Chevy 350 rebuilt motor running rough. Thunderbolt IV Distributor off a tooth?

[wmschenck]

I'm a former aircraft mechanic and learned a long time ago that a good mechanic looks to the experts for help if he can't figure out why something is not working right. That is why I'm here, looking for advice from this great forum of experts.
My freshly rebuilt Chevy 350 feels like it is running on 7 cylinders. Details: Brand new rochester Q-jet from Flying Fish Carburetors, fresh gas, new water/gas filter, gas lines blown out prior to startup, new distributor cap, rotor, plug wires, and AC Delco R43TS plugs. I have a Thunderbolt IV ignition system with the ignition module mounted on the riser. I can get the engine to crank up and run, but it runs very rough at idle and shakes the heck out of the boat when I attempt to rev it up. I've triple checked the plug wires for proper routing, and hooked my induction timing light to each plug wire to verify each spark plug is firing. It does not matter where I advance or retard the distributor, the engine never smooths out. This motor is an aftermarket engine that I rebuild because the previous engine builder built it as a high compression engine (realized this after I torn the motor apart to try to figure out why I kept getting a knocking noise).

My theory is (1): maybe my lifter pre-load is too tight. While building engine on engine stand, using engine manual, I set the engine to TDC on compression stroke and tightend half the lifters to one full turn AFTER I could no longer spin the push-rods. Then turned engine one full revolution, I finished adjusting the remaining lifters. (2): I'm one tooth off on the distributor. Can anyone explain in good detail the proper way to set the distributor? Does it matter what positon the ignition sensor mounted on the distributor (wires from the ignition module connects to) is in? When viewing the distributor straight down and looking towards the bow, the ignition sensor is positioned at the 6:00 position. Part of good troubleshooting is knowing all the facts. I hope I have provided that for you. Thanks again for this great resource.

 

[dan t.]

Re: Chevy 350 rebuilt motor running rough. Thunderbolt IV Distributor off a tooth?

If you can set the timing without stretching the wires a tooth off doesnt matter,I suspect you have the valves too tight,to set them running and keep the mess to a minimum do it like this. Pull 1 valve cover, start the motor , one at a time back off the valve untill it rattles,then tighten until it is quiet, leave it there and go to the next. when the whole side is done to 0 lash shut it off, turn each valve down 3/4 of a turn, put on the cover and do the other side the same, I have been setting chev motors this way for over 30 years and it is foolproof and keeps the oil spray to a minimum.

 

[dannyual767]

Re: Chevy 350 rebuilt motor running rough. Thunderbolt IV Distributor off a tooth?

I'm with dan t. I think that you've got your valves too tight and they're not closing all the way. Readjust them the way he described and see how things go.

 

[6meter]

Re: Chevy 350 rebuilt motor running rough. Thunderbolt IV Distributor off a tooth?

You're one tooth off cause the oil pump rod slot needs to line up with the dizzy. Dizzy position is only for looks. To make it look good, pull the dizzy and stick a long screwdriver down the hole and turn the oil pump rod ccw about 10*. That should line your dizzy up just to look good, but changed nothing as far as running. As said, if you can time it you're ok. It might just not "look good". At idle, one at a time, remove a plug wire from the cap, the cyl that nothing changes when removed has a problem. Make sure plug wires are 1-8-4-3-6-5-7-2. New carbs need to be adjusted. Valves could be to tight. Check plugs. Good luck.

 

[wifisher]

Re: Chevy 350 rebuilt motor running rough. Thunderbolt IV Distributor off a tooth?

You say that you rebuilt it to lower compression. What did you change to do this? Did you check what kind of cam is installed?

 

[Bondo]

Re: Chevy 350 rebuilt motor running rough. Thunderbolt IV Distributor off a tooth?

and tightend half the lifters to one full turn AFTER I could no longer spin the push-rods.

Ayuh,... I agree with dan t too.....
You need to remove the slack in it...
I suspect the valves were too tight, at the point you could no longer spin the push rods...
By the time the spin is gone, the lifters are collapsed...

 

[wmschenck]

Re: Chevy 350 rebuilt motor running rough. Thunderbolt IV Distributor off a tooth?

Well today I pulled all the plugs and turned the engine over while sticking my finger into each spark plug hole to check for compression. Only # 5 had any compression! I was floored! I had set the pre-load twice exactly as the Mercruiser book suggested. I could not get the engine running, so I removed both valve covers and back all of the lifters off 1 and 1/2 turns and put the valve covers back on. I turned the key and the engine fired up and ran smoother than it has since the rebuild. I allowed the motor to warm up then shut it down and installed an old valve cover that I had cut the top out of the night before. I re-started the motor and adjusted the pre-load per the "running method" and as I see Dan T's suggestions today. The motor is finally running fine and I was actually able to get the motor down to around 700 rpm and adjusted the timing to 4 degree BTDC. The motor runs nice and smooth now and revs up nicely. Although my boat is a 19 foot 1987 Chaparral with a 95-00 crate motor block that has been rebuilt, what is a good recommened rpm and timing degree for this boat. I can't really go with what the book calls for because it is based on a 260 HP 350. I have no idea what the HP is with this new motor.
I was very leary about re-using the cam. It is a roller cam with roller lifters. My machinist said it would work good. He specializes in Mercuriser repairs for 20 years. I replaced the high compression heads with 333882.....74-80...350/400......76cc chambers heads. Again, my machinist recommened this for my motor.
I can't thank all of you enough for your time and wisdom. Dan T, I do like your suggestion on how to do the running valve adjustment. There really is no need to continue running the motor once you have removed all the valve's excess valve lash. Just shut down the motor and tighten each lifter an additional 3/4 turns! I certainly hope I can contribute my mechanical and boating skills to this great forum in the future. In the mean time, if anyone needs any advise on how to keep there 747, C-5, C-17s in the air, drop me a line! Aloha, William

 

[LAC_STS]

Re: Chevy 350 rebuilt motor running rough. Thunderbolt IV Distributor off a tooth?

I believe the correct timing is 8 degrees BTDC with 89 octane fuel or above, and 2 (or 4 forget) degrees BTDC with below 89 octane fuel.


With 76 cc heads your probably around 260 HP or so. The smaller cc heads give more horsepower which is what a vortec has. I dunno why your macheneist reccomeded it but id bet he has a reason for doing so.

The smaller cc heads would have gave you more power. I think the Vortec 350's are rated around 300 HP.

 

[dan t.]

Re: Chevy 350 rebuilt motor running rough. Thunderbolt IV Distributor off a tooth?

Glad it worked out for you,pretty sure the timing setting is 8 initial which should give you 30 total,you might want to try 10 inital for 32 total, if it doesnt ping leave it there. The 882 head wouldnt be my first choice of head, they are quite thin and tend to crack between the valve seats, a far better casting is the 993 but it is together and running now so go boating and have fun,as long as you dont overheat it you should be fine. And yes I sure would like to check out the inside of a 747 engine