jetting question Options

[last337]

Okay so I had another thread troubleshooting these carbs but I've found the jetting to be the culprit. I am looking at Bob Tomlinson's suggestions and he is suggesting bw a 1.45 and 1.55 for the main jet. I currently have only a 1.15. I have a 52 for the idle and he recommends a 55. What do you guys think? I am running a stock 2.0.

Also, any suggestions on a vendor for jets?

[ThePaintedMan]

Okay so I had another thread troubleshooting these carbs but I've found the jetting to be the culprit. I am looking at Bob Tomlinson's suggestions and he is suggesting bw a 1.45 and 1.55 for the main jet. I currently have only a 1.15. I have a 52 for the idle and he recommends a 55. What do you guys think? I am running a stock 2.0.

Also, any suggestions on a vendor for jets?

As long as you still have some adjustment (at least 1-1.5 turns out) on the idle mixture screw, the idle jets should be fine.

Glad you took Kevin's advice! It wasn't till I read Tomlinson's manual that carbs started to make a whole lot of sense to me - and jetting logic becomes a piece of cake. The 1.15s are stock main jets for most new Webers or knockoffs - way too small. Whether you go with 1.45 or 1.55 is more dependent on altitude and ambient temperature. With a 2.0 at sea level, probably just split the difference and go with 1.50. If you buy from Aircooled.net, you'll pay a tad more, but John has a return policy that lets you swap out jets if you decide that 1.45 or 1.55s are more what you need.

[rhodyguy]

easier to keep track of what's happening and what you've done if you just use a continuation of the original "carb thread".

[last337]

easier to keep track of what's happening and what you've done if you just use a continuation of the original "carb thread".

Oh sorry about that. I will remember that next time. Thanks for all you guys help!

[last337]

So do you guys think a 150 is what I should go with on main jets?

[michael7810]

My 1911 has 135 mains and AF is 12.5 at WOT (IDF40 w/32mm vents). Not sure how that compares to what a 2L would need.

[last337]

I spoke to a guy at dirtyracingandrods and he seemed to think going from 115 to 150 was a pretty dramatic step especially if the car ran before. He was asking me to diagnose the progression and he seems to think I should only to a 120 first. CB has a chart that specifies 130 so I am not really sure what to do at this point.

[ThePaintedMan]

I spoke to a guy at dirtyracingandrods and he seemed to think going from 115 to 150 was a pretty dramatic step especially if the car ran before. He was asking me to diagnose the progression and he seems to think I should only to a 120 first. CB has a chart that specifies 130 so I am not really sure what to do at this point.

Of course it runs. The idle circuit does all the work before you get about 2000-3000 R.P.M.s, and even 115s are supplying some fuel. The only way you will know for sure that the jet sizes are the perfect for the given engine and ambient conditions is with a wideband air/fuel ratio gauge. Even then, they will change as conditions change. This is why drag racers change jetting throughout a weekend (and ostensibly why IDFs and Dellortos are made to facilitate easy jet changes).

It sounds like you're just trying to get the car to run better and get it in the "ball park." The ball park for you is dictated by the tables you have already read, which suggest for an aircooled engine, at your given displacement, somewhere between a 145-155 main. Keep in mind that a 2.0 flows very well compared to a 1.6, 1.7, etc., so those tables really are more of a sliding scale. Tomlinson dedicated a lot of time and energy to make sure his numbers were applicable for aircooled engines across the board, so I trust him (and much of this was before modern AFR gauges existed).

115 to 150 is a big jump, true. And just because it was true for Tomlinson means it will be true for you. The best you can do is take what has worked and start there. Again, Aircooled.net offers a jet exchange if you're not happy with them. John also has a lot of experience with these cars and might be able to give you his advice, which is far more expert than mine.

As an example though, my 1.7 had 115s and I jumped up to 135s and it still might really be better with 140s... but I like the fuel economy and I keep an eye on my head temperatures.

[last337]

Well from what I understand the main circuit picks up around 3k rpm. That would make sense that it has been undersized this whole time because if I am say running in 3rd and let off the gas then really try to accelerate hard it pops and gets up slowly for a sec then accelerates well which I would imagine is when the accelerator pump kicks in. If I were to accelerate slowly from a stop and drive like grandma you cant tell a difference.

[rhodyguy]

what is the venturi size? (IMG:style_emoticons/default/unsure.gif) engine off, look down one of the venturis. #s should be on the top lip. 36mm=low air speed at low rpm cruise. punch it and there's a delay while the air speed increases, creating the vacuum that draws the fuel thru the supply components.

k

[last337]

Currently this is everything I verified
115 main
52 idle
200 ac
F11
28mm
50 pj

[rhodyguy]

thomlinsons base line guide for dual idf 1835-2000cc.

vent-28mm
main gas jet-145
main air correction jet-220
e/tube- f11
idle jet-55 (prob need 50s. you'll know which works better by the response of the idle air mixture needle screws).

i pulled a 44 i have from A.C.E. these were set up for a 2056 with a carb cam.

vent-32
main gas jet-145
main air correction jet-200
emulsion tube-f11
idle jet-50


did you resolve the fuel pump issue?

k

[last337]

What does the main air correction jet do?

[rhodyguy]

p. 59.

[last337]

So I talked to john at air cooled.net and he said Tomlinson's book is wrong. He think I should keep my mains at 115, go down from 52 to 50 on my idle jets and go down from 200 to 180 on my air correction. He also suggested hooking up the vacuum line on my carbs to my dizzy; I currently have the port plugged on carb and open on dizzy. What do you guys think?

[yeahmag]

I think you really need an LC-1 or LM-2. The LC-1 is very affordable and with ethanol/gas percentages changing this may be a moving target for all of us.

[0396]

I think you really need an LC-1 or LM-2. The LC-1 is very affordable and with ethanol/gas percentages changing this may be a moving target for all of us.

Nailed it.... I got mine from Richard Clewtte and it's saved me many many guess works. I went from 175 main down to 140 main AF ended up between 11:5 to 12:3

http://www.clewett.com

[ThePaintedMan]

I think you really need an LC-1 or LM-2. The LC-1 is very affordable and with ethanol/gas percentages changing this may be a moving target for all of us.

(IMG:style_emoticons/default/agree.gif) That really is the way to go, if you want it to be spot on.

Regarding the idle jet and air corrector jet change - I have received similar advice an a very different engine from John (a 1.7). Not saying he isn't right, but is often more than one way to skin a cat.

A change in air correction jets results in a richer mix throughout the RPM range, but especially at higher RPMS/velocities. You do not not seem to have a problem with getting the idle mix in the ballpark (i.e. setting the best lean mixture at idle), so I don't see the point of changing just the air correction jets. You have a problem with transitioning to the main circuits and it seems like a constant lean condition in that rev range. To me, that suggests you are way off on the main jetting.

Again, take this with a grain of salt a numerically smaller air correction jet *could* solve the problem, but may also leave you with a really rich idle that you can't tune out.

Aaron is spot on - if you want to be picky, an AFR meter is the way to go.

[jmill]

So I talked to john at air cooled.net and he said Tomlinson's book is wrong. He think I should keep my mains at 115, go down from 52 to 50 on my idle jets and go down from 200 to 180 on my air correction. He also suggested hooking up the vacuum line on my carbs to my dizzy; I currently have the port plugged on carb and open on dizzy. What do you guys think?

I agree that Tomlinsons book is heavy on the main jet sizes but he also goes with the larger AC jet which will lean the mixture (F11) at higher velocities. Remember the book was written over 20 years ago for Bug engines. Fuel was cheap and big carbs on small engines were the hot ticket. Those large jet sizes eliminated transition problems with low velocities.

IMHO the 115 and 50 idles are on the small side and might leave you lean at transition. The 180 ACs will then enrichen the mixture (F11) at higher velocities.

My swag at jet sizes with a stock cam would be 55 idles and 120-125 mains with a 200 AC. I have a 25 year old narrow band AFR meter and love it. They are great tuning tools.

[jmill]

A change in air correction jets results in a richer mix throughout the RPM range.

Kinda. It depends on your ET. For the F11, 200 is the base line AC. Above 200 will lean the mixture at higher flows, below 200 will enrichen. AC jet changes won't affect idle at all and will do little to affect transition.

You might want to look into a different ET. The F11 was designed to give a lean transition.