Thursday, September 11, 2008

Dual Carbs - Vintage Style

Exotic carburation! I have to plead guilty to having always been a sucker for it. It may have begun when, as a teenager, I bought an Offenhauser 4 barrel manifold for my Studebaker from a local junkyard for $7. Things didn't change much when I moved on to Harleys. I vividly remember how cool those SU carbs looked on Pans and Shovels with the forward swept elbow.

Of course later when I got into drag racing Knuckleheads, dual carbs became more a necessity than a luxury. But when it came to a street bike, the traditional modification of the heads to hold two carbs held a drawback; that being in the form of a big lack of leg clearance.

Despite this leg clearance issue, the first dual Linkert setup I built consisted of one carb on each side. I used a pair of Linkerts (one on each side of engine going into a common manifold). I ran an M35 (1-1/8" venturi) as the primary carb, with an M74 (1-5/16" venturi) on the other side. I made up progressive linkage so that the M74 didn't start to open until the M35 was at half throttle, but they both reached WOT at the same time. It worked extremely well. Better gas mileage from doing most of your running on the small carb, but when you accelerated you could feel the second carb come in just like the secondaries on a car with a 4 barrel.

Dual Linkert - One Carb Out Each Side

The only downside was that the carbs I used had some wear so that too much air leaked past the throttle plates causing it to want to idle too high. I solved that by putting an auto advance distributor on it (the idle would slow when the weights came back retarding the spark). I think I could have solved the idle by using better carbs, or maybe by disabling the idle circuit on the second carb, but never tried it. Of course with the length of the Linkert, there still were leg clearance issues on the left side carb.

Perhaps a better solution is the method I used on my second foray into dual Linkerts. This manifold is based on a vintage "aftermarket" dual Linkert manifold that I have (and have seen other examples of). This vintage manifold is cast aluminum, and was designed to fit as a plumber manifold. My guess is that the aluminum would not have held up very well with brass seals and plumber nuts, but that is beside the point. The manifold is set up to take a pair of 3 bolt Linkerts, and has individual intake tracts; in other words the front carb feeds the front cylinder and the rear carb the rear cylinder. This coupled with the space limitations for such a design mean that the runners are extremely small, and thus extremely restrictive. The longer "tuned length probably would have given great throttle response and very low RPM performance, but I would bet that a stock 4 bolt Linkert would give better overall performance.

Vintage Dual 3 Bolt Linkert Manifold

Still, this vintage manifold is cool looking, and more importantly served as the inspiration for the next dual Linkert manifold project that I undertook. This next one was at the request of my friend Elmer. Elmer is world famous (seriously) for his Harley restorations, however, some time back he began to get a little weary of building the same bikes over and over. His solution was to start building a few "period correct" bobbers. Dual Linkerts were just the ticket!

Dual Linkert Manifold - Plumber Style

So I built another "prototype", and not just for looks either. I did flow bench testing to confirm that the manifold would be beneficial in the performance department, as well as the aesthetics side. I built it to use plumber fittings, at Elmer' insistence, so that it would be period correct for the original Holister event. It worked out very well, though the one thing I would change would be to cad plate the finished product rather than parkerize it as I did.

Flow Testing Manifold

This was all quite some time ago now (as evidenced by my old Superflow 110 in the picture - I upgraded to the SF600 about 7 years back) I have just now finally started on an aluminum version of this manifold set up for O-rings. Is the world ready for such a manifold, or am I the only sick one out here? I guess that remains to be seen!


vintage said...

i'm looking for a dual manifold for linkert 4 hole if you know of/have one, regards, Ian

St. Lee said...

The Plumber style manifold shown in this post is one I built. I would be willing to build more in the future (but not for the next month - too busy) I have also built the same style from aluminum for using o-rings, but it was much more work. If your next question is how much, I'd have to say I am still trying to work that out.

Unknown said...

I have two cv 40's on my buell. One out each side. Has anyone heard of this? I would love to see if anyone has done it. Never flow tested it but has a noticable power gain. Gets a lot of looks and doesnt interfere with the riding position too much. I just keep breaking carb supports for the left side from vibration.

St. Lee said...

Hi Ryan, CV's have the advantage of being short, and I suppose the footpeg location on the Buell helps with leg clearance also. How did you mount them? Modified manifold?

Greg said...

Hello Lee I really enjoy your website both as a motorcycle enthusiast as well as my being a Christian.I'm considering running two 3 bolt M51 linkerts on a 58" flathead.Its 51 WL cases,4 5/8" stroke,much overbore,ironhead rods,KHK cylinders,cams,intake,and a pair of -52 k-model heads I'm converting to KR combustion chambers per Jim Leineweber,Paul Osborn,etc. direction.currently has a M53 with bomb site venturi.This engine is going in a WL solo that I'm trying to keep around 400 lbs or less.
There's the back story,I'd like to know if the rectangular dual 4 bolt linkert manifold you built features a common plenum or is it divided?I'm near 50 and built several bikes including a 1960 pan that showed 72 hp on a dyno.I weighed it on Government regulated scales and it came in at exactly 480 lbs.I attribute a lot of that to the vintage moly single down tube frame that weighed 21 lbs bare,it being kick start only,and stripped as much as possible and still be operable.Anyway sorry for the long post,and again I love your blog.............Greg

St. Lee said...

Greg, thanks for the kind words and the question. The sheet metal manifold is an open plenum. I built it to run a pair of carbs with progressive linkage so that at low speed/cruising you would only be running on one carb, but get the benefit of both carbs at wide open throttle. When and if I build another I would probably weld in a partial center wall to separate the carbs to the place where the manifold narrows to clear the jugs, which would keep velocity up.

If you noticed the vintage cast aluminum dual 3 bolt manifold from my post, it has individual runners, but they are each so small that it could not help but LOSE performance on a Knuck (which that manifold is made for) compared to stock.

Keep in mind that both cylinders are NOT on the intake stroke at the same time, so if a carb is not too big for a stock manifold, then two of them are not too big for a individual runner manifold.

An I.R. manifold should work fine too, but is difficult to fit between the cylinders. Ideally the runner diameter (or cross sectional area)should remain constant from the back of the carb to the "bowl" at the valve.

Greg said...

Thank you for such a quick response.It is helping me make a decision.I have 2 M51 3 bolt linkerts that are stock other then Indian fuel nozzles with 2 large holes on the lower portion and 3 much smaller holes above well as the Indian idle plugs that has a "tit" on the end.That's the only description I've heard heard for the style of idle plug,I hope I didn't offend you.I work at a machine shop and though not a seasoned veteran I'm capable.I plan to make a manifold out of flat stock as you did.I really don't want to alter any fins,etc. on my KHK cylinders as I paid dearly for them after a long search.Would you humor me with a suggestion on the dimensions and/or volume needed of a single plenum intake.I'm a bit in the dark as the M51 is 1 1/4" throttle bore and the KHK intake ports on the cylinder are 1 9/16" diameter.How would i transition from carb to intake with the 1/4" difference in their diameters?I want to keep the manifold as short as possible for both leg room,etc.I it appears am copying your knuckle racer dual linkert some what as I'm shortening my M51's.I plan to remove just the 1/4" or so breather flange then cut the body back to the point I can just get the needles out after reattaching the flanges by turning down the carbs o.d and the breather mounts I.D. making them a light press fit.I'll then silver solder them on the inside to circumference of the resulting seam between the two pieces,then lastly dress it down so it's as smooth and polished as the rest of the throttle bore.It will result it the same length as a MR3,which is 4 1/4" long.I'm terribly sorry for the long message I just had a lot of information I wanted to share.I understand progressive linkage from 3 deuce setups on various cars in my past.I'm not certain how to approach the fabrication of the progressive linkage on a v twin with it's limited space.
Well sir I look at any information you feel moved to share as a gift,I thank you for your input thus far so I'm happy no matter what.By the way the VL powered by a panhead was a long time dream project of mine.Finances reared their ugly head and prevented me from fulfilling that dream.


St. Lee said...

Don't worry about comment length Greg, doesn't bother me in the least. Also, don't worry about "stealing" any of my ideas; its not like I am in the business of selling manifolds (tho the thought crossed my mind). Now on the other hand if you went into business selling 5/16 Knuckle guides I might be a little bummed ;-)

Besides, hardly anyone really comes up with brand new ideas, we mostly just take ideas we've seen and either refine (or sometimes butcher) them.

If you were to send me a private email (, so that I have your email address, I can send you a couple pictures of progressive linkage that I've used. I may be able to give you some ideas on manifold length and dimensions by doing a computer model with the Engine Analyzer program that I have.

When you are a hopeless gear head like me, a lot of the satisfaction comes just in seeing something really cool being created. If I get to play a small part by providing info, so much the better.

Greg said...

I will be sending you an email directly,I'm feeling quite upbeat I heard back from you so soon.I can weld in a partial wall inside the manifold with no problems.And I'll do as you mentioned and make it long enough to the place the intake narrows at the jugs.A goofy question but am I welding the wall from in between the carb mounting positions then back toward the cylinders or am I starting at the back with the wall and running it toward the carb mounting position?
If you do get the chance to make me a computer model so the intake will be withing the voloume range of perfect I will be very grateful.I guess I already mentioned I wanted to keep it short and tight to the bike as I'm building a little flat track style bike so trying to keep it's profile as narrow as It can work out to be.I'm nearly 50 and have admitted to myself I'll never get over my harley addition :>). As an aside be I want to let you in on my winter project real quick.I've already bought and paid for 2 7"tall X 10" diameter of ductile round bar.It was suggested to me by the owner of king of cubes in Iowa I talk to.He said he uses it as no liner needs added as most quality liners are ductile and that I could also cut my valve seats right into the cylinder deck wit no seats.Paul Osborn suggested 3 1/4" Indian stroker pistons for the bore size,my 1 13/16" KHK intake valves and my 1 9/16" KHK exhaust valves.I believe Paul said with 4 5/8" wheels I would net 77 cubic inches out of a basically stock looking 45 engine.Well sorry to rattle so long.I'll just email you now so as you pointed out will have my email address and quit rattling at you for the day,deal :>).I just feel birds of a feather flock together as the old saying goes so am drawn to the Lee I knw at the present time.

ColtCustoms said...

I've got forward facing DUAL throat DUAL weber carbs (dual throat on the each side of the engine) for my 74 FX Shovelhead. The problem is that the throats are setup for EVO intakes, the 2 bolt design and mine are the the rubber gasket clamps. Besides the manifold differences clearance is a major issue as well. I saw the picture of the cross style intake, something like this might work but the intakes on my heads are angled as well. Looking for a solution, pleeeease help i really wanna use these. thanks.

St. Lee said...

With the limited space between the intake spigots on the primary side(as opposed to the cam side) of the motor on a Shovel, it is going to be tough to get a manifold to work very well compared to the "cross" style that I fabbed for my Knuck. Take a look at the back side of a Shovel manifold, and envision another runner coming off that side. It can probably be done, but performance wise it may not be too impressive. However, I am guessing that what you are really looking for is an impressive look more so than ultimate performance.

Best bet would be to start with a stock style aluminum manifold (such as from S&S) and graft the runners from the dual Weber manifold on to it. You will need to find someone who is a good welder/fabricator for that. Keep in mind that the new opening on the primary side of the manifold does not need to be the exact shape as the runner, but should have the same cross sectional area, with a smooth transition. In other words, you may need to make the new opening in the manifold oval, or rectangular rather than round to keep the area the same.

The success in making the modification look good will depend mostly on the talent of the welder/fabricator. Good luck!

lil dave said...

Lee i have been reading your blog for the last month or two, as a knucklehead fanatic i find it most interesting. Just found your huides for 5/16 valves and even though i just puchsed some chrysler 440 intake for the present knuckle im building I will probably get some since I havent gouged out these heads to 1 1/16 " yet. in about 1990 i built a knucklehead with two carbs , i found some hand cobbeled heads set up for amals and cut all that out , welded , ground, welded , ground until they would accept some fresh delorto's, this motor had 4.5" stroke with .030 over stock pistons, the old Leineweber 2/3 cam and it dynoe'd at 71 HP at 4800 rpm , I told the guy running the dyno go to six grand cause thats when it really pulled. But this new project is what my original dream knuck was to be with two front heads and swept back carbs to facilitate leg clearance. And I wont really have to butcher a set of heads to acomplish this. I think ill use 42 mm mikunis on it , they are really good carbs as far as tunability goes. Now I am wanting to fabricate some roller rocker arms for it , do you know of anyone that sells just a set of rollers and bearings that would be suitable for using in this situation. Thank and Thanks for all the info I have already sponged from your blog

St. Lee said...

Hi David, thanks for stopping by, and for your comments. Sounds as though that 84 incher was quite a hot rod!

For the roller rockers, you might look at Jim's Machining. Their catalog shows replacement's for their roller rockers. Part #2178K is a set of 4 rollers/ #2179K is a set of axles/ #2180K is a set of retainers. Altogether comes to about $63 in their 2009 price list. (I have noticed a whole lot of manufacturers who haven't been printing price lists very often any more - I practically begged Axtell to send me a current price list with my last order, and despite promises, I didn't receive one)

S&S also sells a rebuild kit for their roller rockers, but it is about twice the money because it comes with bushings.

lil dave said...

Thanks Lee, I should have spell checked my last comment. I dont know why I didnt think of Jims, for the roller parts. And once again thanks for this blog. I have spent hours reading and enjoying all aspects of it.

Anonymous said...

Hey man, googling and trying to find a dual carb setup for a knuck, you still building these at

St. Lee said...

Someday I may get into production on them, but not today.

WireAndGum said...

I would like to put an HSR42 mikuni on my 47 knuckle. I am thinking about taking a new replacement plumber manifold, cutting off the flange and extending it to the required length with a new flange mount.

I am looking at the manifold and it has a lot of meat around the corners of the "T". My basic thought is that if I grind the straight corners with a radius it will flow a little better. What is your experience?


St. Lee said...

Your thoughts on modifying a plumber manifold to hold an HSR42 are all sound. There probably won't be enough material in the steel plumber manifold to get a really generous radius, but anything you can give it will be a help. Air doesn't like to go around corners, so the more gentle the turn, the better the chance that the air will follow the turn rather than going turbulent with a resulting loss of flow. Good luck with your project!