I did a favor for my brother in law by rebuilding the engine in his 67 Camaro. I was about ready to give the car back to him but I decided to test run the car to make sure the car performed as it should.

The problem I am having is at idle I can feel an irregular exhaust pulse in both exhaust pipes. It appears that it is maybe one cylinder because one pulse is stronger then the others. This is a dual exhaust without any cross over.

If I rev the engine to around 2200 to 3000 RPM I can hear it starting to break up slightly. When I road tested the car at part throttle it begins to break up and has no power. Sometimes this condition is worse then others, and on occasion it will pull hard with an occasional hiccup.


Everything is either new or reconditioned in the engine except Valve Springs. All parts are brand name ... no generic parts were used.

Here's what I have done so far:

-Tried a different distributor (new points, vacuum advance, condenser, cap, rotor and coil)

-Tried new/different spark plugs

-Replaced plug wires with Accel 8mm solid core wires

-Checked timing numerous times

-Readjusted the hydraulic valve lifters to 1/3 turn preload (Engine running method)

-Cranking compression test around 170 psi all cylinders.

-Cam Degreed and installed straight up

Engine components:
Comp Cam 268H-10 (Close to stock)
Comp Cams Lifters
Original Springs
Stock Q-Jet Carb (Rebuilt)
Stock Intake
Headers w/header mufflers
Power Glide
Stock type flat top pistons
Heads reconditioned with 3-angle valve job and valves hand lapped

I'm beginning to think the valves are not sealing properly and this is causing the poor performance.

ANY HELP or advice would be greatly appreciated!

Thanks
Brian




[Modified by RED92LT1, 3:24 PM 9/2/2002]

 

Is it possible you're getting a misifire condition because of parallel plug wires? Try rerouting them.

 

Plug wires are seperated ... I used the factory seperators and the ones that came with the Accel wires.

Thanks
Brian

 

That kind of sounds like a lifter problem, to me, though I'm not really qualified to make a diagnosis.

I doubt that it has anything to do with the problem you're describing, but is there any particular reason the valve springs weren't changed? Been changed recently, perhaps? Can the stock-type springs handle that cam?

The reason I ask is that I put that same camshaft in a '74 L-48 with the stock springs, after checking them for tension, etc.. Within a few months the springs started breaking. I replaced them with the set that CompCams recommended and didn't have any more trouble. (Well, actually I started breaking the stock rocker arms then, but that's another story..)

Jason


[Modified by needanother1, 5:23 PM 9/2/2002]

 

Jason

As I said I was doing a favor for my brother-in-law and I was trying to keep the costs down. The shop that did my heads said the springs were okay ... and I believe Comp Cams said stock springs would work so I didn't change them.

Whan you started breaking springs how did your engine act? Second question ... did the cam make a difference in your L48?

Thanks
Brian


[Modified by RED92LT1, 3:22 PM 9/2/2002]

 

I would look at the carb more closly. Have you checked for vacuum leaks? Put your hans over the carb to choke off the air while it idles, Does the idle increase or smooth out? Is the choke opeing all the way?

Any chance the wires are on in the incorrect order. 18436572.

Could you has lined up the timing marks off by a tooth? Will the car run better with the timing advanced or retarded to much?

Good Luck

 

I relatively sure I don't have a vacuum leak ... however last summer I found a vacuum leak at the intake gasket which solved a stalling problem when I put it in drive. I can't say for sure that the intake gasket isn't leaking again. I was careful to clean all surfaces and I used a gasket sealing compound.

As far as the firing order ... it's correct and the timing was set with a light and the distributor is in the normal position for the timing. But even if the distributor was off one tooth would that only place the distributor in an odd position without effecting performance.

Also I would think if the firing order was mixed up the throttle response would be sluggish ... in this case throttle response is crisp. But if I increase the RPMs to around 2500 I can hear a misfire that was not there when I initially fired the engine last year.

Thanks
Brian

 

I was actually refering to the timing gears possibly being off by a tooth.

Good Luck

 

I broke three springs before I finally replaced them all. Lucky that I didn't drop a valve. All the ones that broke were on the exhaust side. It wasn't a subtle problem: There was a clearly-audible clacking sound and a miss from the dead cylinder. It didn't backfire really, I guess because the exhaust couldn't get out of the cylinder and no ignigtable mixture could get in.

That's a good mild performance cam, with just enough lope at idle to sound good. I think it's ground four degrees advanced, if I remember correctly. My motor did not have the stock cam in it before the rebuild, so I can't compare it to the original. I had a Performer intake, 1-5/8" headers, factory heads, 3.08 gears and stock converter. All-in-all I think the 260 would have been a better cam choice for me. I wasn't unhappy with the 268, but its power band was a little higher up the tach than the rest of my combination would have preferred. Idle vacuum wasn't real strong on my setup: I had a little fluctuation between 12"-15" at idle, and honestly I never could seem to get it to run right with the Q-jet (which happened to be a reman. unit, not the original). I put a vac-secondary Holley 600 on it, with a four-hole spacer underneath.

I don't have performance numbers from that car, but it pulled hard from 2500 RPM-up. As I mentioned, I think a little higher stall converter or maybe 3.36 gears would have been better.

Jason

Jason

 

I had a similar problem recently and it turned out I had some exhaust
valves intermittently sticking, and when they did, I'd get popping and out
the exhaust and the same kind of irregular pulse you described. The valves
would only stick (not quite close all the way) when the engine was running.
I otherwise had good static compression. The machine shop had knurled
the guides, and they were still a little on the loose side (budget job). Anyway,
had them sleeved with bronze liners and works like a champ - completely
cleared up the problem. An expensive lesson on trying to save some $$.

Dan

 

I had a vacuum leak at the intake gasket last year but thought it might be a valve timing problem so I degreed the cam and made sure it was installed straight up. Any way I replaced the intake gasket and the engine ran fine. However, the carb was running a little rich and fouled the plugs, so I sent it out to be reconditioned. When I got it back I thought I was finished and decided to road test the car and that's when I discovered the performance issue mentioned above.

Thanks
Brian

 

dann085,

These heads were done about 12 years ago. So before I put them on the engine three years ago I dissambled them cleaned light rust off the valve surfaces, hand lapped the valves and reassembled the heads. The only reason I am leaning towards the heads is because the engine was assembled for about two years before it was fired and I darn near checked everthing else. But I certainly could have missed something.


Thanks!!
Brian