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Question 73 RT3 Carb issues, not firing

  • Grahamw
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73 RT3 Carb issues, not firing was created by Grahamw

Hi Group, excuse me if this is a 'read this a 100 times before' subject but my RT3 just won't fire up. It is a recent purchase and recommission but the bike did fire up without trouble initially, repeatedly. One time it cut out and I suspected I had not got the fuel tap turned on [but I can't confirm this]. Since then, no joy, so I have stripped the carb [VM32SH], all seemed in reasonable order but still no luck on it firing up. I have checked for good spark at the plug - all well but I kept getting excess unburnt fuel spitting out of the exhaust flange and filling the silencer when relentlessly kicking the bike over. So I stripped the carb again, suspecting the needle jet setter seal was damaged. I have since fitted a new jet and seal here and a new needle float valve assembly - all genuine Mikuni - and checked everything over again. All seems to be to spec. The bike still won't fire and I am getting the excess fuel spitting out quite freely from the exhaust port and just dribbling onto the floor, looking like a real sad pathetic situation that doesn't involve the relationship of fuel and spark = fire!
I am probably being a moron. Fuel is new and the tank clean, carb is clean, spark is fine, plug is new. Right leg is ruined.
Any ideas please?

cheers Graham - UK
07 Dec 2021 05:20 #1

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Replied by MarkT on topic 73 RT3 Carb issues, not firing

Change the spark plug, gap it correctly...  0..020" to 0.024"

Being it's an RT, check to make sure flywheel key hasn't sheared. 

To clear the flooding...  Leave fuel valve OFF... 

With that much fuel, you might consider getting someone to help push it around in gear with spark plug removed, ignition OFF, and fuel OFF...  until the excess fuel is expelled.

When trying to start, hold full throttle...  do not turn on fuel valve...  when it fires and starts to clear out, then you can turn the fuel valve on.

Always remember to shut off fuel valve when parking...  and back on when riding...check that flywheel key or you'll be chasing your tail and won't ever get it to start if it has sheared.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
07 Dec 2021 05:50 #2

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Replied by Mauser on topic 73 RT3 Carb issues, not firing

Cannot stress enough what Mark says about the flywheel key, these do fail if not installed and tightened correctly. whatever you do make sure you use the de-compressor whilst starting, if she fires early she can bust teeth on the idler and kick start gears....ask me how I know lol.
It won't take much to flood the bike and once flooded she won't start, you really need to get her firing within 3 or 4 kicks ?...where about in the UK are you ?
Yamaha RT2 360 ~ Honda XR650R ~ Yamaha DT175MX ~ Kawasaki KH400 ~ KX650B3

07 Dec 2021 06:02 #3

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Replied by Grahamw on topic 73 RT3 Carb issues, not firing

Thanks MarkT, sounds like the voice of experience....
I assume you are talking about a woodruff key on the shaft though the magneto? I have not been in there yet, so excuse my vagueness. Do I need to get the flywheel off to check? Or I guess I might be able to free-turn the flywheel if a key has sheared off?? You can tell I am a novice, excuse me.

I like your advice about clearing a flooded situation, thanks but I am astonished at how quickly it floods knowing I was starting with an empty carb.

Hi Mauser, I have had some weird noises when kicking the b#gger over and it has still kicked back with the decompressor lever in action but I must have kicked it over 300 times between carb strips with no joy. I am certain all things are connected to the kick starter though and I have seen the clean piston rise and fall easily though the exhaust port, but of course it is still going to do that with the odd stripped gear tooth.
I am based around 20 mins south-west of Peterborough.

Thanks for the advice guys
07 Dec 2021 06:24 #4

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Replied by Mauser on topic 73 RT3 Carb issues, not firing

I think with the cover off you should see if the key has sheared but you might need to remove the bolt first....cannot remember now. Your a bit to far to come and help I'm in the midlands :) I love my RT2.... she an animal lol
Some inspiration for you...
Yamaha RT2 360 ~ Honda XR650R ~ Yamaha DT175MX ~ Kawasaki KH400 ~ KX650B3

07 Dec 2021 12:49 #5

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Replied by RT325 on topic 73 RT3 Carb issues, not firing

If you think its just flooded, drain the carb out the side mainjet just so it's got no more fuel available then kick it with throttle open. Could push it around the yard with plug out & ignition OFF to blow fuel out of the case then try again. Starting rollers would be great for this sort of problem [shall we all buy some].
Flywheel crank key--could eyeball the points opening in relation to 3mm down the plug hole on a piece of wire before top for the piston, might be easiest. Or flywheel nut & two washers removed then eyeball the crank key although 'that' can fool you as can only see whats above the shaft so not a 100% guaranteed ok. My guess is just flooded up.
07 Dec 2021 14:01 #6

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Replied by Grahamw on topic 73 RT3 Carb issues, not firing

Thanks guys for the top tip about the flywheel key....sure enough, the key is sheared. My right leg thanks you [and my sanity].
Guess these keys are a pretty universal engineering spare? Nothing 'special Yamaha' about them?
10 Dec 2021 05:51 #7

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Replied by MarkT on topic 73 RT3 Carb issues, not firing

Some have reported immediate failures using a generic key and that the Yamaha key holds up better.  I've only used Yamaha keys so can't comment.

Also know that the key is more an "assembly aid" to index the flywheel to the crank.**  The real holding power comes from the machined taper.  If your taper is scarred up, you may have more failures.  Minor taper issues can be helped by lapping the flywheel taper to the crank taper.  Major damage may require the attention of a machinist.  And make sure you get the nut tight.  Most problems are probably due to an owner not having a tool to hold the flywheel and not getting the nut sufficiently tight.

I follow the old Husqvarna service method of tightening the nut.  Tighten again after 5 minutes of running.  And then again after 30 minutes of operation. 

P.S.  **the key is not even necessary.  Aftermarket ignitions (VAPE, Powerdynamo, HPI, PVL, etc.) often do not use a key.  A little tricky to install and index but once done and taper engaged, won't move. 
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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10 Dec 2021 06:16 #8

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Replied by Mauser on topic 73 RT3 Carb issues, not firing

As Mark suggests I used some lapping compound from my local garage, saved me buying a tin. Just a smear on the tapper then work the flywheel back and forth till it cleans up on the taper. Some might moan but I did myine up with a impact gun...not come lose to-date lol....least your making progress.
Yamaha RT2 360 ~ Honda XR650R ~ Yamaha DT175MX ~ Kawasaki KH400 ~ KX650B3

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10 Dec 2021 07:15 #9

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Replied by RT325 on topic 73 RT3 Carb issues, not firing

I 'could' show you how tight i do it but might give you bad ideas. Just damn tight & importantly held Solid for a positive feel of nut tightening process. My current crank & flywheel look bad from previous owner damage but never let go as tighten very tight. Ok, here my pic. I'll leave it small. Just hit it to expand. Very old pic.
Last edit: 11 Dec 2021 03:13 by RT325.
11 Dec 2021 03:12 #10

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