MASSIVE flat spot

[Anonymous]

Hi guys,

newbie to the forum. Just got a '78 GS750. It has had a lot done to the engine and running gear but needs finishing off. Bike had been fitted with stage 3 yoshi cams, K&N filter and TTS head. It runs ok at tickover, pulls away (poorly) at low revs and hits a flat spot which is soooooo long, you have to clutch past it before anything happens. I suspect this is a carburation problem but don't have much experience tuning carbs
I was recommended to get some 29mm smoothbores or flatslides but want to see if I can get an acceptable bottom/midrange with the existing carbs before I spend all my available cash!
Haven't checked valve clearances yet but should be ok (engine was rebuilt about 60 miles ago).
Not sure what else could sap so much power from bottom/middle of rev range.
Any advise, suggestions would be greatly appreciated

 

[Anonymous]

EXHAUST??????????????????????????????????????????
???????????????????????????????????????????????????
?????????????????????????????????

 

[KEITH KRAUSE]

Give a list of ALL your engine mod's and what mm size your stock Mikuni carbs are. What adjustments, if any, have been done to the main/needle/pilot circuits? Before any re-jetting, do you have good compression in all 4 cylinders, is the ignition working and set correctly, are the valve clearances good?

 

[SqDancerLynn1]

Sounds like you need to install a jet kit Removing the stock air filter & box to install pods requires custom jetting

 

[Anonymous]

I think you need to check out the carbs you have. When I 1st got my 77 GS750, it started easy, but had a major flat spot as you described. I took the carbs off and found one float bowl completely dry (the float was set wrong) and on another carb, the circlip that holds the needle in place was off and the needle was resting in the jet (not passing any fuel). Just correcting these two items made a world of difference.

Look to see what jet size you have. With all the mods you say the engine has, it better be bigger than the stock 97.5 or 105. I am running a stock motor with an open 4 into 1 and no air filters and I am up to 118 in the jets.

God Bless!

phil

 

[Anonymous]

Cheers for the responses so far, gives me a bit to look at. I plan on removing, cleaning and setting up the carbs once I've sorted the charging circuit. I believe I may be in for an electrexUSA reg/rec & stator! - yes i've been reading the stator papers :lol:
As soon as this damned snow clears I can get started. I believe float level has a lot to do with mid-range power so I'm hoping that will cure things. Not sure what main jet it has, or indeed anything about current carb set-up. Power at the top end seems ok which makes me think (read 'hope') that main jet is ok and it's just poorly setup for everything else.
I hope to start spending a bit more time in the forums, so you're bound to be hearing from me again soon.
I'll keep you posted on what I find out, and thanks again. 8)

 

[pano]

I would just put a hat on

 

[Anonymous]

I would just put a hat on

Ithink he said flat not bald

 

[Anonymous]

I would just put a hat on

Ithink he said flat not bald

Yes, quite so. The FLAT spot is much bigger! (see my postings on www.ThinOnTop.com/forums/ for more info & pictures of the BALD spot :lol: )

 

[Hoomgar]

I've always been told I have a pointy head not a flat one. Although I do have the bald/flat spot. But that is from being tall and hitting my head on door jams

 

[Anonymous]

Well, the plot thickens...

I've just had the plugs out and it looks like the engine is running super-rich (BLACK black) Has issues starting too which doesn't make charging problems too easy to diagnose either! I don't think the carbs are standard, they don't appear the same as the ones in the manual?? I may attach some pictures (gold star for anyone who can identify them). By the way, CRAP isn't a recognised manufacturer! :wink:
Also, the dyna ignition seems to be a bit of a bodge. I've filed out the slots so that you can now advance/retard ignition but I'm not sure what modifications to timing I need to make for the stage 3 yoshi cams???
I also need to pick up a kick-start lever to avoid flattening my battery trying to start the damned thing!

Stood scratching my head for 20 mins today wonding why it is that I like this bike so much (maybe because it reminds me so much of the bikes I fell in love with as a kid). Maybe because I have no brain??? :?

 

[saaz]

If you have the stage 3 cams and TTS head the bike should run very hard up top, but perhaps be a little weak mid/down low unless it has a big bore kit in as well, assuming it is tuned properly. What type of exhaust is on it?

I would contact TTS and get some ideas as to head/cam combination and tuning. The carbs not being standard may be good as they should have been set up to use the mods you have if tuned properly. I did suspect the carbs may not be the ones it was set up for, but some other chucked to sell it, so it does not go well.

 

[Anonymous]

Well.....just had a word with the previous keeper.

Carbs are off a GSX750E (33mm CV???). REG/Rec off CB250!!! - I really can't believe this is a "standard upgarde" as surely CB 250 is 6V??? I'm no tech guru though so please keep me straight on this! I think electrexUSA reg/rec to sort charging as well as thourough wiring check! Carbs seem to run mega-rich although guy says this was only solution to fueling top end without being too lean (and risking major engine failure).
Solutions on a postcard to:

Hope I'm not in over my head competition
bought this cos I love GS's
Cor blimely govnor-ville
brokesville
UK
:lol:

I know I need to spend money to get this running at at least an acceptable level, so please advise me on what to spend it on. I have about ?300/$500.

ps. Bored to 840, originally Yoshi, recently rebuilt with Wiseco pistons and HC rings. Harris exhaust but headers aren't (guessing).
Can I swear here??? :x

 

[Anonymous]

reg rectifier

reg rectifier

CB250N is 12v, and honda made a far better reg rectifier than suzuki did on the 70's gs. Theyve become rare in the breakers now, theyve been pillaged for so many different bikes. If you have charging issues (and the 'dream reg/rectifier is wired in correctly) its maybe time to check out the phases in the alternator are still in good order (measure them with a ohm meter and check theyre in spec).

 

[snowbeard]

unfortunately I can't really make any good suggestions about your machine, you've already surpassed my limited knowledge, but what you said about the kick start interests me. is this something that can really be done or are you just wishing? I would like to have one on my GS650G, but doubt it can be done. am I wrong?

sorry for the thread drift...