Exhaust install and poor performance
 

I just installed an EMI thunder exhaust with 16" long risers on my Baja Force 220. I also changed out the stock intake for a performer RPM manifold. Took it to the lake and it fell all over itself. The motor is a stock 260hp 350. I am pretty sure I have a lean condition I swapped out the quadrajet for a holley 600 I had sitting around. I havent had a chance to test and tune yet, was just wondering if the 600 will be enough carb. I have a jet kit and plan on some t/t time tomorrow.
Any thoughts?

Re: Exhaust install and poor performance
 

Not sure why you ditched the Qjet it is a good carb. On a stock 260 the headers may have reduced the back pressure to much, headers do very little if anything but make noise on a stock setup. With the 600 you can't get enough air in to feed the exhaust, even if you could the stock cam is another choke. Years back I ran into a similar problem on a customers boat, we got it to run decent by choking the exhaust back down to 3" with a home made restrictor.

Phil

Re: Exhaust install and poor performance
 

I have the EXACT same setup other than intake, which is a GM Bowtie high rise intake. Very, very similar to RPM intake other than cast iron vs aluminum.

With the quadrajet I installed next set richer secondary rods - I believe DA's? Don't quote me, memory.

Boat had 22 ignition module, I set inital advance to 12 degrees, using 89 octane fuel.

Engine picked up good power.

Quadrajet 'took a dump' and I installed new BG650 vac secondary marine carb. Played with secondary springs, and engine picked up even more power. I ended up with the super soft 'white' spring, but this is a very light boat (about 2800lbs wet) so your experience may be different.

I highly recommend using the Fel-Pro 1204 intake gaskets that block off the exhaust crossover, thus removing a ton of heat (more important on aluminum intakes)from the intake and stopping the exhaust gas dilution from these exhaust ports.

I agree, the exhaust will not add hp but will remove about 80lbs from boat. However, it may require carb tuning which may be why MOP lost power with the exhaust change ?

Also, yes, the cam on these motrs is very restrictive, enough so that installing a set of good heads without replacing the OE cam will net close to nothing, however, it will respond to better intake as you have done.

Summary - I have gained near 5mph GPS with intake, intake gaskets, exhaust, and tuning. Again, small little 2800lb boat wet with a pad. All boats are different and thus will respond differently.

Hope this helps.

BTW: The 600 is totally fine airflow wise, the carb itself will prob just need to be tuned in. Also, assuming you have the spreadbore flanged RPM, make sure the Holley carb seals to the quadrajet pattern on the intake. If it doesn't, Edelbrock sells a $8 plate that will make it seal. If you need the part # I'll dig it up for you.

If you have the squarebore flanged RPM intake, you had to use a spacer/adapter for the Quadrajetthat probably screwed things up in the tuning/hp department.

Re: Exhaust install and poor performance
 

I forgot to add, if you have a 'heat stove' operated choke, you prob don't have it connected anymore to carb. On quadrajet, wire up choke rod to keep choke door open. If you don't, choke will prob stay closed most of the time and really mess things up.

Re: Exhaust install and poor performance
 

Is the timing correct after you installed the intake. You should not lose any performance. I would also be surprised if you see any increase. Takes allot of additional HP to see 1 MPH.

Re: Exhaust install and poor performance
 

Check The Power Valves On The Carb You Had Sitting Around

Re: Exhaust install and poor performance
 

well i made it to the lake with the new carb.
same stumble on take off. Did a full throttle run and shut it down and pulled a plug. Lean
checked the jets they were 63's. Changed to 67's and gonna try again tomorrow.


Timing is at 8BTDC, which is factory. With the quadrajet I had it at 15BTDC which stumbled a bit out of the hole, but didnt ping on the top end.

I noticed someone said something about performance, well with the quadrajet on there and the boat not running right, I pickup up about 500 rpms and around 4 mph with my 19 pitch 4 blade prop, when it actually would pull hard out of the hole, It was on plane much faster than before. I can feel the difference in the boat.

This is only the first stage of the build, with it being summer time I am trying to keep it running so it can be used. Next month come the heads and cam and another carb.

Thanks for all the help

Re: Exhaust install and poor performance
 

bring it down and put it in at toronto

Re: Exhaust install and poor performance
 

had to work today or I would have brought it to the river.
Ran it tonight on the lake, #68 jets, slight stumble around 2500, full throttle takeoffs fairly smooth. Gonna play with the secondaries tomorrow, and maybe jet up to #70s.

Re: Exhaust install and poor performance
 

hope you get it running i hate tuning when its this hot

Re: Exhaust install and poor performance
 

why would a small block not produce more hp with aftermarket exhaust, but a 7.4L get up to 30 hp increase over stock exhaust?

Re: Exhaust install and poor performance
 

There is definetly more power. In my experience anytime you make a motor breath better it makes more power!!

Didnt make it to the lake today, it stormed, gonna try tommorrow. Anyone out there have a small block with exhaust and aluminum intake running a holley carb?
Just curious of the jet sizes being used.

Re: Exhaust install and poor performance
 

500 RPM, 4 MPH..... I don't think so, Tim.

Re: Exhaust install and poor performance
 

YEP
Stainless Marine Exhaust, RPM Air Gap Manifold, BG Carb but sent to Nickerson Performance to straighten out. Runs like I stole it.

Re: Exhaust install and poor performance
 

Found the problem today, pulled all the plugs after a run, found one with cracked ceramic and another that I dont think was firing. Put new plugs in and hit the lake and it ran great.
Thanks for all the input guys!

Re: Exhaust install and poor performance
 

I'm confused by this too.
Bad tach, speedo ? Using GPS ?

Or did we install lower pitch prop ? Or a ton of slip ? :eek:

Re: Exhaust install and poor performance
 

Using regular gauges to monitor.
Before the changes I could get about 4300rpm and 45-48 on my speedo, this is just a regular speedo, not gps. I have the original review from Power Boat magazine, and this boat outfitted like mine with a 21p ss prop should see 55 @ 4800. After the upgrades with the same prop I can get 4800-5000rpm and between 50-52 on the speedo. I have no idea how accurate the speedo is. Based my observations on the readings of these.

Re: Exhaust install and poor performance
 

BTW I am using a 21p ss prop