Idle does not stay constant, why?

[AUSMHLY]

It seems my 64 BJ8 does not idle consistently; even after a tune up.

After a tune up, idle is set to about 850 when warmed up. Car runs great.
Sometimes after driving a while, when at an intersection the idle will change and settle anywhere between 500-1000.

Why?

Is this normal when the car is warmed up or hot?
If not normal, what should I check?

Carbs were rebuilt by Joe Curto a couple years ago. I have checked for vacuum leaks and do not have any.
New cap, new rotor, pertronix, wires, plugs are good.

Thanks guys.

 

[Patrick67BJ8]

It seems my 64 BJ8 does not idle consistently; even after a tune up.

After a tune up, idle is set to about 850 when warmed up. Car runs great.
Sometimes after driving a while, when at an intersection the idle will change and settle anywhere between 500-1000.

Why?

Is this normal when the car is warmed up or hot?
If not normal, what should I check?

Thanks guys.

It could be worn bushes on the throttle shafts of the carbs. Also rubber jets/diaprams starting to go bad/leaking.

 

[AUSMHLY]

It could be worn bushes on the throttle shafts of the carbs. Also rubber jets/diaprams starting to go bad/leaking.

Thanks Patrick,
Joe rebuilt the carbs and would have replaced the worn bushes on the throttle shaft I'm assuming. Also I think the carb cleaner would have found there was a problem there if so.

What about the distributor being worn out?
Would that affect the idle?

 

[tahoe healey]

A dizzy with wear on the shaft will wobble microscopically and make it difficult to tune but should not change the idle speed. If you are using petrolnix a worn shaft should not be an issue. Is your temperature rising when the idle increases?

 

[Healey 100]

My Healey 100 has never had a consistent idle: it tends to slow down as the engine heats up. I have always accepted this as standard behavior for SU carburetors. I had an MGB many years ago and it did the same thing. I have the fast idle cam set on my Healey so I can pull out the choke cable partially to kick up the idle when it slows down too much. This workaround has served me well for years. I don't think the problem has anything to do with the ignition. Check all those balls, sockets and bearings in the throttle linkage too. Any binding or weak return springs keep the throttle plates from closing consistently. If blipping the throttle lowers the idle, you probably have binding linkage.

 

[Patrick67BJ8]

Thanks Patrick,
Joe rebuilt the carbs and would have replaced the worn bushes on the throttle shaft I'm assuming. Also I think the carb cleaner would have found there was a problem there if so.

What about the distributor being worn out?
Would that affect the idle?

Check the distributor with a dwell meter and if it is bouncing around have it rebuilt. Not sure if the idle would vary as much as yours is now. I never paid that much to my idle but I did have it set at around 800/850. The AC compressor kicking in/out changed it but I had my summer time idle set around 1000/1200.

 

[CLEAH]

AUSMHLY,

I have been having the same problem with my Sprite, and it seems to be heat related. When it is hot out, and once the engine is at operating temperature, the idle slows and slows, and can sometimes nearly stall. I pull out my choke cable a bit to speed it up, but it can be unnerving in traffic wondering if the car will keep running. This past weekend I was out for at least an hour with the outside air temp at around 39 or 40 degrees F, and the idle stayed strong at all times. I noticed that the engine temp was down a good ten degrees also. So, all that said, I don't yet know what the solution is, but I think it is related to heat. Never had the problem on the BJ8.

 

[AUSMHLY]

A dizzy with wear on the shaft will wobble microscopically and make it difficult to tune but should not change the idle speed. If you are using petrolnix a worn shaft should not be an issue. Is your temperature rising when the idle increases?

Hey TH, how are you, hear Tahoe got snow the other day.
Temp does not increase. I am using Pertronix.

 

[AUSMHLY]

Healey 100, cleah, maybe the idle falling when the car is hot is normal?
Maybe some others will chime in about their Healeys and if most do this.

 

[John Turney]

Mine's pretty constant. It may vary a bit, but never enough to make me think it will stall, unless I'm in Tahoe.

 

[Keith_M]

Mine's pretty constant too. I would check to see if the throttle return springs on the carb linkages are hooked up right and not stretched out. Also check to be sure the linkages are all moving easily (as was suggested earlier).

 

[glemon]

My experience has been similar to others, nothing wrong with the ignition or carbs, but the idle will speed up a bit as the oil and engine warms, then slow down as it gets really hot, my 100 did this, as have other SU carbed, cars I have owned. Of interest the noted behavior with idle speed and engine heat all but disappeared after I rebuilt the motor on my 100, and as I recall same thing on my TR4a many many years ago. Maybe a tired engine loses compression when it gets hot and the oil gets thinner or something?

 

[EV2239]

Worn butterfly spindles and carb bodies is usually the cause if jets are centered correct and dashpots not sticking

 

[Chris_Masucci]

I had this problem once when the light spring on the distributor weights for mechanical advance was not tight enough initially. The advance would bounce all over the place at idle speeds and so would the idle. Replacing the spring with one with proper initial tension solved my problem. I never noticed when it was cold with the choke out and fast idle cam in play, but once warmed up and choke pushed in, I couldn't get the idle to stay consistent.

 

[bob hughes]

Interesting topic.

Since I rebuilt the engine on my BJ7, I have had an idling problem, it dies at traffic lights. Bill Rawles cast his wizened eye over it and tuned up the carbs and it ran great for a while but now it is playing up, I thought it may be dirt in the fuel line/carbs but Bill suggested turning up the idle speed a bit - now revs at 1000+. Come to think of it, I had similar problems before the rebuild. So - I know that it can't be a worn engine, the dizzy and carbs have not been re-furbished so it could be either one or both. It is running rich (as proven on start up when there was a slight pop in the exhaust and a black mark appeared on my friends white garage door the size of a dinner plate ) so I am going to adjust the mixture just a tad, this may have an effect on the idle.



Bob

 

[steveg]

My issue is: driving the car at speed on the highway (regardless of ambient temp); pull up to a stop and the idle is 1500 which will over a period of several minutes settle down to 750. Diesels after shutdown. Carbs otherwise run fine and appear similar on Fuel/Air meter.
Carbs rebuilt several yeas ago. Distributor rebuilt by Advanced to 25D specs (has pertronix) only a few years ago. Carbs clean inside and will do the "thunk". No apparent binding on the linkage.

Gasoline around here is 5-8% ethanol - wonder if that could have an affect.

 

[57_BN4]

Steve, does your engine have proper valve guide seals fitted? Any movement in the guides will allow oil/fumes/air into the inlet which changes the idle quite a bit.

40559 now has chevvy valve guide seals and will idle so low that the tach barely moves off the pin, some of which I attribute to tightly sealed guides. Recently I have also done the same mod on a very original Tricarb which had the symptoms you describe above so it will be interesting to see how the idle changes. Still waiting on a few small items to arrive before running it. We are also fortunate enough to have mostly avoided the ethanol fad here which is very likely a contributing factor to engine performance issues over your side. Andy.

 

[steveg]

Andy,
Don't think I have any special seals on my somewhat tired 29D engine. So maybe that's the issue. Will keep that in mind for the next time I have the valve cover off.

 

[57_BN4]

Here is a pic from earlier this week of the guide seals installed. They require the outside of the guide to be machined down to 0.53" by a special tool, then they slip on and fit inside the inner springs. Some seals only fit with single springs, these are fairly generic smallblock chev ones, Cometic brand I think.

Another thing with this particular engine (82,000 original miles by one owner, never had the head off) is that it has about 2" of silt built up inside the block cooling passages which would have contributed to dieseling and so on due to the increased piston temperatures.

Andy.

 

[andrea]

My issue is: driving the car at speed on the highway settle down to 750. Diesels after shutdown. Carbs otherwise run fine and appear similar on Fuel/Air meter.


Hi Steve
My car have a 123 electronic distributor- SU HD6 carbs rebuilt a little bit more rich than my desire- and NGK LFR5E sparks- r/compression unknow probably more than 8.5
It dieseling at the shutdown -with the fuel at 95 octane, the dieseling disappeared when i use 100 octane fuel..!!