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  1. #1

    Default 78 suzuki gs750e torque issues

    So I've got a 78 gs750e, trying to figure out what might be wrong with it but I'm having zero luck. The bike runs well, idle is about 1100 rpm, pretty steady. The bike will do about 60 mph, and feels like it has no torque. All 4 cylinders are firing, the bike just doesn't feel like it's pulling hard enough. It also struggles with hills. We got the bike a few months ago to get me back on a bike after I got smoked on my harley, and since I'm still not cleared for work my budget with this thing is about spent. I want to know if it could be something more serious before riding the carbs apart since as it is now, It runs. Any tips on how I can troubleshoot this?

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    Wonder if the timing chain is stretched out longer than a whores dream?
    Dusty

  3. #3
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    look here:

    The definitive website for Suzuki GS motorcycles
    25 years old and counting..."

    Click image for larger version. 

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    https://www.thegsresources.com/

    and

    Technical Forums
    Air Cooled
    Suzuki GS, GSX, EFE and Katana

    https://oldskoolsuzuki.info/forums/forum/7-air-cooled/
    Last edited by TriNortchopz; 07-17-2021 at 6:49 AM.

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    Compression test first is a rule. Check for air leaks at any hoses and check for cracked carb boots since the originals are probably harder than woodpecker lips by now. If it has a vacuum fuel valve like my 81 did they can fail and limit fuel flow.

    What airbox and exhaust is it running?

  5. #5

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    It's got the stock airbox and fairly restrictive pipes. I think the previous owner had a set of generic megaphone tips welded onto 2-1 slip ons. It looks stock but it very much is not.

  6. #6

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    Also the vacuum valve wasn't working at all, every setting was like prime, so I rebuilt that and now it doesn't constantly dump fuel. I think its getting too much fuel, the plugs are black and last time I checked them after running it, wet.

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    Strip and dip the carbs. I pull the carb boot clamps off the bike to avoid mangling them (a LONG pair of Harbor Freight or other cheap needlenose pliers is gold for carb work). Take your time not to damage the airbox as the engines are tuned for them.

    There are no Phillips screws on that bike. Use the correct JIS drivers to avoid mangling screw heads:
    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

    Leak check the carbs off the bike with a dummy tank. I leave carb racks sit overnight (outdoors). Riding mower tanks make good dummy tanks (make sure it's CLEAN first) and in a pinch ya can even leave the tank on the mower though most remove with a couple 10mm bolts.

    gsresources has plenty of info. All they should need is pull the bowls. jets, emulsion tubes and fuel mixture screws. I drill out the mixture screw plugs with a manual hand drill but a light touch with an electric drill will do the job. Secure the carbs in a vise etc so they don't move and mark your drill bit so you don't go too deep.

    I've used all the commercial carb cleaners (RIP original Hydro-Seal, it was glorious) and now use undiluted Pine-Sol (not generic pine oil) which doesn't hurt plastics. Simple Green works well too. You can reuse it so I pour it thru a rag in a funnel back into the jug. I clean carbs in my kitchen sink these days because it makes flushing with hot water convenient. Plastic tubs are cheap and the closer yours fits the carb rack the less liquid ya need.

    https://z1parts.net/carb-kit-1977-1979-suzuki-gs750/ is what yours should look like. I save the old jets and any serviceable parts (that leaves you a set to drill oversize if desired). Many bikes of the era ran lean due to EPA jetting so I bought a set of #60 thru 80 drill bits (cheap online) and drill one size over the largest SHANK size that fits the low speed (leave the main alone) jet. The way to do that is clamp the bit in visegrips or a pin vise collet then spin the jet between thumb and forefinger, and when the bit breaks through stroke vertically a few times without spinning the jet to debur.

    A quick google found this very nice writeup which is even better than the factory book (and worth saving for other bikers who have similar carbs which are very common):

    http://gsarchive.bwringer.com/bikecl...rb_rebuild.pdf

    It's mower carb day today for an LT2000 so here's a pic of what I use. Coffee strainers trap all the tiny parts (like S&S check balls and the little fucking washers on many Jap carb mixture screws). ALWAYS tear down yer carbs over a tray in bright light to make damn sure you lose nothing. I save old tooth brushes to agitate any soil (tends to cling to rubber boots after the grease holding it there is gone). I collect stainless restaurant steam table pots and any other items like strainers, and collect old plastic gas cans at yard sales then cut the tops off with zip discs to make tough fuelproof storage and cleaning tubs. The 90-degree bent toothbrush is quite handy. The red tub lives in my sink full of detergent solution for hand cleaning (if I suspect crumbs I dump it outdoors ir down the toilet as sink drains are small). Of course the more soap down the drain the less grease remains in your plumbing so washing off parts in your sink is a win. I leave parts to dry on a tray atop a clean rag or towel.

    This method doesn't hurt O-rings or Viton needle tips, but your needle tips/seats are probably done and the correct rebuild kit solves that. Tear down carbs before ordering a kit for best results.

    If your kit has loose O-rings/seals ya can do what I do and measure OD and thickness with a digital caliper then subtract 2x thickness to get the size. That lets ya buy bags of the fuckers for a cheap lifetime supply if ya decide it's a keeper.

    Those GS are tough and plentiful. I put 30K miles on my 91 GS750E which I bought for 500 bucks (in 1985 no less) because the owner thought it needed a top end when it only needed a carb cleaning. All I did after that was chains, sprockets and oil changes.

    Cleaning carbs on yard/garage sitters is easy and has gotten me many deals over the years.
    Attached Thumbnails Attached Thumbnails carbdip stuff.jpg  
    Last edited by farmall; 07-17-2021 at 2:35 PM.

  8. #8

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    Thanks for all the info. I'm not sure at this point what my plans are with the bike, just that I want to get it running right. Originally I was thinking of chopping up the frame and making a chopper, but it's extremely clean, so im wondering if I just want to restore it.

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    Might as well ride the shit out of it and save money while having a solid backup machine.

    Choppers are fun folk art but they exist to imitate or be Harleys and nothing else is quite the same. If you want another chopper starting with an HD in the first place makes the most sense and ya can sell the Zook much more easily if it's stock since choppers like rat rods are very personal expressions many other people won't dig because they're not THEIR personal expression.

    All I did with my E model was some BMW Airhead bend style bars (they also look like XLX bars but in 7/8" to fit Jap bikes) which have good ergonomics for the E seat and peg layout, Vance and Hines headers (affordable then) and a Dynojet kit. An E would make a fugly chopper unless the frame was replaced which is not remotely worth the cash (or the time unless one is stuck in some third world country with no Harleys). OTOH a clean one could sell for enough to buy a low-end five speed Sportster needing work.

    Chopping most UJMs is reminiscent of all the bad "cafe" bikes past and present. Never marry what ya can pimp.

  10. #10

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    Yeah, right now it's got the big wind jammer front end and touring bins, so it's a pretty comfortable little touring bike.

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  11. #11

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    So I popped open the airbox to see what was going on in there, is there supposed to be a gasket between the filter and the hole leading up and into the carbs? Seems like it has a huge gap between the filter and where I would assume it should seal against. Also, loads of crud sitting in that upper part of the airbox, I'm assuming from getting sucked through that gap.

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  12. #12
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    EDIT, found pics:
    https://www.youtube.com/watch?v=tzNYAZqOVIc

    I seem to remember the frame had foam in the inboard side adhered to the steel but it's been over 30 years. Finding pics that don't suck is a bitch. Ask on the gesresources forum for someone to show a complete assembly's worth of parts. Lots of them have spares as buying parts bikes is popular.

    https://www.youtube.com/watch?v=tzNYAZqOVIc

    You could seal your filter however you see fit (I'd shine a light into the gap and measure it). The filter shouldn't be responsible for the power difference but you could make sure the airbox is clean then take a test ride without that filter. It's newer than the bike and may be some horrid restrictive cheap foam instead of Uni.
    Last edited by farmall; 07-20-2021 at 4:22 AM.

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    had a Virago 535 I bought for $500, it would not rev, no power, PO said the shop couldn't fix it. I pulled out the air cleaner and it ran full out, no more power loss issue. Took it on a dirt flat track, had a bit of fun. sold it a couple days later for $500.

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    Also make sure the choke is actually at the correct setting and there is no leakage (requires removing the choke plunger when cleaning the carbs). See exploded carb pic on page 3 of the VM carb rebuild document for what the plungers look like. (They incorrectly call it a "choke" plunger since a choke is by definition a butterfly which increases vacuum when closed to draw more fuel via operating jets while an enricher dumps more fuel without increasing vacuum i.e. "choking".)

  15. #15

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    I think the choke is working fine. RPMs go up when choke is applied. I made a temporary foam gasket for the gap in the breather and it's like a completely different bike. The carbs could definitely use a cleaning, but its good to know that most of my issues were probably from too much air. I'm going to get some high density weather stripping to make a proper rubber gasket for it, but in the meantime it actually feels more like a 750. Now I just need to sort out why the forks have so much flex. Any info on fork tube that fit this bike? Will any 35mm fork tube work?

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    Everything flexed back then. It wasn't much of an issue. Fork DIVE was typically more a concern. gsresources would be a good place to ask about fork swaps but the answer to needing a modern motorcycle is to get a modern motorcycle, not go down the rathole trying to update an incredibly ancient design. I threw a couple of PVC spacers in mine but I forget the length.

    Of course the fluid's likely not been changed so fresh fork juice could reduce fork dive.

    EDIT: I'd still go through the carbs especially if the EPA plugs are still installed (which mean they've never been cleaned as I've never heard nor seen ANYONE install a new set (which would have to have been a dealer and dealer mechs hate time-wasters).
    Last edited by farmall; 08-05-2021 at 6:49 PM.

  17. #17

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    Was anything changed or worked on prior to losing power? That is often the clue.
    My first thought was cam timing or retarded ignition timing. But fix the obvious first.

  18. #18

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    Quote Originally Posted by farmall View Post
    Compression test first is a rule. Check for air leaks at any hoses and check for cracked carb boots since the originals are probably harder than woodpecker lips by now. If it has a vacuum fuel valve like my 81 did they can fail and limit fuel flow.

    What airbox and exhaust is it running?
    My friend is looking at this '81 GS750e with the same issue. We're gonna start with the compression test first. Got to finish installing the lift kit from 4Wheelonline onto the truck tomorrow before we can clear the garage and take her in.

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