Eliminating low speed stumble

[seniorchristo]

Mt S3 Elan has a 1700cc motor with QED 420 equivalent camshafts from Tony Ingram. He gives them a range of 2000-7000 rpm. This bears out well on my car although I seldom exceed 6000 rpm. However, until recently I had a stumble off idle up to around 2000 rpm. My instincts assumed it needed more fuel but advice from a fellow Elan owner and readings from my AFR gauge said initial acceleration went way too rich. As a result I leaned out the idle mixture and went to 30 (from 40) pump jets. Result: Almost no more stumble. I think further improvement may be possible with a larger bleed. Has anybody else had success with this approach?

[alan.barker]

It's not on a Lotus but i have used a AFM on my 1978 TVR 3000S with a very good result.
Got a Lambda boss welded into exhaust just after the manifold. You can watch the AFM Gauge while driving in real time. Same as you modified engine with different camshaft and had never had the Carb jetted correctly.
Mixture wrong with slow running/ transition jet and bad stumble. Mixture too weak causing pre-ignition when switching engine off.
Buying the AFM Kit was cheaper than a rolling road cession and real time driving conditions.
Alan

[holywood3645]

Which carbs 151’s ?.. and what jetting do you use?
cc?
I have QED 1700cc with 420S spec ….
Runs really well. No stumble, pulls strong from 1700 rpm
I can compare jetting if spec is close.
James

[nmauduit]

Mt S3 Elan has a 1700cc motor with QED 420 equivalent camshafts from Tony Ingram. He gives them a range of 2000-7000 rpm. This bears out well on my car although I seldom exceed 6000 rpm. However, until recently I had a stumble off idle up to around 2000 rpm. My instincts assumed it needed more fuel but advice from a fellow Elan owner and readings from my AFR gauge said initial acceleration went way too rich. As a result I leaned out the idle mixture and went to 30 (from 40) pump jets. Result: Almost no more stumble. I think further improvement may be possible with a larger bleed. Has anybody else had success with this approach?

my take on jetting and cam swaps led to the seemingly counter intuitive consideration : when changing profile to a longer opening (say stock to Q420) one do not necessarily need to enrich the mixture, as fuel of the preexisting mix will be sucked in for a longer time (longer opening) through the otherwise identical head resctrictions (ports, valves), so rejetting may not require a massive change.

I generally found AFR quite useful to achieve a reasonable to good initial compromise, a rolling road (or dyno) being required for fine tuning.

fwiw here is my current jetting in a similar situation:

choke 34
idle 40F8 (attempts to solve 2500-2700 cruising somewhat uneveness with 45F8 to be confirmed by richness)
main 135 (M 130 Air 190 would give 1/2HP extra but richness 13,6 at full revs deemed a bit lean)
air 180
emulsion tube F16 (Keith Franck AB > loss of 5hp) F2 too rich
venturi 35
pump 35
cuve 0.50

[alan.barker]

I think the need for rolling road or not depends on state of tune. Also if you are chasing the last hp.
I think on a rolling road they just stick a probe up the exhaust, which is not the best place. It should be in the exhaust pipe just after the exhaust manifold.
With my AFM Kit i can drive, accelerate, drive uphill under load, cruise in real time/conditions.
On the motorway at high cruising speed i can see when stabilised if it starts to run weak or not.
Once carb is calibrated remove sensor and screw in plug. Use kit on other cars good value for me.
Personal choice of course.
Alan

[seniorchristo]

James and nmauduit

I have the original DCOE 18's with:
34 chokes
45F8 idle jets (I think)
130 main jets
170 air correction
F16 E tubes
130 pump jets
50 bleeds

I started with 140 as recommended by Tony but I still run a little rich at cruise and acceleration with the 130's. My next step is to go 180 on air correction jets. Rolling road dynos are not readily available in the US. And neither is expertise in Webers as most everything is FI.

The readings bounce around a lot, I assume because they are Webers and the carbs are old?

[webbslinger]

My motor is about 1615 cc, with Bean's 550E-114 cams and 151 Webers. I had some low speed stumble that I couldn't seem to eliminate until I switched to F16 emulsion tubes from F11's on the advice of Pierce Manifolds' tech guy. That was back in September of 2021. There is no stumble or hesitation anywhere now, and I haven't changed plugs since then.
My present jetting is:
Choke - 33
Mains - 130
Emulsion - F16
Air cor -190
Idle - 60F8
Pmp - 35
I second all the good things said about afr guages. If you do your own tuning you need one.

[alan.barker]

The position of the idle mixture screw gives you an idea for the idle jet. If screwed out more than 3/4 turn idle jet too small.
You can also log on your pc the AFM figures to study after. I must get someone to show me how to do that (i'm not a pc geek).
Alan

[holywood3645]

My jetting is very close to the QED recommended for a 1700cc 420S (super spec)
Build
Ignition is Lucas Constant Energy electronic ignition system.
Inlet & exh cams fully open ai 106 deg
CR: 10.75 : 1
40DCOE151
Choke 34
Main 140
Air corrector 155
Emulsion tube F16
Idle initially 45F8 but changed to 50F8 (ran better)
No stumbling… pulls strong from 1700rpm
Good luck

[mbell]

The transition phase is also very sensitive to fuel level, adjusting it a mm or two can have a big effect. Generally high means richer and lower leans. So personally I view it as another tuning option on the carbs.

It might be worth checking the fuel level on your carbs (via main jet hole method) and seeing if it might be worth adjusting that.

[alan.barker]

Imho you will always have some hesitation/stumble/fluffing under about 1700rpm with a type qed420 type Cam. The price to pay for more power up the rev range.
I had a piper 285 in a 1600 kent engine. I fitted a standard GT Cam in the end for my Sunday driving.
I think the range for 420 is 2000-7000rpm so if you can get that down to 1700rpm you're not doing too bad.
Alan

[Dellortofan]

A lot of running is done on idle jets up to 3000rpm. So can try fat.
Pumps can go to 45 or 50 on some applications. Beware there are two lengths of pump jet - hard to spot until side by side. Downdrafts used longer...
Cams are good in a specific range. If you can invent one good at low And high speed patent it. Oh that's vvt...