Need Mac ID Info

[Dennis Detweiler]

I purchased a rebuildable Mac. I've only done a partial tear down. Likely needs all bearings and gaskets. Not sure about the cylinder and piston yet. Right now I need to confirm identity:
Carb is a BDC16 single pumper, V-4 reeds.
Block numbers: RKS JS4
Exhaust: Dual port
Intake: Triple port
Stuffer: #58135A
Purchased it as a 91b, but I want to confirm via forum members. Any other info needed to ID?
Thanks

[mcbob]

How about a few pic's

Mc Bob.

[steveohara]

Dennis,

The BDC 16 is a double pumper model so if you have one that only has the single pump stack it is not as built. It will work for gas but not as well as a BDC 14. If it really is a 16 the high speed needle and inlet needle are both oversized to handle alky and if the carb is run with gas the tuning is very tricky as very small changes in the HS needle will change mixture quite a bit. If you are planning to run alky the BDC16 body is best but the single pump will not move enough fuel so you would want to get the double pump stack for it.
Does your block have a thin top fin? The 91B is the same as a B1 except the B1 top fin is thicker than the others. If you can post pictures it will help get you the information you need.
Steve O'Hara

[Dennis Detweiler]

Steve,
Top fin is thick.
The carb is stamped 6-72. Below those numbers BCD16. But, is only single stack. Also, butterfly is missing.
How do I tell if the engine (flywheel) is timed for alky?
I'll eventually pull the head and and mic the crank, cylinder, etc. Is there a book or spec sheet that I can purchase to find tolerances?
I'll take some snapshots of it tomorrow and get them to you.
Thanks for your help.
Dennis

[steveohara]

Dennis,

The stroke on your motor is 1.635", the original standard bore was 2.165", the piston should be flush with the top of the block at TDC or up to .015" below, the correct intake manifold should have an "H" on it near the exhaust, the flywheel should have a "5" stamped on one of the posts, the head should have threads for a half inch reach plug rather than the earlier .375" inch reach. To check the timing you need a simple continuity tester that you can buy at your local auto parts store for about $5.00 The method to check the timing is simple... you disconnect the wire coming from under the flywheel where it plugs into the top of the coil and you connect the clip on the continuity tester to the wire, then ground the other end of the continuity tester against the block or head and rotate the flywheel and you should see the light in the tester go off and on. There is a raised line on your flywheel moulded into the aluminum that sits between the magnets and the light should break when that line is lined up with the left side of the center post of the coil. The method I just described is the "quick check" method and if you decide to get serious you may want to buy yourself a degree wheel and take a more precise reading.
Before you take the motor apart you should pressure check it to see if it has bad seals. To pressure check the motor you should remove the carb and make a solid plate to cover the manifold and you will also need a solid cover plate for the exhaust. I use gaskets under those two plates made of inner tube to assure a perfect seal. One of the plates will have to have a hole drilled and tapped for a threaded fuel line fitting that will be connected to a pressure test/pop off guage.... make sure you poke a hole in the rubber gasket where the fitting is located. Once you make the tools you pressure the motor up to 5 or 6 psi and watch your guage to see if it loses the pressure. It should not leak down more than a pound every 10 seconds. If it leaks you need to find out where it leaks. Common leaks are worn out seals, bad gaskets and holes in the stuffer from age. If you can't find the leak spray water on it or dunk the motor in a bucket of water. Make note of the leaks if any and the timing before you tear it apart. Then tear into the motor and strip it all down.
When you have it all apart I'll give you dimensions for the bore of the rod at each end, crank journal and bearing surfaces, piston clearance, ring end gap, head gasket thickness etc.
Report back when done or if you have questions... good luck
Steve O'Hara

[Dennis Detweiler]

Thanks Steve! You help in making the forum a great place for gathering experienced information. I've rebuilt a couple of Briggs engines and shade tree my autos. I've restored a few cars, it's become too time consuming and expensive at my age. I've always liked karts. I haven't had one since late 60's. Now I have two that I've restored. And a 74 Margay Concept that I'll be restoring in the spring. This engine will be mounted on it.
I'll most likely put new seals all through. The pto bearing has visible play in it. Rod bearing has a couple of needle bearings missing. The whole engine might be junk, but I'm going to give it a good go. Once I get the head off and confirm the cylinder is ok, I'll proceed with it. Probably end up top to bottom rebuild. My worst fear is that this is a bag of parts that was thrown together and sold as an engine. If the block is ok, I can still salvage a running engine in the end. If it's timed for gas, I'll probably go with a gas carb. Are there offset keys as with a Briggs, or does the Mac require a different flywheel?
Thanks again for all the info.
Dennis

[steveohara]

Dennis,

The ignition timing on the Mac engines is easily adjusted but for vintage karting applications you will not need to get into anything tricky for gas or alky. Just the std timing will work fine for either fuel. When you get the motor disassembled you can check the sizes of the critical parts to determine their condition. When you remove the rod bolts and take out the needle bearings you should find 24 of them.... lets hope that you only find 22 or 23 otherwise you have big problems. If all the parts are good and all 24 needles are installed there should be very little clearance between the needles. If you place the tip of a very small screw driver between two needles and try to spread them apart so that there are no gaps other than where the screw driver is you should still only have something like .010" gap.
Take real good pictures so we can see the condition and I'll be able to help you get it bak together in good shape.
Steve O'Hara

[Dennis Detweiler]

The PTO seal is missing. The bearing on the flywheel side is not sloppy. The PTO bearing has some movement (wear).
There are 23 needle bearings. However, they don't set straight on the journal when installed. Looks like there's room for a few more? All bearing surfaces are free of scratches.
I can still see slight crosshatch on the cylinder.
I'll most likely use gas. I'll rebuild the BDC16, sell it and buy a 14. Or trade this rebuildable for another rebuildable.
I'm charging camera batteries and will take pics tonight.
Thanks
Dennis

[Ernie Shores]

Dennis:
LOOK real hard in the cylinder block and find that 24th needle bearing.
As far as carbs go I use a Tillotson HR181B on my macs and they run and idle fine. Look on this REAR site and find how to modify the carb manifold for this carb application.
Ernie

[steveohara]

Dennis,

I suspect that the person who put the motor together last just lost a needle and didn't know it was missing. You will want to get a new set in any event. In the 91 series motors the PTO bearing is a caged needle bearing that tends to feel like it has play but they work fine and I've never had one fail in all the years. Due to the age of these old motors I recommend changing all the bearings to assure they are new and in good condition. They are cheap insurance!
In the cleanup process you'll want to inspect the block and other parts very carefully to assure that missing needle is not stuck in some dark corner just waiting to kill your motor
Does your piston have any size markings on the top indicating it is an oversize version? Do you have a way to measure the piston diameter such as a micrometer or good quality caliper?
When you take/took the yellow side cover off the block that carries the crank bearing you want to pay attention to see of the bearing is still an interference fit in the bearing pocket. If the crank just falls out with the bearing still on the crack it means the side cover is bad.
Back for more later.
Steve O'Hara