Carb Problem: Fuel Spraying into Venturi

[bsandersortho]

Fuel is spraying (not dribbling) into the rear carb at idle and low RPMs causing the rear spark plug to foul.

2009 Superset (760 big bore kit) with dual 38mm Mikuni carbs.

I recently pulled the engine due to a blown rear piston. Flushed the crankcase. Placed new crank seals. Re-honed the cylinders and placed new pistons. Rebuilt the carbs as part of the rebuild (new diaphragms, seals, needle and valves). The pop-off pressure is the same as prior to rebuilding the carbs (48 psi for front and rear carbs). The needle valve is 1.5 with the larger of the two springs that come in the Mikuni rebuild kit (I suspect this is the 115 gram spring).

From reading other posts, some people recommend increasing the pop-off pressure. However, the pop-off pressure is the same as prior to the rebuild. Also, the pop-off pressure is on the high end of the OEM specs (30-40 for a stock 700 according to SBT).

Any thoughts as to why I have fuel spraying into the rear carb?

 

[Quinc]

Check the pop off with a gauge. The oring around the seat could be leaking or the needle tip could be worn and leaking.

 

[bsandersortho]

Checked with pop-off gauge: 48 psi on front and rear carbs.

Also, placed new check valves on fuel pump today in case I accidentally flipped them over during the rebuild (per Mikuni concave side is suppose to be towards pump body).

I intend to put the ski back in the water on Friday, but wanted to make sure I hadn't missed something before I put the carbs back in... just don't have a good feeling that I fixed the problem yet.

 

[Quinc]

Make sure everything is tight and no orings or gaskets are being pinched.

 

[kev.t]

is the lever arm bent upwards (not flush with kidney plate), if so, the diaphragm can hold n+s open

 

[bsandersortho]

It appears to be flush with the kidney plate.

if I have the problem when I reinstall the carbs on Friday, I may try bending the lever arm down a little.

I know bending the leaver arm down (more pressure on the spring) will also increase my pop off pressure (which is already at 48 on both carbs). If vibration is preventing the needle valve from seating properly, the increased pop off pressure may help as well... but not sure why vibration would be the issue since vibration seems the same as before the rebuild.

I am concerned that I am already at the upper limit for the pop-off pressure (performance, engine seizure). Once again not sure this is the issue, since pop-off is the same as prior to rebuild. Has anyone gone higher on their pop-off pressure with a similar set up?

I have read several threads on the fuel spraying into the venturi at idle/low speeds. So I thought I would summarize the troubleshooting options I have identified to this point:

1. Generally related to issue with needle valve and seat:
a. replace worn needle valve/seat assembly
b. make sure lever arm flush with kidney plate (so diaphragm won't hold it open hen installed)
c. check pop-off pressure with gauge and confirm specs
2. Evaluate/replace check valves in fuel pump
3. Make sure all gaskets and seals are in good condition and not crimped/pinched during install
4. Make sure the return fuel line is not clogged

Any other ideas for troubleshooting the issue?

Any thoughts on my current pop-off pressure with this set-up?

 

[Quinc]

Are you running the stock flame arrestors and exhaust?

 

[bsandersortho]

760 big bore kit (Kommnader Industries)
B Factory Pipe
Kommander industries flame arrestors

 

[Quinc]

I think your pop off is going to be too high. Contact Kommander and see what there jetting and pop off recommendation is with your setup.