139QMB 72cc Spark Knock

[edwardtennant]

Here is the audio that i recorded. drive.google.com/open?id=1hWBgs5ov1J5ytkW4czYsAAnMWKfJboVO
Please ignore the belt slap, but the spark knock is still audible. It is very loud so headphone users beware lol. It hurt to make it spark knock for that long, just wasnt sure how loud the audio would be with my phone under the seat

I have done around 5400 miles on this BBK with a 85 Main Jet, Needle in the middle and a 35 idle jet. The spark plug is a nice tan brown colour, and recently (about a week or so ago) when i am approaching 50Mph on the speedometer (so more like 45mph) there is what i believe to be spark knock as you can hear in the audio. When this happens the engine hesitates and does not return to normal operation until the speed drops to about 47mph on the speedo. So far here is what i have done to troubleshoot this

• Cleaned the carb out (3 times now)
• Replaced Vacuum petcock
• Replaced fuel filter
• New Spark plug (Cr7HSA to replace Cr7HIX)
• Checked on centre stand to see if it knocked (It revved very high, much more than i was comfortable holding it at without knocking)

Anyone got any other ideas? Here is my bike setup

Bt49qt-9 Pulse Scout

Generic 72CC kit (not including head) from PedParts

6G Rollers

Stock CVT (Though parts have been replaced as theyve worn out)

CVK Carb With 85 Main Jet

Have always run it on 99 Octane fuel which is the highest I can get in my area of the UK.


Any ideas on what could be causing the spark knocking?

[gsx600racer]

Investing in a CHT guage(cylinder head temp)would be helpful. I have a feeling your setup is running on the hot side. Did you reinstall the engine shrouds(black covers) after installing the 72cc kit ?

I will throw out a few ideas,

Compression too high
Carb tune too lean
Iginiton timing

[edwardtennant]

Hi Scootaholic thanks for the reply, The compression on the engine when cold was about 160PSI when i last tested about 2 weeks back. That was with the throttle held flat out. The plastic engine shorouds were reinstalled with the installation of the Engine kit. My engine isnt running too lean as even with the choke unplugged so it is engaged it still spark knocks. The spark plug with a WOT plug chop is a tea brown. Im not sure how to check ignition timing, is there a way to do this easily?

It just seems strange that it was fine for a few months and suddenly
began to spark knock when flat out approaching 50mph. I have noticed increased fuel consumption and a slight decrease in power, but not sure if its related to this, as it doesnt knock when below 50MPH

[acvw74]

If it weren't for the color of the plug, I would be suspecting carbon build up (from running rich-with enricher unplugged)....changing the compression ratio and leaving "burning" pieces of carbon setting off your fuel charge.

[edwardtennant]

These are some photos I took with a borescope back in October. imgur.com/a/Ztdgp here the cylinder had about 3500 miles on it. I will get some more photos taken tonight if i can. I only ran it without the choke to test it to see if it still knocked which would rule out a lean condition. It was ran for about half an hour without the choke connected at the most. if there is excess carbon in the piston and head , what would be the best way to remove it? Pull the head and clean with solvents to remove it?

[acvw74]

I would expect the cylinder to be carbon free - the piston didn't look to bad either...but that was 2000 miles ago and maybe some bad gas.
What octane are you running? Have you switch brands recently?

Seafoam is probably the easiest way to remove cylinder carbon (I would try before removing the head)...just expect to fog the neighbors a bit.

[GrumpyUnk]

An old trick to remove carbon that is glowing & setting off the fuel:air at the wrong time, or raising the compression ratio too much, causing pinging, is to spritz some water into the intake of the carburetor while running the engine at a fast idle. The water droplets/mist turn to steam, and scrub the combustion chamber and piston crown.
From the image, I suspect there is a good carbon deposit on the top of the piston, but cannot tell exactly as the picture is seemingly out of focus.
OTOH, if it happened 'all at once, being fine today and hosed the next', I would be checking the pickup or trigger coil to see that it has not moved from its normal position. If it moved in the CCW direction, that would advance spark timing, possibly leading to knock or ping.
If you do suffer from pinging, you will have increased fuel consumption and loss of power as the knock is working against normal rotation, and causing the need to open the throttle more for the same amount of power, thus using more fuel.
Make sure oil level is proper as it carries away heat from the piston, cylinder wall, etc, and also check that there is nothing blocking airflow over the cooling fins of the cylinder & head. Blocked airflow can cause hotspots as can missing or damaged shrouds. Even the chain tensioner cover should be considered as if it is gone, air can flow out there instead of being directed across the fins & cylinder.
It would not hurt to check base timing. If the CDI does 'something' it could be advancing timing more than it should. If you were getting overheating, the timing could be too late, and that can/will cause more heat to be dumped into the cooling system as the 'burn' has longer time to transfer heat. Some sites don't like the word for late timing, but it is the opposite of 'advanced'.
tom

[edwardtennant]

Hi guys I think I've found the issue. My front brake caliper wasn't retracting which was putting more strain on the engine presumably over stressing it getting it too hot so it was pinging/ spark knocking. I will test this tomorrow but I now need to get new pads for the disc. As for what octane I'm running I'm running 99 octane, and will try the water misting. I have heard other people doing this on cars and hear it is fine as long as you go slow and don't drown and hydrolock it.

[edwardtennant]

Update it still spark knocks when approaching 50mph, although not as bad. Ambient Temperatures were about 5C and it knocked when getting to 50 for the entire 30 mile ride

[FrankenMech]

Maybe replace the vacuum petcock with a manual one. Low vacuum while under power may be restricting fuel flow.

[edwardtennant]

Another Update, Was driving along after putting in new front pads and wheel bearings. Drove for about 10 miles doing 45-50 mph with no issues, stopped at a local tesco, and came back and the bike wouldnt start. I checked for spark, and it had spark. I checked for fuel, float bowl was full but it wasnt pulling fuel through. I removed fuel line from carb and vac line from manifold and when i suck on the vac line the fuel flows, but when vac line is connected to manifold there is no fuel flow. The starter was turning quickly as usual. It still didnt pull fuel when jumped with a 330A car battery. Any idea what to do now? It felt like it had compression though i will do a test at some point. I am also going to put on new vac lines when i can get back to my bike in the next town over. Anything else i should bring? The spark plug was dry when i pulled it.

[FrankenMech]

When the engine is stopped the petcock will not allow fuel to flow. The fuel bowl will remain full unless there is a leak. It takes vacuum from the intake manifold while the engine is running to open the vacuum petcock. The vacuum petcock does not pull fuel, it only allows fuel to drain through by gravity from the fuel tank when vacuum is applied to the vacuum port. Just having the line attached will not open the petcock. A vacuum petcock is a simple vacuum operated valve, not a pump.

The electric starter should only be engaged for a short time, two to three engine revolutions at a time, with two minutes to cool down in between, or it will burn up.

[edwardtennant]

I should have said, that when i was starting it or trying to i was giving it a chance to cool down. I know the petcock is working as when i suck on the vac line the fuel flows. Should the fuel not flow on the starter? But even with the float bowl full of fuel its not pulling it into the engine. Any Idea why?

[FrankenMech]

The engine is an air pump. If the piston movement is not coordinated with the valves opening and closing little to no air will flow when it rotates.

If air is not flowing through the carb it will not pull fuel through the venturi into the airstream. The air pressure differential between the venturi and the air pressure in the fuel bowl is what causes fuel to flow out into the airstream. If a scoot carb does not have enough intake air restriction to drop the air pressure inside the carb no/insufficient fuel will be drawn into the airstream. Carbs are simple devices. Unfortunately scoot carbs are very crude devices also and the intake filter and airbox restrictions are balanced with jet sizes to deliver fuel to the airstream. That is why common carb scoots are very sensitive to airbox modifications.

There are all sorts of vids and explanations on the web about carb operation. Some of them are even good.

Vacuum petcock operation while kickstarting or on the starter is marginal at best. You can certainly pull the fuel line off the carb and spin the engine over to see if any fuel dribbles out. If the engine is not pumping air there will be no vacuum signal to the petcock at all. The fuel in the fuel bowl is sufficient to start the scoot.

[edwardtennant]

HI frankenmech, I will check to make sure the timing chain hasnt slipped so the valves arent opening at the right time. When I removed the carb and placed my hand over the intake manifold and tried to start on electric start the engine was sucking air in, but even with a full float bowl the engine doesnt suck fuel through the carb.