3.8L miss

[fendersfender]

'85 searay twin omc 3.8L v6's w/ 800 series stringer drives.....starboard engine misses like heck....have changed plugs, rotors, caps, fuel filters, engine ran good at first start, but after idling up the river it began to miss...i pulled plugs out and all are carbon fouled, i put another set of plugs in her and checked timing and leaned out carb by 1 full turn on each idle mixture screw, (4 barrel quadrajet), but engine ran better when idle mixture screws were turned out, richer?? I am assuming that in is leaner and out is richer? correct? carbs were rebuilt last summer and boat ran good all year. I think i will set carb mixture screws exactly as other engine, and take her out and run the p### out of her,,,,,and the chokes are working as they should...no excessive fuel is spilling into carb while at idle...i'm stumped, any ideas???

 

[Don S]

Re: 3.8L miss

That's not how you adjust a carb, that is how you tinker and futz around when you don't have a clue.<br />Have you thought about a vacuum test, perhaps doing a compression test to eliminate any serious problems? Have you changed spark plugs, and please tell me they aren't champions.

 

[KaGee]

Re: 3.8L miss

Sure there is no fuel coming in from the fuel pump overflow?? Sure sounds rich to me. Maybe a choke butterfly not opening right?

 

[fendersfender]

Re: 3.8L miss

don, i adjusted idle mixture screws with timing light with built in tach to achieve highest rpm and then adjusted idle to specs, at idle i hold 15 on vacuum gauge, except for jump when engine misses, compression is 140 across the board give or take a few psi.....i am assuming that carb is junk, as mixture screws are loose in threads...but it seems to me that this would cause a lean condition???? kagee, no fuel from overflow that i can tell, even unhooked it and choke is wide open......i have good spark at dist. to all wires, timing is good at 10 btdc...it has to be a carb problem as new plugs are all carbon fouled...i will have carb gone through or get a remaned carb...ill let you know...hey, that is the correct way to adjust a carb, right?? to adjust for highest rpm????

 

[Don S]

Re: 3.8L miss

don, i adjusted idle mixture screws with timing light with built in tach to achieve highest rpm and then adjusted idle to specs, at idle i hold 15 on vacuum gauge, except for jump when engine misses, compression is 140 across the board give or take a few psi...

Great, your original post had me worried. <br /><br />The good news is you have twins. Swap the carbs and see if the problem switches sides. I may have missed it, but did you have Champion plugs in it?

 

[rodbolt]

Re: 3.8L miss

someplace in the service manual is a chart on how to use that vacum gauge, its a mighty handy tool if you pay attention to it.<br /> it can show exhaust resrtictions,leaking intake valves,broken valve springs and blown head gaskets ,late valve timing and a host of other engine mechanical issues all by learning how to interpet what the tool is telling you. you can try swapping the carb but unless its full of water its not likly to help.<br /> that Q-jet is about as simple as a carb gets but its entirely vacum controled,lose proper engine vacum and it cannot function properly, at 15"Hg at idle you have lost proper intake vacum.<br />your engine should see 18-20"Hg at idle.that little jump on the guage normally goes away when the affected cyl has the ign removed. its another way to aid in isolating which cyl is causing a problem.<br />the tapered primary metering rods on the Q-jet are held in the closed position by engine vacum and opened as vacum decreases by a spring under the valve. if a mechanical loss of engine vacum occurs the carb is to stupid to know why the vacum loss occurs and the spring allows the rods to withdraw from the jets and allows more fuel, making a problem worse and causing a lot of techs to chase ghosts. your 140 compression numbers are decent. its possible you have a riser leaking or a sticking valve or a broken spring, however water in the fuel can play havoc with that q-jet as well. flat spots on the cam or a worn timing chain can produce low intake vacum.<br /> when was the last time the motor ran well and how long did it sit before it ran poorly ?

 

[fendersfender]

Re: 3.8L miss

i run ac-mr44t as per manual, gapped at .035.....engine ran well all last year...fired right up this spring with a miss though (both engines did, i assumed it was from fogging oil) after fresh plugs it ran fine until idling up and down river for 30-40 min, then the miss started....boat has not been ran yet, up to planing speed that is.

 

[KaGee]

Re: 3.8L miss

You probably did, but just in case, did you look for any sign of water droplets on the plugs? I have a 2bbl 3.8. That's how it ran when a manifold started to fail.<br /><br />Another thing I noticed on this engine, the plugs always looked dark when playing with it in the drive. It was only after I took it out and opened it up under load that the plugs lightened up.<br /><br />What happened with your steering problem?<br /><br />BTW... I have coverted my manifolds over to the OSCO center riser configuration, it's the only way to go if you need to put manifolds on it. Let me know and I'll send pictures.

 

[fendersfender]

Re: 3.8L miss

no water on plugs kagee........put another fresh set of plugs in it yesterday and went fishing, trolled for 4 hours and ran at 4000 rpm for 10-15 minutes and she runs perfect. fishing wasnt that good though hahaha......maybe me adjusting the mixture screws helped, but i cant see that making a big difference, it ran good last year.....oh well, it runs good now.....thanks for all the help guys....happy easter