not your everyday carb problem

[hobgoblin351]

Hey guys whats up? It's been awhile, but I finally got the stang painted. and it looks great! Yeah I know I'll get some pics in here eventually. So now it's time to work out some of the bugs.
I'm running a Boss351 with a pretty big cam. An Excellorator manifold with a 750 double pump mech 2nds on top. Sounds like a good setup right? I'm running hopelessly rich. I tried playing with the idle mixture, and rejetting. Still I foul plugs. Changed them 5 times this past summer, always black. I even tried a buddies 650 same result.
The 3 books I've read on Holley's have a total of about 3 pages on power valves. I'm leaning toward them as the source of my problem. All the books say that I should be making enough vac at idle to keep them shut. You raise your RPM's and the manifold vac drops and they open up. My problem is that my manifold vac does not drop off it goes up! I used a friend's gauge and checked every vac port. I'm running close to 10 at idle and it goes up with the RPM's. I'm going to buy a new gauge myself just incase his was screwed up. ( But I dont believe it was. The high vac readings would also explain the tranny shifting problems I'm having.)
Now there were two lines in one of the books that said in certain cases if you're running too small a carb on a big cammed engine this can occur. Is this true? Something about the engine wanting to suck large amounts of air/fuel mix throught a small carb port would lead to very high vac readings that dont back off. I'm thinking that because I cant get my engine to idle under 1000rpm, it never makes the low vac needed to keep the valve shut. Instead it idles around 1200 and the valve is always open. Am I on the right track with this? I'm running out of ideas.

[joinrdie]

The holley power valves are labeled by their vac rating. For example, a 65 opens up when the vac drops to 6.5". Normally, you want to run a power valve that's rated at about half of your idle vacuum. Hope that helps.

As for the vacuum going up with RPM.... I dunno. The idea about the carb being too small makes sense, though; like trying to suck a milkshake through a straw, you build up a high vacuum inside the straw unless it is big enough. I run a 351C with a moderate profile cam, and a 750 seems perfect, so if your cam is huge, it may do better to with a bigger carb.

The thing that confuses me is that the power valves open at low vacuum and put extra fuel into the mixture. So unless your jets are super rich, I would expect you to be running lean since your vacuum is so high. That's obviously not the case, though, since you keep fouling plugs.

[HotRoddin]

Originally posted by hobgoblin351

I'm running a Boss351 with a pretty big cam.

I'm running hopelessly rich.

Changed them 5 times this past summer, always black. I even tried a buddies 650 same result.

If you're running a big cam on the street, fouled plugs are just part of the game. If that motor is idling alot, nothing you do is gonna keep it from fouling plugs except getting rid of some of the duration and overlap IMO

[Dark Knight]

what spark plug gap are you running? you should be running .050 or more with the MSD..

check fuel pressure, you may have to much to the carb, forcing the needle valves to stay open...

oh yeah, how much timing are you running?

[hobgoblin351]

Thanks for the input guys. DKNGHT, I can't be sure about the gap but I dont think it's that big .050 Next time i change them I'll try it. I'm running a pressure reg and it stays at 6, and the carb was just redone so the valves are new. As far as the timing off the top of my head I think it's in the 4*-6* BTDC range.
I thinks hotroddin is on the right track.Because I've been waiting for the paint job to be done it dosent have alot of miles on it yet. Around the block, exhaust shop/body shop/alignment shop. Not too much time to burn off the carbon, and if that is the price I have to pay to run the at cam then so be it. But what throws me is the vacum readings. The more I think about it I realize duh, if the vac is so high the power valve would never open so it cant be the reason it's running rich. I cant get it to idle under 1000rpm. Sometimes when it's warm it gets close. But then you put a load on it, ie;put it in gear it stalls. So i bump up the idle and it runs fine excpet for the 10hg of vac, which goes up with acceleration. I've checked it at the back of the manifold,the port at the back of the carb and even the small one at the front of the carb. Isn't that one supposed to have the vac increase with rpm for a vac advance dist? Is there any chance the carb could have something crossed up? I've never heard of that though.

[Dark Knight]

try bumping up the timing to 10* for a start... part of the prob may be that you have the idle speed turned so high, you're starting to get into the main circuit... try cracking the secondaries open some.. there's a screw underneath the carb..
when you start getting into long duration cams, alot of people drill holes in the primary throttle blades..

however, I think the combination of more timing, bigger gap, and the secondary throttle plates cracked some will fix it.. I never had much of a prob loading up with a 272/282@.050 .640 lift solid roller in my 289 with a 750DP too..

I think with the 351c, you'll need a bigger carb too..

[Rev]

The power valve stays shut with vacuum over that marked on the power valve. (usually 3-6 in. Hg vacuum). They work by using a small diphram that the vacuum works on to keep it closed. Only when vacuum falls below a specified amount does the power valve open due to spring pressure to admit extra fuel when the engine is under high load. If that little diaphram leaks, then the power valve stays open when it's not supposed to (at vacuum above the specified value). That makes the car run rich and not idle well and will give you black plugs. O'Reilly has Holley parts and a power valve is not expensive and is easy to install. I just did one a month ago.

Another thing that can make the car run rich and not idle well is the needle and seat valves in either prtimary or secondary float bowls. When the needle and seats are bad, you can actually see the gas dripping out of the booster ventouris at idle due to the too high gas level in the float bowls caused by the leaking needle and seats. They are also pretty easy to replace or sometimes even just flush out after removal.

My bet is that it's one of those problems. At least that's what it has been for me when the car all of a sudden runs too rich and won't idle well.

Rev

[hobgoblin351]

As far as I know Rev, the power valves I've been using are good. New out out the box from Holley. And the valve seats too. Dknght, the 2nd's are open just a little at idle. I did that the last time I had it apart. My cam is 240* @.050 with .540 lift big but not that big. So if you got that 289 going with that size cam I should be able to figure this one out. I'll finally get some free time this weekend to tyr all of your suggestions and get it tested with a new vac gauge. I'll let ya know how it turns out . Thanks

[hobgoblin351]

Ok , so I bumped back the timing to 10*BTDC and that did smooth out the idle a little. I now have a Holley 650 with mech 2nds on it. When I put the vac gauge ot it, it fluttered around 10 then increased to around 15-17 when I reved it. I hit the books again and founf some more on the subject. It said that running big cams with small carbs will produce this. I'll try the 750 again. But I had similar problems with that one also. Jeez how big a carb do I need? I figured the 750 was a little big but I could live with it. 800? 850? or should I just cut to the chase a buy a ******* Dominator!?