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1999 BF50 idles high after period at WOT

skronkman

Regular Contributor
This one has me scratching my head. The motor starts and runs great. Carbies synched and idle set to 950rpm when warmed up on the hose. Normal cruising around on the bay and drop back to idle no problem. After a good run at WOT (about 10 mins at 5500rpm) and drop back to idle and now it's idling fast at around 1500rpm. Turn off engine and restart immediately and it's still idling high. Turn off and let it cool for a while and all back to normal. I have checked the throttle and linkages and they are not sticking. Could an intake air leak be opening up when hot? Could the carbies be leaning out somehow only when hot? Some sort of ignition advance issue? I've not done any testing yet and wanted your thoughts before I start. Many thanks in advance.
 
While checking linkages, did you push the throttle linkage down against the idle stop with some pressure? Maybe something in one of the carburetors expanded and is restricting it from actually returning all the way to idle.

if not, then maybe the intake manifold surface was not cleaned good or the manifold was not torqued correctly or the manifold gasket is bad.. The engine heat may cause extra air to be introduced.

Recheck all of your gaskets (make sure they are all in correctly) and recheck all the carb and intake manifold bolt torques. Remember to tighten the intake manifold bolts in a circular fashion...starting on the inside and working out.

Mike

The one thing to does affect the idle in some of these older 50's is cracks in the jet set (long skinny tube going up through the carburetors). This usually causes a poor and unstable idle. So, probably is not the problem...but who knows for sure? It normally shows up while syncing the carbs. They are in sync for a few minutes and after revving it a couple of times, the motor is out of sync. You can adjust all day and it keeps going out.
 
I thought that maybe the little dashpot valve I see in the parts blow up could be "sticking". I think I might try bypassing that to see if it made a difference.

Good luck.
 
Thanks fellas. I did press down on the throttle linkages and wiggle them about to see if I could drop the revs. I could push it down and stall the engine but it would return to high idle after restart. I also tried pulling on the manual choke a little while it was high idling but I couldn't drop the rpm other than stall it (I wasn't being particularly delicate though as I was hurrying while other boats were queuing behind me at the ramp). The manifold gasket is new 6 months ago but I reused the carby gaskets after I cleaned them. I'll go ahead and loosen off all the bolts and retorque them and Will rebalance while I'm at it. I'll check the plugs too and see if any of them look different as this might narrow it down to a particular carby. I won't pull off the manifold and carbies just yet, I'll wait and see how the quick and easy stuff works out for me. I'll start another thread about the procedure for setting the mixture screws on multi carbies as I'm curious how others do it. Thanks again hondadude and jgmo, I'll keep the post updated as I make progress but it will be a little slow as this engine only sees the water once every week or two.

on a side note, took my dad out fishing for his first time on my boat this week, he was trying to help out and started up the motor for me while I was getting the trailer. The leg wasn't in the water and I came back to a burning rubber smell, there went my impeller with 10 hours on it. Poor bloke felt terrible, luckily it was at the end of the fishing trip not the beginning.
 
Well, tell your dad it's happened to ALL of us at one time or another and I would rather have someone forget to tilt the motor down and burn up an impeller than have them launch my boat with the drain plug out....And THAT has happened to plenty of us too!
;~) ;~) ;~)
 
Ha ha, yep I have launched and gotten half way down the channel to the bay before I asked my deckie if he put the bung in, his response "I thought you did it!". Luckily my bilge pump is faster than the bung hole leak and all was fixed on the water without drama.

regarding the dashpot (I forgot to address it in my previous post,) i believe it's function is to pressurisethe carby bowls when quickly pushing the throttle down to enrich the mix under acceleration and also temporarily prevents the throttle from fully closing back to idle when quickly throttling down. I hadn't thought of it. I'll do a suck and blow test this week and next time I'm idling high I'll disconnect it to see if it slows it up. I should also be able to see its cam holding the throttle open if it's stuck. Thanks for the heads up.
 
Took the boat out today. Same issue as mentioned. Tested the dashpot and it's working fine, not sticking. Tested all the throttle linkages again and not sticking. Sprayed some starter fluid around the intake manifold and carbies but couldn't hear any change in revs. Think I'll pull them, dismantle, clean and inspect components then retorque the bolts and rebalance. Will do tappet clearance too, just because.
 
Sounds like a good plan. Be sure to check those jet sets for minute cracks....you may need a magnifying glass to see them.

Mike
 
I learned a long time ago to just chuck those jet set tubes and make damn sure the NEW one was clean out of the pack. New teensy oring at the top EVERY TIME too!

Other than that, have you done any temperature testing? Could this puppy be heating up on the hard run enough to lean it out a bit until it cools back down?

Another thought is what might be going on with the timing belt tensioner? Is it in tip top shape? No rust?

Running out of things to check here.
 
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