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86 350 idle issue

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viper2217

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"Hey guys. I have an 86 350 m

"Hey guys. I have an 86 350 merc. That has been giving me idle problems. I had a manifold choke q-jet carb on it that gave me hell for the past few years. It would load up with fuel and kill the motor then to start the boat back up took forever because it was flooded. Last week I purchased a newer electric choke q-jet carb and installed it. This fixed my starting problem however, the boat still won’t idle below 1000 rpm but runs great at high rpm. The set screws don’t do anything they can be all the way in or out and it doesn’t make a difference. The gasket for the carb is new. We tried spraying carb cleaner all around the intake and that didn’t do anything. We busted out the timing light and the timing would be at 8 degrees, flicker down to 0, go back to 8, flicker down to 0 and then the motor would die. We changed out the thunderbolt IV, checked the voltage off the coil (14V), and checked the sensor in the distributor cap. Nothing has helped. Today we were going to check the valves and make sure they are not too tight and check the ground off the distributor sensor. The compression is consistent in all cylinders. The vacuum is 10psi at 1000 rpm and 15 psi at 2000rpm (seems low to me). If I place my hand over the top of the carb it runs way better so I am thinking a vacuum leak. However, the flickering of the timing has us scratching our heads. If nothing works out today I think my next step is to replace the intake gasket (even though I don’t think its leaking)…… thoughts, questions, comments, and suggestions are all welcome.
Thanks, Mike"
 
"checked the sensor in the

"checked the sensor in the distributor cap"

What did it look like? Rusty? There are some tiny bare wires on the assy. that will corrode and give you the same symptoms. That device is the amplifier module trigger.
 
"The vacuum is low. Did the RP

"The vacuum is low. Did the RPM change when the timing check was done; did the timing bounce between 0 and 8 (BTDC, I hope) occur at a fixed RPM?

Did you change the gasket(s) under the carb when you switched the carb?

Did you supply power to the choke from the coil?"
 
"You have a vacuum leak.......

"You have a vacuum leak.........10 inches at 1000 rpm for a stock motor with a stock cam is to low. you should be closer to 13-15 inches.

The timing mark is moving because you can not maintain idle.

If you want to know if your timing chain is bad rev the motor to 2000 and hold it there. point timing light at mark if it stays constant with NO movement (occilation) then your chain is good.

The fact that you hold your hand over the carb which does the same as a closed choke says it wants more fuel to make up for the additonal air that is comming from somewhere.

Did the original carb do this?

If not, its the new carb/gaskets etc.

The two problems are not related, ( choke malfunction/flooding vs wont idle due to vacuum leak)"
 
Thanks for the thoughts guys.

Thanks for the thoughts guys. now for some answeres to your questions. the amplifier module trigger in the distributor cap has been replaced and is the new completely encased in gel type so no rust. BTDC????... yes the gasket was replaced under the carb. and YES i did suppply power to the choke from the coil. should i not do that? the original carb did do this but i thought it was just flooding and thats y it wouldnt idle/start after running. now it starts but still wont idle.
 
"BTDC" Before Top Dea

"BTDC" Before Top Dead Center; timing term

Did you check the timing? What is it set at?
 
i have been playing around wit

i have been playing around with the timing it does not help the problem. i know it should be around 8 degrees.
 
maybe its time to check for a

maybe its time to check for a slipped timing chain....valve timing being off can also cause low vacuum.
 
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