Engine Not firing, suspected carb issue

Hey guys.

I've been a lurker for a while,as I've had my bike in pieces for quite a number of years. I've finally got it all put back together, but I'm having issues with getting it to fire. I've got spark on all cylinders, I've cleaned and inspected the carbs. The bike has a V&H 4-1 exhaust and K&N pods on the carbs. The secondary main jet is set at 135, as I can't find any pilots to replace, so the pilots are at 3 turns out. What should I try? I'm at a loss here.

Wow, is it a Vincent Black Shadow? I've always loved the look of those bikes, and they were wicked fast! :laugh: :laugh:

Seriously, add your bike's year and model to your signature line so we can at least try to be helpful. Ed

Yeah, that would totally help huh? wow... it's an 83 kz750 motor in an 81 kz750 frame.

That engine will have CV carbs which can be quite difficult to tune for pods, so maybe someone here can provide a starting point for jetting. Other than that, when you try to start it do you have choke on, petcock in "Prime," and don't touch the throttle? Ed

I had the petcock in prime, but switched it to run, as I didn't want the engine to flood. I did touch the throttle to see if there was any response.

Got a bit further. The engine sputtered, but the carbs overflowed before it completely fired up. Maybe the timing is off? I'm not sure where to go from here... Thanks

You should find and resolve the carb overflow problem before going any further. There is real potential if the carbs overflowed that fuel may have also flowed through the carbs into the intake runners and down into the cylinders. If that is the case, it will have contaminated your oil. You can try the "sniff test" to see if the oil smells of gasoline, but that may not be 100% accurate.

I cannot speak to CV carbs but below are the overflow conditions encountered on VM carbs. CV's may have similar issues.

Fuel coming out of the overflow hose (the little hose on the bottom of the carb) is the common symptom of the 3 possible problems described below. Remember - NO SMOKING or other activities that could ignite the fuel while working on carbs!

The float valve is not sealing. This may be (and probably is) just a matter of some minor dirt in the float valve and is very simple to remedy and can be done with the carbs still on the bike. Turn off fuel; drain carb by loosening big brass screw near bottom; and remove the 4 little screws on the bottom of the carb bowl. This will enable you to remove the carb bowl. You will then see the float. If you carefully remove the float you will see a stubby little needle that mates with a brass orifice; together, these two pieces are the float valve. Assuming there is no obvious damage to the needle or seat, use a Q-tip and some carb cleaner to clean the seat of the orifice and the needle. Be careful not to bend the tang that is attached to the float. This tang is the piece that the bottom of the float valve needle rests upon, and it determines the fuel level in the bowl, so bending it will change the fuel level. With the float valve cleaned it should no longer leak. To prevent a re-occurrence a quality inline fuel filter should be installed between the fuel tank and carbs. If there was damage the needle and seat should be replaced as a set; they are available.

The brass overflow tube inside the carb bowl is damaged. When you remove the carb bowl you will see a brass tube attached inside and rising toward the top of the bowl. This is the overflow tube and it leads directly to the overflow nipple on the bottom of the carb. Normally, the fuel level within the bowl rises somewhat close to the top of this tube. If the tube becomes cracked or separates from the bottom of the carb bowl fuel will flow through it and out through the overflow hose. Checking the condition of the tube is very easy after you have removed the carb bowl from the carb. Simply hold the bowl level and fill it up near the top of the tube with water of alcohol and see if it leaks. A slight crack in the brass overflow tube can be difficult to find. You can connect a rubber tube to the overflow nipple, fill the bowl with water, put finger over the open end of the brass overflow tube and blow in the rubber end and look for bubbles. If it does the best remedy is to replace it although some folks have found creative ways to repair them.

The fuel level is set too high. This condition occurs when someone has bent the float tang as mentioned above. This condition can be checked using what is referred to as the "clear tube test." The test involves attaching one end of a clear piece of flexible tubing to the carb drain hole and holding the other end of the tube above the bowl/carb joint. When the fuel is turned on it should rise in the tube to a level 2.5 - 4.5 mm below the bowl/carb joint. If the fuel rises higher than that level it may run into the cylinders or over the upper end of the brass overflow inside the carb bowl (mentioned above) in which case it will run out the overflow hose. This condition is corrected by removing the float and gently bending the tang to raise the float valve needle position. Doing this may take several tries before achieving the desired fuel level.

Correcting each of these three conditions involves removing the carb bowl. This task can be made easier if a short screwdriver bit is used, and depending on which carb you are fixing a mirror may help you locate the screws on the bottom of the carb bowl. Also, it is not a bad idea to have a new carb bowl gasket available in case the old gasket is damaged while removing the carb bowl. Ed

The first thing done on any carb tuning is checking to make sure the float levels are correct and the float valve is not leaking. All settings in the carb are effected by the float level so do that first. You chould have either Mikuni BS34 carbs or Keihen CV34 carbs either way, 135 main jets are WAY to big for your engine. There are pilot jets in the carbs which should be either a #37.5 or #40, the needle will not be adjustable.
I would tear the carbs down and make sure they are spotless, set the float levels put in 120 main jets, install the #37.5 pilot jets and set the mixture screws to 1 1/2 turns out.
If it still wil not fire try spraying starting fluid into the carbs while cranking, if it fires then you know you still have a fuel issue. If it does not fire I would suspect an ignition issue.

Baldy,

with the 4-1 exhaust and the K &Ns, the 135 jets were suggested everywhere I could find... I have the keihin cv34's I guess I'll pick up a few different jet sizes and start small and see where it goes. I had checked the float mechanisms, but I did not check float height stupidly. Pikot jets are also pretty much unobtainable (unless you have an idea as to where to purchase them), so I'm kinda stuck with what I have, which makes this a bit more difficult.Thanks guys for the info

Your carbs must be the Keihen CV34 those are harder to find parts for. Let me put things into perspective here regarding jetting. I have a highly modded GPz1100 engine with a 1135cc MTC big bore kit, 4 into 1 KR Kerker pipe with the huge competition baffle, pod filters and huge Mikuni RS36 flatslide carbs. It puts out around 135hp. The jetting I finally came up with for these is mains 138, pilot #40. I do not go by what is "suggested" for jetting usually their wrong, every bike is different but 135 main jets on a stock 750 is way to big.
Another option and the parts are much easier to find would be find a set of Mikuni BS34's. These came on thousands of KZ1000's and the early 82 GPz750. It's the same carbs that Kawasaki put on the KZ1000 Cop bike they made until recently.

emptypond wrote: Baldy,

with the 4-1 exhaust and the K &Ns, the 135 jets were suggested everywhere I could find... I have the keihin cv34's I guess I'll pick up a few different jet sizes and start small and see where it goes. I had checked the float mechanisms, but I did not check float height stupidly. Pikot jets are also pretty much unobtainable (unless you have an idea as to where to purchase them), so I'm kinda stuck with what I have, which makes this a bit more difficult.Thanks guys for the info

I see you have an 83 motor in an 81 frame and the carbs are Keihin CV34. Stock jetting is 120 or 125 secondary and about 60 or 62 primary.For several years I had a Mac 4 into 1 exhaust and K&N pods on my 1980 750/4 E1 and it ran fine with 135 secondary mains and 62 primary mains so I would think you should be able to get your running with that jetting. I dont think there would be a huge difference between the 80 and 83 motors that it wouldn't run. Mine ran better with 140 secondary mains and 70 primaries. Yes the pilots are impossible to find but I never needed to replace those. Eventually I put an air box on the bike but I was aiming for a stock look.

Like Ed and Baldy said, you need to fix the carb issue first. Set fuel level using the clear tube method - very important. I think service fuel level is 4 +/- 1 mm for those carbs but you should check the manual. Make sure pilots are clean, no air leaks etc

One other thing. If you have just put the bike back together make sure that you have the coils wired correctly and the green and black wires are going to the correct coil otherwise you would be getting spark on the wrong stroke. I've done this myself

Good luck

JR- thanks! I just had wiring issues that I sorted out last week. I bought new dyna coils and installed and checked them. I did find the float needles slightly dirty, so I cleaned those, which I think was the overflow issue, we shall see when I reinstall them. I also noticed my idle adjustment was set quite high (open) so I adjusted that. We shall see what happens when I put it back together this weekend. Thanks for the advice everyone!