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rebuilding/rejetting 77 750 carbs, HELP!!!

  • Thread starter Thread starter 77GS750inCincy
  • Start date Start date
vladdmd said:
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
Click to expand...
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/techarticles/reading-spark-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.
 
Nervous yes.... I'm not really intertested in spending the money if I dont have to.... I'm on the reading.
 
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)
 
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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.
 
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rustybronco said:
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)
Click to expand...
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..
 
Both mine and Scots are 100% stockers.airbox and all. Only difference is that Dale had done both my 78 and Scots 78 carbs.. The other carbs were done By wired george carbs. I had just assumed that this is the way the bike is supposed to run due to lack of experience and that maybe it just needed the carbs fully synced. Now since Dale has done two sets of Vm26s and they both run like raped apes i SUSPECT that the carbs on the 77 should warrent some in depth investigating. When i got them back fro Texas all i did was took them out of the box and oput them on...thats the story as it sets tonight...
 
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