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Throttle control

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SAMBA
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Throttle control

Post by SAMBA » Tue Aug 01, 2017 12:31 am

I've had my 1953 33' Chris craft sedan cabin cruiser for five seasons now, and even the first season I found that the starboard throttle had to be advanced farther than the port throttle to have the RPMs match between the two engines. At lower RPMs they were relatively similar; as I advanced the RPMs towards 2800 the starboard side was advanced probably an inch or two (30°?) more compared to the port side. The season the asymmetry has progressed. At this point I need to advance the starboard throttle all the way to the extreme forward position to obtain a maximum of 2500 RPM, while the port throttle control is probably 3 inches from the extreme forward position. I can no longer get starboard engine over 2500 rpm's as the throttle is in the maximum open position. Engine does not seem to be straining at all. If anything it almost seems like the starboard throttle control is stretching or slipping; that is, I have to move it an increasing distance in order to obtain what is actually a diminishing increase in the actual RPMs. Once I get over about 1700 rpms I have to move the throttle like crazy to get additional gain in RPMs.

Is the throttle cable slipping? Does it actually have a component of stretch? Is it time for a new throttle cable? It seems to me this is more of an issue of the throttle control, as opposed to the engine being under a heavier load. That is, the discrepancy between the two sides seems to be increasing week by week over the summer boating season.

On a somewhat related note, what should be the maximum rpm's that I cruise with? At 2800 RPM I make about 17.5 mph. Never do that with more than one or two people on the boat, certainly don't want to over stress the engines. Generally around 2200 rpm's what I am motoring around the lake

Any thoughts/advice would be well appreciated

joanroy
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Re: Throttle control

Post by joanroy » Tue Aug 01, 2017 3:11 am

Sounds like the starboard cable is slipping at one of the connections. With the engine stopped push both throttles full open and check at the carbs to see if their both all the way opened. If not the cable is slipping in either the end connections or the cable jacket clamp connections. Another possibility is corrosion inside the cable so instead of sliding inside the jacket the cable is bending when the throttle is pushed. Disconnect the cable at the carb and work with the throttle to see if your getting full throw in and out.

Lower RPMs are always less stress on the motors and the boat. Old Boat. Take it Easy. Happy Boating!

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Bilge Rat
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Re: Throttle control

Post by Bilge Rat » Tue Aug 01, 2017 6:06 am

You definitely need to look into this. A cable may be coming loose at it's attachment to the carb linkage, throttle lever or may be getting ready to break. That would leave the throttle closed, partially open or wide open at what might be the worst time for that to happen (like coming into a busy dock).
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

jfrprops
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Re: Throttle control

Post by jfrprops » Tue Aug 01, 2017 8:21 am

the two previous replies cover the slippage issue exactly right. As to your speed vs rpm ?.....those figures are about normal for a boat of that age etc etc.

John in Va.
1980 Fairchild Scout 30
19?? custom Argentine Runabout 16'
1954 Whirlwind deluxe dual ckpt 16'
1921 Old Town Charles River 17' (founding Captain, James River Batteau Festival)

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evansjw44
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Re: Throttle control

Post by evansjw44 » Wed Aug 02, 2017 9:25 am

Check the linkage first but I suspect you have an engine problem like a head gasket or burned exhaust valves. Healthy CC flat head sixes were generally set up to top out between 3000 and 3200 RPM.
Jim Evans

SAMBA
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Re: Throttle control

Post by SAMBA » Thu Aug 31, 2017 12:49 am

Thanks so much for the replies. Had my mechanic take a look and cables were actually OK. Turns out that the real issue was that one of the spark plug wires had come off the top of the distributor. Problem solved. At low rpm's the difference was minimal; as I advanced the throttle the engine with only 5 firing needed proportionally tons more fuel and unable to match rpm's of the other engine. My sheepishness at the simpleness of the problem was outweighed by the ease and minimal expense of the repair. Amazing how the boat cruises/accelerates so much better firing on all 6 w/ each engine! (My personal take on the issue is that this likely happened when said mechanic had pulled/repaired and replaced starter on that engine 3 weeks previously, but impossible to say for sure. As I appreciate his timely service, I ate this one).

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