340 No low end power

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dgibby

dgibby

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Need some guidance, picked up 68 Dart and it has no low end power. Runs great, great top end but no launch at all, if I power brake it can’t break the tires loose and it seems to maybe rev to about 1200 rpm at best. The few times it did break loose it was like being on ice, burning the tires off like you would think it would. Here is what I was told when I bought it and what it has.
Was told.. Fresh rebuild to “sixpack specs” mopar purple cam and 72 360 heads 1.88/1.60 valves. I had the manifold off and it is a purple cam but don’t know the specs. Suppose to have a stall converter and shift kit but specs are unknown.
It has.. Torker II manifold, carter avs carb, Mallory mechanical Unilite dist, Mallory voltmaster coil, autolite 66 plugs, look a little lean to me, Hooker comp headers, 3 inch x pipe dual exhaust with magnaflows, 727 reverse valve body and 8 ¼ suregrip (no tag don’t know the gears). Timing set at 13 btdc but i dont know the total. Not sure where to start or if I am expecting too much from it
 
Theres not really much info to go by here. 340 six pack specs dont help. first the only difference is the intake and carbs on the 340 six pack, compression and cam were the same.

Theres lot sof different purpleshafts from small to huge solid cams so that doesnt really help.

The torker will kill some low end so I would start by swapping in a dual plane. How bug is the avs ? should be a part # on it someplace and you need to figure out what gearing it has in it.
 
thats a single plane intake right? if so that may be costing you some low end power and giving it back to you up top.
 
Yeah, I know, not much to go on. Carb is 630 cfm and yes it is a single plane intake.
 
Possible cam degree issue?
That thing should light em right now with all it is said to have in it.
 
Loose the intake. Is the carb an aftermarket carter or an old factory avs ? I would swap the car for a new Edelbrock AVS 650 so you can get tuning parts easier.

The unilite will probably burn out at some point and when it does replacement parts are almost as much as a new distributor. but for now you need to find your total timing also. You may have 13 initial and to much total.

How much vacuum does it have at idle ? is the idle rough or smooth and what rpm does it idle at. Im betting its got a 284-.484" Purple shaft in it which is why it doesnt feel very torque. That cam and 3.21 gears with a stock converter is going to feel like a slug.

Pull the cover off the rear and see what the ring gear is stamped with. Im betting 2.76 or 3.21s
 
Not sure on the vaccum, no lines or advance at all. It is an aftermarket carb. Idle is at 900 and is rough, like a bigger cam would be. I have the stock dual plane intake but not opposed to buying a better one. I know its hard with the limited info I have but I dont want to start throwing parts at it. What seems weird to me is how sometimes it will burn them off but most of the time it seems like it has a rev limiter on it.
 
The single plane is killing your low end.
Need to know your total timming,look for 34-36
Compression check ?
Regardless,if in proper tune,you should be able to drop the hammer,and light em up.
 
1.88 heads are not helping any... sounds like a timing/gear ratio issue or your valves are too tight if you have adj. rockers. u dont have to pull the cover to find out the gear ratio....all you do is mark a spot on your tire as well on the pinon yolk, rotate the tire 1 complete time and while doing the rotation count the # of times the yolk spins. i.e. 1 complete turn of the tire = 3.25 spins of the yolk = 3:23 gear. pretty simple. good luck!!
 
340's like 2.02 heads with a cam, headers, and big exhaust.... im not saying its killing it, but i wouldnt run 1.88's when im building a mild 340....... they came with 2.02 heads stock!
 
340 + torker II + AVS + lean + low initial + 3" exhaust + not enough converter + high altitude = DOG!

Not too many thing working against you here. :)

High altitude can usually take more total advance than at sea level. Put a timing light on it and figure out the total and amount of advance in the distributor, which is easy math, total - initial. It may be able to handle 42-44 total depending on altitude. Make sure you rev the engine until it stops advancing, not some predetermined number.

I'd crank the initial up as far as it will go before kicking back on the starter, then figure how the current mechanical advance will play here. If you can add some initial, it may help your idle quality, especially if you don't know which cam you have. Purple shaft is a generic name covering the entire MP cam line, from box stock replacement to drag race grinds so that's no help on actual specs.

Sounds like the tune up may be off which will help get some of the low end back.

I'd put an RPM, LD340 or air gap on it if it were mine.

72-73 340's had 1.88 valve heads.
 
forced 340 said:
340's like 2.02 heads with a cam, headers, and big exhaust.... im not saying its killing it, but i wouldnt run 1.88's when im building a mild 340....... they came with 2.02 heads stock!
Click to expand...

in 72 they went to 1.88 valves. 2.02s will breath better on the top end, they will not add low end torque.
 
forced 340 said:
the comp was also lowered..
Click to expand...

Yes the compression was lowered in 72 and the valve size went from 2.02 to 1.88s. 2.02s will move more air at the top end, They will not add low end torque and some will actually tell you the bigger valve will hurt low end throttle response.

They also switched to a different passenger side exhaust manifolds in 71 as well as switching to the Thermoquad. Neither of which makes a difference in this case either.

His problem is a big pile of mis-match parts.
 
forced 340 said:
1.88 heads are not helping any... sounds like a timing/gear ratio issue or your valves are too tight if you have adj. rockers. u dont have to pull the cover to find out the gear ratio....all you do is mark a spot on your tire as well on the pinon yolk, rotate the tire 1 complete time and while doing the rotation count the # of times the yolk spins. i.e. 1 complete turn of the tire = 3.25 spins of the yolk = 3:23 gear. pretty simple. good luck!!
Click to expand...
actually they are the only thing thats decent for the street .202s would just finish off the rest of the port velocity.get a dual plane intake,rpm,stealth etc.and a 3310 holley.i'tll spin em then!
 
AdamR said:
Yes the compression was lowered in 72 and the valve size went from 2.02 to 1.88s. 2.02s will move more air at the top end, They will not add low end torque and some will actually tell you the bigger valve will hurt low end throttle response.

They also switched to a different passenger side exhaust manifolds in 71 as well as switching to the Thermoquad. Neither of which makes a difference in this case either.

His problem is a big pile of mis-match parts.
Click to expand...

i totally agree that 1.88 heads are great for low end.... im just saying i wouldn't put them on a 340 that i purposely building... i also agree hes got a lot of mis-matched parts going on!
 
So... trying to keep it simple and work my through, start with..
Get the timing dialed in, find out the gear ratio and converter, upgrade the manifold and then the carb if needed? I do have some options, I have a set of 1970 T/A ARR heads, rockers etc but it doesnt sound like it will help much and I was going to sell them to pay for of the upgrades. I also have the stock intake manifold and I think it has an Edelbrock carb on it, not sure of the model and I'm sure it would have to be rebuilt so using my existing carb would be prefered or am i wasting my time with either? I also have some Thrush welded 2 1/2 mufflers but really dont want to change the whole exhaust system if there wont be that big of a gain. My ultimate goal is to have a nice driver, good power and sound. The thing sounds like it should be shooting flames out the back when you get on it but from a light it cant even brake loose, let alone get out if its own way.
 
my guess is the carb and intake. sounds like a sorry carb. Secondary seems to work, primary might have bad pump, missing parts/springs/ etc. also could be some vac lines/leaks around the intake (check with carb spray) then look at timming and dist. could have the timing chain off and can not move the dist enought to get it out and or the vac advance on the dist screwed up. don't really know but these are the1st things Id check out. $0.02 worth
 
Always start with timing. run as much initial as you can with out it kicking the starter or pinging. Then you need to limit the total timing to 36*-38* at around 2500 to 3000rpms. Im not sure how the unlite works for adjusting total advance. The springs will effect how fast the advance comes in.

If the Carter you are running is an after market one its the same as an edelbrock. Check for numbers on the carbs base near the front passenger side mounting bolt.

Also, Im not familiar with the Autolite 66s. Ive always ran 65s in my small blocks. Anyone know the difference ?

See if you can tune what you have before spending money.
 
Timing and a different intake would make the biggest differences if the base shortblock is assembled correctly. A stock 340 squarebore intake would likely preform better than the torker II, especially below 4500 rpm. Put a 1" spacer on the stocker if you don't want to get a RPM or Air Gap.

IMO, don't waste time putting T/A heads on it.

Try to figure out which cam you really have. A dial indicator on a lifter might help get an idea of which cam is installed. If it's in retarded, you may have an uphill battle on your hands.

Leave the exhaust for now.
 
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