compression and vacuum

I did a compression and vacuum test on my 81 seca 750 today. The compression was at 145 on all cylinders, give or take a few psi. I think this is a good number but I don't know for sure?
The vacuum test is another story. At idle (1100 rpm) with the engine at operation temp the vacuum was reading at 5-6 inch Hg. That seems really low to me.
I suspect its the throttle seals on the carbs but I don't know for sure.

 

Do you have an hanging idle, or any other signs of a vacuum leak? Is the vacuum gauge steady, or does it waver with throttle movement?
Most importantly: Are your valve clearances in spec? Have you inspected the intake boots?

Your compression numbers are good.
From Len's catalog (and the FSM):

650, 750 air-cooled engines:

Minimum: 128 psi

Standard: 156 psi

Maximum: 171 psi

Max. variance between lowest and highest: 14 psi

 

That's good to hear about the compression. The valve clearances are in spec. I just had them done yesterday. 2 valves were very tight and one was quite loose (words from the mechanic). They are all done and the carbs have been synchronized. The intake boots are a bit scaled on the outside but do not leak. If I put a dab of oil on the throttle shaft seals, I can see the oil pulsate a bit when I start the bike. I have a really hard time starting the bike when it's dead cold (below 7 C) and have not started the engine in 3 days or more. The idle mixture is very touchy as well. I suspect that low vacuum signal has to do with both problems.

 

There ya go then. Throttle shaft seals. You'll have to break the rack to replace them.

 

Is the vacuum on each carb 5-6 inch Hg?

Gary H.

 

The manual says for 750's that vacuum at idle should be 7.3 +/- .04 inch Hg

As kmoe said, I bet the discrepancy in vacuum could be at least partially made up by replacing your throttle shaft seals.

 

Agreed but there is no mention of vacuum being measured at each carb. The vacuum test and one reading suggests one carb was measured.

Gary H.

 

I completely misread your question there lol

I thought you were asking IF thats the correct vacuum,

But now i realize you were asking if that was the vacuum present on ALL the cylinders



(also thought you were the P.O.....woops, a bit too much ouzo tonight)

 

yep, it's all cylinders. give or take about 1 Hg difference.

Ouzo is the best!

 

Tin cup or Buffalo Trace for me.

 

if you watch the intake manifold closely you'll see it pulsate too. If there's no problems with how it runs, don't tear it apart to fix a number.
I don't think the Yamaha book states a number, and the Haynes book is, well it's the Haynes book. What are the chances of two seals on each carb leaking the same amount?
Is the YICS port blocked or open and how should it be?
The vacuum is what it is.

 

Polock said, "the vacuum is what it is".

Exactly and should be within a inch across the board which you can get with a good bench synch. Xjbikes.com use guages or the likes to set the vacuum exactly the same at each carb.

Gary H.

 

If the vaccum is too low it indicates a leak somewhere in the system. 5 to 6 in HG is on the low side for any engine. and indicative of an intake leak somewhere (a vaccum gauge can tell you a lot about what is happening inside of an engine). At 5-6 in HG the leak is not bad enough of one to keep the bike from being rideable, but it should be addressed. At the very least keep checking the vacuum at each oil change.

 

I may just keep riding it as is for now. Aside from being hard to start when cold (not an issue in the summer) outside, it runs great. I plan on tearing the bike down and powder coating the frame after this season. I want to get some parts re chromed too, but that can be expensive.

 

Fwiw i do use gauges to adjust the vacuum on my bike. Imo find the leak(s), fix them and move on; or not.

Gary H.

 

You can't adjust the vacuum. It is a function of how well the engine is sealed, and of how well the engine draws air into the cylinders.

 

Correct again. You can adjust the air flow to manipulate the vacuum tho right?

Gary H.

 

One more time, is the YICS tool in or out?

 

A very important question. With the tool out you have three extra holes that other engines do not have. It does not take much of a hole to drop vacuum by 1".

 

Only by addding more holes (leaks), or reducing the size of the intakes or valves relative to the piston size. The former is unadvised, and the latter requires a reasonably deep understanding of physics, and engineering and manufacturing practices (there is a ratio of piston swept area to valve size that has to be within a well defined range in order for an engine to work well). There is no practical reason to do anything other than eliminate leaks on an already engineered engine (unless you are a serious racer).

 

I understand. Thanks.

Gary H.