I recently replaced the heads, carb, and cam on my 76 L48 and am having quite a few problems I am hoping that you all can help me out. Now I am not the original owner and the person prior to me had done the exhaust (True duals), headers, Performer Rpm intake, and a carb (Weber casting but not a weber) After putting everything back together I was having a bit of a rough idle and backfire at WOT. I have checked all the plugs, wires, (yes firing order is correct) timing and they are all good. Replaced the Dizzy (Except the coil), also I verified no vacuum leaks from the carb or intake manifold. I checked the air/fuel ratio on the carb and that is fine as well. Fuel pressure to the carb is good, but still I get backfire. (Even tried the old carb) I have checked valve lash like 2 times to make sure that they are not stuck open (Used EO/IC)

If I retard the timing enough I get rid of the backfire BUT then the engine lacks power and diesels when I turn it off. Advance it and it runs smooth but then I get major backfire. Runs like there is a major vacuum leak or that it is struggling to get fuel, but both seem to be just fine... Since I just replaced the Heads (Used Arp bolts & Felpro gaskets) I have yet to do a compression test. I am guessing I should start there... Hopefully someone around here can help me out.

Major Parts I used,
Holly 600cfm Carb
Summit Racing Heads 64cc
Comp Cam 262/270, Lift .462/.469
Roller Rockers 1.5

Now could the Coil be causing this problem, improper pushrod length, or could it be something I am totally missing? Any help would be greatly appreciated!

Thanks in advance
Justin

The car had a hesitation problem prior but nothing like this.

 

Hi, cofeegod ! Looks like you're a new member...

It seems that you have already checked a lot of things !

Some ideas :
- Hesitations or backfires are often an indication of a lean condition and/or bad timing.
- Bad fuel or too old fuel ? What compression ratio do you have with the new heads now ?
Too much compression / too much timing advance with a low octane fuel will lead to backfires/detonations/preignition.
- You said your timing was good, but didn't tell us what your total timing is ? 32°, 34°, 36° ?
- Did you check accelerator pump system ? Pump response should be instantaneous to the movement of the throttle.
- Did you check float levels in the carb ? Is your Holley 600 cfm a double pumper or a vacuum actuated secondary ? This has an influence at WOT.
- How much vacuum do you read at idle ? Is it stable or do you have reading fluctuations ? Dual or single plane manifold ?
- What kind of AFR Meter did you use ? Narrow or wideband ? Where was the sensor located ?
- Did you check AFR at WOT ? If so, how much did you read ? ( 12,6 or 12,8 is a good starting point ).
- And yes, if you have changed the heads, a compression, or better, a leak down test could help to find / eliminate other possible causes...

 

New fuel using 89 octane
I was told by the tech at Summit Racing that I should be @9.25 compression
Timing is about 34
Haven't checked the pump since it was a new carb, but I did hook up the old carb and had the same problem. The new carb has Vacuum and secondaries on the old one are mechanical.
Idle vacuum was right around 16 but yes it jumped quite a bit it was not steady at all.
I was using a vacuum guage when adjusting the A/F ratio going at what gave me the most vacuum and then moving in a 1/8 turn making sure it held steady. I adjusted both screws at the same time and then even went back all the way in counting the turns just to make sure they were even.

Thank you for the quick response so far. Hopefully in the am something will hit me that I may of missed. I will pick up the part tomorrow am to do a compression test, that is if I didn't give myself a concusion from banging my head against my wall...

Thanks
Justin

 

Ohh yeah I am a new member but I have been lurking on these forums since I got the Vette, <<;; so much great info here!

 

600 cfm is perhaps a bit small for your engine with all the new parts.
You could try to increase the secondary main jets by two sizes at least and see if the engine likes it or not ( even if your spark plugs color is OK )...
And check/adjust accelerator pump cams, too...

 

Most likely you have not adjusted the lifters properly. If you do this with the engine OFF, you can have difficulty determining when the lifter is at "Zero lash". It is NOT when the pushrod tightens up. If you go that far, you have overtightened them and they will hold the valves open too long. The best way to determine the "zero lash" point, with engine OFF, is to lightly shake the pushrod...up and down...while you are slowly tightening the adjustment nut, just until the 'slack' is removed. That is the point when the piston/spring in the lifter just makes full contact with the pushrod. Then you tighten down another 1/2 to 3/4 turn CW and the lifter is set. Try setting them again with that method and see if it runs better.

 

Try disconnecting the vacuum advance and plug the ports and see how it runs. Assuming you have it connected now. Good luck. These problems can test you. But, you are getting good advice from everyone.

 

I agree with 7T1 about the valve adjustment likely being off.
If you dont have much experience with adjusting them with the engine off, lots of people have trouble (especially when they try the "spin the pushrod" method of finding zero lash).

You also might double check the plug wires to be sure they are in the correct firing order. I know it sounds basic, but crossed wires definitely happens.

 

 

I agree....I'd sure start here first.

JIM

 

It would also be a possible cause explaining why your idle vacuum is not steady at all.

 

Congrats U win my challenge! About 3 years ago i challenged cf guys to make it easy for a 1st timer to get an engine running good on their 1st try. Nobody could explain this simple and effective method.

 

Weird had to start a new account since the facebook link wasn't there. Ohh well. Anyways I did adjust the lash going side to side only because the tech from summit racing told me to... SO time to pop of the valve covers again and do them that way (Up and down). I will let you all know how it ends up, at this point if you all told me to run around my car naked while chanting and burning incense I would give it a try...

It does seem like this could be the problem I just assumed it was fine since that is how the tech told me to adjust them (First time with roller rockers) after that I will take a vacuum reading and see if everything levels out.

Thanks
Justin

 

The tech wasn't "wrong". But the 'spinning pushrod' method is too easy to misunderstand. When you do get to 'zero lash', the pushrod will still turn very easily; but, if you try to spin it and release it with your fingers, it will almost immediately stop spinning. Prior to 'zeo lash' it will continue to spin freely, as there is no friction to stop it. Once the lifter piston makes solid connection with the pushrod, it won't spin freely any more. When you set it with the method I've described, you can try to spin the pushrod and you will understand the difference. Hope that's your problem.

 

Update:

Ok I re adjusted the valves the and the car defintly started better. Checked the timing (Initial at idle) and was at 16. checked vacuum pressure and adjusted and got a stead 16 with no jumping. Defintly some improvement, but when I try to go WOT still the same problem and I get some backfire. I was reading Lars post on timing and need will try and set it to 36 total when I get home, I might just pick up an adjustable timing light.

So I was wondering if I measured the push rods wrong and got the wrong length could that be part of the problem? Coil going bad and getting sparatic spark at high rpm? Also now that it is running smoother should I adjust the valves with the covers off? (Using a cover cut out).

Thanks for all the help so far,
Justin

Just got off the phone with Summit's tech line and he was saying that even though the carb I got is "Ready to run" that more then likely the accelerator pump, so going to try that first.

 

I have to wonder if we have a misunderstanding here of the "spin" method - there's no spinning and releasing. Could be the core of the "Holy War"...because I can't see how "spinning" a pushrod would work for anything.

Grasp the pushrod between thumb and forefinger. Roll slowly back and forth between your fingers while slowly and consistently tightening the rocker. The first time you feel ANY drag is zero lash. The pushrod never "spins".

Not restarting this whole acerbic debate - just noting that we may have a misunderstanding of methods that's causing the opposition...which if it is as you're describing it I completely agree that ain't gonna work for anyone.

 

That's what I was talking about in post #5. I've had the same problem with my Holley 600cfm DP, brand new, but cams and pump levers were badly adjusted... and the two pink cams were too small for my application, so I finished with an orange and a green cam.
But I wouldn't be surprised that your secondary main jets are a bit small, too. On my carb, the primary jets were almost perfect but I went 2 sizes up on the secondaries to get a 12.8 AFR WOT.
Changing total timing from 34° to 36° will not solve your backfire problem, it can even be worse after.
I think you should keep your timing at 34° as long as your engine is still backfiring. Many engines with ( fast burn aluminum ) aftermarket heads work well with 32-34°.
IMO, it's now most likely a lean condition problem at WOT, which is not timing related, assuming you're really set at 34° total timing, which still has to be confirmed with your timing light.
So, first, check and adjust accelerator pump cams. Remember that pump response must be instantaneous to the movement of the throttle. Holley web site shows the correct procedure for adjustment. ( Videos and .pdf files ).
If it backfires or hesitates, then goes, try bigger cams and /or bigger discharge nozzles.
First try a cam that will activate the pump sooner, like a red / orange / green one.
If it's not enough, then try #0.035 discharge nozzles. Once you get a small puff of black smoke when you floor the gas pedal, then you have done enough ! Double pumpers and single plane manifolds like a big initial shot.
A kit like this one is a great help :

http://www.summitracing.com/parts/HL...2/?image=large

If it still backfires after that, try larger secondary jets. Without an AFR meter, it's a little like shooting in the dark, but at least, you'll see if it helps or not. Fine tuning will come later.

 

billa...

Therein lies the problem. The "twist it with your fingers until you just detect drag" includes an element of feel which is very subtle, at best. And if you haven't done that procedure before, many only detect a different "feel" when the piston starts to bind up. Thus, folks aren't setting the valves correctly. You can verbally instruct and re-instruct till you're blue in the face with limited success. With the 'twist/spin the pushrod' method, some will get it and many will not. Once they are shown that difference [visually and by feel] it becomes apparent to them...but that procedure cannot be effectively communicated in a verbal manner only.

 

Block ALL vacuum ports, leave lines disconnected, except the dizzy....

try it then.....wonder if you have a vac leak...try pliars on the brake booster line, pinch it closed...

 

justin:
? Please specify exactly which part number summit heads you recently installed ? Also Intake Gasket p/n?

All good suggestions; I'll add you should also check for leak(s) between intake & head(s).