling once hot. Yes I checked for vaccum leaks. I’m done with 50+yr old carbs. I’m buying a brand new carb today. But which one. I’m debating on putting a small cam in it. I plan to drive the hell out of once it runs correctly. I was thinking a 500cfm Edelbrock AVS 2. Is that enough cfm with a small cam? The problem is the AVS 2 jumps to 650cfm which I feel is too much. I’m not sure I can jet it down enough to run correctly. Thoughts?

The specs are:
273 2bbl engine with 2 1/2in dual exhaust
4 spd trans
2:94 rear end
25.4 tall rear tire

Interesting and strange. Lean when cold and rich when warm? ean when cold is usually a misadjusted choke. Rich when hot can be too much fuel pressure or a leaky float. In fact a leaky float may cause both. Hard to believe that 3 carbs did the same thing. Not a bad idea to get a gauge and check your fuel pressure. It should be around 5 psi. to not cause problems.

 

[jedrattle]

Interesting and strange. Lean when cold and rich when warm? ean when cold is usually a misadjusted choke. Rich when hot can be too much fuel pressure or a leaky float. In fact a leaky float may cause both. Hard to believe that 3 carbs did the same thing. Not a bad idea to get a gauge and check your fuel pressure. It should be around 5 psi. to not cause problems.

Yeah I thought it was odd too. I’ve put a new tank, lines, filter and pump in it. I drove it 1100 miles to the Zip Tie drags with no problems after I installed all that. I’ll have check it


Edit: the original carb was shot. Too much play on the butterfly shaft to hold a constant idle. So 2 carbs really but still

 

[dibbons]

I found a 1972 NOS Carter BBD two-barrel online on ebay, works fine (just a lucky find, I guess).

 

[Richard Sweet]

Jed,
To avoid heat soaking a carburetor, I recommend blocking the exhaust ports into the intake manifold, or get one without those openings and go to a manual or electric choke. I found that there is plenty of heat to the intake, like 115*F after 20 min. warmup on a cast iron, 4 barrel intake, on a 273. What kind of chokes have you used so far?

I have found that old carburetors often have sloppy shafts that leak air and fuel and are not worth repairing. Maybe you can find a good new one, unless you've positively decided to go to a 4 barrel. In that case, what 4 barrel intake do you intend to get?

I guess you are familiar with problems with using old gas and the common, poor quality California gas available to us.

 

[jedrattle]

Jed,
To avoid heat soaking a carburetor, I recommend blocking the exhaust ports into the intake manifold, or get one without those openings and go to a manual or electric choke. I found that there is plenty of heat to the intake, like 115*F after 20 min. warmup on a cast iron, 4 barrel intake, on a 273. What kind of chokes have you used so far?

I have found that old carburetors often have sloppy shafts that leak air and fuel and are not worth repairing. Maybe you can find a good new one, unless you've positively decided to go to a 4 barrel. In that case, what 4 barrel intake do you intend to get?

I guess you are familiar with problems with using old gas and the common, poor quality California gas available to us.

I ended up with an old performer rpm manifold. I went ahead and bought an Edelbrock AVS2 500cfm with a manual choke. I have everything to put it together but now it’s thanksgiving and it’s suppose to rain for almost 2 weeks

 

[65Val]

I ended up with an old performer rpm manifold. I went ahead and bought an Edelbrock AVS2 500cfm with a manual choke. I have everything to put it together but now it’s thanksgiving and it’s suppose to rain for almost 2 weeks

That's not the best intake for your application, but it certainly will do the job.

 

[jedrattle]

That's not the best intake for your application, but it certainly will do the job.

True but I couldn’t find anything else. Plus being a 4spd car should help. I have a Weiand stealth intake on my Stude’s 360. I was thinking about swapping them

 

[65Val]

True but I couldn’t find anything else. Plus being a 4spd car should help. I have a Weiand stealth intake on my Stude’s 360. I was thinking about swapping them

You want the smallest intake runners you can find in an intake manifold because of the small displacement of the engine (keeps intake charge velocity up).

 

[jedrattle]

You want the smallest intake runners you can find in an intake manifold because of the small displacement of the engine (keeps intake charge velocity up).

Makes sense. This engine is only going to in for less then a yr, if everything goes right. I just need it running and driving right now. I drive with my foot on the floor most of time so that should help too

 

[Richard Sweet]

Jed,
It's important to match the intake manifold port sizes to the intake ports of the heads that you use. The stock cast iron and the Edelbrock D4B, 4 barrel manifolds match the port sizes and the mounting bolts to the '65 heads.

Another thing to consider is the vacuum advance from a 2 barrel engine distributor might not work with a 4 barrel carb., i.e. there might be too much advance at the higher vacuum of the 4 barrel carb.. Once you get it running, always check how much mechanical advance there is, using a timing light with a dial to read the advance at higher rpms. Then connect the vacuum advance and read how much advance it gives at higher rpms. Don't let there be too much of advance; stay within the specified limit for your engine with a 2 barrel cam. My service manual for the 273 with a 2 barrel, lists the maximum vacuum advance as 10.5*-13.5* distributor degrees at 15" vacuum. Remember that crank degrees are 2X distributor degrees. I found that with a new vacuum advance (supposedly correct) there was way too much vacuum advance and the engine would stumble and not run at higher rpms (too much advance and preignition). It runs fine with only the mechanical advance which gives 19*-20* (crank degrees), max. advance. On an automatic, static timing is 10* BTDC, so I get 29*-30* (crank degrees) total advance with just mechanical advance. With a manual transmission, static timing is 5* BTDC (crank degrees). On Mopar vacuum advances, there is an adjustment screw inside the vacuum nozzle for an Allen wrench. The amount of vacuum advance can be turned down by unscrewing that screw.
I hope that helps you to get it running right.

 

[toolmanmike]

Jed,
It's important to match the intake manifold port sizes to the intake ports of the heads that you use. The stock cast iron and the Edelbrock D4B, 4 barrel manifolds match the port sizes and the mounting bolts to the '65 heads.

Another thing to consider is the vacuum advance from a 2 barrel engine distributor might not work with a 4 barrel carb., i.e. there might be too much advance at the higher vacuum of the 4 barrel carb.. Once you get it running, always check how much mechanical advance there is, using a timing light with a dial to read the advance at higher rpms. Then connect the vacuum advance and read how much advance it gives at higher rpms. Don't let there be too much of advance; stay within the specified limit for your engine with a 2 barrel cam. My service manual for the 273 with a 2 barrel, lists the maximum vacuum advance as 10.5*-13.5* distributor degrees at 15" vacuum. Remember that crank degrees are 2X distributor degrees. I found that with a new vacuum advance (supposedly correct) there was way too much vacuum advance and the engine would stumble and not run at higher rpms (too much advance and preignition). It runs fine with only the mechanical advance which gives 19*-20* (crank degrees), max. advance. On an automatic, static timing is 10* BTDC, so I get 29*-30* (crank degrees) total advance with just mechanical advance. With a manual transmission, static timing is 5* BTDC (crank degrees). On Mopar vacuum advances, there is an adjustment screw inside the vacuum nozzle for an Allen wrench. The amount of vacuum advance can be turned down by unscrewing that screw.
I hope that helps you to get it running right.

You might want to check your vac advance. It may not be adjustable. I don't believe either of my stock diaphragm's are adjustable. @halifaxhops may be able to shed some light on the subject.

 

[AJ/FormS]

The specs are:
273 2bbl engine with 2 1/2in dual exhaust
4 spd trans
2:94 rear end
25.4 tall rear tire
Huston; we might have a problem
(273x5000)/3450=396cfm requirement at 100%VE
As a basic engine the 273 might be a 70%er so; 396x.7= 277 cfm,......... which is a 4bbl spec.
As I recall the conversion factor is .67 so for 2bbl this would be; 277/.67=413cfm 2bbl... so a 500-2bbl is plenty/21% plenty
I think the 2bblcarters were maybe 250 or 230s in those early years, so where would a 230 top out?
First you convert the 2bbl spec of 230 into a 4bbl spec and get 230x.67=155 4bbl spec, then
155x3450/273=1959 at 100% VE which is 1959/[email protected]. So at 2800 the carb might be pulling it's limit, according to the math. That don't mean she hits a brick wall. It just means the engine will start pulling a vacuum in the intake, and drawing the 4bbl spec back to the 2bbl spec
Ok so where does the 230 2bbl spec start choking?
(230x3450/273)/.7=4150 rpm But the engine still doesn't hit a brick wall; that won't happen until the springs lose control or the cam stalls, or I guess the exhaust stalls.. Which with a 2bbl topend might occur maybe, I'm stalling, cuz I'm guessing, possibly 4500, but could be earlier with ancient parts. Could be later too if the PO put something in there. Anyway the carb is still gonna pass the mixture. Of 'course the power is long stalled and gone, and the intake vacuum could be high, but I'll tell ya; see below

In practice, I have installed one of those 230s on a 340 short, with a 318 topend AND 318 cam (1969Sattelite parts,done in 73 or 74) with the onl bolt-on being fenderwell headers (65 Valiant wagon), and because it smoked the 1 tire,lol, it was a pretty sweet ride, for a young 20 year old ,me. These were the parts I had lieing around, and my slanty was burning oil faster than it was burning gas,lol,it had to come out.

OOPs I see I'm a couple of weeks late lol.
Oh well, I had fun.
I always have fun with math.

 

[jedrattle]

Thank you for that info.

I forgot to add I upgraded to the Mopar electronic ignition distributor with an orange box. I gave it some thought and ended up with an Edelbrock Performer intake (I’m going to sell the RPM). Hopefully it will be running by the weekend.