304 base adapter vacuum leak???

[shanespurlock]

So after rebuilding the AMC 304 in my 73 CJ5 , i have run into a vacuum leak issue. When the engine is cold, it runs fine, no vacuum leaks, even when I use starting fluid to test for leaks, all is good. Once the engine is warm it falls on its face and dies. If I keep it running by hand and spray starting fluid anywhere near the front of the carb base, drivers side, or rear it increases rpm, obviously getting a vacuum leak when engine is warm. I resealed everything with new gaskets and still the same problem. I pulled the spacer with the two vacuum ports thats between the carb and intake, and it looks okay, but it looks like there may be some tiny stress cracks around two of the holes for mounting the plate the intake. Only thing I can think is that when torqued down and fully warmed up, the cracks open up and create leaks. Either that or the cracks really are superficial and theres some warping that gets worse when warm.

Anyone have any experience with anything like this? Anyone know where to find a new spacer? I can't find one anywhere online.

Im about to rip my hair out over this.

 

[Hedgehog]

If you have the motorcraft Carb, the 2100 or 2150 with the phenolic spacer between the manifold and carb spacer yes I do have experience. I've found a couple problems. First the spacer mounting holes are VERY tight on the mounting bolts. I open them up a little, until the bolts have clearance. Also the spacer isn't especially flat. I glue fine grit sand paper to a very flat board and carefully sand the plate flat, turning as you go to prevent over sanding on one spot. Then the 3/8" or so gasket/spacer that goes between the phenolic plate and the carb doesn't fit freely over the plates feet. I open the 3/8" gaskets corner holes until the phenolic spacer and gasket fit interference free. I also sand this one smooth to ensure a nice tight fit. After making sure the manifold and carb bottom are clean tighten the carb down slowly making sure not to over tighten one more than the other. Folks have cracked the tabs off their carb making that mistake.

 

[shanespurlock]

Thank you for the input. Mine is the original aluminum adapter, not the phenolic ones I see advertised. I thought about sanding it flat but figured the more precise route would be to take it a machine shop and have it planed, but that costs more than a new one, although the new ones are somewhat generic and don't have a port for manifold vacuum. And since its aluminum and has a casting seam running around the entire thing at its midpoint, and because I have seen casting seams open up under heat (not these exact pieces but other engine parts) Ive decided tomorrow to go to the speed shop and see if they have a billet one that will fit. I really like your idea of opening up the bolt holes slightly. I hadn't thought of that but it makes perfect sense.

 

[Hedgehog]

I'm not surprised that your is different from mine. The phenolic spacer was stock for my engine, I have no idea when AMC had a metallic spacer. Mine doesn't have vacuum ports either. What carb. do you have?

 

[shanespurlock]

Mine is a 73 CJ5 with AMC 304 . The carb is a motorcraft 2100. 1.08 to be exact. When I rebuilt the engine i put a rebuild kit in the carb but the thing had a worn out thermatic choke, fast idle cam was worn and had rough spots, throttle shaft bushings were completely shot. Uremco was out of them, so I ordered a National from AutoZone, it never showed up as apparently AutoZone has suspended all orders from National for not fulfilling orders, so the only other option was an Autoline. Visually it looks like they did a good job...bushings fit nicely, external hardware is all new, not just tumbled, choke operates properly right out of the box. However they are all supposed to be pretuned, and the idle mixture screws were out 5 and a half turns and the fast idle screw was way off. No big deal, but certainly not pretuned as advertised. Really hoping that tomorrow I can find a new adapter plate and get this thing running properly. Was going to upgrade the ignition but upon close inspection the PO installed a Pertronix module, so I think I just need to shim the shaft, as it has about .050 endplay, and I hear .015 is the spec when using the pertronix setup. In case the speed shop doesn't have one, any idea of where I can find an adapter plate for bolting the 2100 to the factory AMC 304 intake?

 

[Hedgehog]

Since the 2100 was a stock carb it should bolt right on with no adaptor plate.

 

[jafo]

Thank you for the input. Mine is the original aluminum adapter, not the phenolic ones I see advertised. I thought about sanding it flat but figured the more precise route would be to take it a machine shop and have it planed, but that costs more than a new one, although the new ones are somewhat generic and don't have a port for manifold vacuum. And since its aluminum and has a casting seam running around the entire thing at its midpoint, and because I have seen casting seams open up under heat (not these exact pieces but other engine parts) Ive decided tomorrow to go to the speed shop and see if they have a billet one that will fit. I really like your idea of opening up the bolt holes slightly. I hadn't thought of that but it makes perfect sense.

Wonder what it would cost to have one CNC'd?

 

[shanespurlock]

I dont know if it was all years but on the 73 AMC 304 , the bolt pattern on the manifold was the small Rochester style but the 2100 is the large Holley bolt pattern. Im told this is common for that era of AMC 304 , but im no expert.

 

[Hedgehog]

Well, I've learned something. You might give Rob at Willys Works a call, he might have the part you need. Willys Works, (520) 888-5082