Cam shafts and off idle performance

[Wooky]

Tech question for those who have played with such things...

I have a Nissan A15 motor fitted to my SJ and suspect that this particular engine has a performance (Hi po) cam fitted.
The A series motors had 3 different cam profiles fitted depending on application and geographical market:
1/ Economy cam - lower power but greater fuel efficiency
2/"Hi Po" cam - higher rev range and power
3/ Mild cam - stock cam after mid 80's with emission controlled carb etc.

The "Hi Po" cam was dropped due to not being able to meet idle and off idle emission regulations as well as having a poor idle and just off idle characteristics.... as far as I can google.

I suspect I have this cam fitted as Idle and just off idle is proving difficult to tune 100%.
General driving the motor pulls through the rev range extremely well.

Symptoms:
Idle is lumpy at the factory 650 rpm but smooths out at 900 - 1000 rpm ( throttle butterfly closed)
Idle is better with a slightly richer mixture
Off idle (1000 - 2000 rpm) with light load (in low range) the plugs carbon foul after ~ 40 to 60 seconds and a misfire develops
General driving, including long cruise at low throttle, the plugs are light tan - greyish.

From what I can see with the plugs it would indicate that they are cooling off below the cleaning temp at low load partial throttle and therefore carbon fouling due to the cam profile requiring a slightly rich mixture.

Is that a fair assessment and would a one step hotter plug help the situation?

[grips]

Ignition timing? Seems more timing related. I have run hot cams in several engines for many years and never had a plug fouling cam related experience.
Check out Cam timing with a degree wheel. That will help to identify the cam via its duration and lift.
Also make sure of your ignition timing and spark quality. They are the normal suspects when fouling plugs.
Air Fuel Ratio also could be out. Check your carb jet sizes and idle circuit.

[Wooky]

Ignition timing is spot on and have new electronic dizzy, coil, plugs and leads.
Vac advance and mechanical advance working OK.

Spark looks good but don't have a spark gauge.
If I increase the plug gap from 0.9mm (stock) to 1.2mm as a test the symptoms get slight worse.

AFR looks OK. 13.5 - 14 throughout the rev range

Haven't degreed the cam as yet (not sure how to do that)

I had also originally suspected the usual suspects (timing, spark and jetting) as I am playing with a bike carb on this motor but just can't get rid of the misfire when playing offroad.
Street driving I would not even know there was an issue... only off road in the really slow crawly stuff.

It is right at the point where manifold vac has fallen off ( so no vac advance) but mechanical has not got much over the base 7 deg BTDC.
Not sure if this could be part of it?.... maybe add a few degrees to the base timing?

[Wooky]

Forgot.....

Other test conducted was done at various ambient air temps.
Does not seem to make to much of a difference between 4 deg C and 20 deg C.

I had expected that the colder air would have performed better if it was a rich condition

[Paul#25]

Try changing the heat range on the spark plugs and try one of the fancy type tip plugs. Most of these iridium plugs are expensive but don't fowl easily and last a long time.

[Wooky]

Try changing the heat range on the spark plugs and try one of the fancy type tip plugs. Most of these iridium plugs are expensive but don't fowl easily and last a long time.

That was the way I am leaning but having never dealt with something like this before wanted to check and see if anyone had anything else to add.
I have just ordered plugs one stage hotter. NGK says that should equate to 70 - 100C difference.

Hope that works.... My OCD is starting to kick in

[grips]

With a hotter cam you need advanced ignition timing a degree or two

[Wooky]

With a hotter cam you need advanced ignition timing a degree or two

Thanks Grips.... I saw two listed specs for the Non - California models.
One was 7 deg BTDC and the Other 10 deg BTDC.
I went for the lower but will give the 10 deg a try.

[Paul#25]

Try changing the heat range on the spark plugs and try one of the fancy type tip plugs. Most of these iridium plugs are expensive but don't fowl easily and last a long time.

That was the way I am leaning but having never dealt with something like this before wanted to check and see if anyone had anything else to add.
I have just ordered plugs one stage hotter. NGK says that should equate to 70 - 100C difference.

Hope that works.... My OCD is starting to kick in

I had a NGK catalogue in pdf format on my computer that had good information on cause and symptoms of plug problems. I'll see if I can find it and post a link here. There might be an updated one on the NGK website.

[Wooky]

Thanks Paul....

I have downloaded one and read through the NGK webpage.
That's where the idea started brewing about plug temps.

[Apocalypse]

I should be hanging out here more often....

I have one of these in my forklift and it runs on LPG. so it doesn't really count, so I must revert to general theory...

I think Grips got most of it.

first of all check your cam timing. I doubt even the factory 'Hi Po' cam is running anything close to a 288, never mind proper overlap.

then check it again. It could be some custom cam with dodgy marks - put a degree wheel and gauge on it to check.

Fouling plugs on idle is a 2 stroke thing, or just stupidly rich because the cam is a tooth out.

and, as grips says... if it's a bit wild, you need to run idle ignition very advanced, and depending on whether you have a working advance in the dizzy, that might over advance at full throttle.

but check the cam timing properly to start.

[Bugger]

Ignition timing is spot on and have new electronic dizzy, coil, plugs and leads.
Vac advance and mechanical advance working OK.

Spark looks good but don't have a spark gauge.
If I increase the plug gap from 0.9mm (stock) to 1.2mm as a test the symptoms get slight worse.

AFR looks OK. 13.5 - 14 throughout the rev range

Haven't degreed the cam as yet (not sure how to do that)

I had also originally suspected the usual suspects (timing, spark and jetting) as I am playing with a bike carb on this motor but just can't get rid of the misfire when playing offroad.
Street driving I would not even know there was an issue... only off road in the really slow crawly stuff.

It is right at the point where manifold vac has fallen off ( so no vac advance) but mechanical has not got much over the base 7 deg BTDC.
Not sure if this could be part of it?.... maybe add a few degrees to the base timing?

What Bike carb are you playing with on this motor ?

Bikes usually have a carb per Cylinder so tunning could be a issue

Or did I read the Bike carb thing wrong

As suggested try degree the cam to know what you got with less guessing


Sent from my SM-G965F using Tapatalk

[Wooky]

Thanks for the input guys....

I am not 100% sure I have the "Hi Po" cam and not even sure how wild that cam actually is. There is very little info on the net about it.
What I can find in the ineterwebs says that the mild cam runs out of steam at 4800 RPM while the "Hi Po" cam runs up to 6000RPM
My motor will pull happily to 6000RPM. This along with the reports of lumpy Idle and off idle got me thinking that this might be a cause.

Will have to check the cam timing and degree the cam to make sure.... new skills to learn

I am also not 100% sure I don't still have a fuelling issue as I have no base line to work from with playing with the bike carb.
The carb is a Keihin CVK40 CV carb off a KLR650. Single carb feeding all 4 cylinders. Getting it to run was easy, fine tuning it has proved to be some what more tricky. At this point the carb is running exceedingly well and giving ~10km/l combine cycle (31"tyres and low gears)

The point where this fouling occurs is ~1/8th throttle and right at the point where the transfer ports cross onto the needle. Normally this is not to much of a problem as this is a transient phase so if fueling is off a bit it is not much of an issue. Normal driving you would not notice the issue at all. It only occurs when you hold constant throttle at that point for 40 to 60 seconds with little to no load on the engine.
The plugs carbon foul and a misfire develops but no real loss of power or acceleration to speak of. Rev it up to 2500rpm and the plugs clear almost instantly. Plugs clear at idle but takes about 30 sec.
With misfire you do get a rich smell from the exhaust (only after the misfire develops, before that exhaust smell is normal) but no smoke at all from the tail pipe...

I am fairly confident that the ignition system is good and both the mech and vac advances are working.

I picked up some BPR4 plugs (stock=BPR5) this morning but will only get to test again on Saturday.

Cheers

W

[HenriSteyn]

The plugs fouling and the rich smell you describe point to too rich a fuel mixture at low revs? Sort of similar to a two stroke bike carb that has its needle worn on the thinner section at the taper.

Plug heat range is only really problematic at high temperature i.e. higher revs.

Hope this helps.

[Wooky]

The plugs fouling and the rich smell you describe point to too rich a fuel mixture at low revs?

That's what I also had in mind and have spent the lockdown weeks playing with the pilot jet, needle (profile and thickness) and needle position combinations to see if I could clear it up. I can lean out that point in the fuelling to clear the fouling but then end up with a huge fuel hole (no accel pump) and the car is almost un-drivable. Current AF ratios actually look quite good (13.5-14) rolling on the throttle slowly through this transition zone. It is only if you hold the the throttle at the 1/8th point for a while does it occur. Also ambient air temps seem to have little impact on the situation.

This lead me to start looking at other factors.

One thing I have found googling the net, is that at idle to just off idle the cylinders are not actually filling completely with each intake stroke and hence run at a lower overall compression. This decreases the combustion temps and allows the plug to cool below its self cleaning temp and carbon foul. This is exacerbated by "hotter" cams...... hence my question here.

My current plan of action is:
1/ try the one step hotter plugs
2/ If that does not work then add a few degrees to the base timing
3/ if the above does not work then check cam timing and degree the cam to see what I actually have in there.