B&S twin question.

[Stormin 4,458]

 Decided to see if I could get the B&S 16hp engine in the Sears to run a tad better. I'd a had go at the carb earlier in the week and changed the plugs. That stopped the spitting back in the carb and improved things. When I removed the old plugs, the front cylinder one was wet. Faulty plug maybe. Hence new plugs.

 Today I found the operators manual on the net and the recommended gap is 30 thou. I'd set them at 25 as a guesstimate. So plugs out and low and behold the front plug was again wet. No misfiring or spitting back though. Engine running fine. My thoughts are a bias on the manifold.  Anyone have any other suggestions?

 I also followed the procedure for setting the carb after refitting the plugs. Again engine running fine. I was going to have a look at the plugs again to see what the front one was like, but I got side tracked. I'll do that tomorrow.

 

[Alan 1,271]

Hi Norm.

You probably have seen these links already.  Think I pointed them out to you last year.

http://www.searstractormanuals.com

http://vintagesearsgardentractors.com/manuals.html

 

[Stormin 4,458]

 Hi Alan. You most likely did. I'll be looking at them.

 

[the showman 4,074]

Look what ive got. 

[Wristpin 399]

 

Is it the old boxer flat twin  or the later vee twin Vanguard?  

Are you sure that it's not just running one one cylinder. With either engine, off load, it's not always immediately apparent.

I'm guessing it's the flat twin with one ignition coil with both HT leads coming from it and over the years I've replaced more of those coils than any other BS ones .  If you have not already tried it, pull off each HT lead in turn and see if it will run on one cylinder in turn , or with it running short out each in turn .  

If it's the Vanguard vee it will have two coils connected by a" diode harness" which stops the coils interfering with each other. Diode failure can produce a variety of interesting symptoms but the " try running on one" test is still a good starting point.

 

 

Edited June 17, 2017 by Wristpin
Clarity!

[Stormin 4,458]

Thanks for that, Angus.  I'll give it a try. It's the boxer twin with one coil.

[nigel 1,874]

I aggree with Angus Norm

[Stormin 4,458]

 I've done as suggested. Pulled the cap off the front plug, at idle and full revs. Ran on the rear cylinder ok and when replaced ran on two. Seems to make no difference. So I guess  the coil is ok. Checked the breather pipes as well. They are fine also. 

[Wristpin 399]

You don't mention pulling the back cap?

Ideally you check the rpm drop with each plug disconnected and if the front pot is in trouble (ignition or valves) I would expect to see a drop or stall when the back one is shorted out.

Edited June 17, 2017 by Wristpin

[Stormin 4,458]

Can't remember if I did or didn't.  I think I did. I'll see what happens tomorrow.

[Stormin 4,458]

 I was wondering about valves. A look at job for the future.

[Wristpin 399]

I'm still inclined to say that it's an ignition problem with valves as a close second. If memory is correct that engine has a single choke carb, i.e. both cylinders draw their mixture from a single source , unlike other engines with twin choke carbs which are, in effect, two carburettors feeding individual cylinders.  The original post says that the front plug is wet , so the mixture is getting there but not being ignited. The lack of ignition could be just that - not enough sparks or, a lack of the conditions required for ignition such as the correct air to fuel mix or low compression - an air leak in the induction passage, leaking valves or more seriously and hopefully not, piston and ring problems.

Diagnosing the cause is a job for logical fault finding . The most simple being a check on the loss of power or rpm by the isolation of individual cylinders by removing  HT leads. Inserting cheap in line spark testers into both leads and watching for regular firing will also tell a tale . After that a compression test , or preferably a leak down test on each cylinder.

My money is still with an ignition issue but long distance diagnosis makes certai assumptions that may not be valid.

 

EDIT

Too hot out side so have dug out and scanned a section of an old manual

https://www.dropbox.com/s/88iutp8r69ppkn7/BS flat twin ignition0001.pdf?dl=0

If it is concluded that the coil is the culprit and its an old points system , the way to go is a Magnetron coil part number 394891

 

Edited June 18, 2017 by Wristpin
additional info

[Stormin 4,458]

Thanks for that, Angus. I'll look into it.