Good evening gentlemen,
This afternoon I removed the air filters and carbs for cleaning and rebuild. Firstly, should the Vacuum ignition take-off be connected to anything? There is a short rubber hose, bored through placed over the port, one to match on the Vacuum advance on the distributer.
Secondly, there are two lines running to each carb connected to a rectangular panel on the top left front corner of the block, right below the exhaust manifold. What are thier purpose? These lines run to the carb after the piston and needle. I will venture a guess and say that this is some sort of temperature controller, help to warm the air before entering the piston? The panels are clearly seen immediatly below the cylinder head connection, rather large rectangular openings, two, side-by-side.
Thirdly, what I assume is the intake manifold, that's where the carbs lead to before entering the cylinder head anyways, the manifold has about six threaded openings on top, all plugged with bolts, except for the center one, which has a tube connection and hose attached, that run nowhere (the end of the hose has a bolt in it). What should this hose be doing?
One last question, I have removed the carbs and although I understand the theory behind the carb, I'm a little stumped at how the air-fuel mixture gets past the piston in the carb as while looking it over, I haven't found the mechanism that lifts the piston, other than the hand operated lifting lever? A little carb 101 may be in order so if anyone has any advice or links to sites that can help, please post.
I thank you all for your patience and forewarn you that your replies will probably generate more questions.
Vacuum advance on early cars went to ported vacuum on the rear carb. , later ones went to manifold vacuum from the intake.
You must have a early 70's car... the square box on the block is one of the tappet covers, and has a oil seperator in it ( marginally functional). The pipe is for PCV, and goes to metered vacuum ports on the carbs. There used to be a gulp valve and other emissons stuff connect to the intake ports that are blocked off.
The SU carbs are constant depression units. The air flow through the carb. creates a vacuum in the dash pot proportional to the air velocity. The vacuum lifts the piston and meters the fuel for the given amount of air. This is why the piston MUST be free moving and the metering needle centered in the jet ( although you've got biased needles I'm pretty sure.. and shouldn't have to worry about that). The dampner pistons are there to keep the pistons from 'popping up' and creating a poor mixture situation on sudden throttle movements.
You mentioned in another post that your carbs are AUD405's. Those are 1970 carbs. I have them on my car. The vacumn port is on the rear carb, near the large vertical piston chamber. There should be a semi-rigid plastic tube going from there to the vacumn advance unit on the distributor. This tube is available from Moss, or you can just get a couple of feet of vacumn hose at your local autoparts store.
Whoa, Harlan!! Remind me to e- you when I have carb problems...come to think of it, I have one now...am trying to build a cold air box w/remote cone filter for the HIF-4's I put on my '79 (I want to keep the brake booster)...Believe a 4" hose through the radiator support to a sheet metal box built to fit area between carbs & booster will work..but, it has to taper down from 6"x6" in front of the front carb to only 3/4" thick by 6" high at the rear carb...I put a cardboard mockup on y'day &, revving up to around 2k, she sucks a lot of air...what do you think? Will the "V" shape create problems for the rear carb?
If you consider a VE of 100% (volumetric eff.), the engine will pull about 63.5 cfm through it at 2000rpm. If you run it up to 6K, your looking at something like 190 cfm. Normally you will get a VE more like 85% so, the these numbers will be a little lower.
You don't want to have a different flow regime to carbs, they need to be as close to the same as possible if you want to keep them balanced. If you e-mail me with some pic's of what your doing, I'll be glad to look at it and give you my .02 worth.
think you just answered my question...but, i will take a couple of digital photos of mockup & e to you...probably will have to make up some type individual hose per carb arrangement out to "Y" at K&N filter in front of radiator to keep air cfm equal./
The box that has 2 hoses going to each carb is not an original item. It seems someone was trying to make their own PCV system. Originally, there was a metal fitting attached exactly in the middle of the intake manifold where you now have a hose w/ bolt in the end. The fitting has the PCV valve assembly on top of it. The valve is basically just a rubber diaphragm and spring. Then there is a hose that connects the side of this PCV valve to the front
side panel that you mentioned [tappet or side cover]. This system allows excess gases from the side cover to vent into the intake, and the valve keeps the engine from sucking up to much air from the side covers to prevent it pulling oil through the breather. Moss or VB catalogs have good drawings. It wasn't a well designed system, but I know 3 owners who are using the stock system w/o complaint. Of course, they cleaned up their breathers really well. I helped a freind cut into one and replace the breather brillo pad guts which gave him a brand new breather. It'd be interesting to see what's inside that box you have now. If it was me, I'd clean up the system, find a salvage PCV assembly, and do it the original way, who knows what's going on w/ that box...Good luck and let us know if you find out what's in that box!
I have a '79 converted to twin SU HS4s. During MG Day at Boston's Musuem of Transportation, I saw an immaculate late model B with the Asian repro Hellings & Stellings filters that Moss sells and bought a pair for myself. These do not have much filter surface area, but they look good and definitely clear the brake booster with no other modifications required.
Moss and Brit-Tek also sell cone shaped K&N filters that supposedly will clear the brake booster. Doubtless these are better filters, but they are pricey and I personally don't like the way they look.
4" pipe sounds huge! My Audi has 2.5" tubing throughout and it's rated at 160hp. I definitely like the idea of cold-air intake though. TWM makes backplates and cold-air intakes for different kinds of carbs. Mostly DCOE and such but I did see some SU stuff. Check them out here http://www.twminduction.com/.
you'll have to let the rest of us see what you end up with. This might be a good mod to emulate.
Earlier cars had the 'mushroom' type PCV valve on the intake manifold. In the early '70's they when to the system the split and when into a controlled vacuum port on each carb., and did away with the mushroom.
oops, I guess yours is a '70. Mine is a '65 which had a different setup. You should have one pipe from the separator to a plastic Y and 2 hoses from that to each carb. No PCV valve. Seems a little surprising. The box must be there to replace the Y-fitting. I wonder what difference it makes if any w/ or w/o the PCV valve? Anyway, sorry to give you info on the wrong year.
The hoses comes out of the tappet cover(seperator) and is then Y'd. It then runs to both carbs. The Moss catalog shows both the seperator lines as well as the PCV valve on the intake. I am taking for granted that the seperator is acting as a PCV and that the actual PCV valve is not needed? Comments?
Tony the K&N filters will fit (the rear is a bit tight on and and off) I've got them on my 75 with twin SU's they look good but I've heard really mixed reviews on their performance, from "best air filters there is" to "pieces of crap". Who knows?
Thanks...the guys at Moss are sending me a set of the pricey K&N's to trial fit & expect to get them back (one of the bennies of dealing with them since when, they're great when you get to know them & visit some of their shows)....didn't see the Hellings & Stellings in their catalog....don't really want to go this route if I don't have to as I'll be sucking hot air off the headers...have contacted the folks at TWM site provided by Ameer (thanks, buddy)..waiting for them to contact me back...think there's a way w/o fabricating a whole new set-up, however, might be able to use some of TWM's calculations to build a long, flat filter box & attach a tube to run out in front of the radiator to suck cold air. hmmm...
For what K&Ns cost I think they should clean themselves and produce cool air.I think they look way cool but real pricey.Like has been said before more air does no good if all it gets is hot air from engine compartment or headers.I havent thought about a cool-air-box but like the idea.Thanks for the ideas guys