plug chop results and questions ('79 750)

[gearheadE30]

So I just finished installing K&N pods and an old Kerker 4 to 1 on my 1979 GS750E and am working on sorting out the jetting. I am running a baffle in the pipe. I completely cleaned and rebuilt the carbs with new orings, set float heights (24mm), needles in the 4th position, vacuum sync'd the carbs, installed 125 mains, and set the idle mixture using the highest RPM method. Fuel screws are 1.5 turns out. It's miles ahead of where it was and I had no idea an old 8 valve GS750 could go so well. The midrange loss also wasn't near as dramatic as I expected.

So now that I've ridden a bit and figured out its quirks and checked the plugs a few times, here are the issues I'm having:

-Right off idle, it stumbles a little. I'm guessing this has something to do with the idle mixture, but I haven't been able to make it any better. Checking the plugs after idling shows an even good mixture across the board.

-the choke plungers seem to be sticking a little, and I can't tell if that's messing with tuning the carbs. I pulled them out and cleaned them all, and have no idea why they would stick. I don't want to pull all the carbs off the rack again to try to fix the issue if there's something else that would cause this. They open fine, they just don't always pull themselves all the way closed.

-Midrange is rich. I know I can fix this with the needles, but how much effect does the main jet size have on the midrange? (about 1/2 throttle)

-WOT is very rich...on cylinders 1, 2, and 3. 4 seems only slightly rich. Cylinder 4 is also leaner than the other three at part throttle. Could this be a result of the sticky chokes? Or, possibly, float heights that aren't all exactly the same?

 

[hampshirehog]

I would have a look at those choke plungers before going anywhere else. they need to almost slam closed. You don't need to pull the carbs off the rack - just pull out the link arm and attach each lifter in turn; I bet one is out of line and you'll spot which one is holding back the closing off.

 

[MisterCinders]

After you sort out the chokes, you might drop back some on that main jet. 125 is pretty hefty. See if 122.5 with a bit of needle shim doesn't tighten that up.

On the idle stumble, you might swap those pilots for 17.5's. I have almost the same set up and waffle back and forth between 15's and 17.5's. If you bump up the pilot jet, you should adjust the screws closer to the stock settings (1 turn fuel and 2 turns air, then highest idle on air).

HTH

 

[tkent02]

Look for few of the choke things to be upside down, the little fork thing where the rod attaches to the plunger. 15 pilots usually work, your fuel screws are out way too far. Main jet has no effect until about 3/4 throttle on VM carbs.

 

[gearheadE30]

I didn't realize that the forks could be flipped; I'll definitely check on that. I have a set of 122.5's, so my plan is to fix the choke, check the plugs, then put in the 122.5's and check again, and then use the needles as the last resort, though I do think they will end up needing to be shimmed slightly. Of course, I'm moving to Michigan tomorrow morning, so it might be a bit before I post back with updates.

 

[tkent02]

I didn't realize that the forks could be flipped; I'll definitely check on that.

They fit both ways but they are not quite symmetrical, if some are one way and some are the other, the ones that are wrong won't close their choke all the way.

 

[gearheadE30]

Well, I haven't had internet until today, nor have I had a ton of time to work on the bike, but I pulled all of the fingers out and put them back on the right way. 2 of the 4 were upside down, and it runs much, much better now. When I get some time I'll go out and get some new readings from the plugs. Hopefully everything is a little more consistent.

 

[mvalenti]

now im going to haveta check my choke forks... hmmms....

 

[TheCafeKid]

I was going to say, I had a 550 that the plungers would stick slightly out on and it would play havock with the bottom end, and cake plugs pretty quickly. For the life of me I couldn't figure out why. I'd taken the plungers out, cleaned the. Etc etc multiple times until I realized one day that the little angle that is bent purposely into the actuator linkage someone at some point must have thought WASNT SUPPOSED to bent and straightened it out...that was holding the choke open just slightly....
But, you've found your problem good onya

 

[gearheadE30]

Well, just went out and did some more plug chops. Midrange is a tad lean so I need to shim the needles slightly, but WOT with the 125 mains actually seems to be right on. I was really surprised by that. Cylinder 1 is a little richer than the others, but not by much. I'm going to guess that this has something to do with the float height, so I might try to rig up something to measure and compare the fuel height rather than dry setting them.

 

[gearheadE30]

I put the 122.5's in just for the heck of it because I've been chasing a 75mph cruise issue and nothing else will fix it. Well, the jets did. I seem to have lost a bit of ultimate power, especially below 4500 revs. It's significantly weaker down there now. Top end isn't too far off. Needles are in the 4th groove with no shims, fuel screws at 1.5 turns, and air screws are set to fast idle. I will probably check the plugs again when it isn't 100 degrees out and make sure I'm not too lean, because I can't tell if the bike is running hot due to mixture or just the ridiculous weather. I'm seeing 200 to 210 degrees on the valve cover in traffic, and about 180 degrees going down the road.