Coil question on 750 Sei
Benelli Motorcycle Forum - Benelli Bike Discussions - Powered by vBulletin
Page 1 of 2 12 LastLast
Results 1 to 10 of 11

Thread: Coil question on 750 Sei

  1. #1
    Junior Member
    Member #
    15417
    Join Date
    Feb 2017
    Posts
    8

    Coil question on 750 Sei

    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?

    Coil question on 750 Sei-img_2058.jpg

  2. #2
    Junior Member
    Member #
    15417
    Join Date
    Feb 2017
    Posts
    8
    This pic is a bit better I hope ..

    Coil question on 750 Sei-img_4292.jpg

  3. #3
    Member Cosmo's Avatar
    Name
    Cosmos
    Member #
    13514
    Join Date
    Feb 2014
    Location
    UK
    Posts
    48
    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.
    Coil question on 750 Sei-dsc_0101.jpg

    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.
    Last edited by Cosmo; 03-16-2017 at 08:09 AM.

  4. Remove Advertisements
    BenelliForum.com
    Advertisements
     

  5. #4
    Junior Member
    Member #
    15417
    Join Date
    Feb 2017
    Posts
    8
    Thanks! That is just what I wanted to know
    and you're right about the modern coils. I started to use the Honda coils on my Z1 and XS1100 rides and they work fantastic. And cost is next to nothing..

  6. #5
    Junior Member
    Member #
    16201
    Join Date
    Jun 2017
    Posts
    4

    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...


    Quote Originally Posted by Cosmo View Post
    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.
    Click image for larger version. 

Name:	DSC_0101.JPG 
Views:	175 
Size:	1.03 MB 
ID:	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. #6
    Member Cosmo's Avatar
    Name
    Cosmos
    Member #
    13514
    Join Date
    Feb 2014
    Location
    UK
    Posts
    48
    Quote Originally Posted by jamescheyne View Post
    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...
    That does sound like an overheating coil breaking down the insulation somewhere in the heat. They have a hard time up under the tank and that's why many old british bikes often mounted the Lucas coils under the seat or away from the engine down in the breeze.

    Replacement coils are available, but you would need to consider if you're keeping the points or have an electronica or aftermarket ignition - it would be worth considering improving them. There are like for like replacement coils available in 5 ohm and 3 ohm primaries, search for compatible chinese coils for Honda CB350/500/550 and 750F points ignitions - see note in above thread.
    I don't know where you are but with 30 deg heat it's not here in the UK, but a swift e-bay UK check found this HONDA CB 550 1979 (550 CC) - IGNITION COIL | eBay
    They're not great but are a design similar to the originals, just check your bolt spacings to suit.

    You have I hope, done the obvious and cut down the plug leads to check for clean shiny copper at the connections? The cables will be the first to go with age, corrosion and cracking and they will certainly short out when hot and show the symptoms you describe unless they caps and connections are good and the rubber leads are not cracked with age.
    Aftermarket coils are inexpensive and due to the moulded in leads after 35-40 years, I'd say change them anyway if you have the originals still in service.
    I hope this helps and see how you get on.

  8. #7
    Junior Member
    Member #
    16201
    Join Date
    Jun 2017
    Posts
    4
    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.

  9. #8
    Junior Member
    Member #
    16201
    Join Date
    Jun 2017
    Posts
    4

    Next steps to stop misfire?

    Quote Originally Posted by jamescheyne View Post
    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.
    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.

  10. #9
    Member Cosmo's Avatar
    Name
    Cosmos
    Member #
    13514
    Join Date
    Feb 2014
    Location
    UK
    Posts
    48
    Quote Originally Posted by jamescheyne View Post
    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.
    As you have retained the points, look at the condensers too. Excessive sparking between the points gaps are a giveaway of failing condensers and you may only have them failing when they get hot. They are cheap, so change them if not already.

    If you have checked all the ignition and it runs well when cold, it may be an obvious suggestion, but is the jetting/fuel screws & needles circlip positions set correctly to standard?
    If it runs good when "cold" it may be excessively rich if when the throttle is opened it "stutters" and a fully open throttle or a bit of choke retards progress when hot - this may be a check to do if not done already as an over rich mixture would also foul the plugs eventually. If the bike has ever had open carbs or K&N's, large changes may have been made previously to get it to run without an airbox and you can't assume anything is standard inside the Dellortos.

    I also suggest a hot and cold compression check. You may be unlucky and it is an oil burner and has tired piston rings letting oil by and fouling the plugs - but it is unusual not to have clouds of smoke out the back end if this were the case, so that would be very obvious when it got hot.
    Keep trying and Good luck!
    Last edited by Cosmo; 08-21-2017 at 05:42 AM.

  11. #10
    Junior Member
    Member #
    16201
    Join Date
    Jun 2017
    Posts
    4
    Quote Originally Posted by Cosmo View Post
    As you have retained the points, look at the condensers too. Excessive sparking between the points gaps are a giveaway of failing condensers and you may only have them failing when they get hot. They are cheap, so change them if not already.

    If you have checked all the ignition and it runs well when cold, it may be an obvious suggestion, but is the jetting/fuel screws & needles circlip positions set correctly to standard?
    If it runs good when "cold" it may be excessively rich if when the throttle is opened it "stutters" and a fully open throttle or a bit of choke retards progress when hot - this may be a check to do if not done already as an over rich mixture would also foul the plugs eventually. If the bike has ever had open carbs or K&N's, large changes may have been made previously to get it to run without an airbox and you can't assume anything is standard inside the Dellortos.

    I also suggest a hot and cold compression check. You may be unlucky and it is an oil burner and has tired piston rings letting oil by and fouling the plugs - but it is unusual not to have clouds of smoke out the back end if this were the case, so that would be very obvious when it got hot.
    Keep trying and Good luck!
    Thank you Cosmo. Thi is really helpful. I looked again at the points this morning and it looks like at least one of the points is bit blue. I will certainly replace the condensers as you suggest.

    Concerning the sooty plugs I have been driving the bike recently at about 3,000 to 3,500 revs but I have now run it at 3,500 to 4,500 and the plugs are a much healthier grey colour with much less misfiring when hot.

    Finally, thank you for thinking of oil in the cylinders but the bike has very few Kms on the clock and there is no trace of oil in any of the cylinders, or smoke out the back.

    I'll let you know about the change in performance with new condensers.

    Thank you again - you have been very helpful. What can I do for you? James Cheyne, Coppet, Switzerland

Page 1 of 2 12 LastLast

Sponsors

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Similar Threads

  1. Benelli Sei 750 Rear Wheel Question/Dilemma
    By Orlando6 in forum Classics
    Replies: 8
    Last Post: 06-09-2015, 12:48 AM
  2. Benelli Sei 750 filter question
    By Dutchman in forum Classics
    Replies: 1
    Last Post: 09-08-2014, 02:58 PM
  3. 750 sei
    By richardm in forum Classifieds-For Sale/Wanted
    Replies: 3
    Last Post: 01-03-2011, 02:45 PM
  4. 750 SEI and 900 SEI on Ebay US
    By shamrock in forum Classics
    Replies: 0
    Last Post: 04-16-2010, 05:03 PM
  5. Yet another coil stick question
    By whooshyafuker in forum Tornado Tre
    Replies: 3
    Last Post: 06-24-2008, 05:59 PM

Search tags for this page

benelli 750 sei coil

Click on a term to search our site for related topics.
Google+