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1990 XT600 Engine, 1986 Carb 1990 XT600 Engine, 1986 Carb
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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 21 Sep 2014
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1990 XT600 Engine, 1986 Carb
As the title says! Engine has 22K miles, the carb has 7K miles. The engine starts and runs, but has some minor, quietish backfiring throughout the rev range. I can feel puffs of air from the air intake when it backfires. I also feel it is running a little lean. Valves adjusted, Carb cleaned. PO removed the baffle from the exhaust.

What do you suggest I look at to possibly richen the mixture and secondly to eliminate the back firing throughout the rev range?

T
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  #2  
Old 21 Sep 2014
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Oh, sorry. Yes, I replaced them with new boots. Also the air joints (rubber boots on the intake side) are new. They seem to have a good seal although I am using common hose clamps around each union.
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  #3  
Old 25 Sep 2014
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Boots are too big.
Here's the issue as suggested by Sheva Deva on another site and confirmed just now. I have a 1986 carb on a 1990 airbox and intake manifold; the problem is the intake on the 86 carb is significantly smaller than the 1990 airbox joint.

So now I have to figure out how to make the intake manifold and airbox joints smaller, or make the carb intake larger. I'm thinking of gaffer tape, or cutting the boots to allow cinching down. I will also research the 1986 airbox joints to see if they will fit the 1990 airbox.

Any other ideas? It seems clear that this will solve my rough idle problem.
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Old 25 Sep 2014
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find a rubber hose to fill the gap, cars have alot of different dim to choose from.
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  #5  
Old 25 Sep 2014
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The secondary carb to cylinder manifold is bigger on the 86 than on the 90. I used two primary carb manifolds or you could use the 86 manifolds. THUMPTHUMP
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  #6  
Old 25 Sep 2014
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Thank you for the suggestions! I found a perfect rubber coupling at the hardware store, trimmed it down and it fit inside the airbox joint perfectly. It's a very tight fit on the 86 carb so i think it'll work. Fingers crossed.
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  #7  
Old 25 Sep 2014
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Rubber boot from the hardware store
It fits inside the airbox joint and outside the 86 carb intake.
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1990 XT600 Engine, 1986 Carb-kimg0936.jpg  

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  #8  
Old 25 Sep 2014
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That won't cause any backfiring to the engine unless it isn't the regular form it is normally called . That loose manifold will just allow dirt into the carb. Did you check the coasting enricher diaphragm on the side of the carb?
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  #9  
Old 4 Oct 2014
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Coasting Enricher
jjrider

The diaphragm looks perfect and fits well. I ordered a repair kit anyway that includes a new one. What should I be looking for in the coasting enricher assembly?
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  #10  
Old 6 Oct 2014
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Update
I'll post an update for the sake of those using the search feature of HUBB. Yesterday I removed the carb again (third time) and did another thorough cleaning. (I don't have the coasting enricher kit yet). The engine ran as it did before but I decided to drive it a bit and see what happened. I rode for about an hour off road and on, at varying speeds and rpm. The engine got smoother and smoother. It's not perfect yet, but it's very close. I wonder if the problem was some junk or carbon buildup on the valves that worked itself out with use. Or maybe it just needed some lovin'. Will try again today, and install the coasting enricher kit when it comes. But it's running better than it was. Oh, I also dumped a can of Seafoam in the tank before riding it for the hour yesterday.
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  #11  
Old 7 Oct 2014
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Could very well have been some gunk that is slowly dissolving with the Sea Foam and/or gas. The coasting enricher just has that diaphram that should be nice and flexable with no holes, plus a spring and plunger that should move freely. The passageway should get a good blast of air to make sure it comes out the other end. Not a whole lot to it but can really cause a lot of backfiring when a person lets off the gas.
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  #12  
Old 27 Oct 2014
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Update
Another update for those doing a search on rough running: I noticed at some point that the installation I had made of the carb was not good. The carb was slightly canted to the side at an angle relative to the face of the engine. When I loosened the boots and muscled the carb around straight, the last of the rough running issues were resolved. This is something that could easily be overlooked in a forum response because we would assume that the mechanic had installed everything straight and square. I'm here to tell you to double check!

Went for a long ride this afternoon down the coast and the Zombie Apocalypse Response Bike is performing flawlessly. Now for some off-roading.

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