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Thread: Misfire at idle and with small amounts of throttle

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  1. #1
    Join Date
    Jun 2017
    Posts
    5

    Misfire at idle and with small amounts of throttle

    Hello all,

    I picked up my new Beat today from the shipping depot, a 1991 with about 55,000kms on it. Looks very tidy, but there's an immediate problem that the pre-purchase inspection I had done at the other end of the country never mentioned; a notable misfire at idle and with the throttle just barely pressed.

    I initially thought I was just bunny hopping like a learner driver, but I recalled the other (much more tired) Beat I drove closer to home was smooth as butter.

    The tacho shows the idle is uneven, the needle mostly sits a bit high at 1500RPM and momentarily dips down. It's very difficult to drive smoothly at a constant 50km/h (or 60 or 70 or 80) because the shudder/stumble kicks in just at that semi-coasting throttle position you naturally maintain.

    It pulls very cleanly with your foot down, all the way up the rev range. On overrun it does sound a little uneven though.

    From having a search around, it seems I should be looking in the following places:
    • Distributor (1991! No sign on drivers door of recall mark, if such a thing exists)
    • ECU caps
    • Maybe replace spark plugs?


    Any other suggestions? I did wonder about water in the fuel (as the previous owner kept it in a warm garage, only out on sunny days etc.—it's now spent three or four days on the backs of trucks and ferries and trains etc.). I filled up the half-a-tank that was there, and added a few capfuls of meths in there to boot to absorb any water.

    It does seem odd because apparently there was no hint of this before the car was transported.

    Any advice appreciated!

    EDIT: and I suppose I should add a couple of photos of the thing

    Last edited by Twcwt; 20-06-2017 at 11:15 AM.

  2. #2
    Join Date
    Jul 2005
    Location
    Camberley
    Posts
    98
    Firstly congratulations on the Beat!

    I'd go down the list of usual suspects, starting with the easiest first. Note that revving it might be masking some misfire throughout the rev range:
    Give it a good run, it might just be some iffy fuel that needs cleaning
    Clean inside of dizzy cap and check for cracking, and check the rotor (and its cleanliness)
    Clean HT cables
    Replace spark plugs (they are cheap enough)
    Check air filter (you never know, it could be blocked)
    Check and clean out the air control valve
    Replace the fuel
    Replace ECU caps

    The dizzy being replaced was due to it seizing because of poor oil supply, so it is either completely broken or not.

    Hope this helps!

    Stu
    ! Stuei

  3. #3
    Join Date
    Jun 2017
    Posts
    5
    Cheers Stu!

    Yes very possible misfire is still happening with more foot on the pedal. It sounds pretty clean high up, but everything's firing so quickly and noisily in a Beat it's hard to tell

    I will start working through that list this weekend. The distributor failed because of bad oil feed? I was hoping they were just a sealed bearing that I could replace, damn. Didn't realise there was a separate oil feed.

    Yesterday I started with the ECU, as I was planning to do this regardless and could tackle it inside in the evenings (my garage is unlit, and it's rather wet and cold in the Antipodes at the moment).

    Only one cap that has leaked, but it made a nasty mess. Some new high-quality 105°C 5000hrs+ electrolytics should turn up from RS on Monday, but there is definite damage (and open circuits) on the PCB where the electrolyte has eaten away at the copper. It has wicked underneath the solder mask which will be tedious to fix. Worthwhile, having seen the cost for a new ECU though

    Hopefully it hasn't damaged any other components. There's a transistor array (QM2) nearby; I see with the Japanese cap kits they sometimes include this with the caps, so that's a bit of a worry. It doesn't look like an easily-obtained component.

    You can see the damage in the bottom-left third, out the bottom of C4. That black stain is not just an absence of copper!


  4. #4
    Join Date
    Aug 2002
    Location
    Kent
    Posts
    1,500
    I wouldn't discount a valve issue, I've seen a burnt out inlet valve due to a failed ECU which was causing a poor mixture and subsequently a misfire at idle, so its worth a compression check.

    If you do take the distributor cap off, look for any brown "rust" dust this is the first signs of a failing distributor, they are sealed bearing and takes a lot of stripping of a unit that was never actually designed to be stripped, so I would just replace the distributor if in any suspicion of beginning failure, thou this wont normally cause any other symptom until final failure e.g. snapped cambelt or sheared or slipped camshaft drivegear.
    Steve M
    __________________

    Its all gone Norfolk!!!
    Now in the Lotus position.

  5. #5
    Join Date
    Jun 2017
    Posts
    5
    Thanks Steve.

    I'm hoping it's not an inlet valve; I had a pre-purchase inspection done before I bought the car, and they apparently did a compression check which all looked healthy. I'll keep that in mind if after working through the ignition/fuel/air system doesn't fix it. I have a compression gauge, but it's one of those awful rubber-ended things... maybe acceptable for a crusty old air-cooled motorbike Whilst I didn't drive the car before I bought it, I trust the seller, and the AA inspection surely would have picked up such a misfire. I even spoke with the inspector later the same day to query an issue he raised on the report (Mugen steering wheel off-centre, something else to figure out); he was very positive in his feedback. I would think a burnt valve would happen over a period of time; this seems like something got shaken up on the train/truck/ferry journey across the country.

    I swapped the plugs last night. The ones I pulled out were a grade hotter than standard (BKR5E-11), but looked almost brand new. Insulator tip nice and clean whitish-tan, no erosion on electrodes etc. No leaky plug shaft seals yet, the plug leads/cap came out clean and dry.

    I'll clean up the distributor rotor/cap today. Yes a new distributor would be a sensible idea (new sensors), but they're not cheap or easy to get hold of in this part of the world. I've seen a few folks with more common Hondas rebuild rather than replace (despite not really being designed for that); even a few walkthrough tutorials on the job: https://sites.google.com/site/hondal...ributorbearing and http://techauto.awardspace.com/conte...orexploded.php . I have access to hydraulic presses, bearing pullers etc., seemed nice to stop more landfill. I was wondering about heat/dust from bearing failing causing issues with the sensors, good to know this isn't normally the case.
    Last edited by Twcwt; 25-06-2017 at 02:28 AM.

  6. #6
    Join Date
    Jun 2013
    Location
    Newcastle upon Tyne
    Posts
    51
    I don't know if this is adding anything to Steve's answer, but my Beat was suffering the same issue and I assumed the worst and thought it may be a valve issue. The misfire (and CEL) was solved by

    1) replacing the caps on the ECU
    2) resetting the idle.
    3) setting the timing.

    Hopefully this will sort it for you. You can get a dizzy from https://www.nengun.com/honda/oem-parts-beat if you want to buy new.

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