RPM Drop From Nuetral to Gear

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mopowers said:
Would in improperly sized or blown PV cause a 'popping' off idle?? Seems like it would just make for a super rich condition at idle.
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It's prob prudent to get it to idle, responding to mixture screws, timing set, etc, and if it still pops,, that's next.. the process of elimination..

hope it helps
 
mopowers said:
Well, got the distributor back in. I verified that the reluctor wheel lines up with the pick-up at TDC, which it does. Tried setting the initial timing at everything from 15-30. It picks up RPMs the more I give it, even up to 30*!!!
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Of course it did. You don't have to believe me, but you just proved it to yourself. see post #34 and last paragraph of 45

Power valve if its working should be fine and won't effect idle but if the gasket or diaphram is damaged or incorrect, or the block gasket is leaking then the manifold vac could, nay, will, suck fuel from the bowl or a circuit.

Definately worth trying out your known good carb.

Did you map out your timing curve?

and I agree with those suggesting investigation of the ignition system
 
OK I think I'm getting somewhere. I swapped out the PV for a plug in the primaries and disconnected the PCV (and plugged the port). It fired right up and seems to run A LOT cleaner. I didn't have time to bring it up to temp before I had to leave, but I can tell it runs a LOT cleaner. It actually smelled like a hotrod should instead of just making my eyes burn. When looking at the PV on the bench, it appears to be fine. I tested it with a vac pump and it worked fine. I'm thinking maybe the gasket was leaking? Or maybe there just wasn't enough vacuum to keep it closed at idle. Either way, it's running better. I brought it up to 3500 rpm with any "popping."

Is it possible that the PCV was making it do that? All along I inadvertently left it connected. I guess I can reconnect it and see if anything changes.

I plan on getting it up to temp tonight and see how it runs. I'll keep you posted...
 
Mattax said:
Of course it did. You don't have to believe me, but you just proved it to yourself. see post #34 and last paragraph of 45

Power valve if its working should be fine and won't effect idle but if the gasket or diaphram is damaged or incorrect, or the block gasket is leaking then the manifold vac could, nay, will, suck fuel from the bowl or a circuit.

Definately worth trying out your known good carb.

Did you map out your timing curve?

and I agree with those suggesting investigation of the ignition system
Click to expand...

I believe you.

I haven't mapped the timing curve yet. I haven't been able to get it above 2,000 without it popping yet, so mapping the timing would be difficult. I do know I have 16* initial currently. I also know I've adjusted the distributor to allow for 16* mech advance. Vacuum adv. is and has been disconnected.
 
Good Stuff,, the watery eyes was the give-away...,,

I would hook up the PCV,, if anything, that was helping you offset the rich condition..

Then just get it to run decent, doing the basics,, fancy stuff later.

Might want to smell your OIL, and change it, - if it was flooding with a blown P/V,, then it may have emptied the bowls evertime it was shut off, down the manifold, past the rings, into the oil..

Also with contaminated oil,, the fuel vapors coming up the PCV will make it impossible to tune the carb accurately..

Just a thought

hope it helps
 
PCV can be a major pain when there's not much manifold vacuum at idle. Internally the PCV doesn't shut and so there's a big air leak at idle. There's a small company making an adjustable PCV. I haven't tried it, and I have some questions about it, but its a possibility.

You can put a small hole in a wood or metal plug as a substitute for testing. It won't help much while driving because the hole is so small. The 1970 Chilton's suggests drilling a hole 0.098" - .112" in the test plug for engines 276-350 cid.


IF you discover the problem really is the PV opening rather than a leak or the PCV, there are two possibilties.
1. Throttle blade so far open that the mains have started. I've never been able to get this to happen, but others have said they have.

2. The idle jets are very large and the main jets are very small. The idle jets (aka Idle Feed Restrictions) can be meaured if you have a set of small drills. They are usually, but not always, visible in either the top or the bottom of the idle wells.
With the diameters known, its easy to calculate the area of the IFR vs. the MJ. If the MJ is less than 4 times the area of the idle jet, it can effect downstream flow. In that case, the PV could effect idle. PV and MJ are in parallel. Idle circuit is in series with main circuit.
 
Mattax said:
A little history of why they had so many leakers at that time in is in this archived thread about power valves from the old Innovate forum.
Go down about 1/4 way down the page to post #9 by "Tuner"
Click to expand...

WOW Never knew that............

mopowers said:
OK I think I'm getting somewhere. I swapped out the PV for a plug in the primaries and disconnected the PCV (and plugged the port). It fired right up and seems to run A LOT cleaner......................................... When looking at the PV on the bench, it appears to be fine. I tested it with a vac pump and it worked fine. I'm thinking maybe the gasket was leaking?
Click to expand...

The thing that causes a PV to run rich at idle is not that it's open or closed, unless the engine / throttles are fast enough to be pulling fuel from the main venturies.

What DOES happen is that the DIAPHRAGM leaks, either a hole or improperly crimped, or over-tightened and damaged the valve. Realize that you have fuel sitting on one side of the diaphragm and "high vacuum" on the other. It does not take much of a hole to pull fuel through there.

You won't see this.......only be able to confirm by swapping valves.

There is the possibility that the gasket is leaking, but these are not usually a problem

DO NOT overtighten.
 
Mattax said:
PCV can be a major pain when there's not much manifold vacuum at idle. Internally the PCV doesn't shut and so there's a big air leak at idle. There's a small company making an adjustable PCV. I haven't tried it, and I have some questions about it, but its a possibility.
Click to expand...

This is the exact reason I suggest people disconnect and plug EVERYTHING connected to the carb when tuning the idle. Hook up things one at a time and see if the idle changes.
 
I have the 280-477 mp cam it would not idel very well with the stock 6.5 change it out to a 4.5 on booth carbs i have 600 and a 750 holley runs perfect now with a stock convertor in gear i have 7 inches of vacume,i think its the power valve as said.
 
Popping through the carb can be caused by weak secondary fire. Check coil primary wires to be sure they are on correctly-Reversed polarity. Also check cap and wires. Have seen a few with fuzzed up connections make you feel foolish and "stoopid". Check cylinder leakage. Bad intake seat/valve.You may also have a bad valve spring or guide.
 
my buddy had the same problem with his 340 ..it woudn't idle down and would dies you put in gear .. he thought it was mechanical (maybe a timing chain jumped) so He switched the whole engine with a spare 360 and still had the problem.. well after a months time and two engines and lots of new parts installed ... It turned out to be the simplest thing.. It was a spark plug wire that had burned thru on the exh manifold. it had a dead cyl.
none the less .............he felt pretty stupid after ALL that .
 
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