Tillotson KA7A Carbs, Mercury Mark 20H and 55H

[trident]

Just finishing up a KA7A carb building frenzy, here... this should be my last batch as I'm running out of OEM parts.

I'll be posting a batch of genuine KA7A's, fully refurbished, ready to run, on e-Bay after the Holiday weekend.

Also will post some Alky converted Tillotsons, 1 1/16" KA10A's and a KA5A...

Watch for them if you have a need for these.

[ZUL8TR]

Just finishing up a KA7A carb building frenzy, here... this should be my last batch as I'm running out of OEM parts.

I'll be posting a batch of genuine KA7A's, fully refurbished, ready to run, on e-Bay after the Holiday weekend.

Also will post some Alky converted Tillotsons, 1 1/16" KA10A's and a KA5A...

Watch for them if you have a need for these.

When you list them post the link here.

[trident]

Here is some additional info on KA7A carbs, for anyone who is interested:

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 convert 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).


Note to Lake Racers with 'play' 44's:

Yes, KA7A's are very high priced, which is exactly why I build some from time to time... Why work on something worthless? (Willie Sutton principle)

Don't know what you're running these on but if its a 44, I'd suggest you try KA19A or KA21A carbs. Both run fine with fixed jets, which I'd recommend or 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. The longer taper to the needle gives finer adjustment to the flow, for a given amount of turn, compared to the blunter 20H needle.

[Matt Dagostino]

Jerry...........not good at these updated pm's here on Hydroracer. I tried your e-mail and it didn't work. I am at mdagger57@yahoo.com. Can you e-mail me to start a trail? Thanks Matt

[trident]

And here is a lesson I learned the hard way, when I converted my first 20H, spring of 1960... New KA7A ran like Jack the Bear, but stumbled badly off the corners. We, my dealer and I, figured it out. Later, I found a Factory bulletin recommending this.

Now I set (and lightly LocTite) the idle needle on every KA I build for race motors per below:



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 high speed needle(s) or jet(s).



OK, as promised, the first of my Carbs For Sale listings went out on e-Bay... more to come. You can find them all by searching under 'outboard racing'

[TNT Teen]

Cant find the ads, they sell already? Searched outboard racing, looked at everything posted in the past couple days

[ZUL8TR]

Here are the adds:

Without choke:
http://www.ebay.com/itm/Mercury-Outb...dXR4L9&vxp=mtr

With choke
http://www.ebay.com/itm/Mercury-Outb...1XR4~X&vxp=mtr

[trident]

I looked at it when I posted, I just checked now, and they are all there... A pair of Alky KA10's, an Alky KA5, a KA7A without choke, a KA7A with choke and two different sizes of Grose Jets. More to come later...

When the search box appears, just type ' outboard racing ' and they all appear on the first page of listings.

Sorry you are having trouble finding them...