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AQ151 oil contamination and high compression

T

tjscott

New member
I have a Coronet 21 fitted with a single AQ151 and 270 drive. The engine was supposedly reconditioned 150 hours ago but I've had a few problems since purchase a year ago. Firstly I had an overheating problem which hopefully has now been resolved. The boat is indoors at the moment with the leg in a water tank. So far I have fitted new top hose connector, raw water impellor, exhaust manifold, inlet manifold with weber conversion, pertronix ignition with coil and leads.
I have had a lot of difficulty tuning the new carb (38 DGES), having gone up in stages from size 45 idle jets to size 57 with no success. I have checked for air leaks but found none. Not sure what is going on here.
Then yesterday I found evidence of water contamination in the engine oil (grey and white emulsion and droplets on dipstick). Pulled the (recently overhauled) oil cooler but found no problem whatsoever. Followed with a cylinder compression test (dry and wet - no difference) and found unusual readings of 230, 230, 220, 175.
At this point decided to pull the head. Found that the number 4 cylinder and plug were blacker than the others, but the head gasket looks fine and there is no evidence of any damage, in fact the pistons and bores look like new. The number 4 inlet valve looks as though it hasn't been seating correctly (blackened seat) which may explain the compression variation but not why the other readings are so high!?
Also found that the cam belt idler pulley was seized, the camshaft was about 18 degrees off (36 degrees at crankshaft pulley), the distributor was a spline off and the cam belt HAD A TEAR HALF WAY THROUGH IT. Oh, and the number four exhaust port is very severely pitted.
If anyone can help shed some light on the above and/or advice on how to proceed it would be much appreciated.
 
It sounds like you have multiple problems. Firstly you have to deal with your timing belt issues.

That compressions sounds way too high to me, not sure what is going on there.

I would take the head to a shop and get it checked out, wondering if it has been planned one too many times to give you such high compression?

Another point, prior to ripping the thing apart, it would have been helpful to find where the problem is by first doing a leak down test. A leak down test is more helpful in troubleshooting that a compression test. It will actually tell you where the problem is, whereas a compression test does not.

Hope this is helpful.

Have a great day

Rob.
 
Thanks for the advice Rob. I have taken the head and cam assembly into a shop today for examination. Following a pressure test, they will reseat the valves and re-establish correct clearences after weld repairing the pitted number four exhaust port. They have also recommended replacement of the belt idler pulley and cleaning of the toothed pulleys themselves. On reassembly I'm hoping that the water contamination issue goes away as it could conceivably be a condensation problem caused by repeated slow running in the workshop. Also I'm wondering whether the high compression readings found are a symptom of the incorrect valve timing, although I can't quite think of the mechanism for this.
As I have to fit a new timing belt, is there a good source of reference information available to look at? Thanks.
 
The Weber engineer, Bud Paugy, told me that the 45 was the correct jet for my AQ151C but I didn't believe him and tried, like you, larger jets. He was right all along. However, it is very important to follow the idle mix procedure to the letter. I found that 1/2 turn from closed was the correct (?) opening for the mix set screws otherwise it ran rough. I also had vacuum leaks around both the carb and manifold. Big improvement once I found them. A vacuum gauge plumbed into the intake manifold is a great diagnostic aide.
 
Thanks, that's very useful info. I'm sure that my valve timing having been advanced won't have helped with tuning attempt, but I'll check the carb and inlet manifold interfaces as well. I tried spraying Easystart around the carb to try to find leaks before but this didn't make a difference. Do you have any idea what main jet / air corrector sizes you have fitted in your install?
 
Have had the head checked out and found that #4 exhaust valve needed changing due to severe corrosion as well as repair to the #4 exhaust port. Hopefully this is the explaination for the compression difference. Has anyone else any experience of this?
 
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