Author Topic: To suck or blow? (Read 8781 times)

937ampd · « **on:** July 31, 2013, 12:24:29 AM »

Got your attention? Now get your mind out of the gutter. Lol. Ok I bought an ESR 310 bolt on, going to do a leakdown and was wondering if I should blow air into it with a bicycle pump or suck the air with a handheld brake bleeder putting negative pressure in the case rather than positive? Does it really matter? Any insight on this is greatly appreciated. Aaron P

Polonda · « **Reply #1 on:** July 31, 2013, 12:38:13 AM »

Blow.....then suck. Repeat. :encouragement: In all seriousness most pressurize the cylinder. But I can see benefits to both, such as with the crank seals. If you pressurize it can help the seals seal even if they are worn therefore hiding a possible leak. However a vacuum wouldn't have the same effect and the worn seal may leak. If you have the capability give both a shot!

937ampd · « **Reply #2 on:** July 31, 2013, 08:00:49 AM »

Lol, I figured that'd get somebody's attention. I was worried about possibly blowing the seals but know just about everyone and their momma pumps air into it. Plus 6-8 psi isn't a great amount of air. Just playing it cautious, I don't want to be the reason it fails. Great advice Polonda, I will do both just for sh*ts n giggles.:positive:

rablack21 · « **Reply #3 on:** July 31, 2013, 08:16:07 AM »

I really don't think there is a great deal of benefit of doing one versus the other. I have read the pros and cons, and there is not much in favor of one versus the other. I will say that most of your big name engine builders pressurize instead of using vacuum. There may or may not be a reason for this. Regardless, 6psi for 6 minutes with no loss is a proven method and used by the majority. Can't go wrong with that.

937ampd, You can used up to 8 psi without any issue of damage to the seals. If you are using some sort of small pump, 6 - 8 psi is easily attainable without going over.

broken1 · « **Reply #4 on:** July 31, 2013, 08:43:13 AM »

This is interesting. If you used vacuum to measure negative pressure would the reading be 6-8 inHg?

GO OVRIT · « **Reply #5 on:** July 31, 2013, 09:12:02 AM »

6psi would be 12.22 in hg

C-Leigh Racing · « **Reply #6 on:** July 31, 2013, 10:18:23 AM »

Over the years, 10 psi has been the amount not to go over, so most will go somewhere between 6 & 8 psi.

Years ago how to do a pressure test proper, was to seal the engine openings (exhaust, spark plug hole & intake), pressure it then submerge the whole engine under water. I've never done one like that to date, but I do use a hand sprayer filled with soapy water to check for leaks any where theres a seal or gasket.

Now some may look down on it, but I use Three Bond 1211 sealant on every gasket when building an engine. I'll not take a chance of a gasket leaking, because after all these years have past & as many owners have past parts of 250R engines from one to the other, I seriously dont think theres a perfect gasket surface left.
Now if it was a new machined part, that a German machinist had just perfectly finished, then I would probably trust that gasket surface with only a gasket.

Every engine builder has their own way of pressure testing, no problem with that at all, I just shoot for 7 psi, walk away & leave it & will try for no leak at all. I figure if it sits a hour & still on 7 psi, I've done a good job on the build getting every thing sealed up.
Neil

937ampd · « **Reply #7 on:** July 31, 2013, 11:16:04 AM »

Awesome, great info here. I had a feeling this could open a can of worms with different procedures of leakdown but the general consensus is to pressurize to roughly 7 psi and hold for an hr/spray with soapy water. I did use Honda gasket maker along with the gaskets because as C-Leigh stated the mating surfaces have been around the block and this is a low budget build. Thanks for the replies fellas. Hopefully this holds air and I can pop this new 38 a/s and cm trx5 on and start jetting. I'm having 2stroke withdrawal...

Rupp250 · « **Reply #8 on:** July 31, 2013, 11:24:02 AM »

Like Neil said, I use 1211 as well. It is just hard to find around here for some reason.

rsss396 · « **Reply #9 on:** July 31, 2013, 11:39:52 AM »

I pressurize because its easier than vacuuming but either would work.
IMO I honestly do not think if you lost a couple pounds over a a 10-15 minutes time period on a used motor to be of any concern, new motors with fresh grease on the seals will seal much better than a motor that has time on it.
I have never had a jetting issue/engine performance fixed because I fixed a small leak of that amount (couple psi over 10-15min) after it was repaired

rsss396 · « **Reply #10 on:** July 31, 2013, 11:43:58 AM »

IMO when you use 3bonds 1211 if you have white globs oozing out from the gasket mating surfaces you have used to much, I just barely skim it on by working it on the gasket between my thumb and index finger.

GO OVRIT · « **Reply #11 on:** July 31, 2013, 11:55:32 AM »

^^^^ yep, whatever squeezes out also squeezes in.

937ampd · « **Reply #12 on:** July 31, 2013, 12:32:07 PM »

One last question. BDC or TDC during leakdown?

C-Leigh Racing · « **Reply #13 on:** July 31, 2013, 12:42:59 PM »

At BDC, the whole engine is open to pressure, where as if the piston was up, the pressure would have to squeeze past the rings to get into the head dome area.
Pressure would get there in a short time, but with the piston down it would be no waiting.

Either way is fine, you do it like you want to do it, because its you testing not us, so make yourself feel good.
I'm just glad you got a TRX250R to test :eagerness:.
Neil

937ampd · « **Reply #14 on:** July 31, 2013, 12:56:23 PM »

Quote from: C-Leigh Racing;6729

I'm just glad you got a TRX250R to test :eagerness:.
Neil

I'll raise a glass to that:very_drunk:
BDC it is, thanks Neil and you other guys for the sound advice.