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Kz750n Spectre 1982
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Discussion Starter · #1 ·
Hi all, I have posted on this forum before about my kz750n, it is 40 years old and I know no history of it. Until today it has not run for about 15 years.
I have set the tappets (all good), stripped carbs and refurbished,
Now my problem is it will tick over OK but will only go to 3000rpm, it is early days yet and may just need a good run etc but any advice/help would help me.
I might be asking for help before I need it but any advice would be greatly appreciated.
Many thanks Ian
 

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Ok if carbs are clean, air filter is clean and if supply of fuel is sufficient, then I think it could be:

1) Valve timing. Have you checked compression?
2) Your ignition timing advance is not advancing.
3) If you have CV carbs, the diaphragm is torn or has a pinhole or there is too much friction to raise the slides.
4) make sure that mice or squirrels have not built nests inside the airbox or in the exhaust system. Check for equal flow from all exhaust outlets.

Compression is easy to check with a simple, cheap tester.
Ignition timing can be checked with an ignition strobe light.
 

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Kz750n Spectre 1982
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Discussion Starter · #3 ·
Thanks for rapid reply!!! Compression was checked before tappets were set and not checked since, timing not checked, on my previous resto (XV500) I did have rodent infestation in airbox, in this instance it is not the case. The carbs have had a good looking at.
I am probably asking questions before I have gone through the process of elimination and to which I apologise now, I am not a mechanic and just making enquiries.
I will check all suggestions and reply with the results later.
Many thanks Ian
 

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Since the bike runs now and starts up, your valves are probably okay. Re-check everything anyway. Limited rpm kinda points to carb jet problems or as WFO suggests, carb slides not rising ie and needle jet not rising. Are those Keihin carbs? Does the needle have position notches? Ignition maybe not advancing but it would have already advanced at 3000rpm. Keep checking the carbs.
BTW, what exactly happens when you give it more throttle at 3000rpm? How does it idle? Does the choke work? Curious minds would like to know.
 

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Kz750n Spectre 1982
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Discussion Starter · #5 ·
Yes Keihin carbs, with rock hard rubbers!¡!, it might be i have to remove said rubbers so I can see slides moving when motor running, not really looking forward to removing rubbers as they were a [email protected]#£=rd to get back in and I haven't seen any replacements (soft or hard) to buy,
Chokes are of the butterfly type, very simple and working.
All 4 exhaust headers get hot, therfore firing on all 4.
Don't know if needles are adjustable.
I will confirm what happens at 3000rpm when I run bike again.
The bike has only run for a few minutes (maybe 5) on a remote tank, with loads of positive waves (re Donald Southerland(Kelly's Heroes)) it might just need more running to clear cylinders .
I have no history of the bike other than it was imported from Minnasota to UK this year and purchased by me from the importer approximately 4 months ago with 32000 miles on the clock.
The bike overall is in very good condition (40 years old) and if it had spent all of its life in the UK would now be a pile of rust and corrosion not fit for junk .
Carbs have had basic bench setup, good clean , and have all the correct/standard jets etc . Diaphragms checked and appear OK.
All electrics work as they should .
I have not really checked for vacuum leaks but I can't see anything obvious at this time.
Not been running with airfilter in but would have thought it would rev freely (at least over 3000rpm) even if a bit lean.
No rattles/bangs/clunk's or any unusual noise from motor, some smoke from exhaust but I think that's just the oil I put into cylinders before turning crank over (by hand) when first purchased and should clear (hopefully) when run for a bit longer.
I know this is a lengthy reply but I'm trying to pass on as much information as I can to clarify what I've done and hopefully give anyone a better understanding of what I've done (or not )
Not had new spark plugs or HT leads but it does appear to have a strong spark, plugs gapped at 0.7mm as per manual and does start easily.
Any advice /suggestions please let me know, I am not a mechanic but competent with wrenches/spanners etc and things I feel confident about maybe isn't to someone who actually knows what they are doing!
Best regards Ian
 

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Kz750n Spectre 1982
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Discussion Starter · #6 ·
Just an enquiry, if I were to take the airbox out and use pod filters instead does anyone know what jets I would have to change and what size? All else is standard, just asking.
Regards Ian
 

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Imported from the USA? How much did you pay for that bike? I have some in good shape I'd like to sell.
On your topic, stay away from pods, not good with Kiehin CV carbs! You'll have more problems. Replace the carb rubbers if they are stiff, they should be pliable and easily folded over. Stay with stock air box, least trouble.
Does your bike look like this 1980 KZ750?
Wheel Tire Fuel tank Automotive fuel system Automotive lighting
 

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Just an enquiry, if I were to take the airbox out and use pod filters instead does anyone know what jets I would have to change and what size? All else is standard, just asking.
Regards Ian
If these are CV carbs I would not go this route. It usually results in a world of hurt trying to get it to run right. Have you tried to get new carb boots for it? You do know you can soften the old ones, right?


 

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Discussion Starter · #9 ·
I think you might be surprised at how much I paid, 820 usd (700gbp), the importer has to buy it ,import, pay import taxes and make a profit, I've heard that yourselves over the pond can buy them for next to nothing,
In the UK any vehicle over 40 years old is then considered "historic " and no longer needs annual road tax or annual road worthiness examination, a combined cost of around £100 per year , another plus of owning a classic/historic bike.
If I were to sell mine I would expect to double my purchase price, but to be honest the amount of time and money spent on it I might make £200 profit.
Regards Ian
 

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Discussion Starter · #10 ·
I've heard of trying to soften the rubbers with some sort of silicone, I tried it with a silicone spray I have and let it soak in a plastic bag for a few days but it didn't work at all, if you have a way of softening them then please tell me,
The reason for removing the airbox was to do away with the rubbers as 2 are very hard, the outside ones, I suppose they deteriorated more due to the weather. I would prefer to keep the airbox.
Regards Ian
 

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Just seen other videos and the simplest looks like simmering for about 45min in wintergreen oil, who would of thought that?
Ian
I would not simmer wintergreen oil. Breathing those fumes can be dangerous. Wear gloves while handling it and use eye protection. I used it in a well ventilated area and still used a respirator for protection.

The text in the comments section of the video link provides the details of mix ratios etc. Cheapest source for wintergreen oil will be at a horse supply store.
 

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Discussion Starter · #15 ·
Hi guys, I am absolutely shocked/stunned/amazed, when started just now I still had the same problem of not going above 3000rpm, no vac leaks, ticks over nice, it had me baffled, so I thought I'd put in the air filter because it couldn't hurt, WOW, it absolutely changed it , revs freely upto red line when given a blip on the throttle. Even though I've seen it with my own eyes I still find it hard to believe, I dare say that the more knowledgeable on this fine forum could explain the vast difference between filter in or out, but at the moment I'm ecstatic it works, the exhaust notes are pretty good also.
As for the hard rubbers I shall leave them for now (don't mend it until its broken).
Obviously carbs still need balancing etc, but for now all good.
Also stopped smoking now its cleared out the cylinders, not done compression check but by the way it runs I'm not sure its needed.
Best regards to all Ian
 

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Well that's great news. I was going to mention the importance of the airbox but forgot to.

A lot of time, effort and engineering goes into an airbox. It is not just a hunk of plastic, it is actually tuned for maximum performance and your example has proven its value.
 

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Discussion Starter · #17 ·
Some more little jobs to do, then it'll be good to go, ie oil change and sort out leaky sump plug, hopefully the previous owner hasn't damaged the thread and a dowty washer will fix it,
I am having issues bleeding the front brakes, has anyone any tips or advice? I've bled brakes before and not really had any problems, already put half a pint of new fluid through it and it's still not right, I can pull lever to the bar but doesn't "pump up ", do I need new brake hoses?
Best regards Ian
 

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Stock air box is best. As for brake bleeding...what I like to do is connect a vacuum pump to the bleed nipple and suck the brake fluid through while pouring the new fluid into the reservoir. Gets rid of any air and flushes out the old fluid.
I saw a trick that BMW does at the factory where they pump the fluid through, but then they have special attachments for that procedure.
 

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Discussion Starter · #19 ·
It's not really surprising what you can do when you have unlimited funds for all the best equipment !!!! I only have a small garage/workshop and to put in everything I could use to make life easier for me would fill it 3 times over plus how many times would I use any such equipment? So like many home "mechanics " I make do with what I've got and only buy new/used equipment when absolutely nothing else will do. Necessity is the mother of invention. I have a compressor powered "easy bleed " kit but no compressor, I'm hoping with a bit of enginuity can make it work off a tyre inflator ( very low volume), I don't know if it will work but I can only try.
New braided lines are about £45 for a full front set (3)
Best regards Ian
 

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Making do with little makes you diagnose harder, and I have found also makes you a better diagnostician. You also learn more. The air cleaners maintain a lot of air restriction and the carbs are set up to compensate for that. Take that out, the bike runs too lean, and too much air, not enough gas, the bike will barely run. What ever you do, do not put in "Pods" on the carburetor. The stock air cleaner will remove particulates much more efficiently from the air, and your motorcycle will last 10's of thousands of miles longer. Bikes with "Pods" tend to eat the piston rings up. People say "I've never noticed that?" because they did not keep the bike long enough. I have ripped enough engines to see the long term result. That's a nice looking bike you have there. I spent 3 months in England, outside of Manchester back in 1974. You're not far from some really nice roads to ride on. I really enjoyed the Town of Glossop, up by the Moors. beautiful countryside, motorcycle country, back then anyways. I happen to own a 1973 M.G. Midget. I've blueprinted the motor, modified quite a bit actually, using mostly British parts. I am in the process (long term) of doing some resut repair. I love that car. I drove if 4 seasons, for 7 years before I redid the engine, which has about 8500 miles on it's rebuild. :)
 
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