Printable View
Hi, need some guidance from wise heads. I am struggling with plug readings that are rich on 2 & 3, but lean on 1 & 4. Going richer on the secondary seems to make 2 & 3 richer, which seems counter intuitive.
What is the general wisdom around which carburettor venturi feeds which pair of cylinders?
does your carb not have dump tubes?
No dump tubes as run in racing today, but do have a carb lid with them that I prepared based on the instructions on the Lola registry. Struggled to get them set up when I tried a few yea back. Finally decided that unless you have access to a dyno, it's too hard to tweak them to optimise cylinder mixtures.
Do you think dump tubes might help?
IIRC, if you look closely at the intake manifold, you will see that one carb throat feeds mostly to cyl's 1 & 4, while the other feeds 2 & 3. So you need to richen the one that feeds mostly to 1 &4, or lean the side to 2 & 3.
Yes, you would think so, however I'm finding that the secondary barrel, which looks aligned to cyl's 1 & 4, actually seems to feed 2 & 3 and vice versa. Swapped the 180 (sec) and 170 (primary) jets around and noted a slightly leaner 2 & 3 as a result.
Wondered if anyone else can back this thinking up?
Dumb question: Swapped air-corrector jets, or main jets? 180 & 170 sound like air-corrector sizes. Larger air jet will lean the mixture. Larger main (bottom of float bowl) will be richer.Quote:
Originally Posted by Vne165
Sorry - should have been clearer, was the mains I was referring to.
AC are both 170 for primary and secondary.
OK, another dumb question: do you know that the jet sizes are correct and the one marked 170 was not drilled larger?Quote:
Originally Posted by Vne165
Otherwise I don't understand what's happening.
I haven't had a Weber since 2007, but your issue is contrary to what I thought I knew.
Yes - the jets are good, not drilled.
Agree, it's not what I expected.
I'm running NGK BP7ES plugs.
Carby is in good order, no rattling venturies, float level ok, butterfly shafts ok.
The car run's fine, I am just seeking that little bit extra.
OK, another thought - what emulsion tubes are you running? Some #'s will not correct properly due to too small side vent holes. When that happens the mixture will be too rich.Quote:
Originally Posted by Vne165
I used to run ones marked "50" which had larger vent holes to get enough air flow at high RPM's.
There's a thought - haven't considered ET's - will have to investigate and revert. Thanks for your assistance so far, it's appreciated.
Will get back to you.
I’ve just spent the last couple of days accurately measuring all the jets in all my three carbs. What I have found is that even new jets can be ten points out on what is marked on them. So a 170 can be any where between 165 and 175. My advice is only consider the marked numbers as a suggestion only.
get a set of jet drills and an accurate small micrometer.
Thanks Hawke, good advice.
Still curious to learn about others experience with the way the inlet manifold behaves with respect to each carburettor barrel.
Also make sure the ET's are not full of crud - that can affect jetting. I used a small wire to get dirt out that was packed in their bottoms.Quote:
Originally Posted by Vne165
sc94, DaveW, thanks for your inputs, both good suggestions. I had seen that post before but forgotten about it.
So reporting back as promised:
Primary
Main Jet: 180
AC: 170
ET: F50
Idle: 50
Secondary
Main Jet: 170
AC: 170
ET: F66
Idle: 55
Plugs after cooldown lap and return to pits look as follows"
1: Lean but ok, very white insulator, limited discolouration of the earth electrode, looks to have been hotter than 2 & 3
2: Ok, some slight brown discolouration of insulator nose, earth electrode looks slightly brown.
3. Ok, better discolouration of the insulator and earth electrode. If they were all like this I'd be happy.
4. As per #1
Just to reiterate, the main jets were reversed before the last heat to see if it helped the richer on 2/3 and lean on 1/4.
It looks as if it did based on plug readings.
One more symptom to report - idle prior to race usually normal, post race tends to idle much higher (when hot??).
Vacuum leak? When it’s hot and idling high spray some carb or brake clean around any sealed surfaces or fittings. If the idle drops you found your issue.
To get a more accurate reading of the spark plugs, is it possible for you to avoid doing a cooldown lap? Ideally, you would kill the engine at full load while entering pit lane after a 100% race-pace lap, then coast to a pit stall where you could subsequently remove the plugs for inspection.
I didn't catch this earlier but was the motor always like this or only recently?
If it's been like this since "new" then your porting may be slightly different on the different cylinders or perhaps the gaskets are either covering part of a port or are leaving a gap around one of the ports and causing swirling.
If its just recent, again did you change any gaskets as above or you may have a hairline fracture in the head, especially with the idle change when hot.
FWIW Robby
The intake manifold air leak is a great suggestion as it can cause all kinds of issues. Those small dump tubes are a tuning tool and the length, shape and direction are very important to balancing the fuel mixture.
Reading the sparkplugs is an excellent way to determine individual cylinder performance but it is vital to use NEW sparkplugs when doing this. Once the porcelain is discolored - lean or rich - they are not going to tell an accurate story after any mixture/jet changes. Start fresh or your plug "readings" won't be accurate.
Good luck with this!
Thanks again to all for the helpful suggestions.
Plugs were new, no opportunity to do a plug cut on race days however.
I'll get some pictures and post just out of interest.
I would say the motor has always tended to be lean on 1& 4 - gasket overlap not an issue, very careful to make sure it's nicely trimmed when I install. Butterfly shafts and manifold sealing all ok to the best of my knowledge.
To reiterate, dump tubes not installed.
I have the head at the machine shop now to do a re-face of valves and seats, then a chassis dyno session booked for the 25th.
Will report findings after that.
Once again, thanks to the group.
Vne
you need to check your emulsion tubes. They control air/fuel mixture across the rpm range. Different tubes have different sized holes, and more important, is the placement of those holes in the emulsion tubes. That controls the internal power circuits in the carb.
As purple frog stated in that other thread, it's a balancing act. change the fuel circuit at the wrong rpm, and you will run lean/rich just like you describe.
a factory carb will run an f-50 emulsion tube for the primary and secondary (according to my notes)
One of my racing prepped carbs (without dump tubes) used an F-50 (P), F-6 (S) with main jets in the range you're using
An Ivey tuned carb will most likely have something like an F-5 (P), f-50 (S). It all depends on the fuel you're using, temp elevation, etc..
Im interested to hear where you getting the chassis dyno work done, and if it’s a water brake or inertia dyno.Quote:
Originally Posted by Vne165
Thanks ChrisW52, think I'll leave the F50 (p) and change to an F6 (s) for the emulsion tubes.
Here is a picture of the plugs, 1 to 4 (L to R).
Hawke, I'm going to try West Racing, it's an inertia unit.
Better than nothing.
https://www.apexspeed.com/forums/att...d=106599&stc=1
How are the valve guides and seals on #3?
New exhaust guides and seals prior to last race a week ago.
Head is in the shop now as I ran out of time to get the seats and valves faced.
See what the guys say this week, but not expecting anything out of the ordinary.
It is odd though, I agree. Both 2 & 3 were that colour after qualifying and the first heat, but #2 seems to have cleaned up.
As said previously, unless it's a "clean-cut", idle jetting may be giving a false indicator if you drove into the paddock and then shut it off.Quote:
Originally Posted by Vne165
That rich plug (#3?) is way out there
As a general question, would compression and leakdown tests reveal anything? How about header temps right off the head?
I’d personally run a vacuum test as well, but (without knowing this motor) would think a vacuum leak would show (lean) on all the plugs.
I agree - I wonder if maybe the float level is too high and fuel is spilling over into the intake manifold or idle circuit under some cornering or other condition. IIRC, there was a fix for that other than changing float level, but I don't for sure remember what it was. Might have been soldering shut the secondary idle jet. Someone like QS would know.Quote:
Originally Posted by E1pix
Thanks all.
Engine was new for this meeting, rings bearings etc.
Float level ok (as far as I can tell, no symptoms to suggest otherwise), set just about halfway up the beach in the float bowl.
Standard 98 octane pump fuel is used by the way, not avgas. Standard secondary operating linkage as well.
Possible that valves were passing slightly, as mentioned the head just reassembled and put straight back on without valve work back on - but all looked ok on a close visual inspection.
On a close inspection of the inlet manifold yesterday, noted that the phenolic spacer had some serious overlaps which may have been causing some turbulence.
Have sourced an F6 ET, and will go down a size or tow on the AC's.
Have a 4-channel K-type temperature data logger and a hand held IR temp gun which will be utilised on the rolling road dyno in a couple of weeks.
You mentioned “this meeting,” were you at Philip Island?Quote:
Originally Posted by Vne165
Intelligence included, I have little to go on but have a feeling the valves aren’t adjusted evenly. Just a gut call, but I can’t imagine some intake manifold flow imperfections would make just one plug that horribly rich.
I’ll add you’ll likely see a bigger gain that you might expect when the cylinders are all burning the same.Quote:
Originally Posted by Vne165
Hi E1,
No - was in the West, not Phillip Island. Valves were adjusted correctly, once when building the engine, again after the first heat cycle for a head re-torque.
As mentioned previously, both 2 & 3 plugs looked similar, however #2 seems to have cleaned up better than #3 after I swapped the jets around to smaller in secondary larger in primary. I'm beginning to convince myself it's a combination of incorrect air correction and emulsion tubes causing 2 & 3 to run rich at high speed.
Vne
So there are developments this afternoon.
Took a call from the head shop this afternoon, who were reporting 2 x bent exhaust valves. Which ones and how bad, I don't know. I have had some missed shifts over the last years and one I remember from the weekend before last, but the engine had three seasons on it prior to the recent refresh so could have happened anytime.
I know they were not perfect when I threw the head back on, but must confess I didn't check valves for runout, but neither did I notice anything obvious. So might have happened since the rebuild.
Now to search for valves, not easy to get quickly in my part of the world.
More to come. Living and learning. Love it.
Call Ivey Engines 1-503-255-1123. I would think they can ship them to you. Also get a set of their valve springs. Those springs seem to last forever, and hold their tension.
Valves easily obtained in Oz. If you want compliant FFI valves - Elery Motorsport or Larner’s in Melbourne list them. If you want non legal - just Google it - plenty available at about a 10th of the price.Quote:
Originally Posted by Vne165
PS get a good rev limiter.
Hawk, I believe the Ivey valves were built to the original specifications by the original builder, but Ford would not allow them to stamp Ford on them. You can do better?
Roland, for a legal engine here in Oz, you can only use the very expensive original Ford valves - must have Ford logo - or FFI valves, again with the FFI logo.Quote:
Originally Posted by Roland V. Johnson
Thanks Hawke,
Yes, they are easily obtained from 3,500 kms away.
Shipping being what it is at the moment, that's a week!
The price of living in nirvana I guess.
Hi all,
feedback on the rich plugs scenario, ultimately resolved.
The situation was caused by a combination of the following issues/factors.
- Poor sealing of a couple of exhaust valves, now fixed with new valves and seat refacing.
- Wrong grade of plugs (too cold)
- Incorrect jetting across the range.
Five hours of dyno testing now has the car running the best it has ever been, lambda readings spot on through the rev range, egt's all within 10deg C of each other. Crisp and pulling well from idle, snappy response. A different engine really.
Looking forward to the next meeting, I'll probably be slower as I'll have to learn to drive it all over again.:o
Thanks all the input and advice.
Vne