Compression test results- is this normal?

[Anonymous]

I just did a dry compression test on my 200 (did not have time to do a wet test). It has approximately 2000-2500 miles on it, and I was concerned something might be wrong since I can't get things tuned right wth the carb and timing.

Figures seem kind of low- 142, 147, 145, 134, 135, and 140 from the front of the engine back. By contrast, my worn 170 was 135, 140, 158, 155, 160, and 148 last time I checked.

The 170 in my old '63 comet that had been rebuilt but was well broken in checked in at 155, 160, 162, 155, 153, and 154. Does compression go up as the rings seal? I can see there are still hone marks on the cylinder walls- does this mean the engine is not fully "broken in" yet? Should I be concerned? I'm a bit worried that my lean condition (white-ish plug tips) caused my the continual carb problems I'm having could be doing harm as far as internal wear and/or sealing is concerned. Comments?

I'm still trying to work out the carb situation as I'm on my 3rd reman. carb and they are all worthless. I have no core to rebuild, so I'm about ready to take the jump and buy one from Pony Carbs.

Thanks,
Thor

 

[wsa111]

Thor, the compression is low, but what is your compression ratio & what camshaft are you running??
In my previous engine I had 175# cranking compression. That was with a 67 stock head milled .015 & the block decked .015. The camshaft was 194 degrees @ .050 on the intake. The engine ran strong on the lower end for the parts i had. I need to run a compression test on my new combination.
Give me what you have in the engine & i will be able to give you an intellegent answer. William

 

[Anonymous]

Got a cam in it? Lots of overlap can cause low cranking compression.I wouldn't be too concerned with those numbers, They are relatively close cylinder to cylinder, meaning you probably don't have a cylinder concern... If you still have a problem after a few reman carbs, maybe you oughta look at ignition timing? Maybe the cam timing is off a tooth? That might cause the low compression numbers...You say the thing is way lean? maybe a vacuum leak?

 

[mugsy]

I had numbers about 10-15 psi lower than that on a very old, tired engine. Found out that it was the new Felpro gasket that the PO installed with out milling the head or deck. Did you cc the head and verify the depth of the piston in the block? What type of head gasket are you using? My first thought is compression ratio before bad rings, leaky valves, etc. Check/verify/measure/calculate that first, then look for worn parts.

Bad rings would allow the pressure from combustion to pressurize the engine block. Gaskets, like the pan gasket, would start to leak. You could also see smoke or vapors come from the breather vent too.

tanx,
Mugsy

 

[addo]

I think it's just a CR issue. Thor had good steady vacuum at about 17" from what I recall - unless that's changed recently.

 

[Anonymous]

Unfortuneately, I know nothing about this engine other than it's a c-8 block and c-9 head, it had about 1000 miles on it when I bought it. I don't know anything about what cam or other parts were used, if the head was milled, etc. I pulled the pan and valve cover and everything looks fresh, and the cylinders were obviously at least honed.

There is a little oil seeping past the PCV and the oil breather cap, but not significant IMO. The rear main is leaking, but I think that's due to improper installation on my part. One thing I do notice it the exhaust is a bit "smelly" like my old 170. although it does not smoke. Not sure if all of these engines have bad smelling exhaust no matter that condition, or not, since they don't have modern emissions controlls.

You guys have me a bit worried now, as I'm about to order a $300 carb from Pony. Should I be holding off, or doing a wet test? I just put the old 170 ci 1100 back on and it runs like crap and uses a lot of gas too. I never really had a good baseline to measure from on this engine, so it's all a mystery. I never like to buy used stuff like this and had planned to build the engine myself, but that stupid POS volkswagen golf had to go.....$350 for the engine seemed like a good deal at the time :roll:

Does compression improve after break-in?

 

[Lazy JW]

I also suspect it is a compression ratio issue. I doubt that your numbers will go up very much from where they are now. Did you ever get your distributor to behave? (edit)You will be able to see the hone marks for a long time yet.
Joe

 

[Anonymous]

I've been waiting to find a good carb and then fix the distributor.

 

[Anonymous]

I went 150 miles on the last tank of gas (partly due to carb issues) and I'm worried I've got myself into a can of worms with this used engine. What should I do next if a wet test does not "say" anything? Mill the head? Pull off the timing cover to check if I'm off a gear (several people have suggested this in another thread)? Someone asked what my compression ratio is- how do I determine this?

The instructions for my compression testor say to use 30 wt oil for the wet test- can I use the 10-40 I already have?

I'm about to spend $300 on a carb, and plan to have my dist. recurved as well, but I'm concerned about this motor becoming a bottomless pit Suggestions, anyone?

 

[addo]

That oil will be fine. About a tablespoonful is plenty. You need to do the wet test with the motor at running temperature (thermost just opening), so lay out all your tools as the engine runs to warm up; pull all plugs at the same time too.

Are you sure the trans is not slipping? Also, is the fuel tank filling correctly? The 200 is not an econo-motor; it's unfair to expect miserly rates when you have said your driving is "spirited".

Remember a rebuilt carb should run well on a healthy motor. It's not dead money by any stretch of the mind. A distributor recurve looks feasible enough to DIY on a DS-II (easier than the Bosch!).

Do the wet test and see what the results are. We'll try to steer you from there.

 

[Anonymous]

I'd say my driving habits are average- I don't drive like there's an egg under the pedal, but not like a bat outta hell either I'm usually careful in my driving when I check mileage. I went about 200 miles on a tank of mixed driving just months ago with the worn 170. Trans shifts nice and hard. The 1100 I'm running now uses more gas than even the worst reman. Carter YF I tried. I'll get back to you after I do the test- thanks for the input

 

[Anonymous]

I think my compression guage has had it, because today's results totally confused me. I ran an initial dry test on each cylinder, and they were all lower than the last time- except the first one- 142, 136, 125, 115, 125, 125.

The wet test got me 142 on cyl. #1. First try on #2 got me 180, the 2nd try got me 157 : This seemed odd, as I had 152 and then 157 on #3, 160 and then 150 on #4, 160 and then 155 on #5, and 145 both tries on #6. I'm wondering if the 20/50 I used (it's what I have!) was to thick to work- I used 2 teaspoons as not quite all of it made it down the funnel. The instructions suggested 1 teaspoon of 30 wt. The hose then came off thge fitting on the guage, so I need to go get a clamp and try all this again. It may snow tonight so I may not make it back to the library tomorrow to post the results- I will when I can.

 

[mugsy]

The weight of the oil shouldn't matter that much. All that is being accomplished is to seal between the rings and the side wall. Just a couple of questions for you. Was the carb throttle in full open position? Did you see the engine crank over three times per cylinder? (you should have seen the compression gauge needle move three times) how was the rotational speed of the engine, did it crank over fairly constant? You want to try and reduce/eliminate all variables so the results are as constant as they can be.

tanx,
Mugsy

 

[Anonymous]

Mugsy,

I was concerned that I did not get enough oil in the cylinders as it was so thick not all of it ran down the funnel. Yeah, the engine turns over nice and fast, and I did have the throttle wide open. The only variable I can think of is that the engine temp guage cooled down a bit over the 20 min. or so I was doing this as it's so cold outside. But the numbers were still lower on the first cylinder, wich leads me to think it's the guage. I tried puting a hose clamp on the end of the rubber hose to secure it to the fitting, but couldn't screw the fitting into the spark plug hole after that. Look's like I may need a new guage

 

[Asa]

you could spray a fluid over it to see if it's actually the guage
soapy water should make little bubbles

 

[mugsy]

Sounds like your technique was good.

Hmmmm!