Engine hunting at idle, on Vitesse in hot weather

[daverclasper]

Hi. This issue began a bit back (no work on car around this time). It is heat soak I assume, as happens in slow moving/stopped traffic and also when re starting after maybe 15 mins or so. Will clear when the car stretches it's legs.

Car is not running too hot as such, temp gauge same as it's always been

It will hunt up to 2000 rpm, then drop off and repeat. Maybe not fuel vaporization, as does not fluff up/run rough as such?, as it has done in the past, though a little bit prone to stalling.

Home made carb shield fitted

I thought it was maybe the PCV hoses as were not old but not sturdy and maybe collapsing when hot. Have since addressed this and same issue.

Could it be air entering manifold somehow, after a lot of engine compartment heat?

Cheers, Dave   

[Iain T]

Dave the last time my engine had the same symptoms it was running too rich. Have a look at the plugs and possibly lean it off 1/4 turn on each carb and see if it makes a difference. 

Iain 

[Pete Lewis]

 

Dave  this seems to pop up from you before and the main idea is you are running rich  as Iain suggests   turn the adjuster to lean it 

go 1/4 turn at a time ignore lifting pins and colour tunes   use the ears   , lean it,   does it up the idle  then  youre going the right way 

Pete

[daverclasper]

Thanks. I tried to find the old post to re read it, though couldn't on my own profile, or a general search?.

Will definately try that, now you've reminded me.

It got overlooked, as when the weather cooled it was fine.

Cheers

 

[daverclasper]

Forgot to mention, if I rev at idle, at about 1500 revs, for 5 seconds or so, then it will clear the hunting for a while.

I will lean mixture, though just wondered if this was relevant? 

Cheers

[Iain T]

2 hours ago, daverclasper said:

Forgot to mention, if I rev at idle, at about 1500 revs, for 5 seconds or so, then it will clear the hunting for a while.

No its just clearing its throat! The problem is still there. Have you done all the usual checks? Does the air piston drop to the bridge with a clunk, checks if the jet is centralised and not binding at the bottom. Are both needles set at the same length? Are both jets the same depth from the bridge. You'll need a vernier to measure properly. With all those set you have a good baseline to go forward. 

Iain 

[daverclasper]

Thanks Ian. Will have a proper look. Will also check diaphrams, as I understand this causes over richness.

[Pete Lewis]

diaphragm holes cause a mix of problems   

with a perforation the air piston  will not fully rise to raise the metering needle   but the air velocity under the lowered piston will draw more fuel from the jet 

so holes are a bit chicken and egg  with fuel mixtures  ,  never mind a serious loss of power 

Pete

[daverclasper]

I sold a "colortune" a bit back, as there maybe not that accurate, maybe it could have been useful for seeing if one carb was running a fair bit richer. Will look at plugs, to see if it's showing one carb richer, as the last few miles were in slow traffic, when it was hunting.

Not touched the carbs for ages, as has been running steadly and still does (apart from recent hunting)and returns good mpg, so maybe it's not any mixture(though have leaned the jets by a screw flat)/ throttle base settings as such (though won't assume that), as they normally wouldn't change as such, with my low mileage?.

Cheers

 

[johny]

Dont forget you have the mixture test plungers on each carb Dave. Ok they take a bit of practice to get reliable results but if you do the same with each at least the engine should behave equally in both cases unless there is a difference in their settings....

[Wagger]

I can only echo what is stated above by others, but my 2.5L has SU's. The front three cylinders were running rich and mis-fires would begin if left to tick over for more than a minute. I have gradually adjusted the front carb one flat at a time leaning it off, and this problem is diminishing with each adjustment.

I set the carbs first off using the old 'Listen to the sucking' with a tube against my best ear. (Air filters off). It is running at 650 rpm now at idle on the tacho.

In hot weather, a heat shield between carbs and manifold is a benefit. However, I have had 'Freeze up' in extreme freezing fog at the worst time of year and had to remove these. It was a  ong time ago when we had colder Winters.

[Iain T]

As I said before I would measure the jet and needle, its the engineer in me! Why guess when you can measure then if required variance off accordingly. Just because it was running ok before doesn't mean it was optimum. I put a white Tippex line on my jet adjustment screw again so I have a reference point. 

Iain 

[Pete Lewis]

piston lifting pins   SU or Strombergs 

under the piston the main body on most has a short pin you can use to give an audible test of your mixture 

often the pins are siezed and squirt of oil helpsthey have a small return spring to keep them Off .

when up to temp and with the air filters fitted gentley  yes gentley raise the pin to just touchy/feely the contact with its piston

now raise the piston with the pin just a 2-3 mm  and listen for a hint of idle change

you are not looking for a major change more a hint of change 

if the idle rises about 50 rpm for a couple of seconds its a bit rich

if it falters 50rpm for a few seconds its a bit lean

if there is no change noticed its about right 

doing any checks with the air cleaner removed is a complete waste of time as the cleaner has a big effect on mixtures  it must be in the road running condition

this test is a good guide but needs care  if you yank the pin up expect it to just stall

Pete

 

[Wagger]

Yup, air cleaners fitted after the listening test. My fingers are not sensitive enough since chemo to feel the piston with the lifting pins. I have done my best, but now I'll have to stop in my nearest by-pass layby and check plug colours. No good doing it at home as I am over two miles within a 30mph zone.

My son's 2500s had unmatched needles. That caused a headache until he checked them. Never can tell what PO's did.

[daverclasper]

Thanks all

Wagger, I'm interested in what your plug colour is generally like after a few miles of built up area driving if mixture is correct, if you know that?.  Mine in the past are a bit sooty, with some brown (after a Motorway drive with the last few miles through the city, so no choke affecting it), though are only a very pale grey if a check is done after a good run (assume this is ok if car runs well and engine doesn't run hotter?.

Pete, Using the lifting pin, is the response instantaneous and stays like that. When I tried this method in the past, I seem to remember it would start to stall after maybe a second, whatever the mixture was (even when it was over rich)?. 

[Pete Lewis]

Dave   it only gives a hint of change  if it stalls you have yanked it up too much ....or its seriously wrong 

its just a few seconds of change  123 gone 

Pete

[Wagger]

42 minutes ago, daverclasper said:

Thanks all

Wagger, I'm interested in what your plug colour is generally like after a few miles of built up area driving if mixture is correct, if you know that?.  Mine in the past are a bit sooty, with some brown (after a Motorway drive with the last few miles through the city, so no choke affecting it), though are only a very pale grey if a check is done after a good run (assume this is ok if car runs well and engine doesn't run hotter?.

Pete, Using the lifting pin, is the response instantaneous and stays like that. When I tried this method in the past, I seem to remember it would start to stall after maybe a second, whatever the mixture was (even when it was over rich)?. 

So far. numbers 4, 5 & 6 plugs are light brown for 3/4 of the ceramic and grey on the remainder. The front three were sooty but are now less so. That is why I need to stop and check during a faster run. Back in March, number 2 would need cleaning every third startup. Now it goes more than eight runs without a misfire, so I am getting there. No smoke apparent and no oil loss at all yet in about 300 miles.

[daverclasper]

Well. Did some carb work, checked diaphragms, jet distance from bridge (0.1mm difference). needle bases flush with pistons.

Ran it as had to use car anyway. Worse than ever, popping out of exhaust when the hunting/surging  revs dropping and on over run. Still accelerating cleanly with good power. 

Clutching at straws, I wondered if it still could be the PCV valve arrangement (though had previously checked diaphram, and also replaced the hoses with better ones, as were flimsy and distorting under vacuum).

Anyway, disconnected hose at rocker cover and clamped it. No hunting etc!.

Had to increase idle slightly and idle slightly uneven (as was when I bought car and was breathing open into atmosphere.

I never found any reference to PCV causing this issues, so has taking it out of the equation (though blanked off at rocker end it is still connected to manifold) the cause, or just taken away the symptoms of rich running (car has run way over rich before with out these particular symptoms).

Does the orientation/position of the valve top plate (with the small breather hole make a difference, I cant see how?).

Any checks to do on valve when off the car, eg, blowing, sucking, maybe while blocking inlet/outlet orifices etc?.

Cheers and don't know where the red type came from?

 

 

 

[johny]

Hi Dave, what do you mean 'jet distance from bridge'? This is usually roughly set to get the engine running and then each adjusted individually to get the correct mixture. The jets can end up in a different position on each carb although it shouldnt be by much....

[Pete Lewis]

pcv cap orientation doesnt matte rhere the small hole is 

when disconnected did you clamp /block  the hose   ???    leav it ope and you get air when you dont want  it  an weank the manifold mixture 

agree jet height will vary on an individual basis

Pete

[daverclasper]

Hi

Johnny, I thought I would just check both jet's height, anyway, as recommended by Ian, though had read it's not too critical and ok it is very close anyway.

The hose was clamped at rocker cover end as mentioned

[NonMember]

Is the rocker breather smoky? If the PCV is working it restricts flow so that failing to block or clamp the hose would still allow the engine to run (unlike a hole into the manifold). A failed PCV generally weakens the mixture, which can cause popping, hunting and poor idle, and blocking it off (as you have) will fix that. On the other hand, if the engine smokes a lot, the oil vapour can offset the mixture too rich, plus contaminate the plugs leading to misfires and again, disconnecting it would fix that.

[daverclasper]

Thanks. The rocker breather stub doesn't puff smoke.

Just found time to look at valve. Has about a 2cm split in diaphragm.

Maybe got bigger, as didn't see it when checked it when first started hunting a bit.

Hopefully that's the simple issue 

[daverclasper]

The original diaphragm has a recess in the moulding that corresponds to the space between the valve body and the plate atop of the spring. I guess this aids diaphragm up and down movement without stressing/stretching the rubber too much and maybe works more effectively?.

Anyone tried a flat 0.5mm thickness, flat piece of oil resistant rubber?.

Wondering, as the current one only lasted a few years (usual crap rubber).

Cheers 

[Pete Lewis]

ive seen them cut from old marrigold gloves  and polythene sheet  

they all work 

Pete