Weber settings

[TBG]

I am seeking help with what I think is a simple problem - but being very simple I cannot fix it. I expect Rohan will have the answer or someone else who whizzes about a bit.

I have fitted a brand new QED big valve big port Weber head with QED 420 cams and have changed nothing else apart from fitting wizzy cam cogs to get spot on timing.

The problem is that when accelerating from around 2000rpm to 3000rpm the engine farts and bangs and the car bunny hops down the road. On steady throttle all is very smooth (ticks over at 400 rpm on a touch of the starter) but AS SOON as you touch the throttle all hell breaks loose.

New Lumention ignition, New coil, and jetting as follows. Mains 150, Air corrector 185, Emulsion tube F16, Idle jet 50 F8.

I suspect mixture but what way to go. All was running superbly before head change.

Help please. D

[mbell]

Weber tunning via the internet is probably not advisable.

How ever 150 is pretty big main jet for a twincam and 50 idle is larger than normal. So maybe it's over rich during transition. How did you end up with the 150 main jet?

Have you checked your pump jets are not blocked and ok size?

[TBG]

Mbell - thanks for that. It is a 1700 and something engine and previously we fitted the larger jets to cure a stumble and get more power. I have blown through every known orifice so I think all jets and so on are clear. D

[elans3]

I'd agree, 150 is way too big for even a 1700. i'd have said 125 / 130 would be big enough. I'd try coming down on the main size to 125, bearing in mind the standard is 115. Above could be right , getting too much fuel at progression. I'd say 50 F8 is ok for the increase in bore size, and cams you have, maybe even a little on the conservative side.smaller mains, slightly bigger idle would be my bet. What choke size do you have ?

[webbslinger]

I am waay less than an expert, but in my opinion, a non expert needs an air fuel guage to take the guesswork out of tuning.
I have a 140+ hp motor that is currently using F860 idle and 135 mains. My engine won't run on F8 50s or 55s and it's instantly apparent why when looking at the a/f guage - they are too lean.
Weber (distributed by Redline) has an excellent 6 page handout titled "DCOE-IDF General Guidelines for Low Speed Circuit Calibration". After reading most of the books and I've found these new? instructions to be the most helpfull information I've worked with.

[seniorchristo]

I just built a 1700 twincam with big valves and Tony Ingram cams which have similar lift to the QED 420's. Tony recommends 34mm chokes, 140 mains, and 175 air correction jets as a starting point.
Chris

[TBG]

One further point the static timing is set to 10 degrees. Would this be part of the problem? D

[rgh0]

What size chokes are you using ? 150 mains only useful in 36 or 38 mm chokes in a race engine and even then they don't run well below around 3000 rpm.

The Tony Ingram specs quoted are a good starting point for a 1700cc QED 420 road engine.

cheers
Rohan

[Craven]

Instead of concentrating on adjusting idle mixture at very low RPM, try finding a correct point on the idle mixture screw at 1500/ 2000 RPM range, you may have a progression problem.
FWIW.

[TBG]

Thanks all for some fascinating and useful information. I am using 34mm chokes.

This morning I went into my diddy box of spare jets and fitted 125 main and 155 Air corrector and the difference is amazing. Still some fluffiness but I think I am on the right track. More suggestions so very welcome. I am old and need all the help I can get!! D

[mbell]

You have a few options:
- take it to a rolling road and get it set up
- fit a afr and jets and experiment with some information
- buy some keys and experiment

No one on here is going to be able to tell you the right settings for certain. Might be able to give you some ball park settings to try.

Also what fuel level are you running?

[TBG]

"Also what fuel level are you running?" As per the book settings. D

[mwhitaker]

Hi- I have a 1700 with unknown spec reground C style cams, ported std Weber head and intake manifold with 45 DCOE Webers. With much trial and error my car needs a rich idle jet 65F8, with 130 main jets and 170 air corrector with 45 pump jets and F16 emulsion tubes which I have since replaced with Keith Franck's venturi pump tubes for improved low-mid rpm response. When i was experimenting with diff jet sizes the car would not run well on leaner idle jets with lots of popping and spitting. When I increased the main jets just to 135 it fouled the plugs, so back to 130s. I prob should just go with 34mm chokes but it pulls well up top also so I think I will leave it as is for now. Sounds as if you are on the right track!

Cheers, Mark Whitaker
69 Elan 45/8396

[mbell]

I just have a standard big valve I believe and I am at 50 equivalent idle jets and 115 or 120 mains with fuel level set at 25mm (should help main circuit kick in sooner). Still working on tuning them.

I generally agree with Mark and would be looking at bigger idle and small main jets. I'd suggest try at least 130 and 135 mains and possibly 55 and 60 idle jets.

[elans3]

If you are on 34mm chokes, I'd agree with the above.