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Popping back through carb during warm up.

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Popping back through carb during warm up.
Started at 01:27pm on the 12th November, 2017 by Trew Biker
Trew Biker Subject: Popping back through carb during warm up.
Hi All, I have a 2001 Bullet 500 Clubman S fitted with bellmouth on the carb and Goldstar style exhaust. Once started, it pops back through the carb if I try to give it any revs until its warmed up. Its ok then, just occasionally popping and stalling when I pull off. Any ideas on the cause and how to fix it would be greatfully recieved.
Posted: 01:27pm 12th November, 2017
papasmurf Subject: Popping back through carb during warm up.
"They all do that Sir," Nature of the beast with those modifications. (It could also be running a tad weak, until warm.) My wife's Suzuki VS800 with aftermarket pipes and aftermarket "silencers," needs leaving for a minute or so, until it will tick over with no choke. Not leaving it to warm up and it sounds like heavy gunfire if any throttle is applied.
Posted: 01:36pm 12th November, 2017
Bullet Whisperer Subject: Popping back through carb during warm up.
I would retard the inlet cam by 1 tooth, if it were me, that will probably sort it and make it faster. I shall now wait for the flak ...
Posted: 02:18pm 12th November, 2017
PeteF Subject: Popping back through carb during warm up.
No flak from me BW, I did this to my 350 and it was a definite improvement. A mate of mine has since done it to his 500 with similar results.
Posted: 03:59pm 12th November, 2017
Alan R Subject: Popping back through carb during warm up.
Sounds like you're running weak overall---- you shouldn't be stalling when pulling away when warmed up..Have you fitted jets according to our hosts recommended settings ??
Posted: 04:46pm 12th November, 2017
Mark M Subject: Popping back through carb during warm up.
Paul (BW) is this just for the Indian engines? I was thinking of doing it on my heavy flywheel (500 wheels) Redditch 350 Trials Works Replica, any advantage? Ta,

REgards, Mark
Posted: 04:52pm 12th November, 2017
Trew Biker Subject: Popping back through carb during warm up.
Thank you all for your suggestions. I havent checked the carb settings but I have looked at what they should be so I will do that first. Ill try the cam retard if that doesn't help.
Posted: 05:18pm 12th November, 2017
Bullet Whisperer Subject: Popping back through carb during warm up.
Hi Mark, you could try it - it certainly won't take long to do and won't cause any mechanical harm. On a machine which runs well, perhaps pulling hard from very low rpm through the mid range, but runs out of 'puff' at the top end, i would say give it a try. It may not be what you want for a trials type machine, although the firing pulses would be softer at low / medium engine speeds. Basically, you will lose a little at the lower rpm scale [not much] and gain more than you lose up at the higher end of the rpm scale. The 500 [Indian] Bullets respond well to this in fairly standard form, the 350's also rev more freely and will go faster overall with the change, but don't have the engine size to cope as well as the 500 with the slight loss lower down the rev range - raising the compression ratio on the 350 helps counter this. I tried a late valve timing on a pre war AJS 250 and this sounded the buisiness until I put it in gear and went for a test ride - the engine just didn't have the 'cubes' to cope and it became slower overall, in spite of revving higher in the lower gears, it couldn't quite catch the power band in top gear. Another thing a retarded inlet timing will help with is pinking in high compression engines - the reduced cylinder pressure when the piston is coming up on the compression stroke helps here, all due to the inlet valve shutting later, the engine will be easier to kick start and any electric starter would get an easier life. I think the Indian bikes have an early inlet timing because they seem to like to slog their machines and feel the 'thump' [not good for bottom end life] and it would seem their roads are pretty rough and crowded, so it might make sense to tune them that way. I have comparison videos of my own tuned 350 Indian Bullet and a couple of 500's, as well as that AJS 250 with before and after runs, also a tuned Electra X with Redditch 'S' cams running the inlet retarded by 1 tooth - might be of interest? All sorts of stuff here. Regards, Paul. ... https://www.youtube.com/channel/UCcxcOWPcVIykzWG_lqWuRWg/videos
Posted: 05:39pm 12th November, 2017
Mark M Subject: Popping back through carb during warm up.
Thanks Paul, I've just had the engine down and fitted a higher compression piston as it was a bit flat before so I'll see how that goes first.

REgards, Mark
Posted: 06:29pm 12th November, 2017
Tim NZ Subject: Popping back through carb during warm up.
While the motor is still cold and prior to the engine reaching operating temps, the fuel is not fully atomised with the incoming air. The result is that the combustion process takes longer in a cold motor. Hence, the need for a 'choke' (Or a temp raising of the float level) en-richening function in the low speed carb circuit to provide additional fuel. Thus there is a temporary excess of fuel particles present to ensure consistent flame propagation and complete combustion.
It is this initial longer combustion process that is still occurring when the inlet valve opens that cause the hesitation, or in extreme cases 'Spitting' back even after the engine has reached operating temp. (In which even fit a smaller cut-a-way)
Retarding the cam timing to correct an otherwise normal situation (Lean when cold) carburettion process is akin to applying the hand brake when what is required is backing off the throttle.
Posted: 06:33pm 12th November, 2017
Bullet Whisperer Subject: Popping back through carb during warm up.
Retarding the inlet cams in everything I have done it with, apart from that AJS, has brought benefits. It is easy to do - and undo if necessary and worth a try in my book. The youtube link I gave above doesn't seem to access all of the videos, but this one should work ... https://www.youtube.com/my_videos?o=U
Posted: 06:42pm 12th November, 2017
Bullet Whisperer Subject: Popping back through carb during warm up.
This video might be of interest re exhaust systems, spitting back and idling etc ... https://youtu.be/owIQcjBfDAQ
Posted: 07:04pm 12th November, 2017
papasmurf Subject: Popping back through carb during warm up.
That link does not work either.
Posted: 07:07pm 12th November, 2017
papasmurf Subject: Popping back through carb during warm up.
Is this the right one:-
Posted: 07:09pm 12th November, 2017
Bullet Whisperer Subject: Popping back through carb during warm up.
Thanks for trying, papasmurf, but that last link opens ok for me and shows the 612cc bobber 'Slo Poke' undergoing light experimentation with the exhaust system. Regards, Paul.
Posted: 07:15pm 12th November, 2017
Alan R Subject: Popping back through carb during warm up.
Hello TREW BIKER----OK, so whilst the "Brains Trust" are debating the issue, go to our hosts catalogue and on page 18 you will find the recommended jetting for various different set-ups.......I take it you still have the original 28mm Mikcarb fitted ??........Be honest with yourself as to what air filtering there is / isn't fitted.....
Posted: 08:30pm 12th November, 2017
Alan R Subject: Popping back through carb during warm up.
Hi BW & P-Smurf, that link opened ok for me also---Slo-Poke going over the Level crossing etc..
Posted: 08:45pm 12th November, 2017
Paulk Subject: Popping back through carb during warm up.
"Once started, it pops back through the carb if I try to give it any revs until its warmed up" - are you knocking the "choke" off too soon? "occasionally popping and stalling when I pull off" I would re-check the pilot air screw setting according to the set up procedure not just 1.5 turns out. Also if Mikcarb check float height. Last step if it still stalls after warm - new slide as per Tim. Good luck.
Posted: 08:48am 13th November, 2017
Trew Biker Subject: Popping back through carb during warm up.
Hi Alan R, I will check the jetting according to our hosts and with the bellmouth fitted to the crb. It came on the bike when I bought it in June and I am thinking of fitting a cone filter instead so If there are jetting changes to make I can do them according to the filter I fit. Thanks to the others, the technical information is very interesting and helps a lot if I decide to do stage 2, the cam retard.
Posted: 08:56am 13th November, 2017
Trew Biker Subject: Popping back through carb during warm up.
I have checkrd the carb and the pilog jet and slide are not right and the float is half full of fuel so the level is wrong. Parts are ordered but the float is out of stock so repair may be on the cards. That will be interesting.
Posted: 08:53am 14th November, 2017

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