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A quick tuning question...

  • Thread starter Thread starter cjanderson_90
  • Start date Start date
C

cjanderson_90

Guest
So as I'm getting the bike ready for the year, I break the carbs down, dip them, put in all new o-rings, set the bowl height, replace the intake boot o-rings, and adjust all of the valves. Now when my bike starts, it doesnt want to idle for anything. If I set the idle at 1500 or so I can get it to stay there, but once I start to rev it, its very boggy and dies. Its only if I flog it hard and fast. If i do a slow roll on i can get it to increase RPMs but it stops revving around 6k like it hit a wall. Any ideas? All the jets were cleaned, I'm wondering if its needle placement? The bike has a 4-1 exhaust and the mains are 103s but I'm not sure what the others are. Should have taken inventory when i had them off!
Thanks Guys!
 
Mikuni never made a 103 main jet, you must have read it wrong. If I were you I'd pull those carbs off again and do an inventory of the main jets and the pilot jets to know what you are starting with. Are you running a stock air box? And like said above did you synch the carbs, what are you mixture settings at?
 
I havent yet sync'd them. Im wondering if i need to have the correct jets and needle positions first to sync them or just sync them and make changes from there. Its the stock airbox. Rubber around the covers is good and sealing. Sprayed carb&choke cleaner all over the carbs and airbox and boots while running so no leaks it appears. If I need to sync them first before I check jets and such I will. Just need to buy the tool.
 
Besides the carb sync that was already mentioned, what settings did you use for the air and fuel screws?

In order for us to make some reasonable suggestions, though, it would help to know a bit about your bike.
I see in your sig that it's a '79 750E (thanks for having that in your sig :encouragement:)
Do you have stock exhaust and airbox, or do you have pods and/or a header?

Knowing these details will help us make better guesses how to fix your problem.

.
 
I believe the air/fuel is 1 1/2 turns out? Or whatever the manual says. I can check in a bit. I tuned it all to the manual. It has a 4 into 1 exhaust and the stock air box on it. I cleaned the filter but its going to need to be replaced. What recommendations do you have? The stock filter and prefilter are around $60 i think. Is that normal? Is there a cheaper better option? Id like to keep the stock air box so i can ride in rain, and I would like to keep the bike original. Thanks again!
 
Doesn't the GS750EN have VM carbs?

I'd buy a stock replacement K&N filter in place of the OEM filter.
 
JTGS850GL said:
Doesn't the GS750EN have VM carbs?
Click to expand...
Yes, it does, but he might also have an '80 that was made in '79. :-k

How about it, CJ, which carbs do you have, VM or BS series?

Other clues for '79 vs '80 on the 750: '79 will have an 8-valve head that uses shims, the '80 750 has a 16-valve head that uses threaded adjusters.
Quick clues there are round or rectangular cam end covers.

The '79 brakes are solid, the '80 disks are drilled.

A few other differences, but these will get us going.

.
 
Yes I have the VM carbs. I was leaning towards the K&N Filter anyways so its nice to see that someone agrees with me. Its the 79 with the solid brakes, 8 valves and such. Hope this helps!
 
You might want to go back over the adjustment procedure for the VMs. There are two adjustment screws per carb.
 
Start with the fuel screws (the ones on the bottom) out 1 full turn.

Start with the air screws (the ones on the sides) double that, or 2 full turns.

When the bike is warmed up and off-"choke", slowly adjust the air screw for best idle speed and throttle response.

.
 
With the stock airbox and a 4 into 1 you might be able to go with 102.5's as main jets and stick with the stock pilot, you might have to go up to 105's but you won't know till you get it running and ride it to see what she does. Putting in a K&N filter into the stock airbox will also require a possible adjustment to the main jet or a clip change to the main needle but you won't know till you get the bike running. If it was me, I would pull the carbs off again and check each one to know what is in there, then get it running right before you order the K&N filter. You can do a fairly close bench synch if you don't have a vacuum synch tool yet, it won't run perfect but it should idle ok, but you do need to get it vacuum synched at some point.
 
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