View Full Version : 2GM wonbt rev out amp smokes

08-03-2009, 01:14 AM
"My 12HP 2GM has about 1200 hours up, and revs nicely to about 1800rpm. But any more throttle beyond that just leads to black smoke and no increase in revs. I think it should rev out to somewhere around 3000 (the manual says max is 3400). I've only just bought the boat so can't say if this is a recent change or not.

I've tried running it with the air filter off to eliminate intake restrictions, but no luck. I've been told by the guy who serviced it for the previous owner that he's pretty sure it's not over-propped, and he said the exhaust water trap could be coked up. I've heard of mixing elbows coking up (I'll be checking that this weekend) but not traps. Has anyone come across a coked-up trap?"

08-03-2009, 02:57 AM
"clogged exhaust mixing elbow,kinked exhaust hose,dirty diesel,clogged filters,dirty bottom,something around prop"

08-04-2009, 12:37 AM
"G'day Graeme,
Grants comments are on the money,my bet is on the exhaust elbow being chocked.Check high idle revs prior any other checks,45yrs on the job never seen a carbon clogged stink box ,seen many melted at in&outlets. Cheers, Shakey."

08-04-2009, 01:35 AM
"Thanks Doug. The trap inlet actually was melted a bit, reducing the bore area by about 1/3. I cut the obstruction away hoping it would help, but no deal. The bottom is pretty slick, so the elbow is next, then a chilly swim in Port Phillip Bay to have a look for prop growth or something wrapped around the shaft.
By the way, I read an article about dissolving the elbow gunk by soaking it in dilute hydrochloric acid. Does this sound right?"

08-04-2009, 02:34 AM
"hi grant here the build up i an exhaust elbow is carbon and can be removed with old screw driver,pocket knife,i usually take a combo of tools that can be used probably even a rotary tool but use stones as carbon is bloody hard remember diamonds are made of it,check that the inside tube is not corroded through as this can cause water to flow back into bore!!"

08-04-2009, 02:47 AM
"okay by now you would have taken off the exhaust mixing elbow and discovered it clogged and poked your nose into the exhaust ports and gone holy hell,yes these are carboned up too!! so turn engine in natural rotation direction until vavles are closed for #1 cylinder,you may need to removed vavle cover for this but thats no biggyscrap,grind,file,blast all the carbon off the exhaust port ,get a vaccumm and suck all the crap out ,rotate engine in normal engine direction until vavles are closed for #2 cylinder and do the same then vaccumm out,file or emery cloth the cylinder port face install new gasket and bolt exhaust elbow back on,while you have tappet/vavle cover off tighten head bolts/nuts in correct order and adjust vavle lash 008inch when cold,do an oil change and filter"

08-04-2009, 02:54 AM
Another dumb question: the 1" exhaust hose does a full loop beneath the lazarette before it connects to the stern fitting. This is handy in that it allows the trap to be pulled clear of the engine to do up the clips. But it adds about 1 - 1.5 metres to the length of hose - is this normal?.

08-04-2009, 02:57 AM
"hi again you can turn the engine only in the normal way as if you turn it the opposite way it well stuff the impellar or you could just remove impellar and turn it any way but usually its best to rotate only the designed way as thats the way its been going all its life and wear patterns are formed,but when i did mine i had the water pump off and was always just going pass the tdc as i was looking for clearance on a valve(yes spelt it right that time)that had none so would rotate it back a little the opposite way,not a hell of alot,maybe 3 inches and as impellars wasn't in it wasn't going to stuff it good luck,keep us posted"

08-04-2009, 03:34 AM
"Hi grant,
Iam a couple of hundred ks south of you and know the c 28 -29 very well,gm series are know for chocking the exh elbow, acids are used to clean out the eng & manifold internals i use vinigar, fill eng wait 24hrs then flush.Manifold ete require the chisel & armstrong method,most people nurse there motors much when going back to your berth give it Wot for last 12-15 mins TREAT IT LIKE YOU HATE IT!.When doing v/v clearances if you go past tdc and have to go back always go back at least 1/4 turn so as to take up all backlash on return. Cheers, Shakey."

08-04-2009, 04:06 AM
"hi doug thanks be not my post yes i agree with when returning to mooring run it at about 3200rpm,i was told my a good marine engineer not to let the engine idle in a no load state for to long as some people do to charge batteries etc as this can result in glazed cylinders,the marine engineer said if tried up at marina and you are idling the engine just put the it into gear just enough to make the engine work,and yes after motoring around slowly about every 15mintute give it a good burst uoto 3200 and this well stop any carbon build up hey but this is not my posthttp://www.marineengine.com/discus/clipart/thumbs_up.gif"

08-04-2009, 05:23 AM
Agree totally with your post. MUST pay more attention,apologies Graeme. Shakey."

08-04-2009, 11:19 PM
"Well sadly the elbow is clean as a whistle - sooty black coating, but not clogged at all. After I replace the elbow and gasket I'll have run out of excuses, so it's time for a reluctant chilly swim.

By the way, is there anything special I should put on the gasket? It seemed to have some grey-ish paste stuff which I scraped away. Looked a bit like anti-seize.

Cheers, Graeme"

08-05-2009, 01:47 AM
i would check diesel and change filters and then bleed cheers grant

08-05-2009, 05:09 AM
is engine running on two cylinders as you say it revs to 1800rpm half of what it should and as a two cylinder if one was out it would be 1800rev's

08-05-2009, 07:09 AM
"Hi locke,

Luckey i did not bet my Tom Bowlers on the elbow!Try the following and report.
1. Check full throttle [high idle] rpm in nuetral.
2.Isolate injecters 1 at a time see if 1 produces a greater reaction than the other.
3.Check around inj pump, has any body been playing there?
4.Leave exhaust hose loop in place not a problem unless grimped if fitted with a valve check fully open. Cheers Shakey."

08-10-2009, 07:10 PM
"Well now I'm stumped: the previous owner says this was a long-term problem for him, despite verifying the the prop size, keeping the bottom and the prop clean, regular annual services by the local Yanmar dealer etc etc. I guess the filters are next..........


08-11-2009, 02:43 AM
"hi filters,engine stop working properly eg all the way off,decompression levers not engaged,vavles not partly open,lift pump,injectors cleaned, 90% of diesel problems are fuel(nigel calder marine engineer and author)check if there are any air getting in from previvious people overtighening the bleed screws as these are plastic and get damage when an swang on to tight,bleed low pressure system until all air is out maybe will take more than one go then put in full trottle postion and bleed high pressure side,until just clear fuel always tighten as you are still pumping the fuel out as not to let air suck back into system,i just engage decompression levers and crank on the starter but no more than 30 to 50 secs with engine stop pulled out as you don't want a nasty surpise eg engine starting,check all rubber fuel hose connects and hose clips any chaffing eg against engine compartment and replace,remove the hose clips and check that the hose is okay there, as sometimes the clip cuts in if over tightened,pull hose from tank and if there is a gauze strainer?filter rip it out and biff sorry this is all abit back to front,work from fuel tank along hose to first filter/water seperator then hose to second small engine filter etc,i am surprised you did not do filters first!!!!good luckhttp://www.marineengine.com/discus/clipart/angry.gif"