Starting with mikuni in the cold

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The 74 850 I've been working on has a Mikuni that started ok when it was warm, but now that it's cold all I can get are a couple of backfires and, at most, two or three cylinder firings like it might start. After two or three of those it's back to nothing. But if I let it sit for two or three minutes I can get a few more cylinder firings. I have noticed that there are a lot of cracks in the rubber between the Mikuni and the intake. As I move the carb they grow or contract. Have to replace that, that's for sure, but it did start pretty easy with warm weather. And which is easier to start in cold weather, an Amal carb you can tickle or the mikuni?
 
After trying to start unsuccessfully, check your spark plugs to see if they are wet with fuel or dry.
This will tell you if your choke or idle fuel circuit is working or not.
Definitely replace the deteriorated rubber mounting. If air is entering downdraft of the carb you will not get the proper air/fuel mixture needed for easy starting.
You mentioned using ether starting spray in an earlier post. Be careful, it provides a very volatile mixture that replaces what the carb should be providing.
 
I tried starting fluid early on and wont do it again. It wouldn't start and I sprayed some and it wouldn't start and I sprayed some more and then it took off hard and closing the throttle didn't do much at all and I had to grab for the kill switch. Ether was not a good idea. Where's a good vender for that short rubber intake tube? I had fumbled around trying different things till I got an easy start proceedure for myself. What stopped me before was easy hot weather, full charge (older Boyer) starting -- though there were still running problems when it got warm enough -- a little too rich with the enricher on, a little too lean with the enricher off. All the suggestions I got about that were good. I just had to get time to work through it all. But the one thing I did not see until I got it out in bright daylight and wiggled the Mikuni was the myriad of cracks in the rubber intake tube.
 
The 74 850 I've been working on has a Mikuni that started ok when it was warm, but now that it's cold all I can get are a couple of backfires and, at most, two or three cylinder firings like it might start. After two or three of those it's back to nothing. But if I let it sit for two or three minutes I can get a few more cylinder firings. I have noticed that there are a lot of cracks in the rubber between the Mikuni and the intake. As I move the carb they grow or contract. Have to replace that, that's for sure, but it did start pretty easy with warm weather. And which is easier to start in cold weather, an Amal carb you can tickle or the mikuni?

Replace the rubber adaptor. You're probably getting air in. Are you using choke? When correctly tuned, she should fire up with the Mikuni first kick using the choke. I certainly wouldn't go back to the Stone Age with Amals. You are looking at a lot more work to keep them in tune. My Mikuni was on the bike when I got it over 60,000 miles ago with minimal upkeep.
 
Throttle must be completely closed to start, and once it fired give it a little. If it doesn't respond, your idle circuit is still partially blocked.
 
Hi Concours. Warm, it starts as you say, but yesterday it was cold (below freezing) and the proper cold start proceedure for the Mikuni didn't work. I'm going to replace the short rubber tube between the carb and intake. It has all kinds of cracks in it I've noticed. I'll see what that does. Then idle circuit is next.
 
Its not wet sumping badly is it?, colder oil slowing cranking speed
I sometimes get a similar result and the only way it will start is full throtle
 
I will check wet sumping. Note though, it's kickstart and occaisionally it does make like it's going to start -- firing three or four revolutions, then it just quits. I've kicked it so much and gone through those hopeful false starts enough so some of that oil had to have gotten pumped out. And the false starts tell me it is willing and able.
 
Absolutely replace the rubber connector before trying any more starts. Not only does it make it hard/impossible to start, if it does run the fuel mix will be so lean from the air leaks that you risk burning a hole in your pistons. Drain your sump. Most likely oil has drained down through
the oil pump and this will tend to bog down the crank with cold oil. Don't ever top up your oil tank until you are sure the sump is empty.
A Mikuni is a pretty fail-safe carburetor. If the pilot circuit is not totally gunked up from sitting for months/years the starting procedure is to
push the enrichment lever down, kick through once or twice slowly with NO THROTTLE, and then a mighty kick with NO THROTTLE.
A warm engine should not need the enrichment lever down.
 
I have had very good luck with the following procedure...KEY OFF...kick it through 4 times then key on. I've also noticed that all these british bikes are slower to start in cold weather anyway.
 
Hey Joe, I've got a mikuni on my 750 and it starts in the cold (-5 degrees Celsius here today). Put the choke on and it will start in a couple kicks. It does take longer to warm up to a point that you can shut the choke off though. I have had issues previously when it wouldn't start and would backfire, turned out to be a broken connection in the boyer.
Cheers!
 
That is the first I've heard of the problem being the Boyer. At this point I'm finding out a lot of things about this 850 I bought. I'm going to do the easiest thing first ( replace the short rubber tube between carb and intake) then dig deeper. But can you tell me how you found the fault in the Boyer? Mine has started backfiring in the cold, but that could be for carb reasons as well.
 
Every Norton I've owned with a single Mikuni seems to take exactly two kicks to start. (Warm to coolish weather) I always turn the key on in case that rare occasion on starting on the first kick. My procedure is to turn the gas on for about 30 seconds, push down the enrichener, key on and kick. As others have mentioned a cracked rubber mount can cause harder than average starting. And the cracks aren't readily visible, but when you bend the rubber you can see them. I consider them an expendable item, like oil filters, brake pads, etc. And, depending on the weight of the oil, and how much is in the tank versus how much is in the sump, in colder weather, you can get a pop or two on those first two kicks before it goes.
 
Single Mikuni here too ... similar procedure ... turn both taps on , key off ,engage enricher , take seat off , two kicks , turn right tap off , enricher off ,turn key on ,kick once , starts no matter the temp , check oil is circulating, put seat back , ride ... been this way since 2004 when Mikuni was installed never fails except with cracked wire on Boyer pickup , it was from way back so replaced with mark IV ... great since
Craig
 
Your using the fuel enrichment lever on the carb to start it, correct? If you can warm weather start without it, that indicates it's set too rich from the get go. Also air leaks at the manifold will lean things out making it much harder to start in the cold.

What's the initial reason it's not starting...need more info plz

The 74 850 I've been working on has a Mikuni that started ok when it was warm, but now that it's cold all I can get are a couple of backfires and, at most, two or three cylinder firings like it might start. After two or three of those it's back to nothing. But if I let it sit for two or three minutes I can get a few more cylinder firings. I have noticed that there are a lot of cracks in the rubber between the Mikuni and the intake. As I move the carb they grow or contract. Have to replace that, that's for sure, but it did start pretty easy with warm weather. And which is easier to start in cold weather, an Amal carb you can tickle or the mikuni?
 
Starts pretty easy in warm weather with choke on and full charge on battery. In cold weather (around or below freezing) choke on and lots of almost starts, three or four firings in a row then dies like too lean, with some strong backfires on some kicks. When it starts in warm weather it wants to stay on choke to keep running, but acts like it's too rich. I'm replacing the rubber adapter between manifold and carb as a first step.
 
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