1988 yamaha 200hp knocking at lower RPM's

bigshrimpin · #1 September 10th, 2003, 12:51 PM

I am having trouble with my 88 yamaha 200hp knocking at low RPM's. The knocking seems to begin when I put the engine in gear up to 2200rpm, after running the engine for 15 - 30 seconds in the 1400 to 2200 RPM range the knocking goes away or if I accelerate past 2200rpms the knocking will also go away and won't return until I restart the engine. Whenever I turn the engine off and then start it back up the knocking is back (right in that RPM range). The knocking usually doesn't last more that 15 seconds after the engine has been running.

Other history:

I recently rebuilt the carbs and replaced the fuel pumps, fuel filter, and lines.

Any ideas?

I am going to try running higher octane fuel.

I am also wondering if it could be a clogged low speed jet one of the carbs? I honestly don't recall if the engine knocked like this before I rebuilt the carbs. I only ran the engine a few times before I did the carbs.

mrn714 · #2 September 10th, 2003, 01:18 PM

Suggest you test the compression first. Use a screw in tester. All readings should be within 5-7 lbs of each other. If not, you may have a blow by / piston problem that temp corrects, but will go bad soon. If all is well with that, make sure all plugs are properly set and that fule is not contaminated. Other then that, I am not sure without hearing the sound you have described.

If you did not learn anything today, you should have stayed in bed. Umm, boats.

bigshrimpin · #3 September 10th, 2003, 08:48 PM

Thanks Thom.

Compression looks pretty good.

120 120

125 123

112 112

Just put in new plugs. Made sure that the plugs were gapped correctly (per manual).

I suspect that the engine is getting too much fuel during this period (when the engine knocks it's usually followed by a larger puff of exhaust.

What is a blow by piston problem?

fish4970 · #4 September 11th, 2003, 05:46 AM

Piston blow-by is when the rings do not seal

against the cylinder wall good enough, and allows hot gasses to seep past them into the crankcase

sometimes when an engine warms up to operating temperature the piston / rings expand enough to

lessen the amount of blow-by

DjPro · #5 September 11th, 2003, 06:31 AM

what model 200hp? I have an Evante 200hp same year. The reason I ask is that you need resistor(sp) plugs for that motor. If not they can create a problem with the CDI and cause an alarm problem.

bigshrimpin · #6 September 11th, 2003, 07:52 AM

Model Number is L200ETXG.

How can I tell if this is a blow by problem?

wouldn't the compression be lower if the rings weren't sealing?

rabijunco3 · #7 September 11th, 2003, 07:20 PM

Did you add Ring Free to your fuel? Maybe you have carbon buildup causing detonation? The bottle specifies a "decarbonizing" concentration and a 1 oz./ 10 gal. of fuel for prevention of carbon buildup. How many hours on the motor? Ring Free is $90 per gallon.

Capt Ken · #8 September 11th, 2003, 07:41 PM

During Yamaha's early years in this country, they had a problem with the wrist pins in the pistons. They would wear and cause piston slap. They replaced the older style piston with one with larger wrist pins. They really caused problems with the Excel engines due to knock sensors installed in case of low octane knock. The wrist pin slap would set off the sensors and the timing would change on the engine.

KNOTTY THOTS · #9 September 11th, 2003, 08:24 PM

I had the same problem with my motor. It is a 2001 yamaha 150. No power loss but a knocking at low rev's. I took it to a Yamaha dealer and we found that the problem was at the # 6 cylinder. We pulled the head and found fillings inside. It seams that the oil pump was leaking air due to a pinched O ring. Sill saving to repair. Try some of those long Q tips they use for cleaning tape heads and see if you go through the plug holes and see if you have any metal fillings on the cylinders walls. Good luck!!