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Reconditioning carbie manifolds

Discussion in 'Technical and Troubleshooting Torque' started by mattb, Feb 13, 2008.

  1. I know it’s orthodoxy that you should just replace them when they’re cracked, but at $220 for the replacements on a cheap project bike, I’m wondering if it’s possible to recondition the rubber manifolds (between the carbie and engine). (Besides, orthodoxy is more a 'common opinion' than actual truth, when it comes to MCs.) The XS400’s manifolds are full of little cracks (I mean they look like a piece of shattered glass!), which I suspect might one possible explanation for some of the aberrant plug conditions I’ve come across on it (sometimes fouling, sometimes lean). Anybody have any ideas? I’ve wondered if there’s a glue-type substance that I could paint them with….


  2. ive never tried myself but maybe

    paint them with rubberized under body car paint,like 40$ 4 litres from trade shop?

    fill cracks with silicone based gasket glue?
  3. Here's my free advice. Buy new boots. A multi cylinder motorcycle engine is hard enough to tune as it is without damaged carb boots, and the benefits of new, perfectly sealing boots cannot be underestimated!
    Aftermarket ones are available, so scour around. All you need is the correct diameter and bolt pattern, boots from other bikes can and will work if these are correct.
    Money spent on things like carb boots, carb rebuild kits and ignition components is worth every cent, especially on an older bike with a lot of kilometres on it.

    Regards, Andrew.
  4. Re: Reconditioning carbie intake rubber

    Firstly, it's not the manifold, but the intake rubber which is cracked. The manifold is the metal part which connects the rubber to the cylinder head.

    Secondly, silastic smeared on the outside should work well enough. Don't smear it on the inside!

    Use black, engine gasket grade silastic.

    You could also use self-amalgamating tape, which is a butyl (natural) rubber with a separator between the turns of the roll. When you wrap it around an object and stretch it it forms an "impossible to unwrap" seal which is air and waterproof.

    I cannot vouch for its fuel-resistant properties, though. The SAT takes about 5 to 10 minutes to join to itself after wrapping. You can buy it from electrical wholesalers or specialist antenna installers - it is used to seal F connectors on external aerials (TV type especially).

    I would use SAT over the top of silastic.

    It might pay to look at different, nore recent bikes to see if anything uses a similar intake rubber which could be used or adapted.


    Trevor G
  5. If they are cracked, chuck em in the bin. No matter what you do, they are not going to improve. They will leak air, causing a lean mixture. When that happens, your pistons won't even be good for ashtrays, cos they will have a hole in them. New rubbers are cheaper than pistons.
  6. Just a thought -but are you sure they're leaking. Might be worth making a couple of simple blanking plates, pressurizing the manifolds & dribbling some water/dish detergent over them to see if they actually are leaking before lashing out & spending the big buck. Test before replacing :) :)
  7. I'm of the "just replace it" group, but if you are after a left of centre answer I believe you can buy rubber moulding kits from hobby shops.

    You would have to be confident the rubber used in them was temperature and fuel resistant and was of a similar density.

    On the up side there are not many types of Jap boots, so you could just take yours to a wreckers and see what they had.
  8. As a "get you home" measure, I had good results with a couple of wraps of duct tape (the plastic stuff, not the fabric backed) when the wife's evil Commando split its intake boot far from any replacement. It worked and lasted for weeks, even with the gigantic (non-standard) Mikuni wobbling about under the influence of parallel twin vibes.

    As I had no idea where the boot had come from, I took it to one of the more helpful local spares suppliers when business was quiet and he scratched around in his stockroom until he found something with similar flange and throat dimensions. Didn't cost much. Certainly not the equivalent of $220.

    By the way, are you still interested in that coil?