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Looking for 2 KA7A Carbs, do not need chokes or any of the connecting linkage. Do not need to be in operating condition. 314 741 0563 .
Art K
Art K
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Art, are these for an authentic restoration, or for current operating usage?
Reason I ask is because a pair of fixed-jet KA5a carbs from fishing engines such as the Mark 28 (and several others) might be easier to find and acquire for just operating an engine. The KA5s have the same throat and venturi size, and (I think) the same idling circuit. John Schubert might still have the deeper discharge nozzles that were available to update the KA7a when the original discharge nozzles needed improvement for the 20H conversions.
You can either remove the fixed jet and retrofit an adjustable high-speed needle valve and seat, or just enlarge the fixed jet to accommodate your operating requirement. The fixed jet in my Mark 28 was in the 0.061"-0.063" range, as I recall, and my conversations with some of the D-Mod guys indicate that some of them used fixed-jets as much as 0.010" bigger. These bigger jets might be for the larger KC series of carbs ... don't really know.Last edited by dwhitford; 10-14-2015, 01:22 PM.-
I re-visited the two KC-series carbs that I refurbished. The KC5A carb was on a 1966 4-cylinger Merc 650 (65hp) and had a factory 0.072 inch main jet. The KC7A carb was from a similar vintage Merc 1000 (100 hp) or Merc 1100 (110 hp) six-cyl engine.. The jet in the KC7A was 0.065 inch.
I did not get these carbs straight off the engines that I am citing, but am relying on published data about which Merc engines used which carbs.
The 6-cyl Merc 1000s & 1100s had a substantially smaller bore than the 1966 4-cyl Merc 650, which probably accounts for the smaller fixed jets in the 6-cyl engines. My Mark 28 ((22 cubic inches) from the 1960s had a fixed jet in the range of 0.061-0.063 inch; ... my recollection from 40 years plus fails me. I used that fishing engine on a 185-pound Aluminum DuraTech commuter boat. With just me in the boat, the engine always sounded fluttery, as though too rich. Old Mercs do that when they are over-revved, and I had only the stock factory propeller on that engine.
While pulling college-age water skiers with that rig in Florida, however, (a proper load), that flutter went away.
So my comments here should put any carb experimenters for any of the 2- or 4-cyl Mercs in the ballpark for Mod racing.Last edited by dwhitford; 10-15-2015, 07:53 AM.
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Would be interested in a set of KA5A carbs at a reasonable price. Collectors have driven the price of KA7A carbs to astronomical highs.
Art KComment
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Looking in the Mercury Master Service Manual the KA5A are pretty close but the KA2A is even closer. I have tried them all and I still think the KA7A are the best.Originally posted by Art K View Post
Larry MacComment
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Larry, I just ck. Ka7A & the KA2A specs. all matches up, jet and orfice size. I think the big difference is the way the ckoke is done. I do have some KA2A carbs. I think i will use them after a rebuild Thanks.
Art KComment
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Maybe the reason the KA7A is better is the replacement deeper discharge nozzle. I adapted a KA5A to my Mark 20-powered play-racer (a Foo-Ling) a coupla springs ago and got the replacement discharge nozzle from John Schubert. The nozzle from John definitely extends deeper into the carb body. I think the idea is to improve carburetion in the turns, where centrifugal force tends to fling the fuel to the outside, away from the bottom entry of the nozzle. The float bowl on all the Tillotsons is on the wrong side for left turns anyway.
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I think what I remember is the length of the venture is longer on the ka7a . I don't think it makes much difference .I know we tried both and didn't see anything. You need to put a long dump tube in so it doesn't burble in rough water.
FLYER
Leigh,
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Tillotson KA7A Carbs. Here we go again with the urban myths. Let's dispel some with facts...
These ‘racing only’ model Tillotson carbs, were the only production KA series carbs supplied with an adjustable high speed needle valve with removable seat, an additional internal fuel passage and the mounting flange holes slotted to fit the Carter stud spacing.
They were supplied on the Mercury Mark 20H Conversion and Mark 55H Conversion (converting from the original Carter carbs) and as OEM on Mark 55H-1 racing motors.
They were based on the early long intake casting, like a KA2A, or KA9A, shared some internal parts, but had a front mounted external choke. That long intake was the reason for that odd shaped choke spacer casting common to these carbs.
They were equipped with an additional fuel passage from the float bowl to the main nozzle well that bypasses the adjustable high speed jet. That adds a 'fixed jet' component to the fuel flow and, in effect, turns the high speed adjustable jet into a trimming screw, as fuel still flows with the adjustable jet closed completely. As a result, these do not covert easily to run with fixed jets.
Also the main fuel passage from the float bowl is larger than on other early KA service motor carbs, and has a threaded brass plug externally instead of a lead plug to close that passage.
An update Performance Kit #1399-1620? (which included a brass full throttle butterfly and a much longer base (deeper) main nozzle) was available later.
Another update was the ‘racing only’ solid brass inlet needle seat and steel needle assembly #1399-593? (all metal, no neoprene or whatever insert).
Hope this helps.
Jerry
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Note to Art K.
Yes, KA7A's are very high priced, which is exactly why I build some from time to time... Why work on something worthless?
Don't know what you're running these on but if its a 40 or 44, I'd suggest you try KA19A or KA21A carbs. Both run fine with fixed jets and are easy to convert to adjustable jets. I'd use the KA7A needle seats, with the long taper service motor Mk30 or Mk55 AJ series adjustable needles, etc., as these carbs do NOT have the jet bypass passage as on a KA7A.
I'd also convert to full butterflies, of course.
Jerry
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OK, lets address the flat spot issue Chuck raises... BTW, a KA2A and a KA9A are the same except for the idle tube. I prefer the KA9A tube.
KA Carb Idle Screw Settings...
Yes, the fishing motor service manual says to set them at 1 turn. That's fine for fishing motors and then you fine tune for best idle from there...
But we don't idle. These are race motors which accelerate really quickly with 2# flywheels and tiny props on really light boats.
When a motor is required to accelerate suddenly, it needs more fuel than for 'steady state' running at that same rpm. This is why many auto carbs have accelerator pumps.
Set the KA idle screws to 2 1/2 turns and Loctite them with a small drop of 290 if you are running KA's on a Quicksilver Unit on a race boat. This applies to KA7A and any other KA model you are using for this application.
Sounds counterintuitive but the idle needs to be way rich for racing with KA's. They'll four cycle at part throttle, but accelerate hard with no stumble and go hard when you punch it!
We need this because these carbs have no accelerator pumps, as they were designed for a fishing motor where they don't have to accelerate as hard as we ask of them... so we compensate by running richer than required for 'steady state' running to provide the needed extra fuel for hard acceleration.
BTW, rich starts easier, too.
Then adjust to optimum on the high speed jets... when they are correct, a cold motor will start and come up on plane with no hesitation or stumble. If you get a stumble coming on plane or accelerating hard off a turn, go slightly richer on both high speed needles or jets.
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Larry is correct when you get down to the nitty gritty of the idles holes and such... I probably should have said 'basically the same' as both were used on the same motors, 1960-61 Merc 400, and I don't see any significant difference in running. Both had the same type choke setup and long air intake, and interchanged with no changes to the motor required.
BTW, the 9 idle tube is an eensie bit bigger ID.
The 9's superseded the 2's, which carried over from Mk58A. A 400 was a 1960 model 58A equivalent.
Good motors back in the day. I ran one in very early OPC.
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On that KA21A vs. KA24A question, I just don't know... as I never built any KA24's for racing. I had used the KA21A's BITD, as way back, in the early FE rules, on a Merc 44, you were required to run OEM fixed jet carbs... why, I have no idea.
But I built a nice set of KA21A's with fixed jets for my offset plug block Merc 500 44 Mod. I won with those carbs in FEH once at Dayton. Then I built several sets for others for EMH and also for 'playmotor' early 44's in 55H cowls on D quickies. So I know those run well...
I ran KA19A's on my 40 Mods, which were Mk35A type blocks, and also on my 44's prior to switching to KC16A's. I had great success with the KA19's on those 40 c.i. D's.
Also note, there are KA21's up through D suffix, which are more like the KA24A venturi shape. I never had any of those either.
I know one of Lon Steven's motors for Lee Dingman had the late type venturi's and that motor ran well, but I never worked on it.
Why not build a set and see? I don't see any reason they shouldn't work.
Jerry
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Hey, Jim, JP183W, good to hear from you...
Much nicer, less crowded shop since we moved to Stockbridge on Lake Winnebago. I'll see your brother and Amy this week for dinner...
Drop in when you're back in the state. We'd love yo see you.
Jerry
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Pm SentComment
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"Keep Move'n" life is catching up!
No man's life, liberty or property are safe while the legislature is in session.Comment
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