possible exhaust leak

[thistle3585]

I just replaced my timing set and when I set the timing it wasn't jumping around like it was. So, now moving on to my next issue on the list. I think I have an exhaust leak in or around the headers. They symptoms are a tapping noise from drivers side valve cover, similar pattern/noise from drivers side exhaust pipe, back firing when letting off accelerator as well as down shifting, thicker exhaust that smells like gas and I have some trouble starting it. I also have very low vacuum. If I partially cover the carb then the vacuum comes up and it idles better. It also runs better when I use the manual choke. So, I figured it has to be the header leaking.
I pulled the header off and noticed that two bolts weren't as tight as the others, so thinking that was the problem I cleaned the gaskets and reinstalled the headers but the problem didn't change. I did notice that the header bracket looked like it was bowed out in the center a little bit but the bolts are tight now. Could I have overtightened them and bowed the header? Trying to decide whether I should put my focus towards the headers or do you think there may be something else going on? Would the tapping noise be a faulty lifter? They are Hedman style headers.

 

[Bent]

What venturi size is your MC2100? It will be cast on the drivers side of the fuel bowl. It sounds like the carb might be too big. If it is it will probably backfire out the carb if it is too big.
Very low manifold pressure can also be caused by a timing chain that is one link off. How did the chain link count come out after rotation?

 

[mriplaybass]

What venturi size is your MC2100? It will be cast on the drivers side of the fuel bowl. It sounds like the carb might be too big. If it is it will probably backfire out the carb if it is too big.
Very low manifold pressure can also be caused by a timing chain that is one link off. How did the chain link count come out after rotation?

I agree, you may be off on the timing gear alignment. Did you line up the marks during the install? You could also just be off on the distributor timing. Did you retime using a timing light with the vacuum advance hose disconnected and the line plugged? If this was the same carb that was on before you did the timing set, and there was no problem then, your problem is more than likely timing.

 

[thistle3585]

The carb looks to be a 1.21. I did go over the timing chain and counted the links per the manual and it came out correct. I had this problem before changing the timing setup. Changing the timing set was suggested as a possible solution. Yes, I did set timing with light at 5 degrees. Manual calls for 3-7. I've had this setup running well in the past.

 

[mriplaybass]

1.21 is a pretty hefty carb compared to what I have. I'll just give you my specs since mine runs great. I have a 79 AMC 304 w/2100 w/1.08 venturi. I redid mine for a high altitude trail ride back in July. I changed the jets from 49s to 45s, and I set the timing at 15 degrees. When I got back to Wisconsin, which is roughly the same elevation as you are at, it still runs fine, but I don't quite have the acceleration that I did with the 49 jets. Might go to 47s and back off a few degrees of timing. Though 1.21 is more than you probably need, I still lean towards timing. Are you sure the marks on the crank and cam gears lined up? I never put much faith in counting links when all you have to do is line up 2 marks on the gears. I would try advancing your timing a degree or two at a time if you are absolutely certain the timing set is correct, and see if that solves your problem. 15 degrees is probably too much, as I said, mine is set up for high altitude.

 

[thistle3585]

I bought the 1.21 about 6 months ago and that was what the carb guy recommended. I have an earlier AMC 304 which has more HP than the '79 that was original. Not sure if that is why he suggested the 1.21. I can run the timing 10-12 but doesnt that make it run more rich? I don't know what jets are but can look.

 

[mriplaybass]

The jets are below the float. You have to remove the float to get at them. You can get a set for around $7 to $10 at places like EBay, but I wouldn't mess with changing them unless they are way off (like under 45 or over 50). You are right. I forgot that the older engines have more HP. I would think your carb should be a good match. I don't think a few degrees of advance will make the engine run richer. I know that at 5 degrees, mine is very sluggish, even at low altitude.

 

[thistle3585]

I was very diligent about the timing set install, so I am comfortable that it is correct. Ill advance it some and see how it goes. I was thinking jets were either 47 or 49 but will check. So, what are thoughts on the tapping which seems to be between cylinders 3-5.

 

[Bent]

I think the 1.21 was the carb ford used on the 390. Maybe a little big for the AMC 304 .
If you turned the crank shaft clockwise until the cam shaft sprocket timing mark was at the 3 o'clock position, and you counted 20 pins between the two timing marks, I would agree that your valve timing is correct.
As far as the tapping, its hard to say. I sounds like you have already ruled out an exhaust leak. Next I would check for excessive lifter preload. To do that you want to take the valve covers off and hand crank the motor. As one valve begins to be pushed down, stop. The other valve for that cylinder will be fully closed. On the valve that is closed you should be able to rotate the pushrod between your fingers. You need to do the check before that engine has been run that day so the lifters are bleed down. Check all 16 pushrods. If you find one that you can't rotate then you need to install a shim under that side of the rocker arm bridge. Sumit Racing sells them. The pack contains different size shims. Just keep increasing the shims size or doubling up shims until you get the preload you want. Good luck.

 

[mriplaybass]

I reread your original post, and see that the new timing set solved the "jumping around" problem. Sorry to have pushed the timing so much. I agree with what Bent suggested. A few other thoughts. Since you say you have low vacuum, I assume you have checked for leaks. As to the back firing, could the distributor cap or coil be cracked, or plug wires be arcing? As you asked before, it could be a lifter. You should be able to see that following Bent's procedure.