Make a small vacumn leak and idle smooths out

[junkyardjeff]

I got this 351-W thats in my 55 sunliner and it never idles as smooth as it should,since I am going to be driving it to Daytona next month I was wanting to go over it and get rid of a high gear shifting issue which I think is vacumn related.  I was checking vacumn readings at the trans and carb and got the same readings but when I disconnected the gauge and caused a small vacumn leak the idle smoothed,I dont remember ever having one do this before and where should I look for the problem.

[junkyardjeff]

At idle with the motor warm the gauge is reading at 18 but it will slide into 3dr gear at light throttle and will act normal when I am in the gas some,at idle I have to rev the motor up to get the vacumn wipers to move so something strange here.  I just had another thought after posting,it has a aftermarket carb spacer with 4 individual holes and should I grind a slot between the front two to equalize the vacumn between them as I think some factory carb spacers are made.  Its on a factory cast iron 4 bbl intake if it makes any difference.

[wayne petty]

is that a Holley with a barely blown power valve??? or even a fully blown power valve diaphragm..

if you turn one of the idle mixture screws in... does it start to die..   or continue idling...


i just had to rebuild the 4180 on my 460...   now it idles so smooth.. gets much better fuel mileage...

i did find the 4180  base plate idle mixture screws on it... like to be at one turn out on the rear... 2-1/2 to 3 on the front... i have not found a factory sealed base plate to count how many turns out  ...

i have not finalized the adjustments yet...  need another idle screw tool... it gets TOO HOT in there once the thermostat opens...   for some reason the back of my arms is tender to 180F blasts of hot air off the radiator...

[junkyardjeff]

I can turn the idle mixture screws in and it will want to die and I did replace the power valve a couple years ago,I did get the vacumn up to almost 20 and when I speed the motor up a few hundred RPMs the vacumn goes up to 24 which I think it should drop some.  What can a retarded cam do to a motor as I am wondering if the person I had build the motor screwed up something when putting on the timing chain,it was supposed to be a RV cam but has a 302 firing order so who knows what I have.  I found that the carb has those grooves in it to equalize the vacumn so thats not the problem but I am begining to think it could be related to the cam or installation of the timing chain,its weird that the motor does not have the highest vacumn at idle so something is wrong.

[wayne petty]

try to find a bigger flowing pcv valve...

i have never found a flow rating chart .. so you will have to find the ones that look like they will fit and look at the restrictor size in the inside bottom...

don't go to big..   and watch for excessive oil consumption...

i have found some with way oversized PCV flows that burned oil at 400 miles per quart.. after 4 rebuilds... i figured it out..  i only worked there..

[junkyardjeff]

I am going to change the valve covers since the PCV does not fit in too good,since the motor was in my 55 sunliner I tried to make the motor look like a Y block and used 289 valve covers. The baffles hit the wider rocker arms on the mid 80s motor so I had to crimp the baffles some and the PCV sticks up a little,I will get the valve covers sealed up so the motor gets some vacumn and see how it does and will look into another PCV.

[junkyardjeff]

I am beginning to think its a cam or installation issue as the motor has the highest vacumn reading at about 1200 RPMS which with a automatic trans and no special converter will be impossible,if I really advance the timing the idle will smooth out and run great but will ping and rattle and not restart when hot so I am thinking the cam timing is retarded or it has a slightly radical 302 cam as if I play with it some it will sound like it has a hotter cam.  It was supposed to be a rv grind due to I was going to have a tall gear and a automatic trans and just be a highway cruiser,it just might have to wait untill I get back from Daytona if its the cam and I am thinking of pulling the front of the motor off to see if its installed correctly.

[Mikej]

Are you useing full time vacuum to the distributor?

[junkyardjeff]

The distributor only gets vacumn when the throttle is opened.

[chimp koose]

I would put the vacuum advance to manifold vacuum not ported as you have it. this will raise your rpm at idle slightly ,allowing you to close the throttle more to achieve proper idle speed. This in turn will raise the manifold vacuum helping out your wiper issue,as well as helping with driveability issues.

[junkyardjeff]

I will give that a try.