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I have a 1990 VFR750 with only 15,000KM ( 10,000 MILES  ) on the speedo and have a simple but perhaps strange question

How do you determine that the carb intake rubbers are bad and are in need of replacement?

Is it the number of miles they have on them ( hot/cold cycles ) over the years?

The age of the Rubbers?

How can you tell by looking at them that they should be replaced

Thanks

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My $.02 would be to flex then a bit and observe for any cracking.  They're pretty tough and not all that flexible,  but any cracks would indicate that they could possibly leak air and cause issues.  Heat cycles and age would contribute to cracking but you have low usage on your side.  My guess is that being such low km they're likely fine. 

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My own ‘91 has 50,000 miles on it... I took the carbs off for a strip and clean last year. The rubbers weren’t cracked or anything but they were a little hard. This made them difficult to remove the carbs, but a smear of Red Rubber Grease made the reinstall and absolute doddle; slipped right back in.

 

If yours are cracked then you’ll have to replace them as they’ll pull air in. But if they are just hard, Rubber Grease is a good way to mitigate that. Don’t use normal grease - it kills rubber! I think rubber grease is basically vegetable based. HTH. 

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Soften the rubber in Wintergreen oil solution 

 

1 teaspoon of oil to 300ml of water and boil for 5 minutes ... they’ll be like new, if fragrant 

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You can also use alcohol to dissolve the wintergreen and soak it in bottle overnight. 

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Thumbs and DannoXYZ are on it.

 

I used a mix of Wintergreen and Alcohol and soaked my hard as a rock 85 V65 Sabres intakes in it for a couple of days. Came out pretty much as new. Made the carb install a breeze.

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