aaroncorvette

Hi guys, i read lots of posts on here and have a good idea of where to look, but i want to go back to the start and logically go through things one at a time as this is driving me crazy!!!

Specs
350+30, K/b Hyper pistons flat tops, Edelbrock RPM cam, heads, air gap and carb, ali fywheel, l+b, roller rockers, hooker sidepipes

History -
The engine which came with the car gradually over the course of 3 or 4 years ran worse and worse to the point where it would backfire under load (around 4k in 3rd), took the engine apart and found the combustion chambers caked up with soot, during this time a pertronix dizzy and coil were fitted and part of the fuel line replaced (Collapsed) and the sidepipes fitted, the previous owner fitted the mods listed above

Rebuilt the engine last year with new rings, bearings, gaskets, clutch, sump and pump and reassembled it as above, the car ran fine for 500 miles or so then started backfiring again, seems to be coming out of the drivers side sidepipe, a massive bang at aroung 4-4.5rpm in 3rd at which point i have to let off or feather it and i can't rev past it, it will rev up to 6.5rpm no problem in 1st and 2nd and it will then run bad at idle, this is pretty much the same problem as before the rebuild!

The drivers side pipe pipe insert did fall out once and had to be welded back in (thanks Hooker!) and there is a small blow at the collector but surely not bad enough to cause the engine fall flat on it's face?

I also did have some problems setting the lash, it seemed 1/4 turn either way would be the difference between it starting or not, being a hydraulic cam i would think this is a little too sensitve?

If i change the plugs the car will run alot better (for a while) then it's back to playing up.

The idea for me doing this is obviously to get my car fixed but it will be helpful for anyone else in a similar position

Right i'm going out to take the plugs out and i'll report back, in the mean time any obvious ideas anyone?

Thanks guys, this may take a while to solve, but i'm determined to sort it out

Here's a video, my red '71 rag top at the strip, 1 minute into the video, in 1st and 2nd i'm keeping up with my mates Zr-1 put in 3rd and BANG! (can't hear it on the video but everyone there heard it) put it 4th leaned on the gas for about 500rpm and it would have backfired again so i put it in 5th and cruised across the line!!!!

http://www.corvetteclips.co.uk/site%...d%20Round6.wmv

RobRace10

That doesn't sound right. How are you setting the lash as it sounds like you have the lash to tight. If it won't start the valves must be open and with a hydraulic cam that sounds like you are putting to much pre-load in them.

I had one that would backfire on me and it was the plug wires cross firing. Have you tried new wires?

midyearvette

are you sure you dont have solid lifters on a hydraulic cam???....your lash settings dont make any sense.....good luck

aaroncorvette

Hi mate, i've reset the lash approx 3 times, it will start and run fine usually but if i then add a 1/4 turn the car won't start and has no compression and if i take off 1/4 turn it won't start either?? I'm setting the lash hot with a buddy of mine who knows his stuff

This is interesting, the car is parked in the garage and hasn't been started for a month, the last thing we did on the car was to check the carb was squirting properly, this put a fair bit of fuel through it

Now i've just pulled the plugs and they are all a bit sooty, electrodes are tan/sooty, all of them are dry except no 5 and 7 which have been cleaned by petrol???? I've looked and the valves are shut on 5 and 7

and this is the same bank which is misfiring, i've got a suspicion the lifters have had it?

aaroncorvette

I know, i've not got any room to manouvre with the lash, very wierd, they are Hydraulic lifters (Edelbrock performer RPM cam kit)

...and another thing the rockers are easy to rock when the valves are closed, this is with them set up per Edelbrocks instrucions, surely there should always be Tension on the rockers with a hyd cam even with the valves shut?

midyearvette

you should have ample room for adjustment..something is wrong here...im not trying to say that this is the entire problem but i would check out the lifters by trying to push the plungers.....parts get mis packaged all the time and visually some solids look like hydraulics...hell you should have more room than that....did you check rocker arm geometry??...you should not have any problems with your adjustments...good luck

ps...remove a valve cover and while the mill is idling push down on the rocker top with the heel of your hand , that should move the plunger down after a few revolutions of the mill,.....if not, then the lifters are stuck or the wrong ones.....

aaroncorvette

I did check the geometry, i've been very careful and done everything by the book, i'll go out and try to push a lifter down

Matt Gruber

are u setting the valves engine idling?
if not, there is your mistake!

aaroncorvette

Yep, i've just tried to push a lifter down and i can't but i need to do this with the engine running or hot so the lifters are pumped up?
I take back what i said about the rockers rocking about, i can move them side to side and there is preload on them

I'll gap the new plugs and put those in and get her running tomorrow as i've got to go out for my mates b'day shortly

Any ideas on why no5 and 7 plugs are wet and none of the others?

OMF

First off, do a compression check...that will tell you lots
Second, run the engine in a dark garage or at night, and look all around for ignition sparks going to ground....make sure to run the rpm's up to about 3000-3500 while doing this.
Third, What plugs are you running....sounds like they may be too cold and are fouling after a period of time.....that may be whats wrong with 5&7

midyearvette

go back and re read post #6....you dont have to set them ( the valves) running unless you prefer to....sounds like you could have solids on a hydraulic cam or is it a solid cam??

MotorHead

Try a different distributor, I had the same problem, turned out to be the distributor

BB72

Two things caught my attention. You're fouling plugs because it runs better after a plug change and you have a loud band in your sidepipe. Gas is getting into your exhaust from a miss and igniting, probably from fouled plugs. What do your plugs look like, if they're coated in oil deposits, you have a bigger problem. If they are clean and fouled then it's probably too much gas. Check your carb. Just my 2cents from past experience.

Rob

7of69

Take your valve covers off and use a remote starter switch and watch the valve action, I found a flat cam lobe this way after trying everything else for a backfire. Disable your motor from starting.

aaroncorvette

Great stuff guys, i'm running champion 12yc (can't remember the exact name), the ones that Edelbrock recommends

Plugs are dry and a bit sooty, look at my earlier post

I was thinking of changing to a solid roller cam anyway, any recommendations? something one notch up from the performer rpm cam i have at the moment, i'm not really keen on this cam, seems to have loads of duration and not much lift

i'll do some work on the car today and get back to you

aaroncorvette

right i've just done a cold compression test on the pass side with the throttle closed

No - 2 - 145
No - 4 - 145
No - 6 - 145
No - 8 - 135

Then with the throttle open

No - 2 - 135
No - 4 - 140
No - 6 - 120
No - 8 - 130

Cam specs are - Edelbrock Performer RPM Hydraulic#7102

Duration @ .006" lift intake 308 exhaust 318
Duration @ .050" lift intake 234 exhaust 244
Lift at valve intake 488" exhaust 510"
Lobe separation 112
intake centreline 107 if that's any help


I thought the 2nd results are supposed to be higher? battery was starting to give up towards the end which might account for the 120 reading

Battery is on charge now, i will put the plugs back in, get her hot and then redo the compression test on all 8, hopefully today

Matt Gruber

did u set the valves HOT at idle? how far past "no tap"?

aaroncorvette

Yep, set them hot as per edelbrocks instructions, from memory it was 1/2 a turn or i full turn, but i'm starting to think something is'nt right

Matt Gruber

then it sounds like an ignition problem.

mysixtynine

If I remember correctly the firing order on a small block is 1-8-4-3-6-5-7-2.

Sounds like you have a cross firing issue on 5 and 7 (no coincidence they are right next to each other) you are dumping unburned gas into the exhaust and its finally detonating.

As the RPMs go up it will get worse. Sounds like maybe a bad cap, maybe wires are too close together or your distributor is having issues.

We love to update our cars but when we change allot at one time we sometimes induce new issues.

I would try another distributor and like the idea of watching the wires in the dark for arching. The fact that it seems to be on one bank of the engine does help narrow it down some.