Chevy 350 with a Wieand 144 CID supercharger. The timing is at 4 degrees retard and the car will not start. I am pretty sure it is my lack of vacum lines from _____ and to _____. I have no vacum advance on my distributor and the vacum lines out/in on the baseplate of my carb are plugged up. Can anyone help me fill in the blanks??

Thanks

Nick

 

For vaccum lines whose purpose are to give signals for fueling and spark make sure you sample from under the blower.

 

You should run a vacuum line from UNDER the blower to the vacuum advance. This way, the vacuum advance does nothing while under boost.

Also, set your timing, with the vacuum advance unhooked, to 26 - 28 degrees while revving the engine to ~4500 rpm.

You should never see 4 degrees retarded timing.

Mike

 

I don't have vacum advance. I am also wondering where I hook up my boost gauge. I have heard that I can hook it up into the intake, but I was told it was much easier to T it into an existing vacum line.

Nick

 

you won't see boost above the blower, you need to tap everything in the bottom.

 

I know I have a 3/8 NPT fitting hole in my intake right under the blower. If I tap into that, where does it go from there??

Nick

 

That fitting in the lower intake is where you run the line to your booxt/vacuum gague, if you use one (I suggest you do- at least while you're dialing in the combo so you know how much boost you're making at WOT).

All other vacuum lines (if any- like the PCV valve) attach to the carb as normal. The carb never sees boost on top of a roots blower- just vacuum like on a normal carbureted application (yes, even vacuum advance still plugs into the carb, if you're running it, which you're not). As you have done- plug any unused vacuum ports on the carb (and plug the hole in the lower intake if you're not hooking your boost gague into it- massive vacuum/boost leak if you don't.)

Your timing to get it started should be around 6-8* BTDC until you get things dialed in and get used to the combination. You do NOT want to crank the spark advance way up like on a N/A application as severe destructive detonation could be the result. Nonetheless, 4* AFTER TDC is way too retarded.

On my 142-blown 383 I run 12* BTDC at idle with 14* centrifugal advance (all in my 2800) for a total of 26* advance (excluding vacuum advance- which I use). Conservative, yes. I don't feel like playing on the bleeding edge of detonation. I'm still making 480HP at the flywheel even with the mild timing.

In general, a roots blown motor isn't going to want as much timing as a N/A motor- but it still needs some reasonable amount of advance to run well, as with any engine.

 

If you plug the vacuum advance into the carb, it will advance the timing at all times except WOT. The problem is that it starts building boost at around 1/2 - 2/3 throttle. So, even though you're not a full boost, the vacuum advance is advancing the timing when in boost.

When I had a B&M 144 on a 355, I ran the vacuum advance below the blower to prevent any vacuum advance at any boost level.

Mike

 

So if I need to run the vacum advance from under the blower, would i just tap into the existing fitting in my intake and run my lines from there and then just put a T Line that leads to my boost gauge?? If that is the case, where does the other end of the vacum line end up???

Nick

 

Nick, you can go that route if you like.

However, I can tell you that you can AND I HAVE ON MY CURRENT SETUP, run the vacuum advance from the port on the carb. I use an adjustable vacuum advance canister on my engine and I have found, FROM ACTUAL OBSERVATION OF VACUUM GAGUES MOUNTED SIMULTANWEOUSLY ABOVE AND BELOW THE BLOWER that I don't go into boost until I'm down around 4" vacuum at the carb. I set my vacuum canister so that it supplies no vacuum advance at less than 6" of vacuum at the carb. Therefore, I will never have any vacuum advance kicking in when I'm into boost.

This is easily accomplished using a cheap $15 Crane adjustable vacuum advance canister in my HEI distributor.