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RT-1B CARB ISSUE

  • Mark350
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RT-1B CARB ISSUE was created by Mark350

Took the RT-1B out for a ride, seemed OK thru all the gears under both hard a light pulls. So messed with the mixture screw and it runs best with the screw way out near totally unscrewed? Everything is stock, carb, needle, main jet etc... Any ideas 
03 Aug 2022 13:47 #1

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Replied by RT325 on topic RT-1B CARB ISSUE

Really need a bit more info. In theory mixture only affects idle & slightly above so possible is too big in the pilot jet. But thinking maybe its worn in the needle jet [slide needle & emulsion tube that is] then the pilot could kind of help overcome that a little if you lean it right out as you have. I dunno, so many variables. Oring into the bowl where side main jet feeds is critical but more for an open throttle--well half throttle too i guess. Emulsion tube & needle wear need close eyeballing to see the wear but needle clearance is a good indicator. Just taking wild guesses sorry to start the ball rolling. Choke closing off. Not plastic seal cap holding it off the seat.
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03 Aug 2022 14:54 #2

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Replied by Swoop56 on topic RT-1B CARB ISSUE

RT1's had two separate carb setups . Perhaps you could detail what the various
components actually are in your carb , and we can possibly advise further .
03 Aug 2022 18:02 #3

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Replied by RT325 on topic RT-1B CARB ISSUE

Hey Swoop, how's it going down there. I'm keen to learn the differences, as only know what i know & didn't know 'that' haha sometimes i doubt myself. Cheers.
03 Aug 2022 18:41 #4

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  • Mark350
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Replied by Mark350 on topic RT-1B CARB ISSUE

Well... all the inners are new and stock, all the correct pieces based upon the shop manual; 240 main, 0-4 needle jet, 6CF needle,1.5 cutaway, 30 pilot.   Cleaned it up and checked everything, many times! I reset the mixture screw and got it to run pretty good at 1-3/4" turns out, but now it sputters up near wide open throttle, and then sometimes from 1/8-3/4 throttle. The mixture screw seems to have little effect on the idle speed and or quality. Further out will slightly raise the RPM. Idles great and starts well, hot or stone cold.   Cir-clip is on the second needle jet groove from the top, runs worse on the top groove and also on the third. Second groove seems best. I'm at 1000 ft. ASL, cannot imagine the main jet is to fat at this elevation? Plug looks fine, not black rich and not new white either, kind of a dark tan, but I have not done a cut test. Sometimes it runs pretty good, then it sits for a while and this reoccurring intermittent poor running starts up again. Air cleaner is new, no vacuum leaks, new piston, barrel gasket, intake gasket, damn! Any help appreciated.  
03 Aug 2022 19:19 #5

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Replied by RT325 on topic RT-1B CARB ISSUE

Oring's been mentioned i guess & replaced so can feel the tension fitting the bowl.
03 Aug 2022 19:33 #6

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  • Mark350
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Replied by Mark350 on topic RT-1B CARB ISSUE

Oh yes,  nice press fit. 
03 Aug 2022 20:08 #7

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  • MarkT
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Replied by MarkT on topic RT-1B CARB ISSUE

Air screw response sounds normal at idle.  Standard setting is 1 3/4 turns out. 

Standard Needle is 6CF1 clip in 2nd groove from top.

Air screw is nearly useless at anything above idle throttle setting...  so likely a "coincidence" if it seems to change running at higher throttle settings?

Maybe "governor" (timing advance) in flywheel sticks in "start" position (I can never remember if "b" has the advance still)? 

Or points are getting dirty? 
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
03 Aug 2022 20:10 #8

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Replied by Swoop56 on topic RT-1B CARB ISSUE

As you have said , all those parts listed were the correct pieces for an RT1 B .
The 70 RT1 used a different needle , but not relevant here .
If your needle and needle jet were old , I'd suggest replacing , but as yours are new ,
I'm at a loss for suggestions .
As Mark mentioned earlier , I'm also surprised your air screw adjustments are affecting
mid range and top end performance .
What air filter are you running , and are you using the stock Air Box ?
04 Aug 2022 00:55 #9

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Replied by RT325 on topic RT-1B CARB ISSUE

Just a bit more about idle screw. In extreme cases of rich pilot jet & mixture air screw turned right in by mistake, it will have an affect of. Ok i'll start again. If you're on an open throttle bobbing along then shut off for a stop sighn or sharp bend & then bring it on again it'll come up rich then clear & carry on untill the next similar situation. That's an extreme case though but been there seen that when people play with 'that loose screw'. Not saying 'you' but just generaly speaking. Can see you're well versed in how it all works.
Last edit: 04 Aug 2022 02:55 by RT325.
04 Aug 2022 02:51 #10

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