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backfire through carb

Andrew511

Active member
I have a brand new marine 350 on my DD boat. it fires up and runs great. If you ease into it until about 1500 then stomp it it will build power to 3k fast and easy but if you stomp it from idle it backfires through the carb its a new holley 390 2 barrel and it does have a safety so the power valve doesn't blow when this happens. I had the jets increased, bigger squirter and put a 50cc accel pump on it. I wasn't there but the mechanic triple checked timing and adjusted the valves. Im wondering if a different accl pump cam profile might straighten me out. I ordered them and they should be in today. currently it has a black one that came on it. Does anyone have a recommendation on what color I should use? From what ive been reading the position 1 is for a quick squirt for off the line and position 2 is later.
 
Keep in mind your spinning a big prop that has a lot of resistance. Then dropping your vacuum down to near nothing.

Our way to test this accelerator pump / lean condition is to get a small squirt bottle. Put a little fuel say 1/2 ounce in it and when you rapidly open the butterfly's give it a little squirt. If if clears up or improved your on to the cure.

Be mindful of safety
 
SWAMPHUNTER45 said:
Keep in mind your spinning a big prop that has a lot of resistance. Then dropping your vacuum down to near nothing.

Our way to test this accelerator pump / lean condition is to get a small squirt bottle. Put a little fuel say 1/2 ounce in it and when you rapidly open the butterfly's give it a little squirt. If if clears up or improved your on to the cure.

Be mindful of safety

I will try that with the squirt bottle. Is it dangerous to do that? if it back fires will it throw a flame or anything? im not an expert with this stuff by any means. I can run my boat as is but it bothers me that I cant stab it a little to blow the backwash away when coming off of plane without a backfire
 
If you spend time around a race track mechanics do it to aid start up. There is a risk which is why we only use a very small amount.

Be mindful of safety
 
Rich Andrews said:
id get a 4barrell manifold and carb have it tuned by a professional and be done w it.

I have a 4brl manifold with an adapter for a 2brl. I had a 650 4brl on my last engine and I was told it was too much carb for a DD that's why I went with the 390. I made a post on here and someone gave me the exact part number that should of worked for me
 
AS PER JOHN FENNER his recipe for a 350 DD Vortec

To be honest, a 390 Holley 4V will perform much more efficient than the 500 2V, yes it will need the secondary metering plate kit, a set of 68 jets in the primary and around 70/72 In the secondary metering, more money but in the long run it will pay for itself in fuel and performance.



I would play with what you have now and see if you can get the stumble out of it. Read the plugs and if you can adjust to the roll on type throttle your set.
 
I changed the cam on the accel pump to the yellow that came with the 50cc kit and the son of a bitch ran great. The mechanic that had it for 3 months gave me a box of parts he tried and it was in there so I tried it. I ran it for about 10 mins on the trailer and seemed great. Gonna give it a go on the water tomorrow. Is there any thing negative I can get from too steep of a cam on the carb? The 50 cc kit only came with two. One brown and one yellow and it would lean pop with the brown
 
Lean will pop.

If it is taking the fuel you found the fix.

Run it and see how she acts. Take a look at your plugs after a few hours.
 
SWAMPHUNTER45 said:
Lean will pop.

If it is taking the fuel you found the fix.

Run it and see how she acts. Take a look at your plugs after a few hours.

Sounds good man. I’ll throw some new plugs in it before I run it this weekend. If I remember I’ll take some pics of the plugs after a ride or two
 
390 is 4V 350 is 2V, now, you need a #31 squirter, the cam from a 500 Holley and most likely need to have the pilot jets drilled out a bit.
 
I had this same situation 2 weeks ago! I, too, am not a mechanic, lol. I replaced the fuel pump, fuel pressure regulator, and spark plugs. None of these corrected the problem. My boat was built about a little less than a year ago also. I became quickly frustrated and began looking for any source of excess air. After checking my intake and carb bolts, I found 5 that were lose, not falling out lose, but 2-3 turns lose. I lightened them up and prayed for the best. I took it out the next day and it never missed a beat. Not saying this is your problem, but I would probably check and rule out the small things first, unlike me who spend a couple of hundred dollars shooting in the dark.
 
Check your timing. I had the same issue and my timing was retarded. Soon as I added more timing the issue went away.
 
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