Misfire or backfire

[SweeperForce]

Today at the lake I ran into something new. At idle and anything above 3/4 throttle she runs like a top. But say at 1/2 throttle I'd be cruising along and BANG. I thought I hit a log, it was that loud. it keeps doing it every 2-3 minutes. Anyone have an idea of what is going on here and more importantly how to fix it?
Thanks in advance,
Tom

 

[pnwboat]

If it's a backfire, I'd take a look at the plug wires etc. Make sure the primer bulb gets firm after 4 or 5 squeezes.

 

[Jiggz]

You need to determine with certainty it is a backfire and not some kind of detonation or pre-ignition, which can cause serious damage to the motor. High carbon build up in the cylinder and piston can also cause detonation or pre-ignition. You need to do a visual on your spark plugs for signs of carbon build up. If you know for sure it is a backfire, you might want to double check your timing settings.

 

[jerryjerry05]

Sounds more like the clutch dogs wore and it's a jumping??

 

[jimbo_jwc]

Do a compression test as I found a cracked cylinder bore sleeve on a old 120 hp that ran like you mentioned. It was cracked the whole length and when enough gas got into the water cooled exhaust chamber it would ignite /explode . The stainless cooling plate when I disassembled looked like someone had beat it dimpled with a ballpein hammer and it was really carboned up,

 

[SweeperForce]

I did a compression test and got the following: 1) 149 2) 142 3) 144 4) 145 5) 145 All plugs were dry and clean except for cylinder 2 and 3. They have a few drops of brownish oil on the top. No carbon build up. My only other problem, not sure it is related but is when I come back to the dock after an afternoon out, it stalls out in neutral. My rpms are dead on when leaving the dock; neutral@1100 and idle in gear@800. I replaced all the recirculating tube last spring(2013). It seemed to help a but but now getting worse again. I'll check the bulb next time its running. Thanks, Tom

 

[Jiggz]

Dry and clean plugs do not necessarily correlates to proper operation. Sure it indicates proper combustion but it can also mean two different things, lack of lube due to lean fuel settings or water intrusion (steam cleaning). Desired plug condition should indicate light to dark (but not black) brown discoloration on the face of the plug with a little wetting (but not dripping) from oil. In your case, #2 and #3 seem to indicate the desired plug conditions. You might want to check the idle fuel oil mixture screw setting on the bottom carb. If it is already set to 1 turn out increase it to 1 1/8 and see if that will improve your idling. And maybe on the top carb also.

 

[SweeperForce]

No Title

These plugs are four years old and have roughly 40 hours on theme. It is recommended to replace after 100 hours. Considering the problem I'm having is this a logical starting point? If this don't help then I will readjust the idle mixture screw. Here is a picture of my plugs.

 

[Jiggz]

The middle pics shows plugs' faces with particular pattern of concentrated arc almost in the shape of a clover leaf. This is quite normal due to cold fouling deposits build up in the gap which itself is not normal. Cold fouling is usually caused by improper fuel oil mix (too rich of fuel and less oil), overrich carburetion (especially if you are operating at higher altitudes and using normal size jets) or too much choking during starting.

Try cleaning the plugs and test them again to see if things improve. If not, double check your timing advance which should be 30 BTDC dynamic or 28 BTDC static. I noticed you are using BUHW plugs which are appropriate for Thunderbolt systems. However, if you have the Prestolite system you will need the BUHX instead.

 

[SweeperForce]

I do in fact do have a Prestolite magneto. Lucky an Advanced auto part near my house had five of the NGK BUHX. The reason I was used the wrong plug is because that's what was in when I bought it. Anyway since I was going to replace the plugs I might as well put in the right ones. Besides it only cost me 13 and change for all five. A huge thanks out to Jiggz for bringing this to my attention. Tomorrow I'll install them and keep you posted. Thanks, Tom

 

[Jiggz]

Well, if that is the case that you actually have a prestolite system and uses BUHW instead of BUHX, you will surely recognize improvement in idling and WOT speed with BUHX, hopefully!

 

[jerryjerry05]

The surface gaps can last as long as 10 years or more.
I'm cheap and didn't want to buy new.
Just take them out and clean them.
Champion plugs came with the motor. Try to use them.

SPARK PLUG (Champion # UL-18V) (89A Thru 90A)
		SPARK PLUG (Champion # L-76V) (90B Thru 92C And Serial # 0E000001 Thru 0E093699)

 

[CreekCruiser]

Today at the lake I ran into something new. At idle and anything above 3/4 throttle she runs like a top. But say at 1/2 throttle I'd be cruising along and BANG. I thought I hit a log, it was that loud. it keeps doing it every 2-3 minutes. Anyone have an idea of what is going on here and more importantly how to fix it? Thanks in advance, Tom

I had a similar issue, turned out the key way between the crank and flywheel had sheared, causing all kinds of problems. After I pulled the flywheel and saw loose metal I realized what was happening. Got the new key way for $16 or so locally, back to running normal. Hope this helps

 

[SweeperForce]

Sorry for the late response, I was in the hospital earlier in the week, all is well now. I was able to get the boat out before I went in. So far the only thing I did was change the plugs to BUHX not BUHW. it ran good at all rpms, even when coming down from wot, where it usually stalls out when slowing for a no wake zone. I did still have thumping sound like I hit an underwater log. Only happands at around 1/2 throttle(3000rpms). When I'm able to get the biathlon back out again I'll redial the air/furl mixture screws. Tom

 

[Jiggz]

Need to listen carefully on the "thump" if it is some kind of gear slip or misfire. I doubt it is a backfire but with the proper plugs you still want to check the settings of the idle and the idle mixture screws.