one for all you motor heads..??

[tirefryin-Bob]

Here's the skinny, have a 440 +.060 runs real rough at idle (Almost stalls) but will smooth out above 1200 rpms. has a rebuilt carter 750 AVS, stock compression pistons, mopar hp/road runner cam, stock duel exhaust, rebuilt stock 906 heads, mopar elect. ignition. timing @ 10' btc.
Must be a vacuum leak right?? "Wrong", well lets say if it does I can't find it, I sprayed carb cleaner while it was running all around the carb & intake with NO change in eng. rpm or the rough idle.... I hooked up my vacuum gauge to the intake and here's were it through me for a loop, only 5 inches Hg of vacuum !!!! So far everything I've read points to a severe vacuum leak or valve timing (not ignition timing) being off, well since I can't locate a leak must be valve timing. My first though was bad or jumped a tooth on the timing chain, so off comes the timing cover only to find the double roller chain is still tight and spot on TDC with gear timing marks pointing right at each other..
So if I don't have a vacuum leak and the valve timing is correct, What ELSE could be causing such a low idle vacuum???
above 1200 rpms the engine vacuum climbs up to 16 inches inches Hg.
Please HELP!!!

 

[daredevil]

Is this the problem that some drill holes in the throttle blades for? Go to Diamondback racing engines and read the white pages.

 

[tirefryin-Bob]

Yeah Buck, I thought about the air/fuel mix before and tried adjusting them but to no avail - nada... I also removed the top half of the carb to take a look at the fuel level thinking the same thing that maybe it was a fuel pump or float issue but the carb bowls were full and level!!!
DareD, I'm not too sure drilling a couple small holes in the primary butterfly blades will help in this case, it's just a stock HP cam. I've done this in the pass with high lift / long duration cams for extra vacuum, even then those engines would still pull 12-16 inches of vacuum. this 440's only pulling 5 inches in stock form...Grrrrrrrr:fit:

Any possibility that this original rebuilt carter 750 AVS is the issue??? Thinking of putting a new edelbrock 750 AFB just to see if that'd fix it???? any thoughts?

 

[TheDemonator]

Have you checked the distributor? Is the vacuum advance hooked up? Is it hooked up to the correct vacuum pressure port? If so... is the advance working correctly?

Distributors can cause many problems!

The timing light can tell you that you are right on the money... but the vacuum advance can through it all out the window.

Try a "borrowed" distributor to be sure... or perhaps a lighter set of advance springs may be in order?

Remember... it takes fuel, air and fire to run... so it has to be either the carb or the distributor... especially since you can't locate any leaks anywhere.

 

[tirefryin-Bob]

Hey guys, Thanks for some more suggestions. Yeah Demonator, I plugged off the dist. vacuum advance to take that part out of the equation. No difference!!! It may be possible that the ECU could be causing this??? It's a fairly new "orange" MP unit, Thoughts..
Buck, I am starting to lean to it being the Carb. even if it's a rebuilt. The possibility of a original 38 year old carb has maybe seen it's day. I did pick up a NEW edelbrock 750, and I just got all the gaskets to put it back together. I'll up-date after installing new carb!!:banghead:

 

[black64]

Have you checked vac hoses like power brakes or heater vac hoses for cracks?

 

[tirefryin-Bob]

yeah, I've removed and plugged all and any vacuum hoses / ports going to the carb or intake manifold. pcv, pist. adv., brake booster & heater / vent vac hose, etc. - (NO change)

 

[TheDemonator]

This may be a stupid question, but does your timing gear set have multiple key ways in the gears?

If so... they may not be indexed correctly.

If they have only one key way... then that can be eliminated.

And old school trick is to put a vacuum gauge on the car then rotate the distributor while the engine is running until you get the highest vacuum reading at idle... then back it off just a bit.

Then check to see what your timing marks say it is running at. If your engine idles better, and accelerates properly, then it would indicate improper timing gear alignment.

If it idles better with acceptable vacuum, but will not accelerate properly, it would most likely be carburation... or an improperly adjusted vacuum advance.

Either way... it should be easy to troubleshoot in a matter of minutes!

I know... you are saying to yourself, "yeah, right"!

 

[sumbit]

Did you just put this thing together or did it used to run alright? Have the heads been milled or the block decked or both? Swap carbs but try to get/borrow one thats about right for your motor. Good Luck

 

[Meep-Meep]

Some of your intake push rods are too long and holding the valves open just slightly, and this can be verified with a compression check. It's possible the intake valves have been sunk into the head more than what's acceptable, thereby changing the push rod length requirement. Get about .050" of shim stock or whatever and stuff it under the shafts to prove the concept. If it works then get some adjustable rockers or push rods. With adjustable rockers you want 1 turn from zero lash.

 

[sumbit]

Your 906 heads came without hardened seats. Were new hardened seats installed when they were rebuilt? If so you should be ok there. Good Luck

 

[tirefryin-Bob]

Yeah harden seats were installed when the heads were rebuilt. Heads were only milled .008. on a side note I picked this car up about a year or so ago and it always ran rough @ idle - but smooth as butter on the road, it wasn't until it started over heating this summer that I started looking into the cause and decided then to go a head and take the motor down to the block, I had the heads redone with a new hp cam and timing chain/gear install, installed a new 180 T-stat and fan clutch eliminator, I'm sure the over heating issue is resolved but not this rough idle with no vacuum...
Interesting thought with the push rod length Meep, They're stock rods so if that's a possibility then the only thing I can think of that would cause that would be the block being milled a great deal???

 

[Meep-Meep]

You said you checked for vacuum leaks. How about the brake booster (if you have one). Pinch off the hose while it's running. There's really not much left to check.

Otherwise, you owe it to yourself to do a compression check. It won't take that long and you will get a lot of info. I had the issue with my 66 Ford truck (352 FE engine). It would run fine cold but start idling like crap when it warmed up. The only thing left to check was compression, so I drove it in the garage and started pulling plug wires while it was idling to find the affected cylinder/s. I shut it off and immediately did a blow check and found several low cylinders. That's when it hit me. I threw some washers under the rocker stands and it ran perfectly. All this was done within the hour while it was acting up. An engine with 75 lbs of compression in a cylinder or two, with the rest being in the 150 range, will usually run fine going down the road but have a miss at idle.

 

[tirefryin-Bob]

UPDATE: New edelbrock 750 carb AFB has NO change. Still has only 5 in. of vacuum and will hardly idle. going to do a compression test next, if that checks out okay then I'll look into a new distributor and swap out the orange ECU. Everything else beside the block, pistons & rings have been replaced or rebuilt!!!!:drowning:

 

[sumbit]

Any chance you could have some sort of an exhaust blockage? That'll kill your vacuum at idle too.Is the stock heat riser still in the RH exhaust manifold? If the heat riser is restricting the exhaust your vacuum and idle will suffer. Just a thought....

 

[tirefryin-Bob]

Compression test complete.. They look pretty low to me, all cylinders were between 105-110psi.
Isn't normal more like 145-165psi??? and if so could this be causing the problem?? :mad_6: (see original thread post)..

Sumbit, I haven't yet disconnected the exhaust..Not sure if that's it with it running so good at highway speed..

 

[sumbit]

That compression is on the low side, but if they are all close to each other that's ruling out one or two open valves. The 5 inches of vacuum is what's whack here.

 

[Meep-Meep]

Agreed, low compression pressure, but all even, will run smooth. Also, 105-110 is waaaaay to low for a Magnum type engine (but good for air cooled VW with 7.5:1 CR!). My 8.0:1 CR 911 Porsche engine blew 150-160 PSI and that's a result of camshaft timing.

The low vacuum is curious indeed and points to only a few possibilities. Still wondering about the valves, but since the CR is so even it would be hard to believe they are all barely off the seat the same tiny amount. You could try the compression check under the same conditions as the previous check but with the rocker shafts loose and off the stands by .050" or so (or you might put a piece of shim stock under it so you can snug the bolts down and keep things from flopping around while the engine cranks over). Probably only need to verify on one cylinder. If the number goes up then guess what!!?? Just to make sure, you did the comp check with the throttle open and all plugs out, correct?

.....Or do you know for sure TDC marker is TDC? Perhaps the timing is really in the tailpipe! Try to find TDC with a rod through the spark plug hole and check against the timing marker.

.....Or (and I'm starting to think this is a good possibility) is the cam timing way off causing the intake valves to close very late. A tooth off on the chain perhaps??

 

[tirefryin-Bob]

Man 8.0:1 is waaaay toooooo low.:sad:. I've thought about shimming the rocker shafts like you said Meep just to verify, like you said - hard to believe that'd be it with all cylinders being the same. And yes I've doubled check my TDC & timing chain gear marks, There right on!!!
Starting to think maybe I inherited a bored out 440 that had been rebuilt with low compression pistons and also had a past over heating issue that may now have poor piston ring seal..Remember I starting looking into this rough idle that it's always had when it overheated when returning from a show, that's when I discovered a 160 T-stat..:mad_6:. Hmmmmm - fishy
If shimming the rockers has no change I'm thinking of dropping in a different short block with new pistons & rings.. I think this .060 over 440 block that now looks to have real low compression pistons, bad rings and hardly any vacuum @ idle has seen it's day and has come to an end..:gah:

 

[Meep-Meep]

Before you tear it down try a leak down test. If you suspect the rings this will pinpoint it.