Rough Idle

Reply Subscribe

Thread Tools

Search this Thread

Mar 14, 2009 | 12:14 PM

JimT

Thread Starter

Burning Brakes

Joined: Nov 2006

Posts: 893

Likes: 7

From: Orange Park Florida

Rough Idle

All has been well with my '72 350cu automatic (650 Holley carb) until thursday. Went for a short drive and car began acting erratic - idles rough (normal 700rpm) but stopped in gear, engine wants to die. Brought her home and began looking for the problem. I found - and this sounds crazy - that when in park, engine idling at 750rpm, if I depress the brake pedal, the engine drops 500 rpm. I did this many times to verify that this really happens and it did it every time. I have power assist brakes so I am thinking an engine vacuum problem? I also noticed that when I switch the heater fan off/on it affected the engine rpm (sounds crazy but it happens). An electrical problem? I've disconnected several of the vacuum hoses to see if the engine runs smoother but no luck. Has anybody seen this before? Any suggestions?
The timing is set at 12btc, new plugs (yesterday), new plug wires (several months ago). The only recent work on the car was last weekend when I replaced the front brake calipers.

Mar 14, 2009 | 12:28 PM

jdp6000

Melting Slicks

Joined: Jul 2005

Posts: 2,740

Likes: 3

From: Aurora Ontario

If you have power brakes it could be the plastic valve in the front of the master cylinder. It should be one way only....you should not be able to blow through it from both sides. Did you try disconnecting that at the engine and plugging?

Jim

Mar 14, 2009 | 05:47 PM

JimT

Thread Starter

Burning Brakes

Joined: Nov 2006

Posts: 893

Likes: 7

From: Orange Park Florida

I have removed the brake booster hose from the carb and plugged it. Still runs rough and depressing the brake pedal causes no reaction. I've removed each vacuum hose one at a time (pcv, lights, tranny module, vacuum advance) and plugged them but still no difference. I'm looking at the carb now. It appears that when a load is put on the engine (brakes, placed in gear. etc) the engine cannot recover. I can put a vacuum on the vacuum advance and do see an increase in rpms. The engine is pulling at the vacuum advance port about 5lbs pressure. This is driving me crazy.

Mar 14, 2009 | 07:03 PM

jdp6000

Melting Slicks

Joined: Jul 2005

Posts: 2,740

Likes: 3

From: Aurora Ontario

Do you have an EGR valve? If so test it. Push up on the diaphram from below and see if it changes the idle.

Jim

Mar 14, 2009 | 07:32 PM

CheezMoe

Melting Slicks

Joined: Mar 2006

Posts: 3,456

Likes: 102

From: Piedmont Va

St. Jude Donor '11-'12-'13,'19-'20

I would suspect you have a leak in the vacuum system somewhere. Disconnect all external vacuum circuits. Connect only a vac gauge to an intake port a read vac@idle and check against you engine spec. If the problem persists and/or vac is low you have a carb or intake related problem. If the problem cleared after disconnecting the external circuits, begin re-connecting them one at a time until you find the culprit. You have already proven it's not the booster itself.

peace

Mar 14, 2009 | 09:11 PM

JimT

Thread Starter

Burning Brakes

Joined: Nov 2006

Posts: 893

Likes: 7

From: Orange Park Florida

I've removed all the external vacuum hoses and blocked the ports off, checked all the hoses and haven't found a leak yet. I am going to go through it all again just to make sure all ports are blocked and start taking vacuum readings to try to narrow it down. I'm beginning to think I have a carb problem, possibly a blown/leaking gasket, hopefully not a manifold gasket. It's a vacuum problem, just hard to pinpoint. I'll keep you posted.

Mar 18, 2009 | 07:37 AM

JimT

Thread Starter

Burning Brakes

Joined: Nov 2006

Posts: 893

Likes: 7

From: Orange Park Florida

I've gone through the engine - compression tests and all is good. All hoses are good - no leaks, plugs have been changed, timing had not been touched and I've checked the distributor internals and all is well. I've replaced the fuel lines (both the "S" lines) and have excellent fuel pressure. While removing/replacing plugs, I noticed that all the plugs on the right side were fouled. I have found that while the car is running - fast idle - the front right carb throat is steadily dripping gas instead of a mist, way to much, and shortly thereafter the car stalls. I'm removing the carb this morning and try to find out what is happening. Stuck float? Bad metering? I'm not a carb expert (Holley 4160) but here's hoping!

Last edited by JimT; Mar 18, 2009 at 03:58 PM.

Mar 18, 2009 | 04:06 PM

JimT

Thread Starter

Burning Brakes

Joined: Nov 2006

Posts: 893

Likes: 7

From: Orange Park Florida

Found the problem. I bought a rebuild kit and tore into the carb. I found silicone inside the metering block in the right side idle mixture chamber and in the timed vacuum port channel under the gasket.The only way silicone could be introduced into the carb is during the manufacturing process. My carb was purchased new, and has never been disassembled, or worked on by any mechanic. Apparently some of the the silicone would come loose and float through the carb, thus causing intermittant problems. I'm on the road again!