PDA

View Full Version : carb theory



mandodave82
06-06-2016, 10:21 AM
So here's a question. 82 gs 750 mikuni CV carbs that's been rejetted for pods. Everything is cleaned, no vacuum leaks anywhere. I think my main jet is a 47 and my piliot is a 119. My 1 and 2 spark plugs showed they were burning clean (not lean), 3 and 4 showed burning rich. I adjusted the mixture screw on 3 and 4 and now they show there burning clean
Thorottle response is ok turning up slowly, but when you flick the throttle hard 3 times or so it dies, or trying to get the throttle wide open it dies.
If I let it idle for about 20 seconds I can twist the throttle hard 2 or 3 times and it respnds great but by the 3rd or 4th time it bogs out the engine. I can let it idle another 20 seconds and repeat with the same results.

Questions
Hypotheticaly If I taped up my pod filters to restrict air flow and the engine responded great with no bogging, would that mean I'm not getting enough fuel when there not taped?

How does the slide and needle attached to the diaphragm play into throttle respnse? What role does that thing play lol.

Could the carbs be good the way they are set and it be my timing that needs adjusted?

I hope I made sense

farmall
06-06-2016, 11:48 AM
How is fuel flow to the carbs? Still got the stock vacuum fuel valve? Any other possible restrictions?

mandodave82
06-06-2016, 12:30 PM
There's a new petcock on the bike, and I put a clear inline fuel filter on it. When I primed it after emptying the bowls it flowed very freely. While runing the inlinefilter remains half to 3/4 full.

mandodave82
06-06-2016, 1:56 PM
I had my jets mixed up in my beginig post. My mains were 117, I just moved up to the 120, it did help a little. I can't remember what the piliots are. I think they we're 50 and I dropped to a 40 something when I first got the bike.

When opening the butterflies, does the main jet supply all the fuel at that point or does the pilot jet still supply fuel during throtle opening?

Biskyxj
06-06-2016, 9:19 PM
Your pilot jet supplies fuel from rolling the throttle from zero to roughly 1/4 throttle. Then as you transition into more throttle the main jet takes over.

Your accelerator pump is there to literally squirt fuel as you make a fast throttle application, like the one you are describing.

Take a look down the throat of your carb as you wick the throttle and make sure it squirts everytime.

It could be possible that your accelerator pump diaphram is torn or even out of adjust ment. Sometimes the accelerator pump is adjustable.

Westboundbiker
06-06-2016, 9:21 PM
I've posted this elsewhere, but review these:

http://www.caferacer.net/forum/attachments/technical/8769d1408111516-cb500-four-4-1-exhaust-dilema-jet-chart.jpg
http://kawtriple.com/mraxl/carb/jettingeffects.jpg
http://www.jackbhart.com/firefly/bing2.jpg
http://www.gadgetjq.com/keihin_tuningparts.jpg

mandodave82
06-06-2016, 9:35 PM
Thanks man! What's the throttle valve cut away? And the needle jet? Is that the slide and needle that's attached to the diaphragm?

Westboundbiker
06-07-2016, 6:16 AM
https://www.howtomotorcyclerepair.com/how-to-tune-throttle-valve-cutaway/

You likely won't change that- most guys just change the pilot jet, needle position, and main jet, and can get close enough. There's enough detail around on the web to get things tuned in dead on, though.