C'mon Gurus, where are you?

david f martin /

I've been having a problem with my bike for awhile now, and I've asked for help and gotten nothing yet. I am new to this list, so most of you don't recognise my name, and maybe you're just reading the regular posters, or maybe this is some kind of clique that I just haven't broken into yet...

The problem: The bike bogs at about 75% throttle. If I twitch the throttle a little, sometimes it clears up. It doesn't usually do it going uphill. I put about 25 miles a day on my bike, so this problem not only slows me down going to and from work, it is really starting to aggravate the shit out of me.

This is what I've done so far: Adjusted the carb this way and that... Changed plugs, checked gap with the dealer. Looked for a timing adjustment, but haven't found one yet. Pulled the exhaust, cleaned the exhaust port, drowned the muffler with carb cleaner and poked at it with a coat hanger. I blew thru the muffler, and it doesn't seem to be restricted. Opened up the carb and cleaned both jets. Adjusted the oil injection this way and that. I've been running premium gas in it.

I know this has to be a simple problem that many of you are familiar with. Someone step up and help me out, please!

dave

BTW, the bike is a Yamaha QT50 with CDI ignition. Very simple, and mechanically similar to most 'peds out there.

Hard to say.

The problem I see ( for us to help you)... is that you've already done most all of the things I would do.

It is very hard to diagnose somebody elses problem without having it in your own hands to try it...words just don't do it a lot of times.

The other thing is that most of us only have experience with at most 3 diff peds...so I can only give general info about others.

It is also hard to diagnose a problem that doesn't happen all the time.

You sound like you are mechanically competent...so I figure you will find it if you keep trying things.

The best thing I can say for a sometimes bog at 3/4 throttle would be to check that the fuel flow is good from the petcock and that the carb float will open smoothly with no rough spots.

It can't hurt to buy a small bottle of gas treatment and put 2 capfulls in your next full tank of gas.

Re: Hard to say.

Doug Davidson /

HI, FRED ABOUT COVERED THE NEXT STEP, I am very parcial to synthetic oil mix. Something you dont think about too much is the gas cap being vented, it can build up a vacuum and give problems, try it with the cap a little loose, and being 2 stroke the engine temp....run lousy cold. An old fix on carbs is to rev up motor and put hand over carb to choke out completely and uncover before it dies out, this will suck thru any tiny particles of dirt ect that get in the tiny passages...do in a well vented area, it will look like something on fire! Doug D.

Re: C'mon Gurus, where are you?

Reeperette /

Ok, this is a guess...and only that, I do believe the response has been slow in comin cause most of us are still chewing on this one....

I'd say there is some possibility of the throttle cable itself being mis-set, in that once you go past 75% throttle, it's pulling the gizmo that sets the mix up/out/over too far, and this causes the mixture to go too rich and "flood" a bit....and what you said about twitching the throttle convinces me that it's at least worth a look.

Give it a try, surely it can do no harm to ensure the cable is correctly set, and it eliminates one more possibility if that isn't it.

-Ree

Re: C'mon Gurus, where are you?

Ron Brown /

David,

I'm with Fred, you have done so much it is hard to know what to sugest.

First off, unless you have increased compression, regular gas is just fine.

Did the bike run good at one time and then develop this problem?

Did the problem appear all at once or gradually?

The term, "bogs", covers a multitude of ills. Does the engine feel like it just runs out of power or does it mis-fire. Does the exhaust note change, if so, how?

Have you tried removing the air cleaner and air box and tested that way? Did anything change?

Have you tried applying a little choke when it "bogs"? What did this do?

Given the answers to all these questions, we may still not be able to help you, but it will give us some direction.

Good luck,

Ron

Re: Have you tried...

Dave:

Have you checked to see that the throttle cable really does open the throttle slide all the way at full throttle?

Also the jets in the carb are half of the solution. The passages inside could also be blocked. Check the post titled "motomarina" a few back on how to clean these passages.

Two strokes are fairly simple and don't have much to go wrong, but when it does it can be frustrating to try to figure it out. You have an otherwise good running 'ped, so stick to it.

And in closing I will add that including the make and model will always get the attention of the "Specialists" on this board.

Jim

Re: C'mon Gurus, where are you?

david f martin /

Thanks for the ideas. I'm starting to suspect the throttle cable adjustment, because the bike does have an electric rev limiter... I had to reroute a few cables to install the aircleaner (missing when I bought it)... THAT's when the bike started running like crap.

I started out posting to the specialists, and did get some good help from Ashley, but that's about it. I'm looking for all the good free info I can find, since the manual for my bike seems to be impossible to find. I'm used to working on VWs, and they don't have things like rev limiters on them...

Thanks again. I'll keep you posted, and maybe we'll all learn something...

dave

Re: C'mon Gurus, where are you?

Ron Brown /

Dave,

If the problem started with the air cleaner, suspect it first.

This sounds like the classic, too rich, 4 cycling problem.

Ron

Re: C'mon Gurus, where are you?

david f martin /

..."Yes, the classic (fill in the blank) problem..."

That's what I'm looking for!

Ok, I put a brand new aircleaner box and element on the bike, and that's when the problems started. I had to reroute cables to install it, and that's why I suspect the throttle cable. I cleaned the oil out of the filter to try to de-restrict it, if that was indeed the problem.

The bike was equipped with an airfilter new, so it should run fine with a new airfilter on it, right?

dave

Re: C'mon Gurus, where are you?

Ron Brown /

Dave,

I have a couple of identical Motobecanes. The had not run for tears when I got them but were in good shape and low miles.

When I got the first one running, it would do 25 and then start to 4 cycle. Removing the air box and filter fixed it. Adding any part of the air box/filter assembly wuold reduce power and increase 4 cycling. Even the filter housing with no filter and air box made a difference.

When I got the second one running, it did not have this problem. I discovered that the main jet was larger in the ped with a problem.

The scenario offered on this site was that the larger jet was fitted to restrict the top speed in the US and the dealer who sold one of them was smart enough to change the jet when he sold it, the other one was not.

So, maybe your ped came with too large a jet and the air cleaner was discarded for more power, or the air cleaner was discarded and a larger main fitted for more power, or mayby not.

Anyway, give it a try. I am in favor of air cleaners with oil on the element. Your solution may be, like mine, a smaller main jet, but keep an eye on the plug to make sure you are not too lean.

Ron

Re: C'mon Gurus, where are you?

david f martin /

That makes sense. It does act like the plug is fouling or something. It is most certainly a firing problem... However. I called the dealer and asked about stock jet size. He told me 70, so I borrowed a jet drill index from work, cleaned the carbs and checked the jet against the index. It is indeed 70. I guess I'll pull the aircleaner this evening and see what it does on the way to work in the morning.

Thanks,

dave

Re: C'mon Gurus, where are you?

Ron Brown /

Dave,

I never heard of a jet drill index. I was under the impression that jet numbers were an expression of fuel flow in liters/hour or bbls per fortnight or something and that was a function of the length as well as diameter of the hole in the jet.

I have heard of jet guages but they were allways for a particular make/model of main jet.

Enlighten me please, this sounds like a handy kit.

Ron

Jet drill index

david f martin /

I work in a VW repair shop... This is a tiny box of tiny drill bits, with the bits labeled in jet sizes. I think it is a gauge size, since the lowered number bits are the larger ones.

The dealer told me that my stock size jet is 70. I used the dull ends of the drill bits to clean my jets, and sure enough, the drill labeled 70 was the one that fit.

dave

Re: Jet drill index

Ron Brown /

Dave,

I think that what you have as a set of small number drills as jet sizes increase with greater numbers. As in more volume per unit of time.

If you examine your jet carefully, it should have the number stamped on it somewhere.

For reference, a number 70 drill is .028" diameter.

Hope your "no air cleaner" test went well.

Ron

Re: Jet drill index

david f martin /

I took the element out of my aircleaner box today at lunch, and drove my bike home. Geez, what a difference.

The bike is going much faster now. It still does that "bog" thing if I give it too much gas all at once, but it is much easier to control (still by twitching the throttle). I think I'm actually bumping up against the electronic rev limiter going downhill.

Needless to say, my trip home from work this evening was much more enjoyable. Still, I want my bike to last, so I don't really like the idea of running it without an aircleaner. At least I have the box on it now, which I didn't have when I got the bike. That oughtta at least keep rocks and squirrels and stuff out of my carb...

I guess this means I need a smaller jet, as you said, so I can put my aircleaner back on. Does this mean my bike is going to be slower as a result?

Being a VW mechanic, I know what can happen when you build more engine than the drivetrain and brakes can handle. I'm not interested in hotrodding the bike, because I want it to last for a long time... Besides, I'm used to being passed in my old VW bus, anyway...

I am interested in putting a bigger exhaust and carb on it, though. No one advertises performance equipment for my bike specifically, so I'm wondering if this stuff is made specifically for brands of bikes, or is it pretty much universal... Maybe some issues with mounting, but otherwise usuable on most 50cc bikes... Anyone know?

Thanks,

dave

Re: Jet drill index

Ron Brown /

Dave,

Congratulations on finding the problem.

You are correct about needing a smaller jet. Read the number on the existing jet and try going down a couple of sizes. You need to reduce the main until the four cycling problem goes away. Make sure you do a plug chop afterwards to make sure you are not running too lean.

You will have slightly less power than running without the air cleaner but you should be able to run a lot longer before the engine self destructs and your gas milage will be improved considerably.

The symptom you describe when you snap the throttle open is normal. What happens is that you open the venturi in the carburetor wider than the airflow can support and there is not enough pressure drop to flow gas into the air stream.

This is why the constant velocity carburetor was invented, or in the case of big Detroit V8s, the gas pump, which arbitrarily dumped some raw gas into the intake when you floored it.

You need to "feel" how the engine is pulling and gradually open the throttle as your speed increases. When you get used to it you will find it really helps accelleration.

As for speed mods, I know nothing about your bike. If you have a rev limiter then I suppose you have to be able to get rid of that, or change sprockets to get any more speed. Bigger carb and matching exhaust will help with power. Keep in mind that if your air cleaner is allready restrictive, more flow will only make it worse.

You may want to see if you can cobble some kind of air cleaner that will support your existing jet before trying anythin else.

Good luck,

Ron

Re: C'mon Gurus, where are you?

Brandon E. /

This is just a suggestion, but try running low octane gas in it. Premium is a high octane gas, which is meant to ignite in hotter conditions so it doesn't pre-detonate in high compression engines. Maybe it can't fire at High RPM's. I could be wrong but try it, it shouldn't hurt anything.

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