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Re: Wierd jetting issue Mikuni [Re: Hillbilly bike] #776151 06/11/19 8:15 pm
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kevin roberts Offline
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yes, i think.

the reason i have to show 11.5 on the gauge isn't because that's what's burning, it's because my head is inefficient, so to burn at 12 or so i have to waste mixture through the pipes. likerunning at partial choke all the time. i haven't measured fuel mileage in over 30 years.

Quote
I stop in deserted section and full throttle it through the first three gears to 6500 rpm, the gauge reads 12-1.I did it several more times to confirm....Open the throttle in fourth ,the gauge drops to 10.8 during the longer pull and creeps up near 12 at 6000 rpm....


i don't understand ^^^this, unless you do have reversion but over such a narrow rpm range that you're going through it too quickly to matter when you open the throttle quickly.

obviously the solution is to ride at full throttle all the time. you'll have more fun that way anyway.


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Re: Wierd jetting issue Mikuni [Re: kevin roberts] #776161 06/11/19 9:16 pm
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Yeah, i don't understand either.... so I can roll open the throttle in third gear at 4000 rpm and the A/F drops from 14 to 12 and stays there up to 6500....I do that in fourth and drops to 10.5 and stays there pretty much, Same for 5th full throttle roll on but I have not pushed it beyond 80 because I see cows near the road or there's another sharp bend coming up....


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Wierd jetting issue Mikuni [Re: Andy Higham] #776166 06/11/19 10:01 pm
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Andy,
I like this analogy to the 2- strokes. We think of our old Brit iron as having mild cams, but in fact, they all have some overlap at TDC on the exhaust stroke which will cause some reversion. All of the testing I have done has in fact shown a tendency to rich at some point in the cycle (albeit, only bikes with an A/F gauge can be verified), and it could be due to a similar situation as depending on lengths of intake and exhaust, back flow thru the carb may be causing the apparent over rich mixture. On our race bikes, tuned to take advantage of helpful reversion at high rpm, the problem becomes worse.
Tom


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Re: Wierd jetting issue Mikuni [Re: Hillbilly bike] #776177 06/12/19 12:44 am
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The valve overlap on this engine is 43 degrees... The valve timing in this engine, intake and exhaust lobe centers are pretty much the same as T120 Bonnevilles that were noted for a nice linear power band..Been my experience that this cam timing and having the carbs close to the head doesn't result in reversion issues .. However, the stock two into one T140D exhaust pipes and collector on this bike might create some reversion.......


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Wierd jetting issue Mikuni [Re: Hillbilly bike] #776206 06/12/19 6:08 am
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Air correction jets? What size are you using?


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Re: Wierd jetting issue Mikuni [Re: DavidP] #776218 06/12/19 10:07 am
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Originally Posted by DavidP
Air correction jets? What size are you using?

Good call...but 32-36 MM Mikuni TM's don't have air bleed jets..........I'm going to ride around more today forA/F readings...Once the engine is warmed up the performance from idle to wide open feels flawless to the rider. Engine responds instantly and linear to throttle movement..,better than the AMAL Mk2's....But the richness in wide open high gear is puzzling..
What also is pleasing, the modifications I did for tight piston to head squish has worked out...This engine has an actual 9.2 static compression and using 90 octane non ethanol gas there is no signs of detonation....


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Wierd jetting issue Mikuni [Re: Hillbilly bike] #776225 06/12/19 11:28 am
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Don't be afraid to try smaller main jets. I told you on an earlier post that my TR25 was still rich at mid throttle and at WOT at 5-6000 rpm on a 160 mj, but I forgot that I actually bought 4 more jets and actually am now running a 140 mj. It would seem that your 650 would have a stronger vacuum signal that my 250, so you may be able to lean it out more now that you have an A/F gauge to check.
Tom

Last edited by koncretekid; 06/12/19 11:40 am.

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