IDF Flat spot
 

can only assume i have to play with jetting but here is the story.

Been playing with the IDF's cause i want to get rid of a flat spot.

If you slowy drive up it is smooth, until you hit about 3500 revs. It is about 30-35kms/hr in first or 45-50km/hr in second. At this point if you hold it there it burbles loudly and falls on its face. If u shift before this point it is as smooth as. It is just sooner or later you have to cross this point
You can easily drive through this flat spot but it does make for a bit of on/off of the carbs.
I like it but the wife doesnt
I dont think it is accelerator pump cause it runs smoothly up to that point.

Blocked progression ports?

Jetting at the moment is
40 idles 1 1/2 turns out
120 mains
180 airs
32 vents
F11 E tubes
Stock valved 040 heads

When we had dyno day a while back i had the same idles and 130 mains.
The graph said i was rich on the mains.
13? on idles and 11 after that.
So i have dropped to the 120's


The carbs are IDF 36's and as such dont have access to air bypass screws.

Would different E tubes give a smoother change from mains to air coming in? This is where the problem lies.


If you just floor it it takes off and pulls hard.
If you rev it in neutral (no load) it revs nice and smooth up past 5ooo no worries

Barry

what type of distributor do you have, ive heard that an 009 dizzy will create a flat spot, but there are ways to fix it up.

yes - i have a 009 but i dont think that is the problem.

Long story but i ended up with the 1776 and a 34 pict in the car for a while and no flat spot.

Pretty sure it has to do with the duals

Hi,
Your 40 idles seem very very small! I'd go for 55 idles with 32 venturis...
Also the air bypass srews are hidden under a lead or alu plug. Remove those and they can be reached.
Good luck,
Walter

I agree defenantly your idles are way too small, and so are your main jets. Your engine is starving for fuel.

The reason the engine works well when you floor it is because it is getting extra fuel from the acceleration pump. Also reving from an idle is the same.

Don't worry about stuffing around with emulsion tubes unless you can get a lot of dyno time for free. Just getting your jetting closer will make a big improvement anyway, and F11's will be as good as the alternative F7's that you might be considering using.

I'm using 44idf's on my 1641cc engine setup as follows

55 idles
130 mains
210 airs
34 mm venturies
F11's.

My engine has been dyno run with an exhaust gas analyser and its right in the mark up to 6800 rpm. The distributer is a bosch 009 set to 32 degrees total advance, with high octane fuel.

In general what I have found with a 009 is set it to the max advance you can get prior to pinging, then back it back of about 2-3 degrees.

With a 1776 i'd suggest going back to square one with the following settings , and try changing things.

55 idles
135 mains
190 air correction.
F11's.

With 32mm venturies your limiting your top end RPM potential to about 5500 RPM on a 1776, but you will get better down low performance than with larger venturies. Keep the 32's if your engine isn't built to rev.

Have a look at the webber tuning technical document that you can download from www.aircooled.net as well. It will give you a better understanding of how your idf's work at diferent RPM ranges, and how jettting overlaps.

I wouldn't recommend messing around with the air bypass valves unless the've already been messed with. Read the aircooled.net document, it will tell you how to set these up. But you'll need a balancing meter and a flow bench to do it 100% correct....

Good luck,

Ian.

it sounds like your idles are fine if it runs as good as you say but i would bump your mains up to atleast 130 because 3500 rpms is where the transition is from your idles to your mains and your airs haven't even kicked in yet as they come in about 4000-4500 rpms. if sydney is at about sea level then your airs should be fine but the mains are pretty small even for 36 IDF's. does the car fall on its face if the peddal is pressed half way down the you just let it slowly rev up through the 3500 rpm range? if so its definately your mains. just to make sure, have you checked your float level and balnaced your carbs with a sync tool? if not your wasting your time with jets as those are the preliminary steps to making it work right. hope this helps.

Im running a T1 1776cc with o4o heads, engle 110 cam, 009 dizzy, flamethrower coil and twin 40mm weber carbs.
Right now I can't remember all the details on the carbs.
I had kind of the same problem but in worse situation.
the engine was running perfectly at idle and revving, but when it came for realy smooth in traffic driving i had a huge flat spot from almost 2000 rpm to about 3100 to 3300 rpm.
then i was running 115 jets main on 32 venturies (it was only for a day and just to pass for emmulsion card) and the problem was solved with 120 jets main. At the very first time I was running 122 jets main but fuel consumpion was way over normal and expected.Now with 120s I have a range of about 350 kms from full tank to completely empty. Im running my engine strictly on Shell V-Power racing gasoline which has a 99+ octane rating which is considered to be the best gasoline in Greece.

I hope I enlighten you in some way

Chris (beetle1303)

Serious flat spot on a weber 40 IDF
 

OK...I know what you're going to say...
Well, I don't really.

I 'm running a weber 40 IDF on a 1600 dualport.
It is a pain in the ***. It has a flat spot when I accelerate or when I'm just sitting at a light. Or when I start the car and I have to gas it a little before I can move. All I here is VVVVVVRRRRROOOO)OOOM!!!!!!!!
SPut sput sput...VVVVRRRRROOOOOOOOOMMMMMMMMMM!!!!!
go..and once I'm on the highway...it's ok....well untill I accelerate again.
flat...go flat go...


Any suggestions? I've tuned the jetting just right to get it this far...Is there something else I can do? Something that will help get rid of that flat spot? running an electric ful pump also.

HELP!

On a 1600?
Get a set of 28mm venturis; problem solved :D
If not: try more options of all the variables you can change :p