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Discussion Starter #1
I wonder if anyone has experienced anything similar to my MC17 problem.

It's been running and felt ok, but the compression has always been low (about 100 psi on all cylinders). So I decided to strip it down and have a look at the pistons, rings and bores. There are small scratches on the pistons but everything measures up fine. Only 0.02mm wear on the pistons and 0.01mm wear on the bores. So the problem must be elsewhere I guess. I had the head and valves vacuum checked and that is all really good too. So I am left with the timing, is it possible that I somehow managed to get the timing slightly out, by a single tooth or something? And would this affect the compression so much? I have been using camshafts with smaller lobes from a different model that I bought from Japan, not sure exactly which bike they came from. The camshafts, timing angles, lobe angles etc. all seem to be exactly the same but something has to be causing the compression issue.

I have OEM piston rings on the way, the bores have been honed, new pistons are also coming although I might not use these in favour of the originals depending on how they look when they arrive. The crankshaft bearings were ok and new replacements were not readily available. Conrod bearings were the same. So I am ready to put the engine back together, just want to be sure.

Anyone have any similar problems ever? The bike started and revved fine, was OK lower in the revs but lost power higher up. 30hp max. I have an original CDI and unrestricted new version, which of these is my best option?

I'd greatly appreciate any advice on what I should focus on. I want to get the engine back to it's original power at least.

Thanks
PART_1576150411075.jpeg
 

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MC19 cam info, unfortunately, I've missplaced the valve lift specs, but, by memory, the MC19 has .08mm more lift than the 250 Hornet
intake opens 19 BTDC
closes 33 ABDC
Exhaust opens 36 BBDC
closes 11 ATDC
Overlap of 30.00 degrees and has in Intake Duration of 232.00 degrees. The Exhaust Duration is 227.00 degrees.
Lobe Separation Angle is 99.75 degrees. The Inlet Cam has an Installed Centerline of 97.00 degrees ATDC.
The exhaust cam has an Installed Centerline of 102.50 degrees BTDC.
Max HP @ 16,500 rpm (although Tach error would have you believing it was closer to 18,000 rpm)

MC17 / Hornet / 250/4 cam info
intake opens 10 BTDC (@1mm lift)
closes 40 ABDC
Exhaust opens 30 BBDC
closes 10 btdc
Overlap of 20.00 degrees and has in Intake Duration of 230.00 degrees. The Exhaust Duration is 220.00 degrees.
Lobe Separation Angle is 102.50 degrees.
The Inlet Cam has an Installed Centerline of 105.00 degrees ATDC. The exhaust cam has an Installed Centerline of 100.00 degrees BTDC.
Maximum HP @ 15,200 rpm (expect the same lies from the tach)

I'm also interested how the "vacuum testing" was done ? I find it difficult to think how you sealed the ports completely, A leakdown test would be better, ie: pressurise each cylinder individually (on compression stroke, with the crank anchored to avoid the engine turning as the cylinder is pressurised), noet the time it takes for the pressure to drop to zero , repeat after putting oil in the cylinder, if the pressure still drops quickly, you can suspect the valves are leaking (common problem)
A Nissan diesel glow plug / leak down fitting has the correct 1mm x 1.0 pitch for the plug hole
The heads really need to be taken to specialist head service center, with a CNC machine , and the correct minuscule guide pilot to recut the valve seats, and, if needed, skim the cylinder head surface, BUT , piston/valve clearance is CRITICAL !, make sure the head thickness is accurately measured before, & after any Machining of the face is done , you may need a custom MLS head gasket to keep things from kissing each other.
I bet you are glad you decided to spend lots of money on something Honda produced as a "throw away when broken" bike :rolleyes:
 

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Discussion Starter #3
Hi, thanks for your time. What I can't get my head around is that every pair of camshafts that I have seen to have exactly the same marks and cam angles, how can the timing be any different?

I also now have a replacement head that came with camshafts, it was advertised as MC14 cylinder head and camshafts. It's silver and the lobes have slightly more lift than the camshafts I was using. Am I better off to use the new head and camshafts together or is it going to be okay to just use the camshafts?

The local engine shop have the vacuum valve tester and it's what they use on all the engines they test. They insist that the head and valves are in good shape.

I have a proper leak down tester that has a percentage scale that indicates the exact proportion of leaking air, was under 10% as I remember, but I'll check it again.

Not too much money spent so far, and it's such a cool motor to work on. It's pretty low mileage so I hope it's worth the time I'm investing in it.

Maybe I'll get this new cylinder head cleaned and checked and valve stem seals changed.

Your advice is appreciated. Can't thank you enough.
 

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Hi, thanks for your time. What I can't get my head around is that every pair of camshafts that I have seen to have exactly the same marks and cam angles, how can the timing be any different?
Lift & profile variants

I also now have a replacement head that came with camshafts, it was advertised as MC14 cylinder head and camshafts. It's silver and the lobes have slightly more lift than the camshafts I was using. Am I better off to use the new head and camshafts together or is it going to be okay to just use the camshafts?
It sounds like a MC19 head / cams , MC14 usually relates to the frame number, not the engine.
The only way to ensure correct valve / piston clearance is within specs with your "new" cams, is to check with plasticine , but be VERY careful, roll thin strips, turn the engine very slowly, 2 complete revolutions, measure the clearance with your "old" cams installed 1st, then compare with the new cam clearance (I can't remember what clearance I settled on, but as I was lowering the c/r I made a 2.8mm mlc gasket, I think I had .8mm clearance with the Hornet cams, but the head had been planed too many times

The local engine shop have the vacuum valve tester and it's what they use on all the engines they test. They insist that the head and valves are in good shape.

I have a proper leak down tester that has a percentage scale that indicates the exact proportion of leaking air, was under 10% as I remember, but I'll check it again.

Not too much money spent so far, and it's such a cool motor to work on. It's pretty low mileage so I hope it's worth the time I'm investing in it.

Maybe I'll get this new cylinder head cleaned and checked and valve stem seals changed.
I wouldn't bother , my seals are way past their use by date, but the engine burns no oil

Your advice is appreciated. Can't thank you enough.
The shop that did my headwork had trouble with the exhaust Inconel valves, as inconel requires agressive cutting, the 3.5mm valve stems flex way before the cutting can be achieved, out od 36 valves machines, 18 were ok, the rest, almost ok, but these still leaked @ 5 psi (test procedure included a solid perspex sheet "head gasket", and condoms for the ports
 

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Discussion Starter #5
Hi again. And thanks again.

The shop reckon the angles and profiles of the cams are the same, lift being the only difference.

Are the heads made differently to create the difference in overlap too? The 'new' cams give 0.2mm more lift than the originals, 0.6mm more than those I have been using. I'd rather not have to do the wax/plasticine testing unless maybe I could reuse the head gasket, they take a while to arrive here in Finland. Ah, on that subject, I couldn't buy MC17 head gasket as it's discontinued but used MC19, could that make any difference?

BTW. The audio frustration on your video had me in stitches. I've not been to Aus for a few years and had forgotten how much I miss the attitude and accent. I've a few mates there but not had the chance to visit since the kids were born. Priceless.

G
 

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Could your engine be a MC22 ?
I have a few Chinese head gaskets which I won't use, ever, but they might be handy for valve/piston checking ?
Gawd,,which video,,lol
 

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Discussion Starter #7
I guess it could be MC22. Hard to know for sure. But it's definitely low mileage and makes me think it's 100% original. Head is black, no DOHC markings.

The run2 video. LMFAO
 

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Oh, the dreaded run 2, Wilso (co rider) was testing the blow off valve, (leaky intake valves ignited the methanol / air mix in the plenum) Run 3 was worse
Somewhere on either this forum, or https://2fiftycc.com/index.php there is a great build thread explaining the problems with mixing up MC22 & MC everything else innards, I do remember the MC22 oil filler is on the rear of the clutch cover, not in the middle like MC everything else's
Black eh ,,mmmmmm ?
 

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Discussion Starter #9
It dawned on me today, that the service manual says that the compression should be:

13.0 kgcm2 @ 400 rpm

Does the starter motor actually turn it that fast? I believe most crank at about 200 rpm on the electric start.

What do you reckon? Maybe my numbers are OK.

Motor is back together now, gonna measure the combustion chamber at TDC to calculate the ratio anyway.

More soon.
 

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Discussion Starter #10
I've collected some info, I'll but it all down here and see what anyone thinks.

CBR250R MC14 Valve Spec.
In Open - 10° BTDC (1mm lifted)
In Close - 40° ABDC (1mm lifted)
Ex Open - 30° BBDC (1mm lifted)
Ex Close - 10° BTDC (1mm lifted)

CBR250R MC17(H) Valve Spec.
In Open - 18° BTDC (1mm lifted)
In Close - 34° ABDC (1mm lifted)
Ex Open - 36° BBDC (1mm lifted)
Ex Close - 11° ATDC (1mm lifted)

CBR250R MC19(J) Valve Spec.
In Open - 19° BTDC (1mm lifted)
In Close - 33° ABDC (1mm lifted)
Ex Open - 36° BBDC (1mm lifted)
Ex Close - 11° ATDC (1mm lifted)

CBR250R MC22(L) Valve Spec.
In Open - 19° BTDC (1mm lifted)
In Close - 33° ABDC (1mm lifted)
Ex Open - 36° BBDC (1mm lifted)
Ex Close - 11° ATDC (1mm lifted)

CB250 MC23(Jade) Valve Spec.
In Open - 10° BTDC (1mm lifted)
In Close - 40° ABDC (1mm lifted)
Ex Open - 30° BBDC (1mm lifted)
Ex Close - 10° BTDC (1mm lifted)

I have been using (I think) MC23 Cams, so I've also drawn up a diagram to show the difference in overlap etc.

Your opinions appreciated. MC19_MC23_ValveComparison.jpg
 

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Thanks. Where do I find such info? Or maybe I just rely on you to help.

l8r
The hard way,:icon_rolleyes: I was chasing "hard to start" problems, so place a "spare" bottom end along side the bike (just a set of cases & crank), plugged the bike's CDI in and wound it over, my spark magically re-appeared, so I put a hand held tach on the crank, 550 rpm,,,then slowed the rotation speed with a gloved hand around the alternator stator until the spark event disapeared, 400 rpm seemed to be the cutoff point.
This did not cure my starting problem, so I bought a new (Chinese) starter motor, problem solved, it seems the stock starter motor dragged way too many amps, which also dropped the cranking voltage below 11.5 volts, which the CDI won't accept & still function (I had a 400 cranking amp Harley battery at the time also)
 

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Discussion Starter #14
The hard way,:icon_rolleyes: I was chasing "hard to start" problems, so place a "spare" bottom end along side the bike (just a set of cases & crank), plugged the bike's CDI in and wound it over, my spark magically re-appeared, so I put a hand held tach on the crank, 550 rpm,,,then slowed the rotation speed with a gloved hand around the alternator stator until the spark event disapeared, 400 rpm seemed to be the cutoff point.
This did not cure my starting problem, so I bought a new (Chinese) starter motor, problem solved, it seems the stock starter motor dragged way too many amps, which also dropped the cranking voltage below 11.5 volts, which the CDI won't accept & still function (I had a 400 cranking amp Harley battery at the time also)
Gold dust. Thanks. I have one of those starter motors 'in stock' never used. I'll try that when I retest the compression. G
 

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Discussion Starter #15
Got it back together, and retested compression, exactly the same. But I checked the cranking speed and its 200 rpm about. Tried 2 starter motors, and a large fully charged van battery, still the same.

How does cranking speed affect compression? Does it crank faster with other plugs in?

Rings are new. Cylinders honed. Camshaft timing checked. Valve clearance bang on.

Maybe I need to forget the compression testing and just get it put back in the bike and dyno it.

Thoughts?
 

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Discussion Starter #17
I did the test while cranking with the throttle open all the way, also tried it with the airbox off and held the slide open, also tried it with the carbs off.

Anyway, I put it all back together and this time I reverted everything back to standard; main jets, idle jets, slide washers, exhaust etc.

Did a quick run on the dyno yesterday evening and I the result is below. The dyno has a pretty slick drum (needs to be have the knurling redone), the boss says that it's a few percent off. But was 40hp at the wheel, so over 45hp at the crank. It has much less low and mid range lag and I'm just waiting for the snow to finally melt before I test it on the road.

Thanks all for your help. Incidentally, I now have 6 sets of camshafts, MC14, MC17 and MC19. And I also have a fully refurbished MC19 cylinder head, and a spotless set of MC19 carbs . . . just in case.

Gracias amigos

CBR250RH Dyno.jpg
 

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Hmm. Check out that dip at 13-15K and the rich condition in the same range. Kind of pulling this out my arse - but can you remove a washer under the needles to fatten it up as the raising slides pull the needles out of the emulsion tube? You would know after one pull whether it was better or worse...
 
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