GlenYappa

I built a brand new engine to go in my 84 Camaro. Since this is my first engine build I decided to crank the engine with a 600 cfm Edelbrock carb and a points distributor (New parts I am going to move to my '72 blazer) so no computer to worry about. Some quick specs: 4 bolt 350, AFR 190 heads, SLP headers, Weiand blower (the small emmissions legal one), hydraulic lifters with roller rockers 1.5 ratio, and currently an Edelbrock 600 cfm Performer carb.

It cranked right up and idled pretty good. I tried to go up to 2500 RPM for the break in period but I am getting backfire through the carb. I didn't want to push it so I shut it down especially with the blower. I decided it was timing so I tried to change the timing and it got a little better but still had a carb backfire. I change the vacuum port I was using on the carb from full vacuum to timed vacuum to the distrib. Seemed to run much better but still a carb backfire. At this point I have pulled the valve covers and reset the distrib. with #1 at TDC. With distrib on timed vacuum port it seems to run pretty good but I still got a carb backfire. At this point I am thinking about checking cylinder pressure. I can't see how combustion is getting in the intake unless a intake valve isn't closing all the way ( valves adjusted too tight?) or if fuel is pooling in the intake manifold (I think this is unlikely unless my brand new carb has issues)

Does anyone have any suggestions?

Glen

ede

might want to run the valves again, or just back them all off a bit and see if it improves, also change the timing a bit more.

five7kid

iTrader: (14)

Is the carb power circuit set up for a blower?

GlenYappa

I was under the impression that since this is a draw through system I wouldn't have to make any mods to the carb except maybe rejetting.

five7kid

iTrader: (14)

This is what I thought:

In a draw-through system, when you open the throttle manifold vacuum as seen by the carb will be high because the blower is "sucking" air through the carb - keeping the mixture lean because the power circuit sees high vacuum. But, cylinder pressure is higher which requires a richer mixture.

In a blow-through system, manifold vacuum as seen by the carb would be low because the blower is "pressurizing" the carb. When you open the throttle, manifold pressure increases/vacuum decreases, the carb sees this and the power circuit goes to rich mode.

Or do I have it backwards?

GlenYappa

Looks like I had some intake valves not closing all the way. I pulled the plugs and 3 showed signs of problems. A compression check confirmed the valves were not closing all the way on some cylinders. I reset all the valves and compression looks good. I should be able to crank tomorrow. I thought I had primed the hydraulic lifters enough when i set them but I guess I was off on a few. I will update when I get to crank. I had to turn over the engine a lot without doing a proper breakin. I hope my cam is ok. It seemed to be during the testing.

Glen

Abird4u2nv1

iTrader: (-1)

U might also wanna check your timing before you go and just crank it over. Bad timing could do that among some other thing that you don't wanna do to a new engine. Trust me I've seen it.

five7kid

iTrader: (14)

The lifters don't have to be "primed" to be set correctly w/o the engine running. The plunger spring is quite adequate to let you know when you've removed all the lash - adjust with the lifter on the heel of the cam lobe for each valve.

Connecting the vacuum advance to the carb is a bad idea for the same reason the power circuit won't operate properly.

I strongly recommend you call the Holley tech support people and discuss with them what you need to do to get this thing to work right. I pitted at Bandimere last weekend next to a guy with an early 70's Nova with 350 and small Weiand blower. He told me he called those guys, gave them all the info about the car, what he wanted to do with it, our altitude out here, etc.; they told him what cam to get and how to set it up when he put it together. When they sent him the system, the carb was already calibrated. He's adjusted the idle, that's it. The car runs low 12's up here in the sky.

I realize you've got an Edelbrock, but you can be sure Holley knows about those carbs, too. Or, call tech support at Edelbrock.

GlenYappa

From everything I have read, since the roots blowers are a draw through system and the air/fuel is not compressed until it goes through the blower all the vacuum lines on the carb work just as if the blower wasn't there. All the carb sees is vacuum, no boost. I know now that my valves were set wrong on a few cylinders. I have reset the timing again. If it revs well I will set the timing and finish breakin.

EDIT: Even though the increased airflow will affect the tuning of the carb as you said earlier using the vacuum port on the carb for the distrib should be fine.

five7kid

iTrader: (14)

From http://www.holley.com/HiOctn/TechSer...fo/SCTech.html :

"Power valves provide additional fuel when there is no vacuum at the base of the carburetor. However, in a blower application, there will always be some vacuum, so the power valves will not function properly. You will need carburetors that have a "boost referenced" power valve circuit. Holley "Supercharger Carburetors" are specifically designed with this feature."

GlenYappa

five7kid: I totally believe what you are saying and I may need to hook things up differently. The instruction with the blower which is made for stock applications doesn't say anything about this topic at all. I will keep that info in mind when trying to tune this thing.

Quick update: reset the valves, compression check and it cranked up but with an obvious blowback problem (just air though NO combustion like before). Had an exhaust valve to loose (an accidental high rev made it more loose)! reset valves again being very very **** about the whole process. Cranked right up and purrs like a kitten. Gonna set timing tomorrow and finish a breakin run. I can't believe I haven't killed my cam.

I appreciate the help.

five7kid

iTrader: (14)

Sounds like progress.

Good luck with the finishing touches.