Engine Question
Thread Starter
Instructor
Joined: Sep 2010
Posts: 229
Likes: 1
From: Grants Pass Oregon
Engine Question
I have a '78 L48 w/auto. When I bought it, the PO said it had sat for 6 yrs. Ran like crap. Changed cap and rotor, plugs and wires. Engine runs better, but shakes alot. When I give it a little gas it smooths right out. There is no hesitation. I checked vac. at manifold, steady 16". I ajusted the mixture screws and the best i could get was 17",but now it hesitates when i punch it. Engine was rebuilt some years ago. PO said it is now a 355 but he could not remember what pistons were put in it. In some of the papers i got with the car shows the Hedman Hedders, flowmasters, and the cam is a Wolverine Blue Racer = 204' 214' .50" Dur. 270' 280' AVD. DUR. .280 .295 Cam Lift .420 .443 Valve Lift 107' 117' Lobe Center What does these numbers mean ? .050 cam timing BTC 5-/ ABC 29 / BBC 44 / ATC 10- ? I checked the timing and it is set at 8* @ 500 RPM. Does this sound right ?
Pro
Joined: Apr 2008
Posts: 596
Likes: 6
From: Irving Texas
There's a lot of questions you're asking but the whole question is about the rough idle.
Your last statement says 8* @ 500 rpm. This idle is kinda low...I prefer 700-900 while the car is in park and 600 while in drive. I also recommend a lot more initial advance...like 11-13 degrees at idle.
I'm willing to bet your problem lies in the advance feature in your distributer. It's probably rusted and or sticking. Buy a new one........most HEI distributors are under $100 and are worth it..........unless you know how to lube the shaft and advance weights.
Your last statement says 8* @ 500 rpm. This idle is kinda low...I prefer 700-900 while the car is in park and 600 while in drive. I also recommend a lot more initial advance...like 11-13 degrees at idle.
I'm willing to bet your problem lies in the advance feature in your distributer. It's probably rusted and or sticking. Buy a new one........most HEI distributors are under $100 and are worth it..........unless you know how to lube the shaft and advance weights.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,112
From: Crossville TN
Rebuild/repair the distributor to make sure you don't have timing problems due to operating faults in the distributor mechanism. Also, verify that you don't have any significant vacuum leaks in the system (compare the smoothness and vacuum level at idle condition with vacuum system lines disconnected and plugged, as well as connected for normal operation). Vacuum differences for that test will indicate leakage problems.
If you still have problems, consider removing the heads and evaluating need for a valve job.
If you still have problems, consider removing the heads and evaluating need for a valve job.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
what kind of carberator dose it have? it could be that the mechanical advance is stuck but would be easy to check, rev it with the timing light set at 23 degrees advance, it should start to move towards zero, i second that your idle speed sounds a little slow.
Thread Starter
Instructor
Joined: Sep 2010
Posts: 229
Likes: 1
From: Grants Pass Oregon
Rebuild/repair the distributor to make sure you don't have timing problems due to operating faults in the distributor mechanism. Also, verify that you don't have any significant vacuum leaks in the system (compare the smoothness and vacuum level at idle condition with vacuum system lines disconnected and plugged, as well as connected for normal operation). Vacuum differences for that test will indicate leakage problems.
If you still have problems, consider removing the heads and evaluating need for a valve job.
If you still have problems, consider removing the heads and evaluating need for a valve job.
I have a Q-jet, after hooking the vacuum line back to dizzy and turning down idle [ back to 800 rpm ] , engine seems to run better. Took her out, punch it, BIG hesitation. One thing, the mixture screws don't seem to change anything after 2 1/2 turns.
The reason i asked about the Cam was because the Emission control info on car says:
Timing: 6*@ 500
Curb idle 500 [DR]
Fast idle 1600 [P] or [N]
Thanks for all input,
Ken
Thread Starter
Instructor
Joined: Sep 2010
Posts: 229
Likes: 1
From: Grants Pass Oregon
Thanks. Ken
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
It sounds like your accelerater pump is stuck, when you look down the carb with th engine not running & accelerate it by hand dose it squirt fuel? My vette had set for about eight years when I got it, the carb was all varnished from fuel that had went bad & evaporated, I bought the kit to rebuild the carb which turned out to be a waste of time and money as that the varnish was restricting fuel flow. I ended up going with a 600 cfm holley. If you take the distributor cap & rotor off you can look at the condition of your weights & springs if needed you can buy a curve kit from morose for about 15$ you can also use a vacuum gun to check that vacuum advance functions.
Last edited by bluedawg; Jan 22, 2011 at 01:26 PM.
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Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
Your vaccum advance needs to be hooked to I beleave the word is ported vacuum, which means that when hooked up at idle there is no vacuum to it. There is a long thread that informs in great length & detail about timing. I don't use the vacuum advance on mine, when I had my car on the dyno it seemed to make the most power without the vacuum advance hooked up & using the lightest mechanical springs. Did you check to see if the carb squirts fuel when the engine is not running?
