Expert: Jeff Grigg - 2/22/2008

Question
QUESTION: Hi Jeff,

I am reposting because I received a reply stating that you already answered me; however this is the first I have evered posted to this site.  Can you help?


I have a 1988 23.5 Chapparrel with a 308 engine. We can't seem to
figure out why it has a hard time getting on plain. We dont carry an
abnormal amount of weight,the RPM's and temp seem fine. We did just change the
thermostate, however at full throattle we seem to bog down. It feels
like it wants to go for a minute then bogs down again.  Any suggestions?
We are thinking maybe going to a smaller faster prop but would
appreciate any suggestions and recommendations for props size for a 23.5
footer.



ANSWER: Scott, sometimes the answers get mixed up. Anyhoo...
Has the boat always been hard to get on plane? Is this a new to you boat or is this a recent occurrence? What is the full throttle rpm?
If it is bogging down at full throttle you could have a fuel problem, check the carb for gumming, check the fuel pump output pressure.
What condition is your ignition in? Clean plugs, properly gapped? Timing advancing properly? Good compression?
If the engine is OK and still is hard to get on plane and the engine at full throttle is within the normal operating range then you may need to go down in diameter to help turn the rpm's up to get on plane.
Post back with all the particulars and what it is doing and we can work from there.
Jeff

---------- FOLLOW-UP ----------

QUESTION: Follow up:

This is a recent occurrence, at full throattle we have a hard time reaching between 28-35 rpms.  The ignition is good and we put new plugs in the beginning of the season.  We did put a new (slightly larger prop on) and a rebuilt Carborator.  Timing looks good, however it is a little hard to get an accurate reading because of some rust.  To be honest we haven't check the compression or think that it is bad fuel because the engine seems to be humming along pretty good. In one breathe she wants to run then it seems like it hits a wall (per say).  Especially in semi rough seas (2-4 seas max) Flat seas she runs good.  Do you have a suggestion regarding prop size for a 1988 23.5 ft Chapparal? It is a heavy boat being an I/O.

Answer
I would go back a step. What has recently changed? The carburetor. Why was the carb rebuilt? Was it done correctly and is it a marine carb. It could be running rough in heavy seas from an improperly set fuel float in the carb. I would check the fuel pump pressure and have the carb looked at again.
Before beginning any tune up always check the engine's compression. It will tell you a whole lot about the condition. A vacuum test, using the same gauge will also tell you the condition. If the engine isn't right then no matter what you do to the fuel or ignition systems it will not run right.
Jeff