Rough Idle

[MGB922]

My 79B developed a problem with rough idle.
Idles about 800, timing 13 degrees, and the vacuum gauge 14-16lbs with a constant drop which indicates valve clearance.
With the crankcase vent hose disconnected vacuum comes up to 20 with no constant drop and idle smooths out.

I'm stumped.

 

[DrEntropy]

Half clogged charcoal canister? Air cleaners blocked?

Is 13 degrees at idle a bit too much lead? I'd go for 10 and adjust the idle up after.

Valve adjustments aren't the only thing to suspect... If the rings are "passin' gas" the resulting crankcase pressure could cause it, too.

Just some "gut reactions". WAG stuff, food fer thought.

 

[MGB922]

Thanks for the thought food.

I thought of the air cleaner and removed it with no change. The vacuum gauge levels out when the hose is disconnected and it just had rings so adding the two I may have a weak ring not broken in yet. With the hose connected the crankcase can't vent and blow-by the rings. With the hose disconnected it can vent.

How does that sound?

 

[DrEntropy]

mmm.... mebbe.

How much time onna new rings? It could be. Is there any other evidence? Oil smoke out th' back when the breather is not connected?

 

[MGB922]

Less than one hundred miles on the rings. No smoke. Probably 30 years on the canisters.

 

[DrEntropy]

I'd be fer waitin' a bit to start hunting "more trouble"... the canisters can be refurbed, BTW. There've been a number of "How-To" posts in past here. I couldn't find one with a quick search but know the info is posted in detail.

Keep an eye on this place, it'll move faster than ya expect, too. Someone will know JUST where it (the info) is and set a link down.

And WELCOME to th' forum!!!

 

[MGZT260]

Cracked cylinder liner can cause idle problems as well as multi-carb balancing probs (personal experience)

When you are removing the vent line where are you removing it from (not having had smog restriced MG's not sure where it goes) But looking at pics on Moss you have what appears to be th emost complicated setup smog-wise of all. Air pump. EGR valve, Gulp valve, anti-runon, check valve, air injection into manifold. All those things are designed to make the car run as lean as possible. Bad news is it may be impractical to start replacing all those expensive parts that may have been designed to have car run rough at idle anyway (had domestic '79) Good news is that in many states a car this old is exempt from annual emissions test and if so you could (maybe not legally) re-work the hose connections so that it works more like a '69 than a '79.

Practically speaking, I believe, that when you disconnect that vent line you are allowing additinoal air to enter th emix, which is to say you are leaning out what may be an over-rich mixture (for whatever reason) Is that hose or another hose internally collapsed from age and heat that shuts off a necessary air flow? Could be. But be cautious in allowing teh engine to run leaner than spec for long periods or you will be working on the valves (or burned piston) shortly.

Back to charcoal canisters an dwhat they do. They are designed to capture excess fumes from fuel system and prevent them from entering atmosphere. They ar enot designed to reinsert these fumes into the burn cycle. They are, howver, connected by vacuum to teh combustion cycle. The worst thing a person can do for a charcoal system is to top off the fuel tank, trying to get as much fuel as possible into the tank as this can over time push raw fuel into the canister, saturating the charcoal and then allowing th eexcess raw gas to enter teh intake side - hence a rich mixture that appears 'cured' by allowing extra air through uncoupled venting system.

 

[tony barnhill]

What's the compression on each cylinder?

What carb setup are you running on your '79?

How much of the original emissions are still on the car?

 

[MGB922]

Yes I have the worst and to top it all off I was looking at one of John Twists vidios and he pointed out that the Califonia cars have the vacuum hose routing decal on the drives front. Didn't know that

The crankcase vent goes to the back side of the Zenith 175CD. I used to have a diagram of the carb but haven't been able to find it. Had a gulp valve failure so I took a suggestion from someone on another forum to remove the air pump. Anti run-on, EGR, and ELC are left.

 

[tony barnhill]

IMO, if you're going to remove 1 part of the emissions, remove all parts that affect the engine.

There's a diaphragm in the carb that might be cracked.

 

[PAUL161]

I bought charcoal for a canister redo at a fish tank store. Their easy to do! PJ

 

[tony barnhill]

Yep, here's a link to a "how-to" article on my website: https://www.theautoist.com/carboncannister.htm

 

[DrEntropy]

THAT'S th' one I was thinkin' of!! :laugh:

 

[MGB922]

Problem solved. I have a manual choke conversion and the linkage to the valve popped out so it was running extremely rich. I think I will clean/repair the evap canisters though. I figure they're original so it couldn't hurt. Thanks for all you're help.