Dr Big

Full Version: popping banging on overrun ccm suzuki 644
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I've had my ccm suzuki 644 for a couple of years now,it runs very well except it has always banged and popped a lot on the over run when throttle is completely shut and tickover is at 1500rpm but if i increase tickover to 2000 rpm it cures the problem.--2000rpm is really to fast-- I've carefully checked the exhaust has no leaks.bike has 2mikuni bst33 carbs seems to be setup in a similar way to drbigs.Hhe 2 mixture screws underneath the carbs are now 3 turns out but this does not seem to make any difference.---does anyone have any idea how i can cure this problem other than an ultra fast tickover? I haven't tried to adjust any thing else on the carbs.
Thats how it it supposed to be..... just leave it as it is Thumbup
That cant be right.Spoils a really nice bike.neighbours are complaining.---I dont mind the popping,its the occasional loud backfire thats the problem, just as i'm pulling into my drive.
Had this myself a few times
Almost certainly it`s air being sucked in at the silencer/downpipe connection that is causing this.
You need to reconnect your silencer using gun gum or similar
Make sure the header pipe has a good connection at the exhaust port as well
Trouble with the CCM pipe is that it has springs holding together on one of the joins to allow it to move. Remember when you roll the throttle off you create more vacuum which in turn draws more fuel through the idle circuit hence chucking fuel into the hot exhaust.

You have a few options as I see it:
Check idle mixture is not over rich (sounds rich to me if it actually backfires)
Change down at lower RPM.
Buy earplugs.
Buy the neighbours earplugs.
Tell the neighbours to F@#K off.
Sell it to me Thumbup
well today I rechecked my exhaust.I couldn't find any obvious leaks, but concentrated on that spring joint,reasembling it with exhaust assembly paste. I just tried it out--- no obvious change.---
my pilot screws are 3 turns out,does that seem likely to be too rich? I would have too weak ( needing bigger pilot jet ) might be the problem ) I'll try turning them in a bit to weaken the mixture,if that doesn't work I'll investigate getting bigger pilot jets.
do only bst33 pilots fit,or will other mikuni jets fit?
There are variations in jets such as the angles of the openings and the overall size even if the hole is the same size. I recommend you buy the jets from a reputable dealer that understands the model of carb. That said, If you have the standard setup then I think you need to look for other problems elsewhere which would cause the backfire. All CCM's pop on over run which is more noticeable due to the exhaust which is not like one from a mass production giant like the japs we know & love. You have the "gubbins" which messes them up plus the TPS needs setting correctly which affects the timing.
I know I'm showing my ignorance Nug but what and where is ''the gubbins''
Its a series of vacuum hoses etc which need removing & blanking. cant remember exactly what to do as it was a good few years ago that I did it. You may find some archived CCM riders forum posts if you google it. Sorry I cant be more specific. Your pilot screws are a long way out I reckon. Larger pilot jets wont necessarily help as all you are doing is allowing the pilot screws to be leaned off. Do you still have the airbox lid fitted & is the can standard ?
I would set the carbs to factory settings (including the needle heights) then start from there. Use the Suzuki freewind xf650 setting as a baseline as that is the spec of the motor.

Another thing to consider is the float heights. It can have an effect on the fueling. They also suffer from the float valves leaking and flood in the motor. I had this happen on mine.
The needles wouldn't seat correctly when on the sidestand. When you left the fuel on it would sometimes flood & fill the cylinder with fuel !

It may have already been messed with as most owners liked to do "the gubbins".
yeah.gubbins has gone. I think these carbs are bsr 32's not bst 33's. haines has them at £7.25 each ---pricey for pilots.----- thanks for the advice.
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