Cured the low end stumble finally!

[sparkfly88]

I found a set of 4 diaphams on ebay for 80 $...thats not TOO bad.

[Schming]

I found a set of 4 diaphams on ebay for 80 $...thats not TOO bad.

I paid $84.80 shipped for four diaphragms for my '80 G from JBM Industries.

They fit perfect and the site has excellent tutorials.

http://jbmindustries.com/Yamaha650.html


YMMV HTH

[sparkfly88]

There's no top lock being installed back in???

Really? just trusting rubber to hold it self?

I see now, those have a reinforced section they call the rubber tire...

[Schming]

There's no top lock being installed back in???

Really? just trusting rubber to hold it self?

I see now, those have a reinforced section they call the rubber tire...

Yep, the "tire" is a substantial piece of rubber and is a very snug fit.

[sparkfly88]

Mine rotate pretty much freely

[Schming]

Mine rotate pretty much freely

Mine were tight and had only 23k on them but had pinholes right where they sharply bend on the top edge of the carb body:



...so I used a dremel to sand a slight champher on the inside edge where the diaphragm makes that bend and also brushed some silicone grease on that area to lube the movement there.

[MPittma100]

Super Glue

Yep, the "tire" is a substantial piece of rubber and is a very snug fit.

I bought same "rubber tire" type 2 years ago. The instructions included the option of using Super Glue to the attachment point of the slide. I did that and there is no looseness at all and these slides have been doing their job as advertised.

The only issue is there is no line up tab on the new rubber so you have to insure that the vent in the slide is positioned properly - to the front as I recall.

The manufacturer of these diaphragms is JBM industries. His website:

http://jbmindustries.com/Yamaha650.html.

Mike

[mharrington]

Gas proof and flexible. Under $6 sold at hardware stores and autoparts stores.

[sparkfly88]

I'll give that stuff a go.

I did notice in the morning on the way to work when its under 60*, it runs beautifully.

But in the afternoon when its over 70* it stumbles on low throttle.

[81xsproject]

If you have "pin" holes and are having stumbling issues at steady state, low throttle running, it is likely not related to the diaphragms. Before everyone, jumps my sh**, just let me say that you should indeed fix that issue asap. However, anyone who's watched TC's video of the carbs on a ride, looking through the inlets, knows that the slides don't move except under changing throttle and very heavy throttle. The slides also don't have any effect on the pilot circuit, which is where you state you are having issues. Your sig says you are running stock jets (is that size, brand, or both?), and have a K&N. You might try a 45 Mik pilot and see if it helps. How are your shaft seals? The conditions you are describing are also where the intake has the strongest vacuum (besides decel). Bad seals might be letting a little air by leaning the mix. Good luck.

[sparkfly88]

If you have "pin" holes and are having stumbling issues at steady state, low throttle running, it is likely not related to the diaphragms. Before everyone, jumps my sh**, just let me say that you should indeed fix that issue asap. However, anyone who's watched TC's video of the carbs on a ride, looking through the inlets, knows that the slides don't move except under changing throttle and very heavy throttle. The slides also don't have any effect on the pilot circuit, which is where you state you are having issues. Your sig says you are running stock jets (is that size, brand, or both?), and have a K&N. You might try a 45 Mik pilot and see if it helps. How are your shaft seals? The conditions you are describing are also where the intake has the strongest vacuum (besides decel). Bad seals might be letting a little air by leaning the mix. Good luck.

I'm running stock size jets, but not Genuine Mikuni. the pilots have the correct amount of holes in em.
I guess I could try different pilot jets..

[81xsproject]

That would be a great start. Get some Miki's in there and never go back!

[motoman]

I'm running stock size jets, but not Genuine Mikuni. the pilots have the correct amount of holes in em.
I guess I could try different pilot jets..

Your not running stock jetting if their not Genuine Mikuni. This DOES make a difference in spite of nay-sayers.
FWIW, a K&L main jet marked 110 is equivalent to a 115-117.5 approx. hole size of a Genuine Mikuni. The internal machining angle on the backside is also different on a K&L.
The difference WILL exist as long as Mikuni patented rights exist.
Bottom line, they are NOT the same and will NOT work the same.
Get Genuine Mikunis, both pilots/mains/air-jet...............then come back here with any running issues that may exist.

[skids]

If you have "pin" holes and are having stumbling issues at steady state, low throttle running, it is likely not related to the diaphragms. Before everyone, jumps my sh**, just let me say that you should indeed fix that issue asap. However, anyone who's watched TC's video of the carbs on a ride, looking through the inlets, knows that the slides don't move except under changing throttle and very heavy throttle. The slides also don't have any effect on the pilot circuit, .

Good points!

[motoman]

Good points!

Partially.............since metering rods are attached to slides, they DO come into play associated with the pilot circuit. More so with the early model carbs having the cross-over in the two circuits.