Author Topic: engine timing  (Read 271 times)

Online brotow

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engine timing
« on: August 20, 2017 - 05:13:04 pm »
I have a 340 6pk engine it was rebuilt .030 over. Comp cam 20-224-4  grind#crs xe274h-10,  duration .050 230 intake,  236 exhaust, lift .3250 intake, .3270 exh.  lobe separation 110.0.  Gross valve lift.488 intake, .491 exh.  I set the timing to "1970 340  4bbl (factory manual)" 5 btdc +_ 2 1/2 degrees.  engine idles at about  15-1800 any lower it would stall out. In gear (automatic) I have to give it gas to prevent stalling. Acceleration is a dog, no power.  Should the timing be more advance with this cam? I know nothing about cams, or could it be the carbs which were rebuilt by the engine rebuilder.
« Last Edit: August 20, 2017 - 05:24:24 pm by brotow »
70 Chall. vert.(in process) - 73 Amx -74 Javelin- 87 Wrangler-02 Sebring conv.




Offline Bullitt-

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Re: engine timing
« Reply #1 on: August 20, 2017 - 05:24:08 pm »
Couple of things are universal that apply to all engines you should try...  some cams need more RPM at idle but you should be able to get down to 1000RPM

Follow brother CP's timing method
http://www.cuda-challenger.com/cc/index.php?topic=4430.msg33565#msg33565

Adjust the idle screw so your transfer slots are square (I pilled my carb off to set..)
http://www.superchevy.com/how-to/45638/
Wade  73 Rallye 340..'77 Millennium Falcon...13 R/T Classic   Huntsville, AL
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Online 73_Cuda_4_Me

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Re: engine timing
« Reply #2 on: August 20, 2017 - 06:23:44 pm »
I have the exact same cam in my 340, and it needs 18-20 advance at idle to get any good vacuum...

If you are running stock dizzy, you'll need to disconnect vacuum advance if used, and I made a limiter plate to restrict mechanical advance to 36 degrees all in at 3000 rpm...

You are running into advance on idle when you try to set it... as soon as it lugs just a hair, advance cuts way back causing it to want to stall...
 :2cents:
1973 Plymouth Cuda BS23H3B567783

R11 V6X EN2
M21 M25 M31 M88 N41 N42
V1X U B41 C56 G37 J54
JY9 A6X9 0 703 501616
E55 D34 BS23 H3B 567783


Online 1 Wild R/T

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Re: engine timing
« Reply #3 on: August 20, 2017 - 07:04:34 pm »
Crank in at least 16 base, probably 20..... But in turn you need to shorten the total so you wind up in the 35-37 degree range....

http://www.4secondsflat.com/Mopar%20Mechanical%20timing%20limiter%20plate.htm
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Offline cudabob496

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Re: engine timing
« Reply #4 on: August 20, 2017 - 07:39:55 pm »
yep, as you increase timing BTDC, you will need to lower idle speed, or it will
go to the moon.
72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.
Up to 4000 RPM, Warp Speed, Up to 5000, Rediculous Speed, Up to 6000 RPM, Ludicrous Speed!

1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

Online 73_Cuda_4_Me

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Re: engine timing
« Reply #5 on: August 20, 2017 - 08:24:49 pm »
If you are still running stock vacuum advance dizzy, you will run into the problem that I did when upgrading the cam... the initial timing needed by cam for best vacuum at idle is around 18-20 degrees, leaving a total of about 16 degrees more to get to the total advance recommended by 1 Wild, so you have to limit how much total mechanical advance to do - making or buying a limiter plate...

What you will also run into is the higher idle needed with that cam is just past the start of mechanical advance of the stock springs in the dizzy. You can set the idle with the cam in park or neutral, but when you put it in gear, engine rpm drops about 200 rpm, and the mechanical advance falls off, retarding the timing and dropping engine rpm even more (stalling, unless you pedal it).

The problem with the plate available on line is that it does nothing to prevent that low rpm mechanical advance. Stiffer springs on dizzy change curve too much for a small block.

There is an easy way to accomplish both, though... I made my own plate from electrical box cover, and shortened advance pin slots by .125"... but you shorten them on the INSIDE part of the slot, not the outside. This preloads the springs, shifting the start of mechanical from around 700-800 rpm up to around 1200-1300 rpm, and still provides a total of 16 degrees total mechanical, on top of the 18-20 degrees needed for cam initial timing, for a maximum around 34-36 (If you don't, you'll get spark knock at cruise speed when you try to speed up, from too much advance at 3000 rpm...

I'm running a 70 340 engine, stock bore, pistons, heads, and intake, but cam, headers, and eddy carb. (the 73 #'s matching 340 is on engine stand awaiting stroker kit rebuild $$$).

Please note that my mods are for a 73 electronic ignition dizzy with vacuum advance can... the vacuum advance alone was 17 degrees at crank, so disconnected and went mechanical only, 18 initial to 34 total (the 73 dizzy has 15 degrees mechanical at dizzy, and note that dizzy advance is 1/2 of what total crank advance is, so stock unit added 30 degrees on top of initial timing... Since 8 degrees advance at dizzy is 16 degrees crank timing advance, that is what I set slots up to provide on top of my 18 degree initial)

Some pix of my plate attached...
1973 Plymouth Cuda BS23H3B567783

R11 V6X EN2
M21 M25 M31 M88 N41 N42
V1X U B41 C56 G37 J54
JY9 A6X9 0 703 501616
E55 D34 BS23 H3B 567783

Online brotow

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Re: engine timing
« Reply #6 on: August 20, 2017 - 08:53:15 pm »
Great advice. I am also running a electronic dist. I don't lnow what year or even if it makes a difference. I really don't want to pull the dist. I'm going to see how to limit the mech advance by some other method
70 Chall. vert.(in process) - 73 Amx -74 Javelin- 87 Wrangler-02 Sebring conv.

Offline cudabob496

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Re: engine timing
« Reply #7 on: August 21, 2017 - 12:19:25 am »
just set the initial timing at 10-12  BTDC, and you may see a good improvement.
But lower the idle speed at the same time.
Could also be a carb idle adjustment issue.
And try all this without Vac advance working/hooked up.
run high octane gas with some octane boost to prevent detonation.
« Last Edit: August 21, 2017 - 12:25:34 am by cudabob496 »
72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.
Up to 4000 RPM, Warp Speed, Up to 5000, Rediculous Speed, Up to 6000 RPM, Ludicrous Speed!

1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

Online brotow

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Re: engine timing
« Reply #8 on: August 28, 2017 - 06:25:55 am »
I have found one advance spring unattached to the counter weight. I'm going to see if that will be the difference I need. I doubt it either way whats the best way of removing the reluctor without breaking it, the manuals says to pry it up with two screw drivers. I tried with the maximum force I felt before it would break.
70 Chall. vert.(in process) - 73 Amx -74 Javelin- 87 Wrangler-02 Sebring conv.

Online 73_Cuda_4_Me

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Re: engine timing
« Reply #9 on: August 28, 2017 - 12:48:50 pm »
The reluctor does pry off, just be sure to watch for the little roll pin that clocks the reluctor to the shaft of the dizzy... easy to lose! You could try using some penetrant on the reluctor, and maybe a little heat.. It will be stubborn the first time you take it off in 30-40 years... mine slides on easily now that it's all cleaned up...
1973 Plymouth Cuda BS23H3B567783

R11 V6X EN2
M21 M25 M31 M88 N41 N42
V1X U B41 C56 G37 J54
JY9 A6X9 0 703 501616
E55 D34 BS23 H3B 567783

Online brotow

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Re: engine timing
« Reply #10 on: September 03, 2017 - 09:07:36 pm »
Here's my update.  Made the distributor limiter plate .125 inside, which I was able to do without removing the distributor from the engine, set timing at 20 degrees at idle 1000 rpm, no vac advance, 35 degree mark on damper sits at about 15 degrees after tdc, giving it higher rpms 2500- 3000 brings the 35 degree mark to about 5 degrees.  Hit the gas tires will lose a little traction....good ,till it revs up  anything more rpm the car is a dog, also no pinging with 93 octane. This 340's damper was replaced by the engine shop with a 318 damper cause of a crack around the hub.  Now I wonder if the damper is incorrect timed form my 340?
  The only info I got from the damper is I believe a vendor/date code 85985-056-0 which is the 56th day 70?  Which would indicate a cast crank 70-71 318 2951673. My 340 should have a forged crank as far as I know that's what the rebuilder was given. Can anyone point me in the right direction? Also would the outer carbs
 not opening up cause this difference? I don't hear the outer carbs come alive.
« Last Edit: September 05, 2017 - 06:14:26 am by brotow »
70 Chall. vert.(in process) - 73 Amx -74 Javelin- 87 Wrangler-02 Sebring conv.