Engine knock. Not normal blaster chatter...

[wheresthericego]

I just got done doing a top end rebuild on a stock blaster. The cylinder is .20 over and I used a wiseco rebuild kit. The cylinder was bored and honed to spec at summit offroad.
There is no air leak, I already tested. The pilot air screw is set at 1 1/2 turns out. The idle seems to be normal. I am running 32:1 93 octane fuel.

You can hear the normal blaster engine noise and then there is a knock which is not a consistent sound like the engine noise. It knocks every few seconds..

I took the blaster out to get some break in hours on it today. I did not ride hard never going above 50% throttle and not putting the engine under a lot of load.
After the engine got warmed up and I took off I noticed that it seemed like the engine was having a knock which was audible even over tire/engine noise.
When I came to a stop the engine seemed like it was wanting to stall and the throttle felt dead almost.

When the top end was out, the bottom end did not have any resistance and rotated very easily. I am thinking that there may be a problem with a bearing or something since the engine was starved for oil (most likely the previous owner did not use injection oil)

When I put everything back together I lubed everything generously with 2 cycle oil and the engine rotated over with no problem.

I'm going to pull the head tomorrow to see if the piston has any slap and has started to score the side.
What is the maximum allowable distance between piston and cylinder wall?

Do any of you guys know of anything I should check?





I do not have a video of the knock I might be able to get one tomorrow.

 

[scotj77]

i have had 2 cranks that moved freely but were shot! you need to check the rod for play up and down. even if there is no up and down there might be too much side to side. there is a manual on here that is dowloadable and printable so you can find all the tollerances there. buy a clymers manual if you don't have one. great book to have

 

[Awk08]

the downloadable manual is in here............
http://www.blasterforum.com/engine-13/my-general-troubleshooting-maintenance-30347/#post358200

my preference for piston to cylinder is .004

but more than likely it's the crank or bearings which possibly didn't show it's ass with the worn out, low compression, previous top end

did you so the proper heat cycles before riding it ?????

 

[wheresthericego]

After installing the top end I started it and let it warm up to normal temperatures and gave it a couple slow small revs then shut it off and let it cool. That was tuesday. Today (thursday for future reference) I started it, let it warm up and took it out for its first ride.
I never went over 50% throttle and never put the engine under a lot of load..

 

[Awk08]

at least 3 hot to cold heat cycles are the norm around here

 

[Zwiser]

? how do you check piston to wall clerance

 

[sicivicdude]

You measure the diameter of the bore using a bore gauge at about 9 points around the bore (top middle and bottom around the diameter) then measure the piston diameter about 10mm above the bottom of the skirt at its widest point. Subtract the diameter of the piston from the diameter of the cylinder and you have the piston to cylinder clearance.

 

[liljatownboy]

the downloadable manual is in here............
http://www.blasterforum.com/engine-13/my-general-troubleshooting-maintenance-30347/#post358200

my preference for piston to cylinder is .004

but more than likely it's the crank or bearings which possibly didn't show it's ass with the worn out, low compression, previous top end

did you so the proper heat cycles before riding it ?????

any reason to run .004? i run .002-.0025 and it runs amazing

 

[Zwiser]

ok so the cylinder measures around 72.4 around all the points and the piston measures 72.4 also

 

[liljatownboy]

? how do you check piston to wall clerance

easy way so you dont have to get all the special tools is to put a strip of feeler gauge on the cylinder wall intake side then exhaust side and put the piston in with no rings if the piston drop right through go up on the feeler gauge till its slow on sliding through the bore or stop but not to were you cant move it

 

[liljatownboy]

ok so the cylinder measures around 72.4 around all the points and the piston measures 72.4 also

you want to measure 15mm above the bottom of the skirt and measure there

 

[sicivicdude]

wiseco specifies .0022" so you're running about right. .004" allows less warmup time if you're running a forged piston but it means more piston slap and generally less time between rebuilds meaning your top end will be worn out of bore sizes quicker. It's a trade-off

 

[Zwiser]

what special tools would i need

 

[sicivicdude]

A 72.5mm piston is ACTUALLY 72.45mm .05mm is the .0022" clearance so the cylinder is 72.5mm and the piston is a fraction of an inch smaller.

Remeasure your top end and make sure you're actually at 72.4mm.

 

[sicivicdude]

6 Piece Technical Measuring Set - Specialty Measuring - Hand Tools

6" Digital Caliper - Calipers & Dial Indicators & Micrometers - Hand Tools

Those are a cheap way to measure a lot more than cylinder to piston clearance. Obviously, you don't have to get them from harbor freight but about any source for cheap tools will work.

 

[Zwiser]

ok its like between 72.4 and 72.5

 

[liljatownboy]

the tools i use are a micrometer and t-gauges

 

[Zwiser]

lol this is the exact one im using 6" Composite Digital Caliper - Calipers & Dial Indicators & Micrometers - Hand Tools

 

[Zwiser]

and la sleeve did the bore job

 

[brandoz28]

that's not good enough in any stretch to do accurate piston/wall clearance measurements