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Can someone explain timing to me? Having issues.

8K views 100 replies 13 participants last post by  Dajerseyrat 
#1 ·
I think I am having timing issues on my motor. Ill give you the specs first:
355 SBC
10.3:1 compression
64cc heads
Lunati 550 lift 288 degree cam
Weiland 17530 manifold
Holley 750 CFM double pumper
456 gear
powerglide
Car weighs 2800lbs
28" tire


Ok, now this engine ran fine in my old camaro all I changed was the distributer to a stock type HEI with an MSD 6al2, and the carburator from a 850 dominator to a holly 750 street carb with choke.

Now the issue I am having is when Im crusing under 3500 rpms and stab the gas the car almost shuts off and falls on its face then takes off.Also does this from idle and in a neutral rev will occasionally backfire and stall, did that today actually..It also has a tendancy to break up until 400 rpms but I havent done a full WOT pull yet.

After a part throttle cruise When I pull the plugs the front 2 cylinders and rear 2 cylinders look very rich and all the middle cylinders look much leaner, they could be fouled cause I did have a stuck needle and float at first.

The carb is completely stock other than I went from 28 pump shot nozzles to 31 and it seemed to help a little bit.

The way I set the timing was #1 cylinder to about 10degrees BTDC and dropped the distibuter in, car started up fine, dialed in about 12 degrees timing at idle which is what I was running. I however have no clue what my curve would be or total timing.

Could someone explain the difference of initial timing, total timing and mechanical advance?? Im a bit of a novice at this...
 
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#2 ·
If you are sure of timing check some of these things.

Module in distributor
Springs on mechanical advance. May be to heavy
Crack in the cap
Possible vacuum leak around distributor
Electrical connections
Try setting total timing @ 32 to 36 degrees
Make sure distributor isn't a tooth out
 
#3 ·
stinger460 said:
If you are sure of timing check some of these things.

Module in distributor
Springs on mechanical advance. May be to heavy
Crack in the cap
Possible vacuum leak around distributor
Electrical connections
Try setting total timing @ 32 to 36 degrees
Make sure distributor isn't a tooth out
Im not sure of the timing at all thats the thing..
 
#4 ·
Dajerseyrat said:
Im not sure of the timing at all thats the thing..
You said early on you set ("dialed in") the initial timing at 12 degrees, I assume with vacuum disconnected and plugged?

To state a specific timing number, you must have a timing light, and should be able to find the other timing numbers by watching the timing pointer as you rev the motor. That will give you the advance at different rpms and when it stops increasing, that's your total timing.
 
#5 ·
68NovaSS said:
You said early on you set ("dialed in") the initial timing at 12 degrees, I assume with vacuum disconnected and plugged?

To state a specific timing number, you must have a timing light, and should be able to find the other timing numbers by watching the timing pointer as you rev the motor. That will give you the advance at different rpms and when it stops increasing, that's your total timing.

I may have set it with the vacume line on.. :drunk:

Any idea why some of the plugs would be rich and some decent??
 
#9 ·
A bad accelerator pump in the carb could cause the same symtoms. Is the vacuum diaphram in the distributor working?
Disconnect the vacuum line while timing is always a good idea but if you are on ported vacuum and at idle there shouldn't be any vacuum on the line anyway, unless you have the carb jacked up and have the idle speed screw turned in a lot to keep it running.
 
#10 ·
oldbogie said:
Lots of mayhem occurs between the carb and any given cylinder. There is no such thing as a "simple change" regarding what you did. You've struck in places that require a lot of massaging to get the tune correct.

Bogie
Yeah, Im concerned if i timed the car properly to begin with, this is the method I used.

1) found TDC on cylinder #1
2)rotated the crank till the timing tape showed about 10-11 degrees before TDC
3)dropped in distributor and wired the cap in the proper firing order
4)got car started and set the timing mark to about 12 degrees at idle

I ever did check the total timing, I guess I assumed it would be close to what I had previously which was 34-35.

Did I do this much correct? lol.
 
#11 ·
Dajerseyrat said:
Yeah, Im concerned if i timed the car properly to begin with, this is the method I used.

1) found TDC on cylinder #1
2)rotated the crank till the timing tape showed about 10-11 degrees before TDC
3)dropped in distributor and wired the cap in the proper firing order
4)got car started and set the timing mark to about 12 degrees at idle

I ever did check the total timing, I guess I assumed it would be close to what I had previously which was 34-35.

Did I do this much correct? lol.
Pretty much except for the assuming.
 
#12 ·
willowbilly3 said:
A bad accelerator pump in the carb could cause the same symtoms. Is the vacuum diaphram in the distributor working?
Disconnect the vacuum line while timing is always a good idea but if you are on ported vacuum and at idle there shouldn't be any vacuum on the line anyway, unless you have the carb jacked up and have the idle speed screw turned in a lot to keep it running.
car idles about 1000-1100 rpms.
 
#13 ·
F-BIRD'88 said:
The stock HEI timing curve is all wrong for the cam in the motor.
The HEi distributor needs to be set up to run nice with the big cam.

Exactly which cam you got?

The big cam wants much more than stock initial idle timing.
To allow this you have to shorten the mechanical advance travel 'from stock
+/-20deg to around 12deg.

This will allow you to set the idle timing at around 24 deg and the max will be 36deg at max advance.
Then reset the carb thottles. Install new spark plugs. Yours are fouled.

Vac asdvance will need to be set up and modifed/limited.

Most Stock HEI's are set up for a motor with EGR.
Your motor does not and will need a different vac advance curve.
A crane adjustable vac adv can w limiter is recomended.

search my posts on limiting the HEI's mechanical advance travel.

If the cam is very big (248@.050 ++) you may need to simply lock the mech advance out and run fixed 36deg timing (and some vac advance for cruising)

Once you get the distributor set up you can look for a manifold vac leak.
Actually even with this cam, the car always idled fine at 12-14 degrees. As far as the vacuum advance on my old distributor, I never hooked it up. How would you go about limiting the distributor curve on a distributor that you dont really know the curve to begin with..I knew I should have just bought the MSD digital one..lol
 
#14 ·
willowbilly3 said:
A bad accelerator pump in the carb could cause the same symtoms.
That is the first thing I thought of too! It really sounds like a "stumble" do to no fuel and a huge vacuum leak do to the butterflys being wide open.

Brian
 
#16 ·
Dajerseyrat said:
Yeah, Im concerned if i timed the car properly to begin with, this is the method I used.

1) found TDC on cylinder #1
2)rotated the crank till the timing tape showed about 10-11 degrees before TDC
3)dropped in distributor and wired the cap in the proper firing order
4)got car started and set the timing mark to about 12 degrees at idle

I ever did check the total timing, I guess I assumed it would be close to what I had previously which was 34-35.

Did I do this much correct? lol.
I've not looked up your cam as the data supplied requires a lot of hunting on my part to relate it back to a part number. But at .55 inch lift and 288 degrees this is a pretty big shaft. The intake you have I'm guessing is a Weiand 7530 Team G not a 17530 which is a laughing gas nozzle.

Lets get to igniiton timing: advance in amount and rate is a function of the mixture density inside the cylinders. That is how many molecules of fuel and air per cubic measure, has nothing to do with rich or lean mixtures. High density mixtures burn faster than low density mixtures. Therefore, low density mixtures need more time ipso-facto, therefore, more advance sooner in the rev range. Big cams cause low density mixtures under the torque peak and also feature low manifold vacuum. This drives two things: 1) You need more advance sooner, that means a lot more of base and a lot less in the adjustable portion to get at a total of around 34 to 36 with your heads and compression probably all in by 2000 RPM given your gearing the engine should tolerate this. 2) The cam timing results in very little manifold vacuum, therefore, running a vacuum advance is a waste of time as it just cannot provide enough advance far enough into the throttle opening and RPM gaining cycle to be useful, so just get rid of it and pump up the base; or is that bass we want to hear.

The Team G is a good but quirky intake, it takes either an accident of good fortune or a lot of tuning expertise to get it to work as well as it can. If you haven't, I'd recommend you install the plenum divider made for this intake, see the Holley site, I just don't remember its part number. This will help with the cross cylinder mixture stealing. You may want to get a thickness range of spacers to see if increasing plenum volume also helps stabilize the cylinder to cylinder mixture distribution. I'd use a divided spacer to maintain the separation of the right and left side cylinders as well as a couple open spacers to see which works best. Part of this can be attributed to the smaller capacity carb where the flow exiting the throttle bore has more velocity and is not as given to making short radius turns as would the slower velocity from a 850 CFM carb, so increasing the plenum volume will slow the flow making it easier for the flow to change directions.

For the cam and gearing your compression of 10 something is too low. The key would be to get the compression up around 11 to 11.5 for the street and 12 to 13 if all this sees is the strip. The low gearing lets the motor spin up easily so that balances part of the detonation problems and if you really crunch the squish/quench clearance between piston and head down to .035 to .040 inch you can get a lot more performance out of your gasoline's octane rating. Keep in mind the valve to piston clearance when doing this.

So the up front fixes are:

1) Run up the advance, with a base of about 15 to 20 degrees.

2) Reduce the amount of centrifugal to about 20 to 15 degrees all in by 2000 RPM

3) Eliminate vacuum advance.

4) Install the Weiand plenum divider for this intake.

5) Buy or make a selection of carb spacers with and without a plenum divider and test for best result. Jet changes and carb/ignition tune changes may be required.

6) Get the compression up at least a ratio and put the squeeze on the squish/quench clearance at the same time. Make sure that doing this doesn't collide valves and pistons.

7) Jiggle the cam timing, some advance will help clean up the bottom end rev range.

Bogie
 
#17 ·
Dajerseyrat said:
I think I am having timing issues on my motor. Ill give you the specs first:
355 SBC
10.3:1 compression
64cc heads
Lunati 550 lift 288 degree cam
Weiland 17530 manifold
Holley 750 CFM double pumper
456 gear
powerglide
Car weighs 2800lbs
28" tire


Ok, now this engine ran fine in my old camaro all I changed was the distributer to a stock type HEI with an MSD 6al2, and the carburator from a 850 dominator to a holly 750 street carb with choke.

Now the issue I am having is when Im crusing under 3500 rpms and stab the gas the car almost shuts off and falls on its face then takes off.Also does this from idle and in a neutral rev will occasionally backfire and stall, did that today actually..It also has a tendancy to break up until 400 rpms but I havent done a full WOT pull yet.

After a part throttle cruise When I pull the plugs the front 2 cylinders and rear 2 cylinders look very rich and all the middle cylinders look much leaner, they could be fouled cause I did have a stuck needle and float at first.

The carb is completely stock other than I went from 28 pump shot nozzles to 31 and it seemed to help a little bit.

The way I set the timing was #1 cylinder to about 10degrees BTDC and dropped the distibuter in, car started up fine, dialed in about 12 degrees timing at idle which is what I was running. I however have no clue what my curve would be or total timing.

Could someone explain the difference of initial timing, total timing and mechanical advance?? Im a bit of a novice at this...
You need to verify that what you are seeing at the damper and timing tab for TDC is actually TDC.
DETERMINING TDC will allow you to be sure the timing tab and damper are correctly indicating TDC.

MAKE A TIMING TAPE to see what the total timing is, w/o needing to use a dial back timing light. You can also buy a timing tape, they're not very expensive.

Using the engine w/o knowing how much total timing there is is a dangerous practice. If it's overadvanced it will be detonating. Detonation can kill an engine just a quickly as over revving it and if the exhaust is too loud for you to hear the tell-tail sounds of detonation, this can be a disaster in the making.

Initial timing is the timing at idle, w/o the mechanical OR vacuum advance adding anything. It is set by rotating the distributor while watching the timing marks w/a timing light.

Mechanical timing is the timing supplied by the centrifugal weights and springs under the distributor cap. This is also w/o adding in the vacuum advance but does include the initial advance, obviously.

The mechanical advance should by "all in" by no more than about 3000 RPM. This means that increasing the RPM will not cause any more mechanical advance to occur. Given the relatively light weight of the ride along w/the rear gear ratio, the 'all-in' RPM can be considerably lower than 3000 RPM- but that's a safe place to start from. It's always advisable to start out in safe territory then work your way up from there.

The mechanical advance rate is controlled by the weights and springs. Since aftermarket weights leave a lot to be desired, mostly the stock weights and center "cam" in the HEI distributor is retained and the springs are changed to quicken or slow the mechanical advance rate.

Total timing is the initial timing plus the mechanical timing added together. Like the other timing values, this is measured w/o the vacuum advance added in. It's important to set the total to a safe setting. Depending on the heads you have this might be anywhere from 32 to 38 degrees BTDC (before top dead center).

As far as using a vacuum advance or not, this will depend on a couple things.

An example for using one would be if the carb was idling poorly and you wanted to close the transition slots to regain control of the idle mixture using the idle mixture screws. Using a vacuum advance that supplies about 10° of added advance, hooked to manifold vacuum, will allow the primary throttle blades to be closed because the idle speed increases with the added timing. The vacuum advance needs to be adjustable (stock units were non adjustable) so it can be set to supply advance a couple inches of vacuum below the idle vacuum in gear (if an AT, in neutral if a stick shift) so the advance will be stable and all in while the engine is idling. This will help keep the carb primary blades from being opened too far to get the idle speed where it needs to be. If the blades are opened too far, the idle quality and off idle response will not be good. (they will become non responsive if the throttle blades are open too far- and opening the blades too far can happen w/a big cam because of the poor idle caused by the lean idle mixture Bogie mentioned above).

Another benefit is w/the added advance the engine will idle both smoother and cooler. If you do not have any issues like this, the vacuum advance can be eliminated.

AFA that intake manifold goes, you'd be better off w/an RPM IMO. AFA the stumble/bog goes, first thing be sure the accelerator pump is actually pumping fuel the instant you open the throttle. There needs to be a bit (0.015") of clearance between the pump arm and the pump lever when the throttle is wide open so the pump diaphragm isn't damaged.

But I believe that adding more initial timing will help this the most- 10 degrees just is not enough initial timing. But remember- adding initial timing also adds to the TOTAL timing. This has to be taken into account, in order to keep the total from being excessive- which can kill power and damage parts.
 
#18 ·
MARTINSR said:
That is the first thing I thought of too! It really sounds like a "stumble" do to no fuel and a huge vacuum leak do to the butterflys being wide open.

Brian
Im leaning towards this, i checked timing yesterday, 10-11 at idle and 36 at 3500 rpms.
And considering i had fuel pump issues and backfired throught the carb at least 3 times prior...lol
 
#19 ·
F-BIRD'88 said:
Tell me what cam you got in that motor and I can tell you how to set up the distributor.

12-14deg is not going to be enough for a big racey cam.

You can make this a very long drawn out thing ( vague responses lack of info) or a
short easy project, your choice.

What Lunati cam is it?
Lunati, see attached cam card. Thanks in advance. Also i found that all my valve lash was set to 22 thou on both sides.
 

Attachments

#20 ·
Dajerseyrat said:
Im leaning towards this, i checked timing yesterday, 10-11 at idle and 36 at 3500 rpms.
And considering i had fuel pump issues and backfired throught the carb at least 3 times prior...lol
Check/set the timing with the vacuum advance hose dis-connected from the vacuum advance canister and plugged.

Change the mechanical advance springs so that the total mechanical timing is around 36-38 degrees and is "all in" between 2500-3000 RPM. One light spring and one medium spring should work for that.

Increase the initial timing to 16-18 BTDC with no vacuum advance.

Limit the vacuum advance to about 10 degrees and after setting the initial timing hook the vacuum advance up to a full manifold source. The idle RPM will increase when you hook up the vacuum advance and should be lowered by adjusting the idle stop screw. Set the idle below 800 RPM in DRIVE. 650-750 is best. It will idle around 950-1000 RPM in PARK.

If you get a 'dead' spot when going to WOT from a stop, increase the accelerator pump nozzle two sizes and re-try.
 
#21 ·
Frisco said:
Limit the vacuum advance to about 10 degrees and after setting the initial timing hook the vacuum advance up to a full manifold source.
This is the part for some reason I am not fully comprehending. When you say limit the vacuum advance 10 degrees, what are you exactly doing to accomplish that? Perhaps Im just doing it with different terminology.
 
#22 ·
Dajerseyrat said:
This is the part for some reason I am not fully comprehending. When you say limit the vacuum advance 10 degrees, what are you exactly doing to accomplish that? Perhaps Im just doing it with different terminology.
ADJUSTABLE VACUUM ADVANCE CAN KIT- Crane #99600-1.
DETERMINING TDC will allow you to be sure the timing tab and damper are correctly indicating TDC.

MAKE A TIMING TAPE to see what the total timing is, w/o needing to use a dial back timing light. You can also buy a timing tape.

Check your PM box, Jersey.
 
#23 ·
F-BIRD'88 said:
once you have modified the distributor fort he timing curve I outlined and installed new fresh spark plugs of the correct type for the heads, gapped at .035" it will idle much cleaner and respond to the throttle much better.

The carb may need a bit further set up.
4.5" power valve. re set primary and sec throttles for correct idle transfer slot exposure at idle. on all 8 barrels.
should idle around 800-900rpm (neutral)

Again leave the vac advacen disconnected for now.
Just work on the mechanical.

what torque converter is in this car?
If you tell me what heads you got, i'll tell you what plugs to buy.
Im running a set or 64 cc World 1 sportsman heads, 3800-4200 converter and I usually run Autolite AR135's with no issue for the past 3 years. Like I said all of my issues started when I changed the carb and dizzy. As of right now, Im actually leaning towards the carb as being the culprit.
 
#24 ·
F-BIRD'88 said:
Another poster mentioned getting the plenum divider for your Weiand manifold. I don't got a whole lot of specific experience with this exact manifold but I do think the divider is a good thing for you.

If your manifold has the machined slot in the carb flange to accept the weiand divider:

Get one and try it. Should help low end mid range power and throttle response on this manifold. Especially for street driving.

Your holley 750carb is designed to work with a function PCV system.
The PCV should be hooked up to the 3/8" vac port on the carb base
and the opposing valve cover should be vented.
Oil control baffles in the valve covers under the PCV and breatehr holes is critical. Oil must not be allowed to migrate up the PCV into the intake manifold.

Eliminating the PCV on this street carb creates many idle /off idle tuning problems including throttle response.
Hook it up and use it on all street driven cars with carbs that have a 3/8" PCV vac port on the base.

The PCV is a critical part of the carb's idle circuit.

It's a lot lot easier to hook it up and use it as designed then do the required complicated idle circuit fuel recalibration to correct the idle/off idle ttransition circuit if you eliminate it.
I researched this prior and did add the PCV system for this reason.
 
#26 ·
Dajerseyrat said:
Uhhhhh, hehehehehehe, whoops... :p didnt ground it either..lol
While good grounds are important, if the engine is starting up, it's grounded.

What you want to be sure of is the clamp to the distributor body is able to ground it. This means no paint can insulate the clamp/bolt/distributor body. The HEI coil has its own ground strap or wire that is part of the wiring plug at the distributor.

HERE is one of many threads on grounding the vehicle.
 
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