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1977 GS-750 re-jet. Here is where I am at.

  • Thread starter Thread starter TBC
  • Start date Start date
T

TBC

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This is a follow up to my post from a couple of days ago. It stopped raining so I was able to get out for a bit today. My bike has Emgo pods and a DG Street Performance 4 into 1. Intake boots were fine but I replaced O-rings anyways. New O-rings in carbs. Carbs completely cleaned. Bench synced but not vac synced yet. Stock jetting except for 127.5 mains ( stock is and was, 100 ) and jet needle in 4th position from top. Float height set to mid range at 26mm ( manual says 25 to 27mm ). Petcock has been removed and checked. It's clear. Valve clearance checked and within specs. Plug colour seems fine around 1/3 throttle with clip in 4th position. Still bogging when I roll the throttle quickly ( not whacking it wide open ) wide open off the line. Engine will die out if I hold this and not release. Bogs in higher gears to lesser degree. Held at full throttle up a hill for about 30 to 45 seconds and did the kill switch/close throttle/pull in clutch bit. Bike wouldn't go past 120 KPH by the way but would go well past that on level road. The hill wasn't any particular big deal. Plugs are lean still. I wonder if you have got out on your bike yet, Cafe Kid, to see where you are at? I am wondering how big a main jet to go for now or would float height be an issue for WOT. Low speed stuff ( 60 Kph in 4th gear with maybe 1/8 throttle ) is maybe, maybe, a tad rich on 1,2 and 3 ( dark tan colour. No hint of black )while #4 is completely sooty coloured. At the time the fuel screws were 2 1/4 turns on 1,2 and 3 and 2 turns on #4. Have turned #4 in to 1 3/4. All air screws are 1 1/2 turns. Rock solid idle at 1100 RPM. Doesn't fluctuate and doesn't hang up when I rev engine. Might be able to get back out later. I'm here to learn so comments are most welcome.

Tom
 
so you did a plug chop when it was bogging out? and the plugs were lean? but its a little rich every where else in the band....

i would say try moving the needle one half or one full position richer and see how it handles after that....

if its doing it enough to kill the engine i would try boggin it till it dies, then pull the plugs quick and look at them, if they are wet, too rich, if they are white or dry looking than too lean...

just my thoughts.
 
I should mention that the bike has always bogged out like this with WOT inputs for as long as I can remember, and I've had the bike since 1983. Don't remember if it did it with the stock 4 into 2, though. I put the pipe on it shortly after I got the bike. I just put the pods on a few weeks ago due to the boots on the air box side being shot and not being able to get new ones. The bike bogged with the stock air box setup and also when I had a K & N in the stock air box. This was all with the 4 into 1 on it. Next time I get back out I will go somewhere secluded, roll on WOT off the line and let the bike die and stop and check the plugs.

Tom
 
80GS750 said:
did moving up to the 127.5 main help at all?
Click to expand...

Yes. When I first put the pods on I did nothing to the carbs because I wanted to see where I was at. I did a plug chop going up a big, long hill close to home. If I tried to open it up past 1/2 throttle going up the hill it started to bog out, die and slow down. It was all the bike could do to maintain 80 Kph up the hill! Bike ran terrible. Took the carbs off and raised the jet needle one notch to the 4th position from top. Much better, but still not right. Tried 110 mains. Too lean. Put in the 127.5's which I believe were suggested by Cafe Kid awhile ago. Better but not there yet. As Cafe Kid has mentioned, there are no jet kits for this bike so we are on our own trying to sort it out. My bike actually runs great but it's the WOT inputs. I really don't have any issues with driveability at all.

Tom
 
Seems to me that it's the mains that are too small. Don't mess with float height - leave it as per spec and work around it.

What size mains to try next? You'll have to have 'a feel' for this, whether you work up by 2.5 each time or jump sizes. Could be a PITA and not cheap as I had a 750 that needed 140s (I think, may have been even more - it was a long time ago).
 
If the 127.5s are still lean, Id go to 130?s and try there. The 127.5s sound like they are VERY close. Maybe even try moving to the 5th clip on the needle first. Keep in mind, our needle jets are bigger than the later model VM carbs, so our jump isnt going to be quite as big... you could also try to bracket the size, knowing the 127.5s are too lean (if the needle move doenst help) try the step up from the 130 (132.5?) and if thats too rich, then you know 130 is spot on...
 
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hampshirehog said:
Seems to me that it's the mains that are too small. Don't mess with float height - leave it as per spec and work around it.

What size mains to try next? You'll have to have 'a feel' for this, whether you work up by 2.5 each time or jump sizes. Could be a PITA and not cheap as I had a 750 that needed 140s (I think, may have been even more - it was a long time ago).
Click to expand...


I wish I did have a feel for this sort of thing, but I don't as this is my first time re-jetting. Not sure what I will try next but I will get a couple different sizes. Jets are only $5 Cdn each so it won't get really expensive, especially if you consider taking it to a performance shop with dyno. I am guessing that would run me $300 to $400?? Can buy lots of jets for that and I get to learn lots while doing it myself.

Tom
 
i would move the needle first, then get bigger mains if you have too....

you never know, the 5th clip position may be just right!!!
 
I've been through this battle before. My experience has shown that if it can still bog the engine out completel when you open the throttle, needle position will not be engough to solve the problem. It sounds like you are close, 130 might do the trick.
 
80GS750 said:
i would move the needle first, then get bigger mains if you have too....

you never know, the 5th clip position may be just right!!!
Click to expand...


Okay. This is good stuff guys, but here's my question. At full throttle the needle is up out of the way and the main jet is supplying the fuel, right? Why would changing the needle position have any impact at this point? I appreciate all the input guys. Just waiting for a call back from a local shop to see what they have for richer jets. They had the 127.5's in stock so......

Tom
 
TBC said:
Okay. This is good stuff guys, but here's my question. At full throttle the needle is up out of the way and the main jet is supplying the fuel, right? Why would changing the needle position have any impact at this point? I appreciate all the input guys. Just waiting for a call back from a local shop to see what they have for richer jets. They had the 127.5's in stock so......

Tom
Click to expand...
Well, without even having my bike on the road yet, i went ahead and ordered 130s and 132.5s based on your info. Lil help back my way and i appreciate it :)
 
the needle may come out of the jet all the way, but it is still in the passageway and based on the taper of the needle, it still limits the flow.....

moving it one more position will pull the needle that much higher out of the way, allowing more room in the passageway....
 
80GS750 said:
the needle may come out of the jet all the way, but it is still in the passageway and based on the taper of the needle, it still limits the flow.....

moving it one more position will pull the needle that much higher out of the way, allowing more room in the passageway....
Click to expand...

But the smaller restriction, therefore the one which limits the flow is the main jet.
 
Okay, so here is an update. I put in 135's and went for a ride. When I did WOT input off the line in first gear ( I have been doing this by the way at around 2500 to 3000 RPM ) it still bogged but did not necessarily die. Before with the 127.5's it always died. Always. When it didn't die out but bogged down, I held WOT. It would clear itself and rocket off. I think I am really close. I have not touched the jet needle and it is still in #4. Based on what it was like with the 127.5's and what it is like now with the 135's, I am off to pick up 140's. I might be right or I might be wrong. I'll find out and report back guys.

Tom
 
130's or maybe even 132.5's is where you will probably end up. 140's is crazy talk maing.
CafeKid needs to get that thing running too. Damn that G.
 
TBC said:
Okay, so here is an update. I put in 135's and went for a ride. When I did WOT input off the line in first gear ( I have been doing this by the way at around 2500 to 3000 RPM ) it still bogged but did not necessarily die. Before with the 127.5's it always died. Always. When it didn't die out but bogged down, I held WOT. It would clear itself and rocket off. I think I am really close. I have not touched the jet needle and it is still in #4. Based on what it was like with the 127.5's and what it is like now with the 135's, I am off to pick up 140's. I might be right or I might be wrong. I'll find out and report back guys.

Tom
Click to expand...

K, just a note here... You may find that you are going to have this problem no matter how big a jet you're going to have...the VMs will NOT deal with wacking the throttle open like that well to begin with. You're basiclly aleviating all vaccum from the motor when you do that. This is WHY they switched to CV carbs (well, one of the big reasons anyway) Because that slide, when you wack the throttle open, is suddenly WIDE OPEN, the motor cant deal with all the air at once. It cant pull it. Suddenly there is no vaccum to pull gas thru the jets. This is why they also used VM28 "Pumpers" on some of the other bikes, and race bikes. When you wack the throttle open, there's an accelerator pump (hence the nickname Pumpers) that shoots a shot of gas down the throat of the carb, so it can get fuel to bring the vaccum back up. The 26's dont have this, So it takes a second for it to catch up. Put your 127.5s back in, move the needle, if THAT doesnt work, try 130s. 140s is, IMO on the way high side of the realm of possibility...
 
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As TheCafeKid says, you can't whack open slide carbs that aren't fitted with a pump, though you can roll the throttle on pretty quick if you're jetted correctly.

I think I'd be tempted to do a 2 or 3 cylinder plug chop - pull 1 or 2 plug leads and go for a ride with the throttle wide open - running on 2 or 3 cylinders means you won't be going 100 mph! Hit the kill switch at WOT and read your plugs on the cylinders that have been firing. At WOT your mains totally overwhelm every other part of the carb function so your reading will tell you which way your mains want to move. 140s are on the high side of average for a setup on a 750 but as I said in my earlier post well within reason.

CV carbs without a pump can bog too. Massive difference between a CB900F (no pump) and CB1100F (pump) where throttle response is instant.

The CVs were the Jap answer to meeting emissions levels. All the racers were of non CV design and if you know what the Japs copied - the British SU carb of the 1970s and earlier - you'd be amazed that the Jap bikes out perform a duckbilled platypus - ask any Morris Marina owner!
 
hampshirehog said:
As TheCafeKid says, you can't whack open slide carbs that aren't fitted with a pump, though you can roll the throttle on pretty quick if you're jetted correctly.

I think I'd be tempted to do a 2 or 3 cylinder plug chop - pull 1 or 2 plug leads and go for a ride with the throttle wide open - running on 2 or 3 cylinders means you won't be going 100 mph! Hit the kill switch at WOT and read your plugs on the cylinders that have been firing. At WOT your mains totally overwhelm every other part of the carb function so your reading will tell you which way your mains want to move. 140s are on the high side of average for a setup on a 750 but as I said in my earlier post well within reason.

CV carbs without a pump can bog too. Massive difference between a CB900F (no pump) and CB1100F (pump) where throttle response is instant.

The CVs were the Jap answer to meeting emissions levels. All the racers were of non CV design and if you know what the Japs copied - the British SU carb of the 1970s and earlier - you'd be amazed that the Jap bikes out perform a duckbilled platypus - ask any Morris Marina owner!
Click to expand...

True HH, however, the CVs are LESS likely to bog simply due to their design. As Ive explained before, and was explained to me very understandably, the reason CVs are considered a more STREETABLE carb is simply because of the way they work. They use more or less a dual valve set up, with a real set of butterflies as primaries, and the slides act as, what they are, vaccum controlled secondaries. when you wack the throttle open on CVs, the butterflies fly open, but there is still some blockage of air flow due to the slides, so the motor still retains a decent amount of vaccum. As the vaccum increases further, the slides start moving UP, allowing more air, more draw, more power, LESS bog... The VMs do not have the primary butterfly valves, but use the slides AS the primaries. SO...whacky whacky...boggy boggy... Class dismissed :)
 
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