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Backfiring Through Carb...


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#1 TitanNeil

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Posted 19 May 2019 - 11:32 pm

Ugh. There is finally some nice weather outside and I am in the garage struggling to make my '91 850 run properly. I did a buncha stuff to it - new head gasket, proper valve shimming, replaced timing chain. I discovered how much water a gas tank full of stabilized fuel will develop over a Canadian winter (about 1/3 cup!)...

So the bike rolls over fine, there is spark (although it seems weak, bluish yellow rather than fat white) but I keep getting fuel pouring out when I go to start it up. It starts on one cylinder and when I play with the choke and throttle the 2nd kicks in then fuel comes out the overflow. I am an expert at bodywork/tank removal now, so I rigged up a separate fuel bottle to save me from that nonsense as I don't seem any closer to figuring this out.

Needle and seats, right? They are clean and nice. Now with the tank out of the way I can see the left carb backfiring upwards - and the over flowing hasn't even started. Did get the timing chain 180 degrees off? Is a weak spark to blame for intermittent firing? Any suggestions? I have had the float bowls off 3 times yesterday and today and no change...Is the left carb just dumping fuel in and causing the backfire?


I also replaced the emulsion tubes which were ovalled...



#2 Studley Ramrod

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Posted 20 May 2019 - 01:40 pm

It could be a sticking/worn float needle valve, or possibly a worn/damaged gasket or o ring that is causing it to overflow.


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#3 TitanNeil

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Posted 21 May 2019 - 09:55 pm

I pulled the bowls off and inspected the needles and seat, they are super clean. I don't know if they could possibly hang up, as I was tapping the carb with a rubber mallet when it was running and it made no difference, kept backfiring.



#4 Studley Ramrod

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Posted 22 May 2019 - 09:33 am

The backfiring could be due to not having set the pilot screws and balanced the carbs.  Difficult to know for sure though.


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#5 TitanNeil

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Posted 30 May 2019 - 08:42 pm

Back at it have carbs off and have 2 questions:

 - I found a large chunk of the graphite(?) header-to-muffler seal in the workshop, and looking at the joint there is soot that has blown out. I assume it isn't sealing well there at all..could that cause backfiring through carb?

- I replaced the emulsion tubes with OEM Yamaha tubes. When at rest, are the main needles supposed to close off the opening? If I put a hose on the main jet and blow through it, it almost makes no difference if the needle is 'bottomed out' or not in there at all - air goes through nonetheless. Correct/not correct?



#6 Studley Ramrod

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Posted 31 May 2019 - 09:48 am

Doubt it, more likely to be the pilot screw settings and/or a carb balance.  Might also be air getting in through the carb inlet rubbers, you need a good tight seal and the rubbers need to be without cracks and still rubbery.

 

Correct.


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#7 TitanNeil

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Posted 31 May 2019 - 02:31 pm

Thanks for your answers, Studley. I appreciate it. I will report back after checking the cam timing again and setting pilot screws and checking carb balance.



#8 TitanNeil

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Posted 03 June 2019 - 12:23 am

Well, i'm out of ideas. I checked everything over again, including swapping float needles between carbs. Float levels are spot on, timing is confirmed as good, carb balance appears to be right, and still I have backfiring throught the left carb.  Any, repeat any, suggestions appreciated....



#9 dapleb

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Posted 03 June 2019 - 04:19 am

What "spot on" float levels have you set?
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#10 aircooled68

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Posted 03 June 2019 - 05:57 am

Anything like mine was?? https://youtu.be/IV1uQq37ySw

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#11 MipeTDM

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Posted 03 June 2019 - 09:02 am

Do both slides move up and down together, when you operate the throttle with the engine running, with the fuel filter box off so that you can see?
If not, there might be something wrong with the membranes, e.g leaking.

BTW backfiring indicates a lean condition: too much air or too little fuel.


Also, my experience is that aside from worn float needles, carb overflow can also be caused from tank rust particles. A good fuel filter prevents that from happening.
I was reminded of this again, due to the large amounts of water you collected over the winter.
Maybe also a partially blocked idle jet causes your lean condition. But check the easy stuff first (before disassembling everything)

Edited by MipeTDM, 03 June 2019 - 09:03 am.


#12 Studley Ramrod

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Posted 03 June 2019 - 10:15 am

Try adjusting the pilot screw.


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#13 Robodene

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Posted 03 June 2019 - 10:52 am

A distant memory now but I seem to remember leaning out the carb as much as possible (dropping the needle) and getting that effect. It went on to seize the engine next! (Not TDM). Hope you can solve it soon.
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#14 TitanNeil

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Posted 03 June 2019 - 11:52 pm

Anything like mine was?? https://youtu.be/IV1uQq37ySw

Yes, just like that. Hard to start, probably only running on right cylinder and left one popping when trying to rev it up. Is there a cure?


Try adjusting the pilot screw.

Is it an air or fuel screw? They are both 2 turns out right now. Would it make a difference at higher rpm, as it pops when I try to rev it up?



#15 dablik

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Posted 04 June 2019 - 07:42 am

Anything like mine was?? https://youtu.be/IV1uQq37ySw

 

The cause of his problem was the bike sitting and the carbs gummed up, he left a few litres of petrol in the tank and left a can of Seafoam in a few days after running the bike briefly which cleared it up, did'nt work for wicki recently though.

 

PS aircooled68 is currently lounging hotter climes so doubt he'll be back on soon..


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#16 Studley Ramrod

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Posted 04 June 2019 - 09:06 am

Yes, just like that. Hard to start, probably only running on right cylinder and left one popping when trying to rev it up. Is there a cure?


Is it an air or fuel screw? They are both 2 turns out right now. Would it make a difference at higher rpm, as it pops when I try to rev it up?

 

I suggested adjusting the pilot screws to see if it made any difference to the backfiring through the left carb.  You should also try adjusting the carb balance, again to see if it makes any difference.  If it doesn't then you probably have a blockage somewhere, like Aircooled did.  If it does make a difference then you'll have sorted it.


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#17 Robodene

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Posted 04 June 2019 - 02:23 pm

As there seems not to be an answer yet, is it worth considering an electrical fault? You mention a weak spark. I have experienced mysterious and intermittent backfiring which was eventually found to be a corroded wire. There is much to said against this as a solution, but your checks have shown everything else seems ok. Just my pen'orth.

PS I made this suggestion on another forum and he just wrote in to say all well after changing rectifiers (which he thought ought to be ok anyway) and clean/greasing all leccy cotacts. (Sorry for the hard sell...maybe irrelevant).

Edited by Robodene, 04 June 2019 - 03:03 pm.

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#18 TitanNeil

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Posted 05 June 2019 - 10:01 pm

Success!!!

 

After going through the carbs thoroughly, nipping the spark plug leads back a bit on both ends to get a better connection, and crossing my fingers it appears the backfiring is gone! It did it a bit on startup (I think the left bowl just fills a bit slower than the right) but then it cleared up and now idles and revs as it should.

 

I'm very grateful to the good folks here who posted their suggestions, advice and experience to help me out with this. Studley, dapleb, aircooled, Mipe, Robodene and dablik.....thank you. This is one happy Canadian who will now be touring on the (probably) only TDM850 in Ontario....



#19 Robodene

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Posted 06 June 2019 - 08:22 am

Hooray! These challenges are all very well but you can do without them.
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#20 Studley Ramrod

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Posted 06 June 2019 - 08:48 am

Happy biking. :)


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