Strange problems with a K&b 7.5

[WhenU_C_meComin_RUN]

HI, I have a 20 cajun bullet rigger with a k&b 7.5 in it. This boat humps really fast, when it is running. Since I replaced the picco 20 with the 7.5 I have gone through 4 sets of bearings!

History on the motor..

I bought it new from a hobby store ,It was new never run and saved in oil for years.

I broke the engine in on 15% with 4 tanks and 2 tanks of 30%.

This motor had alot of compression and when it got "loose" I started running 60.

She blew the first set after 3/4 of a gallon of 60%.

Second and third set came 8-16 oz later, now I'm on the 4th, and I would like to find out why before I blow them. If anyone has any tips on why please help.

Thanks in advance, I'm OUT or parts!

 

[Bob Kensill]

What fuel are you running?

 

[WhenU_C_meComin_RUN]

A 60-18 mix blended by Earnie Leflor of the Texas impba, It is good fuel I use it on my sport 40 with 7.5, my 20 and 40 seaduders with cmbs, and my 60 rigger with a k&b 11cc. All run fine, except for this engine, bought new a long time ago, and self destructs every 30 mins.

 

[rhomodel]

There must be a cause for the bearings to go. Usually it's imbalance or running too little oil. Have you checked the crank for misallignment, prop weighted and balanced, motor securely fastened in hull?

 

[Preston_Hall]

When you say the bearings blew, do you mean that they literally came apart? Or are the just rough or bumpy feeling? Do the bearings have steel cages or phenolic? Steel is bad on the big bearing and OK on the small but phenolic is prefered. Are you taking the dust shield off the bearings? If not, you should.

 

[WhenU_C_meComin_RUN]

Motor mounts are tight, prop is balanced, I am off about an 1/8 of an inch on my shaft alingment, there was no way to correct this problem without drilling the tub and kicking the front mount up a little, this amount does alarm me but I have ran as much as a 1/4 off in the past with no problems , just not with this motor. I'm not sure what a dust shield is so I must not be pulling it off, the bearings are new k&b that I bought when I found out they were going to quit making these engines, I bought what I thought was a 5 year supply and now I'm down to my last set. I'm going to the pond friday after work and run her one more time, there are some other engines that I am looking at buying, if my calculations are correct she will blow after 12-14oz of nitro. I did an engine swap and put it in a aeromarine sport 40 , this boat has been nothing but good to me, not to fast but always finished the race, if it blows up in the sport then I know it was not the set up in the rigger.

 

[Eric Bourlet]

If your out 1/8 inch fix it. Back when I ran K&B inboards I found that the 3 piece case can be a problem make sure that the bearing housing is in good and strait. I know some have even glued them in to minimize the case flexing.

 

[not2bright]

someone did a test and they said if you epoxy the case together it will add a few hundred rpm. i cant remember where i heard it though