1989 EXUP firing problems

Thermicman · Post by **Thermicman** » Thu Sep 18, 2003 1:39 am

Can anyone help, please? I have a standard 1989 EXUP which has done about 35,000 miles and I have owned it for 13 years. Two months ago, I started the bike (which is kept covered in a dry shed) from cold and, even after the choke was pushed in, it only ran on 3 cylinders. I drove 10 miles to work and home again on 3 with the bike refusing to chime in on 4. On taking out the plugs, I found one very slightly darker than the rest and found that this one didn't spark when I turned the engine over. I tried it on the other leads and got the same result ; no spark. So I went out and bought a set of new NGKs and fitted them. Result = Bike immediately back to normal again!! Two months later and less than 1000 miles on the new plugs, I had the same problem this morning when I started the bike from cold. It sounded a bit roughish on the choke and when I pushed the choke in, it still only ran on 3 cylinders. Both times this has happened, the bike was running perfectly the last time it ran and before I put it away in the shed. This has never happened before and the bike has been totally reliable. The leads, caps and coils are all stock and original. I don't think that it's a fuelling problem because on fitting the new plugs, the bike started and ran on 4 cylinders immediately. I have yet to go hame and take out the plugs again, but thought that I'd ask the 'Hood for any thoughts??? Thermicman

flyingcircus68 · Post by **flyingcircus68** » Thu Sep 18, 2003 5:51 am

I would start the investigation by checking the carbs out. It sounds like you might have clogged jets that would cause 1 cylinder to misfire, or it could be a problem with rich running. The float level might be high, or the choke might not fully seat and cause rich running. Also check for unrestricted venting on the float bowls. Check the carb sync while you are at it. After carb sync, pull off the carbs and look at the relative positions of the butterflies on the carbs. They should appear even. If they are uneven after adjusting for even vacuum, that could point to tight valve lash on the cams. Tight lash would not allow full seating of the valve and foul the plugs.

Thermicman · Post by **Thermicman** » Mon Sep 22, 2003 5:05 am

Thanks for the tips flyingcircus68! I took the plugs out and it was number 3 that was duff. I replaced all 4 with the new NGK iridium plugs (expensive!!) and also all 4 leads with NGK Racing silicone leads which have a long straight HT cap fixed to them as standard. The result = perfection!! Now, when I come to a very slow 2nd gear corner and take it with the revs down to 1500, the bike pulls away very cleanly and not at all "stuttery" like it used to before with the old leads until the revs got past 2- 2500.

Ton up · Post by **Ton up** » Thu Dec 04, 2003 6:03 pm

Thermicman, my '89 has the same stutter below 2500rpm. Yamaha tell me I can't just replace the leads- they supposedly won't come out of the coils. Did you replace the leads and the caps. If so, how did you go about it? Any info would be appreciated!

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