Carburator Fix – Help! 750 LTD S1 -82

Home Forums All Things 750 Twin Technical Garage How To Fix It Carburator Fix – Help! 750 LTD S1 -82

This topic contains 32 replies, has 4 voices, and was last updated by  Ankargren 1 day, 11 hours ago.

Viewing 15 posts - 1 through 15 (of 33 total)
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  • #24589

    Ankargren
    Participant

    Hi Everyone,

    After last year overall, i started the bike after a couple of month in the garage. To my surprise it did not run as when I left it the year before the season. In the beginning it only misfired now and then. After running approx 50km that summer i gave up and left it in the garage. The only thing i changed before I started it that summer was the spark plugs (to BP6ES ans the manual state).

    So I started my troubleshooting and a week ago I started with changing the coil cables and spark plug caps (NGK 5kohm) in hope of it would solved the problem. Tried and still misfire in low rpm and not able to increase the rpm without the choke. However, when running the choke fully and partly out it does not misfire and I can rev without any problems with the only backside the idle is VERY high. Back in the garage and my next thought was that the problems must be in the carburetors. Tared it a part as shown in the picture, but then I am out of ideas. What should I check? My plan is to ultrasonic wash the parts but I can’t find any obvious problems for why it run as it did. Do anyone have any ideas? The diaphragms looks strange but no holes or similar in them. Some dirt around a few parts but still see all jets are not clogged. How sensitive can it be?

    To summarize and to give you guys info that may help with ideas what can be wrong;

    The bike was running as it should when I left it before the winter. The only thing I changed during the first winter was the spark plugs.

    • Running new plugs BP6ES and caps NGK 5ohm)
    • Fuel is new
    • Petcock is overhauled with new seals and membrane.

    Any ideas what can be the issue? I do not think its the petcock since it runs OK with choke? Can bad diaphragm do this effect? What parts in the carburetor can cause this issue?

    All help VERY much appreciated since my patience for the bike start jingling, which is sad since I doubt there’s not many bike left in this outstanding shape. I spent a lot of money to restore in to original condition.

     

    David

    #24593

    kaptainkwak
    Keymaster

    Hi David
    Do not despare. I am sure we can help. My first thought is the fuel ‘varnishing’ with being stood over winter. Ultrasonic cleaning would solve that problem. The issue is definately fuel starvation on the pilot jets. This could be for several reasons with varnishing being one. Assuming the pilot jets are good and the right size the 2nd most likely reason is an air leak. Have you checked the air box to carb and carb to head rubbers are in good order and clamped properly? If they are perished they need to be replaced as they can leak air. If they are over tightened on the clamps they can become oval and not form a seal.
    One way to check the carb to head rubbers is to start the engine. Turn the tickover screw up so it will run without choke. Lift the carbs and push forward. If there is a leak there this should close it and the revs will increase.
    It is also worth making sure the pipes from the carbs to operate the fuel vacuum tap are in good order and well sealed.
    I hope the above gives you new things to investigate.
    Letus know how you get on.
    KK

    • This reply was modified 4 months, 1 week ago by  kaptainkwak.
    #24598

    Ankargren
    Participant

    Two more questions that came up, how should the idle jet screws shown in the picture?

    Also, anyone tried out the JBMindustries diaphragm replacement?

    David

     

    #24599

    Ankargren
    Participant

    Correction, how should the screws bet set to start with it?

    Sorry for the bad spelling πŸ™‚

     

     

    #24600

    kaptainkwak
    Keymaster

    There is no need to apologise for any spelling.

    The screw indicated is the fuel mixture screw and NOT for setting the idle speed. It determines the mixture of fuel and air that comes out if the carbs. Idle speed is set by adjusting the bigger wheel screw position underneath the carbs towards the back.

    To adjust the mixture each carb will need to be set seperately. If memory serves me well it should be between 1 3/4 and 2 1/4 turns out. Screw it down fully then back it out 1 3/4 turns. Do this on both carbs. I then use a colortune spark plug and start the engine. This lets you see the colour of the burning fuel. Adjust the screw until it is a nice blue colour. Repeat this on the other carb and the mixtures will be correct. After this task has been performed the carbs will need to be rebalanced.
    If you do not have a colortune plug adjust the screw until you get the fastests smooth tickover. Back it off slightly and bring it back to smooth. Again repeat on the 2nd carb. This may require further tweaks to get it spot on.

    • This reply was modified 4 months, 1 week ago by  kaptainkwak.
    #24603

    Ankargren
    Participant

    Thanks a lot Kaptain! This gives me new things to check. I noticed however that one of the rubbers between the carburetor and air box was rock solid and not very tight against the air box. I put them in some brake fluid in hope of getting them softer. I still can’t see its SO sensitive but I may be wrong. My experience is only 2 stroke engines and snowmobiles so can’t really say anything about these four strokes πŸ™‚

    I plan to change out the diaphragm just to ensure they are in good order, did anyone try the JBM ones? Looks like the only alternatives if not pay huge amount of money.

    Do you know a place where I can buy the new rubber boots for the intake and air box to carb?

    /David

    #24610

    Ankargren
    Participant

    Kaptain, any chance that you had time to look at my last reply? πŸ™‚

    #24611

    kaptainkwak
    Keymaster

    Hi David
    I believe the JBM diaphrams work well but I have not tried them myself. There was one of the German members who was making carb rubbers for the 34mm carbs. I will make enquiries and get back to you.
    One other way of sorting the airbox to carb rubbers is to use universal car steering rack gaiters and cut them down to suit.

    #24614

    Ankargren
    Participant

    Thanks a lot, I will wait and see if if there’s a german solution to my diaphrams πŸ™‚

    Let me know!

    #24634

    Michael
    Participant

    Hi David,

    good to see you here and Andy supporting you πŸ˜‰

    I have good news for you both and all the other LTD-riders.

    Our german member “Vau” is going the launch another series of air intake boots (LTD airbox to carb rubbers) shortly.

    We just started to collect the names of interested people and the minimum number of boots to make is 5 to 7 pairs. As I can see there is enough interest in new boots and currently the estimated price is 80,- Euros per pair (plus shipping).

    I have two sets out of the first series and they are perfect !

    Please stay tuned and Andy has already ordered a pair (or even more?).

    David, we will make sure a pair is for you πŸ˜‰

    Cheers, Michael

    https://www.z750twin.de/forum/viewtopic.php?f=6&t=2544&start=190

    #24646

    Ankargren
    Participant

    Hi Michael, Nice to hear from you again! I order the JBMΒ diaphragm and will see how it fits, get back with pictures, it is way cheaper then trying to buy new ones so will give it a shoot.

    Regarding the boots, I put them into DOT3 fluid for 2 weeks and now let them sweat for some while, one of them is as new and soft, the other one is OK but in some areas still rock solid, but still no problem get them on and tight, i think. When they are installed I will get back to you if I need new boots πŸ™‚

    My problem now is the humping on idle so I am order a Ultrasonic cleaner and hope it will help.

    David

    #24647

    Michael
    Participant

    Hi David,

    besides carb issues, bad idling can be caused by loose springs in the ignition timing advancer.

    Just remove the ignition cover and turn the advancer with a lot of feeling. It shouldn’t have any play in rotation direction, as the two small springs turn it back to the starting point. If you can turn it just a little bit without any force against the springs, the springs are worn and need replacement. The only thing is, these springs aren’t available as original spare parts. You have to find other solution to fix it.

    A heat shrink tube on the pins that hold the springs may help.

    Cheers, Michael

    #24670

    kaptainkwak
    Keymaster

    That’s a good point Michael. We are currently looking into getting new springs manufactured. I will keep everyone updated of any progress.

    #24684

    Ankargren
    Participant

    Would these spring cause it to hump also through the who rev range? My problem is that is will idle without the choke on, but as soon I try rev it even when its hot it start to hump. If I put the choke on it will rev as normal but with high idle rpm……

    The bike has only 22000km on it (honest km).

    #24733

    Ankargren
    Participant

    Tonight I replaced the diaphragms, really impressed by the fit and quality of the JBM stuff. Next on line is the ultrasonic wash of the carburetor (the supplier sent me the wrong washer so waiting for a new one).

    I really hope the wash will fix my problems πŸ™‚

     

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