need tuning help on 340

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j.d.duggan

j.d.duggan

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what I have is a 1971 340 with stock cam,360 j 188-160 heads,stealth intake,650 DP holley carb,long tube headers with 3" all the way out the back.MSD box,electronic ign. dist.with what I think is a regular coil.4 speed car.this is a street car(not intended for track use)323's in a 8 3/4 sure grip.These were parts that came off a bit-of-a-radical 340 that was in the car when I got it.So that's why I am using the parts listed.what I need to know from you guys is a good starting point on things like,plug gap?running autolite 65,initial timing,currently set at 10* BTDC,I do not know what jets are in the carb,but it seems like it puts out way to much gas for the motor its on.what jets would you use for a starting point? It starts up really nicely like it is,dose'nt make full rev before firing off,just did engine break-in and drove about 1 mile so not much more info I can give you any thoughts would be appreciated,even you guys that always have to say throw this away,throw that away.really just wanting some good starting points.:thankyou::thumbsup:

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What is the list # 4777, 4777-1 if it 4777 pri jet is 71 sec is 76
4777-1, pri 67, sec 76 try not to go more than 4 jet sizes in ether direction
these are what the carb came with new.
also 4777-3 pri 67 sec 73.
 
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I don't understand the? I'm sorry but the only # I found on the carb were on the breather throat and it's a 76650 3261
 
On the front of the choke tower there will be the #( list 4777 )

it could be a 80783 also if it is a 650 dp
 
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Get the initial timing and total timing right before you spend 5 minutes adjusting the carb. Timing FIRST!!! 10* is not enough. Only adjustments to mess with on carb are the float level and idle mixture after idle is speed is set. Being a 4 speed makes some of this easier.

JMO, a 340 with a stock cam likes 14-16 initial. If it has any larger camshaft than stock, that initial timing number goes UP. You'll have to limit distributor mechanical advance when giving it more initial so your total number is not out of bounds.

I'd get it warmed up and set initial at 18*, reset idle speed. Shut it off and see if it starts OK when hot. DO NOT drive it hard until you get the total timing number sorted out. The idle should clean up a bit with more initial timing.
 
thanks for the input crackedback,what should my total be kept to?34* that's where I had it set for the break-in.what will the dist.advance,total,from initial?No vac advance.
 
If your compression is 9.5:1 or so and you have a stock cam with 3.23 gears your probably fine with an initial timing of around 10 - 12 degrees and a total of 34....right where you are.
You can recurve your distributor to get a bit more initial and keep your total timing around 34 but you are not racing so keep it simple, especially since you said it starts and runs great.
 
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keep in mind that the motor has a 20 min. breake-in and about 1 mile on it.the compression ck was 155-170 PSI.what would your guess be on comp.?
 
j.d.duggan said:
keep in mind that the motor has a 20 min. breake-in and about 1 mile on it.the compression ck was 155-170 PSI.what would your guess be on comp.?
Click to expand...

Your good there I wouldn't want more than 175 - 180 psi cranking hot, all plugs out, throttle blocked wide open.
 
Yes your around 9.5 :1 and iron heads are sensitive to detonation especially with the short duration stock cam.Be careful advancing your timing...you can get detonation without hearing it.
12 \ 34 \ 52 should serve you well.
 
A 71 340 should be 10.5 c.r. in reality probably closer to 10.0:1. The heads should be close to the same cc's just 1.88 vs 2.02 stock.
 
i calculated the compression ratio with 72 cc heads and stock pistons around .020 above the deck and i got around 9.5:1.
but i could be wrong.
 
Idle is too vague when talking about setting timing.
The 12* at idle will work when you get the engine idling around 650 - 700 rpm.
14 - 16* at idle is good if the engine is idling higher than that, say 750 - 950, which often is the case when first getting an engine running.
Both assume a stock or close to stock distributor setup.
 
52 would be the total cruising with vacuum advance connected.
Some guys don't use vac advance but it will improve efficiency for normal driving, and most recommend it.
 
ir3333 said:
Yes your around 9.5 :1 and iron heads are sensitive to detonation especially with the short duration stock cam.Be careful advancing your timing...you can get detonation without hearing it.
12 \ 34 \ 52 should serve you well.
Click to expand...

I agree
Especially with your listed cylinder pressure. IMO with iron heads, you are flirting with disaster.
A manual trans marrys your crank to the rearend, unlike a torque convertor. So if you find that the car is jumpy at low speeds low rpms, and you have to clutch it to get it to stop, then back off the initial timing. I run 14, but only because I have a dash-mounted, dial-back, timing control. That device has a range of 15 degrees . I set it to allow plus 7/minus 8. Around the parking lot and in parades, I can take out up to 8* and that settles her right down. I have the same pressure as you, and I run aluminum heads.
If you really have 175psi plus, and 3.23s with a 2.66low,I'm pretty sure you are gonna have this jumpiness problem at 14 to 18* timing; Unless you just never try to drive slow-enough for somebody to walk alongside the car. Your current slowest speed is probably about 6mph. To get to 4, will require an in-gear and idling speed of 430rpm; good luck with that. And even if you could get down there; at 14* your power pulses would be so strong your car would be bucking like a wild horse. Retarding the timing will delay peak cylinder pressure, and soften the hit.
But what if you say that situation will never present itself. Fair enough. Then 14 to 18 is a good starting range, but, before you get to that, get your Transfer slots synced up, and let the engine tell you what it wants for idle-timing.
 
AJ/forms,I don't understand your above statement about a 2.66 low?what is this?The compression # were cold engine,throttle slightly open,all plugs out and 4-5 revs.on each cyl.what would the total advance of a dist be.I don't have vac advance at this time.:BangHead:
 
Don't worry about vac advance right now.
Get a tach and check the timing at idle rpm, then 1000, 1200, etc. to 2800 or 3000, and 4000 if you want.
Damage from detonation will happen when there is high load on the engine. Not when idling in neutral or cruising.
It (detonation) will first show as tiny metal specs on the spark plug porcelain. A little worse and you will hear it 'ping'.
Keep your ears open, and if your concerned, stay out of wide open throttle, especially in 3rd or 4th gear.
 
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j.d.duggan said:
AJ/forms,I don't understand your above statement about a 2.66 low?what is this?The compression # were cold engine,throttle slightly open,all plugs out and 4-5 revs.on each cyl.what would the total advance of a dist be.I don't have vac advance at this time.:BangHead:
Click to expand...
2.66low is the low gear ratio in the A833manual trans. The ratios are 2.66-1.92-1.40-1.00. The starter gear in your combo is 2.66x3.23=8.59 which is fine for a stock 340, but perhaps a little tall for a cammed up one..... unless it has plenty of cylinder pressure, like yours seems to have.
The starter gear ratio, together with the tire roll-out(circumference or TC) can be plugged into a formula and with either the mph or the rpm plugged in, the other can be determined. The formula reeds;
mph= (rpmxTC)/ (1056x R1xR2)
where; the Rs are the ratios; 1056 is a constant; TC is Tire Circumference;

Example; mph= 6800x85(a 27"tire)/1056 1.40x3.23=121mph

or mph= 600x85/1056x2.66x3.23= 5.6mph

the constant 1056 is a conversion factor that converts all the math to the correct mph.
 
well I got on to some other stuff and haven't done much on the tune,timed at 14 now and about 35 total 18" of vac at idle,startes alright at that setting and seems to run nice and cool,180.hade the tranny out this last week trying to get rid of my vibration issue with no luck.new flywheel and pp balanced at mechine shop didn't help much,guess I'll live with it.anyway I'll drive it a little and get back on playing with the tune in a week or two,a little burnt out right now and it's time to start fishing.:steering:
 
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