Jetting and timing question

[pearljam724]

I’ve been tuning my 360 for a while now. It wasn’t until I installed air/fuel gauge in the past month that I realized my jetting was too rich. I have a larger unknown cam. Even though my gauge reads that it was very rich. Prior to the gauge the engine responded a lot better to a lot of advance timing and a lot of gas ( 101/104 jets). But, my gas mileage is very poor at 9-12 mpg.
I believe the large jets were the reason why I could never time the engine below a lot of advance timing at idle ( roughly 40 - 50 degrees of advance. With or without vacuum hooked up. Does that makes sense to anyone ?
Now that I can tune the carb more accurately using the air/fuel gauge and installing smaller jets. It’s responding well to 89 primaries and 95 secondary jets. Does down sizing the fuel typically mean that most engines timing will need to be retarded to compensate ? I also think retarding timing will improve starting conditions when switching to smaller jets, is that right ?

 

[yellow rose]

What carb? I can't think of a reason you'd need jets that big unless it's a Carter or something. Either that or you are making HUGE power.

Also, what ignition and what timing?

 

[Mattax]

What carb? I can't think of a reason you'd need jets that big unless it's a Carter or something. Either that or you are making HUGE power.

Also, what ignition and what timing?

LOL. I was thinking it might be a 3 circuit 4500. It also could have garbage in the passages. Machining chips.

I believe the large jets were the reason why I could never time the engine below a lot of advance timing at idle ( roughly 40 - 50 degrees of advance. With or without vacuum hooked up. Does that makes sense to anyone ?

Should not be related on a 4150 or 4500

Now that I can tune the carb more accurately using the air/fuel gauge and installing smaller jets. It’s responding well to 89 primaries and 95 secondary jets.

the AFR gage will give you some idea of the the effect of any change. That's the main thing to use it for.
Be aware that change in timing can cause an apparent change to AFR because the combustion changed, not because the AFR changed. AFR gage uses a sensor that is effected by leftover O2.

Does down sizing the fuel typically mean that most engines timing will need to be retarded to compensate ?

Generally a less dense fuel mixture burns slower and so needs more timing.

I also think retarding timing will improve starting conditions when switching to smaller jets, is that right ?

The main circuit jetting has little to do with starting conditions for any of the major carburetor types we use.
If there is no choke, then they should have nothing to do with starting.

Maybe a few general relationships will be helpful.
Where the timing ought to be depends most on the compression and engine speed for a given engine and cylinder head.

from: The New Distributor (Session 136), the Master Technician's Service Conference

At starting and idle rpm, there is more time (in fractions of a second) for the combustion to reach maximum just in time to put maximum leverage on the crank.
As rpms increase there is less time for the burn before the piston is past the ideal postion. So the spark is initiated earlier in the crank motion.
However in most engines the burn improves as rpm's increase. That's why the mechanical advance is not always a straight line.

The density of the fuel and air mixture also effects the speed of the burn.
At steady cruising the engine has little load and can run on a leaner mixture.

from: Ignition System Analysis, Master Technician's Service Conference 1969

Adding throttle from stready cruising should lean out the mixture even further, until 1/2 or 3/4 throttle.
All of the lean combustion situations burn slower so need more advance, usually done with vacuum advance.

More than 1/2 or 3/4 throttle requires richer mixtures and vacuum advance (if used) needs to go away.

 

[pearljam724]

What carb? I can't think of a reason you'd need jets that big unless it's a Carter or something. Either that or you are making HUGE power.

Also, what ignition and what timing?

Edlebrock AVS Thunderseries. I realize the jets are way too big now. I have original type oil filled coil, original distributor with Petronix ignition added to replace points. Up until I lowered the jets. It wouldn’t run right if I retarded timing beyond 40-50 degrees advance at idle. It runs perfect there and I put 5k miles on it at that timing. But, it’s strange to me that people claim they time their small blocks at 15 to 19 degrees in advance at idle. I can’t get any where near that timing. I have to guess what exact degree it is based on inches because it’s so far advanced. It’s a good 3-4 inches off the timing pointer plate. That would put it roughly at 40 plus degrees in advance. If I try to time it near 15-20 degrees in advance just playing around with it, it nearly stalls. My engine is 9 to 9.5 compression at best. It’s not a big horsepower engine. But, it’s not a tird small block either. I haven’t retimed it, since installing smaller jets.

 

[Mattax]

The idle rpm is probably too high. Everyone says 'idle' but its almost meaningless without knowing the RPM.
Depending on the damper and pointer, only having 15* marked is common. Some engines less.
If the damper is a stock mopar diameter, buy a timing tape or two and stick it on.

As far as the large jets, with a Carter design the jet is the wide open throttle enriched condition. The jet + rod combination is the restriction on the main system. So together they effect AFR when cruising at 50, 60 or 65 mph or more. Exactly when they take over from the low speed system you'll find by systematic testing.

 

[Mattax]

One more possibility since its an unknown engine. The timing mark is correct? '69 to '70 timing mark switched sides or some other reason its not correct. Might just want to verify that. Bring #1 to TDC and check that the mark lines with zero.

Here's a couple mechanical timing curves based ion th efactory specs.
Notice how different timing is if measured 1000 rpm instead of 600 rpm.

Always measure timing with the vacuum hose plugged. Even if using ported vacuum, it can sometimes pull a little advance so its best just to plug it when measuring. In your case I'd leave it plugged until the base timing and carburation are figured out. Then reconnect it.

 

[pearljam724]

The idle rpm is probably too high. Everyone says 'idle' but its almost meaningless without knowing the RPM.
Depending on the damper and pointer, only having 15* marked is common. Some engines less.
If the damper is a stock mopar diameter, buy a timing tape or two and stick it on.

As far as the large jets, with a Carter design the jet is the wide open throttle enriched condition. The jet + rod combination is the restriction on the main system. So together they effect AFR when cruising at 50, 60 or 65 mph or more. Exactly when they take over from the low speed system you'll find by systematic testing.

I understand how to tune the carb in relation to the jets, metering rods, springs, etc. The reason why I installed those larger jets was the car actually runs a lot better. But, it’s pretty rich based on the gauge I installed much later. To no surprise, it’s a gas hog too. 9-12 mpg However, the gas mileage didn’t change more than a couple miles per gallon when it had the original 95/98 jets. Since installing the gauge. I’ve found the secondary jets are way too rich. Which makes sense, because all carb manufacturers ship carbs jetted too rich for liability concerns. I dropped to 95 primaries and 89 secondaries. It’s running good there and gauge is confirming the projected ratios that anyone would want. I always set my idle at 1000 rpm according to my tach. Maybe that’s a problem. I only set it at 1000 because the idle sounds better to me and 8-900 seems a bit too low. Even though I do understand that’s where all engines are suggested to idle. I just think that’s too low.

 

[pearljam724]

One more possibility since its an unknown engine. The timing mark is correct? '69 to '70 timing mark switched sides or some other reason its not correct. Might just want to verify that. Bring #1 to TDC and check that the mark lines with zero.

Here's a couple mechanical timing curves based ion th efactory specs.
Notice how different timing is if measured 1000 rpm instead of 600 rpm.
View attachment 1715479951

Always measure timing with the vacuum hose plugged. Even if using ported vacuum, it can sometimes pull a little advance so its best just to plug it when measuring. In your case I'd leave it plugged until the base timing and carburation are figured out. Then reconnect it.

It’s a 72 / 360. It has a timing pointer on both sides of the engine passenger and driver. It doesn’t matter which side you choose to use. Its the same on both sides. The timing is correct, original dampener and it hasn’t slipped.

 

[dukeboy440]

It’s a 72 / 360. It has a timing pointer on both sides of the engine passenger and driver. It doesn’t matter which side you choose to use. Its the same on both sides. The timing is correct, original dampener and it hasn’t slipped.

Did you also adjust for daylight savings time?

 

[pearljam724]

Did you also adjust for daylight savings time?

View attachment 1715480666

You’re thinking my distributor is turned 180 degrees out ?

 

[dukeboy440]

You’re thinking my distributor is turned 180 degrees out ?

No. I'm saying you can't have two timing indicators and BOTH be correct...

 

[MileHighDart]

If you want to actually see what your initial timing is set at you need to get a dial back timing light.
I picked up a nice one at a pawn shop for $13.00