Intake Manifold = Bad Design

[ed_in_az]

With Carb Cleaner I found a slight leak on the bottom rear manifold seal. After taking the flanges off and checking for fit that bugger does not sit flush. It depends on the rubber seals to fill slight gaps on the bottom where both the front and rear hit the heads. I used Preachers old cylinders to do my conversion so I don't know if the manifold did a perfect fit on my old cylinders, but I doubt it. The only way to get a proper metal to metal fit would be to grind the intake manifold to match up with the heads. So with new gaskets installed I'll find out tomorrrow if the leak is gone. But with minor variations in cylinders / heads these intake manifolds aren't perfect fits.

Has anyone else noticed this?

To me it looks like a design just begging to leak.

But, I sure hope it doesn't.

[sportysrock]

Install the manifold, seals, flanges, and bolts just snug. Center the carb and manifold with the carb support or hamcan and snug up the bolts the best you can. Then to finish tighten the bolts you remove the carb so you can get at 'em. Then button it all up and you should be good.

[t.c. Johnson]

It's a piss poor design. It's almost criminal that these bikes don't have twin carbs by now. But if they insist on only one carb, then they could certainly have come up with a better design.

[ed_in_az]

Originally Posted by sportysrock

Install the manifold, seals, flanges, and bolts just snug. Center the carb and manifold with the carb support or hamcan and snug up the bolts the best you can. Then to finish tighten the bolts you remove the carb so you can get at 'em. Then button it all up and you should be good.

Yep that's just what I did last night. I'll check for leaks today. I think they could have still used a compression design gasket, but maybe one that overlapped like the one at the base of the Carb. I would think that would seal any gaps better.

I guess I now have one minor complaint at my bike.

[chrishajer]

The manifold does not fit perfectly in between the intake ports. That's why they have the rubber gasket and the flanges, to take up the gap. The problem with them leaking is not the design so much as the gaskets gets out of position on installation, or they just get dried out and leak.

On the Iron Heads and Shovel Heads, you need to align the heads with a drafting triangle to get them close enough to seal. If you think the Evo style XL intake design is bad, you should take a good look at the Iron or Shovel intake manifold.

FYI, they changed the intake on the '07 EFI models

--Chris

[ted]

I usually put a thin coat of silicone grease on the gasket. That way it will move a bit while being tightened and fill any gaps better. The silicone grease you use on spark plug boots works real good for this.
Ted

[ed_in_az]

Good info on the grease.

I suppose the new manifold isn't backward compatible.

Anyway it appears mine doesn't leak. It idles and runs better when cold now. I'll bet the engine will run a little cooler overall too.

Off to ride now!

Holiday traffic is awful though.

[sportysrock]

If it was sucking air, then it should run cooler now (richer - as designed, rather than leaner - and leaking)

[IronMick]

On the ironheads the trick is to FULLY install the manifold BEFORE tightening down the rocker cover/cylinder head assembly; this brings the cyl heads to the manifold. Then you get a perfect seal, especially with the o-ring seals.

This is the technique in the FM. Unfortunately many mechanics do not do it this way and you get leaks.

[Shu]

I use spray white lithium grease on my intake seals too. Then you can tighten the intake down and still rotate it to install the carb correctly. The key is to check the back of the carb (the flange into the intake seal) and make sure it is perfectly installed in the intake when the support bracket (or a/c back plate) is in the correct position. Once everything is lined up perfectly the gap you described is still there, but minimal and yes the seals and flanges will seal it up.