mopowers
Well-Known Member
Wow, it must be beer thirty. That was a total brain fart on my part.
RPM Drop From Nuetral to Gear
- Thread starter mopowers
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Wow, it must be beer thirty. That was a total brain fart on my part.
mopowers
Well-Known Member
The mechanical advance moves very smoothly.
I've looked all over. No vacuum leak that I can see.
I set the float levels to where fuel is level with the bottom of the sight screw hole with it running. With the screws out, I can bump the car's fender and have a tiny amount of fuel spill out.
Interesting you brought up the ''popping' sound. I have noticed that as I bring the RPMs up slowly in neutral, it does pop a little out of the carb. It does it when I rev the engine quickly as well. What could this be???
All 4 of the mixture screws will kill the engine if I run any of them in all the way. I started with them set at the factory spec of 3/4 turn out (as per Holley tech line). The engine runs best at 7/8 turns out. Smells pig rich though.
I have flipped the set screw for the secondary throttle position so I can adjust it from up top. I have the secondary blades set the same as the primary. It seems as though if I open either the primaries or secondary blades than it's already at, it's too much and I can't get any response from the idle mixture screws. That's why I'm curious if drilling small holes in the blades would help. I do have extra throttle blades to play with.
At this point, what's the next step??
73Scamp318
Well-Known Member
I think that if the distributor is not stock, your mechanical advance springs may be too light. It might be if the advance curve starts in below your idle speed, when you put it in gear, your RPMs drop, you lose advance, and the RPMs continue to drop until it stalls.
I'd give some heavier advance springs a try before drilling on the carb.
I'd give some heavier advance springs a try before drilling on the carb.
mopowers
Well-Known Member
Can I get heavier springs at the autoparts store? Maybe in the performance section? Anyone have a part number?
mopowers
Well-Known Member
Do generic Mr Gasket chevy distributor spring kits work or are the springs different?
C130 Chief
Mechanical Genius
I don't normally advocate connecting the vacuum advance to a manifold source, but in your case (lumpy cam) it might help. It won't cost anything to try it out anyhow.
Also, don't get too hung up on the timing number until you've verified your timing mark with a piston stop. The best selling product in the Mopar Performance catalog was timing tape for a reason.
Also, don't get too hung up on the timing number until you've verified your timing mark with a piston stop. The best selling product in the Mopar Performance catalog was timing tape for a reason.
vac leak or carb. you don't need anymore intial timing, you are borderline high allready!
In neutral, there's no load, so you can keep adding timing and the engine loves it. You can then lean it to match and the engine loves that too. Then you put it gear and it goes - Ugh, the load is killing me. It has no power when set that way.
Try setting the timing around 16 to 18. Then, using your vacuum gage, adjust the fine idle fuel mix for best vacuum. or best rpm if you don't have a vac gage. On an automatic, in theory set fuel mix for best vacuum in gear. In real world, it's easier to set in neutral, then richen it about 1/8 turn and check it in gear.
Since there's a bunch of unknowns in the engine, especially the cam, you'll probably have to experiment with a different initial timing and see which has the least drop in rpm as you go into gear.
I do agree that it if the timing is changing with rpm, that makes it a little more difficult. You can figure this out by with a timing light and tach. Start as low as it the engine will go and the measure timing 100 or 200 rpm. Personally I like to chart the curve out all the way until it starts advancing.
The other issue you may be fighting is the throttle positions. As you open the throttle you not only add air but you feed in more fuel from the idle circuit. How did that happen? Because the throttle blade moves up the transition slot. The idle circuit feeds both the transitition slot and the idle hole. Once the throttle is too far open, there's not enough transition slot left to help the off idle. So it is very important when adjusting idle speed not to let much more or less than .020-.035" of the transition slot showing beneath the primary throttle blades. This is explained with diagrams in Urich and Fisher's Holley Carburetors and Manifolds.
Personally, I've found its worth the time and trouble to take the carb off and set the transition slots so they look square. At the same time, I take note of how many 1/4 turns of the idle speed screw from not touching to .040" of the slots are showing. That way I'll know later on if I'm getting out of range. The purpose of drilling holes in the blades is to get the throttle blades back in position. Cracking the secondary blades a little does about the same thing. This also explained in Urich and Fisher's book.
Recap: Establish what rpm the timing starts to mechanically advance.
Set/check the fuel level as someone else mentioned.
Set the throttle blades so the transistion slots look 'square'
Try adjusting for best POWER at idle using initial timing around 18 degrees, then again say 16 and 20.
PS. You can get by with a light advance spring in the dizzy. It means you have to be willing to tweak the timing back to your initial every time the rpm changes until you get where you want. Advantage is easy starting and one less project. Disadvantage is its another variable to deal with while tuning, and the idle may wander a little more than if it had stronger or shorter spring (or shorter slot).
Try setting the timing around 16 to 18. Then, using your vacuum gage, adjust the fine idle fuel mix for best vacuum. or best rpm if you don't have a vac gage. On an automatic, in theory set fuel mix for best vacuum in gear. In real world, it's easier to set in neutral, then richen it about 1/8 turn and check it in gear.
Since there's a bunch of unknowns in the engine, especially the cam, you'll probably have to experiment with a different initial timing and see which has the least drop in rpm as you go into gear.
I do agree that it if the timing is changing with rpm, that makes it a little more difficult. You can figure this out by with a timing light and tach. Start as low as it the engine will go and the measure timing 100 or 200 rpm. Personally I like to chart the curve out all the way until it starts advancing.
The other issue you may be fighting is the throttle positions. As you open the throttle you not only add air but you feed in more fuel from the idle circuit. How did that happen? Because the throttle blade moves up the transition slot. The idle circuit feeds both the transitition slot and the idle hole. Once the throttle is too far open, there's not enough transition slot left to help the off idle. So it is very important when adjusting idle speed not to let much more or less than .020-.035" of the transition slot showing beneath the primary throttle blades. This is explained with diagrams in Urich and Fisher's Holley Carburetors and Manifolds.
Personally, I've found its worth the time and trouble to take the carb off and set the transition slots so they look square. At the same time, I take note of how many 1/4 turns of the idle speed screw from not touching to .040" of the slots are showing. That way I'll know later on if I'm getting out of range. The purpose of drilling holes in the blades is to get the throttle blades back in position. Cracking the secondary blades a little does about the same thing. This also explained in Urich and Fisher's book.
Recap: Establish what rpm the timing starts to mechanically advance.
Set/check the fuel level as someone else mentioned.
Set the throttle blades so the transistion slots look 'square'
Try adjusting for best POWER at idle using initial timing around 18 degrees, then again say 16 and 20.
PS. You can get by with a light advance spring in the dizzy. It means you have to be willing to tweak the timing back to your initial every time the rpm changes until you get where you want. Advantage is easy starting and one less project. Disadvantage is its another variable to deal with while tuning, and the idle may wander a little more than if it had stronger or shorter spring (or shorter slot).
Inertia
Well-Known Member
While you have the carb off, double check that the base gasket matches both the manifold and carb, with lots of overlap, - some manifold/gskt combos expose a small leak at the secondaries.. and when running, with vacuum, opens it more..
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Mattax is correct.
My understanding of it is this: When you have a big cam with excessive overlap
it allows more of the exhaust residual to contaminate the incoming charge so it displaces both the clean air and fuel required for a good stable combustion event.
This leads to a lean enviroment that requires you to run alot of initial because lean mixtures burn slower. You can try to richen up the idle to see if it helps.
Valve size here also plays apart. The bigger the valve the greater the valve curtain area and the more reversion you will experience.
I just took out a cam that was 256 @.050 and had over 320 advertised duration and 100 degrees of overlap in a 360. The car ran fine with the smaller 318 heads and small valves and the idle was managable but as i put bigger valves in it got worse. When i went to 587's with a bigger valve again it became too much so out it came.
I would check to see what cam you have if you havent already. Make sure your ignition system is up to scratch and check you cylinder pressure.
Hysteric
My understanding of it is this: When you have a big cam with excessive overlap
it allows more of the exhaust residual to contaminate the incoming charge so it displaces both the clean air and fuel required for a good stable combustion event.
This leads to a lean enviroment that requires you to run alot of initial because lean mixtures burn slower. You can try to richen up the idle to see if it helps.
Valve size here also plays apart. The bigger the valve the greater the valve curtain area and the more reversion you will experience.
I just took out a cam that was 256 @.050 and had over 320 advertised duration and 100 degrees of overlap in a 360. The car ran fine with the smaller 318 heads and small valves and the idle was managable but as i put bigger valves in it got worse. When i went to 587's with a bigger valve again it became too much so out it came.
I would check to see what cam you have if you havent already. Make sure your ignition system is up to scratch and check you cylinder pressure.
Hysteric
mopowers
Well-Known Member
Thanks for the input. I have verified TDC on the balancer. It's 1 degree off, but I have the balancer marked correctly to compensate.
mopowers
Well-Known Member
Thanks for the input. First on my list is trying to get this thing to idle in gear so I can check the timing and verify it doesn't change from when in neutral. I'll try to find a strong advance spring first, just to hold the advance a little stiffer to rule out that variable.
I've definitely got a handle on the throttle position of both primaries and secondaries. I started with them square and have opened them a small amount equally to get it to idle in neutral. Every small adjustment I make, I make it to both sides as per the Holley tech line.
mopowers
Well-Known Member
That was one thing I have checked. Everything seems sealed very well. I'll spray carb cleaner around the base while it's running just to triple check.
Thanks!
mopowers
Well-Known Member
Thanks for the input guys. I've definitely got a plan of action for the weekend when I get a chance to get my hands dirty.
It's interesting how only a couple people suggested the convertor's to tight...
Keep the suggestions coming! I appreciate it.
It's interesting how only a couple people suggested the convertor's to tight...
Keep the suggestions coming! I appreciate it.
I have about 8.5-9 inches of vacuum in neutral, which drops down to about 6-6.5 in gear. If the engine is cold it drops even more. I installed a 4.5 power valve to be safe. With your stock convertor you could be pulling it down even further. Might want to try a bit smaller PV.
mopowers
Well-Known Member
Can anyone explain the 'popping' out of the carb as I bring the RPMs up?? Is this caused by a lean condition, or a rich one? Too much overlap?
I always thought engines did this when they didn't have enough timing advance. That's definitely not the case here.
I always thought engines did this when they didn't have enough timing advance. That's definitely not the case here.
mopowers
Well-Known Member
That's definitely on my list. I've got a 4.5 in my carb box I plan on trying.
Thanks, sir!
Sounds like its igniting before the intake valve shuts. Almost certainly the immediate cause is timing too advanced.
I'm all for measuring timing rpm relationship before opening up the dizzy. The springs effect the rest of the curve too, so I like to reduce the number of times I have to open up and modify. If your engine likes a quick curve, then filling in the inside end of the slot is the better way to get the initial. Just my preference. There's lots of ways to skin that cat.
Torque converter 'tightness' should have little impact at idle speeds and such low torque. The 'stall speed' goes down with less torque applied.
Power Valve has no impact on idle in most cases. The primary restriction for the fuel is the Idle Feed Restriction. It is usually much smaller than the main jets. When the area of the upstream restrictions are 4 times larger than the IFR it really doesn't matter if upstream flow is increased.
The 'square' transition slot is just an easy starting point. The best opening will vary a little depending on slot width, idle feed and air bleed restrictions, and engine.
Hysteric - Thanks for sharing your experience with that. Very interesting.
I agree with your explanation about the impact of exhaust gas dilution. In addition, my opening paragraph is true even for stock cams. In neutral, power to overcome internal friction, the alternator and the water pump is all that is needed. Lower power conditions allow for leaner mixes, which go hand in hand with more advance for the reason you explained. But it isn't enough power to do anything else. When I was just learning to tinker with cars, I had stock 6 cylinders and its the same deal (except I didn't understand why it would seem better with more and more advance in idle and leaned out, and then lousy when put in gear). Not 'til Shrinker explained it on the old Innovate forum did I really understand.
mopowers
Well-Known Member
Thanks for the input! If I can get this thing to idle in gear, I'll be checking the timing and vacuum while in gear. Hopefully I can locate some stiffer advance springs to play with.
moper
Well-Known Member
Replace the power valve with one lower and then re-tune the carb completely. I'd go even lower than 4.5 because you may not have that until you dial the carb back in. I think your timing's fine for now. IMO it's the carb that's the problem more than anything else.
Inertia
Well-Known Member
It's even more interesting that none of the known trans gurus,, haven't agreed,.. perhaps because it's not a converter issue,, ???
I've also run cams over 300 duration,, with stock converters, and they would idle in gear,, - a little higher than normal in neutral,.. but nowhere near your problem..
hope it helps
Inertia
Well-Known Member
^^^^ try this,,
Of course, putting that new manifold might be the cure in itself..
Have the intake manifold locator pins been removed.. if not,, make sure the manifold has holes for those pins..
What kind of popping? Is it very rhythmic and popping every time a cylinder fires? If it is, you have something keeping an exhaust valve shut. Wiped lobe, collapsed lifter, bent pushrod, bad rocker.
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