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Carb Theory - Main Jet Effects

  • Thread starter Thread starter redliner1973
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redliner1973

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Question: My 'GK has always fallen flat at full throttle since I took ownership. I have since rebuilt the carbs, synced with Carbtune, and mixture set with colourtune. I run stock #115 main jets, stock exhaust and intake. Back off the throttle alittle, and speed picks right up. this tells me the main jets are too small/lean. So, back to the theoretical question. Looking at the Suzi manual, fuel goes in through the main jet, gets mixed with air, dependent on how high the needle is, then goes up through the top of the needle jet assy, and out into the carb body to the engine. At full throttle, the main jet exclusively meters fuel, but still goes out the top of the needle valve and into the carb body, to the engine. If I went to 117.5 mains, would more fuel then go through the needle jet therefore ever-so-slightly richening that up as well? It seems to be the case If I took out the main jet completely, that a whole bunch would go through the needle jet and be stupid rich even under full throttle. I have needles that cannot be adjusted, and wondering if, by replacing the main jet to cure the full throttle flat spot, that it won't slightly richen the midrange (which is controlled by the needle valve) due to more fuel allowed in the needle jet area? Sound logical?
 
Changing to a larger main jet will not change the mixture of the needle circuit. It would have a small effect at the overlap point, (where the needle gives way to the main) about 3/4 throttle position, but after that you would be completely on the new, larger/richer main. A larger main jet will slightly delay the point of overlap. The needle will have to rise a little more before it gives way to the main.
Fuel flow will be regulated at the smallest opening to fuel flow along the entire main/needle jet passage. As the tapered jet needle rises up through the needle jet, the space between them is smaller than the hole in the main jet, but is increasing. Approximately 3/4 throttle position, the space between them becomes larger than the main jet, so now the main jet regulates fuel flow completely.
 
Ok thanx, that makes good sense. Second question: Am I right on my diagnosis of the flat spot at full throttle?
 
Hi. I went back and made a small explanation correction just as you replied, so that's why the edit note shows up.
The "roll off" at full throttle is a common test. Your results (bike accelerates) suggest the mains are small.
BUT, I have to wonder because you say the bike is completely stock. We all know even factories can make mistakes and that stock jetting can be lean. So yes, a larger main could help here, at 3/4 throttle and above.
As long as the rest of the bike is set up correctly, which means several things, then larger mains makes sense.
You say the carbs are clean, synched, etc. Two thought come to mind first.
Do your carbs have an adjustment/measurement for "fully opened throttle valve position"? If they do, and the throttle valves are opening just a bit past their fully opened position, the bike will accelerate harder if you back off a little from full throttle. Check your manual for this adjustment first.
Next, are you sure the float levels are set correctly? Just a little too lean will cause the same problem.
 
When I had them apart, I checked float level exactly as described in the Suzi genuine manual. I also remember looking at full open position, as I had reservations about pulling off the throttle blades (as described in the manual - I used the suzi manual and pics from the Carb cleanup series) none were "over-opening" so to speak. The carbs got the full treatment, O-rings, float valves, etc. They were not really bad off, just the float needles were leaking, and the fact that at 23 years old, they prolly needed it anyhow. I was just kind of suprised that the problem did not go away. BTW, for info, both boots and O-rings were replaced with Genuine Suzi items 18 months ago. the problem was there before that. I never worried too much about it because I am not a hot-rod when it comes to riding, but it does become inconvenient to have this hesitation/bog when your using everything to pass a car on a shoulderless two-lane road! But Suzi and I are heading to Sturgis in August, and I am in the process of fixing all the little 'things' before then.
 
Well, sounds like you've checked everything off. Hope the larger mains fix you up.
I do have two more thoughts though, you never know.
Does your model come from the factory with all 4 mains the same size? Some have larger mains in the middle cylinders. Your manual says the mains are all 115?
Also, is your fuel line 5/16"?
 
Yes, the mains are all #115's. Fuel line is 5/16, no restrictive-type filter inline. I just wanted to make sure that I was going in the right direction before ordering the mains. Thank you for your advice.
 
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