South West Bikers | Motorcycle Forum

Am I right in assuming that the spent gases from the 4 cylinders on my bike are exited through their individual downpipes?

i.e. do the gases exit through their own ports, cylinder 1 to port 1, cyl 2 to port 2 etc. or are the gases from all cylinders mixed then exited through all the pipes?

There is reason for my query, I am trying to find out as much as possible before the bike goes to the mechanic. I bunged each port with paper and started the engine with a little rev. All 4 papers popped out. I am assuming that that means all 4 cylinders are functioning.

Firstly, am I correct in my assumption?
Secondly, is there a way of measuring the force of the exiting gases to confirm how well the cylinders are operating?

Or should I not bother?

each cylinder exits through there own down pipe, the exhaust gases combine further down the system.

The best way to check cylinder liner/piston/rings is to carry out a cyl compression check.

That's a mechanical test, if you think it's running on 3 it's more likely to be a sparkplug or coilpack.

Thanks chaps,
would all the papers blown out if one of the cylinders was naff?

Yes. All this has done is proved that the cylinder is drawing air through but even a damaged or worn out engine would probably displace some air. So you haven't really proved anything.

A leak down test would show how efficient the rings and valve seats are working.

What is actual problem?

Hi,

As above the only way to test the true compression is with a tool that screws into the spark plug hole, (pictured) it will give a true compression from each cylinder, you have to disable the fuel supply ect first so you don't run the risk of it spraying out of the cylinders. I trained as a mechanic when I left school and forgotten more than I can remember so there are probably lots of other steps to carry out before testing.



It depends on what the issue is though really,

Darren

Thanks Layne,

I am trying to assess whether to throw money at what could be a dud bike. The exhaust was totally non salvageable and also has a broken stud which needs to be extracted. That said, even with all the noise I think it is making another sound that is similar to not running on all cylinders. I have bought an exhaust ( a new one was similar price to a used original ), but need to get the stud removed before fitting. That in itself is probably going to be a couple of hours labour.
I was trying to find out if I could assess the potential cost to justify having it done. Unfortunately it sounds like I have to get the exhaust fitted to hear more.

Can you get the new exhaust on tight enough for it to run?

if you can, do so and fire it up. Running on three will sound faily obvious. One running, pass your hands over (but not touching !!!) the headers - the one not firing will be cool compared to the others.

If a single cylinder is duff, check for spark from spark plugs, the spark plugs themselves, and the HT leads. You can confirm whether one has failed by moving said items to a different cylinder and seeing if the fault follows it. If a pair of cylinders are duff, then it is likely to be furtheer up the system - coils or ignition pick-up.

Obviously check that all cylinders are getting fuel too. The dead cylinder spark plug should be wet when withdrawn. If the plug is dry, and showing a spark when tested, then it's time to start looking at the carbs .... clean float bowl, check float level, needles, jets, etc. .......

The main thing is to be logical, start with the easy things first then go further only when each step has been eliminated.

Very useful, I'll see how far I get. Thanks.

I'm nowhere near you but there must be someone around you who has a compression tester. *IF* you can get to your spark plugs it is a very simple test to do and would rule out major mechanical issues leaving you with the electrical/fuel ones which are easier to fix but harder to track down.

take a trip to lewis motors in chard, he is a genius with bikes and always used to be quite reasonable on prices and information.

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