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O.k. got my timing issue fixed, boat runs awsome. Idles @ 800 neutral, 700 in gear. Runs strong across the board. Dialed in I saw almost a 8mph gain but after a long run when coming in to dock if I pullit into neutral it idles up to 1800-2000 rpm. :confused: really fun too dock I wont put it back ing gear revving that high so its like running a crash box. Only thing I can think of is that timing is getting stuck at parcial advance cause that will bring up the idle. Anyone seen this problem ever. Motor is a BBC.

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Advanced timing would not cause the engine to stick at high RPM but if stuck at that RPM it would be advanced because of the higher RPM. Is the throttle linkage not returning? Perhaps the secondaries staying open just a bit after being opened all the way?

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I follow what your saying but advanced timing at and idle will bring up the idle (at least a little) cause when setting the timing on the motor if I turn it up from 8 to say 10-12 deg. the idle raises. :confused:

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Vacuum leak (don't think that would bump it up 1000rpm?) or something hanging up and not letting carb return to idle position?

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you guy's are right I stand corrected (type without thinking) timing alone prolly wont do it. With very little vacuum lines the only thing I can think of is carb base gasket. Whitch would make sense that it would only leak when its real hot, since everything expands. Thanks guys I'll check that tonight.

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i had a similar problem last summer, it was my throttle return spring.... held the throttle just enough to run 3-400 rpms higher.. just a thought

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check throttle return springs, cable, and make sure plates arent sticking. Also maybe choke is stuck on?

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I have a similar problem in both engines, just not as bad. After a hard run they like to go to 1100 -1200rpm at idle. This seems to last a couple minutes then then start dropping back down to normal, which is about 800rpm. I have fooled with both carbs several times and know the butterflies are closing properly. I am leaning towards heated air going into the carb from a hot engine room. Doesn't really make sense to me because I think that would create a rich a/f mix (air less dense) and therefore lower idle. Usually lean causes a high idle. Just thinking out loud here and looking for ideas myself.

BT

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I am having the same problem on A 1990 454 and I believe it to be the secondarys stuck open!

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Racingbird may have a thought with the choke- if choke is hooked up is it working properly and if not hooked up how is it disconnected? might be the idle step up flopping around (yes flopping around is a technical term) if that moved and the choke plate didn't close that would bump up the idle a lot?

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interesting. my scorpion with twin small block 260's does the same thing. after a hard run the idle hangs at about 1500. wonder if there is any one perticular component the we all have in common. mine are basically stock except for cams and intakes. i have the stock T-bolt ignition (old style).

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I'm running a BG carb w/no choke. could be not related but it never did it before I started messing with timing. Well actually I set the timing then had to adjust my idle stop a little to dail in idle so I guess It could be hanging up.

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i wonder if they may be running a bit rich (loading up just enough to idle at 800), then after a run they clean themselves out and idle higher. just thinking. i hate carb problems!!

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That crossed my mind too, but coming into the harbor its a long no wake zone so by the time I pull it out of gear its been idleing for a long while and would start to load up again wouldn't it.

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Heat build up is the common thread I see here. Guess I am the only one...

BT

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O.k. lets say its heat, next Q is how does that raise my idle?

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Is your motor supercharged?

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Mine does this also, if I'm going into a no wake zone I have to shut the engine off then start it back up then it idles at the right rpm

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No areo, it is n/a.

Don't know the answer Just once, but if we follow that train of thought I could come up with: (Note these are ideas not facts)

thinner oil so oil pump consumes less hp. coincides with normal pressure drop when oil is hot

hotter air charge?

Higher internal fuel bowl pressure due to temp that causes idle circuit problem in holleys. Never had this problem on my qjets (hmmm)

hot tbolt module that when hot advances timing?

Please add on....
BT

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Maybe it would help if each of us with this phenomenon listed specs. Maybe there are other commonalities. Mine are:

mark 4 - 454s
tbolt 4
800cfm holley double pumpers
msd 6 al amplifiers
mechanical fuel pumps
closed cooling

BT

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SBC 383
whipple charger
merc thunder bolt iv and merc dist
holley 1050 dominator
over transon exhaust
never runs over 150 degrees water temp

does anybody elses go back to nomal idle if you shut it off then start it right away? mine does so i doubt its heat related.

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I never tried aero but that is a good fact. I'll check it out this weekend.

BT

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I have the same problem and it is probably a hotter intake manifold heating up your air/fuel and causing it to atomize better, just like the exhuast crossover was designed for. My heads do not have the crossover, are any of you guys running with a crossover?

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No crossover here. I run the edelbrock rpm performer intakes. No splash pan under the intake to keep hot oil off the underside in the valley area either, so I am sure it gets hot under the intake runners. But why would the a/f mix atomize better when hot?

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My guess is the fuel vaporizes bettter with higher temperatures. The exhaust crossover was designed to smooth out the idle so it just stands to reason that without the crossover we are going to have some issues with idle. What temp t-stat are your running? Try pulling out your t-stat for a few runs and see if that helps, if it does we have located your problem.

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By the way
SBC 350
Dart Iron Eagle Heads , 64cc, 180cc intake runner, 2.02/1.60 valves 9.7 CR
Crane Blazer rockers
custom hydraulic cam 218 226
Edelbrock performer intake no exhaust crossover with a oil baffle under it.
holley vac sec 750
HEI distributor
GLM exhaust manifolds 4" thru hulls.
The first incarnation of this motor made just over 400 hp , after a certain unpleasantness I had to machine the heads and ended up with higher cr and stepped up the cam a few degrees so I am guessing I am right around 415-420 hp, no dyno sheet after the rebuild. After a hard run my RPMs jump up from 700 in gear to around 900, I just live with it.

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Don't think its heat related, cause It never did this before I found my timing problem(not advancing) so it used to run alot hotter. My h20 temp sits at 170-180 is that o.k. for a boat motor or too high?
454 .030 over
eagle rotating assmbly.
extreme marine cam
Harland Sharp rockers
B.G. carb.
Gill dry offshore manifolds & risers
merc 420hp heads & intake

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I run around 140-160 degrees with a 140 degree t-stat, what temp t-stat are you running? I don't think 180 degree H2O temp is bad for a motor but it will beat your oil up a lot quicker.

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1990 454 thunder bolt ignition, Yes, mine will stick at about 2000 when backing off the gas, if I shut it off, and start it up again it returns to normal. stock quadrojet. I am surprised this if such a common problem.

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not sure what temp stat.
If I shut it off and start it right back up it still idles up. Maybe if I wait a few min. before restart. I'm gonna work on it tonight so I'll let you all know what I come up with.

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are marine distributors vacuum advance?


thinking out loud; coming off throttle, vac advance sticks, shut engine off, distributor goes back to idle???? or did i just waste a couple braincells?

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good thought, some may be but mine is centrifugal (MSD). I had thought that may be the problem though, weights hanging up after swinging out or something? but we've come to the agreement advance won't make the Idle go up that high.

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I have a stock electronic merc dist and a thunderbolt ignition, so mine problem is not distributor advance. Also I'm using dart alum pro one heads with no thermostat

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Well It turns out it was a combo of timing and vacuum. I found a small vacuum leak in the carb gasket. This brought the idle up to about 1100, well the advance weight return springs I was running were too light so at that rpm it alwready began advincing to about 18 degrees. This took the idle the rest of the way up to 2000.

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Good job, but that dos'nt explain mine or possibly others, ours returns to normal when you shut it off then restart.

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I still think you're guy's issue may be the timing weights not returning fully to base timing. When you shut down. they retract the rest of the way. Take a timing light with you next time it does it keep it running and throw the light on just to see.Worth a shot no?

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No timing weights, stock electronic merc distributer, advance is in the box.

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I forgot you said that, I'm sorry. I guess my find doesn't help you. But its got to be in timimg or lean mixture from a vacuum leak. If you shut it down than restart, will the idle stay down or sit low for a min. than start to climb? I think that was part of my vac leak. cause if I shut it down and restarted it the idle stayed right for a little bit. Prolly just cause It was loaded up from the restart and than richened up the motor enough to cancel the vacuum leak for a few seconds till the motor cleans up. :( :confused:

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Nope, mine will idle right where it should and stays untill the next time I run it hard.I'm leaning to a sticky pcv valve? I duno.

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It must be some kind of vacuum thing, something sticking then when the vacuum goes away it returns, kinda like Just Once's sticky mechanical advance theory.