Hobbit vm20 tuning help..

Hey all-

Just recently plopped a vm 20 (with mlm intake) on my hobbit (dr70 kit, puchonda head, proma...)

Has the stock main jet but I put in a 15 pilot jet (as opposed to the 22.5) The bike runs fine when it is choked. It also rides fine off the choke, as well as will idle fine. But while riding , when coming to a slow/stop, the bike dies-can’t even keep it alive by gassing it. The bike acts almost the same regardless of which pilot jet is in. Any help would be appreciated as I have no experience with these carbs

Thanks

Re: Hobbit vm20 tuning help..

My hunch is that the "stock main jet" is too rich for your setup. You need to tune it. What does your plug look like?

Re: Hobbit vm20 tuning help..

Dark brown bronze ish

Re: Hobbit vm20 tuning help..

Bill Mazzacane /

> taylor Vogel Wrote:

> -------------------------------------------------------

> The bike runs fine when it is choked.

That means the engine is running lean from either your jetting or a vacuum leak. Or a combo of both jetting and leaking. But often the problem is the low jet.

If you were trying to clean your old carb, you should replace small low jets, do not try to clean a low jet.

> taylor Vogel Wrote:

> -------------------------------------------------------

> Dark brown bronze ish

Plug chops are a waste of time!!!

Unless you have a source for leaded gasoline? But for the rest of us that use Unleaded, the only color on the spark plug is deposited by your 2 stroke oil.

Bill M.

Re: Hobbit vm20 tuning help..

Dang it . I've been doing it wrong for the last 50 years .

I always clean every jet in every carb I take down . Amazingly , they sure seem to end up doing their job , flawlessly .

And , I go by the color of whatever gets deposited on the plug . Doesn't seem to matter which type of fuel gets burned .

Re: Hobbit vm20 tuning help..

Who doesn't clean low jets lol

Re: Hobbit vm20 tuning help..

Bill Mazzacane /

> JBOT Admin Wrote:

> -------------------------------------------------------

> Who doesn't clean low jets lol

Obviously you are just an enthusiast. A pro doesn't waste their customers time by risking a comeback and trying to save the customer few bucks on a part that you can not guarantee is clean.

An enthusiast has time to waste with trial and error with diagnostic procedures because they are inexperienced.

Lets say Taylor came into your shop with his Honda Hobbit for: "Engine only starts and idles while the choke is on." You charge him a diagnostic fee, and quickly discover that his carburetor is varnished and needs a carb overhual to fix it. According to Taylor, the stock low jet is a #15, and (in this case) varnish has clogged the low jet.

You can soak the parts, scrape off the varnish, and use air pressure until you see light through the jet, but you can not guarantee that the low jet is 100% clean. If the main jet was not 100% clean, (with some remaining varnish), the engine will still run fine. But that is not the case with a low jet. Varnish has a thickness that will restrict a low or pilot jet.

Low jets are incremented in 0, 2.5, 5.0 (only .001"/size) or 0, 5, 10, 15 (only .002"/size). Even if varnish was only .001" thick, that is still .002" inside a hole, and that .002" is one jet size leaner than whatever size you started with. A .002" layer of varnish is a best case scenario. Mechanics or enthusiasts have not developed the ability to look through a small jet and determine if there is still a layer of varnish inside.

The most important fact is that there is no jet drill small enough to clean that low jet properly until you get a #35 low jet (it will almost work with a #80 jet drill) But the drill is actually a .34mm drill.

(Your not going to catch me trying to push a blunt or sharp wire from a wire brush to scrape out varnish from a low jet).

That is why you should replace the most popular 2 or 4 stroke low (pilot) jets from #15 to #35.

==================================================================

If Taylor's Honda was a 4 stroke, there are a few other details that must be attended too. You must assume there is gasoline that leaked from the carb, into the intake, past the piston rings and finally the fuel drained into the engine oil.

Diluted engine oil will quickly foul a new spark plug, and now you have related comeback after only charging the customer for a carb overhaul because you didn't see the big picture and plan ahead.

===================================================================

When was the last time you changed the spark plugs on your car or 4-stroke street bike?? If your car or street bike doesn't burn oil, the spark plug porcelain will be white because of the unleaded pump gas.

When I started wrenching on Kawasaki KZ street bikes, the spark plugs were always tan to chocolate brown because of the lead in the gasoline. In California we tend to do smog related changes before the other 49 states.

When companies began phasing out leaded gasoline in 1985, the spark plug color charts were rendered obsolete unless you ran leaded race gas.

Didn't you suspect something was wrong when you tried a plug chop within a short distance with new plugs? Why was there only a little color?

Bill M.

If you are a pro, your customers often only have a weekend to play with their toys. They have a right to be upset if their toys don't run as expected. They will complain that their friends rode their toys, while they sat around a campfire or something.

Re: Hobbit vm20 tuning help..

Haha fuck you. my mopeds are faster than yours, ya old ass capitalist bitch

I clean low jets all the time and my customer (me) loves it.

Matter of fact, I've worked on Taylor's bike twice now. Fixed a bunch of shit and charged him almost nothing. So take that no low jet cleaning bullshit and shove it straight up your fuckin dick hole.

Tldr: you can clean low jets without any issues

Re: Hobbit vm20 tuning help..

All that said, to the OP.

A fat idle jet helps with cooling a lot. I try to keep my idle jet as big as I can within reason so when coming off the throttle the bike has a nice rich place to hang out. If you're lean on your idle jet and you happen to go lean on the main or needle, you're asking for a seize.

And yeah, if you want help jetting you know my number

Re: Hobbit vm20 tuning help..

Joe Schuitema /

Have you tried adjusting the idle air/fuel screw on the side or the slide needle? If its bogging like that and won't catch itself, its not sucking in fuel. It can be an airleak around the reeds or carb o-ring. Main jet doesn't matter in this instance, you could run without one and not notice, until you gave it more than 3/4 throttle.

Re: Hobbit vm20 tuning help..

Could be an air leak by the o ring? As there is no gasket or anything between the carb and the mlm intake... on this note-is it normal for the intake to get super cold and have condensation on the outside whilst the bike is running?

Also, have not messed with the needle..what could I do to it to perhaps remedy my situation?

Re: Hobbit vm20 tuning help..

Joe Schuitema /

If the o ring is in good condition and the intake surface is flat, it shouldn't be leaking. Cold and condensation is normal. Richen the needle(move the needle clip down) a notch or two and see how it reacts. Also, try turning the idle air/fuel mixture screw in or out a half turn and see how the bike reacts.

Re: Hobbit vm20 tuning help..

I didn't see any mention yet of how many turns out on the air screw you have set? When I had too large of a pilot I had to turn the screw way in the keep the idle from going to hell after coming to a stop and loading up at red lights. But then because an oversized jet was being adjusted out of the effective range of the air screw my idle would hang high for 5-10 sec after coming to a stop. I just couldn't win until I dropped to a 10 jet. That's on a TM24 though so YMMV. Also the choke is not a true choke. It's an enrichment and it's effect changes throughout the throttle range.

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