Leak down %

[Motorhead]

Interested in a general concensus in what is the allowable percentage for a warm 06 Attak motor.

Changed my stock drivers and the also stock 1 1/4" x 136" (closed window every 3rd), for a 1.25" HD shaft with Wahl extroverts and a fully clipped same style track, At the same time I also needed to move my front suspension mounting location etc.(M20 Air ride) to accommodate the extroverts.

Prior to to the changes this thing rocked on stage 1 (5 PSI), no prob running the dream meter to 120 on very short runs and have had it to 130 on not so long runs prior to letting off. Stage 2 was simply crazy on trails.

After the changes I noticed loss of speed and RPMs. On stage 1 it will hang up at about 108 and sometimes if held open for longer it may start climbing or maybe not !! Stage 2 is also a diff, but with all 250 + - it will pull through this hang up spot and keep on pulling but there is still that noticeable difference even though the dream meter has seen 145ish on ice with more left.

I have checked and tested many things on the sled,both clutches have been worked on to gain some of the RPMs back, chain case and bearings, checked both drivers and track pitch (2.52), tight track, loose track. have made sure that the track and secondary turns free with belt off, etc etc. I did find a wye pipe problem during the track change out,one of them is partly crushed, will change that out this summer, but again it was like that since day 1.

So this brings me to the LEAK DOWN test.
Bought a kit and performed a test on the warm motor and found these from 1 thru 4. 5%, 5.5%, 9.5%, 10%.
At first I thought there it is, a bad motor, so I called 2 local Yamaha shops of which 1 is the regions premier boost shop, to ask what % is allowable. They both said 10% is average and they would not go after anything in the motor until after 20-25%. Also heard that 5% is above average!!

So here I am wondering what others are finding and what you think about a good % for a warm Apex motor,

 

[sledheadgeorge]

I've got 2 to 8 psi leak down and it runs fine. I think up to 10 is acceptable with boost. Have almost 17,000 miles on it. I know the extroverts might slow you down a bit as well as the m20 suspension change. Try pressurizing you charge tube and see if there is a leak some where.

 

[Motorhead]

Thanks for the reply! My boost gauge line comes from the intake manifold and whereas the boost gauge still indicates 5 and 10 on stage II, I feel it is making same boost! Does that sound right ?

I agree that the extroverts and the M20 may be adding to the problem. Not enough time left this season for testing or riding. Probably the last ride of the season this coming Sunday.

New Y pipes, and back to the original drivers and shaft this summer I guess, if the motor really is OK at 10% or 10 PSI lost @ 100 PSI. OH by the way around 6500 miles on sled and it does not use any oil, but I do get some oil vapor from the CC breather.

 

[Motorhead]

Hey Sledheadgeorge, you may have something there. I will look for the leak on my charge tube etc.

I first thought that my boost gauge line originated from the throttle body/intake area, but after thinking about it, the line that I was thinking about comes from my controller. Not sure where the boost gauge line originates from, but I shall find out later on this summer when I tear into it for exhaust replacement. I plan on another leak down test with intake and exhaust removed so I can listen for leaks from valves or base.

Is it possible for my boost gauge to show boost but not have any making it to the intake ?

 

[ghost rider]

I had 3 to 5 and was told 8-10 is acceptable.
If you are losing that much boost is the charge tube you would hear it

Barry

 

[Motorhead]

OK thanks ghost rider.