Curious engine, low compression


Crazy Canuck
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Re: Curious engine, low compression

Post by Crazy Canuck »

Any updates to this?
Forrest Miller :thumupp:

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CBX1000chris
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Re: Curious engine, low compression

Post by CBX1000chris »

Hello Herdygerdy and Crazy Cannuck,

To be honest I didn't look at the ring orientation on all of the pistons but the ones I did check were correct although the gap alignment was way off the recommendation from Weisco. As I think I mentioned early on this isn't my bike, it belongs to a friend who has built a special using a Metisse rolling chassis, not one of the early motocross chassis, (Steve McQueen said "THIS RIG IS THE BEST HANDLING BIKE I'VE EVER OWNED"), but the later MK5, see link below. On the subject of Steve McQueen, it might be worth watching Guy Martin's Great Escape next Sunday (8th December).

https://www.motorcycleclassics.com/clas ... cz12mazbea

Imagine the above with a CBX motor fitted, very nice. I wish I could insert a picture or two (not sure how to do it anymore). The exhaust goes 3 into 1 then 2 into 1 under the seat and out through a hole in the tailpiece. I think Lord Flashheart would have said "Well, well, well, if it isn't little Bobby Parkhurst...saucier than a direct hit on a Heinz factory!"

Anyway, I'm waffling. I took the head and barrels to the work shop. Both the head and the barrel have some warpage and the number three and four cylinders are scored, enough to present a problem. So I'm weighing up the options and decided we could:

1. Get a replacement barrel and bore it to suit the new pistons that were fitted to the bike
2. Over bore to the next size and get new pistons
3. Fit new liners to the number 3 and 4 cylinders

At this point I though it best for the owner the talk the workshop and and discuss the options (money). They have decided to fit new liners and use the existing (as new) pistons. They agreed to lightly hone the other cylinders and I must check to see what they decided to do about the warpage. I did ask a question earlier about how much could be skimmed from the head and barrel before squish became an issue. I did have one reply from daves79x, thank you Dave.

If you have to skim a lot from both head and barrel can you skim enough for the reduction in squish to be taken up by a second base gasket? Until recently I was racing late model TZ250s and had a wide range of Yamaha base gaskets to vary head volume. Sometimes I'd run two gaskets to get the volume I wanted without issue and TZ base gasket sealing is slightly more critical than CBX!

The bits are still in the workshop, as soon as they're ready I'll build it back up and let you know the outcome.
Have a great day everyone,
Chris.
p.s. I probably ought to learn how to post an audio.........

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Re: Curious engine, low compression

Post by Stevo »

text
Last edited by Stevo on Thu Feb 06, 2020 6:48 pm, edited 1 time in total.

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Re: Curious engine, low compression

Post by Stevo »

I had this same problem with my CBX, I live in Australia and bought a US imported bike, the engine was running reasonably, I noticed the airbox had lots of mud in it, as did other parts of the bike, I thought probably been under water. Anyway striped the bike pulled the head and barrels off, no signs of water in the engine. Honed the bores new rings, valve job, carbies overhaul, reassembled. Very hard to start, if I tried to open the throttle it would stall, check valve time with a degree wheel, slightly retarded but not a tooth out, ignition timing etc. Did compression test, very low 70psi. Bought the rings of Ebay, thought maybe they were crap, pulled motor again, new quality rings, checked everything again, wet tested the head. Compression did not improve, the bike was very hard to start cold and had no power, I went for a ride of about 100 miles, started to improve, kept riding every weekend, after a month running pretty good, that was a year ago now runs fine, I feel it may be a little lean, does not run well until fully warmed up in cold weather. Have not rechecked the compression, have not bothered as it runs so good now. My favourite bike at the moment, love it and will only sell when I am too old to ride.

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Re: Curious engine, low compression

Post by daves79x »

I think your valves were just re-acquainting themselves with their seats, mostly. I've seen this exact thing happen on several CBX engines that have been sitting for a long time. Probably not the best way to approach it, but nothing to lose in trying.

Dave

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Re: Curious engine, low compression

Post by CBX1000chris »

So here I am, sat head in hands, feeling pretty sorry for myself.

I got the head and barrel back from the engineering firm last Friday. As agreed between my friend (the owner of the bike) and the company, new liners were fitted to cylinders 3 and 4, the remaining cylinders were glaze busted. I rebuilt the engine without any headaches and crossed my fingers.....

The engine tries to run on cylinders 3 and 4 but the remaining cylinders appear to fire intermittently. At this point there is no exhaust system fitted so I can see black smoke exiting the exhaust port on cylinders 3 and 4 (rich?), the other cylinders have an occasional blue flame. I should mention at this point I do have a fire extinguisher within arms reach!

I did a compression test and got the following figures,

1 - 93 psi
2 - 110 psi
3 - 110 psi
4 - 122 psi
5 - 92 psi
6 - 110 psi

Clearly this is low but I would have thought there was enough there for the engine to start, maybe not? If compression is the issue and 3 tries to fire shouldn't 2 and 6 also? If the bike did start hopefully these numbers would come up a bit as the rings bed in (ever the optimist).

At this point I'll mention again this engine is to be fitted to a 'special' so was bought as a unit, we've never heard it running. It arrived with new Wiseco pistons fitted, 0.5 mm oversize, it was clear looking at the pistons that they had never run. I don't know whether the cases were split as part of the last rebuild. Trying to start it feels like trying to start my own CBX before I serviced the carbs, it tries and tries but doesn't quite make it. To start my bike I had to give a few blast from the accelerator pump before cranking, fully servicing the carbs resolved this. I feel like I'm missing something so here's what I've checked so far.

With cam lobes on #6 pointing out cam lobes on #1 are pointing in.

I had to compromise on valve timing, exhaust camshaft punch mark was either just above or just below the head surface, I opted for just below as this looked closest, previously it was just above. I guess this will be my next thing to alter.

Valve clearances mostly 0.003", a few 0.004" and a couple 0.005".

Static ignition timing was spot on. F aligned with case joint, pip on rotor aligned with pip on pickup 1,6 (the one at 9 o'clock).

With #6 at TDC putting a screwdriver between the pip on the rotor and the pip on the pick up causes #6 plug to spark, similarly for #5 and #4. This is important because the harness was made for the bike, it's not the CBX original. So the spark is just before TDC and is the right spark for the right cylinder.

Carbs stripped and cleaned, particular attention paid to idle circuit including removal of the very handily press in jets. All of the small drillings near the butterflies were clear when tested with cleaner.

New plugs fitted.

With the engine running on #3 and #4 there was a very strong spark on #5 and #6. The spark was weaker when cranking but this is expected.

It''s a while since I've worked on a bike with a mechanical advance mechanism and I couldn't work out why I couldn't turn the rotor by hand, it turned out it was seized. I stripped, cleaned and lubricated it but it made no difference to the running of the engine, it had seized in the 'resting' position so the timing was OK. Just for a moment I thought I was on to something only to have the devil fart in my face once again.....

So there we are, I guess I'll go and have another look. Any thoughts of possible next tests, and whether it ought to at least run with those compression values, would be appreciated.

Regards,
Chris.

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Re: Curious engine, low compression

Post by daves79x »

I'd be looking at the carbs. You have quite enough compression for the bike to start.

Dave

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Re: Curious engine, low compression

Post by CBX1000chris »

Hello Dave,

Thank you for the quick reply, it's reassuring that you also think the bike should start with those compression figures. Given the black smoke from #3 and #4, suggesting richness, what might make the carbs deliver too much fuel? I set the float height to 15.5 mm, some were out, but everything else looked to be OK, right jets in the right places etc. However they had been worked on by someone who was slightly less than careful. In particular the pilot screws were all over the place, some had two o rings, one had no steel washer, the threads were stiff such that the #6 screw was miles out of adjustment, at least 4 turns out. I suspect someone had thought it had bottomed out then backed it out 2 turns when it hadn't bottomed out at all. But it was visibly different to the rest of the carbs and clearly wrong, for someone not to notice that is a bit worrying. What really piss** me off though is the way some folks round screw heads and leave it for the next poor sod to get them out, grrrrrr! If it wasn't such a time consuming job I'd take the carbs off my bike and try them. Maybe I should take the anti-denial tablet and just do it, at least I'd be able to eliminate the carbs from the equation.

Is there anything about the standard air box design that influences the carbs? At the moment I have them open but when the bike was delivered to me it had 6 individual K&N filters. I remember a bike I owned having something on the rubber inlet manifolds that needed to align accurately with an opening on the carb or it was all over the place, can't remember which bike it was though, might have been a ZXR750K1 (flatsides). At the very least I'd expect the mixture to be leaner with no filters but I could be wrong.

Right, onward. Refreshed with a cuppa I'm back on the case.

Regards,
Chris.

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Re: Curious engine, low compression

Post by CBX1000chris »

Hello again,

I reached the point where I'd checked everything at least three times so decided to start comparing my own, running, CBX with my friend's bike. The first thing I checked was the valve timing, both bikes were identical. Static ignition timing was pretty close too between the two bikes. I read in a different post about how not having the earth lead connected to the coil mountings could cause random electrical problems so I checked the bike, the coil mountings were not earthed. I knocked up a short lead to the battery -ve terminal and ........, it was exactly the same.

I was laid awake in the wee small hours of Sunday morning when I had an idea. Some time ago I bought a timing disc for use on an old British bike and I decided to make an adapter so I could mount it on the ignition shaft, basically a piece of 6mm studding and a spacer. The diameter of the disc was just right for me to lay an engineers 6" rule on the crankcase, just behind the cylinder. Running the strobe off #6 cylinder with the engine turning over on the starter but no plugs fitted finally gave me something to work with, the ignition timing was all over the place, something not visible with the static test. Repeating the test on my own bike was as expected, ignition occurring at the same point for each revolution.

I couldn't resist swapping the stator and pickups off my bike to my friends and it started on the button, sweet as a nut. Just to remind everyone this is a special I'm working on with a bespoke harness. Also, and here's the killer, the lead from the pickups to the 6 pole connector had been extended.

Thinking the extension may be acting as an aerial I removed it and crimped on new terminals. At this point I didn't push the terminals into the shell but pushed them individually into the connector that runs to the ignition module, the coloured sleeves that identify which cable goes where were missing and as the cables are the same, two blues, two yellows and two reds you can't tell which is which (I relied on the existing order). When started the bike ran, but badly. I swapped around 2 of the pickup wires and it ran sweet as nut.

I'm now not sure whether the problem was the cable extension acting as a aerial (and I crossed the wires when I connected up the pickups) or whether the wires had been crossed when the extension was added, either way, this was definitely the problem. The irony was the cable didn't need extending anyway! It is interesting that the cabling from the pickups has a woven outer, not knowing the cable specification I'm not sure if this is for electrical noise immunity or heat protection?

Setting up the ignition timing with the timing disc on a CBX was something new to me and very useful. I followed the manual and set the timing to 10 degrees BTDC, it was slightly different to the static set up.

So I'm almost home and dry. All I'm left with is an annoying occasional extra spark (flash of the strobe) when the bike is running with my friends pickups and stator, this is very visible on the timing disc because the disc is flashed 120 degrees out of phase to the #6 cylinder flash. It only does it every few seconds and isn't very bright, however it doesn't do it with my pickups fitted. I'll have a very close look at the pickups and stator tomorrow and as a final test may fit my friends stator to my bike and see if the fault moves too.

The bike looks and sounds great though. I can't work out how to post a picture on the forum, can someone help please?

Thank you to everyone who has contributed to this adventure.

Regards,
Chris.

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Re: Curious engine, low compression

Post by Rick Pope »

<<I can't work out how to post a picture on the forum, can someone help please?>>

Try this: http://photoposting.is-great.net/?i=2
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Re: Curious engine, low compression

Post by Larry Zimmer »

click 'gallery' over near the logout thing on the right side
click "ICOA photo album'
click 'upload'
on that page there will be an empty box to enter the file OR click browse to go to your computer files to find what you want
click that photo and 'open' or enter
Continue this for however many photos you want
click 'submit'
Now the photos appear in the gallery
click each photo to open them
in the information under the photo when you open it, you will find a number looking something like this: album/xxxx/album. Write that number or remember it
NOW, in your post, wherever you want that photo, click 'album'. enter that number Repeat this in your post for however many photos you want to do.

OR, YOU CAN DO THIS. SEND ME COPIES OF YOUR PHOTO(S). PUT YOUR POST ON THE FORUM AND I CAN EDIT THEM INTO IT FOR YOU! cbxlarry@gmail.com
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Re: Curious engine, low compression

Post by EMS »

Before you start, make sure you know how to RESIZE your picture. Most of them will be too large, either pixel x pixel or total file size - both have to meet the requirement - and the picture loader will not accept them.
In the section "new users start here" is a tutorial:

viewtopic.php?f=104&t=5748
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To be misunderstood is the rule.

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Re: Curious engine, low compression

Post by daves79x »

Really glad you found the problem, but have to say that you really did end up needing to do it all yourself. With all manner of messed up wiring, any diagnostic efforts by the group gets are pretty useless. We have to assume a few things and that is one of them. Presented with your bike, seeing the wiring mess, that’s the first thing you’d suspect. Guess we didn’t take you seriously concerning how screwed up the wiring was

Dave

CBX1000chris
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Re: Curious engine, low compression

Post by CBX1000chris »

Hi Dave,
My big assumption, and we all know not to do that, was that the ignition wiring was OK. The static test, touching between the ignition rotor and pickups, gave me six sparks, all on the correct cylinders. Only when the engine was running did it all go to pot, we live and learn. It was a great challenge though and the feeling of relief (not satisfaction) when it fired up was amazing! The engine needed to come apart anyway to repair the bad liners, if we hadn't done that it may now be running but smoking like an LC. Happy birthday by the way, 350LC is 40 years old this year.

Larry, I'll send you some pictures taken in my garage, hopefully the sun will shine tomorrow and I'll wheel it outside.

Regards,
Chris.

CBX1000chris
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Re: Curious engine, low compression

Post by CBX1000chris »

Hello all,
Hopefully images will embedded in the message!

Without any clothes on
7955

7953

7951

7950

The bike sounds awesome with this pipe. Gerry had it made for the bike.
7952

This is how I timed the bike, not sure if it's the best way but it seemed to work. You can just make out the 6" rule taped to the cases behind the cylinder. In the picture the disc is on my bike from when I was comparing the two.
7954

Regards,
Chris.

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