Low vacuum causing an off-idle miss

[leavittt]

I was hoping to get some information on those vacuum reserve cannisters. I don't know much about them except some guys use them to help their accessories powered by vacuum when they run larger cams. Here's the problem. Both me and my brother have basically the same truck-I have a 1970 with a 390 strokered to a 447 & he has a 1969 with a 428. Both engines have Holley 750 Street HPs with mechanical secondaries and both have Edelbrock RPM intakes. Also they have similar cams that are pretty big. I have an Edelbrock RPM and he's running a solid roller Comp. The problem is they both have really low vacuum (6-7 Hg @ idle). I put my engine together myself & he had a reliable & reputable shop put his engine together. We've checked for vacuum leaks throughout and there are none on either truck. I've even changed the intake gaskets twice on my motor in about 500 miles just to make sure. The only thing we can figure is that this is caused by the combination of parts we are running. Both trucks run perfectly except at off-idle at about 1200-1600 RPM. There is a lean miss here. It feels like there just isn't enough vacuum to draw the fuel in. If you give it more gas, it clears right up. My question is whether or not those vacuum reserve cannisters will do anything for engine performance or are they just for power brakes & other parts that run off vacuum? Like I said I don't know much about them. Any info would be great. Thanks.

[Dragon]

A vacuum tank needs high vacuum levels to charge it. They won't really power anything like power brakes and anything except maybe the old chebie vacuum wiper system. Vacuum advance needs to be interactive to the engine not to the storage tank. The vacuum advance depends on air passing over the venturi port in the right front barrel. Air velocity creates the vacuum to open the advance. There is a myth that the vacuum advance port must be removed from the dizzy when checking timing. Has anyone put a gauge on there to see what vacuum is on the line? Nothing, nada, nyet there is no vacuum on the advance port at an idle or even until the engine gets to about 2000 rpm with a load on it.

The lean miss is the carb transition circuit. The transition circuit is a cross between idle circuits and main circuits. They can't be altered at home. Radical cams with no vacuum kill the idle circuit as they depend on hard suction to 12 to 18in to suck fuel out. The mains need air velocity to pull the fuel out of the main wells. You have velocity, nope, you have high vacuum, nope, at that rpm. The transition circuit crashes.

Your low vacuum readings is because of the cams. Your cam is kind of weak though for your engine size to produce that low of a vacuum on your engine size. Single pattern cams are not all that hot for good power.

You say the trucks are basically the same but engine wise they are radically different. You most likely should stop hunting for the non existent vacuum leaks. After looking at your cam the intakes has a lot of air coming back because of valve timing until the cam comes into it's power band.

Not knowing cam data on his I would only take a guess that a solid roller cam is going to be really lopey at an idle.

[mrtleavitt]

Ya the only reason he stated that we've hunted for vacuum leaks is so that a million people would chime in and say "you have a vacuum leak somewhere." Lot's of good info Dragon, thanks.

[leavittt]

Thanks for the info. So, is this low vacuum a fairly common problem on FEs that are set up like ours? I've noticed the richer you set up the idle mixture, the less noticable the miss is. Is this about the only thing that we can do to "cover up" the lean miss besides changing cams? Another question I had- What do you think about a multiport fuel injection setup where the individual injectors are set closer to the combustion chamber? Will this fix the miss since the fuel is being "sprayed" into the intake instead of relying on vacuum to draw the fuel in? I know this wouldn't change the vacuum at all but to me it seems that this could help fix the problem caused by low vacuum at this RPM since there isn't such a big gap from the point the fuel enters the engine to where the fuel is burned. Any thoughts on this?

[Dragon]

Any radical or high lift cam in any engine will cause the loping miss. Fuel injection of any type except stock GM types will help some but they need the inputs altered as the exhaust will go lean then rich and the computer will over compensate. But the flat lean out at the engine would go away as that is steady ramp up signal the computer can deal with

If you want to try and tune this out yourself.
http://www.summitracing.com/parts/PRO-67217/
Then you have to get their air bleed kit to do the tuning.
http://www.summitracing.com/parts/PRO-6 ... refilter=0 and
http://www.summitracing.com/parts/PRO-67245/

[leavittt]

Cool. Thanks for the help. I've been meaning to order a complete kit of air bleeds for my carb so I can fine tune the mixture. Me & my brother share a wideband oxygen sensor so it's easy for us to see how much the tuning affects the mixture. I've been eyeing one of those Edelbrock fuel injection kits but it'll take a while before I scrape together the funds. Until then, I'll just keep tinkering with the carb & get it running as good as possible.

[Dragon]

I would stay away from Edelbrock Just get the fuel rail and manifold then go get a Ford EEC IV computer system and EDIS system and tie them together to run ignition and fuel. If I get the job I am applying for to close my business I will start on my EFI and EDIS 380 cid Aluminum FE with EDC heads (352 interceptor machined combustion small chamber heads) I will buy the fuel rails from Edelbrock and use them for the spacing pattern to modify a highly modified Streetmaster Intake manifold to EFI.

[mrtleavitt]

So what do you have against the whole Edelbrock kit? Any personal experience? Just curious