Quote:
Originally Posted by martyminnesma
ok new update on this very frustrating on-going issue. let me review of what goes on with my truck. at any temperature, but worse when it's cold the truck falls on it's face when i get too much into the throttle. from a dead stop, if i try to punch it, instead of roasting the tires like it used to it just fails. it pretty much completely stalls and then backfires out the carb. if im cruising in a low rpm and try to give it some throttle it will miss and backfire out the carb and just stumble in general, until it kicks down a gear and then it rockets to life. also gas mileage has gone to ****. the 7 and 8 cylinders seem to have a much much heavier carbon build up than the rest as well. those spark plugs were pretty much solid black when i pulled them out, all other six were clean, maybe a little rich, but still an extreme difference from 7 and 8. also when i pulled the heads off a while back i noticed the carbon build up was extreme on 7 and 8, and then gradually/progressively got better as you looked at the cylinders closer to the front 1 and 2.
things I've done to diagnose this;
- re-did the grounding on the engine block with a heavy gauge wire, cleaned off all connection points and made sure they were tight
- re-adjusted the valves while running, they were all fine to begin with
- re-torqued the heads, all was fine
- installed a new MSD distributor. (this made a big improvement. better curve, higher output etc. definitely ran better but main issue was still there)
- new spark plug wires (also a big help as one had burnt open, but issue is still there)
most recently this past week i finally gave in and brought the truck to a dyno shop. he had air fuel ratio sensors on both exhausts, vacuum gauge, fuel pressure gauge etc.
the first thing we noticed was that my air fuel mixture was incredibly rich, especially under load and low rpm's. now part of this I know is due to the terrible 700r4 that i have. normally a great transmission, but i let the shop convince me to get a hydraulic lock up. which is awful. it locks up every gear and makes the transmission extremely slow to decide when to switch gears either up or down. but with some testing we figured the spark plug gap was too large and as the edelbrock carb soaked the spark plugs they would begin to miss. sounded logical to me. we decided the edelbrock carb was junk and I would spring for a holly. so he has my truck for a few days and i get back there, 1900 bucks later. it definitely ran better. the holly has much better response and the air fuel ratio was where it needed to be. he said he couldn't get it to miss on the dyno, but its also hard to hear in there because the dyno is so loud. oh, he also found that the spark plugs i had didn't have a long enough reach. so he replaced them with the proper NGK plugs and gapped them properly.
so i drive it home and it seemed to run good, i thought i still heard an issue but i wasn't sure. by the end of day two of driving it it seems im back to square one. i can hardly give more than a quarter throttle when cruising without it stumbling and backfiring. this is so god damn frustrating.
any ideas what i should check? i'm at my wits end here, ready to drive it off a cliff.
my best guess is the intake valves on 7 and 8 aren't sealing properly because of carbon build up. anyone have any ideas why the rear cylinders would get more fuel to begin with? why would 7 and 8 be rich but 1 and 2 be lean?
anyone know where i can get an attachment that will allow me to hook up my compressor to the cylinder head to test the valve seats? any know of a way to clean them without taking the engine apart again?
thanks for any help you send my way. much appreciated.
|
You spent $1,900 at the dyno shop and still have the same/similar problem?? The dyno guy should have been able to tell you if you have the wrong timing curve in the distributor. You said it was a new MSD distributor? The paperwork that came with the distributor should tell you what the current timing curve is, giving you a baseline so you know what you need to do to correct the timing issue, if in fact timing is your issue.
As has been said, there are a lot of different points of view when it comes to vacuum advance. In my case, my vacuum advance is not used. The GM engineers said not to hook it up and I get all the total advance I need. My motor runs great. I get way too much total advance when I use the vacuum advance.