Hard to start when warm ('81 CB900C) - Motorcycle Forum
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post #1 of 10 (permalink) Old 04-07-2010, 10:09 PM Thread Starter
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Hard to start when warm ('81 CB900C)

I had an old wet battery die this winter. It wouldn't hold a charge, so I swapped it for a better battery from another old bike I have. Since the first day I started the CB900C this spring the bike does not want to start when warm. Choked, not choked, whatever. It sounds like the starter is cranking fine but won't fire. Of course, in the past my '80 GS750L would sound like it was cranking fine, but wouldn't start until I put a jumper on it (warm or cold). That's the bike the replacement battery came out of. Different but related issue I think.

At any rate, the original bad battery from the CB seems to have a lot of JUNK in the bottom, so I think I'll spring for a premium maintenance free battery instead of trying to refurbish this one. Any arguments against this?

Disregarding the GS for now, do you think a new - hard cranking battery will fix my problem with the Honda?

Other Info on this CB900C: I've changed the oil/filter, both tires, and cleaned my K&N air filter and airbox this spring. I'm on my 4th tank of premium gas. The first 3 tanks were heavy with Seafoam like I do every spring. I've been finding hills and roll-starting when necessary. I have a 90 mile round trip commute. This Honda CB900C was a great bike last year (bought it Jun 09) and it ran flawlessly last spring.

I've put 6k on it since I got it. It had a fresh tune then... I guess I'll need to tighten my camchain and check valve clearance too... could this have something to do with it?

My problems are similar, but not the same as another thread, but I can't post a link. I've used the search function in this forum and manually browsed 15 pages deep to try and find the solution before bothering you all.

I'm digging this forum. Ride safe.
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post #2 of 10 (permalink) Old 04-07-2010, 10:32 PM
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I think I'd spring for a new battery. It might be ok enough to turn it over but might not have enough juice to turn it over AND give it enough snot for good spark. Plus if there is a bunch of junk in the bottom it may be shot or on it's way to being shot. Better safe than sorry on that one.

If it turns over easy when hot but won't start check your compression when hot. If the valves are too tight cold, when it gets hot they will be even tighter and can possibly hold the valve slightly open killing the compression and making it hard to start. It could be electrical so check the spark when hot. IF you get weak spark then you'll have to test stuff like the coils and related parts. If you get a nice fat blue spark i'd go with the valve clearance (after a compression check) or clogged pilot jets in the carbs.

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post #3 of 10 (permalink) Old 04-09-2010, 11:01 AM Thread Starter
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I'm pickup up the new premium maintenance free battery today. I'll post-back if that solves the problem.
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post #4 of 10 (permalink) Old 04-11-2010, 01:10 AM Thread Starter
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After intalling the brand new battery I've successfully started the bike 3x while hot. Hopefully this is it. I'll make another post if this doesn't end up fixing it. Otherwise, a good battery did it.
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post #5 of 10 (permalink) Old 04-11-2010, 11:17 AM
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Batteries are rated by how much current they can deliver for cranking; perhaps the battery you subbed in for the failed one had a lower rating, and couldn't keep the voltage high enough for spark generation.
Sealed batteries (AGM type) often have higher capacity in the same size package, so will crank better than the recommended flooded battery. I've noticed that new bikes are starting to be fitted with sealed batteries from the factory, now. I found one to fit my 800, and it works really well. Your new battery will probably serve you for a long time.

Rick
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'68-7? CB450 and '98 Valkyrie Tourer, too
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post #6 of 10 (permalink) Old 04-12-2010, 10:58 AM Thread Starter
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Thanks Sol - good tips! It's about time the industry started producing sealed batteries for these applications as well. It's just a shame that it costed me $95 (there were cheaper ones to be had, but these are the batteries my mech has in all his bikes)... Last year I picked up a premium battery for my diesel-auto that only put me out $110.

Anyway, after my 40 mile commute this morning I stopped to fuel up as I always do. The first time I cranked for 2 seconds and got nothing. It fired up on the second crank (after about another 2 seconds). Still harder starting than I'd like, but no biggie. Definetely tolerable after several weeks of roll-starting when hot.

Cold starting is crazy fast now though. This morning it was a brisk 49degF and it almost seemed to start before I actually touched the button. Ok, that's an exageration - but it did come so fast that I barely made out any crank! That was nice.
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post #7 of 10 (permalink) Old 04-12-2010, 02:34 PM
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Does it crank more slowly when hot, or just not want to fire?

Rick
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post #8 of 10 (permalink) Old 04-12-2010, 05:31 PM Thread Starter
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Nope, just doesn't want to fire. It cranked well prior to the new battery, but now it cranks like mad! I can't tell much difference at all in the way it cranks (cold or hot).

My mech and I both suspected that perhaps it is out of spec (tight) on the valve clearance. The problem there is that if I don't have compression (due to tight clearance) then roll-starting shouldn't work either. This really has us confounded.

In any event, it does start now at least - so I'm grinning. Any input would be stellar though.
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post #9 of 10 (permalink) Old 04-12-2010, 07:00 PM
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The big difference in roll-starts vs. crank starts, is the spark is hotter, because there is more voltage. If your ignitor is hot, it may have issues with a lower voltage, but the valves are certainly worth a look, especially if it hasn't been done in a while. Also, the pick-up for the spark timing could be responding to the heat, especially if the gap between it and the reference on the rotor is a little off.

Rick
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post #10 of 10 (permalink) Old 04-13-2010, 10:35 AM Thread Starter
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No problems in the last 24 hours. I tiny bit more difficult still when hot, but I'm still smilin'. The valves were checked out a year ago...

Makes sense on the roll-starts - and it also leady me back to the battery being the problem all along. I think this one is solved, even though it was odd. I'll be sure to add more info if the status changes in this regard.

Safe riding all!
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