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  1. #1
    Senior Member PyroMcnoob's Avatar
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    Carb Sync VS. Valve Adjustment

    I need the mechanics' opinions. My Gen2 700 is runnin' strange, idling rough and hesitating on take off. I need to know which is the better (and cheaper) route, carb sync or valve job. I'd heard valve jobs on the gen2's are a pain in the neck, and I have a buddy who has a carb sync kit. He says he talked to some forum folks and they told him specifically NOT to do my carb sync until my valves had been adjusted; I think he made that up.

    So what do you guys think? Get the carbs sync'd, or pay someone to do the valve job (cuz I'm pretty much NOT a mechanic)

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    Uber Guru jasonsmith's Avatar
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    I guess it really depends on what kinda maintenance you have been doing on the bike. When was the last time your carbs were cleaned, valves checked, carbs synced, air cleaner changed, fuel filter, spark plugs etc. Back to your question though I don't think it would do any damage to sync your carbs if your valves are out, it may just be a big waste of time. When was the last valve job and why do you think it may be the valves?

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    Member deanc's Avatar
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    Definitely valve adjust before the carb synch. OTOH,~if~ you've done a valve adjustment recently, probably shouldn't need another one soon.

    Your buddy was right. Valve adjust first. This is per the Honda Common Service Manual; the Honda factory service manual; and the Clymer manual.

    Valve adjustment is more expensive. A newbie or home mechanic might take on a carb synch, that's not too difficult with the right tools; but leave the valve adjustment to a pro unless you have lots of wrench time.

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    Uber Guru Lgn001's Avatar
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    FWIW, the reason valve adjustment is usually checked before the carbs are synced is to ensure that all cylinders are mechanically "the same", with regard to their ability to displace air. If the carbs are synced with the cylinders displacing varying amounts of air, each cylinder will produce a different amount of power. If the differences are great enough, it will feel rough. As jasonsmith already pointed out, it won't hurt anything, but it is kind of pointless unless the carbs are really far off.

    It could be that new spark plugs and a carb sync might make a world of difference. If you don't know when the valve clearances were last checked, however, it probably should be done. It's a lot easier than rebuilding or replacing a motor.

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    FWIW, the reason valve adjustment is usually checked before the carbs are synced is to ensure that all cylinders are mechanically "the same", with regard to their ability to displace air.
    The valves, floats, etc. should be checked ahead of a carb sync. This check is usually just acedemic if your machine has been taken care of.
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    Valve "Job" usually means head removal, grinding of the valve sealing surfaces and lapping of the seats.

    I think you refer to a "valve adjustment" which is verifying and adjusting the gap between the valve stem and the rocker surface that forces the valve open. As the engine surfaces wear, you can have valves that aren't opening evenly. It's a miniscule amount and unlikely to have a signficant affect on vacuum balancing. On paper - it absolutely affects the balance of the engine. But, with the type of gauges used by the backyard mechanic bought at Harbor Freight etc, most backyard mechanics couldn't adjust each carb in a precision enough of a manner to know the difference. If a valve was far enough out of wack to have a significant affect on vacuum available to it's respective carb, there would be noticable clattering and vibration in the engine.

    The bigger issue is that neglecting periodic valve adjustments will cause premature wear and tear on the valve train like mushrooming of valvestems for example. Secondly, you have to remove the carbs to do the valve maintenance/adjustments. AND, whenever you remove/install the carbs, it is important to do the vacuum balance because handling of the carbs will cause them to get out of sync.

    So, sync'n carbs before a required valve adjustment would be a waste of time. UNLESS you wanted to balance them as part of trouble shooting. You could certianly sync the carbs to see if it's related to the problems you mentioned in the engine operation. I would suspect however you likely just need some routine maintenance; valve adjust, carb clean, spark plugs, fuel filter, carb balance/sync etc.

    Edit: sorry, I reread your post and see you referred to the maintenance as both a valve job and a valve adjustment so I'm sure you were aware of the difference.
    Last edited by tinkerinWstuff; 01-22-2010 at 01:09 PM. Reason: add
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  7. #7
    Uber Guru squirrelman's Avatar
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    YOU should do a proper compression test first with the engine warmed up.

    Rear cylinder on top, front below.....



    Last edited by squirrelman; 01-26-2010 at 08:18 PM.



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  9. #8
    Uber Guru Lgn001's Avatar
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    Quote Originally Posted by tinkerinWstuff View Post
    ... If a valve was far enough out of wack to have a significant affect on vacuum available to it's respective carb, there would be noticable clattering and vibration in the engine. ...

    ... Secondly, you have to remove the carbs to do the valve maintenance/adjustments. ...
    Not to be an insufferable pain here, but I felt I should point out the following for the sake of clarity;

    If the valve clearance was too tight, you wouldn't hear it, and a tight exhaust valve is generally what can lead to a catastrophic failure. And FWIW, I can't remember any bike where the carbs have to be removed to check the valve clearances. Did you mean if you have to do a valve job, as in grind the seats and valves?

    I will make an exception to carb removal for clearance check; newer Toyota V6 engines need to have part of the intake manifold disassembled to remove the driver's side valve cover. That's not as bad as having to remove the entire intake manifold to replace the starter motor on the Tundra V8's, however. I hope Toyota doesn't decide to build a motorcycle...

  10. #9
    Senior Member PyroMcnoob's Avatar
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    thanks for all the info guys... seems I may be paying someone to do a full-on tune, because quite frankly I can't have my daily ride out of commission for the length of time it'd take me to do the work. I basically suck at anything involving tools haha
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  11. #10
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    Quote Originally Posted by Lgn001 View Post
    And FWIW, I can't remember any bike where the carbs have to be removed to check the valve clearances. Did you mean if you have to do a valve job, as in grind the seats and valves?
    I made a general statement and probably shouldn't have. Maybe I could have said "you MAY need to remove the carbs..." The OP is riding a gen2 700. I assume the gen2 is pretty darn close to my gen1 500 where you need to remove the carb bank to get the engine out of the frame. Removing the engine from the frame is by far the easiest way to do a proper valve adjustment.

    But to your point - my 2003 Shadow 750 probably doesn't need the carbs removed.
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  12. #11
    Uber Guru Lgn001's Avatar
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    Quote Originally Posted by tinkerinWstuff View Post
    I made a general statement and probably shouldn't have. Maybe I could have said "you MAY need to remove the carbs..." The OP is riding a gen2 700. I assume the gen2 is pretty darn close to my gen1 500 where you need to remove the carb bank to get the engine out of the frame. Removing the engine from the frame is by far the easiest way to do a proper valve adjustment.

    But to your point - my 2003 Shadow 750 probably doesn't need the carbs removed.
    Wow. That surprises me, for some reason. Honda is usually pretty decent about making things serviceable. Except for VTEC engines and linked brakes and, errrr... Never mind! :)

  13. #12
    Uber Guru squirrelman's Avatar
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    The photo i submitted above should be sufficient to dispell any idea that carbs need removal for valve adjustment.

    Radiator, however, must be removed, but it's no big deal.



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  14. #13
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    Quote Originally Posted by squirrelman View Post
    The photo i submitted above should be sufficient to dispell any idea that carbs need removal for valve adjustment.

    Radiator, however, must be removed, but it's no big deal.
    Can't tell from your photo if the engine is still in the frame. Is the gen2 700 that different than the 500 1st gens?

    It's easier to remove the engine than the radiator. (at least on the '85 500) I've done it both ways.

    As I said, you can adjust the valves with the engine in, but when you get to the point where everything is removed to gain access to the valves, you're 10minutes away from having the whole engine on the bench.

    You'll spend more than 10 minutes f'n around trying to make fine adjustments to the valves and doing it properly. Therefore, it's far easier to just remove the motor.
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  15. #14
    Senior Member PyroMcnoob's Avatar
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    Well blokes, got my valve adjustments all done ^_^ and didn't even have to remove the motor from the frame.... Had my buddy help me, learned a lot and would feel confident doin' it myself in future...

    Just for the record, on the gen2 VFR700 this is fairly easy... all it takes is a feeler gauge and a couple hours... as it happened, my valves were all still within service limits, needed tiny tightenings but even my buddy Bell was impressed with how litle was needed...

    Turns out, though, that if you forget to re-attach a vacuum line when you put it all back together, the bike won't start hahaha

    All in all, decent day's work though. Now to get my friggin' taxes done so I can get plastics on that thing! can't ride it to work when it rains due to exposed wires (thanks to my bike bein' sans plastics at the moment)
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  16. #15
    Uber Guru Lgn001's Avatar
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    Way to go! After you've done it once, it gets easier. Were you able to check the carbs and spark plugs?

  17. #16
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    way to go pyro! Did you find things to be very far out of wack?
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  18. #17
    Senior Member PyroMcnoob's Avatar
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    nah, all the clearances were still within service limits, just a tad loosey goosey... and I'll be doin the carb sync next weekend
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    Uber Guru squirrelman's Avatar
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    [QUOTE=tinkerinWstuff;235997]Can't tell from your photo if the engine is still in the frame. Is the gen2 700 that different than the 500 1st gens?"

    Engine is in frame, and access is excellent after radiator is removed; rear cylinders, no problem at all.....

    Having an engine that needs to be removed from frame to set valves is a serious design flaw, IMO.



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    Senior Member PyroMcnoob's Avatar
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    Quote Originally Posted by squirrelman View Post
    Engine is in frame, and access is excellent after radiator is removed; rear cylinders, no problem at all.....

    Having an engine that needs to be removed from frame to set valves is a serious design flaw, IMO.
    True, once you drop the radiator off, the valve cover is easy peezy... and the rear valve cover was just as easy, just pull the hose off the back of the airbox.
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  21. #20
    Senior Member PyroMcnoob's Avatar
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    Got my carbs bialed in too ^_^ had my buddy bring his sync kit over and tweak my stuff a bit... took the airbox off cuz I was investigating a cracked bracket, had nothin' to do with the sync

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  22. #21
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    did you find them to be very far off? You'll have to give the final report on what you've found with all the maintenance you've done and how it's all panned out when you get out and ride it.
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  23. #22
    Uber Guru squirrelman's Avatar
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    Good job on the valves, but you guys screwed up the synch operation !

    Never remove the airbox base from the carbs unless you need to remove a single carb from the set or replace fuel tube O-rings, NEVER !

    Without the base plate tying the carbs rigidly together they can move around slightly ...and that WILL effect the synch. As soon as the airbox is replaced, synch will change.

    It's a PIA, but access to the synch screws is from underneath between the "V"

    I Don't think you followed directions in the service manual or you would have done it properly ......without removing airbox.



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  24. #23
    Senior Member PyroMcnoob's Avatar
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    i put the airbox back once i opened the PDF of the service manual

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