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With the often best power recommended 36* centrifugal timing setting the timing at idle on my 327 comes in at 32* (vac can connected). .. Engine has the LT-1 cam. Considering the cam the engine at idle seems ok at 950 rpm, but somewhat on the rough side. In addition, adjusting the idle screws on the QF Slayer carb to as lean as the engine will tolerate to the point where RPM starts dropping off yields a still too rich fuel condition (easily smelled). .. I'm wondering if the somewhat rough idle is masking an ignition miss condition. Is 32* total idle timing too much using the LT-1 cam?
Opinions appreciated, thanks.
John
I know this is a stupid question..but do you know for a fact that the harmonic balancer is good??? And what I mean by good is that teh outer ring of the balancer that has the notch in it that you are using for timing is correctly clocked where it should be.
What is the CFM's of the QF carb you have on it???
When the engine is running and you CAREFULLY look down into the primaries....is it dripping or sucking fuel that you can visibly see???
I have not worked on any QF carbs...but they look much like a Holley...so I wonder if they utilize a power valve and IF that power valve is not damaged.
I first use a piston stop tool to confirm that the harmonic balancer TDC mark is properly coordinated with the "0" on the timing chain cover scale. I've never encountered a balancer outer ring that has "slipped" to a new position. However, I've found that many have been replaced over the years with a balancer from a later or earlier engine. Chevrolet has moved the small block balancer TDC mark and the timing scale several times since 1955.
I then inspect the distributor with the cap off to make sure the advance limit bushing is in place. Since the factory used a flimsy rubber bushing, it's often missing in an old distributor. If it's gone, I install a brass bushing on the limit pin. I also look at the advance springs to make sure they're stock. Many have been replaced with weak aftermarket versions that allow some mechanical advance at idle.
Next I disconnect the vacuum advance canister. It should NOT be functioning when you check total advance at high rpm or initial advance at idle.
Using a dial-back timing light, I set the total advance at 36 degrees at 4,000 rpm. Even the stiffest GM advance springs should allow full mechanical advance by that engine speed.
I finally check to see the resulting initial advance that gives 36 degrees at high rpm. Usually it's in the range of 12 to 18 degrees with the vacuum canister disconnected.
Vacuum advance canisters have different vacuum operation ranges. The canister that works best with the LT-1 cam is a GM "236" that gives about 16 degrees at 8" of vacuum. The Standard brand aftermarket version of this canister is stamped B28. I have it connected to a "full-time" vacuum port so that the canister is functioning and fully advanced at idle with the LT-1 cam. This cam makes 13" - 14" of manifold vacuum at 800 rpm with the stock valve lash settings.
The strongest idle with this cam has an air/fuel ratio in the 13.6 to 14.0 range. I don't notice any particular exhaust smell with this A/F ratio.
The best spark advance map setup for the LT-1 cam is same as OE on the 365/375 HP 327s, with the 30-30 cam... 24 centrifugal all in at no more than 2500 and 12-16 initial to achieve the Chevrolet Power Manual recommended range of 36-40. Go as far as you can as long as there is no detonation.
A 8"/16 degree VAC (B28 equivalent to OE 236 16) should be connected to a full time vacuum source, which should yield total idle timing in the low thirties, which is about right for high overlap cams. Due to extreme exhaust gas dilution they need lots of idle advance and a rich mixture on the order of 12:5:1, which may yield exhaust odor. FI engines can idle a little leaner due to more even fuel distribution.
Typical LT-1 idle behavior set up as above is 12" @ 900 with a noticeable lope.
Since it is difficult to get high overlap cam engines to idle stably below where the centrifugal advance starts, total WOT advance should be set (VAC disconnected and plugged) a few hundred revs above the point of max centrifugal in the range of 36-40 as far as possible as long as the engine doesn't detonate.
...can't help you with the idle mixture issue other than to repeat that high overlap cams need a rich mixture for best idle quality.
Search for a post started by me: Tuning vintage Corvette engines for maximum... if you want additional info on how to set up an optimum spark advance map for any vintage Corvette engine configuration.
Here's how I set timing on a small block.
Using a dial-back timing light, I set the total advance at 36 degrees at 4,000 rpm. Even the stiffest GM advance springs should allow full mechanical advance by that engine speed.
There are several SB and BB distributors that don't bring in total centrifugal until as high as 5100, like late sixties 300 and 350 HP SBs. If you set total WOT advance at 4000 on one of these you will be over-advanced.
Check the shop manual/AMA specs for OE specs then test with a dial back light. It may have been changed over the years, and bringing it in faster will yield better performance and fuel economy as long as it does not introduce significant detonation.
I was just out in the garage running engine and rechecking timing numbers:
The QF carb is fairly new, rated 600 cfm. No fuel drip observed with engine idling. When the engine was rebuilt, using the piston stop method to verify damper number marking to timing tab showed '0' to be almost dead on - within one degree.
Manifold vacuum at idle shows 13.5 on the gauge @ 950 rpm. The vacuum can in the distributor is a B26. A brass bushing is installed on the plunger end. Distributor advance weights are aftermarket as are advance weight springs. Lighter tension springs are being used. Original factory advance weights long gone.
With engine running the dial back shows 35* mechanical advance @ 3000 rpm, vac can disconnected. Engine idle with vac can disconnected shows static timing @ 17*. .. Experimenting with static timing down to the 10* - 12* range, engine idle does smooth out some. Turning in idle mixture screws with static timing @ 12* and vac can hooked up doesn't seem to yield much change with a leaner fuel mixture at this timing setting as compared to before. Thinking to experiment more with this.
Hopefully, getting something close to 375 hp timing numbers will help with the idle issue.
Next up might be to limit vac can advance to 12* to back off idle timing and keep mechanical advance in the 36* range.
Thanks, guys.
John
I first use a piston stop tool to confirm that the harmonic balancer TDC mark is properly coordinated with the "0" on the timing chain cover scale. I've never encountered a balancer outer ring that has "slipped" to a new position. However, I've found that many have been replaced over the years with a balancer from a later or earlier engine. Chevrolet has moved the small block balancer TDC mark and the timing scale several times since 1955.
I then inspect the distributor with the cap off to make sure the advance limit bushing is in place. Since the factory used a flimsy rubber bushing, it's often missing in an old distributor. If it's gone, I install a brass bushing on the limit pin. I also look at the advance springs to make sure they're stock. Many have been replaced with weak aftermarket versions that allow some mechanical advance at idle.
Next I disconnect the vacuum advance canister. It should NOT be functioning when you check total advance at high rpm or initial advance at idle.
Using a dial-back timing light, I set the total advance at 36 degrees at 4,000 rpm. Even the stiffest GM advance springs should allow full mechanical advance by that engine speed.
I finally check to see the resulting initial advance that gives 36 degrees at high rpm. Usually it's in the range of 12 to 18 degrees with the vacuum canister disconnected.
Vacuum advance canisters have different vacuum operation ranges. The canister that works best with the LT-1 cam is a GM "236" that gives about 16 degrees at 8" of vacuum. The Standard brand aftermarket version of this canister is stamped B28. I have it connected to a "full-time" vacuum port so that the canister is functioning and fully advanced at idle with the LT-1 cam. This cam makes 13" - 14" of manifold vacuum at 800 rpm with the stock valve lash settings.
The strongest idle with this cam has an air/fuel ratio in the 13.6 to 14.0 range. I don't notice any particular exhaust smell with this A/F ratio.
i hate to hijack this, but i have a few Qs. and i see some knowledgeable people watching this thread.. sooo, i know my way around a hotrod, but im trying to keep this as stock looking as possible with as many factory parts as i can. with that being said. i have a 65 l78 with a k66 dist with the second design l88 cam . i have my total set at 36*-38*. with my canister plugged.. the can is a 201, which i thought was correct for my car. but now im wondering. my original 3124 carb has the throttle blades drilled for idle air and i have a 3.5 power valve in it.
my questions are, should i just keep the can plugged?
is the 201 can correct?
should i not worry about running a can with the l88 cam as i am doing?
has anyone run this cam, and what did you/they do?
First if you think your car is missing, give it some rpm and it should become obvious!
I I am not a fan of tuning vacuum at idle! The car isn't driven at idle! But for comfort at stop lights, I guess you guys can do that if you like! It is kind of like learning how to stand up, before you learn to walk and WOT is running! You guys that tune for good idle are wanting to be at your best just standing up! But then I am a notorious fuel wasting throttle blipper when sitting at stop lights!
I used to read my plugs a lot, but I cheat now with AF Gauge now and I run the engine thru the rpm range to see what the gauge does at idle, then in transition, then at full advance! I am racist, so I only use Holleys! I even plug my power valve just for tuning only and not recommended for driving on my jets at steady state speeds! It is hard to read the AF alone on an AF gauge because of the power valve enrichment kicking fuel for transition to heavy load! But I have real weird tuning tricks! I am a little different!
Where is your advance coming in at, because I am not going to look up your vacuum advance cannister? So I will trust what has been shared so far!
Optimium tuning is far different than setting your tune from a book!
You want to work into your improvements resulting from your tuning efforts and sometimes go back and forth between timing/advance and carb/idle/fuel mixtures/jetting changes and then back to timing changes/advance to adjust for the newer changes to fine tune it more and back to carb! Your driving style should determine the true tune that ends up on your unique engine! Two engines can be built close to the same specs, but run best at different tuning setups!
Etc etc
Last edited by TCracingCA; 01-20-2017 at 01:17 AM.
i hate to hijack this, but i have a few Qs. and i see some knowledgeable people watching this thread.. sooo, i know my way around a hotrod, but im trying to keep this as stock looking as possible with as many factory parts as i can. with that being said. i have a 65 l78 with a k66 dist with the second design l88 cam . i have my total set at 36*-38*. with my canister plugged.. the can is a 201, which i thought was correct for my car. but now im wondering. my original 3124 carb has the throttle blades drilled for idle air and i have a 3.5 power valve in it.
my questions are, should i just keep the can plugged?
is the 201 can correct?
should i not worry about running a can with the l88 cam as i am doing?
has anyone run this cam, and what did you/they do?
thank you troy
Playing with something that is marginal and more race bred is tricky! You can employ tricks to make a radical cammed engine more tamed! Using a vacuum actuated carb could improve the drivability and for idle and idle mixture sensitivity your throttle blades were drilled! I figure you are going for a low vacuum enrichment circuit because the engine has low vacuum at lower RPMS like at idle???? An L88s drivability would be most affected or changed by the advance and retard of the timing and the jetting needs to be spot on! And the power valve helps the engine like a switch (on and off), so allows fuel enrichment on engine need during transition and run! Actually too low a numbered valve can open too late causing hesitation and lean surge at medium throttles! Vacuum gauge is good for tuning for the best power valve selection! The higher number valves start adding fuel enrichment sooner and can cause your fuel economy to suffer too!
Again is how are you driving this car? Are you trying to putt around on a cruise, or are you hammering it off of the line and looking for instant response from your tune?
A high overlap cam often idling at higher speeds draws fuel from the main metering system! So the low vacuum can open the power valve and result in fouled plugs and poor throttle response!
You our want to make sure you tune to avoid the engine killer, which is lean mixture induced detonation!
Trying to have your cake and eat it too or whatever that saying is difficult on a marginal engine to be used on the street!
Last edited by TCracingCA; 01-20-2017 at 12:20 AM.
I first use a piston stop tool to confirm that the harmonic balancer TDC mark is properly coordinated with the "0" on the timing chain cover scale. I've never encountered a balancer outer ring that has "slipped" to a new position. However, I've found that many have been replaced over the years with a balancer from a later or earlier engine. Chevrolet has moved the small block balancer TDC mark and the timing scale several times since 1955.
I then inspect the distributor with the cap off to make sure the advance limit bushing is in place. Since the factory used a flimsy rubber bushing, it's often missing in an old distributor. If it's gone, I install a brass bushing on the limit pin. I also look at the advance springs to make sure they're stock. Many have been replaced with weak aftermarket versions that allow some mechanical advance at idle.
Next I disconnect the vacuum advance canister. It should NOT be functioning when you check total advance at high rpm or initial advance at idle.
Using a dial-back timing light, I set the total advance at 36 degrees at 4,000 rpm. Even the stiffest GM advance springs should allow full mechanical advance by that engine speed.
I finally check to see the resulting initial advance that gives 36 degrees at high rpm. Usually it's in the range of 12 to 18 degrees with the vacuum canister disconnected.
Vacuum advance canisters have different vacuum operation ranges. The canister that works best with the LT-1 cam is a GM "236" that gives about 16 degrees at 8" of vacuum. The Standard brand aftermarket version of this canister is stamped B28. I have it connected to a "full-time" vacuum port so that the canister is functioning and fully advanced at idle with the LT-1 cam. This cam makes 13" - 14" of manifold vacuum at 800 rpm with the stock valve lash settings.
The strongest idle with this cam has an air/fuel ratio in the 13.6 to 14.0 range. I don't notice any particular exhaust smell with this A/F ratio.
Damn I hope the crank to TDC in building the engine is good tin relation to your timing gear and cam! and then you won't have to worry about where your mark on the balancer is after the fact?
And you aren't grabbing distributors out of the junkyard are you? That is too advanced for most of these guys!
12 to 18 is quite a spread! If you tune anything that walks in the door, then I could maybe see the possibility of such a wide recommendation, but again too advanced of a spread for one guy's particular best to look for advance! I would hate for the guy to go, oh I am between 12 to 18, so I must be good!
Coming all in at 4000 on a hotter cam is super late! That is a long way off for full performance capability? Curious why so lazy? I personally think you should be all in at the optimum rpm range low end of your selected intake manifold essentially!
Last edited by TCracingCA; 01-20-2017 at 01:02 AM.
for those vehicles with stiff factory distributor springs, cannt you make the adjustments with one spring removed?
It is best to have a selection and step up in testing steps from
heavy heavy
heavy medium
medium medium
medium light
to light light to see what your engine likes the best or what feels good to you!
Same for your initial! Bump 1 then 1 more till you like how it runs!
Gas quality and spark off of fuel combustion efficiency are big factors to how early or advanced you are starting you combustion mix under compression stroke for best firing and power tuning!
Last edited by TCracingCA; 01-20-2017 at 01:16 AM.
You mention this is a new engine, if you used the newer Hypereutectic pistons you will need to back the timing off by 2 degrees compared to cast or forged pistons. The Hyper pistons reflect more heat back into the chamber.
When the hypers were released it took a while to figure out they needed less timing.
If forged work well at 36 degrees the same build with a Hypereutectic will perform best at 34.
You mention this is a new engine, if you used the newer Hypereutectic pistons you will need to back the timing off by 2 degrees compared to cast or forged pistons. The Hyper pistons reflect more heat back into the chamber.
When the hypers were released it took a while to figure out they needed less timing.
If forged work well at 36 degrees the same build with a Hypereutectic will perform best at 34.
I had to put my iPhone on charge, but this is a perfect example of where the engine could be built to a compression ratio, using a certain sized cc chamber, and piston style, but the material difference of the piston material causes a 2 degree change in what advance best to run!
The harmonic balancer issue came up a few posts earlier. I have to admit I've never seen the outer ring move to a new position. At least no permanently.
Keep in mind that the outer ring can move as your rev the motor. As the rubber wears the elasticity may increase. When you drop back to idea the outer ring returns to it's normal position.
It's easy to check the ring for wear.
GM did some testing a few years back and found that at 7,800 rpm one damper allowed 2.0 degrees of crank twist. The second damper they tested allowed 1.7 degrees of twist, and the third had allowed 1.25 degrees of twist. And these were all brand new units.
If the crankshaft is moving that much imagine how much a worn out outer damper ring might move. Yes, it will alter your timing.
A stock replacement will be fine. There are some issues with the after market performance dampers.
01-18-2017, 03:23 PM
power timing and idle mixture
But then I am a notorious fuel wasting throttle blipper when sitting at stop lights!
