results of compression test GM305 1978(not running WOT)

[Haut Medoc]

Re: results of compression test GM305 1978(not running WOT)

Hi all
Well, today i went for a run to check some more things.I warmed up the engine
and i caught 3000RPM.I was traveling for two minutes until the engine started loosing power and then died.Now i thought is a good moment to see the results of a warm compression test.And to my surprise the starboard side head that counted 145-140
-133-127 with the engine cold, now counted perfect(all of them 150-155).What kind of SURPRISE is this????
How do you explain that?All the valves of that side had problem and now dissapeard? It doesn?t sound logical.(The port side when the engine was cold had the three cylinders 150-155 and the last aft 145 and with the warm test they remained the same).Do these numbers may be affected by a leak to the intake manifold which is the most possible reason for my troubles?I think now i have to take out that weer square bolt of the intake manifold and connect the vacuum gauge somethin that i should have done many months ago.(Why there is never enough time the first time.........Don S says!!!!)
Anyway i need your opinions
(Carb, fuel pump,distibutor exept rotor ,gas tank, filters,coil checked or replaced.Ater dying the engine starts up again if you wait for a while and then the next dying will come sooner.)

This is a fuel issue....
Did you try running an external tank with fresh fuel?....
I would.....

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

I' ve tried an external gas tank without succes.
What if due to an input manifold leak it can't suck the proper mixture in?
What about the compression results?
Forgot to mention that valves and advance timing adjusted.

 

[Don S]

Re: results of compression test GM305 1978(not running WOT)

One thing that causes compression to change like that is sitting for a long time (or even a short time if salt water) with water in the cylinders,
What is the condition and age of your manifolds and risers. How does the gasket surface look. Is it all there or just too thin to seal?
By all means do the vacuum check, but I don't see that as something that will change enough to stall.

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

new risers ,manifolds and gaskets.

 

[Don S]

Re: results of compression test GM305 1978(not running WOT)

Are you still running a points type ignition? And remind me, is this an OMC, Mercruiser, or VolvoPenta?
You've messed with this thing long enough, let's get it fixed.

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

no water in oil

 

[Don S]

Re: results of compression test GM305 1978(not running WOT)

no water in oil

It doesn't take water in the oil to have water in the cylinders after shutdown. Even a bad gasket surface on a NEW manifold or riser will cause the problem. There have been several posts on this lately.

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

ok,i don't dought but i said just after the shutdown i did the second compression test.i had no water in the cylinders.Something noticeable that i didn't mention is that at wot the sound is not clear and the secondary air choke (at the top of the carb and aft )never opens more than some mm.In addition when i turn it a little by the side rod a little clockwise(i think i close it) the sound gets more clear and the rpm go a little higher

 

[Don S]

Re: results of compression test GM305 1978(not running WOT)

Do you have a new carb, or did you rebuild it?

 

[jy118lfd]

Re: results of compression test GM305 1978(not running WOT)

I give you the results of the compression test.
1-150,3-150,5-150,7-145
2-147,4-145,6-135,8-125
According to the specs everything is ok but i don't know whether the gradual decrease of compression at the starboard side is a mater of coincidence or indicates something.What is your opinion?

Any chance you checked the 1 3 5 7 side then the 2 4 6 8?

Looks like the battery was not cranking the engine as fast on the latter side. That would also explain why the numbers came up after you ran it.
Anyway as stated before its not the problem and by the sound of it in your last post you may have a carb or cable problem

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

When i read the low compression 127 when the engine was cold ,#8 cylinder ,i returned to the port side and checked again the 150-155 cylinders .They remained the same.(I had my battery fully charged).The carb 4bbl Rochester was rebuilt by a specialist,i returned to him second time ,the problem the same and then he sent me another carb to check and the problem the same.I told him that at idle in gear the port side screw works and the engine dies when i adjust the idle mixture but not the same for the other screw for the port side and he replied that this means that a vacuum leek or a exhaust restriction may be the problem.

 

[Don S]

Re: results of compression test GM305 1978(not running WOT)

OMC, Volvo, or Mercruiser engine?

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

mcm 228 gm305 mercruiser

 

[Bondo]

Re: results of compression test GM305 1978(not running WOT)

I think now i have to take out that weer square bolt of the intake manifold and connect the vacuum gauge somethin that i should have done many months ago.(Why there is never enough time the first time.........Don S says!!!!)

Ayuh,..... Just so you Know,......

That Pipe Plug goes into the Water Jacket,...... Not the intake plenum......
You Can't Check the Vacuum from there......

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

a)So,how do i test the vacuum?
b)I think something is going on with the oil lubrication.I think this would explain why the one head gave me lower compressions when cold.But after 2 minutes at 2800rpm if the engine's loss of power and then stop was due to oil shouldn't i notice high oil pressure or high temp or the valves to hit ?Next step i intend to conect a pressure gauge directly to the engine and see what happens to the oil presure when the engine dies .Immediately after the dying i will pull the high tension of the coil and put it to the ground to see if there is spark because when it dies if i crank right then it doasn't start but if i wait for a while then it starts normally.
c)In which cases does the circuit braker is energized?Because i remember that a time that the engine died i turned the key on and the starter didnt crank(but this hapened only one time).I thaught that the pistons got stuck but again after a while the engine started up .
Thanks in advance for your opnions

 

[fendersfender]

Re: results of compression test GM305 1978(not running WOT)

i agree with earlier post...its fuel....sounds to me like its starving...which explains why you get more rpm with secondaries closed, with them open, it creates lean condition....re-check or replace all filters, check anti-syphon valve, how about fuel tank pick-up?, is tank vented correctly?, i've even seen bad hose from tank to pump, check to carb, float level?, sticking inletvalve....something is causing this thing to starve for fuel....should be no other reason than a fuel delivery issue to cause your problem

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

Now i'm almost sure it's a carb issue.I went today to check if that squared bolt at the top of the manifold was for connecting vacuum tester or to check water passage as Bond-o said.The engine at idle and when i was taking that bolt very slowly out i heard a psssssss and the engine took more than 200RPM .(i had already bloked the choke plate open).I' m sure when i adjust it myself following the service manual the engine will reserect.If the weather helps me in a couple of days i'll report you the results before starting accusing the carb "specialist".
Thanks for your replies

 

[Don S]

Re: results of compression test GM305 1978(not running WOT)

a)So,how do i test the vacuum?

Post a picture of your intake manifold, then WE can see what is going where.
Also post pictures of the manifold with and without the carb.

When you installed the carb, did you have a thick or thin gasket? Was it new or used?

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

Don S i'm afraid of harming the base gasket as after the rebuilt from the "specialist" i used the new one and i've already taken the carb out 2 more times,so last time i used sealant to be sure about good attachment.The gasket was the thick one with the wide opening(not the 4 holes) but this time i think i'll fix it without the leaking test.I've already put a weight of 95gr at the edge of the double air valve and it didn't open since the manual says 70-90gr.But if i fail certainly i'll take it apart and send pictures.

 

[Bondo]

Re: results of compression test GM305 1978(not running WOT)

Ayuh,...... I'm Wrong,.... That appears to be a Vacuum port.......