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Hard to start hot engine, engine occasionally cuts out (CB250RS)

Discussion in 'Technical and Troubleshooting Torque' started by jack_1313, Mar 7, 2011.

  1. My bike has recently started playing up and I hoping that someone here will be able to help me identify and possibly solve the issue.

    Background info:

    It’s a 1982 Honda CB250RS with about 35 000 ks on the clock, assuming the odometer is genuine. I’ve had the bike for about ten months and have had no major mechanical problems. The bike runs well and has always been easy to kick start. I have made some effort to maintain it including getting it serviced and frequent oil changes.

    In the last week or two it has developed the problem described below. Since the bike is my only source of transport and also a source of much enjoyment I’m very eager to sort it out.

    Symptoms of the problem:

    *Bike is very reluctant to kick start when the engine is warm. It takes quite a few kicks to get it going, and it might produce several load popping sounds first, which I assume is the engine backfiring. Once going, it might cut out again, often as soon as I twist the throttle, at which point I have to start trying to kick start it again. Once on the road it is usually ok.

    *Bike still kick starts first time with no problems when the engine is cold. This bike has always been easy to kick start cold, and even easier to kick start hot.

    *The engine occasionally just cuts out while idling at the lights, particularly when I twist the throttle to take off (see above). This happens perhaps once every two days. It gives no sign of struggling whatsoever before cutting out, and until now has kick started again right away. Sometimes when I twist the throttle to take off it seems like there is a moment where the engine misses a beat, but then recovers and revs up as normal. The engine has cut out while idling before, but until now it has been so rare that it hasn’t been worth worrying about (once every few months).

    Info on potential solutions:

    My thought is that there is some problem with the fuel/air intake system. It’s quite interesting that the bike starts fine when cold. I initially suspected a problem with the ignition system, but I’ve looked at the spark plug and it seems to be sparking fine with every kick and I don’t know of any reason to expect different behavior from the ignition system when the engine is hot.

    *The air filter is cleaned regularly and doesn’t look dirty, so I doubt that is the problem.

    *The carburetor was cleaned out less than a year ago by a mechanic.

    *The petrol filter was installed less than a year ago (non-standard for this bike), and does not appear to be blocked in any visual way. I will replace it anyway but I’ll have to cut the fuel hose to get it off.

    *There is an oil leak from somewhere on the cylinder head due to a worn o-ring. My mechanic has looked at it before an assured me that it’s nothing to worry about.

    Any ideas on what could cause this kind of behavior? All help is much appreciated. Unfortunately I’m not much of a home mechanic as this bike has been my first exposure to automobiles.

  2. Very hard to diagnose without being there. It is possible for the coil to give a weaker spark when its hot, given the bikes age there's a distinct possibility that could be the problem. The other possibility that comes to mind is that a partially blocked idle jet is causing a lean mixture, but that it starts OK when cold because the choke overrides that. I'd start by replacing the plug and draining the carb. If that doesn't help, move on to cleaning the carb, then replacing the ignition coil and plug lead. Suggested order of work is based on least expensive first. Cleaning the carb is easy to do yourself if you do a bit of research and work carefully in a clean environment.
    Nice bike BTW, my brother recently sold his after 12 years ownership, great commuter, very economical and fun to ride.
  3. just needs a carbie clean/tune.
  4. Some of the old late 80's 250 honda's would cut out if the wire running down the front frame down tube was too near the exhaust..
    Didnt That one take a while to find..](*,)
  5. Problems when warm can also be a result of the ignition pickups breaking down when they get hot. Used to be quite a common problem with 1st generation, post-points bikes.
  6. Thanks for the help guys, I'll start by working through the tasks that the mole suggested. Another possibliy was voiced today when I was buying a fuel line, which is that the valves could be too tight, causing issues when the engine is hot. However, I'm not sure why that might cause the engine to cut when, once started, the throttle is opened. The valve clearences were checked and adjusted about three months ago though. Since that's a task I don't know how to do, it will be last on the list of things to investigate.

    Drained the carberettor this morning and found a small amount of dirt in the drained petrol. I guess this is normal though?

    One other thing: is it possible that the fuel line and filter could become blocked with air? I had a look at the fuel filter today when the bike wasn't starting and noticed that the level of petrol resting in the filter itself didn't reach the tubing ports on either side due to an air bubble. The filter was probably 2/3rds filled with air. I noticed this the other day too and thought it might be odd.

  7. An air lock in the fuel line is possible, but I wouldn't worry too much about the fuel level in the filter. I've used quite a few over the years, without problems, and I've never seen one full. If you're concerned, ity shouldn't be too hard to bleed some/all of the air out.
  8. "Drained the carberettor this morning and found a small amount of dirt in the drained petrol. I guess this is normal though?"

    If you've got a filter in the line, you shouldn't be getting dirt in the fuel, even a small bit in the carb can cause idle problems. The hole in the idle jet is very small. Did it run any better after you drained it?
    Very old fuel lines can break down and drop little bits of rubber into the carb. To check if fuel is getting through the filter OK, turn the tap off, take the fuel line off the carb and put it in a small jar, turn tap on and watch the flow.Check jar for dirt coming through as well.
  9. Sounds like it's running lean or an electrical issue.

    I'd start by stripping & cleaning the carbs, 'cos that won't drive you 4 types of insane like diagnosing electrical problems will...

    If that doesn't fix, start replacing spark plugs, coils etc.
  10. Hi guys, the problem was solved last night!

    It turns out it was a spark plug issue after all. Although I tested the plug both hot and cold and found that it sparked every time, putting in a new plug solved the problems instantly, much to my surprise. I guess there are two possibilities:

    There was a loose connection between the spark plug and the line. It did feel a bit loose, and perhaps that was somehow being exacerbated when the line and plug were hot.

    It could be that the plug was giving a weaker spark when hot. I'm not sure if this is possible, but I certainly would not have been able to identify that the spark was weak myself.

    Total cost of solution: $4!

    Thanks for all the help. This was a very positive outcome.

  11. I don't know how u do it on bikes but if it's adjustable (should be on an old carby bike), try taking a bit of static timing out of her. Costs nothing to do. just mark the current position and retard it a couple of degrees.

    Edit: just saw that u fixed it! Yep never hurts to throw a set of plugs in if you have any doubt about ignition or fuelling. If it makes no difference u have a decent set of spares anyway.
  12. The spark is less likely to when its in the cylinder, as the compression in the combustion chamber increases the resistance across the plug gap. Also, when coils get old they tend to break down when they're hot.
    Note that your solution may not be quite that simple. It may be that your coil is not performing 100%, so you need a good new plug to get a spark. Possibly, replacing the coil may have had it work OK with the old plug, who knows? If it happens again while the plug is in good nick, suspect the coil. Meanwhile, enjoy the ride. (y)
  13. 30 odd year old coil? Could well be past its best.