weber port to cis port issue

[advtracing]

How things, need a quick answer to a question. I have a set of webers arrive and the manifold has been polished and flowed and is 38mm port at the bbase. Now I haven't removed the cis yet but the books give me a 36mm port. So what issue if any could this pose. Will the large to small cause disruptive air flow and issues or do I have the post size on the heads wrong

[fred cook]

A smaller port on the engine side will interfere with the air flow more than the other way around. Once you pull the CIS manifolds and verify the port size, you can either swap out the mainifolds for a set that matches or use something to seal the port near the valve and open it up with a grinder. Of course, you will have to be absolutely certain to catch all of the metal chips. You could dip a small piece of shop rag in oil and with the intake valve closed, stuff the oily rag down in the port. Once the grinding is done, vacuum out as much debris as possible and then carefully pull the rag back out. The oil should catch just about all of the chips. Then, using a small hose attached to a shop vac, suck out any remaining chips. The ports are fairly easy to see into with the intakes off so you should be able to see when it is really clean. Mask off that port and move on the the next. Be certain to tape off the ports not being worked on. Of course, the best way would be to remove the heads, but that may not be feasable. Good luck!

[advtracing]

Cheers Fred, sound drastic getting the grinder out, as the cis pistons and came are staying put for now I have been advised that it might not cause to much issue. Best bet is I think do the set up, give it a ttry and if it doesn't work well swap manifolds

[Vin-barrett]

I have 42mm webers on the late cis 34mm small port intakes. Pmo manifolds matched to 42mm on top. intake side is 36mm.36mm pmo insulators between the intake and cylinder head with a taper down to compensate for the change in size. I am satisfied with the results

[advtracing]

Originally Posted by fred cook

A smaller port on the engine side will interfere with the air flow more than the other way around. Once you pull the CIS manifolds and verify the port size, you can either swap out the mainifolds for a set that matches or use something to seal the port near the valve and open it up with a grinder. Of course, you will have to be absolutely certain to catch all of the metal chips. You could dip a small piece of shop rag in oil and with the intake valve closed, stuff the oily rag down in the port. Once the grinding is done, vacuum out as much debris as possible and then carefully pull the rag back out. The oil should catch just about all of the chips. Then, using a small hose attached to a shop vac, suck out any remaining chips. The ports are fairly easy to see into with the intakes off so you should be able to see when it is really clean. Mask off that port and move on the the next. Be certain to tape off the ports not being worked on. Of course, the best way would be to remove the heads, but that may not be feasable. Good luck!

well fred you are correct and i need to enlarge the intake ports . what is the best tool and grinding bit to produce the correct curve and should i polish the surface once done

[timmy2]

Why bother, just give the weber setup to me and I'll use them on my ROW '78 with large ports for you!

[kerthunk]

I think you would be best off with tapered insulators or a different manifold. You need insulators anyway to prevent heat soak into your carbs. It is far cheaper to buy either of those than it would be to replace a head that you accidentally mess up. Richard at PMO should be able to help you get either one.

[fred cook]

Originally Posted by kerthunk

I think you would be best off with tapered insulators or a different manifold. You need insulators anyway to prevent heat soak into your carbs. It is far cheaper to buy either of those than it would be to replace a head that you accidentally mess up. Richard at PMO should be able to help you get either one.

See if you can find a set of the plastic insulators that are designed to match the Carrera intake to a CIS head. They should be close to the dimensions you want to match and can be modified away from the engine to keep debris out of same. If this setup does not perform to your satisfaction, then the only other choice would be to pull the heads for porting or swap out for a set of Carrera heads with the larger ports. Check with Summitt Racing for a suitable porting tool. A small shop grinder like a Dremel Tool won't be up to the task. It's my understanding that you would want to leave a slightly coarse finish in the port to help induce a swirl to the intake charge to help mixing. Good luck!