Plug Gap

[whale]

I need a little help with plug gap.....What is the best gap for a Twin Turbo ( Race Aero ) on a 468 BBC with Aul., Heads. Thanks for the help.....wayne All right guys let me make this more difficult. I have a 21 foot Rayson Craft Day Cruiser with a 468 BBC with 28 gears, 400 turbo tranny and a Twin Turbo set up. Boat runs fine untill it gets above 2800 rpm then it stumbles.....misses and will drop 3 to 500 rpm......does not make boost. Engine runs fine below 2800 rpm......Does anyone have any ideas or has experienced this before........Carbs are fine....present plugs are NGK-9s at .040

[Snowboat]

What is your ignition system? Mags .018"-.020".

[Turboelimtim]

Whale,
Without knowing any other details, sounds like it might be fuel starvation. It could be many things. You just have to eliminate things it could be by testing. If it were me, these are the steps I would go through. 1) I would start by making sure all cylinders are getting spark with a timing light. Then make sure the timing is set correctly. I run mine at 10 deg advance at idle, advancing to 36 deg total by 3000 RPM. I run an MSD retard box that sets the timing back to 30 deg total advance above 7 psi boost. 2) After eliminating timing/spark problems, a compression check to get a rough idea of cylnder condition is the next thing. 3) Do you have a mechanical or electric pump? If it is electric, you can disconnect the lines to the carb(s) and run them into a gas can. Switch the ignition/pump on for 60 seconds. It should pump at least 1 gal in a minute. If it doesn't, it is most likely the pump, the filter, restriction(s) in the lines or tank feeds or it could even be a lack of tank venting. If the fuel flow checks out OK, I would check the carb bowl inlet valves. Also, if the stone filters are in the inlets, I would remove them (assuming you have a water separator filter). Make sure the fuel level in the bowls is correct (just below the sight opening). The only way I know of to check a mechanical pump is to cut the engine immediately after it stumbles and check the fuel level in the bowls. If you can see the fuel just below the sight holes, it is not fuel starvation. One issue with doing this is that it is not good for the turbos to shut the engine down like that. You should always idle it for a while prior to shut down to allow the turbos to wind down before cutting their oil pressure. A couple of times should be OK though. 4) If the fuel supply is not the issue, I would check the intake systems ability to hold pressure. The way I do this is to disconnect the turbo compressor outputs from the intercooler and block them off. Then I remove all pre-load from the intake valves to close them. Then I pressurize the intake manifold to 15-20 psi and see if it will hold pressure by reading the boost guage. If it won't hold static pressure, it may not hold dynamic (boost) pressure.
There are many other things that it could be. I'm just giving you some ideas what it might be based on my somewhat limited experience. Hope you find the problem and please let us know if you find out what the issue is. Good luck, Tim :idea: :)

[whale]

Thanks for the input.....Ing. sys. is MSD with boost retard and the timing set at 30 total .....system is also set to 8lbs of boost..........electric fuel pump , 8 lbs of pressure flowing good no problem there ....carbs are fine, fuel level is good......eng., is new....could the problem be in the ign. system....coil, rev limiter, I will check the compression and re gap the plugs ....and check for fire on all cylinders.............thanks again, wayne

[whale]

Thanks to all .......the problem was a faulty boost controller...........wayne

[Turboelimtim]

Boost controller as in wastegate? or MSD unit? The MSD was going to be one of my next guesses, but I figured it wasn't the issue since you never made boost to activate the timing retard in it. My boost control is an unusual system (PFM) that chokes the path between the turbos and the intake manifold as boost increases. It originally limited boost to about 6 psi until I put washers in the springs to increase the boost to around 12 psi. Is yours similar, or typical wastegates. Tim

[whale]

Thanks Tim, yes it was a MSD Boost Retard System set up in 1978 with Hobbs Switches to lower the timing. The system had a 3 step system for a total of 9 degrees of retard. I do appreciate your help and coments.....we are not sure how it was doing what it was , but it was droping numbers 5-6-7 cylinders out they were carbon foulld.......but when we disconnected the unit all was well......................wayne