308 Leak down test - What does Good/ Bad look like

Hi,

I have searched the archives and not come up with any numbers and can't find anything in the workshop manual.

I have a 79 308gt4 which I am going to do a full top end rebuild on in coming months

Before I start I also want to check pressures and leak down numbers.

Does anyone have the correct suggested procedures ( hot or cold) and numbers or % I should expect, how long to do the test for etc.

Background

The engine has 70K on it, recently have some smoke on acceleration, (After a heavy engine cleaning!! - it is backfiring slightly through the range and not pulling properly- I am about to rebuild the carbs starting this wekend!, I assume this is a significant contributor to smoke(wrong fuel mix etc?) - engine was not smoking and runnign smooth before I tried to clean it!, smoke puffs really only on acceleration with a load and is not present at idle and reving through the ranges in neutral

Any comments/ suggestions appreciated.

Want to make sure everything is OK before I take the top endsoff with engine in situe!

 

This is from a FORZA "Shoptalk" -- has some leakdown numbers towards the end (I can confirm that at 15% loss it's time to fix/repair):

 

Sounds like water under the cap or laying against the plugs at the head and shorting the plug wires to the engine. Just saw this happen on a Diablo. Dry all the igintion components. This is sure to be the problem if you pressure washed it. My 2 cents

 

yes, clean out the plug wells before you go further.
Philip

 

The engine cleaning was done a few weeks ago, so the enigine is well and truely dry by now, still have the backfire issue (things were significantly worse afre the cleaning!) so the water is no longer the issue now.

Still thinks the acrbs got some crud into them during the clean down so i am going to strip and rebuild anyway. But more concerned that the smoke might be something more sinister.

 

Hey Tom
Couple of thoughts.

The accel pumps on the Webers deliver a pretty healthy dose of fuel, so you don't surprize me when you indicate that you get some smoke out the back on accel under load. Next question is what color is it? Grey/light blue is usually fuel enrichment, dark grey/black is usually burning oil.

How much oil is it using?

Second, the water can stay in the plug wells long, long time, and while it will vaporise with engine heat, will then re-liquidize if it can't escape, to repeat over again. Ask me how I know. The answer on this is twofold. First, to pull the plug caps, clean it off and blast the wells, plug tops and caps with compressed air. Second, is to put on the early style boots which are larger (looking more like a can) than the later "donut" style.

Third, on your expectations for leak down and compression, I've posted by own data before, but here it is again. All this with stock cams on a 77 GTB with 40,000 miles (when done). Compression (warmed engine) ranged from 185 low to 195 high. Note, hi-lo range is as important as absolute level. This was done 18 months ago, just before we put in the big cams. Leakdown (I believe with a warm engine, but I don't know) was done 30 months ago as part of a PPI. 7 cylinders 4 - 6%, one was 12%. Tech's view was "not bad, better than a lot of newer cars, not bad enough to do anything to the motor. 15% is a threshold" Tony's view in the Forza article posted by Steve is similar.

Last, I found rebuilding the Webers an interesting (therapeutic?) exercise and the sight of fresh, clean carbs on the intake manifolds will put a smile on your face. My advice is to do it thoroughly, use carb dip, do the needle valves [note sizes], new gaskets and o-rings throughout, base gaskets as needed and then you know they are good. Pierce can help you with all the parts. Get these first. While apart, the most critical thing to check for us shaft wear and the condition of the bearings. If there is any seepage around/ability to wiggle the shafts where they go through the carb body, you've got to address it. I am told more of a problem with the earlier cars (1960s) than the later ones due to the use of a bearing. When you put them back together and have them on the car (don't they look great now), before you leap in and start it, turn the ignition on and watch the throats carefully. It is easy for a float to get stuck and, if left unchecked, to cost you a hydra-locked engine. Also, make sure the throttle plates go vertical (and no more or less) when the throttle pedal is fully depressed.

Hope this helps
Philip

 

If you got water in the distributors or spark plug wells it will still be there a year from now. Those guys are giving you good advice, pull the dist. caps and plug wires/boots and blow them out. Heat and or time will not do it, they are well enough sealed that they will just develope their own weather system inside.

 

The water will stay there for a while. I had the same problem on my 77GTB and that was after hand washing...not power washing the car.

Water gets past the rubber spark plug seals. It will just hang around close to the spark plugs and as water or as steam it is enough to allow a short at that plug. The seal design was changed on the injected 308's, so it was mainly a problem on GT4's and carb 308.

Try drying everything out first...all plugs out.

Cheers
Simon

 

To add to the process of drying out everything, install new plugs just to be sure that none of them misfired enough to glaze over. If this happened, you'd certainly have erratic/ poor performance.