I seem to be stumped for the moment. I have a 66 SS chevelle with a brand new engine. It is a 454 block with a 427 crank and solid cam.

The problem is that the idle will stay right at 850 then creap up to 1100 and then it will drop down to 500 and want to quit. It only changes when coming to a stop. It will idle fine for a wile then Ill come to a stop and it will want to die sometimes and others it will be idling fast.

I have changed the PCV valve, the distributer is a brand new Accel GM HEI with a hypertec coil. The plugs are new as are the plug wires. the carb is a freshly rebuilt Holly 780 4150 (stock L-78 carb)with 3.5 power valves and corect float level. The fuel pump is a 110 gph @6.5 psi Holly. The carb seems to be working within the range of the idle mixture screw adjustment (I can get it to run lean and rich).

I noticed that if I put the car in nutral or with my foot on the clutch, rolling down the street, when I put on the brakes, the idle will drop off.

The only thing I can think of at this point is maybe the fuel pump is suplying to much pressure(i dont have a regulator between the pump and the carb) with the higher head pressure at the pump when I apply the brakes.

Any sugestions on where to look would be greatly apreaciated.

Will

I have seen this many times before,it's likely a problem between your intial timing a little on the retarded side along with the mech & or vac advance just on the threshold of kicking in all due to the lower idle vacume from a longer duration perf cam.
This is due to having to jack up the carb idle to a point where it starts to transition off idle along with the mech/vac advance just begining to come in or go out if comming down to an idle.

Try this,if you have a perf cam bump up the intial tining to approx 14-16 deg at approx 600-650 rpm(vac advance plugged),then rev it up to check total mech advance which should be 36-38 deg total (vac advance still plugged),then hook up the vac advance to a ported port on the carb and not a full vacume at all times port from the intake. Now your idle speed should need to be reduced some,reset the idle mixture and you should be all set.

If you do all this and it does not help let us know so we can dig deeper into other areas to correct it.

It sounds like your setup is ok including the pwr valve which can handle a low vac idle and just a little tuning will work.

Let us know how you make out.

Scott

When the carbs were rebuilt, were the floats replaced? This can be an issue with either brass or composite floats, and I always recommend replacing the floats when doing a rebuild. Composite floats will become saturated over time, and fuel will eventually find its way inside a brass float as vibration and electrolysis eat away at the soldered joints.

The floats are brass and I inspected them for wear and damage and found them to be ok. I got the carb from PAW around 10 years ago, Holly was remanufacturing carbs for original rare cars. Sence my car was a matching numbers L-78 car (stil got the block with a broken #1 bore) I bought one.

Scott, this brings up a question about ported ports on the carb. On my carb there are 3 ports, one for the PCV valve, one for the choke diaphram, and one that the vacume advance hooks up to. The one for the choke is in the front of the carb was hooked up from the factory, the one for the PCV (3/8") is right to manifold pressure. All are in the bottom plate. Is this the right port for the advance diaphram?

Im going to check this stuff out tommorow and Ill post what I have now and what happens when I change it.

For the record my cam specs are as follows.

cam lift int. .320 exh .335
valve lift int. .544 exh. .570
lobe ctrs. int 109 exh.119
adv. dur. 300int 310 exh
.050 dur 238 int. 248 exh

Thanks again for the input

Will

thr062, try installing a 'vent whistle' in your rear metering block. Holley sells two lengths, use the shorter one. I'd type out an explanation but if you just think about it for a minute, you'll figure it out.

Good luck,

Louie

Vacum leak will cause idle to up and down. And as Louie was saying about the shorter vent tubes, I have the short vent tubes and cut the tops of the tubes on a 45 degree angle so my low air cleaner lid will not block off the vents tubes. Dave

Actually, I was referring to the plastic 'vent whistles' that attach to the metering block and extend horizontally into the float bowl above the float, not the metal vent tubes that are pressed into the main body and extend vertically up from just slightly above the accelerator pump nozzles.

Louie

LouieHammel:
Actually, I was referring to the plastic 'vent whistles'
Hi I think Holley calls them " Vent Baffles" I installed them in my carb a few years ago, I didn't relize there was two different lengths. I have the long one's I think. Correct me if I'm wrong but these vent whistles or baffles stop the gas in the fuel bowl from splashing up and out of the metal vent tubes on cornering & braking. What is the diffence in between the short ones & the long ones?

[This message has been edited by dsr (edited 11-07-2002).]

Sounds to me it could be a vacuum leak or a bad Master cylinder. I had one go bad awhile back...when I hit the brakes the car would try to shut off.

Since you already have vent baffles in your metering blocks that eliminates that question. The long version of the vent baffle is for center-inlet fuel bowls and the shorter one is for side-inlet bowls. Not knowing which style bowls you have, I suggested the short ones because they will fit either bowl style.
Yes you are correct in stating that the bowl vent baffles help prevent fuel slosh into the venturis during hard launches and hard braking.
I think that CHELKAMINO had a good idea about checking the condition of your power brake vacuum diaphragm. When it leaks, it is a big leak.

Good luck,

Louie

Well after a week out of town, I finally got around to messing with my car. Scott, I found (just like you said) my initial timing a litle retarded, around 10 deg. I set the timing to 16, initial and got around 37 total (still with vacume pluged). I pluged it back in and reset the idle. I havent had a chance to go out and test it as it is raining cats and dogs and I would be having way too much fun not to get pulled over.

Interestingly enough , when I set the idle to 650 RPM just the "lump" in the idle was enough to activate the mech. advance a bit. I have one black spring and one silver spring in the dist (strongest and second strongest).

Ill keep you posted and thank all you guys for the help.

Will

[This message has been edited by thr062 (edited 11-09-2002).]

Well I had a chance to go out and try it out. It is still doing it. One thing I found is when it is trying to die, if I shut the engine off, then start it up with out touching the gas petal, it idles normal. Im still open for ideas.

Will

Will,if you shut the car down and then it idles down ok after you refire it sounds like you have a problem with either the vac advance (or mecahnical as mentioned below) line hooked to a full on vac port at all times,so try to find a carb vac port that increases in vacume as the throttle is opened up with no or very little vacuume at idle.

Also,your mechanical advance can be either hanging up,the springs are too weak,the weights are too light and or heavy. Any imporper mix of these items can make the mach/advance still be active at a low speed not being able to retard/settle back down from engine speeds above idle so get the stock weights/springs and start from there to get the idle corrected. If this fixes it then you can slowly re-tune your mech advance to come in quicker with the lighter springs/weights (36-38 deg total no vac) to get a happy medium point before you run into the surging up/dn idle again along with staying away from detination too.

Check your breaker plate too to see if it's hanging up at times.

Another suggestion is to start the car and drive it untill its fully warmed up and shut it down. Then restart it so it will hopefully return to normal idle as you said it dies,then unplug the vac advance and rev it up a few times to see if it returns to a normal idle,if yes its your va advance which needs to be hooked to a vac source thats not active at idle. If your carb does not have a vac source thas not of at idle you could leaving the vac advance pluged off for now and dial in a couple more deg initial timing to help eliminate a soggy feel from idle just as long as you don't detinate in the process.

If it does not return to the normal idle then its likely your mech advance thats either hanging up or needs to be dialed in correctly as i mentioned above.

Let me/all of us know how you make out.

This is lots of fun being able to help/chat with (chevelle) folks on the west coast when i am up here in NY state on the east coast,way cool.

Good luck

Scott

[This message has been edited by SWHEATON (edited 11-10-2002).]

[This message has been edited by SWHEATON (edited 11-10-2002).]

[This message has been edited by SWHEATON (edited 11-10-2002).]

Well the dist. is a brand new Accell unit. The vac. advance is hooked up the the port on the carb. It was doing it with the old dist as well. Ill check out the stuff you talked about and get back to you guys.

Will

Never mind!!! I just saw what your fuel pressure was!!! But did you try checking your vacuum???


[This message has been edited by CHELKAMINO (edited 11-11-2002).]

There is a chance that I might have found the problem. I was looking at the carb last night and found the secondary shaft to be a bit on the loose side. With the thing running, it seems like I can duplicate the problem. When I have a little more time Ill screw around with it a more and report back.

*edit*

Well I think the problem might be that with the shaft being loose and no lubercation, it would not return to the stop every time. I just sprayed some LPS on the shaft and Im going out to try it out.

Will

[This message has been edited by thr062 (edited 11-12-2002).]

I DONT THINK ITS YOUR LINKAGE BECAUSE YOU SAID IT WHEN YOU SHUT THE CAR DOWN AND RE-FIRE IT WITH NO THROTTLE THE IDLE SETTLES DOWN AND IS FINE AT THAT POINT WHICH TELLS ME IT'S LIKELY STILL AN ISSUE WITH VACUUM STILL APPLIED AT IDLE TO THE VAC/ADVANCE AND OR YOUR MECH ADVANCE STUCK OR NEEDS RETUNING WHICH I OUTLINED IN MY PREVIUOS POST ON 11/13.

Scott

Oooopppsss,sorry about the CAPS for all you sensitive types.