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Post by strick on Jun 15, 2008 17:42:43 GMT -5
Now the the bike is running (engine and carb rebuilt), it seems to fall slowly back to idle. The mixture screw does not seem to tune it out. I can raise the idle with the mixture screw and I can choke off the idle, but nothing seems to allow tuning the slow return out.
I have a stock main and and pilot, K&N filter, 10:1 Wiseco, and large header exhaust.
Any thoughts or suggestions?
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Post by stevec5000 on Jun 15, 2008 18:12:23 GMT -5
If you want the speed to fall back to idle faster you will have to get the throttle to close faster.
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Post by chew652 on Jun 15, 2008 19:06:37 GMT -5
Now the the bike is running (engine and carb rebuilt), it seems to fall slowly back to idle. The mixture screw does not seem to tune it out. I can raise the idle with the mixture screw and I can choke off the idle, but nothing seems to allow tuning the slow return out. I have a stock main and and pilot, K&N filter, 10:1 Wiseco, and large header exhaust. Any thoughts or suggestions? This usally signals an air leak or lean condition. With the bigger pipe and K&N you might have to richen things up.IE bigger pilot jet ,raise the needle a notch or two ,maybe a bigger main jet. www.siue.edu/~rsutton/SR/carbmod.htm Check for air leaks first. |
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Post by strick on Jun 16, 2008 0:22:40 GMT -5
Thanks for the replys. I was so excited this weekend to finally ride the bike I didn't even think about checking for air leaks. However, NO backfiring!
All the throttle presets are set properly, so the throttle return is working correctly. Even all the cables were siliconed and checked for free movement.
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Post by wotavidone on Jun 16, 2008 2:00:07 GMT -5
There is a diaphragm in the stock carb that lets a little bit of fuel in on overrun. You shut the throttle, the manifold vacuum goes up, opens this diaphragm and thus lets in a bit of extra fuel. If it is slow to return to idle due to a lean condition, this is worth a look maybe?
But, I agree that standard jetting might be a bit on the lean side for your mods.
Mick
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Post by jimnmad on Jul 8, 2008 1:00:06 GMT -5
I replaced the diaphragm on the bottom of the carb. The triangular one , not the square accelerator pum. There was no change. Did anyone ever get a resolution? What was it?
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Post by strick on Jul 8, 2008 9:43:46 GMT -5
I have been too busy to yank the carb back off. The return to idle is slower than I would expect, but doesn't 'race' when the throttle is released. I will get to it and correct it and will post back. But right now I needed to get my bike street legal and get a plate on it.
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Post by StewRoss on Jul 9, 2008 2:22:42 GMT -5
I'd be tending a little toward the over-run diaphragm myself...just a thought... SR  |
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Post by strick on Jul 9, 2008 17:54:15 GMT -5
Can you explain 'over run diaphragm'? Are there two diaphragms: one accelerator pump and one = ?
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Post by jimnmad on Jul 9, 2008 22:36:17 GMT -5
Hi, I replaced the triangular diaphragm on the bottom of the carb. It made no difference. It did have a hole in it so it may be contributoory. I got a rebuild kit from the Old Bike Barn and replaced everything that I could except for the float needle assy since I was having no problems there. I have a 1979 and it had the factory lock out on the pilot adjustor screw. I removed the lock out and screwed the needle in 3 full turns before removing it and replacing the O ring on it. I put it back in and gave it 1 1/2 turns. I was then able, after I got it running to use the throttle stop screw and the procedure for setting the pilot adjuster to FINALLY get a proper idle. Without the very slow return to idle. I never changed the throttle stop and the fast idle came on rather suddenly. Maybe an old blockage finally came out? I have owned this bike since 1982 and it only has 10,000 miles on it. It has never idled as well as it does now. It still will pull from idle through full throttle. I wonder if the pilot was parialy plugged, open too far,or what. I have always adjusted the idle with the throttle stop because the pilot didn't seem to have much effect. Anyway, it works for now ,and I know a lot more about carburetors
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Post by aero on Jul 10, 2008 3:22:08 GMT -5
Sluggish return to idle is usaually down to several things. The carb sticking, lean mixture, or tight inlet valve clearances.
Sounds like you had a lean idle mixture if its cleared itself.
The overrun valve shuts off the fuel supply when there is a high vacuum in the inlet (throttle closed with the engine turning at higher then idle speeds) to stop fuel being drawn into the engine. Its an emissions control as well as stops popping in the exhaust
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Post by StewRoss on Jul 10, 2008 3:35:08 GMT -5
Hi Aero, Yes, struck me that it could possibly not shutoff and in turn allow some fuel through ...I am much happier replacing these carbs when they malfunction with something like a 38mm Mikuni or similar. Simple and straight forward....one cable no pump diaphragms or any other type for that matter and they need next to nothing to keep running for many years trouble free. Here in Aus it would cost you about $340 for an opening cable to be modified to suit and the cost of the carb jetted to suit as well. Small price for simplicity and a probable performance gain. Especially if you free up the exhaust flow and put in a high comp piston as well. Thanks to a German site for this pic...   SR  |
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