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  #1   IP: 72.234.132.13
Old 11-13-2005, 10:04 PM
ReeseJ ReeseJ is offline
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Intake Manifold Gasket?

My engine cranks well but doesn't start (good spark & compression). There is plenty of fuel in the float bowl of the carb. I pulled the carb and replaced all jets with the Moyer kit (Zenith carb) and blew all the passages out although everything already looked clean in there (the carb is only two years old!). I then reinstalled the carb and, sadly, there was no difference at all. I did not see any fuel in the air intake and the spark plugs look dry. However, if I spray a little carb cleaner into the intake, the engine fires right up and runs strong for a few seconds. Hummm...could it that there isn't any vacuum? Maybe a hole in the intake manifold gasket? I hate to think so, since I'll have to pull the engine to replace it (there is only 2 1/2 inchs of clearance on that side of the engine on my Catalina 27). Any suggestions?
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  #2   IP: 69.72.48.60
Old 11-14-2005, 02:48 PM
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Reese,

With the flame arrestor housing removed, you should see fuel puddled in the intake throat of the carburetor after approximately 15 to 20 seconds of cranking, as long as the choke valve is completely closed. Since you're not seeing fuel in the intake throat after cranking with the choke closed, it appears clear that you're not getting fuel to flow from the float chamber, into the main passage, through the main jet, and up through the main discharge nozzle. Sorry, but it appears that we're simply missing something in the carburetor.

I don't believe you have a vacuum leak or the engine wouldn't run so well, albeit for only a few seconds, while starter fluid is being burned off.

Don
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Old 11-16-2005, 09:04 PM
ReeseJ ReeseJ is offline
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Thanks Don. I hope you are right. I just ordered a new carb from you so I'll give that a try. I can't think what I might be missing from the old carb - I used all the pieces in the kit when I did the rebuild. The same problem existed both before and after the rebuild.

PS: I only cranked the engine for five seconds or so at a time so maybe it wasn't a long enough time to check for the presence of fuel in the carb intake. I was concerned that prolonged cranking might be detrimental (I do keep the water intake shut off).
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Old 11-25-2005, 02:33 PM
ReeseJ ReeseJ is offline
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Still the same...

I was excited to get the new carburetor and put it on, but had the big letdown of finding that nothing had changed. The fuel still gets nicely into the float chamber but after 20 seconds of cranking there is no evidence of fuel in the intake under the back flame arrester, and no evidence of fuel on the plugs. The engine still runs nicely for a few seconds when I spray a little carb cleaner into the intake. Compression on all cylinders is great. So, what's left to consider now? All I can think of is a hole in the intake manifold gasket or a rust hole in the manifold itself (but I can't see any evidence of that). Any further suggestions would be greatly appreciated! Jeff
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Old 11-25-2005, 04:33 PM
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Jeff,

You sort of have us over a barrel! How do you know fuel is filling the float chamber, and are you sure your choke is closing completely?

If I were on your boat, knew that the carburetor float chamber was full of fuel, the choke completely closed, and still no fuel puddling in the intake throat, I'd hold my hand over the intake throat and ask you to turn the engine over on the starter for a couple seconds. I would expect to feel a lot of suction on my hand, and there should be a lot of fuel puddling in the intake throat of the carburetor. Actually, if there were ignition, the engine should start before much fuel would have a chance to puddle.

If I didn't feel any suction to speak of while holding my hand over the intake throat, I'd do a simple compression check by holding my thumb over each spark plug hole while you cranked the engine on the starter again.

If I felt good compression (unable to hold my thumb over each hole) and no suction at the intake throat, you may indeed have a big suction leak. I consider this to be unlikely, however, since a vacuum leak of such a proportion would make it difficult to start the engine even with starter fluid.

Please get back to us after these checks. It's always valuable to the group to see how these episodes turn out.

Don
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Old 12-02-2005, 12:29 AM
ReeseJ ReeseJ is offline
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Hi Don,

I got back to the boat today and ran the checks you suggested.

I know that fuel is in the float chamber of the carb since when I loosen the drain plug fuel comes out. This was true with the old carb as well as the new carb - no change on that.

With my hand over the carb intake, I could feel no suction while cranking. (I probably should have checked that before I got a whole new carb!).

I then did the thumb test over each spark plug hole, and found a strong burst of compression from each hole. PS: I had done a compression test just recently and got readings between 95 and 100 for each cylinder.

I then did a visual inspection of the intake manifold. The space is tight and I couldn't see all of it, but everything looked pretty normal other than some surface rust at one end.

What would you advise now? How can I know if there is a blown gasket or if there is something wrong with the intake manifold itself (I really can't think of anything other than those two possibilities). Either way, it's bad news since there isn't enough space in the engine compartment (Catalina 27) to get the manifold off the engine without pulling the engine out.

Thanks for your help with this puzzler! Jeff
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Old 12-04-2005, 07:47 AM
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Jeff,

Wow, now you really have us on the ropes!

The spark plugs were still installed when you did the intake suction test, right? It's very difficult for me to believe that you have suddenly developed a massive leak in the manifold gasket to account for absolutely no suction being developed at the intake throat of the carburetor. It's almost easier for me to envision a blockage developing within the exhaust system. To check this out, you'd have to remove the flange from behind the manifold and repeat the intake suction check.

Can you think back to the last time the engine ran OK, and then tell us anything that might have changed between that time and the time the engine first refused to start?

Hopefully someone else will step in with some thoughts.

Don
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Old 12-05-2005, 12:38 AM
ReeseJ ReeseJ is offline
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Wow! Don, you are a genius!!!!!!

I hadn't even thought about it, but your comment on the possible blocked exhaust system got my mental gears turning. Hummm...the engine was working when I took it to the boatyard a few months ago. After that, it wasn't. What happened there? Ah yes, I remembered.... there was no flap on the exhaust port (located just above the waterline) and I was worried about following seas pushing water up the exhaust pipe. I did ask them to install a flap.

I checked today, and sure enough, there was a heavy rubber flap on the exhaust port. I rotated the flap so that it wasn't blocking the port, and with bated breath cranked the engine... Yes!!! It started up very nicely!!!!!

So, I ended up rebuilding the carb and then even purchasing a new one, all because I didn't think this one through very well. Of course, I still don't know exactly why a rubber flap on the exhaust port would have such a pronounced effect on the intake of air through the carburetor - but clearly it does. Would you care to comment on that, Don?

Anyhow, much thanks for saving me from the work of pulling the engine!

Jeff

PS: I noticed some light colored smoke in the exhaust. Of course, the engine hasn't been run at all in months and only about an hour or so per month over the last few years - perhaps that has something to do with it...
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