74' 250 inline 6 ... help!

[ampcarfan]

Idles good, won't take throttle. Pops intermittently through the carb (fuel not flames), can get it to rev in nuetral by cheating the throttle with quick/short pumps.

Rebuilt the carb, (No change) but the accel pump would hang on the spring and get dislodged. installed the old accel pump... squirts a stream (not spray) if you throttle it quickly.

New points and cond. Dwell is at 32deg and doesn't vary at idle.

Timing is 6btc and doesn't vary at idle.

Installed a new rotor button, cap, wires and plugs .035

Installed a new coil (wasn't sure how to check this, but it has 8 volts if I put the neg lead of the volt tester on the neg side of the coil and pos side of the tester to pos side of the batt)

tried removing the tach lead from the coil, no change.

Checked the carb and base for vacuum leaks.

Disconnected the fuel line and stuck it in a water bottle and turned it over... filled it half way in 15 secs. (Strong spurts)

Tonight I checked and double checked compression
1) 160
2) 160
3) 170
4) 165
5) 170
6) 160

Plugs were black and sooty, but not wet.
I'm totally stumped guys, any input would be greatly appreciated!!!

 

[Howard Sterndrive]

Re: 74' 250 inline 6 ... help!

triple check the firing order

to check power to the coil, put the red voltmeter lead on the positive coil terminal, and the black on a GOOD GROUND.

 

[ampcarfan]

Re: 74' 250 inline 6 ... help!

Thanks HS. What should the values be when checking the coil voltage? 12v not running (key on), 6v running? This deal has me really baffled. One thing I did notice when checking that the dist advance springs were free and working is that the dist shaft has some play to it. If the points are on the flat part of the cam and I wiggle the shaft pretty firmly, I can see the points open slightly... could this be causing the popping and lack of power???

 

[ziggy]

Re: 74' 250 inline 6 ... help!

understanding i'm not the pro....

Rebuilt the carb, (No change) but the accel pump would hang on the spring and get dislodged. installed the old accel pump

that don't sound like a totally successful carb. rebuild. + did you set all specs to the mercruiser specs in the s/m? from what i understand. some carbs. just aren't rebuildable.

One thing I did notice when checking that the dist advance springs were free and working is that the dist shaft has some play to it. If the points are on the flat part of the cam and I wiggle the shaft pretty firmly, I can see the points open slightly

ya could take the dist. to a shop and have them bench test it. dwell variation would affect timing.
have ya gone through the trouble shooting guide in s/m #2. pp 10-15. pretty long list of possibilities.

 

[ampcarfan]

Re: 74' 250 inline 6 ... help!

understanding i'm not the pro.... that don't sound like a totally successful carb. rebuild. + did you set all specs to the mercruiser specs in the s/m? from what i understand. some carbs. just aren't rebuildable.

All set to specs. Agreed that putting the old accel pump back in doesn't constitute a complete successful rebuild. I think my next step is to try to locate "just" the pump and see if that cures the problem.

ya could take the dist. to a shop and have them bench test it. dwell variation would affect timing.
have ya gone through the trouble shooting guide in s/m #2. pp 10-15. pretty long list of possibilities.

Pulling the distributor and having it checked and possibly rebuilt seems to be the only option as there are non available... Have done many of the checks mentioned in the s/m, but after reveiwing it again, Accel pump is the first thing they mention. gonna start there.

 

[captk]

Re: 74' 250 inline 6 ... help!

does any one still bech test and rebuild distributors anymore.The place i used here in Cleveland stopped doing it almost 20 years ago. be nice to find a good shop.

 

[cedarjunki]

Re: 74' 250 inline 6 ... help!

having it pop* i would assume you crossed a couple plug wires...

 

[Howard Sterndrive]

Re: 74' 250 inline 6 ... help!

coil + should have 12V cranking, 8 to 10 with key on.
referencing good ground or batt neg.

if you saw 32 deg dwell thats good.
and if it doesn't change more than 1 or 2 degrees when you rev it up. I doubt distributor wear is a serious issue.

what firing order did you use?

 

[Silvertip]

Re: 74' 250 inline 6 ... help!

If you can move the shaft enough to open and close the points it very well is at least part of the problem. With the engine running and a dwell meter attached, blip the throttle quickly. A worn distributor bushing or shaft should cause the dwell angle to change. I also feel you did an incorrect accel pump install or used incorrect parts.

 

[Fishermark]

Re: 74' 250 inline 6 ... help!

Rebuilt the carb, (No change)

Sometimes carbs just can't be successfully rebuilt. If you are sure of the firing order, then I would suspect a worn out carb that needs to be replaced, not rebuilt.

 

[ampcarfan]

Re: 74' 250 inline 6 ... help!

I disassembled the carb pre-rebuild and soaked it in a gal can of carb cleaner, am 99% sure I rebuilt it correctly as I have been rebuilding those Roch 2bbls since I was in HS..lol The accelerator pump is definatly speculative tho. I will do the rev test with the dwell meter attached as soon as the weather breaks. I double, but not tripple checked the firing order 153624. Thanks all !

 

[ampcarfan]

Re: 74' 250 inline 6 ... help!

UPDATE- Had some time this evening and triple checked my firing order 153624 and all is good. Was alone so couldn't check the voltage cranking (+ tester lead to + side of coil & negative side of the test lead to Ground) But I have 8.2 V running, but only 3.1 V key on/not running? Starts quick, even cold. Pulled the valve cover and ran it, ALL the rockers are moving well and the same. Checked the valve springs as well. On a side note, the inside of this part of the enging looks like new.. amazingly clean, no sludge or dark spots, everything is light grey.

Any thoughts on my next step??? Inline visual spark tester??? The fact that the plugs (AC MR43T) I put in 3 weeks ago were black and sooty but not wet when I pulled them to do the compression test leads me to think it's running rich/weak spark?

I'm stumped...

 

[ampcarfan]

Re: 74' 250 inline 6 ... help!

Sorry, I forgot to add that I put the dwell meter on it to check for variance while running and it held steady (32 degs) even while changing rpms and revving it to 2000 (as best as it would...lol)

 

[ampcarfan]

Re: 74' 250 inline 6 ... help!

My thought patterns are progressing... If I put an inline spark tester on and spark is good (anybody know what color it should be?) then the carb HAS to be junk... there's nothing left...? It did have alot of weird gooey sludge in it.

 

[littlebookworm]

Re: 74' 250 inline 6 ... help!

You've pretty much ruled out ignition as your cause, though those voltage readings are a little low. If it were my boat, I'd do the following: Replace the carb, dis-assemble and clean each battery connection - including both ends of the battery leads, and try a different battery. In my mind this would clear up your problem. Let us know the finall outcome. Good luck. Hy

 

[ampcarfan]

Re: 74' 250 inline 6 ... help!

Thanks Hy, Batt is new and charged (12.8v not running) as are the connectors. Voltage on the coil is correct running (8V), but not sure on the 3v not running/key on..
It seems to be running a little rougher as I progressivley test, even at idle.
thinking the carb is junk

 

[ampcarfan]

Re: 74' 250 inline 6 ... help!

UPDATE: Finally got the new carb yesterday and installed it. Exact same problem. Idles ok, runs a little rough above idle, pops through carb and is resistant to taking throttle.

Purchased a spark tester and I have a weak and intermittent spark no matter what plug wire I hook it up to.
I'm thinking even though my dwell stays steady, it HAS to be the excessive play in the distributor shaft causing this???
ANY thoughts or input would be welcomed...