No wake zone low speed miss
 

I installed anew long block about 15 hrs ago ( stock ) and last night took boat out ant in no wake zones would develop a miss after a bit . Seemed to idle fairly smooth, and if I advanced throttle it pull clean and ran fine . Its stock 350 with thru prop exhust and (a Holley 4175 vac secondary carb. When I started it that morning I pumped throttle a couple times and it backfired once but then started. What are symptoms of hurt power valve? Can backfires still hurt those? One terrifying thing is at ramp whe n I rise up and went to get truck when I retsrateted it turned hard for a split second then started and I thought maybe water in cylinders. But started fine at house to flush it and I started it 3 more times since and no issues and cranks fine. I had underwater lights and stereo running so may e just battery was drawn down ). Exhust is about 1.5yr old and tested it with alchol when I did motor. Sorry for long post.

With a low speed miss and a backfire I suggest checking the timing, then pull the plugs and see what they tell you

Could be timing. Could be reversion from a bad/loose exhaust manifold gasket. When my carb was too rich it had a miss at idle.



Edited: failed comprehension on my part

Might recheck your exhaust, a small leak at idle could cause a miss that would clear up with more exhaust pressure.

Sounds like could be bad power valve in carb.

Just started it and cranked fine and ran it on the hose for a bit. It seemed like it loaded up and idled rough. If could advance the throttle and clear it. Ran it till it warmed up and then let it idle a bit and **** it off without clearing it. I will pull plugs tomorrow and see if they are sooty or worse wet.

the symptom,s sound like reversion.did you pressure test the exhaust system?

FWIW, if I pull the plug out of my O2 bung it's shocking how much water squirts up out of the hole, but when I leave the plug in and kill it first the plug comes out dry and sooty. That just demonstrates how much a leaking exhaust gasket could also suck up some water. With the backfire up through the carb though I'd guess the PV is dumping huge gas in there flooding it at idle.

I didn't pressure test the exhust. To make story longer, the old motor got water in cylinder number one and at first suspected failed exhust. Pulled them off intact (risers in place) and filled them with water and never leaked. Hooked up water and pressurized clock and exhaust port around valve guide on one leaked. ( motor had 1600hrs of salt sayer use so assumed head errorded ). Manifold and riser surface a were clean and flat and overall they looked ok, but they are GLM and I know some peop,e hate them. I guess I will pull carb apart and replace PV to eliminate that.

Good to know, so if I am following even if plugs are sooty I may still have a lake?

What gen small block is this ? How new is this carb ? How long was it sitting if not new ?

This is a bare stock 350 motor, don't see how reversion could be an issue

It's all stock basically, only mod is a 4175 vac secondary All dodge. I think a gasket leak or pinhole could cause reversation even with stock cam etc.
I started it before work and cranked fine and ran little grumpy. So advanced throttle to like 1200 when it was chugging and turned idle mixture screws in. They typically at like 3/4 turn out and are very senistive. Like a couple degrees changes the speed a lot. I turned them both in till seated and although it didn't want to idle well at all it didn't stall. That seems weird it would run at all with both of them fully seated.
More history, Carb is about 10 yrs old and hasn't been apart since I installed it and did some minor main jet changes with old motor. Didn't sit long as it ran fin before the old motor head cracked. Also, I looked up my maintance spreadsheet and manifolds and risers are about 3.5 yrs old, I thought they were newer than that but they looked ok. ( I know appearance of the passages is not the total picture though)

even a bones stock 350 can revert water if it has an exhaust manifold with an internal crack that allows water to enter the exhaust stream.

I'd do the simplest and easiest first. Sounds like you could be loading up on incorrect timing. YES, backfires and Holley PVs do NOT like each other. Although the newer PVs seem to be more resistant, they are still vulnerable to backfire. Do you have the right one in there in the first place? Holley PV are marked on the flat side with a number, which is the Vac load. You need to determine your vacuum at idle and make sure you have the right one in. I'd change PV (for fun), set float levels, reset my idle screws, Check for vacuum leaks (double ck to make sure you covered all non-used ports--don't ask! LOL!) double check timing and take it out and have a beer before I'd get all tied up in reversion and all that complicated stuff. If you know for proof positive the fuel and spark are right, and you still have the same situation, THEN start looking around at other stuff. JMHO, I could be wrong!

Longer than 5 or 6 months ?
I'd still go in and give everything a good cleaning. Carb cleaner and stick red straw in everything while blowing out both directions. Don't forget to remove idle mixture screws and clean thru all (4 each side) air bleeds. Blow out with air gun just after. See if there is a fuel filter in the carb inlets - some of these have them. Check needle and seats...I've had plenty with ust a few years use have bad o-rings and or tips on the needles.

Cap and rotor okay ? No, I don't care if fine last time it ran. LOL.

You didn't answer what Gen motor this is,so, does this motor have roller lifters or flat tappets ?

What's your initial and total timing ?

Has the subject of a collapsed lifter come up yet?

:party-smiley-004:

I ordered a rebuild kit for carb from local speed shop and pick it up tomorrow. Carb only sat about a month while I was doing long block. It's a Vortex with roller lifters. ( 1999 was original motor). Timing is 8 deg btdc, at least that's where I put it when I dressed it. I can't recall what the total is, it is the stock preto lite distributor with weights,,Cap and rotor have about 100hrs on them. When I got home from work pulled spark plus out and they were fairly black sooty and smelled like fuel. No water on them at least best I could tell. PV is currently a 5 and plan on replacing it with that but may buy a couple others when I am picking up rebuild gasket kit.

why did the old motor crack a head,did it freeze or overheat?

I assume it just had eroded water jacket, it was raw water cooled and had about 1600 hrs of salt water use. Head got a crack I. Exhaust port right around the boss where the valve guide pokes thru the exhaust port.

What plugs ? Weights in a '99 stock distributor ? What intake gaskets and bolts did you use ?,

Before getting too worked up about the carb and ignition, check the manifolds/risers. Sounds like it's getting some water in from somewhere.

Plugs are ac delco mr43lts. Yes, it's a volvo penta and srock distributor has mechinal advance. Long block came with cast iron intake already installed so no idea what gaskets are there. I removed a couple intake bolts to attach throttle cable bracket but torqued them and used sealer. Forgive my ignorance but wouldn't that put water in oil and not intake port? Oil appears to be clear and no milk shake.

I thought he said the exhaust manifold were 1.5 years old and tested prior to install. If it was same set the eroded head was on, I'd think that would be first check, but........hoping for him it's something in setup.

Daily update, first off I checked my maintance records and the exhust is not 1.5 yrs old, they are 3.5 yrs old. Time flies. But when I did the long block I inspected them and they weren't terrible. ( am super anal about flushing ) but they are also GLM and I know some people hate them. After work tonight rebuilt carb but was dark so did t I stall it yet. Tested PV with hand vaccum pump and tested fine but I did it find the secondary metering plate was loose, like all the screws were not even snug . I could see some burnishing on the steel plate itself and some of the finish was bugged away , I assume from relative movement with the body. Hoping that was the issue ( seems like would allow fuel to go places besides thru the metering passages).

So I dropped it in the water this afternoon and carb rebuild didn't fix it. But had more chance to troubleshoot and brought more tools. While in a wide open part of ICW I set throttle at the setting where it chugged the worst and removed plug wires one at a time and when I got to # 7 no change. Had a timing light along and when I checked it the spark was intermittent on that cylinder at best. Pulled cap off and it looked like death. Wire brushed it as a bandaid and it ran smooth and had good spark according to the light. Cap and rotor are about a year old and have 160 hrs on them and don't know why they look so bad but I bet anew one fixes it.

That is some good old fashioned troubleshooting. Seems to be a lost art these days. Most guys seem to just change parts.

Great work!

Congrats! KISS principle is always a good start!

Admittedly I am a bit of a dork and a engineer as my profession so throwing parts at stuff isn't in my nature. Not to say I dont do stupid stuff, I just do it slower and in methodological nature. Ha