backfire on startup

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alan hodge

alan hodge

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bike starts first kick but...last time or two bike let out a kapow from left muffler on startup (with very quick and slight puff of black smoke) when warm still feel very slight misfire when put hand over left hand muffler outlet... has pazon and single mikuni .. battery is somewhat fishy ordered a new one would weak battery make backfire on startup? also both plugs somewhat sooty the mikuni is 32mm Sudco straight out of box have read they are jetted too rich bike runs good otherwise...I use nonethanol…...thanks for any re-marks
 
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Well I would say as it’s just on one side might indicate a problem on a side specific item. Spark plug and coil spring to mind. It really is a matter of standing in front of the bike with a cuppa tea in hand and formulate a logical plan to try and isolate/understand the problem.
 
Try and get the miss to move to other side one component at a time and you will find the issue ...
 
put new NGK BP7ES plug in and no kapow and no misfire carb is very rich as out of box coil new as is 90% of bike. jet suggestions for Sudco kit??
 
Where is needle clip , maybe move it to leaner position? should have 5 grooves try moving just one position at a time .... Up is the direction to go
 
Alan , I have a VM34 on a 750 .... from memory the slide is 2.5 , the air jet is 1.0 , the main is 220 , the rest I would have to look .... bet some one with a more similar bike might chime in .... if not I will pull it apart and get back to you , I knew I should have made notes , dah .....
 
No trouble should be able to have a look Thursday once our Grandson goes home .... that is unless you get info first ....
 
Suggested jetting for a VM32 on an 850 is: 25-30 pilot jet, 3.5 slide, P-2, P-4 needle, 6DH2 needle, 230 main jet, air jet 1.0
 
There you go Alan ! hope you got that stuff layin around , slide seems a bit large , I will take a look see in my Mik box ...
 
maybe my memory has failed because I thought carb is 32mm but could be 34mm how I tell difference? whole thing came as kit with manifold etc.
 
Size: The bore size is etched on the intake face of the casting that the choke (enrichener) lives in, very hard to see. You can also measure the ID of the bore with a caliper. I would think that Sudco would have sold you a 34mm VM kit. Jetting for a stock 850 engine would be 3.0 slide, 2.0 air correction, 159 P-2 needle jet, 35 or 40 idle jet, 230 or 240 main jet, 6DH3 needle with clip in position 3 (mid). The slide and air correction jet are as supplied from MIkuni. The carb also came with a 35 idle jet which should be OK, some of my customers use a 159 O-8 needle jet and a 230 main. These suggestions are for sea level, if you are above 5000' post the range you ride in and I'll update my suggestions.

Best
 
Does your Pazon ignition require a resister component, like resister spark plugs, or resister plug wires

if so do you have resister in place....

get some new plugs anyway since yours are dirty, fouled? resister plugs would be NGKBPR7es - the R is for resister
 
here's what I gonna do...…...this may 'spark outrage' as the talking heads say on cable news but...…..here goes my spiel my BSA and Triumph call for Champion N3 but because I ride low and slow I use N4....one heat range up and plugs look OK...I have tried NGK BP7ES and Champion N7YC in commando and both brands look sooty....as I said SUDCO kit jetted..I ride the 850 low and slow as well never over 55 and on back roads on Sunday mornings to clear my head.....that's how I roll.....I am going to give NGK BP6ES a try...one heat range up....ride for about 30 miles and see how they look...
 
Why not jet it correctly? I would. I would buy a handful of pilots and start from there.,
Buy the jets here https://www.jetsrus.com/
If you can turn the mixture screw out more than 2 1/2 turns and it doesn't falter, you're too rich.
My 750 Combat didn't clean up till I got it down to a 25 pilot and a 240 main.

BTW, one of the best investments ever was the Mikuni carb tools.
I use them on EVERY carb, including Amals.
They have the correct driver tips. I haven't buggered a screw or jet since getting them.
And, they ONLY come out for Carbs, nothing else.......
 

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put fresh BP7ES and backfire gone also put fresh battery in the one i had was not holding charge very well rode bike sunday it ran great plus still a little sooty....also on the spark plug scene i ordered some BP6ES ones but a day later spotted some Champion N4G on ebay for $9.95 a pair so ordered those too rain coming over the next five days so wont get to ride but when sun returns will use the N4Gs
 
Are the NG4 hotter than the called for champion 7’s I’m using now , I have had great luck with ngk BP7Es as well ... did end up with some ngk knockoffs that put me off them , so back to original seem to work better than what I remember , maybe I got bike tuned better ....
 
I think the slide is your problem. Use a #3. And go back to the NGK BP7ES. The needle jet is a P-0 or P-2
 
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