SuzukiSavage.com - Print Page

SuzukiSavage.com
/cgi-bin/YaBB.pl
General Category >> Rubber Side Down! >> Carburettor unstable performance
/cgi-bin/YaBB.pl?num=1727718508

Message started by Bjorn on 09/30/24 at 10:48:28

Title: Carburettor unstable performance
Post by Bjorn on 09/30/24 at 10:48:28

I did change my petcock to the Yamaha one, still the fuel delivery is very unstable more so lately (1997 model in Europe):

Engine starts OK cold, runs even without choke soon,
in the beginning accelerates OK sometimes for many minutes,
sometimes only problem at full throttle high RPM,
then can (nearly) stall, but when this problem it runs better when warm using choke, but worse and worse.

I use fuel without ethanol, previous owner used E10 for 1 year/last year.
I checked using start spray: no false air leak.

I suppose there is a need for carburettor cleaning, maybe a dirty float bowl problem? Not too high or low like described here: https://www.vintagebikebuilder.com/setting-mikuni-float-levels.html

Can the bottom of the carburettor be removed while in MC?
Maybe first only remove the float bowl drain plug to get out some dirt?
Not spraying in carburettor cleaner through drain plug hole, because it destroys rubber gasket.

I did buy these rebuild kit and jets but wonder why my Mikuni carburettor underside looks different than most pictures?

Maybe my carburettor is not standard and parts will not fit? (see below)

Has anybody tried low cost alternatives like these?
"unbranded alternative type" Mikuni VM38mm VM38 VM 38mm Round Slide Motorcycle Carb Carburetor VM38-9
https://www.aliexpress.com/item/1005003070271940.html
https://www.aliexpress.com/item/1005007039437665.html 40 mm?
https://www.ebay.com/itm/184829373498
https://www.mapcycle.com/vm38-9-mikuni-vm38-round-slide-38mm-carburetor.html
https://mikunioz.com/product-category/jets/main-jets/?v=13b249c5dfa9
https://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html

Title: Re: Carburettor unstable performance
Post by Bjorn on 09/30/24 at 10:50:34

My carburettor pictures

Title: Re: Carburettor unstable performance
Post by Bjorn on 09/30/24 at 10:51:14

left side my carburettor

Title: Re: Carburettor unstable performance
Post by Bjorn on 09/30/24 at 10:51:44

right side my carburettor

Title: Re: Carburettor unstable performance
Post by Bjorn on 09/30/24 at 10:52:32

my carburettor rebuild kit (not tested yet) and jets
https://www.aliexpress.com/item/1005004516162406.html
For Suzuki 1986~2019 version LS650 Savage (Boulevard S40) motorcycle carburetor repair kit with vacuum diaphragm

https://www.aliexpress.com/item/33018387858.html
Replacement Large Round Carb Main Jets For Mikuni N100/604 Carburetor #155 160 170 175 180

https://www.aliexpress.com/item/1005006288806849.html
Pilot Jet Idle Slow Jet N151.067 M5x0.8mm Thread for Mikuni CV Carb

Title: Re: Carburettor unstable performance
Post by verslagen1 on 09/30/24 at 14:00:38

79515449553B0 wrote:

I suppose there is a need for carburettor cleaning

that's the 1st thing I would do.

Quote:

Can the bottom of the carburettor be removed while in MC?

yes

Quote:

Not spraying in carburettor cleaner through drain plug hole, because it destroys rubber gasket.

correct

Quote:

I did buy these rebuild kit and jets but wonder why my Mikuni carburettor underside looks different than most pictures?

Maybe my carburettor is not standard and parts will not fit? (see below)

looks like mine.

as far as kits go, they vary a lot because the jets change over the years and differ as well for countries.
I'd rather clean the original jets and use them if the jetting is correct.
if not, change only 1 at time to gage if it is correct.

Title: Re: Carburettor unstable performance
Post by ThumperPaul on 09/30/24 at 14:36:15

What Verslagen said!

Most people would caution against using cheap knockoff jets like in those Ali express kits. Buy genuine Mikuni is the main suggestion.

If you decide to go the cheap knockoff route, those main jets are the right style Mikuni jets. BUT they are all too big. The smallest one in the kit is a #155. It might work (but on the rich side). A #150 is about all you need even if you have a better flowing muffler. If you live in higher elevation, a #145 might be just right.

The pilot jets (slow jets) are the right style, but you’ll only need a #47.5 or #50. All those other ones are just a waste.

For quality genuine Mikuni jets, try JetsRUs.com.

If you want to try a cheap knockoff Keihin slide carb, I’m running a PWK38. Link to my trials and tribulations here. —> http://suzukisavage.com/cgi-bin/YaBB.pl?num=1714330191

This style slide carb requires a different throttle cable (or modified stock), and you’ll have to use a different air filter or intake tube. Same as the Mikuni VM36 slide carb needed changes.

Title: Re: Carburettor unstable performance
Post by LANCER on 09/30/24 at 16:45:11

Buying genuine Mikuni jets & parts is very important, it saves time, money and frustration, since other providers sell parts that may not be usable, many don’t even have any identification on them so you have no idea of what they are. If they are marked, what do they mean ? If you have difficulty jetting & tuning, and ask for help, how are we to help if we are unable to know what you’re working with.

If you decide you want a Mikuni VM carb, I put kits together, and that includes a custom throttle cable, plus more.

Title: Re: Carburettor unstable performance
Post by ohiomoto on 09/30/24 at 16:56:03

1. Are you sure you got a good petcock and is it properly installed?

2. I agree with varslagen1. If possible, avoid aftermarket kits and start with stock jets.

3. I would only replace the jets if they are obstructed or I suspect they could have been modified.

4. The jets from the top of the carb can get mixed up with the main and pilot jets.

5. It could be the slide. If it loses vacuum or sticks it can cause intermittent lean conditions.

6. Good Luck!

Title: Re: Carburettor unstable performance
Post by JOG on 09/30/24 at 19:24:56

I dunno what you paid for the petcock, but if it isn't the real, OEM petcock, it's probably not going to solve your problems. Last time I got one it was about thirty dollars and that was a Long time ago.

Title: Re: Carburettor unstable performance
Post by Bjorn on 09/30/24 at 20:59:08

Thank you all for good answers:
Verslagen1 ThumperPaul Lancer ohiomoto and JOG

I live in Norway and often companies in USA will not ship to Norway,
or the shipping cost is extremely high,
and in addition the high toll import customs fees are added here on top of all.

Title: Re: Carburettor unstable performance
Post by Dave on 10/01/24 at 04:38:31

I have cleaned a lot of stock carbs, and most often it can be done without the need to replace any parts.

The most common issue is the pilot jet is gummed up and blocked.....the center hole is very small and becomes clogged quickly. When a bike is not run for a while (or has frequent times of sitting), the fuel evaporates in the float bowl and the pilot jet gets gummed up by the fuel residue. Cleaning often requires you to poke something in there to get the pilot jet open - soaking or blowing on it alone doesn't get the jet open.....I use a guitar string as anything other kind of wire is often too large.

The float bowl gasket is pretty durable and often isn't damaged - but it would not hurt to have one on hand. The TEV diaphragm is often very stiff and may work better with a newer replacement....you can get aftermarket ones much cheaper than the stock one. The idle mixture screw can be a bugger to get out if the threads are corroded - it is not uncommon to strip the screwdriver slot trying to get it out......when that happens I have to drill the brass screw and use an "easy out" or "Torx" bit to remove it (then I clean up the threads with a tap and install a new mixture screw).

So.....sometimes the carb cleaning process is routine - and sometime that brass mixture screw can turn the job into an intense session that requires a good amount of tools and skill.

Title: Re: Carburettor unstable performance
Post by ThumperPaul on 10/01/24 at 05:24:37

PS - Perhaps too obvious, but when you installed the Raptor petcock valve, did you plug or cap off the vacuum line/port on the carb?

Title: Re: Carburettor unstable performance
Post by Bjorn on 10/01/24 at 07:39:05

I did plug the vacum line.

Now I dismantled the carburettor, it did not look green or especially dirty inside, but the jets was not clean enough I beleive but difficult for me to see.

The slide look worn on the side: if you hold the carburettor upside down shall the slide spring be able to push it fully closed?
Maybe low power spring combined with worn slide is one of the problems.

Title: Re: Carburettor unstable performance
Post by JOG on 10/01/24 at 08:36:46

A sticking slide will create a variable. I'm Not a carb guru. I'm a little dangerous to have working on one. BUT, if you're thinking the worn slide might be a problem, is there any roughness you can feel? Inside the carb?
I've never tried this, but if I wanted to See how rough a surface is, I'd try getting a thin piece of paper and lay on it, and rub it with a pencil.
Or,maybe, get the slide clean and dry,no fingerprints, and rub the raw spot with a pencil.
Will that fix it? No, but if it runs right until the throttle increasing and decreasing causes the up and down motion to rub the graphite off,, you will have your very own

Well, There's yer Problem

moment.

Title: Re: Carburettor unstable performance
Post by ThumperPaul on 10/01/24 at 09:18:59

Clean or replace the jets when in doubt.

Take a pic of the slide. It sounds like it’s sticking from being worn/rough. Take a pic so we can see how bad it looks.

If you flip the carb upside down, the slide won’t necessarily open completely. The spring won’t compress enough for it to completely open. If the slide won’t move at all when flipped upside down, it’s sticking. It should at least move a little and clack around on the idle screw. Be gently clacking it around so you don't break or damage the idle screw. The spring compression shouldn’t be strong enough to hold the slide completely closed.

Note: The spring acts more to help close the slide after you’ve rolled off the throttle. It offers a little resistance when rolling on the throttle. Most of the slide action is vacuum-driven (thus CV carb).

Title: Re: Carburettor unstable performance
Post by Bjorn on 10/02/24 at 11:24:48

Success! Thank you all for all advices, including Thumper Paul: my slide work like you explained it should do.

I saw the main jet (size 125) and pilot jet (size 47,5 both are original "Europe-sizes" 1997) was dirty.

I changed both jets to:
main jet size 145 and pilot jet size 50,
everything now works perfect with my Dyna "open muffler".

My carburettor float bowl does not have the normal drain plug screw for easy draining during winter storage:
Maybe running the engine with the Yamaha petcock in off position until engine dies is good enough for long term storage?
+ using som "fuel stabilizer additive" in gasoline before winter storage? Like this:
https://www.thansen.no/scooter-mc/olje-rens-og-pleie/putoline-produkter/tilsetningsstoffer/putoline-fuel-stabilizer-150-ml/n1070286/pn1070287

Title: Re: Carburettor unstable performance
Post by ThumperPaul on 10/03/24 at 11:39:16

Way to go!

Dang - tiny little #125 European main jet. Talk about limiting performance to save a little savage smog! A #145 is much better. You'd probably really like a #150 or #152.5! But going from a 125 to a 145 probably already seems like a major upgrade!

I live in Houston Texas so I ride right through the winter. I don't know what winter storage is. ;D One of our Yankee members will have to answer that one. Maybe Sta-Bil fuel storage additive (or that expensive Norwegian stuff you found).... If I lived up north, I'd have to find a way to go VROOM VROOM in the garage for 5 minutes once a week to keep the fuel moving through the system.