Engine Knocking Sound (writeup)

I'm on my last string before i (i dont know) but i'm very pissed off at my TB 88 kit because of the knocking sound it makes under stress. I've re adjusted the valves time and time again, and the knock still comes back. So far i've heard my engine make 3 different clicks - 1: the TB click which is a given, 2: The valves poorly adjusted (took care of that) and 3: The more of a knocking sound that i can't get rid of. I've heard that the TB cam might have too high of a lift.. any comments on that? Some people are also saying that the valves might be hitting the piston... hopefully that's not the case. I'll attempt to explain the tick the best that i can...:

1. It only happens when the engine is nice and toasty

2. It gets worse as the engine gets hotter

3. It only happnes under a load (bogging it). When it's really hot (peak hottness) it will do it while reving it up while stopped.

4. It wont happen if i roll on the throttle and rev it out.

5. The ticking sounds like if you were to take two marbles and smack them together (wierd, but that's what it sounds like)

That's the best i can explain it in words... i'll try to get a sound clip up if anyone's interested.

Here's what I"M thinking of doing/replacing:

1. I'll try putting my stock cam in and seing if that takes care of the click - That will varify that the lift on the TB cam is too high (right?)

2. I could order HD valve springs if i'm suggested to.

3. If i really need to, i could get the race head (last option)

PLEASE think about what this ticking could be! Post everything you think it might be (please serious answers only) Like i said i'm on my last straw and i've passed the torch on to you guys! :bowdown: :drinkup:

That will varify that the lift on the TB cam is too high (right?)

The way to do t his is called "claying the piston". take the head off and use childrens modeling clay to fill in the dished out valve reliefs in the piston dome. Time it, Button it back up and torque to spec. The turn the engine over by hand a few times. Take it back apart and check the clay. If there are marks in the clay from the valve hitting, how deep are they? Do they go all the way throught the clay and actually touch the piston? If not what is the clearance? (I am not sure what the clearance should be so check with a reputable engine builder) Now, if you are usnig an agressive cam and stock springs, the springs could "Coil bind" and or float the valves allowing them to hit the piston even though the clay shows it is not. Contact the Cam manufacturer to get their opinion. Dratv for example sells special springs to go with their cam to prevent it............. good luck!

What octane gas you running, sounds like detonation.

Double Check You Chain Timing To Make Sure Your Tdc And The Cam Gear "o" Is Located At 9:00...

I Tried The Heavy Duty Springs, I Used Xr 70 Springs... Mistake... A Honda Dealer Installed Them And They Ended Up Binding And Ruining A Cam...

I Spoke With Tb Regarding The Springs And They Recommended I Remain Stock. Even With A Rev Box

Just A Quick Question... On Assembly, Id You Put On Assembly Lube On The Cam Lobes?? If You Didn't This Can Also Cause Premature Wear Which Will Knock/click...

Did You Install A Hd Oil Pump??? If Yes,did You Prime It???

Did You Adjust The Valve Gapping While The Engine Was Hot??? If Yes This Will Also Cause A Ton Of Problems.. I Would Recommend Checking Your Cam For Scoring!!

Hope This Helps Somewhat...

What octane gas you running, sounds like detonation.

That was my first thought too.

Thanks for the replies! Every one is saying to use higher octane fuel (i have been giving the thing 87oct for a while!) I will try the 100oct gas from 76 and see if it likes that. Also, i will contact TB (trailbikes) which is the maker of my cam and CDI and i will ask them if my setup is ok for the cam. I know the timing is right on because it runs GREAT when it's not under a heavy load. I'll re post after i try the better gas.

THAT IS YOUR PROBLEM, you cannont run 87 in the any 88 bigbore, especialy the TB stock head 88. The piston has a popup on it which raises the compression to 13:1 and must have at least 91octane. If you switch to 91 or 93 pump you will be just fine as that is for certainly your problem.

Ok i'll try that asap! Thanks

Well i did what everyone told me to do and got some 100oct. fuel and now it runs GREAT! No knocking and it's faster too! Thanks everyone for your help!

Funny thing is I have some of those same problems while running 93oct gas. My valves are in spec. cam chain is tight. Everything was installed to what the directions said (twobros.com) all the bolts are to tech spec. Yes im running the hd oil pump. And another thing which makes me ponder. I can kill stock 50s in low end but on a high speed streight they can keep up. Im running 15/35 gearing too. Maybe its a carb problem and its cutting out too early?

With 15/35 gearing you should be able to run about 45mph top speed. 88 kits are not for top speed, they are extra power to just get you to top speed much much quicker. On a tight mini track top end isnt really a big deal. Are you running the stock rev box, maybe hitting the rev limiter, or it could be your carb isnt adjusted right, is it fluttering on the topend or what. Give some details so I can troubleshoot.

I have the tb rev box and the bike does not sputter when in the high rpms. There arent many mini tracks around here so I have to race against the xr70s-ttr125s on big bike tracks. I can pull most of them out of the corners but I have no chance in catching them in the long streights.

Hmmm, that is odd. Does it seem like the bike is revving all the way out or like its gettin to a rpm range and signing off, the thing should rev until you think it sounds like its going to blow up. The only thing I can really think of is that maybe you got your cam timing off by a tooth, which would make it run great on bottom but then sign off up top on the rpms.

Nope its running great all the way through the rpm range. I dont really like running it in such high rpms that the valves would float, but high enough it sounds like it will go kaboom. My dad and I checked and rechecked the cam after installing it and its right where its supposed to be. Maybe im just expecting more out of this thing that what it actually can throw out.

what size carb are you running on there?

Lol, I think you may just be expecting to much out of it. You are just running a stock head 88 kit, with the small stock head valves they will sign off on power in the upper rpms. You could alway upgrade to the Race head and give em some trouble then hehe.

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