View Full Version : Problem with motor rebuild-too much blowby
8th February 2005, 01:19
OK-I got everything together. Ported heads with heavy springs, 12 to 1 Wiseco pistons, combustion chambers cut to give 10.8 to 1 compression, Andrews N8 cams, carburator rejetted to 45 slow, 190 main. The motor starts easily and runs smoothly. The carb needs a little more fine tuning to eliminate some carb farts, but the bike idles well. Everything is normal as the motor warms up, but when it gets hot, blowby from both head breather holes is a steady stream. I currently have the holes open using the stock backing plate with breather bolts to hold the carb in place, awaiting a new carb bracket for my NHRS breather assembly. The motor sounds good and has no usual sounds. When I turn it off when hot, the breather hisses like it is relieving pressure. After it cools, the blowby is gone again until the motor heats up. Is this normal with new pistons and rings? Anyone have any advise.
8th February 2005, 01:34
The "steady steam" and hissing raises a flag with me. It sounds like your building up pressure in the heads somehow. Maybe the oil drain holes are plugged up or somethin', making the oil puddle up in the heads. After the bike is shut off for a while, it gradually drains down. (although, it doesn't seems likely that both sides would plug up) I'm sure you'll hear other ideas from the experts here. I'm just guessing.
8th February 2005, 01:43
Whatever the issue is, its only when the motor is hot and it is uniform on both front and back heads.
8th February 2005, 07:11
Blowby, in some degree, seems to be a problem for anyone running a hotter set-up. Don't know all of the specifics just yet but I found this site helpful. Sounds like this company has found a way to deal with it. I bought an extended idle mixture screw from them and they seem like good folks with solid products.
8th February 2005, 07:34
I will have to ASSUME a couple thing here. :wonderlan
1) You had the piston fitted, machined and honed to the cylinder. ;)
2) Your compression calculations are right on. :censor
3) The ring ends are not lined up. :yikes
The only place it going to get that amount of blow-by is from the rings not sealing.
Either you have too much end gap, too much pistion play or the rings were too tight, and when they got hot they swell up and cracked.
I am sorry to say that any way you look at it, your answer in going to require you to pull the cylinders to inspect. :cry1 :frownthre :cry1
Hope this help.
8th February 2005, 09:13
As the bike been ridden anywhere yet or is it still in the workshop???
With that sort of compression I would not be surprised at some blow by untill the rings are bedded in properly......
Another thing to add to what Jeffy just said (his points are valid ones) ....what year is it???? and did the umbrella valves get replaced???
8th February 2005, 13:05
The motor was brand new-ran less than a minute before I torn it down. Its an 05, so it has the new breather valves in it. My machinist checked the cylinder to piston fit with the jugs in torque plates. I did the rebuilt and checked each ring in its respective cylinder. They all fell into the middle of the reccomended specs in the service manual and also withing Wiseco's recomendation, and I installed them with the top ring at 10 o'clock, the second ring at 4 o'clock, the oil ring retainers opposite one another and lined up with the pin.
To be clear, I am not getting any oil in the blowby, just light smoke that looks almost like steam. That doesn't start until the motor gets warmed up good. Not a drop of oil has come out. The exhuast is burning very clean.
I have driven the bike about two miles up and down my driveway in first and second gear at speeds under 25 mph. I am thinking that maybe the rings just haven't had time to seal yet. I will have tags on it today or tomorrow and will take it out on the road. Any other thoughts would be appreciated.
8th February 2005, 13:19
As I said ... bed the rings in first before ya panic..
Warm it and ride a couple of miles
Take it out to about 50mph in top
Crack the throttle till it gets to about 70 then back off the throttle fuly closed..
Repeat this every couple of miles
Ya wanna do it at least half a dozen times
The bore only has the hills on it for a short while... it's imperative that the rings are pushed hard into the bore while the hills are still there.. this beds the rings in properly..
Any engine I've done this on has NOT had an oil or blowby problem
8th February 2005, 16:59
Stevo, Thanks for the advice. I will give it a try.
9th February 2005, 04:28
I followed Stevo's advice. I got the carb bracket and head breather setup installed with the HurricaneFlow air filter. Did about 6 or 7 50 to 79 mph roll ons. It was dark out when I pulled in, but in the light of my garage, I couldn't see any more of the vapor blowby coming out of the breather filter. The light of day may reveal more, but I think everything is OK. The hissing sound is also gone. Replacing the stock breather bolts with tiny little holes for the bolts in the Drag Specialties kits with big holes stopped the hiss. I have to get some break miles and then its off to the dyno to see what all these parts got me.
Thanks again for the advice.
9th February 2005, 04:36
Good ta hear.....
If ya took a cross-section of the bore it would look sorta like he cross-section of a file, just a lot finer...
The initial bedding in of the rings takes the tops off the hills and gives you plateaus.
In the first stages before these plateaus form it is easy for the gas to get past as it's hard to seal.....
I'm not going to go much further as there has been numerous engineering papers writen on the subject of correctly honing a bore and running it in.... and it gets a lot more complicated than I feel like getting at the moment..
10th February 2005, 20:36
I put about 10 miles on it last night without a sign of the blowby vapor. Now I have to get the carb and timing right. I have a 45 slow jet and 190 main with one loop clipped off the spring. I think its still a little lean on acceleration so I am going to try a 195 tonight. It came with a 180 stock, so I don't think that a 195 is too much for the cam, heads and compression that I am running. Do you?
10th February 2005, 21:31
195 main jet .... Jeeeze, that's pretty big.
The only way your going to be absolutly sure is to put the bike on the rollers. You need close to 13 to 1 fuel/air ratio at wide open throttle (WOT) right on up to the rev limiter.
11th February 2005, 01:32
I still have the stock needle though and its pretty thick. The plugs are still white and it just doesn't feel like its getting enough gas. On the other hand, the 190 I am running could be too big and its actually sluggish from too much fuel. That just doesn't seem likely to me with the plug color and the exhuast being very clean. I will be on the dyno in another week. I just like my dyno guy to think I know a little about these things before I show up.
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