jet advice again

[Rocket Ron]

I'm looking advice on jet sizes guys.

I just installed some 40 mm vents in my 48's and there is now a real flat spot / stumble going on mid range now

I'm not very good with carbs and wondered what sizes i should have now I've stepped up on the vent size

currently i have  f7 tubes with 160 main jet and in the idle side i have 120 holder with 55 jet

engine is a 2276 with 42 x 37 heads and 8;1 compression and k8 cam in a full weight bug

thanks in advance for your help

Regards Ron 

[Bruce]

The only person I know of that has been able to make a 55 idle jet work in an IDA is John Plow.  However, he also uses F11 tubes, so that will radically alter where the changeover is to the main cct.
I recommend you try a 60 idle jet.

[Rocket Ron]

thanks bruce i'll try that

[Jim Ratto]

Hi Ron, try the 60's but if the motor comes on strong, idle wise, after 3/4 turn out from seated on mixture, you may need to step up to 65. My 2165 is "in between" whn it comes to idle jets. It runs better on 60's than it does on 65's, but it took some footwork with air jet (holders) to get the 60's to carry powerband thru until mains took over. The 65's were good in that respect, but they would cloud over @ idle and get soft.
I've never got anything under a 60 idle to run correctly in 48IDAs, on any size VW motor.
My jetting in my car is as follows

60 idle
110 air holder
165 main
170 air
F2 tubes
2.00 needles

all w/ 40mm vents

[Bruce]

Ron, you are not restricted to 60s or 65s.  The advanced student of IDA jetting will try custom made 63 jets.

[Rocket Ron]

It did run with the 55 idle jets and never had a flat spot before I moved up on the vents size, so it is quite interesting with what you said with regard to a 55 idle.

I've bought a few jets which are on their way and I'm going to give them a try at the week end, but I'm interested in the function of the emulsion tubes and the result of using different ones could you explain their function to me please

Thanks Ron 

[Fastbrit]

You might find this interesting. OK, it is weighted towards Holleys and he like, but all carbs function basically the same:
http://www.popularhotrodding.com/tech/0611phr_the_secret_life_of_carburetors/index.html

Also, read this page:
http://dimequarterly.tierranet.com/articles/tech_carb_tuning_guide.html

"The natural tendency is for the mixture to become richer as airflow through the venturi increases, creating a stronger vacuum in the venturi. This effect has to be balanced to maintain a perfect mixture at all engine speeds. This is accomplished by leaking a small amount of air into the fuel passage between the main jet and the booster venturi, through the combination of the primary main air bleed jet and an emulsion tube, shown in figure 3A. Imagine drinking water through a straw. Now prick a hole in the straw - you have to suck a lot harder to get the same amount of water. This is the function of the air bleed jet in the main fuel circuit. The emulsion tube is located in the main well under the air bleed to set the point in the development of the venturi vacuum where the bleed hole is opened up. That point can be varied by changing the position of the holes in the sides of the emulsion tube - higher holes mean that the air starts bleeding into the fuel stream sooner, lower holes mean air starts mixing in later. "
 

[Rocket Ron]

Thanks Keith but I'll have a read tonight as I'm at work as we speak,

So which way does the numbering run, is it lower number = lower holes  ??

Ron

[Fastbrit]

There is no direct correlation between the F number and the function on Weber emulsion tubes.   There are variations in diameter to be taken into account, too.

Here's a good feature written by John Connolly on aircooled.net:
http://www.aircooled.net/gnrlsite/resource/articles/jetting.htm

[Jim Ratto]

I have some notes buried somewhere on emulsion tubes I have tried and monitored with AFR

off the top of my head:

F11 too lean on acceleration (ran worst of the tubes I have)
F7 same, but better than F11
F4 too rich as rpms climbed, smaller main made hole in mid transtition
F2 runs best idle to 7000 in my car

[John Maher]

Re emulsion tubes, this might be of use.....

"Hold the tube upside down (main jet up).
The top of the emulsion tube now represents the top of the rpm range and the bottom end the bottom of the rpm range.
Where there are holes the mixture will be leaner.
Where there are no holes it will be richer.
You should be able to home in on the right tube now really quickly."
David Vizard

IMO your current engine specification is too tame to justify 40mm chokes.
FK8 and 8.0 CR aren't going to pull strong enough on the aux vents to deliver a well atomised mixture to the combustion chamber - possibly the reason you just put a hole in the performance curve.
Tinkering with emulsion tubes can help smooth out weak or rich spots but will never mask a 'carbs are too big' problem.

F7 and F11 are the most common e-tubes used on VWs.
F7 will tend to richen the bottom end and add fuel during acceleration. It has no holes at the the top (bottom if you have it upside down as per Vizard's comments).
F11 will weaken low rpm and throw in less fuel during acceleration (holes at the top).

To complicate things further, if you've spent time dialling in the mains and air correctors, an e-tube swap will throw those out too!   

I've spent plenty of dyno time experimenting with various e-tubes thinking I could dial in a pair of IDAs as good as EFI.
Countless gallons of fuel laterI decided it can't be done - well not by me anyway 
There's always somewhere in the range AFR strays into 'far from ideal' territory.

Before swapping e-tubes, make sure the float levels are set correctly.
This has a huge effect on throttle transition and general running.
Also it's not just the hole position in the tubes that affects performance: the diameter of the tubes varies along their length.
This alters the amount of fuel held in the fuel wells (thinner e-tubes = more fuel).
Insufficient fuel in the e-tube well, whether a result of incorrect float level or too thick a tube diameter, can contribute to poor acceleration response, flat spots etc.
Likewise, too much fuel on acceleration will bog the motor.

On many engines fitted with IDAs you have to accept dialling in the air/fuel mixture for best performance can only be achieved within a specific part of the engine's operating range.
It's not going to be great everywhere, regardless of 3rd progression hole mods and other tricks.
If you're only interested in wide open throttle in a narrow rpm band you can pretty much get exactly what you want.
That's why they work great for drag racing.

Disregarding looks/cool factor etc, I'm sure you're aware IDAs are not the best carburetion choice for your engine specification.
Being realistic, a pair of 44IDFs or Dellorto 45DRLAs (if you can find them!) would deliver more average power and torque, better driveability throughout the rpm range and improved mpg.
Probably not the answer you wanted to hear 

[leec]

Ron,
My 2276 with wedgeports and 10.3:1 CR runs IDA's with 40mm vents (as recommended by Matt Keene, similar to a JMR Motor also)

Might have the 37's if you need to borrow them?

John, would you recommend going to 44 vent for racing? Ive done a 12.1@108mph so far?

Lee

[Rocket Ron]

Ron,
My 2276 with wedgeports and 10.3:1 CR runs IDA's with 40mm vents (as recommended by Matt Keene, similar to a JMR Motor also)

Might have the 37's if you need to borrow them?

John, would you recommend going to 44 vent for racing? Ive done a 12.1@108mph so far?

Lee

Just taken the 37's out lee, but thanks anyway mate.

John

I have installed the 40's as I thought that the 37mm vents would be restricting in the inlet track. The car is notably quicker and feels stronger with the 40 vents fitted when its on the cam but the hole is in the transition period from about 2500 to 3000 rpm. I have set the floats with the berg tool and these are spot on.

The car has run a 14.2 and it was in an effort to dip into a 13 that I have changed the vents.

I live quite rurally and don't spend extended time sitting traffic so I could live with a so so idle but I'm looking for a decent pick up and good wide open throttle if all this is possible at once.

Currently the motor feels like it bogging at the 2500-3000 rpm and a bit lean at extended periods of wide open throttle so I was going to try a 60 idle and 200 airs with 180 main.

I did try a set of idfs on a smaller motor but found these gave very poor mpg even compaired to the larger motor with ida's

I'll let you know how I get on with the new jet set but if all else fails I'll have to fit the turbo efi kit I've got in the garage 

thanks Ron
 

[Jim Ratto]

my opinion:

while the emulsion tubes will have an effect on eliminating BIG holes iin powerband, it will not be as pronounced as if car is jetted incorrectly or if vents are too big.

A 2276 turning 6000 rpm should use what a 39mm vent has to offer. Let's call it 40mm.

I have to ask what your distributor is doing? A lot of times guys will point blame at carbs, and sometimes rightly so, but often the carbs get blamed due to their "multiple adjustments factor" ("if I just turn this screw a little, use this jet, etc....") when it comes down to weak ignition or lazy advance curve. My car got a lot more balls off idle by going to locked advance (30 deg).

And is your "bog" or "flat spot" occuring just as you tip in to throttle or do you have to "wait" for the power to come in? If it is just a hiccup as you lean on it, the problem may lie in the accelerator pump circuits. First thing I would check there is a worn cam and roller driver for acc pumps. If the rollers have flats worn into them it delays the acceleraotr pump output for a split second, but you'll feel it. The motor won't like it either. I've heard some nasty detonation just from the rollers being worn.

I think 180 main jet is way too big. But jetting cars over the internet is not soemthing we can really do. You have to take good notes and go test it,

best of luck getting it to scream
Jim

[JS]

my opinion:
A 2276 turning 6000 rpm should use what a 39mm vent has to offer. Let's call it 40mm.

Jim, would head flow not etc. be a big factor in this equation?

[John Maher]

Hi Lee,

Matt told me he'd used a similar spec to the old Nigel Fleet motor. It's a good street/strip combo. Great to see you wringing the best out of it 

Can't give you a definite answer to the venturi question but in my experience I've never found any worthwhile gains when using a choke size the same diameter as the inlet valve - at least not on an engine similar to yours.

44mm chokes might pick up a little peak bhp BUT you need to be wary of losing power either side of the peak.
If average power through the rpm range you use down the track is less, you lose ET, even if peak power is up.

You might gain some bhp opening the chokes to 42.5mm or 43mm but ideally this is best carried out on the dyno.

=======================================

Ron,

60 idles will almost certainly help - you probably have too big an AFR gap between idle and main circuit. That might be enough to eliminate the hole.

Often on milder IDA motors I find a smaller air corrector helps deliver a more consistent AFR. This requires the use of a smaller main, which helps clean up lower rpm when running at WOT - an area that tends to go massively rich when the top end is running max bhp AFR.
In extreme circumstances I've gone as low as 130 on the airs but more often end up somewhere between 160 and 170 on engines making less than 200bhp.

Air correctors have much more influence on the top end (bigger = leaner).
With your proposed 180m/200a combo you'd be trying to lean out the top end with a large air corrector to compensate for a main that's way too big.
If you suspect the mixture at 2500-3000rpm is going rich (almost certainly is), try a smaller main to lean it out and switch to a smaller air corrector to enrich what you've given up at the top end.

It's difficult to suggect specific jet sizes - every engine has its own quirks. My recommendations won't necessarily work with your combination.

You should hook it up to a decent AFR monitor.
Ideally run it on a rolling road or dyno with someone who can interpret the results and zero in on the optimum jet combination.

Re IDFs retrning poor mpg - must have been set up wrong or something else was off.

[Jim Ratto]

my opinion:
A 2276 turning 6000 rpm should use what a 39mm vent has to offer. Let's call it 40mm.

Jim, would head flow not etc. be a big factor in this equation?

it is based only on single cylinder volume (cc) and rpm. Not that "best" venturi will be landed upon by using this equation, but it will get you in the ball park.

Headflow, I think of it as a "limiter", meaning to get "x" size cc cylinder to turn "y" rpm, you will need "a" amount of intake mixture, or VE. The venturi, in my opinion, shouldn't be thought of this way. In my opinion, the venturi diam should be determined by signal strength it sends to circuits over rpm they will turn. Without the stong signal, the rpm attained will probably not matter anyway.

I totally agree with John Maher on the air corrector thing, it is interesting to run "___" motor on a small(er) main and note response, but then equip stack with a very small air corrector (130 like he said), I recall running 150 x 130 in my F2's with 40mm throats and it ran well on throttle. This was before I went to 110 idle air hole and the problem was the canyon between idle and mains. From the low rpm (under 2500) if I stomped it, (and the lower rpm I did this the more pronounced it was) there was a hesitation. The AFR would show the engine not getting what it wanted too. going to 65 idles masked that problem, but made for a inconsistent idle. Going back to 60 idles cleared that, but left hole. Going bigger on mains (155) caused mains and idle to do each other's task, too much.... steady state low rpm was miserable (buck buck buck). Finally took Sarge's advice and drilled a set of 60 idle holders out to 100, then 110, now back with 60 idle. Now it had a nice hard edged, clean idle, but no hole as it went into main. Now I went and settled on mains that would come in nicely and hold a good AFR under load on highway and then selected air correctors that would keep it clean as RPMS went (lower gears). A long time ago, my old boss told me "you lean it out a little on top with a bigger air.... see what it does"
He was right, on some, but not all motors. Seems the smaller, milder motors liked the bigger air correctors (esp IDF motors) but most of the more visceral IDA motors with big ports, long overlap (W130 and bigger) liked the 170-185 airs.

John, thanks for posting, I always dig reading your advice and experinces. 

[Rocket Ron]

Thanks guys this is great reading and thanks for taking the time to help

My carbs have done about 5000 mile now and have been well lubricated since nearly new as I was warned about worn shafts early on but i'll take a look when the lights better later on today 

Also I had a look at my current air  jet and its a 125 too small?

I think I'll start easy with the bigger 60 Idle then see how I go before I get too off track.

Hopefully the jets will be here today then I can play 

Regards Ron

 

 

[Rocket Ron]

just dropped the 60 idle jets in and the cars 95% better on transition I.E. 2500 - 3000

I may try a 65  or open out my 55  alittle over 60 for interest and report back but I could easily live with how its running now, cant believe how much difference there is just stepping up that one size.

Just shows how finely balanced carbs are when they are set up well

Thanks again guys, its been an education and very interesting so far

[Brown-nose]

Here's a good feature written by John Connolly on aircooled.net:
http://www.aircooled.net/gnrlsite/resource/articles/jetting.htm

quote from the aircooled.net article

"What you want to do is this: get the engine fully warmed up. Now, with synchronized carbs (remember?), turn the idle mixture screw on one cylinder in until you hear the engine misfire. It means THAT CYLINDER is too lean, and it doesn't have enough gas to run properly. Turn the mixture screw back out until the engine idle smoothes out again, and then go out another 1/2 turn. Repeat for the other 3 cylinders. "

In the weber tech book it says to turn the mixture screw back out until the engine reaches MAXIMUM revs,no more. which method do you guys use ?

[Bruce]

", turn the idle mixture screw on one cylinder in until you hear the engine misfire. It means THAT CYLINDER is too lean, and it doesn't have enough gas to run properly. Turn the mixture screw back out until the engine idle smoothes out again, and then go out another 1/2 turn."

If the misfire when the screw is in indicates that cylinder is too lean, then when the screw is turned out to the point where that cylinder begins firing again, then that cylinder's mixture is right.  Turning the screw out from there just throws more unnecessary fuel into the mix.  All that does is put out the fire since there's no air for that extra fuel to burn with.

[DKK Ted]

Whats that rule of thumb which determines the size of the vents from the size of you intake valve? Doesn't the cam determines that also, say a FK 87 to a E110, in this case a K-8? I assume that would get you in the ball park, you could go up one size or down one size. Right? 

[Rocket Ron]

quick update

I've changed the 55 idle jet to the 60 which helped a lot, then tried a 65 no real improvement so went back to the 60. then changed to 170 air and 165 main which feels much better higher up the rev range but has put a hole, all be it a smaller one than I started out with, in the transition area between idle and the main coming in so I'm going to try the 65 again or the 55 to see which way to go. I think Bruce said right at the beginning of this thread a 63 idle may be better.   

Jim and John you were spot on with the 160-165 main with the 170 air

BTW my original 125 air was drilled to more like a 170/180 

[Brown-nose]

", turn the idle mixture screw on one cylinder in until you hear the engine misfire. It means THAT CYLINDER is too lean, and it doesn't have enough gas to run properly. Turn the mixture screw back out until the engine idle smoothes out again, and then go out another 1/2 turn."

If the misfire when the screw is in indicates that cylinder is too lean, then when the screw is turned out to the point where that cylinder begins firing again, then that cylinder's mixture is right.  Turning the screw out from there just throws more unnecessary fuel into the mix.  All that does is put out the fire since there's no air for that extra fuel to burn with.

thanks Bruce,i hoped you would comment. from my own (in)experience with my carbs,i can make the cylinder drop(miss),then with a slight turn out it will fire ,then a little more out and it will pick up to its max rpm....then if you go out with the screw a little more it will even out but will lose that "eager" sound and start to feel/smell rich. the transition from "firing" to "rich" is about 1/2 a turn. do people suggest the "smooth then 1/2 turn" thing as just a general "safe" guide ? or just to have all their mixture screws in a similar position ?
lastly,should i be aiming for the "just firing" point or the "max rpm" point on the screw ?
thanks

[leec]

Hi Lee,

Matt told me he'd used a similar spec to the old Nigel Fleet motor. It's a good street/strip combo. Great to see you wringing the best out of it 

Can't give you a definite answer to the venturi question but in my experience I've never found any worthwhile gains when using a choke size the same diameter as the inlet valve - at least not on an engine similar to yours.

44mm chokes might pick up a little peak bhp BUT you need to be wary of losing power either side of the peak.
If average power through the rpm range you use down the track is less, you lose ET, even if peak power is up.

You might gain some bhp opening the chokes to 42.5mm or 43mm but ideally this is best carried out on the dyno.

=======================================

Ron,

60 idles will almost certainly help - you probably have too big an AFR gap between idle and main circuit. That might be enough to eliminate the hole.

Often on milder IDA motors I find a smaller air corrector helps deliver a more consistent AFR. This requires the use of a smaller main, which helps clean up lower rpm when running at WOT - an area that tends to go massively rich when the top end is running max bhp AFR.
In extreme circumstances I've gone as low as 130 on the airs but more often end up somewhere between 160 and 170 on engines making less than 200bhp.

Air correctors have much more influence on the top end (bigger = leaner).
With your proposed 180m/200a combo you'd be trying to lean out the top end with a large air corrector to compensate for a main that's way too big.
If you suspect the mixture at 2500-3000rpm is going rich (almost certainly is), try a smaller main to lean it out and switch to a smaller air corrector to enrich what you've given up at the top end.

It's difficult to suggect specific jet sizes - every engine has its own quirks. My recommendations won't necessarily work with your combination.

You should hook it up to a decent AFR monitor.
Ideally run it on a rolling road or dyno with someone who can interpret the results and zero in on the optimum jet combination.

Re IDFs retrning poor mpg - must have been set up wrong or something else was off.

Thanks John, I will stay where I am for a bit then!

Lee

[Bruce]

.....i can make the cylinder drop(miss),then with a slight turn out it will fire ,then a little more out and it will pick up to its max rpm....then if you go out with the screw a little more it will even out but will lose that "eager" sound and start to feel/smell rich.

I don't see that with my carbs.  Even if I did, I wouldn't be allowed.  Annual smog testing restricts rich running like that.  When you smell the exhaust, you are smelling unburned fuel.  Proof that it's rich.  Running rich at idle serves no purpose, unless you're trying to cover up a lean transition somewhere.
If you like to give it an extra half turn, beware.  Some carbs react differently than others.  A Dellorto has a super fine idle mixture screw, so that a ¼ turn has little effect on the mixture.  The IDA mixture screws have a very coarse thread.  ½ turn with an IDA is a huge change.  Next is the position an IDA's mix screws end up at.  If you checked an engine that was perfectly tuned, you will find the mixture screws will usually be ¾ to 1¼ turns from seated.  A ½ turn more is a HUGE % of the existing setting.

[63 ripper]

If you checked an engine that was perfectly tuned, you will find the mixture screws will usually be ¾ to 1¼ turns from seated.  A ½ turn more is a HUGE % of the existing setting.

you turn your mixture screws out from missfire just until the rpm picks up again,or when the rpms reach maximum ?

[Jim Ratto]

Whats that rule of thumb which determines the size of the vents from the size of you intake valve? Doesn't the cam determines that also, say a FK 87 to a E110, in this case a K-8? I assume that would get you in the ball park, you could go up one size or down one size. Right? 

Determining venturi selection is based upon cylinder volume and rpm desired. An Engle 110 in a 2276 isn't going to make for an engine that needs 40mm+ venturies, let alone 48IDA Webers. Likewise, an FK10 in a 2276 isn't going to be setup to run on 28mm venturies in 40mm IDFs.

If you have engine "x" with a 320 degree cam in, and engine "y" with a 294 degree cam in it, all other variables (except CR) being identical, these two motors will make peak HP @ two different RPMS. The venturi size needs to match the RPM requirements.
The 320 degree motor might peak out @ 6800, the 294 @ 5500, so venturi diameter needs to be selected accordingly.

[Jim Ratto]

Whats that rule of thumb which determines the size of the vents from the size of you intake valve? Doesn't the cam determines that also, say a FK 87 to a E110, in this case a K-8? I assume that would get you in the ball park, you could go up one size or down one size. Right? 

Determining venturi selection is based upon cylinder volume and rpm desired. An Engle 110 in a 2276 isn't going to make for an engine that needs 40mm+ venturies, let alone 48IDA Webers. Likewise, an FK10 in a 2276 isn't going to be setup to run on 28mm venturies in 40mm IDFs.

If you have engine "x" with a 320 degree cam in, and engine "y" with a 294 degree cam in it, all other variables (except CR) being identical, these two motors will make peak HP @ two different RPMS. The venturi size needs to match the RPM requirements.
The 320 degree motor might peak out @ 6800, the 294 @ 5500, so venturi diameter needs to be selected accordingly.

2276 Engine Y with peak HP @ 5500 would run w/ 37mm
2276 Engine X with peak HP @ 6800 would run w/ 41mm (I would err to 40mm)

[Brown-nose]

Jim,how do you do your mixture screws ?
with idle jets,if you are out more than how many turns from seated until your jet is too small...2 1/4 ?
or too big 
thanks