K&B 7.5 Inboard

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Kez

Well-Known Member
Joined
Feb 27, 2003
Messages
1,230
I have a K&B 7.5 drum intake engine that I want to install in a sport 40. But the rear housing won't clear the mount. I can still use the engine by swapping the drum housing with a disk back plate. Just wondering how did the K&B 7.5 disk engine run compared to the drum version? What is the recommended disk clearance? Is 0.0015" OK?

Thanks,

kez
 
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K&B 7.5 disk rotor motors ran like a bat outta hell, but they had some weak points...the disk plate front housing would crack where the set screw threaded to hold the carb sometimes, and i think there were problems with the case cracking from time to time....i never experienced the second problem, just heard that it happened...i ran the red carb on mine and it ran just fine back in the day....i dont remember the clearence for the disk rotor, but if Rod Geraghty reads this, he will tell you.....Rod KNOWS K&B Motors!
 
Bill's right on this one. I've got a disc rotor version and it ran great, IF you don't run it lean. I ran mine too lean and it blew the conrod through the bottom of the case. If you don't have a disc backplate good luck as, according to the website, MECOA NO LONGER MAKES THEM!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

You can only get the drum backplates and the matching case from MECOA and that's ONLY IF they have them in stock. Rod G may have some parts stashed so if you need anything K&B, contact him first :)
 
Kez said:
I have a K&B 7.5 drum intake engine that I want to install in a sport 40. But the rear housing won't clear the mount. I can still use the engine by swapping the drum housing with a disk back plate. Just wondering how did the K&B 7.5 disk engine run compared to the drum version? What is the recommended disk clearance? Is 0.0015" OK?

Thanks,

kez
Click to expand...
The disc engine is the one to have. To keep the case from cracking, use a one piece motor mount. Then use rubber isolators to attach that mount to the boat. If you really want to have some fun, get Rod Geraghty to massage the motor for you. The price is reasonable and I guarantee that boat will fly.
 
Thanks all for your replies. I have two drum 7.5 engines but I have never had an early 7.5 disk version so I can not compare the two. From the photos, it appears that K&B only changed the intake. Everything else seems to be the same.

Bill, would epoxying the carb in help reduce the cracking tendency? If I use the red carb, I will still be able to undo the two screws that hold the back housing.

HJ, I have a back housing with disk. The disk looks very beefy compared to some and appeared to be balanced quite well. Unlike some engines in that vintage, the drive pin is also beefy. That thing should resist wear and last.

Dr. Hydro, I always use the Octura Humdinger mount or solid mount for the crankcase cracking reason. Almost all mounts available today are two-piece. The engine lugs/case take all the stress. The Octura mount cradle the engine and that is why I use them.

Does anyone know why K&B went from disk to drum intake? Was it because of power? or reliability (cracking?)

Thanks,

kez
 
I don't have an answer on the change over but one thing you need to be cautious of is disc orientation. There are two crank pin slots on my engine's disc so that the engine could be set up to run backwards for use in twins. You need to make sure that the disc is set up in the correct slot or your engine will not run properly
 
Kez, just use a little blue loctite on the setscrew that locks the carb to the intake and DONT overtighten the setscrew...also, if you look at the backplate on the opposite side, you will see that it is cast molded with the same set up to drill and tap for a screw if the other side cracks.....im NO engine expert to say the least, but ive talked to engine guys about the disk vs drum rotor thing and what they say is it takes less power to turn a light weight low mass disk, than a drum rotor, BUT that disk rotors require more maintenance than drum rotors..which are usually pretty bullet proof....maybe someone else with ALOT more knowledge than myself will chime in on this subject! I will say that with proper maintenance, disk rotor engines will last a long time if your willing to take care of em.....
 
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Granted its not a KaBoom motor but I do have a Picco 67 disc rotor from the early 90's still in running condition. Its had many many gallons of 60 percent ran thru it and it is asking for more.

To check the disk gap use a crisp dollar bill between the housing and the disk the dollar bill should pull out with slight drag on it.
 
Bill Gibson said:
K&B 7.5 disk rotor motors ran like a bat outta hell, but they had some weak points...the disk plate front housing would crack where the set screw threaded to hold the carb sometimes, and i think there were problems with the case cracking from time to time....i never experienced the second problem, just heard that it happened...i ran the red carb on mine and it ran just fine back in the day....i dont remember the clearence for the disk rotor, but if Rod Geraghty reads this, he will tell you.....Rod KNOWS K&B Motors!
Click to expand...
.002"
 
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