vacuum leak? how to find?

[aribert]

First a little background. I put a Clifford 270M cam into my '78 200 engine. I bought this cam in 89 and ran it for about 25K miles in the early '90s in a stock 170 in my Falcon. Along with the cam, my 200 also had the head milled .060, valve pockets blended, valves unshrouded, chambers balanced and the carb inlet opened up ( http://i290.photobucket.com/albums/ll27 ... rface3.jpg ), along with prior headers, Weber 32/36 carb, '73 dist. When this cam was in my stock 170 it was very mild. With the bump up in compression, I was expecting a loping idle, but now I am unable to keep the engine running at less than (guessing) 800 rpm. BTW, to get the engine to run, I advanced the timing to 15 BTDC (harmonic balancer mark is good, it seems to want even more advance), screwed the idle speed screw in **all the way** (I can clearly see a major gap between the carb throat and the air flap) and upped the idle jet size. It feels and sounds like a vacuum leak.

I used an unlit propane bottle/torch and was not able to tell a difference in engine speed - even when I had the torch tip directly over the carb inlet. So I hooked up my narrow band A/F gage (I realize that it will not tell me much near the middle but it will show if the carb is at one of the extremes). The A/F gage is reading very rich - I would expect lean if there was a vac leak. Could this be why the propane did not speed up the engine.

I was expecting to have to rework the carb jetting with the change in cam and compression but I did not think I would have trouble to just keep it running.

 

[CobraSix]

15* BTDC isn't that far off. Stock for an auto I believe was either 10 or 12* to begin with. If you had your idle screw turned all the way in, what is your idle speed? For it to be running would likely require you to be running over 1500 RPM at idle.

Try backing the timing back to 10* or so for now. Get the car running with the idle screw opened 1-1/2 turns from closed. I would then use something like starter fluid to spray around the carb and the base. And just like with your propane, listen. Problem with using propane is it disperses really quick and would likely have that much effect on your idle through a carb.

But, I think you are right, you are running rich, which will explain the closing of the idle screw and the timing. You really need to get the RPMs at idle down. It can be a viscous cycle. If you can't get the car running at a low RPM, then that is a different set of issues.

 

[JackFish]

(I can clearly see a major gap between the carb throat and the air flap) and upped the idle jet size.

So you are not able to close the throttle plate all the way?
That would mean it's not into the idle circuit.
I drilled a hole in my throttle plate, but my engine is rather lopey at around 750 rpm.
That may be as good as it gets with that cam. I have the 264/274/112 advanced 2 degrees.

 

[aribert]

Thanks for the replys.

I've managed to get it less rich at idle by going back an idle jet only one size larger than before the engine changes with the idle mixture screw about 1 turn out from seated. Advancing the timing allowed me to achieve a 850 rpm warm idle and I was able to back the idle speed screw out 1/2 turn to bring the idle down to about 725 - 750. I too have considered drilling a hole in my throttle plate - I have done so on another vehicle that I fuel injected where the donor FI was for a 2L and the recipient engine was 2.3L and the air idle control was not able to keep up with a cold engine and rapid throttle lift. I just don't understand why the engine **needs** (wants) so much more air at idle - displacement has not changed.

I'll continue to look for a vacuum leak a bit this Saturday and but I am coming to the conclusion that there may not be a leak - especially if it is consistently running rich. To qualify that, so far I have not had the car away from the garage by more than a mile or two and have not had it over about 45 mph. On Saturday, I'll take it up on a freeway to see how the A/F mix is at cruising speed. Before the mix appeared to lean out at 60 to 75 mph (depending on the air temp). I really want a wide band A/F system to speed up the jet tuning but that i$ not happening in the next few months.