High idle for no particular reason

Hi guys,

My 1982 XJ750 runs fine at highway speeds. However when I come to a stop light it slowly increases rpm. I can take my hand completely off the throttle and it will slowly increase up to 3000 rpm.

I decided to clean the carbs - could not see anything blaringly wrong, until I closely examined the diaphrams. Three of them have pin holes - I can see light shining through when I hold a flashlight behind it.

So I have 2 questions... 1) is this the problem causing the high idle? And 2) Where do I get new diaphrams?

Thanks
Steve

 

aparently you can use some rtv silicon on pin holes this video explains the fix is good for a couple of hundred kms

 

after you use some ultra black on pin holes if that doesnt work i would look at vacuum from petcock to carby or leaks at intake rubbers heres a link to diaphram rubbers aparently a bit of work and the right glue to fix to the slide otherwise you will have to buy them as complete unit upto 240 each wich you can buy on this site under supporting vender..this is just the diaphrams ....http://www.ebay.com.au/itm/BN-Set-of-4-Carb-Slide-Diaphragms-Yamaha-XJ650-XJ750-80-83-HITACHI-HSC-32-HSC-33-/222030653296?hash=item33b20ee770:g:dn8AAOSwx-9WykTD

 

You can also use a condom , cut a small piece use a little rtv put a light smear put the piece over the hole and put a light smear over this again. As funny as it sounds it works

 

Would a piece of a bicycle inner tube work? I HATE using a condom. Lol

 

Possibly, but a latex condom is about the same thickness or a latex glove perhaps...

 

Pinholes in the diaphrams will not affect the idle. It will make the bike hesitate when accelerating and make it run lean on that cylinder.
Your idle problems are most likely a air leak on the intake side of the carbs. Throttle shaft seals and carb to head boots are the #1 leak spot. Air leaks make your engine act like the throttle is being opened. The longer you sit at idle the faster it runs. Sometimes "blipping" the throttle makes it return to normal but you still have to fix it.
Check for intake leaks by using some sort of flammable spray( carb cleaner, starting fluid etc) around suspected leaks. Only spray a very little, don't drench it. If there is a leak the RPMs will increase noticeably.

 

Thanks for all the feedback. I was able to find after-market diaphragms from JBMIndustries.com for about $20 each so I decided to replace all 4. I'm not convinced this will fix my racing idle problem so I'll have to do as mlew advises and try to find the vacuum leak after I put it all together this weekend.