Carburetor YS 637S Repair subject again

Cacti_Ken · Post by **Cacti_Ken** » Wed Nov 21, 2012 2:36 pm

I'm not sure what I have. My YS637 has a different spring in it as compared to the one that comes in the kit. I have the kit from MWM.
Im talking about the diaphram spring on the right side (opposite the side that has the metering rod).

My engine has developed a hesitation when accelerating from take off.

I have always been satisfied with the performance of the carburetor up till now. Except the fact that I always had to prime the intake before I could get it started.

So I don't know whether to re-use the spring that was in it or use the one in the kit.

And The Throttle shaft Seal, which way is it inserted? I am questioning how the carb was last put together. The seal was inserted with the groove lips outward.

This Way?

OR This Way?

Post by **wesk** » Wed Nov 21, 2012 5:57 pm

You've got the right spring on the metering rod diaphragm.

The accelerator pump springs are a bit harder to sort out.

Ron F's kit is shown below:

The Midwest kit (dual application for both carbs) I received about 2006 is shown below.

All the parts illustrations in the manuals show the M38's YS637S with the significantly shorter and heavier spring used on the accelerator pump.

The YS950S illustrations I have seen show the slightly longer and lighter spring in the metering system and the just slightly shorter (actually longer than the YS637S) and heavier spring in the accelerator pump system.

As for the throttle shaft seal I install the lips out. I do this because the air flow will normally be from the outside in due to the vacuum in the throttle body area. The lip can protect from vacuum leaks going in and dirt goin in with the lips facing the threat.

Cacti_Ken · Post by **Cacti_Ken** » Wed Nov 21, 2012 8:06 pm

Thanks for the reply Wes.
I will install the seal with lips outward.

Your Ron's kit has the same springs that the kit I have from MWM.

In your photo of the MWM kit you have two metering rods.

I have everthing else in the kit as you show.
In mine, there is only one metering rod, and the pump spring is the short stout one.
Sorry I can't lay it out as I have already install some of it.
I also compared the meter rod diameter in the kit with the one in the carburetor. The rod in my carb is .070, the rod in the kit is .068

Post by **wesk** » Thu Nov 22, 2012 1:58 am

The Midwest kit has two metering rods because one is for the 637S and one is for the 950S. Carter kits and stateside kits should have the rod part number on one of the flats of the rod.
75-826S for the YS950S
75-595S for the YS637S

Let me dig out the measurements.

Here the measurements are:

Cacti_Ken · Post by **Cacti_Ken** » Thu Nov 22, 2012 9:31 am

It appears my YS637s had a pump spring and a metering rod designed for the YS950S in it.

Per your info, the springs and metering rod in the kit I have are correct for my YS637s.

Thanks for all the pictures and info Wes.

"Happy Thanksgiving"

Ken

Post by **wesk** » Thu Nov 22, 2012 10:29 am

A happy Thanksgiving to you and your family as well.

Cacti_Ken · Post by **Cacti_Ken** » Fri Nov 23, 2012 1:09 pm

Quoting William Bendix from The Life of Riley, "What A Revolting Development This Is"

As I continue the rebuild on the carburetor and install the diaphrams I discover the holes aren't punched right. They are as close to half a hole off. NUTS!

guess I'll have to repunch the hole