Topic: Overly flexible inlet rubbers

[Nerdos]

Hello fellow SOHCers,

I have a 72 750K3 which I have more or less restored but, despite the bike looking the part there are a few performance issues.  The bike doesn't start well, doesn't idle for more than a minute and when it is running it is kind of asthmatic and hesitant which has suggested lean running .  I've been through the carbs three times now, jets are clear, airways are clear as per Hondaman's book, air screw, needle setting and jets are all stock (40 and 120 I think but can't remember right now as I type this).  There are no vacuum leaks as far as I am aware (I wafted some propane around the inlet rubbers with no change in RPM).  So, I am almost out of options other than coils but as I was observing the bike at tickover on the centrestand today I noticed that the inlet rubbers (which are new David Silver pattern parts) were visibly pulsing with the inlet stroke of each cylinder.  The original rubbers were rock solid so I binned them when I was rebuilding the bike so I don't have any reference point as to how these rubbers should behave.

Is visible flexing of the inlet rubbers (deflection of a couple of millimetres in the radius) normal or could it be the cause of or contributing to my problems?

Cheers,
Nerdos/Paul

[martin99]

Flexing of the rubbers is normal. I would suggest you start from the beginning and do a 3K tune up, that way you can at least verify one system is working before you move on to the next.
 

[BobbyR]

I noticed the pulsing also. They look like little hearts beating. Are you sure the choke is closing all the way?

[Nerdos]

OK, in an attempt to bring closure to this thread, I went back to the basics as advised.  The pilot jets which were A) brand new and B) K&L items were removed and replaced with the original 1972 items.  The originals had been clogged but with some carb cleaner to start with followed by some prolonged poking with some wire filaments and plenty of inspection with a cheap jeweller's loupe I got off ebay I got them cleared.  Two of the new items were already clogged which regrettably takes me back to the internal condition of my tank.  I was hoping not to have to line it but I fear I may have to.  Plenty of threads on the site for that of course.  The petcock is brand new Honda and I had hoped that any traces of rust would be handled by the screen filter in petcock bowl.
Thanks for the advice rendered on this particular question.

[BobbyR]

You have done your due diligence and discovered the cause. The particles getting by the new petcock screen must be quite small. Perhaps an inline filter would be easier than lining the tank which can have some downstream problems. Or a good tank cleaning?

[Djfob]

I would also recommend a good tank cleansing and inline filter. Drywall screws to remove rust then soak in metal rescue worked well for me.