Carburetor Problems

Hello,
I got my parts today. I changed the oil and filter and put in a new air filter. I replaced all the O-rings, float valves and pilot jets in my carbs (VM 28s). I set my float levels to 4mm. The problem is it now runs worse than before. It won't idle smooth and back fires. The idle knob doesn't seem to have much effect.
The old float valves were 2.3's and the new ones are 2.0's would that cause a problem?
Any ideas?
Thanks

Here's an update/ more info.
It runs rough and backfires at low rpm 1000-2000 seems to run fine up around 3-4000 rpm. After a minute or two of idleing it dies out.
At full choke it runs rough but with a lower idle 12-1500 if I ease the choke down to half it picks up rpm to around 3500.

Guessing carb pilot circuits aren't yet as-new perfect.



Good Fortune!

Bike was idleing fine before I put the new O-rings and float valves and airscrews in it. When I cleaned them I hooked them up to a pump and pumped diesel through them for four days. Circuits were all clear. I blew them out with compressed air and carb cleaner.
The only big change I made today (besides the new parts) was to set the float levels at about 4mm below the top of the bowl. The bowls were only about half full before I set them.
I believe I've got the air screws adjusted wrong. I've got them turned out one and an eight turn. This is a Canadian spec bike so no pilot screw just the air screw on the intake side of the carb and no accelerater pump.
I'm thinking its running rich at idle as its smoking some and doesn't take much/any choke to start. If its been running for a few minutes you have to open the throttle to get it to start.
Should I turn the air screws out more. When I took them apart the two outside carbs #1 & 4 were a turn and a quarter and the two inside carbs were a turn and a half.

NovaNewfie wrote: ...won't idle smooth and back fires....

Back fire through the carb may result from an imperfectly sealing intake valve, often due to insufficient valve clearance.

Or might result from a timing advancer sticking in the advanced position.

Good Fortune!

Patton wrote:

NovaNewfie wrote: ...won't idle smooth and back fires....

Back fire through the carb may result from an imperfectly sealing intake valve, often due to insufficient valve clearance.

Or might result from a timing advancer sticking in the advanced position.

Good Fortune!

Thanks for the advice. The problem I have with your advice though is that it was idleing fine until I adjusted the float levels and put new valves and O-rings in the carbs today.
I looked at the timing advancer and it shows no signs of sticking or corrosion.

NovaNewfie wrote:

Patton wrote:

NovaNewfie wrote: ...won't idle smooth and back fires....

Back fire through the carb may result from an imperfectly sealing intake valve, often due to insufficient valve clearance.

Or might result from a timing advancer sticking in the advanced position.

Good Fortune!

Thanks for the advice. The problem I have with your advice though is that it was idleing fine until I adjusted the float levels and put new valves and O-rings in the carbs today.
I looked at the timing advancer and it shows no signs of sticking or corrosion.

Given correct fuel levels, the new float valves and o'rings are imo unlikely to cause the degraded idle performance and/or back firing, and would suggest there being some other reason.

Good Fortune!

If "Back fire" means popping through the exhaust on deceleration, it's often caused by too lean (for whatever reason) carb pilot circuits, or possible air leak (from whatever source).

Maybe the carbs aren't perfectly seated (for whatever reason) in the carb holders, and thereby allowing an air leak.

Good Fortune!

Took my carbs back off again today. Cleaned out pilot circuits with carb cleaner and compressed air. Verified float levels. Removed ignition advancer (looked like new) and cleaned and greased. Put back togeather. Still running rough, rough Idle and backfiring through the exhaust at idle. Seems like the motor is fighting itself.
Took carbs back off. For shits and giggles I pulled out my digital calipers and measured the new air screws in relation to the old ones. The new ones measured 2.99mm on the round shaft above the taper and 2.12mm at the end of the taper. The old ones measured 2.92mm on the shaft and 1.86mm at the end of the taper. I put the old ones back in. It seemed to run a bit better hardly noticed any backfiring.
Took the bike for a run. Came back after about 30 miles and the idle was high. I set at 1000 rpm approx, after the ride it was idleing up around 3200-3400rpm.
Its not idleing smooth but I have no idea how smooth or quiet these motors were to begin with.

When I use the choke on full choke it seems to idle very high. close to 4'000 rpm, when I take the choke off it will drop down to less than 1000 and die. If I just open the choke slightly it will run around 1500 rpm.
Is it normal for the choke to make it run that high?

Yes, you should only use the choke when you start a cold motor and then quickly back it off as the engine starts to warm up or it will run at very high rpm. After about 2 minutes or so you should shut the choke all the way off; don't run a warmed up engine with the choke on at all. After the engine is warmed up use the throttle stop screw (the single big knob on the carbs) to adjust the idle to about 1,000 rpm. Once that is set, you shouldn't need to fiddle with it again. Ed

Thanks Ed.
I know thats the way it should work. I set the idle the other day to idle at a thousand rpm and after a run of about thirty miles the bike was idleing at over three thousand. I lowered the idle down to around 1000 again.
The next day I started it with the choke and it revved to 4000 on full choke. I cut it back to just barely open on the choke and got it to idle at about 1500. When i took the choke off it was idleing so low it died.
I cann't seem to find a "set it and forget it" place on the idle adjustment.
It seems like between full choke and barely any choke there is a spot where it will pick up 2-3000 rpm around half choke. I don't think it should do this.
Most other engines i've had the choke would only raise the rpm 500-1000 rpm.