rebuilding/rejetting 77 750 carbs, HELP!!!

[TheCafeKid]

Hi guys, I am trying to figure out what adjustments to do on my 77 gs 750. I added pods and High compression forged ARIAS pistons, and I retarded the timing about 3 degrees. Stock exhaust. Bike is running, yet at full throttle it bogs down. My first project and learning as I go. I already.. rebuilt the top half with ARIAS high compression pistons, added a copper gasket, had a valve job done, re did the electrical, duneage's RR and 81 GS 850 magneto and RMstator's stator, rebuilt front and rear calipers.... Carbs scare the crap out of me. A shop wants 300 to jet and sync the carbs.... seems like a lot. I was told that I am running stock jets. Plugs do NOT look white. They like they have normal wear and tear. Also in terms of exhaust heating up (I dont know if it matters) number 2 and 3 heat up first, with 4 then 1. Could that be from poorly calibrated carbs? Also since I am breaking in the engine, I should have an even compression +- 15 psi, before I jet right?

...Overwhelmed.

-Vlad

HI Vlad, SOunds like youre pretty close. Maybe a move of the needle clip is what you need? Its not as easy as the CV carbs to do that, as you have to pull the throttle arm out to pull the slides, and then you'll have to resync, but its not all that tough either..

[vladdmd]

So then, Is it possible to operate with pods and high comp pistons, with standard exhaust on stock jets?. I thought changing jets is a must..

[Octain]

Wouldnt wide open throttle as he described be in the main jet and not the needle?

[vladdmd]

Actually maybe someone jetted it way back... The bike runs with no pods at all too. Not sure if that means anything.

-Vlad

[TheCafeKid]

So then, Is it possible to operate with pods and high comp pistons, with standard exhaust on stock jets?. I thought changing jets is a must..

Well you have High COmp pistons.. but retarded timing. The plugs arent white.. and they will tell you if its lean more than anything other than the bike itself. Im not a motor whiz, I will be the first to admit. The High Comp pistons and timing, I dont know what that equates to, but if the plugs arent lean, Then.. I dunno. But bogging out before WOT, or breaking up around WOT, maybe a main, maybe the needle, maybe both! The slide carbs dont quite work the same as the CVs..

[Octain]

VM carbs tend to be a bit more forgiving.

Lets see what CK has to say..

[TheCafeKid]

Actually maybe someone jetted it way back... The bike runs with no pods at all too. Not sure if that means anything.

-Vlad

Well, PROPERLY JETTED, you dont need filters. I can take my 1100ES's filters off and blast all over the place..In fact, it might run just a *touch* better. Dont want the dirt in there tho.. The only way to really know what's going on is to crack em open and see what you have in there for mains, pilots and needle settings. The pilot circuit pretty much controls everything up to about 1/4 throttle, the needle from there to about 7/10s , and WOT is all main jet. If it falls flat on its face at WOT then you know its probably the main. You cant relate RPM, speed or anything OTHER than throttle position to the carbs, because they make no difference. At 70MPH, just cruising with a slight bit of maintenance throttle, you're likely only using the pilot circuit, MAYBE the pilot needle transition, but certainly no more than that. So it may run like a champ. But wack the throttle (something you CANT do on VM carbs, no matter how they are jetted BTW, ask if you dont understand why, ill be happy to explain) and get into the higher needle, or main, and it could fall on its face..

[vladdmd]

I did notice that when I ease into open throttle the engine eventually revs up. If I want a snappy throttle, am I better off with going with a 80 CV carb and put in a stage 3 dynojet kit?

[TheCafeKid]

I did notice that when I ease into open throttle the engine eventually revs up. If I want a snappy throttle, am I better off with going with a 80 CV carb and put in a stage 3 dynojet kit?

Well not necessarily. It will depend on whether or not your carbs, once tuned propperly, still dont perform like you want (VMs you have to ease into the throttle, even when they are jetted right. Snap em open, and you drop all the vacuum in the motor, which pulls the fuel/air mix out of the carbs, and it bogs out for a sec or so) The CV carbs have more of a two stage valve set up. The butterfly valves are operated by the throttle, the slides are lifted by vacuum. Snap those open, it bogs, just for a split second, but recovers quickly. They're a more "streetable" carb for sure. But if you wind it out, run it wide open alot, like on a track, or hot thru the country twisties, and dont do alot of city wacking thru traffic, the mechanical type (VM, RS, etc) might be more your speed.

[vladdmd]

Great Info!! Thanks!! One last one... If, in the appropriate order, my exhaust pipes are ALWAYS heating up in 2,3,4,1 can that mean that the carbs are not synced? Does it mean anything? Also on the fins, same temp distribution. Seems that 2 and 3 are also running a bit hotter than 4 and 1. I am going to get a laser temp gauge.

-Vlad

Also, if anyone needs dental advice, I'm a dentist.. I know about dentistry a whole hell of a lot better than about my GS.

[rustybronco]

Carbs scare the crap out of me.... Plugs do NOT look white. They like they have normal wear and tear. Also in terms of exhaust heating up (I dont know if it matters) number 2 and 3 heat up first, with 4 then 1. Could that be from poorly calibrated carbs? Also since I am breaking in the engine, I should have an even compression +- 15 psi, before I jet right?

...Overwhelmed.

-Vlad

First thing's first... get it out of your head being scared to work on those carbs, you'll be able to work on VM's in your sleep.

first read this... http://www.dragstuff.com/techarticle...ark-plugs.html
it will give you some insight on how and where to read spark plugs.

then we'll get into breaking them down, cleaning, re-jetting and proper re-assembly or, you can go ahead and spend the $300. it's all up to you.

[vladdmd]

Nervous yes.... I'm not really intertested in spending the money if I dont have to.... I'm on the reading.

[rustybronco]

Read it all (links too!), then re-read it.

Chuck Hahn, does your or Scott's bike bog?
(I'm not arguing, I just haven't noticed it on VM's. but then I'm no expert)

[KEITH KRAUSE]

If 2/3 cylinders heat up before 1/4, then that suggests 1/4 isn't sparking immediately and/or spark begins improving with the heat of the motor to help combustion. That should be corrected FIRST before checking the jetting. All basic tuning comes first, then jetting, though tuning does include checking your current jetting to know what you have and if it makes sense.
Check the coil and all connections. Plug caps and their connections. Check the plug lead at the coil and primary side. Check voltage and secondary and primary resistance to see if 1/4 coil matches 2/3 and meets the factory specs'. Be sure plugs are correct and in good condition. Be sure battery is good to do accurate testing. I suggest re-setting the timing to factory spec'. Why retard it? Once you have proper firing then you can jet, if needed.
You still need to be sure there are no intake leaks, the carbs are clean, floats adjusted, AT LEAST a good bench synch, side air screws/pilot fuel screws adjusted, etc. All basic maint'. I also suggest removing the 2 floatbowl vent lines and leaving the vent ports open for best venting. Sometimes poor venting can occur with pods and removing the lines helps eliminate fuel starvation because of poor venting.
Stock exhaust and pods aren't a good match as you're dealing with a part that improves flow and making it work with a restrictive exhaust. If you have STOCK jetting, I'll suggest some adjustments but because of the intake/exhaust combo it can be trial and error. Also, I'm not sure if your '77 has the jet needle e-clip in the 2nd or 3rd position from the top. Knowing the factory setting is important. If you inspect and find jetting that's richer than stock, let us know. If the jetting seems in the ballpark then you can do some testing at the throttle positions that each jetting circuit regulates. If the testing shows you need to make changes then we can help.
The pods should require at least 1 position richer on the jet needles. 2 positions is too rich in my opinion, though 1 1/2 positions, by using an approx' .022" thick jetting spacer may be needed. If your needles are in the factory position, I'd try lowering the e-clip 1 position and test at 1/3 to 1/2 throttle only and chop off/read.
For the mains, if stock is 100, I'd try 120's. That's 4 full sizes up. Maybe someone here with the same set up (and whose testing you can trust) can suggest something else but 120's seem right. I can't imagine anything less than 115's working. Test at full throttle and do what the performance/plugs say.
Try to make the stock 15 pilot jets work by richening the pilot fuel screws underneath first. Be sure the screws are in good condition and the tiny holes in the carb body (that the screw tips regulate) are clear. RECORD where your pilot fuel screws are currently set. Seat the screws lightly and try 1/2 additional turn outward from what you have. Test at minimal throttle position for the pilot circuit (just puttin' around at a steady speed of no more than 35 mph). Generally, the factory sets these screws at about 1/2 to 1 full turn out, so keep that in mind if you see very different settings to begin with. As I mentioned above, the side air screws must be set correctly first, by using the highest rpm method. Know how? They generally end up about 1 1/2 to 1 3/4 turns out.
Generally, if you go beyond 3 full turns on the pilot fuel screws with poor results, then a 17.5 pilot jet is needed.
The carbs should be bench synched as I said but there's no replacement for synching with a vacuum gauge. It's a part of basic tuning/jetting. Sounds like you aren't prepared for that but you can do a good bench synch and if done right, will be good enough to test your jetting and get it right.
I know my post is long and I jumped around a bit but I tried to include everything. Inspect your jetting and check why all cylinders aren't firing immediately or properly first. Let us know what you have.

[TheCafeKid]

Read it all (links too!), then re-read it.

Chuck Hahn, does your or Scott's bike bog?
(I'm not arguing, I just haven't noticed it on VM's. but then I'm no expert)

If they're running the stock airbox, the bog will be less noticeable. But, on the VM or ANY mechanical, single stage slide carb (even on a car, a 2 barrel, floored, will bog, while a four barrel with a vacuum secondary will not so much) will bog out if you wack the throttle full open from maintanence throttle or zero..