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View Full Version : Misfiring badly idle and part throttle, thoughts?



cadunkle
06-14-2014, 08:57 PM
'89 Saltare with 454 rebuilt by PO ~2007. No idea what's in it, but carb is a 2 year old Holley 800 DP, Distributor a Mallory Unilite with surge protector from Mallory wired inline, NAPA marine coil about a year old. Boat ran great end of last season, no issues. Stored with full tank of 89 octane with stabil added after it sat unused for a few weeks. Rocked boat on trailer, ran it a bit and winterized.

Boat was hard to start this season, gas was nasty yellowish and didn't vaporize easily when spraying accelerator pump. Sprayed out carb and air bleeds without disassembling any of it and got it started. Fuel looks better but not fresh. Went out for the first time this season today and it started easy, mix of idle and 2400-2800 RPM cruising for about 20-30 mins then anchored ate lunch, chased wiring under dash since radio wouldn't work, then ran maybe 20 mins at 2500-3000 RPM and came back to idle... Stalled. Immediately restarted but misfiring badly. More throttle and misfiring badly but over maybe 1800-2000 RPM smoothed out. Ran the 20 minutes back to the ramp at 2500+ RPM and it ran fine even up to wide open. Slowly back to idle and it misfires starting somewhere around 2000 RPM or less and gets worse approaching idle, wants to stall but won't quite stall. Sprayed carb cleaner in all 8 air bleeds and after spraying one of the primary air bleeds it runs better but still misfiring. Called it a day and pulled out.

Once home I checked distributor cap and rotor, cleaned off some minor carbon tracking, sprayed air bleeds, adjusted idle mixture screws. It sounded like a 4 cylinder but eventually I got it idling normal. Revved up around 2500 and back to idle, and back to misfiring and sounding like a 4 banger. Lightly working accelerator pumps without opening throttle seemed to slightly smooth it out. Played with carb spraying it and adjusting mixture some more and it's still misfiring but sounds more like it has a big cam. In other words not a steady hit on half of them as before but random misfiring not as bad.

I've never stored this boat will a full tank, usually half or less and topped off with fresh fuel first time out. Previous boats only stored once will full tanks and I recall burning through that it didn't misfire but just felt off, like a little hard to start and not as snappy. The issues resolved themselves with fresh fuel. What gets me is that it runs poorly at idle and lower RPM (though didn't initially) but runs fine at higher throttle and RPM. Wondering if it's a fuel issue given how nasty the fuel in the carb and lines was on first start of the season. Thinking I should clean the carb properly, but not sure if I want to do that now or after burning through the rest of this 50 gal of old gas. Only burned 1/4 tank today and didn't top it off.

Thinking maybe I should change spark plugs (2 yrs on these now), fuel filter (1 yr old), and pull carb to thoroughly clean everything. Almost acting like junk or varnish in air bleeds or idle passages, like it's real lean at idle and just off idle. Wonder if the fuel is a problem though and hate to run a bunch of bad gas (if it is actually bad) through a freshly rebuilt carb. Thoughts?

michael hunter
06-15-2014, 08:35 AM
Sounds like bad gas take a gas sample in a clear jar and see what it looks like.

TitanTn
06-15-2014, 08:42 AM
I think putting in a carb kit would likely solve this.

inair
06-15-2014, 10:44 AM
Last year I left ethanol gas in the boat with Sea Foam stabilizer in it. In the spring it had turned a little ugly. Gas would get through the filter but actually clogged the screen just before the carb with a thick yellowish sludge almost like gelatin. I could idle and get about 1500 RPM and then it would die. In the end some got through the screen and clogged up the carb as well. Your fuel may not have gotten as bad as mine so you could get some cruise time before the problems started to get worse.

michael hunter
06-15-2014, 12:23 PM
Drain and clean the fuel system then rebuild the carb.

CJD
06-15-2014, 05:45 PM
I used to store with a full tank. Now that all we get is gasohol, I only put the minimum I can. It just doesnt store like straight gas, even with the stabilizer. Your symptoms sound like classic bad gas. It seams like ignition for a few minutes, then carb, then something else before going back to ignition.

cadunkle
06-21-2014, 09:46 PM
Got it solved, I think. Just junk in the fuel system. Drained carb bowls and fuel didn't look great but I've seen a lot worse. Pulled the bowls and blocks and found a bunch of sandy grit in the bottom and some on the primary bowl needle no doubt causing my occasional leaky booster that I thought I'd resolved by setting fuel level lower. Cleaned it all out and reassembled. Now she's idling and revving smoothly. Did not have a spare fuel filter so I'll have to order one, not a big deal. I'll just top it off tomorrow and try to burn most of it out since it seems to be doing fine now with the carb cleaned.

I also changed the plugs. Old plugs looked rich and fouled. Was tempted to drop a bit on jet sizes but they could have fouled from dumping fuel whenever the junk on the needle caused the bowl to overflow. I'll run it a bit and try jetting leaner once I've got fresh gas in it. Engine is pretty much a stock PCM 454 so far as I know. Carb is a Holley 800 DP and current jets are 72 primary and 87 secondary which seems in the ballpark. Definitely needs more primary accelerator pump shot though as I've always had either a slight stumble when punching it too quick from idle. Didn't check discharge nozzle size, I'll deal with that once I'd got fresh gas in it and probably start with a more aggressive cam profile and failing that go up a few sizes on the nozzle. Secondary pump is a 50cc which I think is more than this needs so I'll probably change that over to a 30cc.

Hopefully it runs well tomorrow, I have so little time to work on projects anymore since work got crazy. It was a late start to the season and only a few weeks 'till vacation. Three weeks on the lake and it's not usually as quick or easy to get parts there. The reunion is on my mind but not sure I want to do 1000 miles south then 1500 miles north in one weekend!

TitanTn
06-22-2014, 07:59 AM
Glad you figured it out and that it wasn't anything major. So since you saved all that time and money, the trip south to the reunion should be easy! It's downhill anyways...

CJD
06-22-2014, 10:47 AM
Burning all the gas out of the tank...that's what I'm talking about! Can I help?

Also, don't judge the plugs following a misfire condition. They will always be black and wet afterward. If it feels strong, I'd leave the jets where they are. With the loads we operate, you would much rather a bit rich than any lean at all.

cadunkle
06-23-2014, 09:09 PM
Topped off with about 13 gallons of 89. Fired right up from cold at the ramp, ran well all day. Had one issue after running for a while then eating lunch of the hook it started but didn't idle well, was lean and needed about 1/4 turn out on each idle mixture screw. After that it was fine with much time ranging from easy cruise at 2500 RPM to WOT and a fair amount of idle through no wakes. I think it's safe to say it was just trash in the fuel that got to the carb rather than bad gas from age. I've had vehicle run "alright" on 2 year old gas, though that was in the MTBE days before ethanol. I've also seen fuel that was at least 15 years old. That stuff was nasty, smelled like varnish, and when I tried to light it with a torch it took a bit to light and wouldn't stay burning without holding the torch on it.

I'll bring my Holley tuning stuff with me on vacation and get the carb dialed in there. It may well be fine as is but I won't know until I go through it. Should give me something to do and I feel much better tuning when I have my own private dock to tie up at on a calm lake rather than taking up the very limited space at the public ramp on a busy tidal river with a strong current.