- This topic has 17 replies, 6 voices, and was last updated 11 years, 4 months ago by
.
-
Topic
-
I have a 1973 Chevy 350 v8 (in a 1980 k10) that is missing on cylinder 2 and 8( front and rear most cylinder on the passenger side.) It has headers and an Edelbrock 600 cfm carb and intake manifold.
About a month ago I felt a miss that would come and go. It, eventually, became constant and (recently) became worse. Because of the intermittent nature I assumed it may be related to the carb. I had a shop, that I have dealt with for many years, rebuild it. The problem still exists.
Over the last year I have replaced:
-coil
-plugs
-wires
-Dis cap n rotor
-valve guide seals
-push rodsI’m thinking it may be the “HEI” distributor going. Cause I have checked compression, spark, and rocker arm movement. The distributor has a little bit of rust inside and a minute amount of “side to side” play. nothing looks broken.
Any additional trouble shooting ideas would be nice.Thanks
-
Replies
-
Chevy 350’s have a habit of eating cam shaft lobes ( #8 and 6 seem to be common ) how many miles on your cam ? oem or aftermarket ? how is your timing ? with a t-light on it does it stay steady or fluctuate ? do a leakdown test on the suspect cyls. you can use a dial indicator to measure the cam lobe lift
I have no idea how old the cam is. I did (recently) replace the valve guide seals and push rods. And when the miss began I pulled the valve cover and it looked like all the rockers were working properly
I replaced the timing chain not long ago. I set the timing by ear, while someone was dragging the clutch.
The cylinders are fine. 3 rotations gave me 160 psi on all cylinders. They held strong.If you have good spark and compression on those cylinders I would remove the #2 and #8 spark plugs and install them in different cylinders to see if the miss follows.
If still missing on #2 and #8 there must be an intake manifold gasket vacuum leak at those cylinders or maybe a vacuum hose leak if the source is on the intake manifold runner for those two cylinders.
use a infra red thermometer on the header tubes to
compare temps.see if 2 and 8 are lower. does the engine
have vacuum advance? check that the springs and weights
work.also the diaphragm holds vacuum. I would put a timing
light on it. When you changed the carb and intake. was the
intake runner design changed? from 4 hole to open or vice
versa?I replaced those two plugs and the problem persists. I checked for vacuum leaks and didn’t find any.
The Edelbrock carb and intake were on the truck when I bought it two years ago. It does have vacuum advance, and it is working. The problem exists through out the rpm range. Those two header pipes are colder than the others (I poured water on them.) I am confident that the timing is correct. I adjusted it before this problem began. Besides (not to be rude) the timing being off wouldn’t only affect two cylinders.
You might consider connecting a vacuum gauge to an intake manifold vacuum port as those gauges can tell you a lot about an engine and do so very quickly.
If the engine is rough at idle only you also might consider the possibility of a few weak valve springs. I’ve seen this a couple of times on 350s and the first time it really threw me off because there was nothing obvious causing the rough running.
With the valve covers off, I could depress the springs manually on the 2 questionable cylinders and there was a huge difference in tension as compared to the others.
So much for the automotive machine shop checking things like this while reworking the cylinder heads…If the heat is low on those two cylinders.Then either
spark is not lighting or fuel is lean.You may need to jet
up. Have checked for a good spark on those two cylinders?Add a little propane to the carburetor to see if it smoothes out. You could also back off the rocker arms to a light tick on those cylinders to see if anything changes.
I recently put in new valve guide seals and checked the springs with a gauge. I also spun the valves on their seats by hand and ran them each in and out to check for any play in the guides. I pulled the valve covers to check for rocker movement and make sure no keepers had popped loose. The miss can be felt through out the rpm range.
College Man,
I don’t think it’s a jetting problem. It’s a 350 V8 with a 600cfm carb. It was working fine before. The only reason I got it rebuilt is because it was leaking a little bit of fuel out of the side. I have good spark too. It’s right around 40k. It’s also steady. I was thinking it may not be firing at the correct time though. I triple checked the plug wires and they are all going to the correct cylinders. There is a very small amount of side to side play in the distributor shaft inside the cap, but I’m getting strong steady spark.If you put the engine at tdc-#1 and timing marks align.
does the rotor point to #1 at the distributor?providing the miss is at 2 and 8 only I would recommend a leakdown test on those cyls ( perhaps a valve is not seating properly ) would also recommend pulling the push rods for those cyls and make sure they are not bent.
- You must be logged in to reply to this topic.