Left cylinder running rich, single VM36 carb.

[Olly]

Hi, I have an '81 xs650 engine with a single Mikuni VM36 carb. For some reason the left hand cyclinder is running very rich, but the right hand cylinder is ok. Any ideas as to what could be causing this? The left hand cyclinder runs ok if I clean the plug but quickly fouls the plug and misfires. I've tried swapping the plugs and HT leads over and the problem is always the left cyclinder.

 

[hein weijers]

Hello Olly,
Is the left hand plug wet (petrol) or oily? Do you see blue smoke coming out of the left hand exhaust?

 

[fredintoon]

Hi Olly,
you don't say if the engine has been like this since you've owned it, or since the single carb manifold was installed, or if it's a new phenomenon.
Anyhow, it's more likely an engine problem than a manifold problem.
Try a compression test, then a leakdown test then rent a borescope and take a look down the plugholes.
You may be looking at a head job and perhaps a rebore too.

 

[Olly]

Hi, thanks you for these responses. hein, the plug is sooty rather than wet and oily. fredintoon I should have mentioned that I did have a problem before when the engine had the standard bs38 carbs. The left cyclinder would cut out altogether, and appeared to be getting no fuel. At first I thought it was a carb problem and that thought the single carb had resolved it (it's taken a while to get the jetting sorted) but it seems to still be an issue although slightly different symptoms.

 

[Olly]

fredintoon I got a friend to compression test it: 120 on each cylinder (he thinks his gauge reads low). Is there a guide on this forum to doing a leak down test?

 

[fredintoon]

Hi Olly,
use the "search" button to see if the list has a leakdown test.
The 120psi both sides compression test indicates the rings and valves are both working acceptably.
If there's excessive oil in one cylinder I'd suspect that particular cylinder's valve stem oil seals had failed.

 

[5twins]

A compression test isn't a "tell-all". You can get good readings from a bad cylinder. All the excess oil leaking into the bad cylinder can give a false reading. One of the worse engines I ever tore down was like this. Compression readings were good but one cylinder would foul it's plug after 10 or 15 miles of riding. That cylinder was a mess inside when we tore it down, all full of oil.

 

[Olly]

Thank you for the advice, I'll try and do a leakdown test and look for signs of a leaking valve valve stem oil seal.

 

[grizld1]

Yep, a compression test will tell you that you have a problem, but it won't tell you that you don't. I once had a motor (big bore, high compression) that showed equal 180 psi readings on both cylinders. When the right cylinder was pressurized I didn't have to hunt around for the problem; the wind was blowing loud from the right intake, no need to put an ear to it. Leakdown testing showed 20% pressure loss; the intake valve was badly cupped.

 

[Tomterrific]

One more thing to worry about. The design of the single carb manifolds for the XS seem destined to change the mixture from one cylinder to the other. As a stop gap for the symptoms, put a hotter spark plug in the fouling side. The current plug is not running at a temperature to burn off carbon.

Tom

 

[Panabikerxs650]

Watch this video this might help