I set the total timing 30%@2500 rpm and 4% btdc. After finding out that the vacuum advance diaphram was out, Everything (idle, cooling, idle fuel mix, preignition clatter) started working right except for one thing, backfiring under exceleration. Does anyone know what I might check?
Just a quick thought. Did you have the plug wires off? If so maybe two are switched. Depending what pair it may only backfire under light acceleration or heavier. Cracked plug, bad cap or rotor?
Check NoLift's suggetions first. When you replaced your cam, did you degree it? If not, did you install it "straight up"? eg: Timing marks on timing chain gears lined up perfectly. Did you do a cam break in procedure, or just let it idle with the new cam? http://www.v8buick.com/showthread.php?t=168300 With "lots of clacking", you may have hurt something in the valvetrain like a lifter or cam lobe. ElectraJim
Carburetor shots one 3 foot flame with loud bang. Then idles OK. Slow rev is OK. Fast throttle with secondaries.....bang! Scarred the sh.... out of me.
I would recheck everything. Plug firing order, and cap. If it pops back through the carburetor, that usually means it is lean. #1 cause of lean is a vacuum leak. You said you set the total timing at 30* at 2500 RPM. Do you have light springs in the distributor?
The distributor has original springs. The motor is still factory stock except for new cam, valve springs and lifters. I set the timing with advance unplugged at 4 btdc. I gradually increased the rpms to 2500 and the 30degree special mark on the balancer was at the zero timing mark.
That doesn't mean your total will be 30* With stock springs, the total will likely increase beyond 2500 RPM. The only way you can set total advance like that is to use light springs, so that the weights will swing out to their full travel at lower RPM's There is no way to tell how much mechanical advance is in the distributor you are using, unless you can see the distributor number, and we may be able to look it up. I think you have something else going on here with the back firing.
Mine started doing theat when I wiped out an exhaust lobe on the camshaft. Just thought it was worth a mention. -Josh
Like larry says if you can get the dist code we can look up the amount of mechanical timing it has. I use the mr gaskets bushings to limit mechanical, then use medium springs from the crane adjustable vac advance kit. Once I am happy with the performance with the vacuum advance disconnected I then limit the vacuum canister to 8 degrees then adjust with an allen key to change how quick the vac. advance comes on. I find that all 350s like at least 10 initial timing and usually 14-20! The only problem there is that with the HEI I use they add too much mechanical and it pings, so I limit the mechanical with mr gasket bushing kit. Some weld the slots and file them untill they get the amount they want (trial and errror time). I even know some one who set timing at around 25 degrees locked down no mechanical or vacuum then boost the snot out of the engine with turbos with good results.
Are you in a cold climate and do you have an air cleaner on the car? If you suck in large volumes of cold air it will make a perfectly tuned engine backfire through the carb. No air filter or even a very low restriction air filter will aggravate the problem if the weather is cold enough.
Did the backfiring start right after the cam change or did it appear at a later date? If it happened right away maybe your gear set was installed one tooth off and the cam is retarded way too much. Usually though the car will run ok but you will just be down on power big. The carb will make the big power sound(woooo) but you won't be going anywhere. As mentioned if an exhaust lobe wiped carb backfires can happen. You can take the valve covers off one side at a time and watch the valve action while it is running. Look for each rocker to be moving about the same amount. If a lobe is wiped bad enough to cause backfiring you'll notice one not moving as much as the others If it is a factory dist and you set it at factory spec initial timing then backfiring because of timing shouldn't be a problem. Changing springs and such won't help this problem. That is something to do when everything is right and you're looking for max performance. Right now you're looking to fix the backfire. If you put the 30 mark on your damper make sure it is on the correct side and that you are using the factory mark to set initial(don't ask me how I know about that one). Like I said though if that is the same dist you have been using then setting it at what you had it at before and it didn't backfire then you should be ok now. Are you saying you're slamming the throttle open while just standing there in neutral? If so you have to really wail on a Q-jet to get the secondaries to open and if you have a bad secondary pulloff it will let it plop open which will cause a backfire. You can't tell anything from flooring a car in neutral. How is it when you are driving the car and you floor the throttle? If it backfires/big hesitation(one or two pops)and recovers and accelerates smoothly after that your secondaries are just opening too fast. If you accelerate slower and work your way up to flooring it and it works fine right up to the max then you do have a problem in the transistion from primaries to secondaries. If it keeps backfiring(bap, bap, bap...) as you try to hard accelerate then something else is going on.
Thanks for all the help. I set the timing (unplugged vacuum) at zero 0 degrees and stopped pinging or backfiring. The motor seems to run OK. If I re-connect the vacuum line to distributor....the motor pings going down highway under light exceleration. If I go WOT the pinging goes away, must be manual advance taking over??? Does anyone know if pinging can be mistaken for something else? Could the timing marker above balancer be off? Is the round bullet hole on timing marker (6 to 10 degrees below zero) for another timing? HISTORY ON MOTOR: (past 30 days) New spark plugs, New spark plug wires, New vacuum advance Timing marks on crank and cam...perfect. Cam bearings inline and OK. New camshaft: CompCam mild hydralic lift part# 92-203-4 .469 lift 268H New stock lifters: Comp recommended New stock valve springs: New Rod Bearings Good Main bearings Removed RH cylinder head checked wrist pins and pistons on 2 and 4 Check wrist pins on all 8 cylinders from bottom at crank case (engine still in car) Each morning when I start the motor it makes a thrashing sound like all the lifters are calapsed and have to pump back up. After only 4 or seconds it goes away and motor is smooth and no noise. All the rest of the start ups during the day are perfect...no noise, only after setting over night. I have always been able to set timing on most any motor up to 12 degrees advance with out any ping.....this motor will not allow anything over 4 degress and I have to unplug vacuum advance or it pings under acceleration even when set to zero. This is a very tough motor. The motor is probably out of 1978 model Buick. When we first installed it we had cooling issues. It overheated 4 or 5 times. After that I only found one rod bearing on #3 to have been worn on top half of bearing. So I replaced all the rod bearings. They were plastigauged and checked out good with 2 thousanths clearance. The mains checked out with 2.5 thousanths clearance.
You are doing a GREAT job with that 350! Your issue right now is that when you want the vacuum advacne to add timing (under light load cruise), it is not. It is hovever adding vac advance at low rpm with screws everythign up! If you disconect the hose from the vac canister and feel it I bet there is suction at idle and it goes lower as you rev the engine. You want the vac to disapear under load and re-apear when you are cruising down the highway getting good fuel milage. There wil be another port on the carb that has what you need. When you drag race the car you can basically disconnect the vac canister becasue at no time in the drag race will you need vac advance. hope this helps.... BUT I WANT PICS, this car sounds cool as hell and you are doing great work with it.
