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RB25 Compression Test

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Anyone have compression numbers for an RB25 kit?

Bike died during break in (near 100 miles on fresh build) and wouldn't start. After checking ignition and fuel and such and trying kicking and bump starting many times, checked for poor compression (it still felt ok). Checked it with Harbor Freight special and only got 120psi. I thought I had remembered when freshly built it gave 160psi, so I tore it all apart suspecting a stuck ring or something. I didn't find anything wrong with the top end...And then went back to build notes and I either didn't test or didn't write it down, so unsure of memory of 160psi...

So, hoping to compare to someone elses numbers...I was using the long hose on the compression tester which does lower the values a bit.

 

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Crazy would be thinking you should get one value vs another and tearing down a motor for no reason...Ugh.

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i don't think you should compare a run motor to an unrun motor. 120 psi should be fine for compression and would look else where for an issue. cdi or electrical would be my first suspect. 

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Ya, I did swap the coil and pick up thinking it was ignition prior to tearing it apart. Spark was good and unchanged. The way it died felt like the ignition crapped out or the timing slipped so that was the first thing I checked. Then went through the carb, checked fuel flow...Tried a little carb cleaner in carb mouth, kick starting, bump starting...It would fire once and then nothing. Makes me suspect the ignition but I tried two setups and disconnected the kill wire and little spark indicator was showing spark...

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What stator plate and flywheel combination are you using? I've had problems with bgm in the past. The wiring on the stator could be worn/ broken .  Pick up height can be sensative depending on the flywheel used. 

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This is the same bike that I posted about that had that issue, but I thought I had it fixed...It is BGM with AFRayspeed. When first built it would rev cleanly to about 6K. I fiddled with it and added a Kytronic gizmo and it then rev'd cleanly all the way through. As part of troubleshooting, I replaced the pick up and swapped to a second Kytronic gizmo and coil. Same behavior...

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I would right away get rid of the kytronic.  To much of a variable in the equation. I also had an issue of a loose, not completely broken wire on a new bgm stator. If that's the case send it back to the supplier. To much $ for it to be your problem. I would scrap the bgm for an Indian or rayspeed stator or change your flywheel for a bgm. Seems bgm is like apple. Bgm with bgm parts. Not into proprietary nonsense. 

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19 hours ago, TRs said:

I was using the long hose on the compression tester which does lower the values a bit.

 

Technically you might get a different reading by using a longer hose but it would not be measurable with your gauge.

 

For the running issue you just have to methodical about it. Check every connection, jet, flow rate, adjuster there is. Something moved or broke loose. 

 

 

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I hear you Mike, pain is, I can't get it running to diagnose. it just fires once and then quits. I went through everything, going to go through again when I get the chance.

I swapped all the ignition and tried it both with and without the Kytronic. I am getting spark in all situations and the behavior is the same, yet I still think it is something with the ignition as the carb is all clean...I will get more time with it after the holidays.

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This kind of stuff opens the flood gates of suggestions, so I hesitate...  But simple things first.

- Change your spark plug
- Change your cap
- Change your spark plug wire

One at a time.

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That is one I didn't check, the spark plug cap. Thanks!

I was troubleshooting with one of those inline lights that indicates spark and it appears to always be sparking but I don't trust it is sparking strongly enough...

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So, a buddy gave me a BGM flywheel...If using the BGM stator with BGM flywheel, does the pick up height still have to be adjusted?

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This is the first time I have had to adjust a pick up height, I want to say that better controls/standards by the manufacturers could eliminate this headache, but I am sure there is some reason...

And Mike gets the prize..."Something broke loose" is exactly right and the signal was the other thread I started about spitback...One of the reeds failed. Luckily it didn't make it out of the intake, so the top end is good. I have only put about 100 miles on this build, would think they would last more than that. The first 60 or so were without panels and I didn't notice much but then with the panels the reed was probably getting worse and the spray was more noticeable. I thought it was just a big carb thing, but it appears it was the reed starting to fail.

Hope to have it on the road soon so I can do more jetting and more break in, really want to see what this little motor can do!

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Did you experience any backfires through the carburetor mouth? If lost reeds because of that.

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No backfires...

It appears to be an issue with the intake manifold that came with the bike. For those that don't know, I got this bike as someone else's dream build that they never completed except for gathering all the parts. One of the items was a nice CNC intake manifold, I believe from MMW. There is a recess in the manifold flange that fits the reed block. The RB kit has the same recess. When I first put it together, it appeared that the intake flange had enough material to bear down not only on the gasket but also the reed block. But the recess actually fits the reed block, so once the gasket had enough punishment, both reed blocks were bouncing around in the recess. I don't know why both parts have the same recess... I spent this afternoon fiddling with JB weld and filling the recess in the manifold. Going to leave it to dry overnight and bolt it all up tomorrow...

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the reed issue is a problem with the RB kit, I've had this happen to me twice. One of the reasons I'm detuning mt TV3 this winter,

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Here is the bodge to a very nicely made, if not poorly designed part...

Why does the MMW RB intake have this relief? It allows the reeds to bounce around which caused mine to fail after 100 miles, trying it filled with JB weld...

 

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The picture is of a MMW intake manifold. For some reason, there is an inner recess that the reed blocks fit into which matches the recess in the RB cylinder where the reed blocks are supposed to fit into. The RB manifold doesn't have the recess. When assembled, the MMW essentially doubles the thickness needed in the flange region of the reed with only the gasket locating the blocks. Add a little over 100 miles of use and the gasket no longer holds the reed blocks down and they start bouncing around in the extra clearance and all hell breaks loose as the backside of the screws which hold the reeds into the reed blocks hit the manifold and they work loose. I filled in the recess with JB weld to make the manifold support the gasket... 

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Bridge cut out and stupid recess...I think it also moves the carb about half an inch inboard making it clear the panel better...

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I'm late to this party as well, but I can tell you I had the almost exact same issue. Narrowed it down to the stator and/or found out I had eaten a reed. I've got the AF reeds and a spare carbon fiber they had available. I'll try that first (assembling this week in the evenings) and let you know the results.

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