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Discussion Starter · #1 ·
My CB350 has an engine that was rebuilt by PO ~1000 miles ago and I'm starting to notice a lot of blowby. Compression was 175/185 the last time it was checked. It was running rich before I made exhaust/filter changes so there is a bit of carbon buildup. Petcock is shut off everytime, and most of the time a block or 2 from my house. I been smelling/checking the oil every week and I don't think I can smell the difference anymore. Oil has been changed 3x within the last 300 miles. Last weekend was the first time taking the bike onto the highway (finally got the rims trued) and after 20 miles or so I noticed a good bit of oil splatter over the tires and everything it can fling onto. Oil level might have dropped 10% from the top line on the dipstick. There is a good stream of whiteish smoke coming from the breather tube at idle after it is warmed up. I've also checked the oil check bolt and a good little stream comes out at idle so it should be getting adequate lubrication.

I just installed a cheap catch can with stainless scrubbers and after about 30 minutes of riding (~10 on highway) there was about a teaspoon of water or maybe gas in the catch can but not much oil yet. There might be oil after a longer ride. I have it venting through another tube running down the frame and there's still white smoke coming out of there. Any ideas as to where to start to diagnose this? (Side thought, does winter blend gas produce more water?)
 

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The PO didn't leave the baffle plate out, under the cylinder head cover, did he? White smoke is caused by water in the oil. Water is a by-product of gasolene combustion, there is always a small amount of blowby even in engines in perfect condition. Water condenses in the oil during cold start-ups and normally is boiled out of the oil when the engine is fully warmed up. Are you making long enough trips to warm it up?
 

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Discussion Starter · #3 ·
I assume I wouldn't be able to tell if the baffle plate is in there or not without opening it up. I typically ride it for 2 hours or so on the weekends so it there shouldn't be any water in the oil by the end of that.
 

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There is a gasket on either side of that plate, you should be able to spot it there. I don't remember if the plate can be installed backwards or not on the 350, the drain holes in the plate have to face the right direction.
 

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Discussion Starter · #5 ·
Mystery solved... compression a little over a month ago was 175/185. Compression is now 80/115. No change with oil. I put less than 300 miles between the two tests. This was suppose to be a relatively newly rebuilt motor documented by PO on this forum.
 

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Those numbers with a warm engine and throttle wide open?

How do your plugs look? Check your valve clearance and are you sure on the mileage and where did the first set of compression numbers come from?
 

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Also, you mention highway and oil on tires. Remove the cover to the countershaft sprocket(front drive chain sprocket) and check to make sure the clutch rod seal is in place. That could be the source of oil. Are there any oil drips where it's parked?
 

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Discussion Starter · #8 ·
Didn’t go WOT but cranked it until the needle won’t move. Even if its reading lower, the difference between both cylinders is pretty big. Engine was not hot but it wasn’t cold either. Might been 1 hour after I rode it plus a few idling starts. First set of compression numbers were done by myself. We started looking at compression this time because it would occasionally misfire at idle and die. It would run if I have the throttle cracked open to about 2k rpm.
Checked fuel, checked spark, removed filters, checked timing, and finally did compression at the end and here we are.

I replaced the clutch rod seal a week ago. I thought it might been that originally but it is from the breather. I relocated the breather and tied it to the outside of the frame to test and it dropped oil onto my exhaust instead of the tire.
 

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Check valve clearance and recheck compression with throttle wife open, or carbs off
Also comment on the plugs
As for the running issues, might be good to perform the cam chain tension service, then go through timing and carb sync and float check(holes and height), and then spark advance, points wire position and overall condition and plugs/coils in good shape and condenser is grounded.

Edit: is it smoking on startup or driving?
 

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Discussion Starter · #10 · (Edited)
Did valves and cam chain tensioner when timing was adjusted few weeks ago. Plugs look fine. The original set that came with the bike were black but after the tuneup and full length exhaust/uni filters, the new plugs look pretty decent.

Few weeks ago after the tuneup the bike seemed to have ran really strong, much better than when I first got the bike. Cold idle no problem, revved up past 8k easily, hit 80mph. I get a little bit of wobble after 80 so I haven’t tried going any faster but it felt like it had more to give.

There’s no smoke on startup that I can recall. Once warmed up, it will smoke from the breather. I think I was told there’s smoke from exhaust when I downshift too.

Going to do a leakdown if there’s a gauge to rent at autozone
 
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