Compression issue...or something else?

Recently bought a Honda PCX150, 2012 model.

Having a little issue with it - on startup and pull away it has a dead spot until the revs are up, then it’s fine.

I’m wondering - compression issue, or could it be dirty spark plug/air and oil filters/dirty oil?

Something else? No smoke coming out the exhaust and has decent grunt at higher revs, but definitely not as peppy down low as my Fuzzy 125, which had instant and continuous acceleration.

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If I didn’t know the previous owner, I would would change the oil and filter and check the air filter. (not that expensive and gives you piece of mind)
next step would be to take the plugs out and look for discolouring.


I’m sure @Ducked has some advice on this. Possibly need to hook the whole thing up to your refrigerator or convection oven.


I don’t know this machine. Is it carburetted or fuel injection?

My guesses therefore won’t be very helpful (Not as unhelpful as, say, an Obnoxious Fat Hippy who knows fuck all about cars and motorcycles and couldn’t care less, but still fairly unhelpful)

Re the Solution (?) above, this is good basic maintenance BUT

Oil and filter are unlikely to make any difference to a flat spot (Most small motorcycles and scooters don’t have an oil filter anyway).

Air filter is unlikely to make any difference to a flat spot early in acceleration. Carburetted engines are fairly insensitive to air filter clogging, and if it has any effect, it’ll be at full power when the air demand is greatest. It’ll be down on full power and will run rich.

Fuel injected engines, (at least on cars, motorcycles may be a bit less sophisticated) are almost completely insensitive to air filter clogging, since the system adjusts the mixture to compensate for the reduced air supply. Full power will be reduced but it won’t run excessively rich like a carbed bike might.

Checking the plug for signs of running rich or lean MIGHT be informative, but a flat spot is a transient state, while the plug appearance reflects the average of the conditions its exposed to. IF you can hold the throttle in the flat spot for a while, then turn off, and pull the plug, its state might be a better clue.

Changing the plug is also cheap basic maintenance and MIGHT make a difference.

So far, so negative. The fact is, there are lots of potential causes, and the fact that its transient will make it harder to diagnose. Here’s an irritating list, produced by someone irritating, on the Internyet. (Not as irritating as, say, an Obnoxious Fat Hippy who knows fuck all about cars and motorcycles and couldn’t care less, yet is perhaps the most frequent poster in Cars and Motorcycles, but still fairly irritating)

One class of PITA (Not as big a PITA…etc) problems that can cause this, and which I have recent experience with on the Skywing, is the vacuum leak. It got better after giving the carb and intake manifold multiple coats of sunflower oil to seal it off.

With older carburetted systems stickiness of the centrifugal advance and/or vacuum advance could cause the ignition timing to be slightly wrong at some points in the rev range. Valve timing and/or worn cam lobes are another possibility.

I once had a car that would hesitate and sometimes cut out on decelleration. Turned out to be because the core of the high tension lead to the distributor was breaking down. (When it started doing it on acceleration too I investigated and replaced it.) Long shot on a scooter but I suppose it could happen.

Again on carbed bikes (I’ve never worked on FI systems and I’d suspect they aren’t really DIY territory) dirt or gunge in particular jets, or a wear ridge on a fuel metering needle, or a failure of an acceleration pump diaphragm, can cause fuel starvation in part of the rev range.

I have some of my own carb cleaning tricks but I guess you don’t DIY, and its probably FI anyway, so I won’t bother with them. Convection ovens don’t feature (aren’t all ovens convection ovens?).

You might try a fuel injector/fuel system cleaner. No experience with them, but Americans seem to like Techron, which I’ve seen at Costco, though maybe local brands (which you can buy at CPC petrol stations in little long-necked red and yellow bottles-dunno what the difference between these is.) are OK too.

The word is it should have P.E.A (polyetheramine) in it, which Techron apparently does.

It could also be that "“They all do that, Sir” (Only in Chinese). If you get a chance to ride another of the same make, you should take it. The flat spot MIGHT just be a characteristic of that system.

OP, I suggest you take your query somewhere else. Cars and Motorcycles ain’t what it used to be.

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Another possibility, not especially likely but worth considering because its serious, would be pre-ignition / knocking.

I dunno if FI scooters have knock sensors, If it does, it’ll try to protect itself (preignition can wreck an engine quite quickly) by altering the ignition timing, which might be felt as a transient loss of power.

I’d guess they don’t have knock sensors, though, but pre-ignition can sometimes be heard as a sharp tap.

This is more likely to happen if your cylinder is coked-up, which you can get clues about by taking the plug out and shining a torch down the plug-hole with the piston at TDC. You can use a chopstick as a probe.

There are various tricks for decoke without disassembly, Probably the best is spraying de-ionised or distilled water into the air intake (filter removed,) while revving the engine.

Use something like a plant misting hand sprayer. If you just use a cup and your hand slips, dumping a slug of liquid in, you can wreck the engine.

Tedious (took 2L and about an hour on the Skywing) but it works.

Problem solved: faulty fuel pump. Goes like a bat outta hell now. Brakes on a dime. Beautiful bike.!

It’s a 2012 model that’s had the shit ridden out of it no doubt, but rides nicely - good for town and country riding. Not as big as the Majesty, not as small as my Fuzzy. Don’t think it was/is sold in Taiwan, mostly SEA, Europe and the Americas.