I have bored out my manifolds on my 71 2800cs with a 3.5 liter to a "Peanut" shape and have trouble with intake leaks. The shelf on the manifold does not match the rim base of the Weber. Can you only use virgin manifolds and keep the intake openings separate? I am using an original thick Weber gasket. Thanks for any input!
weber 38 / 38 on a 3.5 liter
- Thread starter tiico
- Start date
MMercury
Well-Known Member
- Messages
- 481
- Reaction score
- 5
intake manifold questions
Your description of the porting and the "virgin" gasket/s you have attempted to use is a bit confusing. Pictures might be more understandable.
I am familiar with the shape of the manifold runners, but porting is a black art that requires a lot of knowledge/experience or plain dumb luck to obtain measurable improvements. I am guessing you tried to enlarge things to accommodate the larger displacement. Depending upon what you were seeking to accomplish, this may not have been the best idea. The smaller runners would have probably offered higher gas flow rather than volume and the carbs could have been jetted to work with that setup. In any event, if the mating surfaces are machined flat/plumb, you probably do not need anything but the stock gaskets to seal things. Obviously, if the gaskets do not conform to the parts, trim them. If things were roughed up a bit, then there is always RTV, permatex and the like to seal up the irregularities. I would prefer not using the later stuff, but I have for peace of mind, on a few occasions.
Your question about virgin gaskets is perplexing. As long as the gasket is sealing the circumference of the carb base and the manifold inlets, just about any gasket should be useable. If you are describing the phenolic resin block divider, you can machine, file, carve or sand it to conform to the two surfaces. Port dividers, or lack thereof, on a 38/38 carb should matter little. The two barrels share the common manifold but a few millimeters away. The difference is analogous to a crossover pipe between the banks of exhaust pipes whereas it may equalize things between the banks of cylinders, or in your case, carb barrels. Books have been written addressing the resonance between multiple carbs and manifolding, but if I were you, I would start with getting things running without vacuum leaks and fine tune from there.
hth
Your description of the porting and the "virgin" gasket/s you have attempted to use is a bit confusing. Pictures might be more understandable.
I am familiar with the shape of the manifold runners, but porting is a black art that requires a lot of knowledge/experience or plain dumb luck to obtain measurable improvements. I am guessing you tried to enlarge things to accommodate the larger displacement. Depending upon what you were seeking to accomplish, this may not have been the best idea. The smaller runners would have probably offered higher gas flow rather than volume and the carbs could have been jetted to work with that setup. In any event, if the mating surfaces are machined flat/plumb, you probably do not need anything but the stock gaskets to seal things. Obviously, if the gaskets do not conform to the parts, trim them. If things were roughed up a bit, then there is always RTV, permatex and the like to seal up the irregularities. I would prefer not using the later stuff, but I have for peace of mind, on a few occasions.
Your question about virgin gaskets is perplexing. As long as the gasket is sealing the circumference of the carb base and the manifold inlets, just about any gasket should be useable. If you are describing the phenolic resin block divider, you can machine, file, carve or sand it to conform to the two surfaces. Port dividers, or lack thereof, on a 38/38 carb should matter little. The two barrels share the common manifold but a few millimeters away. The difference is analogous to a crossover pipe between the banks of exhaust pipes whereas it may equalize things between the banks of cylinders, or in your case, carb barrels. Books have been written addressing the resonance between multiple carbs and manifolding, but if I were you, I would start with getting things running without vacuum leaks and fine tune from there.
hth
intake manifold
Thank you for taking the time to reply. By virgin manifold, I mean the the bridge between the two carb openings have not been removed so it is not open like a peanut shape, similar to what 2002 owners do. The carb gasket hangs over on the rear manifold at the rear about 15mm so that it does not make contact between the carb and manifold. The carb relys on sealing to a gasket that hangs over the rear of the manifold. Although I seemed to have sealed the gasket with Elring sealant, It does not seem very permanent.
Next is getting these carbs jetted properly.
Thank you for taking the time to reply. By virgin manifold, I mean the the bridge between the two carb openings have not been removed so it is not open like a peanut shape, similar to what 2002 owners do. The carb gasket hangs over on the rear manifold at the rear about 15mm so that it does not make contact between the carb and manifold. The carb relys on sealing to a gasket that hangs over the rear of the manifold. Although I seemed to have sealed the gasket with Elring sealant, It does not seem very permanent.
Next is getting these carbs jetted properly.
Honolulu
Well-Known Member
shed some light if I can...
IIRC, the reason to siamese or "bore out" the manifold(s) is that the two-hole manifold will jam the throttle plates of other than the stock carbs.
If the cutting is done to match the carbs, there should be no problem. If the mating surfaces are nice and flat, there should be no problem. If you doubt the condition of the surfaces, hmmm, well I'd use some RTV and let it set at least overnight.
Another trick to getting flat surfaces is to clean the manifold mating surfaces well with rough (maybe 60 grit) sandpaper, then smooth on a thin coat of JB-Weld, and set a flat glass plate on the epoxy overnight. Next day, remove plate and the epoxy will have an excellent flat surface. BUT... remember to apply some sort of release agent to the glass before you set it down. I'd suggest a coat of two of car wax, or a light spray of silicone lube, something that you know the epoxy won't stick to. If in doubt, a test run with a spot of JB Weld on the flat surface (don't test on the manifold) will prove the usefulness of your proposed mold release.
If have used this trick on power tools where the front and back platen of a power planer were not in the same plane. Works well.
IIRC, the reason to siamese or "bore out" the manifold(s) is that the two-hole manifold will jam the throttle plates of other than the stock carbs.
If the cutting is done to match the carbs, there should be no problem. If the mating surfaces are nice and flat, there should be no problem. If you doubt the condition of the surfaces, hmmm, well I'd use some RTV and let it set at least overnight.
Another trick to getting flat surfaces is to clean the manifold mating surfaces well with rough (maybe 60 grit) sandpaper, then smooth on a thin coat of JB-Weld, and set a flat glass plate on the epoxy overnight. Next day, remove plate and the epoxy will have an excellent flat surface. BUT... remember to apply some sort of release agent to the glass before you set it down. I'd suggest a coat of two of car wax, or a light spray of silicone lube, something that you know the epoxy won't stick to. If in doubt, a test run with a spot of JB Weld on the flat surface (don't test on the manifold) will prove the usefulness of your proposed mold release.
If have used this trick on power tools where the front and back platen of a power planer were not in the same plane. Works well.