The Stovebolt Site Forums The Shop Area Engine Shop 235 Vacuum at idle

Anyone know how much vacuum should a 235 register at idle? I have a single 2bbl H-W carb on the engine and it seems that it may have too much vacuum right now. I have not measured it yet (job for tonight) but the vac advance is fully engaged as soon as the engine turns over. Is that normal? Also, how can I adjust things to help reduce the vacuum if necessary? The engine is idling awful rough right now. I have adjusted the points, sealed up the exhaust better, and tuned the carb. I am running out of answers so any help would be appreciated. Thanks!

Steve, I'm having the same problem on my 292. I put a new intake with a Rochester 2bbl and now I've got too much vacuum for the advance. If you've got a port that's above the throttle valves on your carb that should be a ported vacuum. At idle, it's supposed to have less vacuum than say, intake vacuum, so it doesn't screw with your advance. On my carb, it seems to have as much vacuum as the intake ports. I've been wanting to switch over to HEI so, I'm gonna do the swap and put on an adjustable vacuum advance and hope this solves the problem. Probably the first thing to check is where you're connected vacuum wise - determining if you've got a ported vacuum source. Hopefully this your problem and will be an easy fix. And if you get your timing squared away you may have to play with your idle and mixture again to dial it all in.

The vacuum port that I am connected to is above the throttle valves. I'm not sure if that makes it a ported valve or not, but I would guess so. I have not tried, but I wonder if it would make a difference if I disconnected the vacuum advance all together? Anyone know if that might help or hinder my attempts? Thanks again.

I'm not sure which 2 barrel either one of you guys are using. You are either using the wrong port or your throttle blades are open too far at idle.
The first thing I would look at is the timing. Assuming that you have already done that, I say you need to check the timing mark on the damper pully. You have to be 100% confident that this mark has not moved. These dampers go bad more often than on most engines because the belts are on the outer ring of the damper. The belts, the alternator, the fan, the water pump are all tugging on that ring and trying to make it break loose. On most V8's the pullys are bolted to the front on the inner part of the damper.

If the throttle blades are open too far at idle you can increase the bleed air a bit by drilling a small hole in the throttle blade and readjusting the idle to get the throttle closed more.
Also make sure your air cleaner isn't overly restrictive.

OK, so why might the throttle blades be open too far at idle? My 2G was a remanufactured one. Was it just assembled that way? Or maybe rebuilt for a different application and thus presents problems? I figured for the most part a 2G was a 2G.
4OnTheFloor, not sure I'm following your hole drilling advice.

"Anyone know how much vacuum should a 235 register at idle?"

*I say you need to check the timing mark on the damper pully."

Maybe on a 292 but not on a 235.

Steve, Do you have instructions for your specific carburetor? Mugsy's suggestion about wrong vacuum port is a good possibility.

Curmudgeon

According to my Shop Manual, the vacuum reading at idle for a 216 should be 17-21 inches. I would assume the 235 is pretty similar.

Try putting on a vac guage on the ported and non ported ports on the carb you are using and start the engine. If I remember correctly the 235ci with the Rochester single barrel pulled about 5-7 lbs of vac at idle thru the normal vac advance hook up. This vac will be different than the advertised direct manifold vac of 17-22. Hope this helps.

FWIW, I concur with 52pu. I hooked a vac gauge to the port on the Roch. "B", where the vac advance from the distributor goes, and I am getting 9" of Vac. Hooked the same gauge to the intake port below the carb and I get 19".

Thanks guys, that gives me a place to start. I plan to check the port that I am connected to tonight and check how much vacuum is being pulled at that port. I will fill you in once I find out where I stand, but my bet is that the port is pulling too much vacuum.

Probably the easiest thing to do to limit the vac using an after market 2 or 4 barrel carb would be to limit the travel of the old vac advance arm by drilling multiple holes along the arm and pinning. Some experimenting will be necessary to find just the right amount of travel without pinging. Ultimately you may have to completely disconnect the vac advance. The newer HEI set-ups you can buy the adjustable vac advance.