SuzukiSavage.com - Print Page

SuzukiSavage.com
/cgi-bin/YaBB.pl
General Category >> Rubber Side Down! >> Carb Tuning
/cgi-bin/YaBB.pl?num=1709040342

Message started by ThumperPaul on 02/27/24 at 05:25:42

Title: Carb Tuning
Post by ThumperPaul on 02/27/24 at 05:25:42

Whoever made this chart got the concept generally correct, but 1/4 is not more than 1/3 just because 4 is bigger than 3.

I can’t remember which burger joint came out with the 1/3 pounder, but the marketing failed because people wanted the “bigger” 1/4 pounder from McDonalds.

Title: Re: Carb Tuning
Post by ThumperPaul on 02/27/24 at 06:16:55

I do have a question about this inaccurate chart - what is being referred to as “Straight Dia”?

My Honda Shadow 750 uses a pair of 34mm Mikuni CV carbs. It has the convoluted stock air box and overly-complicated air intake tubes. The rear baffles on the dual exhaust have been removed.

The prior owner removed the exhaust baffles and there is evidence that someone has been in the carbs. I suspect cleaning only and not rejetting. I haven’t dived in yet to investigate, but I need to get in there and do something to eliminate a lean “dead spot” right off idle up to about 1/8th throttle position.

I’ve adjusted the air/fuel mix screws about 1 turn richer and it helps, but the issues still isn’t 100% cured.

Everyone knows this is a bit of trial and error, but dealing with dual carbs that are wedged in the “V” and have to be synchronized isn’t something I want to do multiple times. The carbs must be removed to do any tinkering.

Stock jetting is front cylinder 40/105, rear is 40/108. A carb tuning/jet selling website suggests 40/108 front, 40/110 rear with 1 shim (washer) on each needle. This set-up is prescribed for slightly modified exhaust (part baffles removed) and a stock air intake system and filter.

The next prescription from the calculator is for lightly modified exhaust AND air intake. It bumps the primaries up to 42 and mains to 110 front and 112 rear with a shim on each needle.

To me, the shims on the needles are going to having the most impact on eliminating the lean condition just off idle and during transition. In a way, this is akin to shimming the needle on the Savage CV carb to reduce afterfire on deceleration.

Sorry for the long post. I’m getting to my other question.

I’m about 3.5 turns out on the air/fuel mix screws. Assuming I find a pair of stock 40 primary jets in the carbs, I’m inclined to go up to 42s (as recommended when exhaust AND intake have been slightly modified). The shims on the needles are definitely getting installed. WOT is good. No real evidence of running too lean up top. But could it be better? I’m debating the 108/110 vs 110/112 change.

Please don’t tell me to do “1 mod at a time”. I can go up on the primary jets from 40 to 42 without an issue - just dial back the air/fuel mix screw to something more reasonable like1.5-2.0 turns out. The shims seem to be the fix addressing the lean condition. What to do with the main jets?

Interested in thoughts, but don’t tell me to do 1 mod at a time. LOL. (I’m not overly concerned about mpg.)

Somebody (one of you gurus) tell me to go 42/110 front, 42/112 rear with a shim on each needle. And if I create a rich condition at WOT, just deal with it and the lower mpg. Note: using WOT isn’t how I normally ride, but some extra oomph is always welcome! Maybe the 108/110 mains is my happy medium and compromise.

Title: Re: Carb Tuning
Post by Gary_in_NJ on 02/27/24 at 15:00:49

Despite the fractional error, I don't agree with almost anything on that chart. The ranges or the effective zones seem incorrect from a "general" point of view, and certainly not correct for most CV carbs. The diagram I have attached is much easier to follow. If you ignore the Slide Cut away curve and the Primary Choke (the bore or Venturi) Curve it is easy to see the interaction between the circuits.

Title: Re: Carb Tuning
Post by ThumperPaul on 02/27/24 at 15:29:43

Gary, your chart definitely makes more sense and agrees with how I think about it. And the fractions are in the right order!

Any thoughts on my dial-in tuning approach?

Title: Re: Carb Tuning
Post by Gary_in_NJ on 02/27/24 at 16:11:10

I agree with your approach. I always get the needle dialed in first since we ride 80+% of the tome on the needle. If you go to a 42 pilot and are able to close the adjustment screw without a drop in rpm, then the 40 is the correct pilot. You wont know until you try. Lastly, before making carb adjustments its a good practice to perform a compression check to ensure that the cylinders are within 10% of each other. If the compression isn’t balanced you’ll never get the carbs correct.

Title: Re: Carb Tuning
Post by ThumperPaul on 02/27/24 at 16:41:14

Thanks Gary. Compression is good. I bought the bike last July with 2600 miles (bike is a 2006 mint condition). I've added about 1700 miles and at about 4300 total. Bike is just now getting broken in. ;)

I'm thinking with the 42s, I should only need to be 1.5-2.0 turns out. Totally agree about the mid range riding 80%. It's unnerving to have this lean condition where it's at in transition.

Title: Re: Carb Tuning
Post by Gary_in_NJ on 02/27/24 at 17:12:00

If the 42 doesn’t get you where you want to be on the pilot/needle transition, then raise the needles.

Title: Re: Carb Tuning
Post by ThumperPaul on 02/27/24 at 17:37:04

That's what I said about adding a shim/washer - raise the needle. That's really the main fix as I see it. The 42s are mostly because I'm turned out 3.5 with the 40s. Between raising the needles and going up to 42s, my transition should correct.

Title: Re: Carb Tuning
Post by Dave on 02/28/24 at 03:58:43

If you are working on a VM carb......the pilot mixture screw is an "air correction" screw, and turning it out leans the mixture. On a CV carb the pilot mixture screw is a "fuel correction" screw and turning it out richens the mixture.

The easiest way to tell which screw you have - is the location. If the screw is located on the cylinder side of the carb slide - the screw adjusts the amount of fuel......if the screw is located on the air cleaner side of the slide - the screw adjusts the amount of air.

Title: Re: Carb Tuning
Post by ThumperPaul on 02/28/24 at 05:43:01

Thanks Dave. Yup! Exactly! I have to consciously think about it because the Honda has a pair of CV carbs and my S40 has a VM36 (ala Lancer carb swap). Once you get it backwards and go the wrong way a couple times, it becomes obvious. lol.

I could probably get away with 4 turns out on the 40 primary jets and get some more improvement, but that’s too many turns out, and then I’d be worried about the screws vibrating out. Plus, it’s not the best fix for my lean transition issue. If I wanted to do 1 mod at a time, I’d raise the needles with a shim/washer. I think that alone would resolve it and I could probably keep the 40s and go with a more normal 2.5 turns out. Service manual calls for 2.375 turns out as the initial setting and then do the idle drop procedure to dial-in the final adjustment. Most of the guys on the HondaShadow forum that have tinkered with their stock bike after a carb rebuild or just wanting to tinker have landed about 2.75 turns out with the stock set-up. None of them are at less than 2.5 turns out (obviously a little lean out of the factory to please the smog gods).

Title: Re: Carb Tuning
Post by DragBikeMike on 02/28/24 at 12:07:19

"I do have a question about this inaccurate chart - what is being referred to as “Straight Dia”?

The needles are not a continuous taper. At the top of the needle (usually about the first 20mm to 30mm), the needle is straight. If you look at a Mikuni needle chart you will see that they provide the diameter of the needle measured at 10mm increments. They provide measuring locations. D1 is the location 10mm from the top of the needle, D2 is the location 20mm from the top, D3 is 30mm from the top, etc. The needles are usually straight at the top portion, so D1 & D2 are usually the same diameter, then at D3 or D4 it starts to get smaller due to the taper.

That straight portion of the needle comes into play at idle and very small throttle openings. As previously mentioned, you should be able to eliminate your problem by installing the larger pilot jet and raising the needle just a bit (0.5 to 1.0mm). Your current mixture screw setting is too far out. The larger pilot jets should correct that. Seems like we are all on the same page.

You are currently running good at WOT so why even consider messing with the main jet.

Gary, that is a nice chart. Where did you get it? I assume you recommended ignoring slide cutaway because the Mikuni CV carbs usually have slides with a flat bottom. However, the cutaway has the most significant effect in the range where the problem is reported. I expect that these particular CV carbs do not have cutaways, and any sort of slide replacement would be cost prohibitive. Hey Paul, when you take the slides out in order to raise the needles, howzabout taking a pic or two so we can see the bottoms of your slides.

BTW, it's intuitively obvious to the most casual observer that the guys who made that chart that Paul posted have a very old typewriter. The "8" anvil is worn out or misaligned such that only one-half of the 8 transfers through the ribbon. Only the right-hand side of the 8 transfers. ;) Gotta cut em some slack.

Title: Re: Carb Tuning
Post by ThumperPaul on 02/28/24 at 15:16:47

Hey Mike. Thanks for the clarification of the "straight dia". I was wondering if that was referring to the non-tapered top of the needle.

I think I may have thrown you off with my talk about the VM36 slide carb. I'm talking about the CV carbs in my Honda. So vacuum/piston diaphragm in the Honda CV carbs. No pics to share.

Thanks for the validation on the larger primaries and raising the needles with shims/washers. The washers i have are almost 1mm think. Should do the trick and correct this annoying and nerve racking lean spot.

It's mostly just a feel that I need to go up 1-2 sizes on the main jets. I don't do much WOT, but when I do it "sounds" like it might be a tad lean. Then maybe it's supposed to sound that way. With partial baffles removed, I think I can go up a size on the main jets.