Author Topic: serious white smoke after big bore (Read 14153 times)

bluefields · « **on:** June 05, 2015, 09:30:52 AM »

Title pretty much says it all.
I've got a 72 cb750 with a 74 head that I just recently had bored over to an 836. I used the ebay big bore kit, ape valve springs with new seals, heavy duty cylinder studs and cycle x street cam (web 41). It runs good, fires right up and pulls strong but it blows a smoke screen behind me. Serious smoke screen. I've got about 40 miles on it and used about a quart of oil. Going to change the oil tonight out tomorrowStill need to fine tune the carbs and double check the timing. This makes me nervous maybe I need to be patient?. Any advice would be great.

hotdog · « **Reply #1 on:** June 05, 2015, 09:43:07 AM »

Something's wrong, that's excessive oil consumption with a fresh top end. There is oil getting into your combustion chamber from somewhere....
Did you install new cylinder head o-rings with the head gasket.?
What head gasket have you used.?
Did you check ring clearances.?
Also the cruzinimage eBay 836 kit needs to have .001" clearance upon installation or your motor will burn oil.

Search the forum for "cheap eBay pistons" also, plenty of info in that thread.

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madmtnmotors · « **Reply #2 on:** June 05, 2015, 10:03:17 AM »

Quote from: hotdog on June 05, 2015, 09:43:07 AM

Did you check ring clearances?

If so, what were the piston to cylinder clearances?

Who did the bore/hone work?

bytio · « **Reply #3 on:** June 05, 2015, 10:05:19 AM »

Have seen this a number of times and almost always piston to bore clearance. Too much

bluefields · « **Reply #4 on:** June 05, 2015, 10:08:29 AM »

Perhaps I exaggerated a bit on the whole quart, really it's more like a half but still... New o rings, head gasket came with the big bore kit. I put a couple coats of the spray copper gasket stuffs on my new base and head gaskets. I've checked and checked and there's no visible oil leaks that I can see anywhere. As far as the cylinder clearance goes, I'm not sure. It was very tight and very difficult to get the new pistons and rings in the new cylinders. I'm sure it's supposed to be so that isn't ruled out. I made sure the gaps were staggered about 120° from each other and all that jazz. The rings in this kit did not have any markings to identify up or down so that concerns me as well.

bluefields · « **Reply #5 on:** June 05, 2015, 10:11:42 AM »

I sent the jugs and pistons off to be bored.
This was the big bore kit:
http://pages.ebay.com/motors/link/?nav=item.view&id=121669847054&alt=web

MCRider · « **Reply #6 on:** June 05, 2015, 10:33:05 AM »

I had the same problem with my fresh 888cc motor. To fix it we (MRieck) went at it with a broad brush, so we don't know which solved it, or combination of which.

Like a smart aleck, I left the guide seals off the exhaust side, like a K0. So Mike reguided it and installed seals. M uses special guides I think as well as special seals.

Had a fresh bore but not sure if it was tight enough. So Mike procured a new to me cyl, resleeved and rebored it. And some top notch rings though I forget the brandname.

Either or both were the cause as it no longer smokes.

bluefields · « **Reply #7 on:** June 05, 2015, 10:40:05 AM »

Thank you all for your insightful and prompt responses. Keep them coming.

hotdog · « **Reply #8 on:** June 05, 2015, 11:23:34 AM »

Plenty of the eBay kits in use with no issues, I just fired mine up last week and its fine - no smoke at all.

As a next step maybe talk to your machinist to find out what clearances he gave the pistons....

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cbr954 · « **Reply #9 on:** June 05, 2015, 12:56:39 PM »

i am suspect of having rings in up side down. The second ring normally has a taper of some sort built in as it also does a fair amount of oil control, if it is in upside down it will scrape the oil up to the top compression ring and flood it with oil. Just a thought.

MCRider · « **Reply #10 on:** June 05, 2015, 01:07:23 PM »

Quote from: cbr954 on June 05, 2015, 12:56:39 PM

i am suspect of having rings in up side down. The second ring normally has a taper of some sort built in as it also does a fair amount of oil control, if it is in upside down it will scrape the oil up to the top compression ring and flood it with oil. Just a thought.

Good point. The second ring is known as the "scraper" ring for that reason, it scrapes the oil off the walls of the cylinder, ideally, scraping down to the crankcase. It scrapes in one direction only.

When the scraper isn't marked, do this: One side will be a lighter shade of pale, the other side is dark. The light side is chrome to resist the heat from above, hence it goes on top. Markings are not needed, if this is known.

It does have a notch or a taper to assist in the scraping. But this need not be recognized as they are color coded, essentially.

turboguzzi · « **Reply #11 on:** June 05, 2015, 02:56:25 PM »

a few points and checks and dont get offended, we dont know you and your experience level:
1. hope you didnt fill oil directly to the crankcase....
2. any chance that the motor is wet sumping? check oil level in tank BEFORE starting.
3. external/visible oil leaks are unrelated to the smoke, it's oil getting to the chambers. when it's really bad then it's 99% rings related, hard to believe it could be guides or guides seals.
4.is the guy who bored able to tell you what clearance he used?
5.did you bring him the pistons or he went by dimension? most shops will not bore unless they have the piston.
6.take out the plugs. is one of the plugs specially oilly or all the same? if one stands out then i would say you broke a ring while installing.

just a few checks you can do before dismounting...

MCRider · « **Reply #12 on:** June 05, 2015, 05:31:58 PM »

Quote from: turboguzzi on June 05, 2015, 02:56:25 PM

a few points and checks and dont get offended, we dont know you and your experience level:
1. hope you didnt fill oil directly to the crankcase....
2. any chance that the motor is wet sumping? check oil level in tank BEFORE starting.
3. external/visible oil leaks are unrelated to the smoke, it's oil getting to the chambers. when it's really bad then it's 99% rings related, hard to believe it could be guides or guides seals.
4.is the guy who bored able to tell you what clearance he used?
5.did you bring him the pistons or he went by dimension? most shops will not bore unless they have the piston.
6.take out the plugs. is one of the plugs specially oilly or all the same? if one stands out then i would say you broke a ring while installing.

just a few checks you can do before dismounting...

Yes, wet sumping could cause it to smoke per what I've read about British bikes wet sumping. But it should stop smoking after running for a while and pumping the oil back to the tank.

Mine was WS also. But I didn't include the symptom because afer the rebuild it had stopped smoking. But it still wetsumped after I had cleaned the pump and stopper.

To eliminate the variable I have since installed Elan's kit for the oil pump.

MRieck · « **Reply #13 on:** June 05, 2015, 08:19:58 PM »

Did you check the guides? they are usually blown out at that point despite what other people say. It sounds like you have other problems as well.

HondaMan · « **Reply #14 on:** June 05, 2015, 09:05:18 PM »

The CruisinImage 836 rings are non-directional. Do you have any idea what clearance the shop gave those pistons? Too many try to give them .0020" or more, which is very close to the worn-out limit for the engine. These pistons needs .0008"-.0012" clearance, no more.

I recently (last year) had to fix an 836 engine just like yours: the owner's machine shop bored them to .0025" clearance. After 100 miles, the bores were nearly .0030" loose. It pumped oil out the exhaust pipes, it was so wet.

The valve guides on these engines are frequently worn, thanks now to ethanol. It can wear them more than .0010" in just 10,000 miles, mine proved it. In 2006 I tore the top end off to measure things after it sat for 5 years after I had cancer: the pistons were at .0016" clearance at 126,000 miles, so they got new rings. The valve guides were all less than .0018" clearance worst case, so I kept them. By 2012, with the ethanol gas here in Colorado, the bike started using oil. I suspected the rings (was wrong), took it down for a full rebuild at 138k miles. The guides were severely worn, which all happened in the 12k miles in between (even using top oil in most fillups!).

Bronze guides will go a long way toward solving this issue. The clearance is very important: I build all my engines to .0008"-.0012" intake guide and .0014"-.0016" exhaust guide clearances, except my own: I went to .0008" intake and .0012"-.0014" exhaust. The performance is stellar, and it is smoother than it ever was before! It so easily tops 100 MPH I have to watch myself: it feels like a liter bike(!) all of a sudden, so I put a 19T front sprocket on it to tame it down a bit.

bluefields · « **Reply #15 on:** June 06, 2015, 03:47:10 PM »

You guys rock. I'm going to have to check the valve guides cause I'm pretty sure that since I didn't replace them they are indeed worn out. As far as the piston to cylinder clearance, I'm not sure. I didn't specify to the guy the exact tolerances (cause I didn't know) so chances are it's all sloppy already with 50 miles on it. Sigh. I plan on doing some tinkering on it tomorrow, I'll check the plugs the and post what I find.

bluefields · « **Reply #16 on:** June 06, 2015, 04:19:38 PM »

Just got with guy, clearance is 0.02.

bluefields · « **Reply #17 on:** June 06, 2015, 04:21:25 PM »

I think he meant .002

HondaMan · « **Reply #18 on:** June 06, 2015, 06:59:14 PM »

Then your best bet, short of rebuilding again with something like the [marginal] 855cc pistons, or another set of cylinders and rings (and correct clearances) is to be sure and use 20w50 oil from someone like Torco, Bel-Ray (use their mineral oil type EXL), or Amsoil's 20w50 Motorcycle Oil (Not for V-Twins, says so right on their bottle). This will help reduce the ring flutter and give the engine a chance. This has always been the "fall back" position for the bikes that got this clearance by mistake, in years gone by: it will last for a while, at least a season or two, until you can have a chance to do it again?

TurboD · « **Reply #19 on:** June 06, 2015, 08:46:40 PM »

Against my better judgement, I will throw my 2 cents in.

I do NOT think the .002 is singly to blame for oil consumption or smoking, I would look elsewhere. Here's why.

I my many years (30+) of rebuilding engines I have reassembled quite a few engines that had much more clearance then we are talking here, and there was no smoking or high oil useage. Some of them may have not gone 100K miles, but they did not smoke or use oil like described here in only a few miles.

I built racing Briggs 5hp engines for gocarts for nearly 10 years, The 5hp Briggs engine uses a piston very close in size to the CB750. It was not uncommon to run .004 -.006 clearance, I even raced engines with up to .010 clearance with ZERO problems or side effects. Yes you read that correctly .010, and won a number of races with them.

I am currently rebuilding a 750A (CB750A Barn Build), It has right at .002 piston to wall clearance. I don't expect to have any problems with it.

Big Jay · « **Reply #20 on:** June 07, 2015, 12:35:49 AM »

Try to see if the smoke is coming from one hole. May have to remove the exhaust for that. If it is, then you may have an oil ring expander overlapped on itself.

turboguzzi · « **Reply #21 on:** June 07, 2015, 01:05:13 AM »

Quote from: Big Jay on June 07, 2015, 12:35:49 AM

Try to see if the smoke is coming from one hole. May have to remove the exhaust for that. If it is, then you may have an oil ring expander overlapped on itself.

blufields, listen to jay (and me too

) pull out plugs and exhaust system and try to understand if all pots are oily or just one. it could be just a mangled ring during installation, a 15 mins check...

bluefields · « **Reply #22 on:** June 07, 2015, 11:10:30 AM »

Out getting oil and filter for oil change today. No luck on finding amsoil, bellray or torco. Did find lucas zinc additive and an oreilys brand 20w50. I'll pull the plugs and let cha know what I figure out.

bluefields · « **Reply #23 on:** June 07, 2015, 03:55:25 PM »

Spark plugs, all equally black and wet. From left to right 4-3-2-1.

hotdog · « **Reply #24 on:** June 07, 2015, 09:00:52 PM »

Your running out of easy fixes, motor out time for you!

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Author Topic: serious white smoke after big bore (Read 14153 times)