My bike sounds like a microwave full of popcorn

[Beaver]

My bike (GS750) has started 'popping' in the mid range revs. I have recently podded and gone up 6 sizes in main jet, which gave no problems to begin with. It comes from the carbs. You get a little 'blow back' coming out of the pods on start up, and then when reving/riding in the midrange it pops almost constantly. It is only under 3/4-full throttle that it stops and really hums. Then when I throttle off it will backfire quite nicely but this time through the exhaust.

I by no means consider the tuning of my carbs to be complete, but I was just thinking that with symptoms like the ones I have that someone could both tell me why it is doing this, and send me in the right direction. I am planning on syncing the carbs this weekend and would like to also check the right proceedure for that. As there are some small details like where the air mixture screws sit that I can't seem to find in Basscliffs resources. I have long hose to move the gas tank, have a sync gauge, a colour tune, a whole day with no commitments, so it would be nice to sort some of my more pressing issues while I can.

Can anyone help shed some light on my problem?

Cheers

Angus

[TheCafeKid]

You're still lean on the needle for sure. Possibly the bleeder needs more fuel too. What is your main size, needle clip position and pilot jet and fuel and air screw setting?

[Steve]

OK, you went up on the main jets, and that's good, but did you also raise the needles?
Depending on whether you have stock pipes or a header, you will want to go up one or two notches on the needle.
You might also want to check the mixture screw settings. Maybe about 3/4 turn on the fuel screw (the one on the bottom) and 1 1/4 to 1 1/2 on the air screw (the one on the side).
Your Colortune will help, but spark plug readings by doing plug chops will be the definitive determination.

I was going to automatically say "check BassCliff's site for instructions", but I just looked and did not see anything related to synching your VM carbs.

To sync your VM carbs: Start the engine, warm it thoroughly, set the idle speed. I like to do it right near idle speed, about 1100-1200 rpm, but the Suzuki manual suggests 1500-2000 rpm. What vacuum gauge are you using? Does it need to be balanced to itself? Now is the time to do that, if necessary. Once the gauge is balanced to itself, stop the engine, connect the vacuum tubes to the intake tube fittings. Start the engine again, still at your preferred speed, observe the readings on your gauge(s). If they are not inline, remove the tops of the carbs and adjust by using the screws there. Loosen the locknut, tweak the screw, tighten the locknut. On the VM carbs, there is no "master" carb, so you can do them in any order you wish. When itis all done. make sure the idle speed is proper (usually about 1000-1100), stop the engine, put the carb tops back on, remove the adapters from the intake tubes, put the plugs back in where you took the adapters out. Now take it for a ride and enjoy your new, smooooth bike.

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[Beaver]

I am running a Marshall Deeptone, but the header pipes aren't as large as say the Kerker I am trying to put together to replace it. The only thing I have done is changed the main jets, as much of what I had read in here led me to the conclustion that I should only raise the needle if nescessary. Which it sounds like it is. So one notch for stock pipes and 2 for headers? I realise that this is only a rough guide.

I have the motion pro (cheaper model) from Z1. I have tried using it before but didn't know about needing to have access to the tops of the carbs. Thanks for the nice description of how to sync the carbs by the way. I am feeling much more positive about going into my weekend project now. Although I might need to add that the bike died on me 200m from home (it was pouring down and I suspect a fuse has blown) so I will have to see how easily I can get it to the workshop first.

I can't remember which jets are sitting in there, I would have to check at the workshop. I am pretty sure that they are 122.5 or 125. I ordered 2 sizes from Z1 and they were the larger of the 2 that I received.

Cheers

Angus

[TheCafeKid]

Well you say it smooths out after 3/4 throttle and if it really seems to run well up there (which by that point you're nearly solely in the main and off the needle) then I'd bet youre either on with the main or at least damn close. A throttle chop at that range and plug read will tell you for sure. If it's dark you're a jet too big light or ashy too small otherwise leave it for now. I'd suggest moving the needle clip down two notches from stock at first blush but without riding or hearing it it's just a guess. Usually I've found one clip possition won't result in spitting but more just surging and slight bucking on steady throttle in the needle circuit. Spitting and popping sounds more than one possition lean. As for your fuel and air screw settings Steves suggestion would be where I would start. Leave the fuel screw alone unless it's drasticly rich on the pilot chop readings and adjust to suit with the air screw. If you get more than three and a half turns out on the air screw and it doesn't seem to make it lean out the pilot circuit any then back off the fuel screw a 1/4 turrn. And start over. ( this ofcourse is assuming your float levels are spot on across the board) once you're relatively happy with all of that then sync the carbs and adjust the pilot air screw for highest idle method. Remember though that if your needle circuit is not responding the way you like don't bother syncing untill it's close. With the VM carbs everytime you adjust the needle clips you have to resync because you have to pull the throttle arm out to pull the slides to get at the needles. Hope this helps

[Beaver]

Wow!

Everything went well, I didn't lose any parts, and managed to figure out where the extra parts that I had left at the end belonged. Hmmm, where to start. The fuel screws were 4,3½,2½, and 2 turns out on each of the carbs to start with. When I went to sync the carbs, with the fluid about ½ way up the vaccuum gauge at callibration, one of the carbs didn't even register it was so far out of adjustment. Needless to say I felt like I had gotten a new bike once I finally got it out on the road. It has never idled so well before and I now have power all the way through the rev range instead of just at 4000 and above (it used to be like riding a 2 stroke with powerband)

The irony is that today is the last day that I can ride it till the spring if I want to be covered by my insurance I suppose now I can finally get started on pulling the back end off.

Thanks guys for your help, it was most appreciated!

Cheers

Angus

[Steve]

Needless to say I felt like I had gotten a new bike once I finally got it out on the road. It has never idled so well before and I now have power all the way through the rev range instead of just at 4000 and above (it used to be like riding a 2 stroke with powerband)

The irony is that today is the last day that I can ride it till the spring if I want to be covered by my insurance

At least you will rest easily over the winter, knowing that it runs well.

Just don't forget to stabilize the gas, keep the battery charged and DON'T start the bike just to hear it run.
In fact, don't start the bike unless you are going to go for at least a half hour ride to warm it up fully and properly.

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