Automotive engine noises can usually be diagnosed quite quickly, especially in the hands of experienced and knowledgeable mechanics…but sometimes the answer is a lot harder to come by.
Take the case of a loud knocking sound on a Harley Davidson motorcycle with a Milwaukee Eight 107 engine…and it is only evident when accelerating under load and even more so if starting from a higher gear. So what could be the cause?
The first thing to do is rule out anything that could be loose on the bike. Heat shields, hose clips etc. Then check for exhaust leaks…as that is what it can sound like too, like there is a hole on your exhaust system somewhere and it is blowing hard.
If and when you have checked for exhaust leaks, look to see if the exhaust flange gaskets may need changing too.
At this stage it is a process of elimination. So the next item will be to check for any intake manifold leaks. Get a spray can of brake/carb cleaner or easy start. Start the engine and let it settle into idle. Then spray a little onto the intake manifold (being sure not to spray any near the intake air cleaner. Check to hear if there is an increase in engine rpm. If so, then you will need to change your intake manifold or replace the gaskets at least.
Now, that indeed may have been the issue or just an incidental and separate issue. So what else could it be.
So the next thing will be to go back to eliminating anything that be a simple fix. Such things and low oil level has been known to cause strange noises. Try higher octane fuel for a tank full or two and see if that has any bearing on this. It could be a combination of any of these things. Also look at spark plugs for any signs of damage. Look at HT leads too. If you have an endoscope (cylinder borescope), try to look inside the cylinders for any abnormal carbon build up on the piston crowns and or any signs of wear or damage to the cylinder walls.
At this stage, nothing has worked, so now onto the sensors. The ACR (automatic compression release valves) and the Knock sensors. Could these be the issue.
Harley Davidson motorcycles with an M8 engine have common known issues with sticking ACRs either in the closed or open position. In the closed position, the engine might be difficult to start. In the open position, indeed there could be a noise as what is being heard under acceleration.
However, before looking at the sensors…you need to determine if the tune on your bike is still working properly. If you are running a completely stock bike without a tuner, then it could be that the mechanical and electrical components are starting to wear out thus causing the fixed tune on your bike to not be so effective, thus causing a loud knocking/clacking/clapping sound on acceleration. If you have a tuner on your bike, then it might be worth trying to perform an autotune to see if that fixes anything. It could be enough to reset the AFR tables (air fuel ratio) and advance/retard the spark timing that works better thus eliminating the noise.
At this stage, still the noise persists, so now to check sensors and connections. Start with the ACRs as they are more common areas of concern. Check the wiring for any signs of heat damage. You will have to remove the fuel tank to do this.
Check the connections are true and follow the service manual for any electrical function tests. It could be a faulty ACR that is causing the problem. A loose connector or at worse it could be the ECM that is at fault.
Many times the ACRs, even those that are faulty or not working do not present any error codes either. With heat damage, the ends of the ACRs can burn away causing a leak which is more prominent under engine load whilst accelerating.
Once all connections have been checked with a noid test light and multimeter, the next job will be to use a 13/16 slotted deep socket to remove the ACR to inspect them for damage. If they are, then this is likely the case.
It could be a combination of the ACRs and the tune that is at fault. The knock sensors can be checked as well, however if they are defective, it is probable there would be other symptoms at other rpms and when not under load either.
The idea here is to keep eliminating potential problems until the solution is found. This could be the fix or it could still be a long road ahead. Anything else could mean an expensive bill at the end as it could be work pistons, bearings, cracked cylinder walls too. Hopefully the solution is a simple one before looking at expensive pathways.
This post will be updated as and when I determine what the root cause of this noise that I have on my bike is.