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Coil question on 750 Sei

4K views 10 replies 3 participants last post by  Cosmo 
#1 ·
Hi Guys ,
last year I bought this one, an early 750. The engine was in a hardtail homemade chopper frame and the rest was stored in baskets in a cellar. After cleaning it looked too nice to restaure and the engine ran excellent and smooth so I am just rebuilding it.
My question is this; the stock bike has 4.7 Ohms standard coils and the engine in the hardtail frame was running on Honda CbX1000 coils with 2.2 Ohms, although with different plug caps and cables (no resistors).
What is the story on the coils? Can I keep using the Honda ones as the stock ones are fried?

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#3 · (Edited)
Hi,
If they are working with good blue spark, stay with them, but if you are changing to an electronic system or similar upgrade check with the manufacturer of the system. Sachse specify that 2-6 Ohm coils are compatible for their electronic system, but points systems are forgiving.

If you're sticking with points any replacement twin lead coil with correct mount spacings for older points based bikes will work just fine from Honda, Kawasaki or Suzuki. Unless you are going for performance, Dynas etc are probably not needed and I believe the more common upgrade is Honda "TEK" twin coils of 3 Ohms or so (generally CBR 600 or similar but mount spacings differ) as they give a strong boost in performance at breaker costs and the leads can be replaced which is usually the main problem with the old ones rather than the copper windings inside. See photo of my 504/820 Sei special to see what I mean.
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Aftermarket chinese coils are readily available for 1970's Honda points equipped bikes with similar spacings, but the quality is questionable and the leads are moulded in so the same problems occur as the O.E. type.

I'd stay with what you have unless they are obviously failing.
Hope this helps.
 
#5 ·
750 sei misfires when hot

1978 750 sei. I have set the carburetors correctly and the bike starts instantly and runs really well until in get hot. That is, after 30 minutes driving in 30 degrees C weather (90F) with frequent stops at traffic lights. It misfires so much that the bike becomes undrivable since the engine cuts when revs drop below about 2,500.

I have heard that a hot ignition coil can cause this. If so, how can I test a hot coil and where can I get a replacement, please?

I'm stumped...


Hi,
If they are working with good blue spark, stay with them, but if you are changing to an electronic system or similar upgrade check with the manufacturer of the system. Sachse specify that 2-6 Ohm coils are compatible for their electronic system, but points systems are forgiving.

If you're sticking with points any replacement twin lead coil with correct mount spacings for older points based bikes will work just fine from Honda, Kawasaki or Suzuki. Unless you are going for performance, Dynas etc are probably not needed and I believe the more common upgrade is Honda "TEK" twin coils of 3 Ohms or so (generally CBR 600 or similar but mount spacings differ) as they give a strong boost in performance at breaker costs and the leads can be replaced which is usually the main problem with the old ones rather than the copper windings inside. See photo of my 504/820 Sei special to see what I mean.
View attachment 21449

Aftermarket chinese coils are readily available for 1970's Honda points equipped bikes with similar spacings, but the quality is questionable and the leads are moulded in so the same problems occur as the O.E. type.

I'd stay with what you have unless they are obviously failing.
Hope this helps.
 
#7 ·
Thank you. This is really helpful. As you suggest I will do all the simple things first and then look at changing the coils. My bike has only run about 25,000 kms from new so I would like to keep it as original as possible. I'm a bit of a nut that way: if someone wants electronic ignition why not buy a bike that has electronic ignition.... Thank you again for your help. P.S. I live near Geneva.
 
#8 ·
Next steps to stop misfire?


Hello,
Thanks you for your help. I have now:
  • replaced all three coils with new ones of original specification,
  • replaced the HT wires and plugs (to NGK D7EAs)
  • re-balanced the carburettors and checked the float levels

The bike runs even better now for the 15 minutes and then the misfiring starts again. The weather is cooler now too so perhaps the ambient heat was not part of the problem. The new plugs get black quickly, however.

Do you have any other suggestions for me, please? The 750 sei is such a beautiful bike that it's frustrating for me not to be able to get it right.

Cheers.
 
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