Engine wont go above 6000 RPM AFR suddenly goes very lean DYNO

[Swedish Chef]

I took my car over to a DYNO yesterday and had it tuned up for road use with some drag strip use too. However, I appear to have come across a strange issue.

I have a video of the issue here
Link


Every single pull we did was consistent and exactly the same every single time.

The engine would get upto 6000RPM then suddenly at 6000RPM it made peek power at 120hp every time but the very second it got to 6000RPM the AFR would go lean from a really nice ARF as expected at full throttle suddenly instantly it would go lean like air was coming from nowhere. This was replicated over and over again and always exactly the same. Almost like hitting a rev limiter.

It has a fuel regulator and was solid throughout, the car had fuelling to both carbs with a T piece.

The spec is:
Steve tims Stage 1 heads
CR 8.59
SLR EXR296 Cam W/ 4 degree advance
1.4 Lifters
82mm crank
90.5 B&P
Mostly Berg or scat parts
The ignition is a Bosh Germany 009 distributor with a pertronix ignitor 3 and Flamethrower coil. (I have set the limiter to 9000 to try to rule this out)

My jetting is currently:
36 Venturi
70 idle jet
170 Main jet
200 Air jet
55 Pump jet

So I tried a few things, Checked timing during the pull but the timing did not seem to go out at 6000RPM, Checked fuelling was perfect to both carbs.

I tried a 180 Main jet but that did not make any difference to the AFR through the pull I expected it to run slightly richer but it was exactly the same. (This seemed very strange to me)

My question is what could be causing this, what should I do about it and what can I try?

Thank you in advance

[jeffrey8164]

What carbs?
We’re all just guessing here but maybe valve float?
I’m not sure what exactly happens when you run out of cam.
My Ghia runs out of cam at about 6500 but I haven’t paid attention to what my AFR gauge read at the time.
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[Swedish Chef]

The carbs are a set of DRLA 45, I belive the 4 degree advance was suposed to bring the power in sooner. It makes peek power at 6000RPM.

The only reason I didnt think its running out of cam is, I would expected the power would taper off before then.

I had considered valve float but would it be that reliable every time? I was thinking the heads were good to around 7000 RPM

[67rustavenger]

[Alstrup]

It shows lean because it stops igniting the fuel.
Replace the distributor and the coil and try again.
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[txoval]

Heads are small for the combo but hopefully Alstrup will read this and provide feedback.

Hope you figure it out

Edit…Alstrup is here

[modok]

What I would have tried first is reduce the plug gaps down to .020 and try again.

[74 Thing]

I think you would be rich with your jetting if you measured and verified those jet sizes.

A miss would cause a full lean reading on your A/F meter, so I would replace the plugs, and the distributor or more importantly the Pertronix III (making sure the coil is compatible with whatever you are using eg points or new point replacement module or new distributor), and if it continued look into the spark plug wires as well.

Got to be ignition related if your fuel flow is good as you say it is.

What kind of plug wires are you running-they could be incompatible.

What kind of distributor clamp are you using and it you are using an anodized one it may be preventing the distributor body from grounding to the case.

Lost of problems with the Pertronix III (make sure the coil you are using is compatible with the ignition module-check ohms on the coil with your multimeter).
https://www.speed-talk.com/forum/viewtopic.php?t=40890
https://www.jalopyjournal.com/forum/threads/pertronix-iii-problems-solved.1116927/

[94touring]

Those patronix 3s need low ohm coils if I remember correctly. Like all the way down to a 0.3 ohm. If you're running a higher ohm, say 3 with less energy behind it, the dwell timing of the module can't keep the coil charged between spark firings as rpms increase. I was recently tuning a car with a module designed for 1-1.5 ohm coils that had a 3 ohm coil installed. I was unaware of this. Around 5500 rpms it stopped pulling, whereas the cam and head work should have kept it going to 6500 no problem.

[modok]

ahem, yeah, I wouldn't recommend a pertronix III for any serious purpose, from what I've heard, but, that's just my own bias and experience. Your results may vary.

[mikedjames]

Going "lean" can be a perfect mixture not firing. It still has all its oxygen so the sensor reads lean. Rev limiting ignition ?

However, do you know what the fuel pressure is when it goes lean ?
It is possible your fuel pump and regulator is not able to flow fast enough..
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[meciscokid]

First I would look at the obvious. If the AFR goes lean in a hurry, then I would conclude that you are burning all of the available fuel available. Throw some more fuel at it! Jump up to 190 or 195 mains and see if you get the same result. You won't hurt it by being rich. If that doesn't help, then start looking for problems.

[mcmscott]

You are running your float bowels empty, you need larger needle and seat, also bore all fuel passages to the seat. This is why going bigger on mains did nothing, put a 190 in there and it will go leaner sooner.
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68 Ghia
67 T-1
65 Notch
02 Mexican beetle
74 Thing
15 Long travel rail
07 Nomad
05 f-250

[oprn]

My Pertronics III distributor had a built in rev limiter. 5500 Rpm if I recall correctly. Remember your AFR meter measures unburnt air not the fuel… if the engine stops firing none of the air is used resulting in a lean reading when in actual fact there could very well be lots of fuel.

My gut feeling from thousands of miles away is you are loosing spark not fuel.
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[74 Thing]

To check the float bowl from running dry when it stops shut down everything including the fuel pump the pull the main jet stack on the dells and use something maybe a zip tie to see if there is fuel still in there.

I still think it is the crappy Pertronix III or coil compatibility.

Let us know what you find out. Lots of good suggestions here.

[mcmscott]

Not a fan of pertronix coils, they usually crap out in more "drivability" situations, but I really don't think that is your problem. I run 2 pumps to my Dells, one to each carb, 300 needle and seats and can still empty the bowels on a hard pass.
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There are no stupid questions, only stupid people,

68 Ghia
67 T-1
65 Notch
02 Mexican beetle
74 Thing
15 Long travel rail
07 Nomad
05 f-250

[Swedish Chef]

Thank you all for the reply’s, This is very helpful.

The fuel pressure is always solid at 3psi as I had is measured and mapped on the dyno and it never dropped.
What Distributor / Coil is recommended?
I see people some people like MSD and others don’t. I will try to swap the petronix system out for something better see what difference that makes.

I originally had a bosh blue coil with 009 and points but that didn’t seem to give great results

[mcmscott]

[slayer61]

[jpaull]

Hydrocarbons