manifold filling with gas/low compression

[starsnstripers]

Well i was trying to start my 1968 250 L6 165hp mercruiser and it puffed and farted a few times then nothing. So i rechecked timing/fuelpump operation/points/fire= all good. Then before i went to start it again i looked in the manifold where the riser attaches(square exh. outlet) and it had about a cup of gas in it (glad it didn't fire whoa) so i took the manifold off and poured some gas down into the carb wondering if the manifold was cracked. No gas went into the exhest area. So my ? is could gass run down between the head and manifold and leak into the exhaust if the manifold gasket is old used? the motor is at a slight angle with the flywheel end lower than the front about 3" difference but i'll level it before i try it again i didnt think that would matter ? not sure. I did a compression test, this moter has sat for maybe two years also. The compression was 90 psi in all cyl's. it calls for 140 psi. do you think its low from sitting and maybe if i get it running things might reseat or? opinions? thanks

 

[Silvertip]

Re: manifold filling with gas/low compression

There should be no raw fuel anywhere except inside the carb. If the engine sat for two years and the fuel wasn't treated the fuel you have is probably worthless and the entire fuel system is varnished and gunked up including the carb. A stuck needle and seat or a float that has sunk would allow the carb to run over and pour into the intake. That condition can only occur however during cranking as the fuel tank is lower than the carb so a siphon action is not possible.

 

[Don S]

Re: manifold filling with gas/low compression

You might see a lot of gas in the manifold if you keep pumping the throttle each time you try to start it. You may also have the engine flooded.
Have you pulled the spark plugs to see if they are wet with gas? If they are wet, pull the coil wire out of the cap and ground the wire. Then move the trottle and choke to Wide open, and crank the engine over (with the plugs out) and let the gas out of the cylinders.
Dry the plugs and put them back in and hook up the coil wire. Don't touch the throttle and try to start it. If it starts, pull the throttle back so the engine doesn't over-rev, but keep it up till it clears out.

If there is a lot of gas in the cylinders, it may wash all the oil off the rings and make it show low compression.

 

[starsnstripers]

Re: manifold filling with gas/low compression

I have the motor on a motor stand in my shop with a clean gas/tank. The plugs didn't seem fuel soaked to me. when trying to start it it seemed like it wasn't getting enough fuel. Also when i started cranking it over i looked in the carb throat and moved the carb throttle and no gas was spraying, like the accelerator pump wasn't working. I took the top off the carb and wiped the bowl out and blew it out with air it wasn't to bad. I ran a tool around the accelerator rubber thinking it may be dry and flared it out a little. Put it back together and then noticed all the fuel in the exhaust. maybe carb prob. I dont like the idea of looking down into the carb when cranking it over. when i did the compession test no fuel mist came out of the cylinders. It's like the gas is going straight into the exhaust without going into the cylinders. Thats why i wodered if it's leaking by the gasket and where the motor was at an angle it runs down the valley between the head and manifold and leaks into the exhaust?

 

[JustJason]

Re: manifold filling with gas/low compression

hmm... i wouldn't start that on a regular engine stand... on a cradle type it's okay but not on a "rear" catching stand...

 

[starsnstripers]

Re: manifold filling with gas/low compression

well im going to block the low end to level the motor. I borrowed it and someone added some larger wheels on one end. any comment on the problem at hand? do you think the angle has something to do with it?

 

[ziggy]

Re: manifold filling with gas/low compression

sorry, i got more questions than answers for ya bg. you have the engine ft. tilted down? that's beleiveable when it's on the stand...
you have a cup of fuel in the manifold where the riser attaches at? how can that be if the ft. of the engine is tilted down?
you took the top of the carb off and wiped the bowl clean? was there any fuel in the bowl before ya wiped it clean and air dried it?
your useing an old manifold gasket? most gaskets are one time use.

do you know what psi the fuel pump is puting out? and have ya replaced both fuel filters.... got any fuel in the sight tube back up to the carb from the fuel pump?
how do you know ya got a spark?
did you do dwell, then timing?

i'm haveing trouble with the idea of fuel in the exhust being visable at the riser area. specially with it tilted down in the ft. yer sure it was fuel? got water hooked up to it?

 

[a70eliminator]

Re: manifold filling with gas/low compression

sorry, i got more questions than answers for ya bg. you have the engine ft. tilted down? that's beleiveable when it's on the stand...
you have a cup of fuel in the manifold where the riser attaches at? how can that be if the ft. of the engine is tilted down?
you took the top of the carb off and wiped the bowl clean? was there any fuel in the bowl before ya wiped it clean and air dried it?
your useing an old manifold gasket? most gaskets are one time use.

do you know what psi the fuel pump is puting out? and have ya replaced both fuel filters.... got any fuel in the sight tube back up to the carb from the fuel pump?
how do you know ya got a spark?
did you do dwell, then timing?

i'm haveing trouble with the idea of fuel in the exhust being visable at the riser area. specially with it tilted down in the ft. yer sure it was fuel? got water hooked up to it?

Yea I was thinking the same thing, could be that it's flooding so bad that cylider pressure is blowing it back there, no? Reguardless ther is no way for fuel to get to a head anywhere but through the carb, you don't have the fuel line hooked up to the pcv port I hope?

 

[starsnstripers]

Re: manifold filling with gas/low compression

I tried to be clear on every thing sorry. My post reads" the flywheel end of the motor is lower than the front of the motor" meaning it was tipped back. but never mind that i'm going to level it. And yes i'm using a used exhaust gasket for now just to start motor. Dwell, points, timing,and good spark to plugs. Carb is getting fuel, the bowl had fuel when I wiped and blew it out. May be the carb. My ?s are 1) Would the angle i had the motor at 3"s between frnt. and back of eng. make the gass run back toward the low end and possibly leak through the exhaust gasket into the manifold. 2) Does the 90 lbs compression affect fuel suction into the cylinders, when it should be 140 psi. I'm assuming the fuel that ended up in the exhaust never went through the cylinders to get there. Thats the way it appears, the plus weren't gas soaked and when i took them out and did the compression test no gas vapor or anything blew out of the cylinders. Thats alot of fuel to be in the exhaust manifold with out the cylinders being flooded out. I'm going to replace plugs anyway. Then put a carb kit into it, get a new exhaust gasket level motor and give it another shot. Didn't notice fuel in oil however i'll change that again too. Oh well, trial and error. thanks for trying guys

 

[Bondo]

Re: manifold filling with gas/low compression

1) Would the angle i had the motor at 3"s between frnt. and back of eng. make the gass run back toward the low end and possibly leak through the exhaust gasket into the manifold.

Nope.......

2) Does the 90 lbs compression affect fuel suction into the cylinders, when it should be 140 psi.

Ayuh,......
If the motor Can't Blow,.... It Can't Suck either.......

I'm assuming the fuel that ended up in the exhaust never went through the cylinders to get there.

Unless something is Broken or Severely out of Wack,......
That's Impossible.......

 

[Windykid]

Re: manifold filling with gas/low compression

You might try a wet comression test. That will tell you if its the rings or valves.

 

[Windykid]

Re: manifold filling with gas/low compression

Also have you gotten anymore fuel in the manifold?

 

[starsnstripers]

Re: manifold filling with gas/low compression

Bondo thats good haha ! yeah the thing is "What"? Could be so out of wack or broke to cause this? Hmmm? whats a wet compression test??? I only know of sticking a compression tester in the plug hole and crank? I havent got the plugs, gasket, oil and carb kit yet. I'll let you all know how it turns out "if i survive" J.K. Thanks again

 

[starsnstripers]

Re: manifold filling with gas/low compression

OH, Bondo does that ayha mean you think that 90 psi is to low to try starting it and may be the issue?

 

[Bondo]

Re: manifold filling with gas/low compression

OH, Bondo does that ayha mean you think that 90 psi is to low to try starting it and may be the issue?

Ayuh,....... That's Correct........

Btw,......
A "Wet" compression test is doing the Same thing,...
After you add a teaspoon of Oil to the cylinder......

 

[Windykid]

Re: manifold filling with gas/low compression

You add a tea spoon of oil to a cylinder, if the compression reading goes up, then the rings are your problem. If it stays the same then your valves are most likely unless there is a big hole in the cylinder.

 

[achris]

Re: manifold filling with gas/low compression

When you did the compression test, did you have the throttle open? If you didn't, that would easily account for the 'low' readings. Also if the engine has sat for 2 years, then I would want to have it running for a while before I gave credence to any compression readings, wet or dry. Engine angle won't affect the fuel flow. The only way for fuel to get into the exhaust without going through the cylinders first is if there is a big hole in the intake manifold through to the exhaust. Using the old manifold gasket (bad idea anyway) would only allow the engine to suck air into the cylinders (intake leak) and/or exhaust out (exhaust leak). If you have had the distributor removed, then check that you don't have the timing 180 degrees out. Check that even if the dissy hasn't been out. And check the firing order too.

Chris.................

 

[Robj]

Re: manifold filling with gas/low compression

When I got my boat it had been sitting for awhile. I did a compression test, and I had two low cylinders, 90 and 120 psi, the other two were at 155. I knew that if I posted those numbers on this site, some would have suggested a rebuild. I thought that maybe it sitting for a few years had something to do with it, so I decided to run it a few times and redo the compression test. After running the boat 4 times, I redid the compression test and this time all 4 cylinders were at 155 to 160 psi. Your compression readings sound odd because all 6 are at 90 psi. Usually with a tired engine you have a couple of weaker holes and there is more variation between the numbers. I would redo your compression test. I believe Don S has a post that contains the procedure on how to do a compression test, you can do a search for it.

Have a great day,

Rob.

 

[Lakester]

Re: manifold filling with gas/low compression

hello:

----------

OH, Bondo does that ayha mean you think that 90 psi is to low to try starting it and may be the issue?

----------

90 is definitely low. personally, i think it would run at 90. but i have never tired it. and that would require everything else to be on the $$! i would rather see #s like 160 on a fresh engine with new rings or about 135-140 on a good condition used engine.

if i may ask, why are u starting it on an engine stand? or trying? never have i started an engine on an engine stand! dangerous, unless designed just for that purpose. i have always started engines in the chassis and with all items for it installed, exh, belts, alt, etc. temp wires, all...

usually an engine on a stand has no silenced exh. so its is loud. TOO loud to listen to it "talking". can hear its ails... etc.

my bet is u have things off. if an engine is timed on the $$ and it has gas, and its mechanicals are good... it will fire immediately. they have been doing it for decdes, and they do it by the millions, daily.

you may have to hand choke it, but the only gas i would give it is off the accelerator pump. if it wont prime off it, with a bit of help from manual choke, the ignition or the cam timing, or valve setting is not correct.

but, that aisde... i am ROOTing for you.

regards,

lakester

 

[Lakester]

Re: manifold filling with gas/low compression

starsnstripes sed:

"So i rechecked timing/fuelpump operation/points/fire= all good."

would u mind telling us how u did this?

a) timing?

b) fuel pump?

c) points/fire

regards,

lakester