Carb Puffing Back

Hi Guys, 

Got my first bike running and idling, but 2 of the cylinders will intermittently puff back through the carbs. It seems to only happen at idle and is maybe once every 15 seconds. I have done some of the main checks I have seen discussed on here but have not found anything conclusive. Here is what I have. 
- KZ650 bored to 717 
- 26mm Carbs off 900
- 115 main and 17.5 pilot jet
- Head was done by reputable tech, he did valve job and clearance (I didn't check his work)
- Carbs fully cleaned, float bowls set with clear tube
- Carbs bench sync then checked with vacuum gauge and all cylinders are 20 + or - 2 cm.wg
- Tried adjusting the idle air screws, started 1.5 turns out and adjusted slightly for highest idle rpm (I find it hard to hear a difference at middle adjustment, could only tell when the cylinder is bogging out when turned over 1 turn in or out from start point)

Any help would be great, this is all a first for me. 

Are the two carbs back firing on the same coil? 
If they are you could have a slight timing issue on that coil. 
That said, lean mixture is the normal reason for back firing through the carbs. 
Suggest you use a colour tune plug to adjust the idle mixture accurately. Also test for intake air leaks on the back firing carbs which would also cause a lean mixture scenario. 

Hey Warren, 
It is cylinder 1 and 3 which were doing it. I check timing statically by rotating with a test light and checking for voltage at the F mark prior to top dead, correct me if this is wrong. I should check with a timing light. With a timing light it should flash at the T mark correct? I also sprayed WD40 all over the new boots and it didn’t affect the idle. What is a colour tune plug?

Colourtune plug is a way to actually see the colour of the combustion of the air/fuel mixture. Blue burn is lean, orange is rich. It comes with instruction on what to look for and how to use. You need the 14mm version. 
www.gunson.co.uk/product/G4074/Colortune-Single-Plug-Kit-14mm

Timing should be on the "F" mark with either a test bulb or a strobe.

WD40 is not great for testing for air leaks use aerosol carb cleaner.

Another thing to check is that the spark plug threads are clean and free from any dirt or grease. They need to be very clean to get a good earth on the cylinder head. 

I've had carbs puffing back twice. Each time the cause was cracked carb to engine boot. I see you mention new boots so perhaps something like  missing or cracked vacuum caps?......if your carbs require them.

Yes, I have new boots and they have rubber caps. I sprayed WD40 and didn't hear anything. Maybe I will try another fluid to check. What have you found works best for locating vacuum leaks? 

sflesch94 wrote:

Yes, I have new boots and they have rubber caps. I sprayed WD40 and didn't hear anything. Maybe I will try another fluid to check. What have you found works best for locating vacuum leaks? 

What worked best was just slow careful examination. Many problems are not immediately obvious and only uncovered after a slow and thorough check. If you are located in USA then perhaps you got that clean air system that we were spared in Canada. From many posts here that system was problematic when it aged. Other things to check would be carb fit in the boots, hose clamps etc

As for sprays I don't think the are much use unless the air leak is large.

PS. My puffing back was also only noticeable at idle and low speed
 

Start gas?

So I got some starter fluid and sprayed all over the boots and did not find anything, made sure all was in good condition. Decided to check timing with a timing light, and the the cylinder which is blowing back mostly seems to have an intermittent spark. The other cylinders have a steady timing light flash, and cylinder 1 and 3 are coming on and off intermittently. Checked all my wiring connections which seem tight. Checked my battery which I just charged and I am getting 12.6V at the battery with ignition on, and only 10V at the electronic ignition power supply. Looks like I need to look into a wiring issue. Does this seem possible that reduced voltage could be causing a firing issue? It is worst at idle and seems to clear up when I rev it up. I am also not seeing a voltage increase on my battery with the bike on, appears to be a charging issue as well..... oh boy. 

When you say "ignition supply" are you talking about the supply at the coil? Yes that big a "Volt drop" will cause issues (weak spark etc) and will probably increase the longer it is running. 

I am checking the voltage at the red/yellow wire which comes from the right handlebar ignition switch and supplies voltage to the ignition module. Seems maybe I have a voltage drop through the harness or the ignition switch itself. 

Volt drop on the coil supply is a common issue due to aging and the number of contact points this supply runs through. A common and "cost effective" solution is to fit a "by-pass" relay to take the coil supply directly from the battery (see below link)

Ignition by-pass relay W - KZRider Forum - KZRider, KZ, Z1 & Z Motorcycle Enthusiast's Forum