Carb backing plates cracking

[poorbiker]

Have a 2004 RK in my shop with a ongoing issue with aircleaner backing/support plates cracking into. Threads in breather holes are fine as are breather bolts,  top motor mount from head to head is good. Threaded breather holes are square to each other. Did not do work on engine was done by shop in Bay area, big comp. & HP. Not stroked. I can find no reason this is happening though my expertise lies more in the pre 1984 era. Anyone with knowledge of similar issues would be muck appreciated...

[1FSTRK]

What backing plate, intake, carb/tb is he running?

[poorbiker]

Apparently several different types, most recent an S&S setup he brought to me. It split in two at a vent hole within 10 miles.
I removed the tank and inspected frame backbone and upper mount all was good. All the other mounts look good though I have suggest he upgrade them. He put his stock OEM cleaner back on and promptly broke it. This is the damndest thing.

[poorbiker]

Bike handles/rides drives great, hands off straight as can be.
In my opinion he has way more hp and torque than he needs but opinions are like assholes.

[poorbiker]

Carb is Mikuni, stock intake I believe.

[1FSTRK]

Sounds like it may need some shims between the backing plate and the heads.

[bobo]

mount the backing plate to the carb and run feeler gauge between backing plate and heads at the point the breather bolts go through the backing plate and the head add shims as needed, try and make both sides equal

[poorbiker]

If anything backing plate could be shaved at point of mounting surface. Carb has play when all is fastened up, not much and no air leak at seal but carb can be moved by hand.
I thought the same at first when I noticed previous shop hadn't relieved cylinder fin where carb bowl sits, I initially thought carb was in bind and causing a stress on backing plate.

[bobo]

try 2 gaskets between carb and backing plate.
back in the days working on panheads and shovels I learned that you should not stress two parts together
one will break

[poorbiker]

I actually installed a new seal from manifold to carb which took out the slight looseness. This is a damn strange deal  owner did end up contacting shop that did motor work in the Bay area and they said that this has been something they have been seeing and while not offering explanation instead suggested some other air cleaner setup which customer ended purchasing but hasnt arrived yet, I dont think it addresses the reason nor do I think it will work.

[kd]

try 2 gaskets between carb and backing plate.
back in the days working on panheads and shovels I learned that you should not stress two parts together
one will break

  on the gaskets.    All of those things change when the engine comes up to temp also. An extra gasket at the base will also move the float  bowl marginally away from the fins. I used 2 on my HPI / S&S base after adding S&S rubber coated shims to the breather bore mounting surfaces. I made the second base gasket out of thinner gasket paper to take up the minimal clearance I ended up with.

[JMHD]

I have seen the back plates crack when the heads have been changed to a 2006 and later cylinder head but the intake flanges were used from the old heads. The old style flanges were stamper F and R , the new style have no markings because they can be used on front or rear. I think they called them asymmetrical flanges. Might be worth a shot to look into.

[poorbiker]

Why, do you think, would the flanges in that scenario cause the issue of cracking backing plates.
I dont know the build history on this engine and dont dont know if these are the same cylinder heads engine started with and haven't looked the the intake as the culprit..

[ecir50]

put a straight edge across the breather holes to see if the heads aren't cocked.

[turboprop]

'Try two gaskets between the carb and the backing plate".

Its a Mikuni - no gaskets.

[1FSTRK]

'Try two gaskets between the carb and the backing plate".

Its a Mikuni - no gaskets.

Mikuni still uses the stock gasket between their adapter and the backing plate.

[JMHD]

If you look at the rear head above the exhaust port and the casting number ends with a -06 then you should have the intake flanges that have no markings. If they say F or R on the flanges you need to change them to the newer 06 and up style. Hope that helps.