Power Problems
<HR style="COLOR: #666666" SIZE=1>Hey all finally got tired of using a garage to solve my problems with my engine. A quick history:
Last year had a new (rebuilt) 302 and C6 tranny put in my baby 72 sport short box. (modified)
about 3 months ago on a return trip my engine started to cut in & out and backfire thru the Carb. But when I pulled over to check it ran fine, later discovered that it would only happen when under a high load or going up a high grade hills. Also noted that my available power is gone. When I could get the needle to disappear to working hard to get it to 100. going up hills takes forever to get 80. I've gone to my rebuilder and talked, here's the work that he's done:
New fuel pump(twice), cleaned and rejet carb, new coil, cleaned & steamed
tank, Blown fuel lines, engine analized turn out good ( they say ).
What I've done suspect the electron points so swapped it with a good distridutor, still acted up. And checked my compression here are the numbers
#1 150 psi good spark plug light brown color
#2 150 Psi same
#3 130 Psi same 87 % of highest.
#4 150 Psi same
#5 145 Psi same 96 %
#6 125 Psi same 83 %
#7 125 Psi same 83 %
#8 105 Psi good spark plug with a very light brown color. 70 % of highest.
None leaked they all held the value listed.
as you can see my compression varies alot, 75 % is 112.5 psi of my max reading. So looks like #8 does not good with #3,6 and 7 following.
I suspect the Camshaft myself but he seems not to agree.
So the question is what can I do now ?
Cheers and nice site from Moose JawSaskatchewan

 

What type of distributor? Have you checked for blockage in the exhaust?

 

Thanks for dropping in, too bad about your problem bcause it can be very flusterating and time consuming. Sounds like a fuel starvation that shows up under a heavier load even with all the work done to the lines and tank, something like a cracked pickup tube in the tank. If it starts, idles and other wise runs fine that should rule out a vacuum leak. Are there any more symptoms or only a problem when under heavier loads?

 

No other real symptom just a slight engine shutter around 55 mph that carries on. I've talked to my garage and he's going to at no charge put in an electric fuel pump because he too thinks it's fuel starvation. Others thought that it could be a valve but my compression check more or less ruled that out. The distributor is the orginal with a electronic module inside, the other that I tried against it was another orginal that had points. I never checked the exhaust yet but my try that one out. Tks all.

 

Check the modulator valve. Maybe it is malfunctioning or the vacuum line is plugged or soft and collapses under load.
My truck recently was bogging down and found tranny fluid in it. and the line. Changed hoses and valve and it worked fine.
Last wed. my truck acted up and was barely running, today I changed carbs and it runs great again. I heard of dirt getting into areas in the carb that people overlook when working on them and they still stay plugged.
Having your compression all over the place says to me the engine is maybe needing some TLC with one at 100 and others at 150 that is quite q difference. 10lbs is not bad.
Goodluck with your engine.

 

Looking at the compression results and seeing that #8 is the lowest, makes me wonder if you should try a leak down test to see if the compression loss is by the rings or either valve. One problem with engines that have the firing order with two adjacent cylinders sequentially firing after each other, is the potential for crossfire -- voltage induced into the non firing cylinder by the currently firing cylinder. Your 302 probably fires #7 and then #8. I've seen many times that if the spark plug wires are run side by side or too close together that 7 and 8 crossfire will happen. This induced voltage fires the spark plug on #8 when #7 fires, which can break rings or take the top of the piston off due to way too far advanced spark. Check your plug wire routing, and even check the firing order, making sure that numbers 7 and 8 are not mixed up, or touching each other. Running a wire loom cover over one of the spark plug wires works, but just routing them away from each other is good too. I learned the hard way and had to replace a motor one time, due to broken rings on #8 cylinder. This could explain lower compression readings on that cylinder.

I've seen these wires mixed up before on a chevy 350 (same firing order just different way of numbering the cylinders) and cause a mid-range shudder and due to its large cam, was harder to detect at an idle. I've also seen intake valves being too tight and when we backed them off on another chevy 350 engine, the backfiring through the carb quit -- it would only pop back when you had it right to the floor or would work it. I know that 302's do not have adjustable valve train, but a leak down test will show any potentially leaking valves.

Some things that cause backfiring through the carb under load is too advanced timing, lean fuel mixture, or even an intake valve not sealing properly or not closing all the way. The shudder you notice makes me think that you should follow the firing order and the wire routing for sure.

Good luck!

 

Tks silverstreak I'll check into the firing order.