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RD4ME

No Snap!

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2 hours ago, RD4ME said:

So that's my list completed - the only thing I haven't done is the timing. I pulled the flywheel and the points looked new - and I have good spark. I haven't found a dumbed-down step-by step guide and I don't have a way to accurately measure the piston position for the advancing of the spark - and I'm concerned I'll do more damage than good! But any advice on where to go next is appreciated!

1) Spark Plug - done

2) New Gas/Oil mix - done

3) New Tranny oil - done used Rotella 15W40

4) Compression test to veriify my foot feel - done

5) Air box check - Done

6) Carb clean - done

7) Clutch adjustment - done

8 Exhaust check/de-coke - done

 

Learn how to check the timing. You don't need to accurately measure piston position to check it is in the ballpark. Here's my suggestion:

Take the plug out.

Take the flywheel cover off.

Find top dead centre using a clean object inserted through the plug hole.

Make a mark on the flywheel rim in line with one of the holes that the flywheel cover screws go into, with the piston at TDC.

Measure around the rim of the flywheel anticlockwise 22mm and make another mark on the rim.

Get a torch and look at the points. Find the flywheel position where the points start to open when rotating the flywheel anti clockwise. It should be close to where your second flywheel rim mark is aligned with the screw hole you chose.

If the points open within a few mm either side of your second flywheel rim mark then your problem is not your ignition timing.

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Thanks Feetupfun for the great explanation - I think I can follow those instructions. But the good news is - I may not need to!

The Snap is Back!!

It looks like I've made a break through. I pulled the carb off to install my new needle and seat - and pulled all of the jets and doo-dads out again and gave it full clean/blow out.

Then when I was adjusting the Air screw - I noticed as I screwed it in further - it revved-up faster. So I screwed it all the way in and took it for a spin up the street - and it felt like a different bike. The front end came-up high with just a blip of the throttle without any pulling on the bars - it was amazing! But then when I screwed out the Air screw to the 1.5- 2.0 turns - it lost its snap again. So I'm not sure if when screwed-in all the way it gets more or less air - and why it needs it. But I guess that's what I need to figure out next - but pretty pleased that I've found the power again!

 

 

 

Edited by RD4ME

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Winding the screw in richens the mixture, sounds like you need a bigger pilot jet. The pilot jet in the TY mikuni is different than most other mikunis which means your local shop won't have 1 in stock, with my modified airbox that breathes a lot better I took the advice on TY trials site & ordered a #37.5 which has worked out well

B&J racing is closer to you than TY trials so I'd see if they can supply you a couple of different sizes

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I ordered a couple of pilot jet from a Yamaha dealer last year, they where cheaper than everywhere else I had look. Maybe I was lucky.

Guy

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OK - great - I'm going to order a #37.5  and a #40 jet. 

I've removed my stock #35 from my Mikuni VM26SS and have measured it. Pretty certain it's the M28/1001 series - but hoping someone might be able to confirm?

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If you have to screw the pilot air screw all the way in to make the engine run correctly and the engine,intake/exhaust/carb specs are stock,then you need to figure out why.Logically if you have to shut off the air flow in the carb to make it run right ,where is the air coming from .? It would follow that there is an air leak somewhere....base gasket,intake manifold or a leaking crank seal are the most likely.

Trying to adjust the carb jetting to accomadate a air leak will be a impossible  task.

 

 

 

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 Could be the pilot circuit is still plugged. The pilot flows out of two holes. One in front of the slide and one underneath the slide. Are you sure both are clear?

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On 11/22/2018 at 6:38 PM, lineaway said:

 Could be the pilot circuit is still plugged. The pilot flows out of two holes. One in front of the slide and one underneath the slide. Are you sure both are clear?

I've had the carb out again and there's no blockage in the jet or the passageways. But it does still require the Air screw to be in all the way to run right.

Bmonk dropped a logic bomb which has me putting the brakes on my bigger jet order. I sprayed carb cleaner around the base gasket and intakes - nothing seems to get sucked in and the revs don't change. I've had the flywheel off and don't see any evidence of the seal leaking - although if air is being pulled in I don't knwo that I would. Haven't had the clutch side opened up yet - so I guess I could go there. 

Just wondering if I'd be doing any harm to it by riding with the Air screw all the way in? I guess if the issue got worse - there'd be no further adjustment - but at the same time it might conceivable work itself through. 

By the way - I had my first "off" yesterday when riding it in minus 11 degree weather. The grass was frozen and slick and I clutched-in and put the front brake on; due to my sticking clutch plates the back wheel kept turning so I pulled the brake harder - locked it and went over. I rolled off with nothing hurt by my dignity! I quickly got up before my wife noticed...she would have been concerned over the big skid mark I made on the grass!    

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If you end up with constant carb problems I would not fiddle with it to much.You can buy a oko flat slide from u.s dealer for 100 bucks.They are made in Taiwan(not China junk).they are a copy of kiehin flat slide carb and work better than the round mikuni.They sell them for 110$ jetted for the ty250 and other bikes.I have used 4 so far on nonstock rebuilds with success including a 96kdx200.I ordered one today for a modified 74 ty250 and spent a extra 50 for the hand polishing and a custom cable kit and was still under 200.

Edited by Junglejeff
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Sorry to hear you had an 'Off'.   When I first started practising, I used to use up about half a gallon of fuel at a time, by bashing around the local fields.   If I hadn't fallen off at least twice, I considered I was not trying.   …  That was at least 50 years ago now...  I'm still good at it, falling off that is.

.

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Now you know why most trials bikes with drum brakes aren’t very powerful in the stopping of forward motion. Practice riding without touching the front brake, a little dab of rear brake helps when taking a tight slow turn....but forget the front keep that for an “oh sh#t moment🙂👍

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On 11/24/2018 at 11:38 AM, RD4ME said:

I've had the carb out again and there's no blockage in the jet or the passageways. But it does still require the Air screw to be in all the way to run right.

Bmonk dropped a logic bomb which has me putting the brakes on my bigger jet order. I sprayed carb cleaner around the base gasket and intakes - nothing seems to get sucked in and the revs don't change. I've had the flywheel off and don't see any evidence of the seal leaking - although if air is being pulled in I don't knwo that I would. Haven't had the clutch side opened up yet - so I guess I could go there. 

Just wondering if I'd be doing any harm to it by riding with the Air screw all the way in? I guess if the issue got worse - there'd be no further adjustment - but at the same time it might conceivable work itself through. 

By the way - I had my first "off" yesterday when riding it in minus 11 degree weather. The grass was frozen and slick and I clutched-in and put the front brake on; due to my sticking clutch plates the back wheel kept turning so I pulled the brake harder - locked it and went over. I rolled off with nothing hurt by my dignity! I quickly got up before my wife noticed...she would have been concerned over the big skid mark I made on the grass!    

As long as the mixture is right there is no harm from poor combustion happening by having the air screw all the way in. However, if the pilot circuit is working normally and the air screw has to be all the way in to get the mixture right, then there has to be unfiltered air going into your engine somewhere which will cause damage.

Have you checked the oil injection port for air leakage? On one of my bikes I had used a piece of plastic tubing to seal off the oil port and the tubing had swollen over time and had started to let air in.

When you look at the LH crank seal, the evidence for a leaking seal may be very small. Even if there is no oil witness there, it can still be leaking.

Sometimes that crank seal will seal in one direction but not the other, so air can go in but no fuel/air mix comes out, so no witness. I've just found exactly that failure mode on my KT250 LH flywheel side crank seal.

One difference between a problem with the pilot circuit and a problem with air ingress is often that with air ingress via a crank seal or the centre joint of the casings, the degree of the problem will be quite variable, but if it is the pilot circuit that is the problem then the symptoms will be fairly consistent.

Edited by feetupfun

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