Major problems - how do you do a compression test correctly?

[twoseater]

Hi - Sorry for what will be a long post but I've really been struggling on getting my '80 running again. I have problems getting it to run after replacing Carb, intake, Distributor, spark plugs and spark plug wires. I have some back firing through the carb at higher RPM (Idles fine) and I've seen gas spurt out the carb as I turn the engine over but don't start it. I've also removed all the air stuff as part of this project (smog pump etc). It runs horrible under load (undriveable). Sooooo I bit off more than I can chew and need some help.

Spark plugs are black - appears to be carbon but I also see traces of oil so I can't be sure if its oil based or carbon. I have timed it over and over as well as others more knowledgable. I also ensured TDC on #1 when I put distributor in. Prior to all the work I did the old spark plugs were a healthy tan.

Now the question: I did a "dry" compression test. I removed all plugs, disconnected the distributor cap, threaded the compression tester in the spark plug hole, matted the accelerator pedal and turn the key for about 5-10 seconds. Is that the correct way or do I turn the key to "bump" the engine and have it turn 1 rev (repeat for 2-3 times). I ended up just leting it turn over for 5 to 10 seconds. Longer I let it go the higher it would get - up to 175. With this method all cylanders measured between 160-175.

I'm optomistic that I don't have more serious issues but just don't know. If I did the compression test correctly, does this rule out valve, piston, head gasket issues?

Could the intake have leak on the inside cause this behavior?

Specifics on new items:
Intake:Edlebrock Performer
Carb: Edlebrock Thunder series
Dist: Accel with high voltage accel coil
Spark plug - Platnum plugs gapped to .045
Spark Plug wires were 8.5mm - crimp your own ends

Thanks!

 

[Jack]

First guess is that the new carb is jetted/adjusted way too rich. The compression numbers look OK. If you continue driving it while overly rich, you'll gas-wash the cylinders and ruin the rings ... compression will fall off. Suggest you also re-check the sp wires and verify that you don't have a couple out of firing order sequence.
JACK:gap

 

[geekinavette]

I agree with Jack. The compression numbers do look ok...it should only take a few revolutions to reach max compression (keep in mind that you only get one compression stroke for every two revolutions). In addition to what Jack mentioned I would also check the lifter adjustments (hydraulic I assume?). If you have a little too much preload you can still measure good compression but get some backfiring and loss of vacuum.

Bill

 

[craig32]

I agree also, check the plugs and wires. I had a 327 that was backfiring and shooting gas out the carbs. I thought I needed to have them adjusted, but turns out 3 plugs were fouled out. Runs great with all 8 now. Easy check first before you tear everything down. Good luck.
Craig

 

[mike weyman]

you do not need to pump the gas for a compression test.if the intake manifold is leaking you will get a bad condition. let the motor idle while you hold an "on" but not lit propane torch, trace around the outside of intake and anywhere they may be a vaccum leak. if it sucks the gas into the intake thru a leak, it will change the idle slightly.hold the torch over the running carb first to determine what the change will sound like.

If this is not it, and your timing is good(what's it set at?)i really believe the problem is the carb.

 

[goertz]

you do not need to pump the gas for a compression test.if the intake manifold is leaking you will get a bad condition. let the motor idle while you hold an "on" but not lit propane torch, trace around the outside of intake and anywhere they may be a vaccum leak. if it sucks the gas into the intake thru a leak, it will change the idle slightly.hold the torch over the running carb first to determine what the change will sound like.

If this is not it, and your timing is good(what's it set at?)i really believe the problem is the carb.

you don't need to "pump" the gas, but you do need to have the throttle wide open for an accurate reading. otherwise you're not getting a fraction of the air it needs to compress. sounds like a carb problem though. It's dumping fuel into the motor.

 

[GerryLP]

Hi - Sorry for what will be a long post but I've really been struggling on getting my '80 running again. I have problems getting it to run after replacing Carb, intake, Distributor, spark plugs and spark plug wires. I have some back firing through the carb at higher RPM (Idles fine) and I've seen gas spurt out the carb as I turn the engine over but don't start it. I've also removed all the air stuff as part of this project (smog pump etc). It runs horrible under load (undriveable). Sooooo I bit off more than I can chew and need some help.

Spark plugs are black - appears to be carbon but I also see traces of oil so I can't be sure if its oil based or carbon. I have timed it over and over as well as others more knowledgable. I also ensured TDC on #1 when I put distributor in. Prior to all the work I did the old spark plugs were a healthy tan.

Now the question: I did a "dry" compression test. I removed all plugs, disconnected the distributor cap, threaded the compression tester in the spark plug hole, matted the accelerator pedal and turn the key for about 5-10 seconds. Is that the correct way or do I turn the key to "bump" the engine and have it turn 1 rev (repeat for 2-3 times). I ended up just leting it turn over for 5 to 10 seconds. Longer I let it go the higher it would get - up to 175. With this method all cylanders measured between 160-175.

I'm optomistic that I don't have more serious issues but just don't know. If I did the compression test correctly, does this rule out valve, piston, head gasket issues?

Could the intake have leak on the inside cause this behavior?

Specifics on new items:
Intake:Edlebrock Performer
Carb: Edlebrock Thunder series
Dist: Accel with high voltage accel coil
Spark plug - Platnum plugs gapped to .045
Spark Plug wires were 8.5mm - crimp your own ends

Thanks!

Yes! Once the spark plugs are fouled, you'll have to clean them or use new ones. If the engine idles, use WD-40 around carburator mating surfaces and intake mating surfaces to check for leaks. If it is running rough and momentarily stops being rough then you have a leak at that spot. The same goes if the idle RPM climbs while doing this check.

Backfiring through out the throat of the carburator is almost always a lean condition. A leak downstream from the carburator will cause a lean condition (i.e. carburator mounting flange or intake leak).

not having the correct firing order causes problems, so check they're correct.

I also had a bad intake leak, and I had to remove the cylinder heads, disassemble them, and clean the valves and chambers. So don't let an intake leak continue for for time, or you'll have problems down the road.

GerryLP:cool

 

[twoseater]

Thanks guys - the spark plug change and wires were a result of trying to fix the back fire. So, unless I have two bad sets I am going to rule that out.

Firing order checked and rechecked - good thought. I also started it for 10 seconds and felt each exhaust manifold - they all felt about the same temp (pretty warm) so I believe they are all firing.

Lean/Rich - got a bit a differing opionions on which causes a carb backfire. Since lean implied a leak I will do the the torch or carb cleaner test first. If nothing shows then I will look at it as if it is a Rich condition.

I have never heard of the torch test - I like it as I don't have to worry about spraying a flammible liquid at the engine.

Car is def off the road until I get this 100% resolved. Missing a nice summer behind the wheel though! Prob is the winters here in Vermont are too cold for any garage time in which to have made this a winter project!

 

[twoseater]

Mike - sorry forgot to respond to your timing question - I started at 8 deg BTDC and have adjusted it higher (12-14). Still had backfires at all settings.

 

[wishuwerehere82]

So with all the info. here in mind:

Backfires at higher RPM's.
New intake, carb, distributer, plugs, wires.
Gas spurts up though the carb.
Timing checked and reset. Still backfires.
Compression tests OK.

Who here besides me thinks that the valve backlash needs to be set or a new timing chain is in order?
I'm leaning towards a new timing chain and gears.

 

[RodsnRides]

I'm with wishyouwerehere82. That's the logical next step. All your posts are good (vacuum leak, etc.)-you guys are on the right track.

Another good point is the carb-sounds like it IS jetted rich. They usually jet them for sea level-what's your elevation?

I have also seen situations where junk gets into the needle/seat area and can cause flooding as well. Packing debris, stuff in the line, etc.

To check for timing chain slop, grab the front pulley/balancer area with both hands and roll it each way. You'll be able to feel the slack. Any much more than about 6 degrees I consider questionable. Good luck.

Rick

 

[72VETTE454]

If it ran fine before the new parts were installed, then it is most likely something that you did or didn't do. Doubtfull that a timing chain and gear set would suddenly go bad, but not impossible. I agree with the carb senario, an excessively rich condition will cause backfires. I would throw the original carb back on, and see what happens. Good luck.

 

[twoseater]

Update: Checked for leaks with carb cleaner - no acceleration so OK there.

Old carb won't fit on the new intake without an adapter plate. The old carb was extremely dirty and the car did not run very well. However, it ran much better then now and the plugs were clean not black like I'm seeing now.

I have two thoughts now:

Carb - looking in during acceleration I see drops of gas and for lack of a better term - splashing. What I'm seeing just doesn't look right so I am researching on how to tune this "works out of the box" carb. Part of why I chose the edelbrock is because I didn't want to mess with it.

Second and more important observation: I put a vacuum gage on the dist vacuum advance I got a solid 20hg. When placed on the intake I got 20hg but wasnt solid AND it would drop as I accelerated. I've read this could imply clogged cats etc. I'm going to retest the vacuum again to verify the results. (I was doing it when my wife kindly reminded me that it was 11:00pm and everyone was trying to sleep)!

Once again - thanks for all the thoughts. I will also check timing chain as it seems pretty straight forward.

 

[Jack]

I put a vacuum gage on the dist vacuum advance I got a solid 20hg. When placed on the intake I got 20hg but wasnt solid AND it would drop as I accelerated. I've read this could imply clogged cats etc.

When you accelerate, manifold vac reading should drop. If you have a plain jane performer (P/N 2101) it should also have the OE Qjet bolt pattern flange ... just my 2 cents, but I find a good condition Qjet is as good a carb as there is for most street chevy and it'll flow about 750cfm.
JACK:gap

 

[brusso]

Carb - looking in during acceleration I see drops of gas and for lack of a better term - splashing. What I'm seeing just doesn't look right so I am researching on how to tune this "works out of the box" carb. Part of why I chose the edelbrock is because I didn't want to mess with it.

I don't believe that you should see fuel spashing at any time when looking down the throat of carb. Except while moving the throtle you should see nice streams of fuel being sprayed and those streams should only occur while you are advancing the throtle. You may have a float adjustment issue or bad/missaligned float needle and seat?

 

[twoseater]

Stomach bug ripped through the house so I haven't had much engine time....I believe the prob is def in the carb. The Primary Venturi boosters basically drip gas into the carb. When I open the throttle, the air valves open and dumps the pooled up gas into the intake. Anyone seen this or have a resolution? I plan on calling Edelbrock tomorrow to see if they have any ideas.

I also need to do some fuel line work to get the old carb back on but will try what 72vette454 and put the old one back on. I thought the bolt pattern was different but Jack set me straight.

 

[RodsnRides]

It sounds like you either have too high a float level or there is some sediment in the carb, per my earlier posting. Possibly rich jetting.

I suspect you have some object between the needle and seat, which doesn't allow the needle to seat properly, hence the carb is overfilling with fuel.

Suggest seeking a warranty replacement, and/or try putting your old carb back on and see what happens. Summit Racing has a Q-jet pump to carb fuel line kit.
Try this:
http://store.summitracing.com/default.asp?Ntt=Quadrajet+fuel+line&Ntk=KeywordSearch&DDS=1&searchinresults=false&N=0&target=egnsearch.asp

Good luck.

Rick

 

[twoseater]

Update - still no go! I am going to put on the old carb, however, the bolt pattern is indeed different. Edelbrock sells an adapter plate so I'll see if I can pick that up. I also replaced the fuel pump in case that was bad.

Called edelbrock support and they indicated that I should see drops of gas that is not atomized. However, that still just doesn't seem right.

Saga continues!

 

[black_81_vette]

I think the edelbrock boys are pulling your leg!! The only time ive ever seen a carb do that is when someone is running a huge duration cam and it screws with the vacume signal the carb gets (but thats a whole other can o worms). If its puddleing up in the intake its way to damn rich! if its the AFB style carb it could have been assembled incorrectly IE bent main jet needle ect hell ive seen one with a missing jet! But the fact its dribbling fuel out the venturi booster area leads me to belive the float drop is not correct (not closing off fuel flow completely) If im not mistaken the AFB has two floats and there is a alighnment you have to watch and a float drop and level setting that needs checked. All in all not a bad carb but it could use some updates.Personally id take it back go order a rebult Q-Jet and drive the wheels of it!

 

[twoseater]

Thanks Black 81 Vette - I did adjust the floats as its a known issue with a manufacture defect. The conversation with Edelbrock was blame everything else, when I explained all that was done, he suggested like many others, that I put the old carb back on. Unfortunately, it won't fit the new intake. I bought the wrong adapter plate today (I can now attach a AVS carb to the original intake) I am going back to auto store to see if they have the reverse. I'm just about out of ideas so this will be my last go at it. I think the AVS is a good carb - I just can't get it working. I may pick up a Holly and try that - first I have to see if Jegs will take the AVS carb back.

The AVS does have two floats and they have been adjusted to the recommended spec.

I chose the AVS for the bolt on an go reputation along with the power matching packages. (Carb/intake/cam etc) It will be a few years before I try the cam after this disaster!