Low vacuuum reading

[Rasputin]

So if I have a low vacuum reading at an idle...less than the lowest of 8 lbs on my morgan carbtune.. could this be a result of valve clearances? The intakes were all about half "clearance wise" of what they were supposed to be and the exhaust was all out as well. I tried many things so far as carbs were concerned and finally decided that maybe valves were the culprit. Any ideas or thoughts would be appreciated. Thanks.

[79XS11F]

Make sure the damper screws are not closed too much. They prevent bounce of the fluid in the tubes but if closed too much they will be too restrictive.
Rob

[dbeardslee]

I rebuilt an SF this past summer, and it was showing pretty low vacuum. While most of them show around 20 cm of vacuum at idle, I've worked on several that were much lower. Just seems to be the way some of them draw. With the carbtune (which doesn't have adjustable dampeners), if your vacuum readings are so low that the rods don't want to move, you can turn it upside-down and use it that way. The scale will no longer be relevant, but it will still show you the correct 'relationship' between the carbs. Also, you sometimes need to hold it at a slight angle until you can see all the rods dancing. If they're not moving, you're probably not getting a good reading.

[Rasputin]

I had to order some valve shims and they wont be here until tuesday. Once I get them in I will recheck and see what I get for vacuum. I did not know you would still get a good reading by tilting or turning them upside down. I will give that a try if needed then. Yes the dampners are not adjustable on the Carbtune but i will look for any obstruction in them as well. Thanks guys

[DGXSER]

Hey Ras,

IMO, check compression. If compression is good, then your cylinders are drawing the volume to create the vacuum at the inlets one way or another. I would say if your compresion is good but your vacuum is extremely low, at least check for a vacuum leak at the intakes.

[Rasputin]

The compression is all within 5 psi and averages about 142 psi when the engine is warm with the throttle open. The carb boots are new and I do not notice any difference when I use a propane torch around the carbs when it is running.
The manual asks for 0.16 - 0.20mm on the intake valves for clearance. I had 0.08mm on three of them and 0.09mm on one. So I am in the process of getting that rectified.
My only other thought was that I may have something plugging the exhaust which would also affect intake vacuum. If the air has nowhere to move it won't move. The bike has sat for about 10 yrs so the possibility of mice or bugs partially plugging the exhaust is there. It does run and I have revved it out a few times in the garage, so not thinking it is plugged badly if that may be the case.
Thanks for the input and I will let you know if I find anything else.

[DGXSER]

Hey Rasputin,

I do not recall the numbers off the top of my head, but the 80-81 model intakes are a smaller gap than the earlier models. So just make sure your using the tighter gap on the intakes. That gap may be it, just making sure you know it is different.

[Rasputin]

Thanks for the heads up DGXSER but my 81LH is a frankenbike as it has the 79SF engine in it. Most of the paint and trim has to be repainted so it will not look like a MNS when it is done either.

[Rasputin]

Well I finally got all the shims correctly spaced. Now to see if it makes a difference with the vacuum or not. Too late to try tonight so will have to put it off for another night.