OK It's be a while since I last had to look at a carb, let alone tune one. And I'm having some problems with tuning this one. I've followed the instructions that Holley provided with the carb and have searched the net and forums, and get pretty much the same info. The problem I'm having is it seems to be running rich. It's not smoking out the tail pipe, but it smells like fuel. It seems to run fine after a warm up, but when I come to a stop and go to take off it lugs and dies. Sometimes it backfires up through the carb and slams the choke shut, but normally only durring "tuning" sessions. I've checked the choke setting and it seems to be fine and working correctly. I haven't found any vacuum or exhaust leaks. No matter what I do with the mixture screws I only have 10-11" of vacuum at the primary carb port and it's steady. This seems low for the engine build I have. My timing light is curantly MIA, but last I checked I set base timing at 10* and it sits about 30* with the vacuum advance hooked up. Points were gapped right and dwell was at 32. Idle is set to 850 rpm, fast idle is 1200rpm. I know it's got to be something basic I'm missing, but I can't put my finger on it.

The motor I've got is a freshly rebuilt '63 352 .040" over bore and only a few miles and maybe 20 hours of run time. It's got TRW cast pistons for a 9:1 comp ratio, Clevite RV cam, Weiland dual plane intake, 1" open carb spacer, Holley 4160 e-choke 600cfm carb, a 4v fuel pump from a 70's 390, and all the rest of the engine parts are stock sizes.

So anyone have any input as to what I'm not doing right? Or what I should check. Any input is good input at this point. I'm going to pick up another timing light tomorrow and recheck my timing. Thanks

 

I've only had one holley so consider the source. Check float levels and power valve.
Hope it helps.

 

First off, you have the vacuum advance hooked up to manifold vacuum, not the proper "Port Vacuum Port" on the passenger side of the front metering block. Timing should not change when you connect to the correct vacuum port at idle. If it does either you are on the wrong port or the throtle valve is to far open at idle. Check manifold vacuum at the port where you now have the vacuum advance hooked to. It sounds like you have either a blown power valve of an internal fuel leak. Idle at 850 rpm is a bit high unless you have a fairly radical cam, which explains the low vacuum. What is "the primary carb port"? I have never heard the term?

 

It's the manifold vacuum port at the base of the carb. I've got the vacuum advance hooked into the port on the pass side just above the adjustment screw. And as I said I need to recheck those timing specs, so I could just be remembering wrong. What should the idle speed be then if 850 is too high? I hope it's nothing internal on the carb, it sucks to tear down new parts. But it happens.

 

First, disconnect the vacuum advance and see if you have vacuum on that port at idle. If you do, there's a few things to do.

One, how big a cam is it? Stock? RV? Lopey idle? Too much cam, and you have to open the throttle plates too far to get it to idle, and the vacuum advance port will be getting vacuum.

Two, with the vacuum advance disconnected, what is the timing at idle?

Not enough timing, the idle will be too low, so you have to open the throttle plates more, and again, you have the vacuum advance port getting vacuum at idle.

Report back when you check these things.

 

First, disconnect the vacuum advance and see if you have vacuum on that port at idle. If you do, there's a few things to do.

One, how big a cam is it? Stock? RV? Lopey idle? Too much cam, and you have to open the throttle plates too far to get it to idle, and the vacuum advance port will be getting vacuum.

Two, with the vacuum advance disconnected, what is the timing at idle?

Not enough timing, the idle will be too low, so you have to open the throttle plates more, and again, you have the vacuum advance port getting vacuum at idle.

Report back when you check these things.

 

OK so it's been a while since I've had time to play with this and I've been dodging rain all day just to get nowhere fast. So I've backed the idle speed down to 700rpm, this has removed the vacuum from the ported nipple at idle. I've kept my base timing at 10*, as this just seems to be a good place for it. I now have a whopping 12-13" of vacuum at idle. I removed the power valve and hooked it up to a Mighty-Vac and it holds vacuum, so that should be good. I did notice that it is stamped as an 8.5, should I change this to a 6.5? I did yet another mechanical timing check and everything lines up like it should, so I don't think there's a problem there. If I advance the crap out of the distributor I can get about 18" of manifold vacuum, but then it's so much advance the engine won't start. It still seems like I have little to no tuning from the idle screws. And I have traded my intake backfires for tailpipe backfires. So my guess is I've traded a timing issue for a carb issue.

The cam is a Cevitte 77 RV cam. Specs are:
Lift: 484/510
Adv. duration: 270/280
Duration @ .050": 204/214
Lobe sep: 107/117
If I can dodge the rain again tomorrow I'll go play around with it some more. I'll see if I can't get enough adjustment out of it to get the backfires gone completely. Thanks again.

 

I have found that if the carb isn't responding to idle mix adjustments the idle is turned up way to high and the engine is running on the main fuel circuit (sounds like you already fixed this problem). The other cause I've found is from being overjetted. I'm pretty sure 1850s come with #65 jets in front and a plate in back the equivilant to #63s. Have you done a 4150 conversion that puts a double pumper type metering block in the back. If you have and your running bigger than #65 jets in it, that may be the problem. I don't know if overjetting the primary bowl can cause the same issue, I"ve only ever messed up with the back bowl.

What is the elevation where you live? If your at 5000 ft or above, you might consider stepping down to #63 jets in the front bowl and rechecking you idle adjustability and backfire issues.