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82 XJ550R Seca won't rev past 8K - Help!

Discussion in 'XJ Technical Chat' started by Mordred, Jul 17, 2021.

  1. Mordred

    Mordred New Member

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    Hey all,
    Wondering if you can help with my problem.
    I have a '82 which I built into a cafe-racer, I attached a photo for anyone interested.
    It pulls up until 7,500 ish RPM, then will hesitate, judder and struggle to rev any higher, with very little power (In any gear), with wide-open throttle, or just sneaking up on it.
    It has had a re tune, and the issue still persists.
    The top-end and pistons were bored and rebuilt recently too, with valves in spec.
    It has pod-intakes and an aftermarket 4-into-2 exhaust, which never caused any significant problems.

    Any recommendations? I was thinking either the floats, or electrical, like a weak spark?
    It starts and idles fine, and runs lovely up until high rpm.
    Thanks for your help.
     
  2. Mordred

    Mordred New Member

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    Trying to get around the spam filter to post this, doesn't seem to like my jpg or imgur links.
     

    Attached Files:

  3. Jetfixer

    Jetfixer Well-Known Member

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    With pods you have to play with jetting , for idle and low speed two sizes from stock will work . But if you go up to much it can cause other issues. Check plug tip color , the other item you could have a coil breaking down ...do the coils have cracks in the case ?
     
  4. Mechanic1978

    Mechanic1978 Active Member

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    ignition advance-- it controls spark timming at set rpm ranges, if its not functioning it will do that. run into this all the time in hot rod engines brought to me, they dont know how to set the points or adjust the distributor for advance timming, and can cause backfires as well. a distributor in an old rod engine is a "mechanical" advance.

    Yours is electronic and is pre programed into the TCI unit. so if it runs exellent up until that range then Id say check out the pickup coil on the end of the crank, test it for its specified ranges, and inspect it for cracks in the plastic casing.

    is it the same issue when cold vs hot?
     
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  5. Mordred

    Mordred New Member

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    Not that I can see.
    I'll talk to my bike guy and see if he has a jet kit in stock - I'll see if it makes a difference!
    Thanks again for you help.
     
  6. Mordred

    Mordred New Member

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    I'll take a look this week at it.
    From what I remember it was spotless, looked like it came right out of the factory.
    I haven't really revved it out when cold for fear of damage, but I'll investigate further.
    Thank you for your help. I'll get a jet kit as Jetfixer suggested, and if that doesn't fix it I'll start testing the pickup.
     
  7. Mechanic1978

    Mechanic1978 Active Member

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    Its either a severe restriction of fuel, or the timming advance. How much of the cylinders were bored? .030,0.020??
    when you enlarge the cylinders you physically alter every other thing in the engine, higher temps, more fuel, potentially more compression etc.. this kind of work will require the jets to be huge compared to the OEM sizes, I'd guess atleast 4 to 8 sizes up on the main jets, and 3 to 6 sizes up on the Idle. also maybe a 1 to 3 size up on the choke/enrichment circuit if it has them.

    Higher compresion will require a tuning in the fuel and timming advance curve and in some cases the adition of a cooling fan and or valvetrain work (larger valves/higher lift cams), thus your OEM TCI (since it ca not be flashed) may not be able to run this engine "properly" any more.
    Your attempting to run a stage 2 modified engine on an oem controller. you have increased the bore displacement, but not the stroke, so that potentially may very well be your "new" semi sorta redline. I doubt it, but can happen, without propper tuning and performance part matchup.

    I have built a plethora of race engines, and here is what I recomend you to use for your ignition system, it is fully tuneable, and will allow you to adjust the advance curve, and you can use your OEM pick up coil. I have built race engines from atv's on alcohol and or turbo, to mopar big block 440 magnums, and seen an 871 roots blower explode out the side, the engine work that you said has been done has opened a can of worms, thats a lot larger that you realize. Most people think a part here, a part there and some adjustments and i'm good. and for professionally designed "bolt on" parts that's usually an acceptable result, but remember this, every single part on an engine is in CO-OP with each other. The preverbial "Link in the chain" sorta speak. and whoever did the work needed to ensure the parts used

    https://imfsoft.com/en/category-control-units/master-mini-ignition-tci#toc-variants

    this unit is known as the "mini master"... and it is!
     
  8. Mechanic1978

    Mechanic1978 Active Member

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  9. Minimutly

    Minimutly Well-Known Member

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    This is the biggest load of nonsense I have ever read on this forum. A rebore will never ever stop an engine revving out. Simple.
     
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  10. Mechanic1978

    Mechanic1978 Active Member

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    so instead of acting like a child and trying to say something is BS, why dont you tell us why you know this to be true...how many modified engines have you built, how many modified engines have you tuned, raced, etc... its easy to sit behind your rebel keyboard, but if you cant explain your theory, its just blahh blahh... bu blah. I know guys like you.
    he has a cylinder bored out, with Oem cams,valves,carbs, raised temps, etc... I would love to see one of your engines on my bench...ohh how much fun that would be...haha..:)

    Here genius, check this page here I found for your education, pay paticular attention to the verry first paragraph about loss of power, and the lower paragraph titled:
    WHY COMPRESSION MATTERS lower in the article.
    https://www.enginebuildermag.com/2018/05/engine-math-for-engine-builders/

    I dont need to prove anything to anyone haw good I am, I know Im that good
    I have been educated and trained by
    General Motors finest from 1994 to 2009, I worked for a Chevy dealership for 6 yrs, and a Buick dealership for 9 years, tooak every class they offered, I was also sent on a paid in full trip to
    "Detroit Michigan the Motor city" for speciffic training courses only offered to each dealerships top 5 techs.

    I have nothing else to say to you in this thread. I dont want to be confrontational or have any issues here. I'm here to help and spread some knowledge. So PM me please, I have plenty of time to put you in your non educated place.
     
    Last edited: Jul 18, 2021
  11. Minimutly

    Minimutly Well-Known Member

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    I've no interest in keyboard arguments, but to justify my reasons for disagreeing here. A simple rebore - without calculating it, is likely to add 5% (say) to swept volume. Sure compression ratio, air volumes will both go up by a similar amount, but only at peak torque. Once past this the increases will reduce. The xj carb, whichever make is a cd/cv device that flows the air required based on the slide position - this in turn is governed by air flow and rpm. at peak torque the slide is midway up maybe? By the time peak rpm is reached, the extra air requirement will be reduced by inlet/outlet restrictions, so the supposed 5% could be 2 or 3%. Are we saying that between 8k and 12k the poor old carb will be unable to supply air and fuel at this tiny extra amount? If so why do the mikunis (unchanged from an xj600) work so well on my 650?
    So a rebore might require a richer/higher needle, maybe main jet to correct any additional fuel requirement, but it's going to be a small correction. Not enough to make the xj hit a brick wall, ever.
     
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  12. Polock

    Polock Well-Known Member

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    maybe, just maybe they were trying to get rid of you for a few days...?
     
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  13. Mechanic1978

    Mechanic1978 Active Member

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    you gave a vague description of the engine work performed, by who(you? a shop?) if it was you, what skill set you have etc.. what all parts are new/old in the engine, was this issue present before the rebuild, happen righ after, or run exellent for a while and then just sudenly happen?
     
  14. JBurch

    JBurch Active Member

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    My 2 cents; to the OP, was a valve job done on the head? You stated the valves are in spec, I will take that as the clearances are in spec. My opinion on why the engine won't rev is that valve spring pressure is weak. If a valve job was done; grinding not lapping; will change the installed height of the springs which will also change the seat pressure; make it weaker; which will also change the open pressure; make it weaker; which will affect how the engine will rev. If a valve job was done, and springs not shimmed back to installed height spec could explain why it won't rev, it will be all hunky dorry , start, idle, rev to a point where valve control is lost.

    My opinion.
     
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  15. Mordred

    Mordred New Member

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    Hey Jburch,
    The valves were lapped and springs adjusted by a qualified shop, and correctly shimmed by myself.
    Hoping it's not the issue - not hoping to take the engine apart mid summer
    Thanks for your help. I'll add it to my list of things to test.
     
  16. Mordred

    Mordred New Member

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    The top end was cleaned up by a shop, valves cleaned and lapped (one replaced). The cylinder walls were bored only to rid it of the hourglass cylinder walls, and pistons were cleaned up and rings replaced.
    On the recommendation of the shop we stuck with the original pistons and same rings, as the wall removed wasn't significant enough to warrant oversize rings.
    The issue was only noticed a couple weeks ago, while on a spirited ride, so I believe it to be a recent problem.
    While I don't have your experience I am competent in my abilities to follow the manual and not screw things up.
     
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  17. Mechanic1978

    Mechanic1978 Active Member

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    Exellent to see someone with a similar viewpoint, and some obvious knowledge that can contribute to the posted issue.. thats similar to where I was going In my later post, but did not know what all parts had been reused. I also assumed the valves were original and just had shim tolerances checked. Also the Op has not responded since he posted, so at this point, all were doing is speculating..

    this is a small snip of the info at the link I provided above, and is the basis of my second/3rd post.

    WHY COMPRESSION MATTERS
    Compression affects power, the octane requirements of the fuel and ignition timing. Higher compression ratios increase the thermal efficiency of an engine and allow the engine to capture more of the heat energy that’s released during combustion. That’s one reason why high compression diesel engines are more fuel efficient than gasoline engines.

    From what I gatered from his limited info, was the only new parts were the cylinder bore, and a piston. I had to assume everything else was OEM. this will alter the compression ratios, thus altering his timming then altering the timing of the advance curv.


    I seen you responded with more info... so cylinder bore or honeing was done. if it was bored, you can't re use the old pistons, the rings will never allow a full seating, and extensive blow-by will occure resulting in lost compression and/or piston slap. do a compression test, and check the crank case for excessive pressure, also check to see if your oil level is disappearing.
     
    Last edited: Jul 19, 2021
  18. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    I have been a professional educator for over 15 years. I still have to prove myself every day. Even experts can be wrong.
    Be humble, because high horses tip over easily.
     
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  19. Polock

    Polock Well-Known Member

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    new plugs are cheap and easy, might be all it needs.
    does that pdf picture have white lines in it?
    looks good
     
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  20. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Boring an engine oversize will not result in a higher compression ratio; in fact, it will do exactly the opposite, it will LOWER the CR, as long as stock style pistons are used (and on the XJ engines, typically you purchase standard oversize pistons, + .25mm, + .50mm, or +1.00mm) and as far as I know and can tell, they do NOT have any extra dome material nor pin location changes to "adjust" the CR back to the stock ratio:

    And in case you are interested, here are the standard compression ratios for the various engines:

    XJ550: 9.5-to-1
    FJ600: 10.0-to-1
    XJ600 Seca II: 10.0-to-1
    XJ650 (except Turbo): 9.2-to-1
    XJ650 Turbo: 8.2-to-1
    XJ700 air-cooled: 9.5-t-1
    XJ700-X and XJ750-X water: 11.2-to-1
    XJ750 air: 9.2-to-1
    XJ900: 9.6-to-1
    XJ1100: 9.0-to-1
    XS1100 9.2-to-1


    Now, boring and fitting new pistons and rings will restore the compression pressure, but of course that is a function of simply installing new, properly fitted and sized parts, and has nothing to really do with bore size......
     

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