Author Topic: Weber carb set-up (Read 150853 times)

scottly · « **Reply #700 on:** November 07, 2015, 06:50:04 PM »

Swapped in the 4.5 aux, and found them to be, once again, much richer than richer than the 3.5, offsetting most of the mid-range gains from the F16s. I also compared the 4.5s with a previous test with the same jetting, other than the F16 emulsion tubes, and found little difference. My gut feeling right now is that 3.5 aux are a better fit for our "small" displacement motors, compared to the 1600-2000cc motors Webers are normally used with. They seem to provide a more even fuel curve, especially with F16s.
The aux number refers to the area of the opening inside the "bulls-eye"; a 4.5 has the area of a 4.5mm diameter circle, and a 3.5 has the area of a 3.5mm circle. The smaller area reduces the amount of "draw" exerted against the main circuit.

BPellerine · « **Reply #701 on:** November 08, 2015, 05:01:06 PM »

excellent scottly,my 3.5s can stay!bill

Tintop · « **Reply #702 on:** November 11, 2015, 05:15:08 AM »

Very interesting info scottly, thanks for doing all these runs. You might find that using a larger pump jet will give more high speed enriching, with the 3.5's. Though you would also have to balance that with more 'spill' so the accel action doesn't go over rich.

scottly · « **Reply #703 on:** November 13, 2015, 07:07:42 PM »

Ran a test with 32mm chokes, just for Jerry's sake.

Next test will be with 50 idles, 100 mains, and 150 airs. I'm interested to see the effect of the 50s at lower RPMs; I doubt they will do much to improve the lean-out at 3300-4300, as that is when the throttle is whacked open and is more accelerator pump territory. Just for the record, 3000 RPM in 4th gear is 37 MPH, and in the real world one wouldn't whack the throttle open under these conditions.

mwasson · « **Reply #704 on:** November 22, 2015, 11:34:08 PM »

What Weber Accelerator Pump Discharge Valve are you using for these tests? Have you did any fine tuning with these or are you using the one that totally blocks fuel bypassing into the float bowl?

I am curious to the effect these have on the A/F ratios as the accelerator pump kicks in.

scottly · « **Reply #705 on:** November 23, 2015, 09:36:05 PM »

Hey Mark, I have used .55 discharge valves for all tests; you can see the effect of the pump at the start of most runs, where the mixture goes rich for a short time. I've been starting the runs at a higher RPM lately, as I'm concentrating on the main circuits for now.

scottly · « **Reply #706 on:** November 23, 2015, 10:06:13 PM »

With an 836 motor, 30mm chokes, 4.5 aux, F16 emulsion tubes, I recommend a 105 main, with an air jet in the 175-195 range, depending on altitude. If using 30mm chokes with 3.5 aux and F16s, or 32mm chokes with 4.5 aux with F16s, use a 110 main, with air jets in the 185-210 range.

BPellerine · « **Reply #707 on:** November 24, 2015, 02:54:41 PM »

so bill bentons carbs read just like your rec jetting,so jetting was good for 836!did you do any driving with any of these setups scott,change in response?

Tintop · « **Reply #708 on:** November 24, 2015, 04:43:07 PM »

Quote from: scottly on November 23, 2015, 10:06:13 PM

With an 836 motor, 30mm chokes, 4.5 aux, F16 emulsion tubes, I recommend a 105 main, with an air jet in the 175-195 range, depending on altitude. If using 30mm chokes with 3.5 aux and F16s, or 32mm chokes with 4.5 aux with F16s, use a 110 main, with air jets in the 185-210 range.

We need to have FAQ to keep copies of individual posts like this. Tough sorting through all the pages just to find one post like this.

Great work scottly.

scottly · « **Reply #709 on:** November 24, 2015, 05:41:24 PM »

Thanks Brian. Check post #1 on this thread.

scottly · « **Reply #710 on:** November 25, 2015, 05:32:20 PM »

I've been sifting through the data, and I thought this graph comparing the baseline with recent tests was interesting. I believe the tendency to go rich starting at about 4000 RPM, and starting to go lean at about 6000 RPM may be caused by the cam timing and overlap? The torque peak is always around 7000 RPM, regardless of jetting.

scottly · « **Reply #711 on:** November 29, 2015, 05:26:24 PM »

A while back I told you guys I could save you some money, and now I'm ready to back that up. With 30mm chokes on an 836, any combination of 3.5 and 4.5 auxiliaries and f11 and f16 emulsion tubes will work nearly the same. There is no need to spend $240 for aux, or $40 for tubes. Use the following charts as a guideline. (With the 3.5 aux, I would've gone down one size on the air jets in both cases, but I didn't have 180s or 205s on hand)

scottly · « **Reply #712 on:** December 08, 2015, 08:59:10 PM »

With the "heat wave" we've had here the last couple of days, I managed to get in a little butt-dyno time on the road. The 9+ increase in both HP and ft/lbs at 5500 RPM is quite noticeable, as well as the nearly 5 HP increase at 7000 RPM, although I barely tapped into the upper RPM range for fear of getting a go-directly-to-jail speeding ticket.

On a dead end road in a lightly traveled industrial area, I whacked the throttle open while idling along in first gear, and the bike responded as if shot of of a cannon; no balks, sputters, or farts, it just picked up and went.

The one thing I did notice that the chassis dyno wouldn't detect was a slight hesitation when cruising at 65-75 MPH: I had felt this before with 40 pilot jets, and corrected it with a switch to 45s, but with the much smaller main jets it seems 50 pilots are in order.

BPellerine · « **Reply #713 on:** December 09, 2015, 02:42:33 PM »

great news scott,my stocker is like that ,if I open it up with the revs up a little bit in first.bill

scottly · « **Reply #714 on:** December 09, 2015, 06:15:37 PM »

Bill, have you ever tried 110 mains?

BPellerine · « **Reply #715 on:** December 10, 2015, 04:53:28 PM »

no I have not scott,my carbs came with 115s ,and did not have any others,always thought I would have to go bigger not smaller with a bigger engine,building a 900 flat top piston engine that will be ready in the spring.bill

scottly · « **Reply #716 on:** December 10, 2015, 05:16:15 PM »

With 29mm chokes and 900 cc, 105 mains may be on the rich side? I have no idea how your emulsion tubes affect your particular tuning, though...

BPellerine · « **Reply #717 on:** December 11, 2015, 01:41:39 PM »

after seeing your results,I am thinking that the 115s will not be too lean,have f11 tubes,will only know when I try it.the thinking was always to go to something like 125 mains but you have proven that wrong.bill

mwasson · « **Reply #718 on:** December 14, 2015, 02:42:45 AM »

I am looking at running F16s and 3.5 aux. I am at 750ft where I live in Germany, and it seldom reaches 90+ degrees in the summer. I am going home in June to change the jets, etc. and get it back on a dyno (or may not need one with all your testing and research!).

I do have a .35 pump jet, but may go to the bigger .50 if you think that is better for a 736.

What main and A/C jet would you recommend as a starting point considering the altitude difference? I do not have a lot of time at home and want to get as close as possible on the first shot.

Thanks for all you have done!

scottly · « **Reply #719 on:** December 14, 2015, 06:38:36 PM »

Mark, I thought you had an 836?? For a 736, use 28mm chokes with similar jetting to my 836 with 30mm chokes, including the 35 pump jet.
As far as altitude, don't worry about it; my jetting is slightly rich, other than what I already mentioned about the air jets with the 3.5 aux. When the altitude goes up, the air is less dense; a given volume weighs less, while gasoline weighs the same. That less dense air pulls less fuel through the main circuit, so the effect is less than most people think, at least as far as A/F ratio goes. The total effect on power output is a hard and fast rule of physics, and can't be overcome with jetting.

Retro Rocket · « **Reply #720 on:** December 14, 2015, 06:46:23 PM »

Quote from: scottly on December 14, 2015, 06:38:36 PM

Mark, I thought you had an 836?? For a 736, use 28mm chokes with similar jetting to my 836 with 30mm chokes, including the 35 pump jet.
As far as altitude, don't worry about it; my jetting is slightly rich, other than what I already mentioned about the air jets with the 3.5 aux. When the altitude goes up, the air is less dense; a given volume weighs less, while gasoline weighs the same. That less dense air pulls less fuel through the main circuit, so the effect is less than most people think, at least as far as A/F ratio goes. The total effect on power output is a hard and fast rule of physics, and can't be overcome with jetting.

736 was a typo Scott, its 836 mate...

mwasson · « **Reply #721 on:** December 15, 2015, 01:37:46 AM »

Scottly, that was a typo... it is an 836.

So, you think the .35 pump jets are sufficient with the F16s and the 3.5 Aux?

You mentioned a rough spot between 65-75 which you thought could be eliminated with .50 pump jets.

BPellerine · « **Reply #722 on:** December 15, 2015, 10:56:01 AM »

I think it was his idle jets that he was going to try the 50s on.bill

mwasson · « **Reply #723 on:** December 16, 2015, 12:23:50 AM »

Bill, I am understanding pilot jets as the accel pump jets, not the idle jets. I cannot see how that would affect the engine when the carbs have progressed to the main circuit.

Scottly, I thought you meant that you were changing the accel. pump jets from .35 to .50 to smooth out the engine at 65-75 mph?

Just need to make a last minute adjustment to ensure I am as close as I can get with the new carbs.

Tintop · « **Reply #724 on:** December 16, 2015, 05:42:12 AM »

+1....seem to be confusing idles and accel jets. As the accel jet acts as a high speed enricher, making it larger will effect the amount of fuel available, but also likely require a change to a smaller 'bleed back' valve to re-balance the initial accel pump action. Not sure that the idle jet keeps flowing fuel once the butterfly has moved passed the progression holes.

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Author Topic: Weber carb set-up (Read 150853 times)