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jetting advice, going from36 to 32 venturi on 2109 engine
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Jens-Ole
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PostPosted: Sun Apr 20, 2014 12:13 pm    Post subject: jetting advice, going from36 to 32 venturi on 2109 engine Reply with quote

Hi,

I'm going from 36mm to 32mm venturis on my 2109 engine. The current jets are 50 idle / 155 main / 190 air which it has been running good and solid but a little on the rich side.

The CSP web page tells me that if I go for 32mm venturis I should go for 120 main, but what about idle and air?
I was wondering if 45 idle / 120 (or 125) main / 160 air could be a start?

Engine spec is:
94x76 on 5.5" rods
40x35.5 CB 044 (more or less out of the box)
comp ratio 10,1:1
CB Eagle 2242 with 1.4:1 rockers
dual 44 IDF's
42mm megerd header with mondo muffler
Very Happy
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75smith
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PostPosted: Sun Apr 20, 2014 3:51 pm    Post subject: Reply with quote

I would start with just the main, but you may end up closer to 47.5, 130, 180

dropping from 155 to 120 is pretty big, even with a vent change
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Hophead
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PostPosted: Sun Apr 20, 2014 5:38 pm    Post subject: Reply with quote

I am curious with that combo why you feel you need to reduce the size of your venturies...

I would keep the 36 mm venturies and drop your main jet to a 145.
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PostPosted: Sun Apr 20, 2014 6:32 pm    Post subject: Reply with quote

With your elevation (average 1600 or so), I wouldnt bump your idles down any further and I would experiment with your mains like Hop suggests. It sounds like your in the range but need fine tuning. I wouldnt change your vents either a 36 to 32 is a too large of a change. Go down on the main one step. Drive it for a day and note changes. If it is still rich, bump down to 145s. When Im close on jetting ( rich or lean), I will drive my ride for at least one tank full to monitor changes/temp/idle-main response/timing and mpg. I suggest this to my customers that have bought an engine from me as well. Gotta really be one with your ride and pay close attention to reach perfection or a trade off of best mpg vs performance.
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Jens-Ole
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PostPosted: Sun Apr 20, 2014 11:20 pm    Post subject: Reply with quote

Well, there are three main reasons for wanting to try smaller venturis which leads to smaller jets as well.

1 is that below 2000 - 2500 it won't do full throttle. The pedal needs to be slowly moved down. In my head that tells me that the carbs are on the large side which leads to issue number 2, or could this just be the accelerator pump that's wrong?

2 is that it's using spark plugs and they are pretty dark.

3 is the most interesting one, i have had a slight over pressure in the case and a customer told me he had the same problem with a 2 liter bus engine and his 2332 IDA race engine. After running both engines in a bench and jetting down the case over pressure was gone.
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PostPosted: Sun Apr 20, 2014 11:48 pm    Post subject: Reply with quote

Well just for testing sake, pop a 55 idle in there and see if that hole goes away. Your set up and heads can support it. Maybe the rich condition your reading on tge plugs is from the mains oversized. Have to dial in idles before u can set mains. Pull your plugs and clean them so that each change will show the new reading.
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smitty24
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PostPosted: Sun Apr 20, 2014 11:59 pm    Post subject: Reply with quote

Weird you post this, I just did the same thing a few days ago. Installed 32s in my 44s (2054cc). I couldnt get decent enough torque in the lower rpms. I prefer lower end power and driveability, rather than more top end. My engine is actually very torquey now...it wants to come loose on the rear end now. Have to be really careful taking off from stops. I got on it a bit hard and went sideways a little. Im still at the testing stage to see what effects it has on temps and such.

I have had multiple sets of Webers on different engines. I think the 120 is too small, despite your elevation. I am at 1k feet and run a 135 main jet, 55 idle, 200 air. From my experience, the 28mm vents require either 115 or 120 mains. 120 sounds way too small with a 32.

I know most the experts say you want a vent size 4-6mm smaller than intake valve size. I intend to try the 34s when funds allow. Might be the best blend of power and driveability.
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Alstrup
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PostPosted: Mon Apr 21, 2014 3:11 am    Post subject: Reply with quote

Jens-Ole wrote:
Well, there are three main reasons for wanting to try smaller venturis which leads to smaller jets as well.

1 is that below 2000 - 2500 it won't do full throttle. The pedal needs to be slowly moved down. In my head that tells me that the carbs are on the large side which leads to issue number 2, or could this just be the accelerator pump that's wrong?

2 is that it's using spark plugs and they are pretty dark.

3 is the most interesting one, i have had a slight over pressure in the case and a customer told me he had the same problem with a 2 liter bus engine and his 2332 IDA race engine. After running both engines in a bench and jetting down the case over pressure was gone.


Take off the 1,4 rockers. and install stock. That cam is not designed for that AFAICT. Keep the 36 mm venturies and get it jetted correct and it will take full throttle at lower rpms. - Provided that the intake ports are not hogged out too much.
Smitty, you need better rear tyres Wink

T
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Quokka42
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PostPosted: Mon Apr 21, 2014 5:06 pm    Post subject: Reply with quote

Yes, I read somewhere you like 5.5" in the rear.

Reducing venturi size to reduce case pressure sounds like an old wives' tale to me. 32mm might be OK with that combo, especially with the limitations of the heads. Open the heads up a bit and you'll need at least 34mm.

The ease with which you can change venturis in the Webers is what made them so flexible and popular, and there are many cases where it really smooths out the response. Any venturi change does change the whole behaviour of the carb and requires rejetting of both circuits, but 120 seems a bit small to me. You need to use what works best with your engine, but I believe most start around 130 with 32s in 44s.
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75smith
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PostPosted: Mon Apr 21, 2014 6:08 pm    Post subject: Reply with quote

Quokka42 wrote:


Reducing venturi size to reduce case pressure sounds like an old wives' tale to me. .


it seems like just a case of re-jetting, a too rich mixture washes down the cylinder wall, reduces sealing capabilities, ta-da magic, too much crank pressure

the changing of the venturi just "hid" most of the jetting changes made also

the reason for changing becuase you can't use full throttle is plausible, it is letting in too much air at once, giving it a weak burn...
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smitty24
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PostPosted: Tue Apr 22, 2014 1:17 am    Post subject: Reply with quote

yeah guys, I need bigger tires! Currently have Kumho 165/80 because it was all that would fit on the back of my long axle short spline KG with gasburners. It is short axled now with Pro Street box...Im going to try and fit some "quality" 205/70 back there in the near future.

Back to comment on the poster's questions...
I like to play with vent sizing and see what it does. I find the sweet size where it gets the best performance as well as mileage mix. I used DRD L5 heads on 2 engines, one a 1904 and the other 2054. On the 1904, with 36mm vents, the engine had NO bottom end. I made countless changes to jetting. Part of the issue was the Web 86b and 1:4 setup. The 36s were recommended for the DRD heads, but that combo just didn't play well on the street until I dropped the vents down. Vents are not the cheapest parts but its worth experimenting with.
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GodJockey
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PostPosted: Tue Apr 22, 2014 2:37 am    Post subject: Reply with quote

I have been thinking about doing this to my 2020cc with 44s. I wanted to go to 34 vents since when I take off and give it WOT the engine bogs for a split second and then it starts to rev up. I'm sure I need to play with the jets and everything else but the I'm sure a little smaller on the vents would help this issue alot though. I just wouldn't know where to start with the rest jets that are currently stock.
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Jens-Ole
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PostPosted: Tue Apr 22, 2014 4:56 am    Post subject: Reply with quote

ok, so if I stay with the 36 vents, put the 1,4 rockers on the shelf and drop down to 145 main or should I try 140 main?
And any suggestions for the air and idle?
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jfats808
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PostPosted: Tue Apr 22, 2014 5:10 am    Post subject: Reply with quote

55/145/190 36 vents. Run it and report back. Your low idle hole should show some signs of better response.
Ive read that the 2242 can use 1.4s so Id leave the 1.4s in there right now. Its a really soft ramp cam w/ 248@ 50. Youll need new CTF pushrods if your going to bump down to 1.25s. Work on your jetting first.
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Jens-Ole
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PostPosted: Tue Apr 22, 2014 5:26 am    Post subject: Reply with quote

So from 50 idle to 55?
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jfats808
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PostPosted: Tue Apr 22, 2014 5:41 am    Post subject: Reply with quote

Idle is always set first. Changing to a smaller/larger venturi depends on combo and how the engine reacts in the current set up first. Granted there are guidelines, I generally wont put a set of idas on a 1600. You know what I mean. Have to decipher first if the engine produces enough air speed to keep the current 36 vents. This is where you exhaust all jetting probabilities. A set of 44s is perfectly capable of working great on your set up.
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You might as well put 10 year build in your bullshit sig, as it will NEVER run. Also your a dick

You can always learn something new, even from a fool.
Check your oil levels routinely!
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Ragman
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PostPosted: Fri May 09, 2014 3:04 pm    Post subject: Reply with quote

I too would work on the jetting and keep the 36mm vents.

55 Idles
150 Mains
200 Airs
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Alstrup
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PostPosted: Sat May 10, 2014 4:43 am    Post subject: Reply with quote

Hello
Apart from the jetting (issue) if there is any, the addition of 1,4 rockers will result in significantly reduced air speed in the ports at lower rpms and also overscavenging. One way to overcome that if high lift rockers are to be used is to advance the cam. Since that is not an easy task when the case is together and assuming that the cam gear is non adjustable, the next step is to reduce valve lift to increase air speed and also back pressure on the exhaust side. This will make it much easier to jet correct and elliminate off idle bog.
For starters I am close to the suggested jetting like 55/140/200

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MURZI
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PostPosted: Sat May 10, 2014 6:56 am    Post subject: Reply with quote

1.4's on a CB2242? What is your net lift at the valve .550? On a stock 40x35 head? That's the first mis match. The second is 1 5/8 exhaust on this engine when you are looking for low end.

If your ultimate goal is low end, I would drop the compression a bit ([email protected] with 10:1...pushing it) so you can add some advance, pull the 1.4's and stick 1.1's on there. Change the exhaust to 1.5", and rejet the carbs.

The jetting of 52-55/140-145/190-200 should be real close. I have used 50 idles on an engine like yours as well. Without a Wideband you will spend tons of money "guessing".
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PostPosted: Sat May 10, 2014 7:57 am    Post subject: Reply with quote

Im with alstrup.200~205 airs 140~145 MAINS. and dont forget the timing, you may need some dist curve work.what dist do you have??it may all be in the dist.....and needd biger airs...and a little less mains.....possiably 1.25~1.33 rockers. ..it just needs tuned.
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