ok guys, I need some thoughts from you that have diagnosed vm26 carb tuning issues.
I have recently restored a 78 GS1000, before restoring, I made sure the bike ran before tearing everything down and all seemed well.
I tune carbs on the regular and I would appreciate any technical insight or anything I am missing.
once I got the bike back together. I did the following to the carburetor.
1. replaced all o rings
2. completely cleaned 2 times over
3. replaced the stock pilot jets with 17.5 pilots (could be my problem) thought it would be better with increased airflow from pods. Plug chops seem normal color as well
4. set floats to appropriate height
5. It already had pods and seemed to run fine before tearing it down.
6. Replaced intake boots and o-rings
7. I have recently synced the carbs and all is good there.
8. Valves in spec
9. The bike already had 4-1 exhaust as well
my symptoms now are:
1. hesitation on bottom end.
2. hiccup when I blip the throttle and spitting sound through filters at times.
3. Just seems the low end lacks power until after 5000 rpm and runs well mid-top
The fuel mixture screw only tunes the carb in the idle to low range as well as the air screw, correct me if I am wrong. and please advise me on a standard starting point carb setting in which I believe to be 2.5 to 3 turns out on the fuel mixture and 1.5 -2 on the air screw. Let me know your thoughts on this.
Just looking for some ideas on things to try and insight from experienced tinkerers and carb guys.
Thanks,
You said the bike ran well before you took things apart. Your list of things you did, if done correctly, wouldn't result in ALL of the problems you list now. The only thing that you list that shouldn't have been done is installing the larger pilot jets. Larger pilot jets wouldn't result in "spitting" out of the pod filters. "Spitting is a classic lean condition.
As for the pilot jets, consider what Dynojet uses in their stage 3 jet kits. Stage 3 meaning for use with quality pods like K&N and quality exhausts like Vance and Hines, Yosh', Kerker. I can't speak for Delkovic.
Dynojet retains the stock #15 pilot jets for the '78 1000. Basically, the freer flowing pods and exhaust make more power at higher rpm's and wider throttle openings while the carbs jet needles and main jets are regulating fuel flow. More air entering and exiting the system equals more power, if jetted correctly. The increased air flow does effect the pilot jet circuit, such as spitting out the filters while idling, but the increase in flow while operating on the pilot circuit isn't significant enough to require larger pilot jets. Richer pilot fuel screw and correct side air screw tuning will work to richen the pilot circuit. I'd install the stock pilot jets. Use only genuine Mikuni parts. Then set the pilot fuel screws (underneath) to 1 1/4 turns out from lightly seated as a starting point. Set the side air screws to 1 3/4 turns out as a starting point. REMOVE the two float bowl vent lines and leave the ports open to increase float bowl venting and avoid fuel starvation under certain conditions. Removing the lines is a requirement in Dynojet's instructions. If the carbs are vacuum tool synched as you said, place the bike on the center stand and warm up fully. Using the highest rpm method, tune the side air screws. Set the base idle to 1,000 rpm's. Start at any carb. Slowly adjust the side air screw in or out to achieve the highest rpm. Then reset the base idle to 1,000 by using the throttle pulley adjustment knob. Go to the next carb and repeat until all 4 are set and the bike should be idling very close to 1,000 rpm's, not 1,150, 1,200, etc. Fine tuning of the pilot fuel screws may be needed but the bike will idle and run without significant issues due to screw adjustments.
You said the bike hesitates at the bottom end and hiccups when you blip the throttle. Those words and description are open to interpretation. "Hesitation", could be rich or lean related, but it's most commonly used to describe a lean condition. A momentary or prolonged lack of fuel. Quality pod filters effect not only the air/fuel mixture, but the vacuum as well. The VM carbs are infamous for poor vacuum control at low rpm's/minimal throttle openings. You can take perfectly operating VM carbs and "blip" them into a stall very easily. At these lower rpm's/smaller throttle openings, pod filters make the VM carbs more sensitive to throttle openings, especially sudden or quicker openings. These throttle openings create a momentary drop in vacuum and this could be the hesitation you describe? I don't know. There's no real cure for it except to be more careful opening the throttle. Modifying the intake and exhaust often results in some loss of "streetability". One other thing that's critical with pods when used with VM's, is to properly oil the filters if they require oil such as K&N's. K&N's are easily over-oiled, resulting in a lack of air flow at middle throttle openings, or lesser throttle openings if really over-oiled. On the other side, the filters could be too "dry", not enough oil. This increases the momentary vacuum loss and some would describe it as hesitation. If you change the pilot jets and make the screw adjustments I mentioned earlier, and you still have hesitation, I'd suspect the jet needle circuit is too lean. Do you notice any hesitation or a surging feeling when at a constant 60 mph in 5th gear? Do you know what jetting changes have been made? Try the above and see where you're at.
Yes, the fuel mixture screw as you called it (correct term is pilot fuel screw), tunes the amount of fuel entering the pilot circuit. It's purpose is to "assist" the pilot jet by fine tuning the amount of fuel. That's why the tip is so fine. Every cylinder is a tad different straight from the factory and this screw allows for fine tuning each cylinder. There is no exact starting point for these screws because they are set at the factory using precision emissions equipment. After the screws are disturbed, we can't set them manually. To me, the standard starting point is 1 to 1 1/2 turns out from lightly seated when running quality pods and exhaust. Then fine tune as best you can which will often result in different settings for each cylinder. The side air screws are ALWAYS adjusted using the highest rpm method I described. I've found 1 3/4 turns out, or close to it, is most common. It depends on your elevation/conditions.
